Publications
2025
Intravenous liposomal drug delivery holds great promise for pharmaceutical efficacy, but faces challenges such as rapid clearance and immune system degradation. PEG-based liposome surface functionalization (PEGylation), currently the gold-standard and most widely-used approach to address these issues, is prone to reduced cellular uptake and accelerated bloodstream clearance (ABC) effect upon repeated administration due to immune activation. We demonstrate a novel liposome surface functionalization using poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC) that significantly overcomes these limitations while maintaining comparable colloidal stability. Such polyphosphocholinated liposomes exhibit tunable cellular uptake, and prolonged blood circulation times, both modulated by polymer length, alongside reduced immunogenicity (lower IgM antibody elicitation) and a diminished ABC effect compared to PEG-liposomes. These polymer-length-dependent properties offer flexibility in optimizing drug delivery systems, positioning pMPCylated liposomes as a compelling alternative to PEGylated formulations with clear advantages for liposomal drug delivery therapeutics.
The extremely low sliding friction of articular cartilage in synovial joints has been attributed to phospholipid boundary layers, lubricating via the hydration lubrication mechanism at their exposed, highly hydrated polar-head-groups, in a medium the synovial fluid where osmolytes, which may modify the hydration layer, are ubiquitous. Here, using a surface force balance (SFB), we carried out a systematic study to elucidate the effect of sucrose, a known osmotic regulator solute, with concentrations csucrose, ranging from 5 to 20 wt%, on the normal and shear forces between interacting phosphatidylcholine (PC) bilayers, both in the gel (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and liquid (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) phases, supported on atomically-smooth mica substrates. Several additional approaches including cryo-transmission electron microscope, atomic force microscopy, small- and wide-angle X-ray scattering, differential scanning calorimetry, dynamic light scattering and zeta potential measurements are exploited to get additional insight into the nature of the sucrose-dependent interactions. As csucrose is varied, a remarkable variation in the friction is observed: a marked reduction in friction is seen at low csucrose, but at higher sucrose levels the friction increases, for both gel and liquid phase lipids. This challenges the expectation that hydration lubrication is degraded by osmotic solutes, due to their competing for water of hydration, and reveals for the first time a non-monotonic effect of a sugar on the interactions, particularly frictional forces, between lipid bilayers. This non-monotonic effect correlates with the bilayer potential, and is attributed to a concentration-dependent affinity of the sugar to the PC headgroups.
Osteoarthritis (OA) is a widespread, debilitating joint disease associated with articular cartilage degradation. It is driven via mechano-inflammatory pathways, whereby catabolic genes in the cartilage-embedded chondrocytes are presumed up-regulated due to increased shear stress arising from friction at the cartilage surface as joints articulate. The enhanced expression of these cartilage-degrading and inflammatory genes leads to tissue degeneration. However, the nature of the stress, and how the cells within the joint respond to it, are poorly understood. Here we show, in a proof of concept study on a mouse model where surgical joint destabilisation has been carried out to induce OA, that the early up-regulation of the matrix metalloproteinase 3 (Mmp3) gene, a member of the matrix-degrading MMP family, and of the interleukin-1 beta (Il1b) gene, a key mediator of inflammatory response, are significantly suppressed when lipid-based lubricants are injected into the joints. We attribute this to the reduction in frictional stress on the chondrocytes due to the lubricant at the cartilage surface. At the same time, Timp1, a compression but not shear-stress sensitive gene, is unaffected by lubricant. Our results demonstrate that cartilage lubrication modulates catabolic gene regulation in OA, shed strong light on the nature of the chondrocytes response to shear stress, and have clear implications for novel OA treatments. Statement of Significance: Osteoarthritis (OA) is a widespread, debilitating joint disease associated with degradation of the articular cartilage, the tissue that covers and protects the joint surfaces as they rotate. Such degradation is due to catabolic enzymes expressed by cartilage-embedded chondrocytes (the only cell type in cartilage) in response to mechanical stress. In this proof-of-concept study in a mouse OA model, we show that reduction of cartilage friction by liposome-based lubricants suppresses the production of the catabolic, OA-related genes in chondrocytes. Our findings provide direct evidence in an animal model that catabolic genes are induced in chondrocytes in a mechanosensitive manner, related to the friction at the cartilage surface, and identify putative novel OA treatments through efficient cartilage lubrication.
Osteoarthritis (OA) is the most common degenerative disease of the joints, causing significant disability and socio-economic burden in the aging population. Simultaneously, however, it is a common occurrence in younger individuals, initiated by joint injuries or obesity alongside other factors. Intravenous and oral pharmaceutical OA management have both been associated with systemic adverse effects, thereby resulting in a growing interest in intra-articular (IA) treatment. IA-administered drugs circumvent the requirement for high dosage, offering immediate access to the site of interest while minimizing any unfavorable effects. Nonetheless, IA-injected drugs, administered in their free form, present low retention time in the knee joint raising the need for multiple injection dosage regimens, while their capability to target the cartilage or specific cell populations is limited. Liposomes, due to their unique characteristics and tunable nature, have proven to be excellent candidates for the management of knee OA. This review explores the last decade's research on the efficacy of various IA liposomal formulations, investigating their multifaceted properties as pharmaceutical carriers, lubricating agents, and a basis for combinatorial approaches paving the way to novel treatment solutions for OA.
We briefly review the organizing principles that nature has selected to reduce friction between sliding tissues in the body to maintain their well-being. This is of especial relevance where large stresses are involved, such as in the major joints (hips and knees), where lubrication breakdown, wear, and degradation of articular cartilage are associated with osteoarthritis, the most common and debilitating joint disease affecting hundreds of millions worldwide. In particular, in light of recent experiments, we revisit the fundamental issue of the mechanisms responsible for lubrication of articular cartilage in synovial joints. We conclude from these direct studies that lubrication by lipid-based boundary layers rather than interstitial fluid pressurization plays a major role in cartilage lubrication. We also review the development of new material properties for synthetic hydrogels inspired by this lubrication mechanism of cartilage, a complex bio-hydrogel, which hold strong potential for novel applications.
2024
Designing plant protein-based aqueous lubricants can be of great potential to achieve sustainability objectives by capitalising on inherent functional groups without using synthetic chemicals; however, such a concept remains in its infancy. Here, we engineer a class of self-assembled sustainable materials by using plant-based protofilaments and their assembly within a biopolymeric hydrogel giving rise to a distinct patchy architecture. By leveraging physical interactions, this material offers superlubricity with friction coefficients of 0.004-to-0.00007 achieved under moderate-to-high (102-to-103 kPa) contact pressures. Multiscale experimental measurements combined with molecular dynamics simulations reveal an intriguing synergistic mechanism behind such ultra-low friction - where the uncoated areas of the protofilaments glue to the surface by hydrophobic interactions, whilst the hydrogel offers the hydration lubrication. The current approach establishes a robust platform towards unlocking an untapped potential of using plant protein-based building blocks across diverse applications where achieving superlubricity and environmental sustainability are key performance indicators.
Microorganisms use toxins to kill competing microorganisms or eukaryotic cells. Polymorphic toxins are proteins that encode carboxy-terminal toxin domains. Here we developed a computational approach to identify previously undiscovered, conserved toxin domains of polymorphic toxins within 105,438 microbial genomes. We validated nine short toxins, showing that they cause cell death upon heterologous expression in either Escherichia coli or Saccharomyces cerevisiae. Five cognate immunity genes that neutralize the toxins were also discovered. The toxins are encoded by 2.2% of sequenced bacteria. A subset of the toxins exhibited potent antifungal activity against various pathogenic fungi but not against two invertebrate model organisms or macrophages. Experimental validation suggested that these toxins probably target the cell membrane or DNA or inhibit cell division. Further characterization and structural analysis of two toxinimmunity protein complexes confirmed DNase activity. These findings expand our knowledge of microbial toxins involved in inter-microbial competition that may have the potential for clinical and biotechnological applications.
Lipid bilayers, ubiquitous in living systems, form lubricious boundary layers in aqueous media, with broad relevance for biolubrication, especially in mechanically stressed environments such as articular cartilage in joints, as well as for modifying material interfacial properties. Model studies have revealed efficient lubricity by single-component lipid bilayers; synovial joints, however (e.g. hips and knees), comprise over a hundred different lipids, raising the question of whether this is natural redundancy or whether it confers any lubrication benefits. Here, we examine lubrication by progressively more complex mixtures of lipids representative of those in joints, using a surface forces balance at physiologically relevant salt concentrations and pressures. We find that different combinations of such lipids differ very significantly in the robustness of the bilayers to hemifusion under physiological loads (when lubrication breaks down), indicating a clear lubrication synergy afforded by multiple lipid types in the bilayers. Insight into the origins of this synergy is provided by detailed molecular dynamics simulations of potential profiles for the formation of stalks, the essential precursors of hemifusion, between bilayers of the different lipid mixtures used in the experiments. These reveal how bilayer hemifusionand thus lubrication breakdowndepends on the detailed lipid bilayer composition, through the corresponding separation into domains that are better able to resist stalk formation. Our results shed light on the role of lipid-type proliferation in biolubrication synergy, point to improved treatment modalities for common joint diseases such as osteoarthritis, and indicate how lipid-based interfacial modification in a materials context may be optimized.
Straightforward design and long-term functionality for tribological considerations has prompted an extensive substitution of polymers for metals across various applications, from industrial machinery to medical devices. Lubrication of and by polymer gels/coatings, essential for ensuring the cost-effective operation and reliability of applications, has gained strong momentum by benefiting from the structural characteristics of natural lubrication systems (such as articular cartilage). The optimal synthetic strategy for lubricating polymer gels/coatings would be a holistic approach, wherein the lubrication mechanism in relation to the structural properties offers a pathway to design tailor-made materials. This review considers recent synthesis strategies for creating lubricating polymer gels/coatings from the molecular level (including polymer brushes, loops, microgels, and hydrogels), and assessing their frictional properties, as well as considering the underlying mechanism of their lubrication.
Transient electric fields across cell bilayer membranes can lead to electroporation and cell fusion, effects crucial to cell viability whose biological implications have been extensively studied. However, little is known about these behaviours in a materials context. Here we find that transmembrane electric fields can lead to a massive, reversible modulation of the sliding friction between surfaces coated with lipid-bilayer membranesa 200-fold variation, up to two orders of magnitude greater than that achieved to date. Atomistic simulations reveal that the transverse fields, resembling those at cell membranes, lead to fully reversible electroporation of the confined bilayers and the formation of inter-bilayer bridges analogous to the stalks preceding intermembrane fusion. These increase the interfacial dissipation through reduced hydration at the slip plane, forcing it to revert in part from the low-dissipation, hydrated lipidheadgroup plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials modification properties.
Fusion of lipid bilayers in membranes is important in processes from vesicle-cell interactions (as in drug delivery) to exosome-cell signaling, while transient transmembrane electric fields are known to occur spontaneously. Two contacting phosphatidylcholine (PC) lipid membranes are known to fuse into one under external electric fields, suggesting that the interaction between them is modified by the field as they approach, prior to the fusion event. Here we measure directly the adhesion energy between dimyristoylphosphatidylcholine (DMPC) and between distearoylphosphatidylcholine (DSPC) surface layers attached to solid substrates both without and with a transmembrane electric field. We find a marked pressure-dependent adhesion behavior in the electric field, which we attribute to fusion intermediates that are formed, shedding new light on membrane electro-fusion.
The outstanding lubrication of articular cartilage in the major synovial joints such as hips and knees, essential for the joint well-being, has been attributed to boundary layers of lipids at the outer cartilage surfaces, which have very low friction mediated by the hydration lubrication mechanism at their highly hydrated exposed headgroups. However, the role of spontaneously present lipid splayslipids with an acyl tail in each of the opposing bilayersin modulating the frictional force between lipid bilayers has not, to date, been considered. In this study, we perform all-atom molecular dynamics simulations to quantitatively assess the significance of splayed molecules within the framework of lubricating lipid bilayers. We demonstrate that, although transient, splayed molecules significantly increase the inter-membrane friction until their retraction back into the lamellar phase, with this effect more steadily occurring at lower sliding velocities that are comparable to the physiological velocities of sliding articular cartilage.
2023
The remarkably low sliding friction of articular cartilage in the major joints such as hips and knees, which is crucial for its homeostasis and joint health, has been attributed to lipid bilayers forming lubricious boundary layers at its surface. The robustness of such layers, and thus their lubrication efficiency at joint pressures, depends on the lipids forming them, including cholesterol which is a ubiquitous component, and which may act to strengthen of weaken the bilayer. In this work, a systematic study using an atomic force microscope (AFM) was carried out to understand the effect of cholesterol on the nanomechanical stability of two saturated phospholipids, DSPC (1,2-distearoyl-sn-glycero-3-phosphatidlycholine) and DPPC (1,2-dipalmitoyl-sn-glycero- phosphatidylcholine), that differ in acyl chain lengths. Measurements were carried out both in water and in phosphate buffer solution (PBS). The nanomechanical stability of the lipid bilayers was quantitatively evaluated by measuring the breakthrough force needed to puncture the bilayer by the AFM tip. The molar fractions of cholesterol incorporated in the bilayers were 10% and 40%. We found that for both DSPC and DPPC, cholesterol significantly decreases the mechanical stability of the bilayers in solid-ordered (SO) phase. In accordance with the literature, the strengthening effect of salt on the lipid bilayers was also observed. For DPPC with 10 mol % cholesterol, the effect of tip properties and the experimental procedure parameters on the breakthrough forces were also studied. Tip radius (242 nm), material (Si, Si3N4, Au) and loading rate (401000 nm/s) were varied systematically. The values of the breakthrough forces measured were not significantly affected by any of these parameters, showing that the weakening effect of cholesterol does not result from such changes in experimental conditions. As we have previously demonstrated that mechanical robustness improves the tribological performance of lipid layers, this study helps to shed light on the mechanism of physiological lubrication.
Hydrogels with pure hydrophilic network have received much attention due to their excellent low frictional behavior. However, the lubrication performance of hydrogels is not satisfied under high-speed condition due to the energy dissipation caused by adsorbed polymer chains as well as the failure of lubricating mechanisms accompanied by the transition of lubrication regime. In this work, interpenetrating double-network organohydrogels were constructed by combining hydrophilic and oleophilic polymer networks to modify the physiochemical properties of surface polymer chains, especially the chain mobility. The oleophilic polymer network spatially restricting the mobility of the swollen hydrophilic network in water, resulted in a low coefficient of friction (ca. 0.01) compared with conventional hydrogels at high speed (0.1 m s−1). Meanwhile, the organohydrogels had superior wear resistance, with almost no wear observed on the sliding track after 5 k cycles of rubbing at high speed. The design concept of organohydrogels can be extended to a variety of low-wear, highly-lubricating materials.
Phospholipid surface assemblies are crucial ingredients in reducing the boundary friction of articular cartilage in synovial joints such as hips and knees, of central importance to their homeostasis and to tissue-wear-related diseases such as osteoarthritis. From the point of view of biolubrication, the very large number of different lipids in joints begs the question of whether this is natural redundancy, or does this multiplicity confer any benefits, possibly through natural selection. Here we demonstrate that particular combinations of lipids present in joints may carry a clear benefit for their lubricating properties. Using progressively more complex mixtures of lipids representative of those in joints, and measuring their interactions using a uniquely-sensitive surface forces balance at physiologically-relevant salt concentrations and pressures, we show that different lipid combinations lead to very significant differences in their efficacy as boundary lubricants. This points to a clear synergy arising from the multiple lipid types in the lubricating layers, provides insight into the role of lipid type proliferation in synovial joints, of possibly evolutionary origins, and may suggest new treatment modalities for osteoarthritis. We identify parameters of lipid-based boundary layers that might contribute to improved boundary lubrication in the light of the present study. Finally, we describe a possible approach based on molecular dynamics (MD) to emulating such optimal lipid combinations, and provide proof-of-concept MD simulations to illustrate this approach.
Transient electric fields across cell bilayer membranes can lead to electroporation, as well as to cell fusion, and have been extensively studied. We find that transmembrane electric fields similar to those in cells can lead to a massive, reversible modulation--by up to 200-fold--of the interfacial energy dissipation between surfaces sliding across the lipid bilayer membranes. Atomistic simulations reveal that this arises from (fully reversible) electroporation of the interfacially-confined bilayers, and formation of bilayer bridges analogous to stalks preceding intermembrane fusion. These cell-membrane-mimicking effects topologically-force the slip to partially-revert from the low-dissipation, hydrated lipid-headgroups plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials-modification properties, and shed new light on membrane hemifusion.
It is well known that lipid membranes respond to a threshold transmembrane electric field through a reversible mechanism called electroporation, where hydrophilic water pores form across the membrane, an effect widely used in biological systems. The effect of such fields on interfacially-confined (stacked or supported) lipid membranes, on the other hand, which may strongly modulate interfacial properties, has not to our knowledge been previously studied. Motivated by recent surface forces experiments showing a striking effect of electric fields on lubrication by confined lipid bilayers, we carried out all-atom molecular dynamics simulations of such membranes under transverse electric fields. We find that in addition to electroporation, a new feature emerges of locally merged bilayers which act to bridge the confining interfaces. These features shed light on the remodelling of confined lipid membrane stacks by electric fields, and provides insight into how such fields may modulate frictional and more generally surface interactions in the presence of lipid-based boundary layers.
It was recently discovered that friction between surfaces bearing phosphatidylcholine (PC) lipid bilayers can be increased by two orders of magnitude or more via an externally-applied electric field, and that this increase is fully reversible when the field is switched off. While this striking effect holds promising application potential, its molecular origin remains unknown due to difficulty in experimentally probing confined membrane structure at a molecular level. Our earlier molecular dynamics simulations revealed the equilibrium electroporated structure of such confined lipid membranes under an electric field; here we extend this approach to study the associated sliding friction between two solid surfaces across such PC bilayers. We identify the enhanced friction in the field as arising from membrane undulations due to the electroporation; this leads to some dehydration at the lipid-water interfaces, leading to closer contact and thus increased attraction between the zwitterionic headgroups, which results in increased frictional dissipation between the bilayers as they slide past each other. Additionally, the electric field facilitates formation of lipid bridges spanning the intersurface gap; at the sliding velocities of the experiments, these bridges increase the friction by topologically-forcing the slip-plane to pass through the acyl tail-tail interface, associated with higher dissipation during sliding. Our results account quantitatively for the experimentally-observed electro-modulated friction with boundary lipid bilayers, and indicate more generally how they may affect interactions between contacting surfaces, where high local transverse fields may be ubiquitous.
2022
Phosphatidylcholine (PC) lipid bilayers at surfaces massively reduce sliding friction, via the hydration lubrication mechanism acting at their highly-hydrated phosphocholine headgroups, a central paradigm of biological lubrication, particularly at articular cartilage surfaces where low friction is crucial for joint well-being. Nanotribological measurements probed the effect on such lubrication of dehydration by dimethyl sulfoxide (DMSO), known to strongly dehydrate the phosphocholine headgroups of such PC bilayers, i.e. reduce the thickness of the inter-bilayer water layer, and thus expected to substantially degrade the hydration lubrication. Remarkably, and unexpectedly, we found that the dehydration has little effect on the friction. We used several approaches, including atomic force microscopy, small- and wide-angle X-ray scattering and all-atom molecular dynamics simulations to elucidate this. Our results show that while DMSO clearly removes hydration water from the lipid head-groups, this is offset by both higher areal head-group density and by rigidity-enhancement of the lipid bilayers, both of which act to reduce frictional dissipation. This sheds strong light on the robustness of lipid-based hydration lubrication in biological systems, despite the ubiquitous presence of bio-osmolytes which compete for hydration water.
Drug delivery via nanovehicles is successfully employed in several clinical settings, yet bacterial infections, forming microbial communities in the form of biofilms, present a strong challenge to therapeutic treatment due to resistance to conventional antimicrobial therapies. Liposomes can provide a versatile drug-vector strategy for biofilm treatment, but are limited by the need to balance colloidal stability with biofilm penetration. We have discovered a liposomic functionalization strategy, using membrane-embedded moieties of poly[2-(methacryloyloxy)ethyl phosphorylcholine], pMPC, that overcomes this limitation. Such pMPCylation results in liposomic stability equivalent to current functionalization strategies (mostly PEGylation, the present gold-standard), but with strikingly improved cellular uptake and cargo conveyance. Fluorimetry, cryo-electron, and fluorescence microscopies reveal a far-enhanced antibiotic delivery to model Pseudomonas aeruginosa biofilms by pMPC-liposomes, followed by faster cytosolic cargo release, resulting in significantly greater biofilm eradication than either PEGylation or free drug. Moreover, this combination of techniques uncovers the molecular mechanism underlying the enhanced interaction with bacteria, indicating it arises from bridging by divalent ions of the zwitterionic groups on the pMPC moieties to the negatively charged lipopolysaccharide chains emanating from the bacterial membranes. Our results point to pMPCylation as a transformative strategy for liposomal functionalization, leading to next-generation delivery systems for biofilm treatment.
Boundary lubrication is associated with two sliding molecularly thin lubricated film-coated surfaces, where the energy dissipation occurs at the slip-plane between lubricated films. The hydration lubrication paradigm, which accounts for ultralow friction in aqueous media, has been extended to various systems, with phosphatidylcholine (PC) lipids recognized as extremely efficient lubrication elements due to their high hydration level. In this work, we extend a previous study (Lin et al., Langmuir 35 (2019) 6048-6054), where a charged lipid- poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) conjugate was prepared, to the very different case of a neutral lipid-PMPC) conjugate. This neutral molecule stabilizes the liposomes by attaching highly water-soluble PMPC to the surface of liposomes with its lipid moieties incorporated in the lipid bilayers. Such neutral polyphosphocholinated liposomes provide a surface lubricity which is well within the superlubrication regime (μ ≈ 10-3 or even lower). In contrast, negatively charged lipid/polyphosphocholine conjugates modified liposomes were unable to adsorb on negatively-charged (mica) surfaces. Our method provides stable liposomes that can adsorb on negatively charged surfaces and provide superlubricity.
The hydration layer surrounding the phosphocholine headgroups of single-component phosphatidylcholine lipids, or of lipid-mixtures, assembled at an interface greatly modifies the interfacial properties and interactions. As water molecules within the hydration layer are held tightly by the headgroup but are nonetheless very fluid on shear, the boundary lipid layers, exposing the highly hydrated headgroup arrays, can provide efficient boundary lubrication when sliding against an opposing surface, at physiologically high contact pressures. In addition, any free lipids in the surrounding liquid can heal defects which may form during sliding on the boundary phosphatidylcholine layer. Similar boundary lipid layers contribute to the lubricating, pressure-bearing, and wear-protection functions of healthy articular joints. This review presents a survey of the relationship between the molecular composition of the interfacial complex and the lubrication behavior of the lipid-based boundary layers, which could be beneficial for designing boundary lubricants for intra-articular injection for the treatment of early osteoarthritis.
ConspectusIn the course of evolution, nature has achieved remarkably lubricated surfaces, with healthy articular cartilage in the major (synovial) joints being the prime example, that can last a lifetime as they slide past each other with ultralow friction (friction coefficient μ = the force to slide surfaces past each other/load compressing the surfaces
Surface-attached layers of phosphatidylcholine (PC) lipid vesicles (liposomes) may reduce the friction coefficient μ (= force-to-slide/load) between the sliding surfaces down to μ ≈ 10−310−4 up to tens of atm contact pressures, as high as those in the major joints (hips or knees). Such friction reduction is attributed to hydration lubrication by the highly-hydrated phosphocholine head-groups exposed at the outer vesicle surfaces. It has been suggested therefore that intra-articular (IA) administration of liposomes as potential boundary lubricants may alleviate degenerative, friction-associated joint conditions such as osteoarthritis (OA), which is associated with insufficient lubrication at the articular cartilage surface. To overcome the problem, common to all nanoparticles, of rapid removal by the mononuclear phagocyte system, as well as to ensure long-term colloidal stability during storage, functionalizing liposomes with poly(ethylene glycol) moieties, PEGylation, is often used. Here we describe a different liposome functionalization approach, using poly(2-methacryloyloxyethyl phosphorylcholine), PMPC, moieties (strictly, lipidPMPC conjugates), and compare the retention time in mice joints of such PMPCylated liposomes with otherwise-identical but PEGylated vesicles following IA administration. We find, using fluorescence labeling and in vivo optical imaging, that when PMPC-stabilized liposomes are injected into mice knee joints, there is a massive increase of the vesicles retention half-life in the joints of about (45)-fold (ca. 300400% increase in retention time) compared with the PEGylated liposomes (and some 100-fold longer than the retention time of intra-articularly injected hyaluronan or HA). Such PMPCylated liposomes are therefore promising candidates as potential long-lived boundary lubricants at the articular cartilage surface, with implication for friction-associated pathologies. Moreover, as lipid vesicles are well known to be efficient drug carriers, such long retention in the joints may enable analgesic or anti-inflammatory agents for joint pathologies to be more efficiently delivered via IA administration using PMPCylated liposomal vehicles relative to PEGylated ones.
The viscoelectric effect concerns the increase in viscosity of a polar liquid in an electric field due to its interaction with the dipolar molecules and was first determined for polar organic liquids more than 80 y ago. For the case of water, however, the most common polar liquid, direct measurement of the viscoelectric effect is challenging and has not to date been carried out, despite its importance in a wide range of electrokinetic and flow effects. In consequence, estimates of its magnitude for water vary by more than three orders of magnitude. Here, we measure the viscoelectric effect in water directly using a surface force balance by measuring the dynamic approach of two molecularly smooth surfaces with a controlled, uniform electric field between them across highly purified water. As the water is squeezed out of the gap between the approaching surfaces, viscous damping dominates the approach dynamics; this is modulated by the viscoelectric effect under the uniform transverse electric field across the water, enabling its magnitude to be directly determined as a function of the field. We measured a value for this magnitude, which differs by one and by two orders of magnitude, respectively, from its highest and lowest previously estimated values.
2021
The direct measurement of forces between atomically smooth mica surfaces down to sub-nanometer separation was pioneered over 50years ago and has yielded deep understanding of a range of interfacial effects, not least the forces that determine colloidal stability and self-assembly, the properties of highly confined fluids, and the molecular origin of friction and lubrication. Here, we describe recent advances, including the use of substrates other than mica, probing the shear properties of highly confined fluids including hydration layers, and the modulation of surface forces by surface-attached macromolecules and amphiphiles, together with microscopic imaging of the surface morphology. These advances enabled novel features such as external potential control of the interacting surfaces, new understanding of lubrication in aqueous and biological systems, the design of novel nanoparticles and surface assemblies for modulating frictional dissipation, and insight into the nature of long-ranged attraction between surfactant-hydrophobized surfaces. We conclude by briefly outlining future challenges and opportunities provided by such direct surface forces studies.
Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently-lubricated surface known in nature, with friction coefficients as low as 0.001 up to physiologically high pressures. Such low friction is indeed essential for its well-being. It minimizes wear-and-tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage-embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio-inspired lubricants is reviewed.
Duncan Dowson, whom this issue commemorates, was a world leader in the field of biotribology, with prolific contributions both in fluid-based and boundary lubrication of biological tissues, in particular articular cartilage, a central issue in biotribology due to its importance for joint homeostasis. Here we explore further the issue of cartilage boundary lubrication, which has been attributed to phospholipid (PL)-exposing layers at the cartilage surface in part. A surface force balance (SFB) with unique sensitivity is used to investigate the normal and frictional interactions of the boundary layers formed by PLs extracted from osteoarthritic (OA) human synovial fluid (hSF). Our results reveal that vesicles of the OA-hSF lipids rupture spontaneously to form bilayers on the mica substrate (which, like the in-vivo articular cartilage surface in synovial joints, is negatively-charged) which then undergo hemifusion at quite low pressures in the SFB, attributed to the large heterogeneity of the hSF lipids. Nanometric friction measurements reveal friction coefficients μ ≈ 0.03 across the hemifused bilayer of these lipids, indicating residual hydration lubrication at the lipid-headgroup/substrate interface. Addition of calcium ions causes an increase in friction to μ ≈ 0.2, attributed either to calcium-bridging attraction of lipid headgroups to the negatively-charged substrate, or a shift of the slip plane to the more dissipative hydrophobic-tail/hydrophobic-tail interface. Our results suggest that the heterogeneity and composition of the OA-hSF lipids may be associated with higher friction at the cartilage boundary layers and thus a connection with greater wear and degradation due to hemifusion of the exposed lipid bilayers.
As a metal (gold) surface at a given, but variable potential slides past a dielectric (mica) surface at a fixed charge, across aqueous salt solutions, two distinct dissipation regimes may be identified. In regime I, when the gold potential is such that counterions are expelled from between the surfaces, which then come to adhesive contact, the frictional dissipation is high, with coefficient of friction μ ≈ 0.80.9. In regime II, when hydrated counterions are trapped between the compressed surfaces, hydration lubrication is active and friction is much lower, μ = 0.05 ± 0.03. Moreover, the dissipation regime as the surfaces contact is largely retained even when the metal potential changes to the other regime, attributed to the slow kinetics of counterion expulsion from or penetration into the subnanometer intersurface gap. Our results indicate how frictional dissipation between such a conducting/nonconducting couple may be modulated by the potential applied to the metal.
2020
Phospholipid-macromolecule complexes have been proposed to form highly efficient, lubricating boundary layers at artificial soft surfaces or at biological surfaces such as articular cartilage, where the friction reduction is attributed to the hydration lubrication mechanism acting at the exposed, hydrated head groups of the lipids. Here we measure, using a surface force balance, the normal and frictional interactions between model mica substrates across several different configurations of phosphatidylcholine (PC) lipid aggregates and adsorbed polymer (PEO) layers, to provide insight into the nature of such lubricating boundary layers in both symmetric and especially asymmetric configurations. Our results reveal that, irrespective of the configuration, the slip plane between the sliding surfaces reverts wherever possible to a bilayer-bilayer interface where hydration lubrication reduces the friction strongly. Where such an interface is not available, the sliding friction remains high. These findings may account for the low friction observed between both biological and synthetic hydrogel surfaces which may be asymmetrically coated with lipid-based boundary layers and fully support the hydration lubrication mechanism attributed to act at such boundary layers.
Osteoarthritic joints contain lower-molecular-weight (MW) hyaluronan (hyaluronic acid, HA) than healthy joints. To understand the relevance of this HA size effect for joint lubrication, the friction and surface structure of cartilage-emulating surfaces with HA of different MWs were studied using a surface force balance (SFB) and atomic force microscopy (AFM). Gelatin (gel)-covered mica surfaces were coated with high-MW HA (HHA), medium-MW HA (MHA), or low-MW HA (LHA), and lipids of hydrogenated soy l-α-phosphatidylcholine (HSPC) in the form of small unilamellar vesicles, using a layer-by-layer assembly method. SFB results indicate that the gel-HHA-HSPC boundary layer provides very efficient lubrication, attributed to hydration lubrication at the phosphocholine headgroups exposed by the HA-attached lipids, with friction coefficients (COF) as low as 10-3-10-4 at contact stresses at least up to P = 120 atm. However, for the gel-MHA-HSPC and gel-LHA-HSPC surfaces, the friction, initially low, increases sharply at much lower pressures (up to 30-60 atm at most). This higher friction with the shorter chains may be due to their weaker total adhesion energy to the gelatin, where the attraction between the negatively charged HA and the weakly positively charged gelatin is attributed largely to counterion-release entropy. Thus, the complexes of LHA and MHA with the lubricating HSPC lipids are more easily removed by shear during sliding, especially at high stresses, than the HHA-HSPC complex, which is strongly adhered to gelatin. This is ultimately the reason for lower-pressure lubrication breakdown with the shorter polysaccharides. Our results provide molecular-level insight into why the decrease in HA molecular weight in osteoarthritic joints may be associated with higher friction at the articular cartilage surface, and may have relevance for treatments of osteoarthritis involving intra-articular HA injections.
The lubrication of hydrogels arises from fluid or solvated surface phases. By contrast, the lubricity of articular cartilage, a complex biohydrogel, has been at least partially attributed to nonfluid, lipid-exposing boundary layers. We emulated this behavior in synthetic hydrogels by incorporating trace lipid concentrations to create a molecularly thin, lipid-based boundary layer that renews continuously. We observed a 80% to 99.3% reduction in friction and wear relative to the lipid-free gel, over a wide range of conditions. This effect persists when the gels are dried and then rehydrated. Our approach may provide a method for sustained, extreme lubrication of hydrogels in applications from tissue engineering to clinical diagnostics.
Hydration lubrication has emerged as a new paradigm for lubrication in aqueous and biological media, accounting especially for the extremely low friction (friction coefficients down to 0.001) of articular cartilage lubrication in joints. Among the ensemble of molecules acting in the joint, phosphatidylcholine (PC) lipids have been proposed as the key molecules forming, in a complex with other molecules including hyaluronic acid (HA), a robust layer on the outer surface of the cartilage. HA, ubiquitous in synovial joints, is not in itself a good boundary lubricant, but binds the PC lipids at the cartilage surface; these, in turn, massively reduce the friction via hydration lubrication at their exposed, highly hydrated phosphocholine headgroups. An important unresolved issue in this scenario is why the free HA molecules in the synovial fluid do not suppress the lubricity by adsorbing simultaneously to the opposing lipid layers, i.e., forming an adhesive, dissipative bridge between them, as they slide past each other during joint articulation. To address this question, we directly examined the friction between two hydrogenated soy PC (HSPC) lipid layers (in the form of liposomes) immersed in HA solution or two palmitoyl-oleoyl PC (POPC) lipid layers across HA-POPC solution using a surface force balance (SFB). The results show, clearly and surprisingly, that HA addition does not affect the outstanding lubrication provided by the PC lipid layers. A possible mechanism indicated by our data that may account for this is that multiple lipid layers form on each cartilage surface, so that the slip plane may move from the midplane between the opposing surfaces, which is bridged by the HA, to an HA-free interface within a multilayer, where hydration lubrication is freely active. Another possibility suggested by our model experiments is that lipids in synovial fluid may complex with HA, thereby inhibiting the HA molecules from adhering to the lipids on the cartilage surfaces.
Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication. This is attributed to the hydration lubrication mechanism acting at the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each bilayer. Micelles exposing such phosphocholine groups could be an attractive alternative to liposomes due to their much easier preparation and structure control, but all studies to date of surfactant micelles have revealed that at relatively low normal stresses the surface layers rupture and friction increases abruptly. Here, we examine surface interactions between three kinds of phosphocholine-exposing micelles with different designed structures: single-tail surfactant micelles, homo-oligomeric micelles, and block copolymer micelles. Normal and shear forces between mica surfaces immersed in solutions of these micelles were measured using a surface force balance. The adsorbed layers on the mica were imaged using atomic force microscope, revealing surface structures ranging from wormlike to spherical micelles. The block copolymer micelles showed relatively low coverage arising from their stabilizing corona and consequently poor lubrication (μ ∼ 10-1). In contrast, the surfactant and homo-oligomeric micelles fully covered the mica surface and demonstrated excellent lubrication (μ ∼ O(10-3)). However, while the boundary layer of single-tailed surfactant micelles degraded under moderate pressure, the homo-oligomeric micellar boundary layer was robust at all applied contact pressures in our study (up to about 5 MPa). We attribute the difference to the much greater energy required to remove a homo-oligomeric molecule from its micelle, resulting in far greater stability under pressure and shear.
Sphingomyelin is one of the predominant phospholipid groups in synovial joints, where lipids have been strongly implicated in the boundary lubrication of articular cartilage; however, little attention has been paid to its lubrication behavior. In this study, we demonstrate that sphingomyelin is an excellent boundary lubricant by measuring the normal and shear forces between sphingomyelin-layer-coated surfaces with a surface force balance under aqueous conditions. Slightly negatively-charged egg sphingomyelin vesicles were adsorbed on mica either by calcium bridging or by charge screening with high concentration monovalent salt. The normal force profiles between opposing egg sphingomyelin layers (vesicles or bilayers) show long-ranged weak repulsion and short-ranged strong repulsion on approaching. Friction coefficients, calculated from the highest load, were (7.2 ± 1.7) × 10-4 at contact stresses of 9.1 ± 0.7 MPa across 0.3 mM liposome dispersion in 0.03 mM Ca2+, and (0.8-3.5) × 10-3 at contact stresses of 7.6 ± 0.8 MPa across 0.3 mM liposome dispersion in 150 mM NaNO3. Similar or slightly lower friction coefficients of (5.3 ± 0.8) × 10-4 at 9.8 ± 0.2 MPa were obtained by replacing the liposome dispersion in 0.03 mM Ca2+ by water. Such low friction coefficients, attributed to the hydration lubrication mechanism, are comparable to those of phosphatidylcholine lipids, which have been widely recognized as excellent aqueous biolubricants. Therefore, we believe that sphingomyelin, in parallel with phosphatidylcholine, contributes to the remarkably good boundary lubrication in synovial joints.
2019
Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-toliquid phase-transition temperatures T-M. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below T-M. Under water (no added salt), excellent lubrication, with friction coefficients down to 10(-3)-10(-4), up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO3 salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO3 was replaced with NaNO3, as a result of the different PEO-mica ligation properties of Na+ compared to those of K+ Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K+ by Na+ ions.
A wide range of phosphatidylcholine (PC)
lipids with different degrees of unsaturation has been
identified in the human synovial fluid and on the cartilage
surface. The outstanding lubricity of the articular cartilage
surface has been attributed to boundary layers comprising
complexes of such lipids, though to date, only lubrication by
single-component PC-lipid-based boundary layers has been
investigated. As distinguishable lubrication behavior has been
found to be related to the PC structures, we herein examined
the surface morphology (on mica) and the lubrication ability
of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-
phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are
among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs)
prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and
shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good
boundary lubrication (friction coefficients ≤ ca. 10−4
) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ±
1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of
two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers
in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of
the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.
Hydration layers surrounding charges or zwitterionic moieties have long been known to play important roles in areas including antifouling and colloidal stability, and particularly over the past 15 years or so, their role in boundary lubrication has been widely investigated. Hydration repulsion because of hydrated ions or polar groups present on surfaces may dominate their interactions at high electrolyte concentrations, so that Derjaguin-Landau-Verwey-Overbeek theory does not apply. Hydration shells, strongly held by the charges they surround, can sustain large pressures without being squeezed out, while by rapidly relaxing, and they behave like a fluid during shear; this may lead to their acting as lubrication vectors with outstanding friction reduction properties. This review considers hydration layers around trapped ions, polymer brushes, and amphiphiles (surfactants and phosphatidylcholines), focusing on their lubrication properties. Finally, we suggest some prospects for further development of current hydrated vectors and designing new hydrated vectors for modifying surface interactions.
The original version of this Article contained an error throughout in which an incorrect symbol was used for the diffusion coefficient: it should be cambria math, italicized, and not bold. These have been corrected in both the PDF and HTML versions of the Article.
The following is a brief Comment on a paper in this special issue, as suggested by the editor. In a recent review in this journal [1] Kekicheff suggested an explanation (of electrostatic origin) for the long range attraction of order tens of nm - across aqueous media between surfaces that had been rendered hydrophobic in different ways (mostly by surfactants). Several experimental studies, all dating to the period 19881996, were used to compare with and support this explanation [1] (see studies cited in figs. 312 of Ref. [1]). However, the review [1] may have missed the implication of more recent studies of the long-ranged attraction between surfaces that had been rendered hydrophobic by surfactants.
We have prepared phosphatidylcholine (PC) vesicles (liposomes) incorporating a novel lipid/poly-phosphocholine conjugate. This both stabilizes the liposomes against aggregation (for example, during storage or when being delivered) and allows them to act as very efficient lubricating elements readily attaining superlubric performance (defined as coefficient of friction mu
Hyaluronan (HA)-lipid layers on model (mica) surfaces massively reduce friction as the surfaces slide past each other, and have been proposed, together with lubricin, as the boundary layers accounting for the extreme lubrication of articular cartilage. The ability of such HA-lipid complexes to lubricate sliding biological tissues has not however been demonstrated. Here we show that HA-lipid layers on the surface of an intrasynovial tendon can strongly reduce the friction as the tendon slides within its sheath. We find a marked lubrication synergy when combining both HA and lipids at the tendon surface, relative to each component alone, further enhanced when the polysaccharide is functionalized to attach specifically to the tissue. Our results shed light on the lubricity of sliding biological tissues, and indicate a novel approach for lubricating surfaces such as tendons and, possibly, articular cartilage, important, respectively, for alleviating function impairment following tendon injury and repair, or in the context of osteoarthritis. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
2018
Meso-porous electrodes (pore width « 1 µm) are a central component in electrochemical energy storage devices and related technologies, based on the capacitive nature of electric double-layers at their surfaces. This requires that such charging, limited by ion transport within the pores, is attained over the device operation time. Here we measure directly electric double layer charging within individual nano-slits, formed between gold and mica surfaces in a surface force balance, by monitoring transient surface forces in response to an applied electric potential. We find that the nano-slit charging time is of order 1 s (far slower than the time of order 3 × 10−2 s characteristic of charging an unconfined surface in our configuration), increasing at smaller slit thickness, and decreasing with solution ion concentration. The results enable us to examine critically the nanopore charging dynamics, and indicate how to probe such charging in different conditions and aqueous environments.
Using viscosity and dynamic light scattering (DLS) measurements, we monitored the changes in the properties of dispersions of chitosan (a cationic polysaccharide) in acidic solution over a period of up to 700 h. Different polymer concentrations, weight average molecular weights, and degrees of deacetylation were examined. We found that the solution rheology and chitosan aggregates continue to change even up to 700 h. It was observed, remarkably, using both capillary and cone and plate viscometry that the viscosity decreased significantly during the storage period of the chitosan dispersions, with a rapid initial decrease and a slow approach to the steady state value. DLS measurements over this period could be interpreted in terms of a gradual decrease in the size of the chitosan aggregates in the dispersion. This behavior is puzzling, insofar as one expects the dissolution of compact polymer aggregates with time into individual polymer chains to increase the viscosity rather than decrease it as observed: We attribute this apparently anomalous behavior to the fact that the chitosan aggregates are rigid crystalline rod-like entities, which dissolved with time from dispersion of overlapping rods (with high viscosity) into solution of individual random coils (with lower viscosity). A detailed model comparing the hydrodynamic behavior of the initial overlapping rod-like aggregates with the subsequent free coils in solution is in semi-quantitative agreement with our observation. Published by AIP Publishing.
Friction at hydrophobic surfaces in aqueous media is ubiquitous (e.g., prosthetic implants, contact lenses, microfluidic devices, biological tissue) but is not well understood. Here, we measure directly, using a surface force balance, both normal stresses and sliding friction in an aqueous environment between a hydrophilic surface (single-crystal mica) and the stable, molecularly smooth, highly hydrophobic surface of a spin-cast fluoropolymer film. Normal force versus surface separation profiles indicate a high negative charge density at the water-immersed fluoropolymer surface, consistent with previous studies. Sliding of the compressed surfaces under water or in physiological-level salt solution (0.1 M NaCl) reveals strikingly low boundary friction (friction coefficient mu approximate to 0.003-0.009) up to contact pressures of at least 50 atm. This is attributed largely to hydrated counterions (protons and Na+ ions) trapped in thin interfacial films between the compressed, sliding surfaces. Our results reveal how frictional dissipation may occur at hydrophobic surfaces in water and how modification of such surfaces may suppress this dissipation.
Imagine supporting a 1-ton weight on your hand and then sliding it along the palm with a slight push of a finger. Pulling off that trick would require, roughly, the level of lubrication that is provided by cartilage surfaces in the major joints of our bodies. Those joints, which enable rotation (or more precisely, articulation) at shoulders, elbows, hips, and knees, are remarkable structures. Indeed, the articular cartilage layers that coat the ends of the bones and slide past each other as we flex our joints are the most efficiently lubricated surfaces in nature. No manmade material can match the ultralow sliding friction, which is a consequence of the lubrication, that cartilage provides at the high pressures and low velocities that our joints experience. Such low friction is essential for their health, as they withstand varied harsh and complex loading, day after day, over a human lifetime.
2017
Poly[2-(methacryloyloxy)ethylphosphorylcholine] (pMPC) brushes provide extremely low friction coefficients up to high compressions, but their use as boundary lubricating layers is limited by the challenge of surface grafting of the chains. Coating by thin layers of cross-linked pMPC hydrogels may provide an attractive alternative. Here we use a surface force balance (SFB) and other techniques to examine the effect of light cross-linking (0.1% cross-linker) on the surface interactions and frictional behavior of grafted-from pMPC brushes on mica, up to physiologically high contact pressures. Atomic force microscopy, X-ray photoelectron spectroscopy, and interferometric surface-excess measurements show little difference between the non-cross-linked (linear) pMPC brushes and the cross-linked brushes prepared under otherwise identical conditions. Normal force-distance profiles between the polymer-bearing surfaces, however, reveal a marked compaction of the unperturbed thickness of the cross-linked brushes relative to linear ones, attributed to the cross-linking which limits chain swelling. The cross-linked pMPC layers exhibit very low friction (friction coefficients of order 10-3-10-4, depending on sliding velocities), similar to the corresponding linear brushes and due to hydration lubrication by the highly hydrated phosphocholine monomer structure. Within the range of our parameters, however, there is a marked qualitative difference in the dependence of friction on sliding velocity vs. While for the linear brushes friction is only very weakly vs-dependent (over 3 orders of magnitude in vs), due to a self-regulating brush interpenetration, for the cross-linked brushes friction increases markedly with vs (∼vs1/2), an effect attributed to suppression of interpenetration by the cross-links.
The boundary layers coating articular cartilage in synovial joints constitute unique biomaterials, providing lubricity at levels unmatched by any human-made materials. The underlying molecular mechanism of this lubricity, essential to joint function, is not well understood. Here we study the interactions between surfaces bearing attached hyaluronan (hyaluronic acid, or HA) to which different phosphatidylcholine (PC) lipids had been added, in the form of small unilamellar vesicles (SUVs or liposomes), using a surface force balance, to shed light on possible cartilage boundary lubrication by such complexes. Surface-attached HA was complexed with different PC lipids (hydrogenated soy PC (HSPC), 1,2-dimyristoyl-sn-glycero-3-PC (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-PC (POPC)), followed by rinsing. Atomic force microscopy (AFM) and cryo-scanning electron microscopy (Cryo-SEM) were used to image the HA-PC surface complexes following addition of the SUVs. HA-HSPC complexes provide very efficient lubrication, with friction coefficients as low as μ ∼ 0.001 at physiological pressures P ≈ 150 atm, while HA-DMPC and HA-POPC complexes are efficient only at low P (up to 1020 atm). The friction reduction in all cases is attributed to hydration lubrication by highly-hydrated phosphocholine groups exposed by the PC-HA complexes. The greater robustness at high P of the HSPC (C16(15%),C18(85%)) complexes relative to the DMPC ((C14)2) or POPC (C16, C18:1) complexes is attributed to the stronger van der Waals attraction between the HSPC acyl tails, relative to the shorter or un-saturated tails of the other two lipids. Our results shed light on possible lubrication mechanisms at the articular cartilage surface in joints. Statement of Significance Can designed biomaterials emulate the unique lubrication ability of articular cartilage, and thus provide potential alleviation to friction-related joint diseases? This is the motivation behind the present study. The principles of cartilage lubrication have attracted considerable attention for decades, and several models have been proposed to elucidate it, however, the mechanism of this ultralow friction is still not clear. In this paper we explore the recent suggestion that its efficient lubrication arises from boundary layers of hyaluronan-lipid complexes at its surface, in particular exploring a range of different phosphatidylcholines (PCs) mimicking the wide range of PCs in synovial joints. The present study suggests a synergistic lubricating behavior of the different lipids in living joints, and potential treatment directions using such biomaterial complexes for widespread cartilage-friction-related diseases such as osteoarthritis.
The lubrication properties of saturated PC lipid vesicles containing high cholesterol content under high loads were examined by detailed surface force balance measurements of normal and shear forces between two surface-attached lipid layers. Forces between two opposing mica surfaces bearing distearoylphosphatidylcholine (PC) (DSPC) small unilamellar vesicles (SUVs, or liposomes), or bilayers, with varying cholesterol content were measured across water, whereas dimyristoyl PC (DMPC), dipalmitoyl PC (DPPC), and DSPC SUVs containing 40% cholesterol were measured across liposome dispersions of SUVs of the same lipid composition as in the adsorbed layers. The results clearly demonstrate decreased stability and resistance to normal load with the increase in cholesterol content of DSPC SUVs. Friction coefficients between two 10% cholesterol PC-bilayers were in the same range as for 40% cholesterol bilayers (μ ≈ 10-3), indicating that cholesterol has a more substantial effect on the mechanical properties of a bilayer than on its lubrication performance. We further find that the lubrication efficiency of DMPC and DPPC with 40% cholesterol is superior to that of DSPC 40% cholesterol, most likely because of enhanced hydration-lubrication in these systems. We previously found that when experiments are performed in the presence of a lipid reservoir, layers can self-heal and therefore their robustness is less important under such conditions. We conclude that the effect of cholesterol in decreasing the stability is more pronounced than its effect on hydration, but the stability is, in turn, less important when a lipid reservoir is present. This study complements our previous work and sheds light on the effect of cholesterol, a prominent and important physiological lipid, on the mechanical and lubrication properties of gel-phase lipid layers.
Combining direct surface force measurements with in situ regulation of surface potential provides an exceptional opportunity for investigating and manipulating interfacial phenomena. Recently, we studied the interaction between gold and mica surfaces in water with no added salt, while controlling the metal potential, and found that the surface charge at the metal may vary, and possibly even change its sign, as it progressively approaches the (constant-charge) mica surface [Langmuir, 2015, 31(47), 12845-12849]. Such a variation was found to directly affect the nature of the contact and adhesion between them due to exclusion of all mobile counterions from the intersurface gap. In this work, we extend this to examine the potential-dependent response of the adhesion and interaction between gold and mica to externally applied voltages and in electrolyte solution. Using a surface force balance (SFB) combined with a three-electrode electrochemical cell, we measured the normal interaction between gold and mica under surface potential regulation, revealing three interaction regimes-pure attraction, non-monotonic interaction from electrostatic repulsion to attraction (owing to charge inversion) and pure repulsion. Accordingly, the adhesion energy between the surfaces was found to vary both in no added salt water and, more strongly, in electrolyte solution. We justify this potential-dependent variation of adhesion energy in terms of the interplay between electrostatic energy and van der Waals (vdW) interaction at contact, and attribute the difference between the two cases to the weaker vdW interaction in electrolyte solution. Finally, we showed that through abruptly altering the gold surface potential from negative to positive and vice versa, the adhesion between gold and mica can be reversibly switched on and off. We surmise that the process of bringing the surface into contact is associated with the formation of a strong electric field O (108 V m-1) in the intersurface gap.
Extending earlier studies where phosphocholinated brushes and phosphocholineexposing phosphatidylcholine lipid bilayer vesicles (PCliposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highlyhydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pcPSNPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PSNP, with the hydrophilic phosphocholines exposed at the NP surface. These were then allowed to adsorb onto mica surfaces, and the normal and shear interactions between them were examined in a surface force balance. On moderate compressions (contact pressures>9atm) the pcPSNPs were squeezed out, leaving a single layer between the surfaces. Shear of the surfaces revealed a large frictional dissipation, with a friction coefficient μ≈0.2, in contrast to our expectations that such phosphocholinated NPs would provide highly lubricating analogues of PCliposomes (for which μ≈10−3 10−4). This is attributed to a number of factors, including the inherent roughness of the NP multilayers at low compressions and the relatively low areal density of the phosphocholine groups on the PSNP surfaces. In particular, at higher compressions, sliding results in energydissipating breaking and reforming of phosphocholine/mica (dipole/charge) bonds at the mica surfaces (and thus high friction), because of bridging of the surfaces by a single layer of pcPSNPs.
Nanoparticles (NPs) which enter physiological fluids are rapidly coated by proteins, forming a so-called corona which may strongly modify their interaction with tissues and cells relative to the bare NPs. In this work the interactions between a living cell and a nano-object, and in particular the effect on this of the adsorption of serum proteins, are directly examined by measuring the forces arising as an Atomic Force Microscope tip (diameter 20 nm)-simulating a nano-object-approaches and contacts a cell. We find that the presence of a serum protein corona on the tip strongly modifies the interaction as indicated by pronounced increase in the indentation, hysteresis and work of adhesion compared to a bare tip. Classically one expects an AFM tip interacting with a cell surface to be repelled due to cell elastic distortion, offset by tip-cell adhesion, and indeed such a model fits the bare-Tip/cell interaction, in agreement with earlier work. However, the force plots obtained with serum-modified tips are very different, indicating that the cell is much more compliant to the approaching tip. The insights obtained in this work may promote better design of NPs for drug delivery and other nano-medical applications.
2016
The surface structure of the trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) on mica and the interactions between two such DTAD-coated surfaces were determined using atomic force microscopy and a surface force balance. In an aqueous solution of 3 mM, 5 times the critical aggregation concentration (CAC), the surfaces are coated with wormlike micelles or hemimicelles and larger (∼80 nm) bilayer vesicles. Repulsive normal interactions between the surfaces indicate a net surface charge and a solution concentration of ions close to that expected from the CAC. Moreover, this surface coating is strongly lubricating up to some tens of atmospheres, attributed to the hydration-lubrication mechanism acting at the exposed, highly hydrated surfactant headgroups. Upon replacement of the DTAD solution with surfactant-free water, the surface structures have changed on the DTAD monolayers, which then jump into adhesive contact on approach, both in water and following addition of 0.1 M NaNO3. This trimeric surfactant monolayer, which is highly hydrophobic, is found to be positively charged, which is evident from the attraction between the DTAD monolayer and negatively charged bare mica across water. These monolayers are stable over days even under a salt solution. The stability is attributed to the several stabilization pathways available to DTAD on the mica surface.
Highly efficient lubricating boundary layers at biosurfaces such as cartilage have been proposed to comprise phospholipids complexed with biomacromolecules exposed at the surfaces. To gain insight into this, a systematic study on the normal and frictional forces between surfaces bearing a sequentially deposited model alginate-on-chitosan bilayer, bearing different adsorbed phosphatidylcholine (PC) liposomes, was carried out using a surface force balance. Structures of the resulting surface complexes were determined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). The liposome/lipid-polymer complexes could maintain their integrity up to high pressures in terms of both normal and shear interactions between the surfaces, which were repeatable, reproducible, and revealed very low friction (coefficient of friction μ down to 10-3-10-4, depending on the PC used) up to pressures of hundreds of atm. We attribute this remarkable lubrication capability ultimately to hydration lubrication acting at the hydrated phosphocholine headgroups of the PC lipids, either exposed at the liposome surfaces or through complexation with the polyelectrolyte bilayer. Values of μ, while low, were roughly an order of magnitude higher than for the same PC vesicles adsorbed on bare mica, a difference attributed to their lower density on the bilayer; the bilayer, however, stabilized the PC-vesicles far better than bare mica against rupture and shear at high compressions and sliding.
Using the surface force balance (SFB), we studied the surface properties of gold in aqueous solution with low electrolyte concentration (∼10-5 M and pH = 5.8), i.e., water with no added salt, by directly measuring the interaction between an ultrasmooth gold surface (ca. 0.2 nm rms roughness) and a mica surface. Under these conditions, specific adsorption of ions is minimized and its influence on the surface charge and surface potential of gold is markedly reduced. At open circuit potential, the electrostatic interaction between gold and mica was purely attractive and gold was found to be positively charged. This was further confirmed by force measurements against a positively charged surface, poly-l-lysine coated mica. Successive force measurements unambiguously showed that once gold and mica reach contact all counterions are expelled from the gap, confirming that at contact the surface charge of gold is equal and opposite in charge to that of mica. Further analysis of adhesion energy between the surfaces indicated that adhesion is mostly governed by vdW dispersion force and to a lesser extent by electrostatic interaction. Force measurements under external applied potentials showed that the gold-mica interaction can be regulated as a function of applied potential even at low electrolyte concentration. The gold-mica interaction was described very precisely by the nonlinearized Poisson-Boltzmann (PB) equation, where one of the surfaces is at constant charge, i.e., mica, and the other, i.e., gold, is at constant potential. Consequently, the gold surface potential could be determined accurately both at open circuit potential (OCP) and under different applied potentials. Using the obtained surface potentials, we were able to derive fundamental characteristics of the gold surface, e.g., its surface charge density and potential of zero charge (PZC), at very low electrolyte concentration.
Understanding the structure of solid supported lipid multilayers is crucial to their application as a platform for novel materials. Conventionally, they are prepared from drop casting or spin coating of lipids dissolved in organic solvents, and lipid multilayers prepared from aqueous media and their structural characterisation have not been reported previously, due to their extremely low lipid solubility (i.e. ∼10-9 M) in water. Herein, using X-ray reflectivity (XRR) facilitated by a "bending mica" method, we have studied the structural characteristics of dioleoylphosphatidylcholine (DOPC) multilayers prepared via drop casting aqueous small unilamellar and multilamellar vesicle or liposome (i.e. SUV and MLV) dispersions on different surfaces, including mica, positively charged polyethylenimine (PEI) coated mica, and stearic trimethylammonium iodide (STAI) coated mica which exposes a monolayer of hydrocarbon tails. We suggest that DOPC liposomes served both as a delivery matrix where an appreciable lipid concentration in water (∼25 mg mL-1 or 14 mM) was feasible, and as a structural precursor where the lamellar structure was readily retained on the rupture of the vesicles at the solid surface upon solvent evaporation to facilitate rapid multilayer formation. We find that multilayers on mica from MLVs exhibited polymorphism, whereas the SUV multilayers were well ordered and showed stronger stability against water. The influence of substrate chemistry (i.e. polymer coating, charge and hydrophobicity) on the multilayer structure is discussed in terms of lipid-substrate molecular interactions determining the bilayer packing proximal to the solid-liquid interface, which then had a templating effect on the structure of the bilayers distal from the interface, resulting in the overall different multilayer structural characteristics on different substrates. Such a fundamental understanding of the correlation between the physical parameters that characterise liposomes and substrate chemistry, and the structure of lipid multilayers underpins the potential development of a simple method via an aqueous liposome dispersion route for the inclusion of hydrophilic functional additives (e.g. drugs or nanoparticles) into lipid multilayer based hybrid materials, where tailored structural characteristics are an important consideration.
Frictional energy dissipation between sliding solid surfaces in aqueous media may proceed by different pathways. Using a surface force balance (SFB), we have examined systematically how such dissipation is mediated by the series of hydrated cations M+ = Li+, Na+, and K+ that are trapped between two atomically smooth, negatively charged, mica surfaces sliding across the ionic solutions over many orders of magnitude loading. By working at local contact pressures up to ca. 30 MPa (∼300 atm), up to 2 orders of magnitude higher than earlier studies, we could show that the frictional dissipation at constant sliding velocity, represented by the coefficient of sliding friction ^^Petroleum Research Fund of the American Chemical Society M+, decreased as μLi+ > μNa+ μK+. This result contrasts with the expectation (in conceptual analogy with the Hofmeister series) that the lubrication would improve with the extent of ionic hydration, since that would have led to the opposite μM+ sequence. It suggests, rather, that frictional forces, even in such simple systems, can be dominated by rate-activated pathways where the size of the hydration shell becomes a dissipative liability, rather than by the hydration-shell dissipation expected via the hydration lubrication mechanism.
Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14 : 0), DPPC (16 : 0) and DSPC (18 : 0) and POPC (16 : 0, 18 : 1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (μ -4) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (μ -4) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (μ = 10-3-10-4). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints.
The major synovial joints such as hips and knees are uniquely efficient tribological systems, able to articulate over a wide range of shear rates with a friction coefficient between the sliding cartilage surfaces as low as 0.001 up to pressures of more than 100 atm. No human-made material can match this. The means by which such surfaces maintain their very low friction has been intensively studied for decades and has been attributed to fluid-film and boundary lubrication. Here, we focus especially on the latter: the reduction of friction by molecular layers at the sliding cartilage surfaces. In particular, we discuss such lubrication in the light of very recent advances in our understanding of boundary effects in aqueous media based on the paradigms of hydration lubrication and of the synergism between different molecular components of the synovial joints (namely hyaluronan, lubricin, and phospholipids) in enabling this lubrication.
2015
Surface interactions across water are central to areas from nanomedicine to colloidal stability. They are predominantly a combination of attractive but short-ranged dispersive (van der Waals) forces, and long-ranged electrostatic forces between the charged surfaces. Here we show, using a surface force balance, that electrostatic forces between two surfaces across water, one at constant charge while the other (a molecularly smooth metal surface) is at constant potential of the same sign, may revert smoothly from repulsion to attraction on progressive confinement of the aqueous intersurface gap. This remarkable effect, long predicted theoretically in the classic Gouy-Chapman (Poisson-Boltzmann) model but never previously experimentally observed, unambiguously demonstrates surface charge reversal at the metal-water surface. This experimental confirmation emphasizes the implications for interactions of dielectrics with metal surfaces in aqueous media.
Jee et al. assert (1) that the effect we measurewhat happens to a lubricant film thickness in individual slip events during intermittent stickslip sliding (2)was already measured long ago (3, 4), and with better resolution. We are bemused by this assertion because in the works cited (3, 4) not only is intermittent stickslip friction, with its characteristic saw-tooth pattern, not shown or measured, but their inability to measure what happens during the individual, fleeting slip eventsthe key finding of our own study (2)is explicitly stated in the papers themselves (below).
Macromolecules, which adsorb or intrinsically form boundary layers at surfaces sliding past each other in aqueous media, are ubiquitous both in technology and in biological systems and can form effective boundary lubricants. Over the past decade or so, hydration layers - robustly bound water molecules that surround charges or zwitterionic groups of different macromolecular species - have been identified as remarkable lubricating elements, sustaining high loads while exhibiting a fluid-like response to shear with extremely low friction. This modification of frictional forces in aqueous systems, based on the behavior of water molecules confined to hydration shells, is the central idea behind the hydration lubrication mechanism, which is presented and discussed in detail in the current Perspective. We describe the nature of hydration under confinement and the underlying experiments revealing this mechanism, focusing in particular on synthetic and biological macromolecules attached to surfaces and on phospholipid assemblies. We also emphasize these recent findings in relation to physiological environments and functions of the human body, such as cartilage lubrication, in which hydration lubrication is believed to play an important role.
Intermittent sliding (stick-slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick-slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4-0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.
Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. We find that surface-anchored hyaluronan molecules complex synergistically with phosphatidylcholine lipids present in joints to form a boundary lubricating layer, which, with coefficient of friction 1/4 ;0.001 at pressures to over 100 atm, has a frictional behaviour resembling that of articular cartilage in the major joints. Our findings point to a scenario where each of the molecules has a different role but must act together with the others: hyaluronan, anchored at the outer surface of articular cartilage by lubricin molecules, complexes with joint phosphatidylcholines to provide the extreme lubrication of synovial joints via the hydration-lubrication mechanism.
The conditions for atom transfer radical polymerization (ATRP) of poly[2-(methacryloyloxy)ethyl phosphorylcholine] (pMPC) chains are modified to enable much more efficient growth of these poly zwitterionic chains from macroinitiator-coated mica substrates using the "grafting from" technique. In particular, we demonstrate directly that achieving a lower level of oxygen in the reaction mixtures through longer evacuation results in the creation of significantly denser and more extended pMPC brushes, with substantially improved interfacial properties both in pure water and in 0.2 M NaNO3 salt solution. Using a surface force balance combined with atomic force microscopy and X-ray photoelectron spectroscopy, we characterize these brushes and determine the normal and especially shear interactions between them. Normal force profiles reveal that the grafting density is independent of the brush molecular weight M, and that the swollen brush thickness L scales linearly with M. Moreover, shear force measurements indicate that such pMPC brushes provide boundary lubrication that, with friction coefficients μ down to μ -4 at pressures P > 150 MPa, is superior by an order of magnitude compared to literature data for polymeric boundary layers, including pMPC brushes described earlier. We attribute this enhanced lubrication to the denser and thicker brush layers achieved in the present study, together with the hydration lubrication mechanism arising from the highly hydrated phosphorylcholine groups on the chains.
Why is friction in healthy hips and knees so low? Hydration lubrication, according to which hydration shells surrounding charges act as lubricating elements in boundary layers (including those coating cartilage in joints), has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood. Here we report the direct determination of energy dissipation within such sheared hydration shells. By trapping hydrated ions in a 0.4-1 nm gap between atomically smooth charged surfaces as they slide past each other, we are able to separate the dissipation modes of the friction and, in particular, identify the viscous losses in the subnanometre hydration shells. Our results shed light on the origins of hydration lubrication, with potential implications both for aqueous boundary lubricants and for biolubrication.
2014
Glucosamine sulfate (GAS) is a charged monosaccharide molecule that is widely used as a treatment for osteoarthritis, a joint disease related to friction and lubrication of articular cartilage. Using a surface force balance, we examine the effect of GAS on normal and, particularly, on shear (frictional) interactions between surfaces in an aqueous environment coated with small unilamellar vesicles (SUVs), or liposomes, of hydrogenated soy phosphatidylcholine (HSPC). We examine the effect of GAS solution, pure water, and salt solution (0.15 M NaNO3) both inside and outside the vesicles. Cryoscanning electron microscopy shows a closely packed layer of liposomes whose morphology is affected only slightly by GAS. HSPC-SUVs with encapsulated GAS are stable upon shear at high compressions (>100 atm) and provide very good lubrication when immersed both in pure water and physiological-level salt solutions (in the latter case, the liposomes are exceptionally stable and lubricious up to >400 atm). The low friction is attributed to several parameters based on the hydration lubrication mechanism.
Using a surface force balance (SFB), we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyltrimethylammonium chloride (TAC) surfactant solutions well above the critical micelle concentration (CMC). The surfactants that were used-C 14TAC, C16TAC, and C18TAC-varied by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging and ranged from flat bilayers to rodlike micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water, and have similar friction coefficients (μ -0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.
The lubrication properties of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) extended supported bilayers were studied and compared to those of surface-attached DSPC small unilamellar vesicles (liposomes) in order to elucidate the effect of phospholipid geometrical packaging on the lubrication and mechanical properties of these boundary layers. The topography and response to the nanoindentation of bilayer-and liposome-covered surfaces were studied by an atomic force microscope (AFM). In parallel, normal and shear (frictional) forces between two opposing surfaces bearing DSPC vesicles/bilayers across water were studied with the surface force balance (SFB). A correlation between nanomechanical performance in the AFM and stability and lubrication in the SFB was observed. Bilayers were readily punctured by the AFM tip and exhibited substantial hysteresis between approach and retraction curves, whereas liposomes were not punctured and exhibited purely elastic behavior. At the same time, SFB measurements showed that bilayers are less stable and less efficient lubricants compared to liposomes. Bilayers provided efficient lubrication with very low friction coefficients, 0.002-0.008 up to pressures of more then 50 atm. However, bilayers were less robust and tended to detach from the surface as a result of shear, leading to high friction for subsequent approaches at the same contact position. In contrast, liposomes showed reversible and reproducible behavior under shear and compression, exhibiting ultralow friction coefficients of μ ≈10-4 for pressures as high as 180 atm. This is attributed to the increased mechanical stability of the self-closed, closely packed liposomes, which we believe results from the more defect-free nature of the finitely sized vesicles.
We review here two recent works on boundary lubrication by macromolecules with the aim of obtaining an insight into the extremely efficient lubrication of mammalian joints. The first work emulates the structure of the cartilage superficial layer by reconstructing films of hyaluronan (HA) and of HA/aggrecan complexes stabilized by cartilage link protein and studies normal and shear interactions between two atomically smooth mica surfaces coated with such molecules using a surface force balance. These molecules have been conjectured to act as boundary lubricants while sliding past each other at the interface between the articular cartilage surfaces and the synovial fluid. It was discovered that the HA/aggrecan/link protein macromolecular complexes provide better boundary lubrication compared with HA alone, most likely as a result of the higher charge density allowing a more efficient hydration lubrication effect. However, the results suggest that such complexes by themselves cannot be responsible for the very efficient lubrication of synovial joints. On the other hand, in the second work, phosphatidylcholine vesicles were shown to provide extremely efficient lubrication at physiological pressures as long as the robustness of the layer is maintained. Normal and shear interactions between two surfaces bearing different phosphatidylcholine vesicles, having the same head group but varying acyl chain length, were studied systematically. In addition to hydration, robustness of the surface layer plays a major role in the lubrication efficiency of the system. Implications of the results of these studies to the lubrication mechanism of synovial joints are suggested.
2013
Using a surface force balance, we have measured the forces between bare (hydrophilic) mica surfaces, and between hydrophobized mica surfaces, in each case coated with the amphiphilic protein hydrophobin (HFBI) from Trichoderma reesei. We additionally characterized these surfaces by contact angle measurements and AFM. The results are consistent with the formation of hydrophobic surfaces exposed by HFBI adsorbed on the hydrophilic substrate, and hydrophilic surfaces exposed by HFBI adsorbed on the hydrophobic substrate. In particular, friction between HFBI surfaces on hydrophobized mica, exposing hydrophilic surfaces, is an order of magnitude lower than friction between HFBI-coated hydrophilic surfaces, which expose the hydrophobic side of the protein to the water interface. This result can be readily understood in terms of the greater hydration level of the exposed outer surfaces in the former case.
We used colloidal probe atomic force microscopy to measure the normal forces between the surface of a silica colloidal particle and a sparse layer of hyaluronan (hyaluronic acid, HA, MW ≈ 106 Da) covalently attached to a planar silica surface, both across pure water and following the addition of 1 mM MgCl2. It was found that in the absence of salt the HA layer repelled the colloidal silica surface during both approach and retraction. The addition of the MgCl2, however, changes the net force between the negatively charged HA layer and the opposing negatively charged silica surface from repulsion to adhesion. This interaction reversal is attributed to the bridging effect of the added Mg2+ ions. Our results provide first direct force data to support earlier simulation and predictions that such divalent cations could bridge between negative charges on opposing surfaces, leading to an overall reversal of force from repulsion to attraction.
Phosphatidylcholine (PC) vesicles have been shown to have remarkable boundary lubricating properties under physiologically-high pressures. Here we carry out a systematic study, using a surface force balance, of the normal and shear (frictional) forces between two opposing surfaces bearing different PC vesicles across water, to elucidate the origin of these properties. Small unilamellar vesicles (SUVs, diameters
The hydration lubrication paradigm, whereby hydration layers are both strongly held by the charges they surround, and so can support large pressures without being squeezed out, and at the same time remain very rapidly relaxing and so have a fluid response to shear, provides a framework for understanding, controlling, and designing very efficient boundary lubrication systems in aqueous and biological media. This review discusses the properties of confined water, whichunlike organic solventsretains its fluidity down to molecularly thin films. It then describes lubrication by hydrated ions trapped between charged surfaces, and by other hydrated boundary species including charged and zwitterionic polymer brushes, surfactant monolayers, liposomes, and biological macromolecules implicated in synovial joint lubrication. Finally, challenges and prospects for future development of this new boundary lubrication approach are considered.
2012
Using a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).
Using the surface force balance (SFB), we recorded the changes with time of the adhesion, normal, and shear interactions between a monolayer of cetyltrimethylammonium bromide (CTAB) on mica and a bare mica surface across surfactant-free water. In this asymmetric case, the bare mica acts a stable probe of the interactions between the two surfaces as the CTAB-coated one undergoes changes with time. As previously demonstrated, when a CTAB monolayer on mica is immersed in water, it reorganized to form bilayer patches, exposing the bare mica surface, followed by a gradual release of free surfactants to the bulk. We probe how this degradation with time affects both the normal force vs distance interaction profiles, and adhesion between the CTAB-coated surface and a bare mica surface. We demonstrate that the CTAB layer leads to a reduction in the sliding friction relative to that between bare mica surfaces, which is reversed only at advanced degradation levels, whereupon an abrupt increase in the friction occurs. This change is ascribed to adhesion between exposed bare mica surfaces, which sets on when the density of CTAB patches is low. The reproducibility of the normal force profiles and of adhesion forces on sequential approaches at the same contact spot indicates that there is no substantial transfer of materials between the surfaces while they are in adhesive contact.
Interactions in aqueous media between uniformly charged surfaces are well understood, but most real surfaces are heterogeneous and disordered. Here we show that two such heterogeneous surfaces covered with random charge domains experience a long-range attraction across water that is orders of magnitude stronger than van der Waals forces, even in the complete absence of any charge correlations between the opposing surfaces. We demonstrate that such strong attraction may arise generally, even for overall neutral surfaces, from the inherent interaction asymmetry between equally and between oppositely charged domains.
In a series of Surface Force Balance experiments, material from human whole saliva was adsorbed to molecularly smooth mica substrata (to form an 'adsorbed salivary film'). Measurements were taken of normal (load bearing, Fn) and shear (frictional, Fs*) forces between two interacting surfaces. One investigation involved a salivary film formed by overnight adsorption from undiluted, centrifuged saliva, with the adsorbed film rinsed with pure water before measurement. Measurements were taken under pure water and 70 mM NaNO3. In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10 mM NaNO3. Fn results for both systems showed purely repulsive layers, with an uncompressed thickness of 35-70 nm for the diluted saliva investigation and, prior to the application of shear, 11 nm for the rinsed system. Fs* was essentially proportional to Fn for all systems and independent of shear speed (in the range 100-2000 nm s-1), with coefficients of friction μ ~ 0.24 and μ ~ 0.46 for the unrinsed and rinsed systems, respectively. All properties of the rinsed system remained similar when the pure water measurement environment was changed to 70 mM NaNO3. For all systems studied, shear gave rise to an approximately threefold increase in the range of normal forces, attributed to the ploughing up of adsorbed material during shear to form debris that stood proud of the adsorbed layer. The results provide a microscopic demonstration of the wear process for a salivary film under shear and may be of particular interest for understanding the implications for in vivo oral lubrication under conditions such as rinsing of the mouth cavity. The work is interpreted in light of earlier studies that showed a structural collapse and increase in friction for an adsorbed salivary film in an environment of low ionic strength.
The forces acting between polymers and polymer-modified surfaces are discussed under a consistent framework. We consider the role of the solvents, their structure, and their properties near surfaces and when confined between surfaces. We then study the manner by which polymers attach to and are structured on surfaces, and how those structures determine the normal and frictional forces acting between polymer-modified surfaces under different solvents. We also focus on aqueous solutions and charged macromolecules, and consider their relevance to biolubrication.
In this paper we review recent work (Goldberg et al.; 2011a,b) on a new use for phosphatidylcholine liposomes: as ultra-efficient boundary lubricants at up to the highest physiological pressures. Using a surface force balance, we have measured the normal and shear interactions as a function of surface separation between layers of hydrogenated soy phophatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion, at both pure water and physiologically high salt concentrations of 0.15 M NaNO 3. Cryo-Scanning Electron Microscopy shows each surface to be coated by a close-packed HSPC-SUV layer with an over-layer of liposomes on top. The shear forces reveal strikingly low friction coefficients down to 2 × 10 -5 in pure water system or 6 × 10 -4 in the 150 mM salt system, up to contact pressures of at least 12 MPa (pure water) or 6 MPa (high salt), comparable with those in the major joints. This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.
We describe here the design of a liquid cell specific for synchrotron X-ray reflectometry (XRR) characterisation of soft matter nanofilms at the mica-water interface. The feature of the cell is a "bending mica" method: by slightly bending the mica substrate over an underling cylinder the rigidity of the mica sheet along the bending axis is enhanced, providing sufficient flatness along the apex of the cylinder as required by XRR measurements. Using this cell, we have performed XRR measurements for a number of systems and in this article we show example results: (1) a cationic surfactant, C 16TAB; (2) a zwitterionic surfactant, C 12H 25PC; (3) a semi-fluorinated surfactant, F 4H 11(d)TAB; and (4) surface complex of an anionic fluorinated surfactant, CsPFN, and a positively charged polymer, PEI. For the data analysis we have taken into account the mica crystal truncation rod, i.e. the reflectivity from the mica substrate, and fitted the data with a custom Java™ based software package. Our results unravel detailed structural information of these soft nanofilms, indicating that this method is suitable for XRR measurements of a wide range of soft matter structures at the mica-water interface.
The roles of macromolecules in living systems as information storage systems (as DNA) and in biochemical synthesis have been much studied and are relatively well understood. Far less is known about their physical behavior at biological surfaces and interfaces. This review considers in particular the roles of polymers in biological lubrication and its relation both to diseases such as osteoarthritis and to remedies such as tissue engineering. The lubricating behavior of common bio-interfacial macromolecules including mucins, hyaluronan, lubricin, and aggrecan are described, and insights into the mechanism of biolubrication are examined in the light of the recently revealed role of hydration lubrication in water-based (including living) systems.
We have used neutron reflectometry to investigate the behavior of a strong polyelectrolyte brush on a sapphire substrate, grown by atom-transfer radical polymerization (ATRP) from a silane-anchored initiator layer. The initiator layer was deposited from vapor, following treatment of the substrate with an Ar/H 2O plasma to improve surface reactivity. The deposition process was characterized using X-ray reflectometry, indicating the formation of a complete, cross-linked layer. The brush was grown from the monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), which carries a strong positive charge. The neutron reflectivity profile of the swollen brush in pure water (D 2O) showed that it adopted a two-region structure, consisting of a dense surface region ∼100 Å thick, in combination with a diffuse brush region extending to around 1000 Å from the surface. The existence of the diffuse brush region may be attributed to electrostatic repulsion from the positively charged surface region, while the surface region itself most probably forms due to polyelectrolyte adsorption to the hydrophobic initiator layer. The importance of electrostatic interactions in maintaining the brush region is confirmed by measurements at high (1 M) added 1:1 electrolyte, which show a substantial transfer of polymer from the brush to the surface region, together with a strong reduction in brush height. On addition of 10 -4 M oppositely charged surfactant (sodium dodecyl sulfate), the brush undergoes a dramatic collapse, forming a single dense layer about 200 Å in thickness, which may be attributed to the neutralization of the monomers by adsorbed dodecyl sulfate ions in combination with hydrophobic interactions between these dodecyl chains. Subsequent increases in surfactant concentration result in slow increases in brush height, which may be caused by stiffening of the polyelectrolyte chains due to further dodecyl sulfate adsorption.
Lubrication by hydration shells that surround, and are firmly attached to, charges in water, and yet are highly fluid, provide a new mode for the extreme reduction of friction in aqueous media. We report new measurements, using a mica surface-force balance, on several different systems which exhibit hydration lubrication, extending earlier studies significantly to shed new light on the nature and limits of this mechanism. These include lubrication by hydrated ions trapped between charged surfaces, and boundary lubrication by surfactants, by poly-zwitterionic brushes and by close-packed layers of phosphatidylcholine vesicles. Sliding friction coefficients as low as 10 -4 or even lower, and mean contact pressures of up to 17 MPa or higher are indicated. This suggests that the hydration lubrication mechanism may underlie low-friction sliding in biological systems, in which such pressures are rarely exceeded.
2011
Mammalian synovial joints are extremely efficient lubrication systems reaching friction coefficient μ as low as 0.001 at high pressures (up to 100 atm) and shear rates (up to 10 6 to 10 7 Hz); however, despite much previous work, the exact mechanism responsible for this behavior is still unknown. In this work, we study the molecular mechanism of synovial joint lubrication by emulating the articular cartilage superficial zone structure. Macromolecules extracted and purified from bovine hip joints using well-known biochemical techniques and characterized with atomic force microscope (AFM) have been used to reconstruct a hyaluronan (HA)-aggrecan layer on the surface of molecularly smooth mica. Aggrecan forms, with the help of link protein, supramolecular complexes with the surface-attached HA similar to those at the cartilage/synovial fluid interface. Using a surface force balance (SFB), normal and shear interactions between a HA-aggrecan-coated mica surface and bare mica have been examined, focusing, in particular, on the frictional forces. In each stage, control studies have been performed to ensure careful monitoring of the macromolecular surface layers. We found the aggrecan-HA complex to be a much better boundary lubricant than the HA alone, an effect attributed largely to the fluid hydration sheath bound to the highly charged glycosaminoglycan (GAG) segments on the aggrecan core protein. A semiquantitative model of the osmotic pressure is used to describe the normal force profiles between the surfaces and interpret the boundary lubrication mechanism of such layers.
Human salivary statherin was purified from parotid saliva and adsorbed to bare hydrophilic (HP) mica and STAI-coated hydrophobic (HB) mica in a series of Surface Force Balance experiments that measured the normal (Fn) and friction forces (Fs*) between statherin-coated mica substrata. Readings were taken both in the presence of statherin solution (HP and HB mica) and after rinsing (HP mica). Fn measurements showed, for both substrata, monotonic steric repulsion that set on at a surface separation D ~ 20 nm, indicating an adsorbed layer whose unperturbed thickness was ca 10 nm. An additional longer-ranged repulsion, probably of electrostatic double-layer origin, was observed for rinsed surfaces under pure water. Under applied pressures of ~ 1 MPa, each surface layer was compressed to a thickness of ca 2 nm on both types of substratum, comparable with earlier estimates of the size of the statherin molecule. Friction measurements, in contrast with Fn observations, were markedly different on the two different substrata: friction coefficients, μ ≡ ∂ Fs*/∂Fn, on the HB substratum (μ ≈ 0.88) were almost an order of magnitude higher than on the HP substratum (μ ≈ 9 and 0.12 for unrinsed and rinsed, respectively), and on the HB mica there was a lower dependence of friction on sliding speed than on the HP mica. The observations were attributed to statherin adsorbing to the mica in multimer aggregates, with internal re-arrangement of the protein molecules within the aggregate dependent on the substratum to which the aggregate adsorbed. This internal rearrangement permitted aggregates to be of similar size on HP and HB mica but to have different internal molecular orientations, thus exposing different moieties to the solution in each case and accounting for the very different friction behaviour.
Liposomes of hydrogenated soy phosphatidylcholine (PC) lipids spontaneously self-assemble in close-packed layers on solid surfaces to form boundary lubricants that reduce the coefficient μ of sliding friction between them down to μ ≈ 10 -4 - 2 × 10 -5, at pressures up to ca. 12 MPa. This strikingly low value is attributed to hydration-lubrication by the PC headgroups exposed at the surfaces of the gel-phase vesicles.
Using a surface force balance, we measured normal and shear interactions as a function of surface separation between layers of hydrogenated soy phosphatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion at physiologically high salt concentrations (0.15 M NaNO 3). Cryo-scanning electron microscopy shows that each surface is coated by a close-packed HSPC-SUV layer with an overlayer of liposomes on top. A clear attractive interaction between the liposome layers is seen upon approach and separation, followed by a steric repulsion upon further compression. The shear forces reveal low friction coefficients (μ = 0.008-0.0006) up to contact pressures of at least 6 MPa, comparable to those observed in the major joints. The spread in μ-values may be qualitatively accounted for by different local liposome structure at different contact points, suggesting that the intrinsic friction of the HSPC-SUV layers at this salt concentration is closer to the lower limit (μ = ∼0.0006). This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.
A surface force balance was used to measure the normal and shear forces between two mica surfaces each bearing an adsorbed layer of porcine gastric mucin ("Orthana" mucin), genetically similar to human MUC6. This mucin is a highly purified, 546 kDa, weakly negative, polyampholytic molecule with a "dumbbell" structure. Both bare (HP) and hydrophobized (HB) mica substrates were used, and forces were measured under 1 and 30 mg/mL mucin solutions, under pure (no-added-salt) water, and under 0.1 M aqueous Na + solution. Normal surface forces were monotonically repulsive in all cases, with onset of repulsion occurring at smaller surface separations, D, in the 0.1 M salt solutions (∼20 nm, compared with ∼40 nm for no added salt). Repulsion on HP mica was greater on surface compression than decompression, an effect, attributed to bridging and slow-relaxing additional adsorption on compression, not seen on HB mica, a difference attributed to the denser coverage of mucin hydrophobic moieties on the HB surface. Friction forces increased with compression in all cases, showing hysteretic behavior on HP but not on HB mica, commensurate with the hysteresis observed in the normal measurements. Low friction coefficients (= Fs/Fn-1). The lower friction with HB relative to HP mica suggests a selectivity of the HB surface to the hydrophobic moieties of the mucin that in consequence exposes relatively more of the better-lubricating hydrophilic groups. This surface-selectivity effect on lubrication may have a generality extending to other biological macromolecules that contain both hydrophilic and hydrophobic groups.
Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature's benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.
2010
A common method for creating hydrophobic monolayers on charged surfaces is by self-assembly of ionic surfactants from solution. Several factors are important in controlling the structure and properties of such layers: the hydrophobic interactions between adjacent chains, the electrostatic interactions between adjacent headgroups, and electrostatic interactions between the headgroups and the surface charges. We have discovered that the surfactant counterions can have a remarkable effect on the hydrophobicity and hydrophobic interactions of a self-assembled layer. The experimental system was stearoyl(C18)trimethylammonium surfactant with iodide, bromide or chloride counterion (STAI, STABr, and STACl respectively) self-assembled onto mica substrates. Changing the surfactant counterions alters the wetting properties of hydrophobic monolayers on mica. Using a surface force balance we have carried out direct measurements of the interaction force between two surfactant-coated surfaces across water, revealing a strong effect of counterion on the normal interactions. Paradoxically, STAI-coated mica has both the highest water contact angle (is 'most hydrophobic') at the same time as having the highest surface charge relative to STABr and STACl. We use measurements of interfacial tension, asymmetric force measurements, and XPS to lead us towards an interpretation of these results and an understanding of the effect of counterion on the structure of self-assembled monolayers.
We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than ∼2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1-2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.
2009
Using a surface force balance, we measured the forces between an ultrasmooth (0.2 nm rms roughness) templatestripped gold surface and a molecularly smooth mica surface. Comparison of these forces in both low salt (conductivity water, equivalentto10-6-10-5 M 1:1 salt) and high salt (10mMKClO4) regimes enabled us to examine the properties of water layers confined between a metal and a dielectric to films of a few nanometers or less in thickness. We find that the long-range forces between gold and mica are similar to those between two mica surfaces, indicating a net effective negative charge density on the gold similar to that on the mica. Differences were more pronounced at small separations, manifested by the larger jump-in distance in pure water and the weaker hydration repulsion in high salt between a gold and a mica surface compared with two mica surfaces. However, despite these short-ranged differences, replacing one mica surface with gold does not measurably alter the viscosity of nanoconfined water layers, either as free molecules or as bound hydration layers, relative to their confinement by two mica sheets.
Using a surface force balance with fast video analysis, we have measured directly the attractive forces between oppositely charged solid surfaces (charge densities σ+, σ-) across water over the entire range of interaction, in particular, at surface separations D below the Debye screening length λS. At very low salt concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at D
Articular joints in human body are uniquely efficient lubrication systems.. While the cartilage surfaces slide past each other under physiological working conditions (pressure of tens of atmospheres and shear rates up to 10(6)-10(7) Hz), the friction coefficient (mu) achieves extremely low values (down to 0.001) never successfully reached by mechanical prosthetic devices. Friction studies on polymer brushes attached to surfaces have recently demonstrated (17) their ability to reduce friction between the rubbing surfaces to extremely low values by means of the hydrated ions and the charges on the polymer chains We propose that the extremely efficient lubrication observed in living joints arises from the presence of a brush-like phase of charged macromolecules at the surface of the cartilage superficial zone: hydration layers which surround the charges on the cartilage macromolecules might provide a lubricating ball-bearing-like effect as demonstrated for the synthetic polyelectrolytes (17). In this work macromolecules of the cartilage superficial zone (aggrecans) are extracted from human femoral heads and purified using well developed biochemical techniques (20). The extracted molecules are then characterized with atomic force microscope (AFM). By means of a surface force balance (SFB) normal and shear interactions between mica surfaces coated with these molecules are examined focusing on the frictional forces between such surfaces at normal stresses similar to those in human joints.
To maintain low friction and low wear in natural synovial joints, adsorbed film formation on articular cartilage surface appears to play an important role in mixed or boundary lubrication regime where local direct contact occurs. Furthermore, the proteoglycan gel layer at the uppermost superficial zone in articular cartilage appears to play an important role in preserving low friction and low wear by the hydration lubrication mechanism even after removal of adsorbed film. However, the interaction or synergistic action between adsorbed film and hydrated film/surface has not yet been clarified. To examine the roles of adsorbed film and gel layer on articular cartilage surface in hydration lubrication, the changes in friction were observed in the reciprocating test of articular cartilage against a glass plate, at repeated rubbing including restarting after interrupting-unloading process. It is noticed that at restarting immediately after loading, hydration lubrication is expected to become effective. The lubricating roles of protein-adsorbed film in hydration lubrication are discussed on the basis of experimental results in both cases with and without surface gel layer. The comparison of albumin and γ -globulin is described in connection with the existence of a gel layer.
This paper presents measurements, using the surface force balance (SFB), of the normal and shear forces in aqueous solutions between polyelectrolyte layers grown directly on mica substrates (grafted-from). The graftingfrom was via surface-initiated atom transfer radical polymerization (surface-initiated ATRP) using a positively charged methacrylate monomer. X-ray reflectometry measurements confirm the successful formation of polyelectrolyte layers by this method. Surface-inititated ATRP has the advantages that the polymer chains can be strongly grafted to the substrate, and that high grafting densities should be achievable. Measured normal forces in water showed a long-range repulsion arising from an electrical double layer that extended beyond the polyelectrolyte layers, and a stronger, shorter-range repulsion when the polyelectrolyte brushes were in contact. Swollen layer thicknesses were in the range 15-40 nm. Upon addition of ̃10-2-10-1 M sodium nitrate, screening effects reduced the electrical double layer force to an undetectable level. Shear force measurements in pure water were performed, and the measured friction may arise from polymer chains bridging between the surfaces.
The very low sliding friction at natural synovial joints, which have friction coefficients of μ
Includes discussion of "Dynamic properties of confined hydration layers" Susan Perkin, Ronit Goldberg, Liraz Chai, Nir Kampf and Jacob Klein, Faraday Discuss., 2009
Insights into the molecular origins of cartilage lubrication could lead to more efficient tissue repair and to longer-lasting prostheses for hip and knee joints.
The importance of understanding the interactions between nanoscale materials and living matter has now begun to be appreciated by an extraordinaryly large range of stakeholders, including researchers, industry, governments and society, all of whom appreciate both the opportunities presented by and challenges raised by this arena of research. Not only does it open up new directions in nanomedicine and nanodiagnostics, but it also offers the chance to implement nanotechnology across all industry in a safe and responsible manner. The underlying reasons for this arena as a new scientific paradigm are real and durable. Less than 100 nm nanoparticles can enter cells, less that 40 nm they can enter cell nucleus, and less that 35 nm they can pass through the blood brain barrier. These are fundamental length scales of biological relevance that will ensure that engineered nanoscience will impinge on biology and medicine for many decades to come. One important issue is the current lack of reproducibility of the outcomes of many experiments in this arena. Differences are likely a consequence of such things as uncontrolled nanoparticle aggregation leading to unpredictable doses being presented to cells, interference of the nanoparticles themselves with many of the tests being applied, differences in the degree of confluency of the cells used, and a host of other factors. NanoInteract has shown how careful control of all aspects of the test system, combined with round robin type approaches, can help resolve these issues and begin to ensure that the field can become a quantitative science. The basic principle of NanoInteract is that given identical nanomaterials, cells and biological materials, and using a common protocol, experiments must yield identical answers. Thus, any deviations result from errors in (applying) the protocol which can be tracked and eliminated, until quantitatively reproducible results are obtained by any researcher in any location. This paper outlines the NanoInteract programme, illustrates key advances, and highlights early successes. (www.nanointeract.net)
2008
Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic- polyelectrolytic diblock copolymer, poly(methyl methacrylate)-block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA-b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 -4 w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (±20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 ± 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.
We use a surface force balance to study shear interactions between adsorbed poly(ethylene oxide) layers (Mw = 150 000 or 170 000) adsorbed onto mica in 0.1 M KNO3 aqueous solution. The shear forces increase with increasing compression of the layers or with increasing shear rates (at a constant compression), though at high compressions or shear rates the shear stress σs appears to saturate. This behavior is attributed to frictional effects that are dominated by viscous dissipation (and possible weak monomer-monomer complexing) between the mutually sliding layers at low compressions or shear rates, and by a substrate-slip mechanism at the highest compressions and shear rates. The shear behavior of adsorbed PEO layers in aqueous electrolyte at the higher compressions or shear rates can be well understood in terms of the attachment mechanism of PEO segments, via ion ligands to the charged substrate (as elucidated earlier, J. Am. Chem. Soc. 2005, 127, 1104).
Using a surface force balance, we have measured normal and shear interactions between mica surfaces across pure water and across 0.1 M aqueous solutions of LiNO3, NaNO3, KNO3, and CsNO 3, both prior to adding polymer and following addition of 1.5 × 10-4 w/w poly(ethylene oxide) (PEO, Mw = 170 kD) and overnight incubation. Our results reveal that while the PEO adsorbs strongly from the KNO3 and CsNO3 solutions, unexpectedly it does not adsorb at all from the LiNO3 and NaNO3 salt solutions. We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively charged mica surface of the etheric -O- group on the ethylene oxide monomer for the case of the more weakly hydrated K+ and Cs+, but not for the case of Na+ or Li+ with their more strongly bound water. A simple model relating the electrostatic energy changes occurring upon such liganding to the experimentally measured hydration energies of the different alkali metal ions supports this attribution.
Using a surface force balance, we have measured the normal and shear forces between mica surfaces across aqueous caesium salt solutions (CsNO3 and CsCl) up to 100 mM concentrations. In contrast to all other alkali metal ions at these concentrations, we find no evidence of hydration repulsion between the mica surfaces on close approach: the surfaces appear to be largely neutralized by condensation of the Cs ions onto the charged lattice sites, and are attracted on approach into adhesive contact. The contact separation at adhesion indicates that the condensed Cs ions protrude by 0.3 ± 0.2 nm from each surface, an observation supported both by the relatively weak adhesion energies between the surfaces, and the relatively weak frictional yield stress when they are made to slide past each other. These observations show directly that the hydration shells about the Cs+ ions are removed as the ions condense into the charged surface lattice. This effect is attributed to the low energies-resulting from their large ionic radius-required for dehydration of these ions.
Prompted by the recent discovery that water and aqueous monovalent Na + solutions remain fluid-like when confined to films of a few molecular layers between mica surfaces,[Raviv et al., Nature, 2001, 413, 51-54; and Raviv and Klein, Science, 2002, 297, 1540-1543] we now extend the previous study by comparing the shear- and normal-force properties of 0.1 M Na +, Cs+ and Ni2+ salt solutions which demonstrate a diverse range of behaviours under confinement. In the case of hydrated Na+ we extend the previous study to higher pressures, up to ∼10 atmospheres, and record similar fluidity of the hydration layers at these elevated pressures. Aqueous Cs+ films under confinement between mica sheets have been found to be unable to support an applied load - that is to say they do not demonstrate any hydration repulsion regime - as a result of their lower hydration energy [see Goldberg et al., Phys. Chem. Chem. Phys., 2008, 10, 4939-4945] which contrasts with the properties of Na+. We show that 0.1 M Ni2+ solution remains close to its bulk viscosity down to nanometre thin films, but does not demonstrate a hydration repulsion. By comparing the properties of this range of cations, with differing valency and hydration, we aim to examine the conditions under which ions serve as effective lubricants and what we call the 'hydration lubrication' mechanism.
The interactions of hyaluronan (HA), a high-molecular-weight linear polysaccharide present in many pericellular coatings, with different facets of chiral calcium tartrate (CT) crystal surfaces are investigated using a molecular force probe. Forces between {011} and {110} facets of (R,R) and (S,S) CT crystals and a HA-bearing surface have been measured in saturated CT solutions. It has been observed that hyaluronan binds most strongly to the {011} facet of the (R,R) crystal, compared with the other facets examined, which is consistent with earlier observations of the adhesion of HA-coated cells to chiral CT crystals. The variation of binding strength among the facets studied is tentatively attributed to the surface structure difference between the {011} and {110} facets as well as to the preferential matching of the local hyaluronan H-bond network to the -OH groups on the {011} facet of the (R,R) enantiomer.
2007
We used the surface force balance technique to investigate the influence of free polymer chains on the equilibrium and dynamic properties of dense polymer brushes, focusing on their extension and swelling under oscillatory shear flow. In particular, we extend earlier results on shear of brushes (Nature 1991, 352, 413) both to additional molecular weights of the brush and especially, for the first time, to the effect of the added free polymer. We present experimental studies on how the equilibrium properties of these polymer brushes are influenced by the presence of free, chemically identical polymer chains well above the overlap concentration, c ≈ 40-50 c*. We observe a strong influence of the free chains on the behavior of the brushes when high shear rates are applied, even when the brushes are well away from contact, indicating a clear coupling between the brush and the free polymer. The onset of shear-induced swelling in our experiments correlates reasonably well with the relaxation properties of the outer regions of the brushes.
Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 ± 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.
We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading-unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.
Analysis of friction versus time traces for stick-slip friction of mica surfaces across solidified films confined between them, reveals that most of the frictional dissipation occurs via viscous heating of the shear melted film during the part of the cycle where the surfaces slide past each other (slip). A much smaller part of the energy dissipation results from the loss of residual kinetic energy at the abrupt end of the slip.
Robust polymer brushes are grown from a planar mica surface (see figure) using atom-transfer radical polymerization. The brushes are characterized by X-ray photoelectron spectroscopy, X-ray reflectometry, atomic force microscopy, optical interferometry, and surface forces. (Figure Presented).
The forces between two adhering surfaces bearing highly extended polymer melt brushes as they are sheared past each other, and between a single melt-brush-bearing substrate sheared across an adhering bare solid surface, were studied using a mica surface force balance with a high shear-force resolution. The melt brushes were created by Langmuir-Blodgett deposition of zwitterion-terminated polyisoprene chains on the mica substrates. Shear of the single melt brush by the bare surface revealed little sliding, deformation, or relaxation of the confined melt brush, under all shear regimes applied in this study. In contrast, shearing of the two melt brushes past each other under the same shear conditions showed a marked shear-rate-dependent, multistage deformation of the sheared brushes. On stopping the applied lateral motion, a logarithmically slow relaxation of the stored stress was observed, which could be quantitatively interpreted in terms of mutual retraction of the entangled tails of the two brushes. The low friction and characteristic relaxation behavior following initial adhesive contact of the brushes developed with time to a solidlike response on shear of the confined chains and was attributed to bridging of chains adsorbed on the opposing surfaces as squeeze-out of the polymer occurred.
The time variation of the frictional force between two surfaces, undergoing stick-slip sliding across a molecularly thin film of a confined model liquid, was examined at high time and force resolution, showing clearly that dissipation of energy occurs both during the slip, and at the instant of stick (via transfer of residual momentum). Detailed analysis indicates that, in marked contrast to earlier suggestions, of order 90% or more of the dissipation occurs by viscous heating of the confined shear-melted film during the slip, and only a small fraction of the energy is dissipated at the instant of stick.
2006
Current models for lubrication of synovial joints, and the nature of the cartilage surface, are briefly recalled. Direct friction studies between polymers attached to surfaces are then considered, particularly the very recent demonstration of extreme friction reduction enabled by hydrated ions and by charged polymers. It is proposed that the extremely efficient lubrication observed in living joints arises from the presence of a brush-like phase of charged macromolecules at the surface of the superficial zone. This phase forms when charged macromolecules, including lubricin, superficial-zone protein, and aggrecan, cross the interface between the superficial zone and the synovial cavity as they are secreted into the synovium from within the bulk of the cartilage, and, in particular, the feasibility of such brush-like surface-phases is examined in some detail. The molecular mechanisms for the reduction in friction are proposed to be similar to those recently revealed using surface force balance studies on lubrication by charged brushes.
Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century. Such monolayers generally consist of amphiphilic surfactants anchored by their polar headgroups; sliding occurs at the interface between the layers, greatly reducing friction and especially wear of the underlying substrates. This process, widespread in engineering applications, is also predicted to occur in biological lubrication via phospholipid films, though few systematic studies on friction between surfactant layers in aqueous environments have been carried out. Here we show that the frictional stress between two sliding surfaces bearing surfactant monolayers may decrease, when immersed in water, to as little as one per cent or less of its value in air (or oil). We attribute this to the shift of the slip plane from between the surfactant layers, to the surfactant/substrate interface. The low friction would then be due to the fluid hydration layers surrounding the polar head groups attached to the substrate. These results may have implications for future technological and biomedical applications.
We have measured normal and lateral interactions across a range of different liquids between mica surfaces using a surface force balance (SFB). The mica surfaces were prepared either by melt cutting using Pt wire and standard procedures in our laboratories or by tearing sheets (that had not been exposed to Pt) off from a freshly cleaved sheet of mica. AFM micrographs revealed the substantial absence of Pt nanoparticles on the melt cut and torn-off mica surfaces. Normal-force versus surface-separation (D) profiles and shear force versus D measurements for purified water (no added salt), for concentrated aqueous NaCl solutions, and for cyclohexane revealed that in all cases the behavior of the highly confined liquids between melt-cut and between torn-off mica sheets was identical within experimental scatter. These results demonstrate directly that interactions measured between melt-cut mica surfaces as routinely prepared using established procedures in our laboratories and in other laboratories are free of the effect of any Pt contamination.
We have measured directly the forces across water between hydrophilic surfaces covered with a random mosaic of positive and negative charged domains. We find a strong, long-ranged attraction between them at a surface separation comparable with the charge domain size (many tens of nanometers). This attraction persists at higher salt concentration, but its range then becomes comparable to the Debye screening length. We attribute the attraction to correlation between negative and positive regions on opposing surfaces, facilitated by the lateral mobility of the charge patches on the surfaces.
2005
Using contact angle measurements, surface force balance experiments, and AFM imaging, we have investigated the process of self-assembly of surfactants onto mica and the subsequent stability of those layers in pure water. In the case of cetyltrimethylammonium bromide (CTAB), the stability of a monolayer when immersed in pure water is found to be dependent on initial immersion time in surfactant, which is likely to be caused by an increase in the proportion of ion-exchange to ion-pair adsorption when incubated in surfactant for longer periods of time. Infinite dilution of the surfactant solution before withdrawal of the sample is found to have little effect on the stability of the resulting layer in pure water. The nature of the counterion is found to affect dramatically the stability of a self-assembled surfactant monolayer: cetyltrimethylammonium fluoride (CTAF) forms a layer that is much more stable in water than CTAB, which is likely to be due to faster and more complete ion-exchange with the mica surface for CTAF. Surface force balance experiments show that when the hydrophobic monolayer is immersed in pure water it does not simply dissolve into the water; instead it rearranges, possibly to patches of bilayer or hemimicelles. The time scale of this rearrangement agrees well with the time scale of the change from a hydrophobic to more hydrophilic surface observed using contact angle measurements. AFM imaging has also in some cases shown an evolution from an even monolayer to patches of bilayer.
Using a surface force balance we demonstrate unambiguously that high-molecular-weight poly(ethylene oxide) (PEO) does not adsorb onto mica from purified water with no added salt, a surprising observation in view of its strong adsorption on mica from aqueous 0.1 M KNO3 solution. Analysis of the force profiles, together with the known complexation of PEO with metal ions, suggests that the polymer attachment to the negatively charged surface is mediated by the hydrated potassium ion acting as a ligand.
The diblock copolymer poly(methyl methacrylate)-o-poly(sodium sulfonated glycidyl methacrylate) (PMMA-6-PSGMA) was end-attached by its hydrophobic block (PMMA) onto mica hydrophobized by a stearic trimethylammonium iodide (STAI) layer, to form a polyelectrolyte brush immersed in water. With a surface force balance (SFB), we extended earlier measurements between two such brush layers for the case of normal and shear forces at different shear rates, surface separation, and compressions between one mica surface coated with STAI or a STAI-diblock layer against a bare mica surface. After coating one of the surfaces with STAI, a long range attraction that results in a jump into an adhesive flat contact between the hydrophobic and hydrophilic surfaces was observed. A very different behavior was seen after forming the polyelectrolyte brush on the STAI-coated surface. The long range attraction was replaced by repulsion, accompanied by very low friction during shear (ca. three orders of magnitude lower than with adsorbed polyelectrolytes). On further compression, a weak attraction to the adhesive contact was observed. From the final surface-surface contact separation, we deduce that most of the polyelectrolyte diblock brush layer was squeezed out from the gap, leaving the STAI layer and a small amount of the polymer attached to the surface. Stick-sliding behavior was seen while applying shear, suggesting a dissipation mechanism caused by the trapped polyelectrolyte.
Superparamagnetic particles offer a new way to probe the kinetics of adhesive processes. Two different scenarios of physical adhesion are studied. The thermal activation of van der Waals adhesion is well described by an Arrhenius model. In contrast, it is necessary to go beyond the Arrhenius description to understand the thermal activation of bridging between colloidal particles by a polymer at equilibrium adsorbance. We show that polymer bridging requires some removal of adsorbed polymer and is strongly influenced by the proximity of a glass transition within the adsorbed polymer.
2004
We use a surface force balance (SFB) to study the normal interactions between polymer brushes, which are self-assembled from solution. They consist of polystyrene (PS) chains in toluene (neutral chains in a good solvent) anchored on the interacting mica surfaces via sulfozwitterionic end groups. The properties of the brush depend on the length, N, of the chain, and the energy, αkBT, with which the end group adsorbs on the surface. In contrast to earlier studies where N was varied, we attempt to vary the sticking energy by using polymer chains with one, two, and three zwitterions attached to their ends. We use the theory of Alexander and de Gennes to predict how the normal force profile should vary with α and N, finding, for example, that the brush height L0 obeys L0 = a ·3/5 · α2/5. Surprisingly, our measurements show that the grafting density does not vary significantly between polymers with 1,2, and 3 end groups. This could be attributed to dtpole-dipole interactions between the zwitterions themselves. It is also possible that the effect could be kinetic; that the brush is unable to reach its equilibrium state because successive polymer chains are hindered from attaching to the surface by those already in the brush. Measurements on longer time scales will be necessary to determine whether kinetic effects are important.
Using surface force balance measurements we have established that polystyrene chains bearing three zwitterionic groups have a higher end-group sticking energy than equivalent chains bearing a single zwitterionic group. In a good solvent, polystyrene chains end-functionalized with three zwitterionic groups form brushes of a higher surface coverage than those bearing a single zwitterion. The increase in surface coverage is slow compared with the initial formation of the brush. Measurements of the refractive index allow us to directly quantify the variation of surface coverage, permitting comparison with models for the kinetics of brush formation based on scaling theory and an analytical self-consistent field. We find qualitative support for associating the kinetic barrier with the energy required for an incoming chain to stretch as it penetrates the existing brush.
In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxation/exchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.
A surface force balance with extremely high sensitivity and resolution for measuring shear forces across thin films has been used to investigate directly the dynamic properties of salt-free water (so-called conductivity water) in a gap between two atomically smooth solid surfaces. Our results reveal that no shear stress can be sustained by water (within our resolution and shear rates) down to films of thickness D = D 0 = 0.0 ±0.3 nm. At short range (D 0, at which the surfaces rigidly couple. Analysis of the jump behavior reveals that the viscosity of water remains within a factor of 3 or so of its bulk value down to D 0. This contrasts sharply with the case of confined nonassociating liquids, whose effective viscosity increases by many orders of magnitude at film thicknesses lower than about five to eight monolayers. We attribute this to the fundamentally different mechanisms of solidification of organic liquids and of water. In the former case, the density increase induced in the films by the confinement promotes solidification, while, in the case of water, such densification (due to vdW attraction between the liquid molecules and the confining walls), in agreement with bulk behavior, suppresses the tendency of the water to solidify.
The persistant droplet motion in liquid-liquid dewetting was discussed. It was found that the droplet continued to move in all direction of the dewetting front for extended periods with an initial droplet velocity varying linearly with the droplet size. It was observed that the persistant motion was due to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front. A model to calculate the absolute values of the spontaneous droplet displacement and its variation with time was also presented.
Using a mica surface force balance, we have measured the interactions between mica surfaces bearing chitosan (a common, naturally occurring, cationic polysaccharide; the average molecular weight of our sample was 6 × 105, degree of deacetylation 85%) adsorbed from acetic acid solution. We also introduced a polyionic cross-linking agent in order to cause gelation of the adsorbed layer. Both normal and shear interactions (the lateral forces acting between the surfaces as they slide past each other under compression) were measured in the two cases (freely adsorbed and cross-linked chitosan layers). Normal interactions between the adsorbed non-cross-linked chitosan layers were similar to previous reports of interactions between such layers; their shear interactions revealed a very low effective friction coefficient μeff = ca. 0.003 at low compressions, increasing to ca. 0.07 at pressures of some atmospheres. We attribute the low friction to the weak interpenetration between the layers arising from steric effects and counterion osmotic pressures, together with the presence of hydration sheaths about the charged polyelectrolyte segments, which are known to provide efficient local lubrication. The higher friction on strong compressions is attributed to bridging effects. For the case of the cross-linked layer, normal interactions revealed longer ranged and more repulsive forces, due probably to the network formation resulting in a higher effective modulus of the layers. Frictional forces between the rubbing cross-linked layer were much higher than for the non-cross-linked chitosan, an effect we attribute to increased segmental friction arising from attractive interactions between the cross-linking points; this is also consistent with our observation that for the cross-linked layers significant hysteresis was observed when measuring normal interactions on approach and separation of the layers, an effect that was absent for the case of the non-cross-linked adsorbed layers. Refractive index measurements revealed an adsorbance of 1.2 ± 0.4 mg/m2 of the polymer on each mica surface, consistent with expectations for cationic polymers adsorbed on solid surfaces.
A surface force balance has been used to investigate the shear behavior and to evaluate the viscosity of water-based solutions confined between smooth surfaces in various conditions. We examine the process of jump-in, across the last few nanometers of thin water films, to adhesive contact between the surfaces. Analysis of the jump behavior indicates that the effective viscosity of the films of the different solutions examined remains similar to its bulk value even when it is confined to sub-nanometer gaps. Independently, we find that the shear stress across the progressively thin films is immeasurably small (within our resolution) down to the adhesive contact. These findings lead us to conclude that the two observations are related to each other and that this should be a general phenomenon.
Langmuir-Blodgett monolayers from end-functionalized polyisoprene (PI-X) were studied in a surface force apparatus (SFA) as a model of a highly stretched brush melt. After deposition on a freshly cleaved mica, two identical brush monolayers (with surface area per molecule of about 170 A2) were brought into adhesive contact in the SFA; then kinetic changes in the film thickness and the topography of the contact were continuously monitored. The spontaneous thinning of the brush melt bilayer was observed. This effect can be attributed to the enhanced lateral motion of the sticking end-groups under the 'contact induced pressure.' The possible model describing kinetic changes in the film thickness is presented. The behavior of the two opposing brush melts and a single brush monolayer in contact with two mica surfaces was compared. Molecular mechanisms involved in the rearrangements of brush melts are proposed for both systems.
2003
Long-ranged forces between surfaces in a liquid control effects from colloid stability to biolubrication, and can be modified either by steric factors due to flexible polymers, or by surface charge effects. In particular, neutral polymer 'brushes' may lead to a massive reduction in sliding friction between the surfaces to which they are attached, whereas hydrated ions can act as extremely efficient lubricants between sliding charged surfaces. Here we show that brushes of charged polymers (polyelectrolytes) attached to surfaces rubbing across an aqueous medium result in superior lubrication compared to other polymeric surfactants. Effective friction coefficients with polyelectrolyte brushes in water are lower than about 0.0006-0.001 even at low sliding velocities and at pressures of up to several atmospheres (typical of those in living systems). We attribute this to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments. Our findings may have implications for biolubrication effects, which are important in the design of lubricated surfaces in artificial implants, and in understanding frictional processes in biological systems.
Recent studies have revealed that, in contrast to non-associating liquids such as oils or organic solvents, salt-free water retains a viscosity close to its bulk value even when confined to films thinner than some 3 nm, indeed down to only one or two monolayers thick. For the case of high concentration aqueous salt solution compressed down to subnanometer films between charged surfaces, the trapped hydrated ions serve to act as molecular ball-bearings, sustaining a large load while remaining very fluid under shear. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study has shown that when charged polymer brushes in aqueous media are compressed and slid past each other, they provide a lubrication that is considerably superior to that afforded by neutral brushes: This is attributed on the one hand to the resistance to mutual interpenetration of the chains due to entropic barriers in the good-solvent conditions, and, on the other hand, to the hydration-sheaths on the charged polymer segments which can act - as noted above - as molecular ball-bearings.
Using high-resolution shear force measurements, we examine in detail the frictional drag between rubbing surfaces bearing end-tethered polymeric surfactants (brushes). The drag attains a maximum on initial motion, attributed to elastic stretching of the chains, which falls by a cascade of relaxations to a value characteristic of kinetic friction. This has a very weak velocity dependence, attributed to chain moieties dragging within a self-regulating, mutual interpenetration zone. When sliding stops, the shear stress across the polymer layers decays logarithmically with time, consistent with the relaxation of a network of dangling ends.
2002
A detailed investigation of a new process of dewetting which takes place at the liquid-liquid interface of partially miscible liquids is reported. We use nuclear reaction analysis and real-time video monitoring to examine in detail the formation of two coexisting liquid films formed by phase separation from a mixture and in particular the subsequent dewetting of one of them from the other. A dewetting front propagates from the edge of the spin-cast sample inward, and it is suggested that a Marangoni flow leads to its initiation and subsequent propagation via a mechanism involving fast rupture of holes ahead of the front. Subsequent growth of holes results in the coalescence of the surrounding rims and their breakup into droplets, whose persistent motion at the interfacial plane is described. This route of dewetting differs from the classical ones involving thermally driven fluctuations or nucleation of dewetting centers, by its relatively short induction time, the evolution of a front, and its unique morphological characteristics. The process is believed to be general and important in thin films of partially miscible liquid mixtures.
Compression of an adsorbed polymer layer distorts its relaxed structure. Surface force measurements from different laboratories show that the return to this relaxed structure after the compression is released can require tens of minutes and that the recovery time can grow rapidly with molecular weight. We argue that the arrested state of the free layer before relaxation can be described as a Guiselin brush structure (O. Guiselin, Europhys. Lett. 17, 225 (1992)), in which the monomer density falls off only weakly with distance from the surface. This brush structure predicts an exponential falloff of the force at large distance with a decay length that varies as the initial compression distance to the 6/5 power. This exponential falloff is consistent with surface force measurements. We propose a relaxation mechanism that accounts for the increase in relaxation time with chain length.
We have studied the properties of layers of end-functionalized poly(ethylene glycol) (PEG) grafted by one end to mica surfaces, and, using a surface force balance, the interaction between two such layers. PEG of molecular weight 3.4 kg/mol, functionalized at both ends with an N-hydroxysuccinimide carbamate (NHS) group, to form NHS-PEG-NHS, does not adsorb on mica. When a trimethylammonium group ((CH3)3N+) replaced one of the NHS groups to form ((CH3)3N+)-PEG-NHS, a surface layer formed on the mica from its aqueous solution, showing that the functionalized PEG had been attached by its (CH3)3N+ end. The properties of the ((CH3)3N+)-PEG-NHS layer were examined primarily by surface force measurements as well as by atomic force microscopy, contact angle measurements, and confocal fluorescence microscopy. The layer was found to be very smooth, and the NHS group was found to retain its reactivity also when attached to the surface, demonstrating that the end-functionalized PEG chains can be used as flexible surface-active spacers for binding amino-containing molecules to the surface. Refractive index measurements yield the adsorbance and hence mean interanchor spacing s of the PEG chains, while force profiles revealed the role of both electrostatic double layers and steric components of the normal interactions at different surface separations D. Shear forces between the sliding surfaces were first observed at compression ratios (2L0/D) ∼ 1.2, increasing markedly for (2L0/D) > 3, where L0 is the unperturbed PEG-brush thickness. This may be attributed both to the relatively low (L0/s) ratio, which allows high interpenetration and therefore high segmental density and viscous shear dissipation between opposite layers, and to bridging effects.
A surface force balance has been used to investigate the viscosity of salt-free (conductivity) water confined between hydrophilic and between hydrophobic surfaces. We examine the process of jump-in, across the last few nanometres of thin water films, to adhesive contact between the surfaces. We analyse the flow of water out of the gap under slip and no-slip boundary conditions at the confining surfaces. In both cases we find that the effective viscosity of water remains comparable to its bulk value even when it is confined to sub-nanometre thin films.
We have measured the shear forces between solid surfaces sliding past each other across aqueous salt solutions, at pressures and concentrations typical of naturally occurring systems. In such systems the surface-attached hydration layers keep the compressed surfaces apart as a result of strongly repulsive hydration forces. We find, however, that the bound water molecules retain a shear fluidity characteristic of the bulk liquid, even when compressed down to films 1.0 ± 0.3 nanometer thick. We attribute this to the ready exchange (as opposed to loss) of water molecules within the hydration layers as they rub past each other under strong compression.
A surface force balance (SFB) was used to characterize the behavior of highly stretched polymer brush melts. Langmuir-Blodgett monolayers of polyisoprene (Mw = 29.9 kg/mol) end-functionalized with a zwitterionic group were deposited onto freshly cleaved mica (areal density ≈ 1 chain/(170 Å2)), and two identical brush monolayers were brought into adhesive contact in the SFB. The changes in film thickness as well as the topography of the contact could be continuously monitored. We observed spontaneous film thinning of the brush-melt bilayer, attributed to the outward lateral motion of the anchoring end groups resulting from the contact-induced pressure on the confined brushes, and a detailed model for this is presented. Refractive index measurements of the confined PI-X melt brushes did not reveal any significant deviation from the bulk value for polyisoprenes, suggesting that possible effects (if any) on the optical properties due to chain orientation were below our detection limit. The behavior of the two opposing brush melts was compared with that where only one brush monolayer was confined between mica surfaces.
An investigation of the time-dependent behavior of the normal forces between two smooth mica surfaces immersed in salt-free water was presented, using a surface force balance (SFB). An equilibrium surface-charge density σ = σ0 was indicated by a long-ranged repulsion on initial approach to adhesive contact. The results showed that subsequent force measurements at times τ following separation of the surfaces revealed drop in σ, followed by contact and separation, but then relaxed back to σ0 with a characteristic time τr = 11 ± 2 minutes.
A simple model that shows an additional attraction between solvated surfactant-coated systems is developed. The model simply calculates the van der Waals attraction between the solvated surfactant layers. This attraction was previously neglected as it was expected to have a small energetic contribution. This is indeed the case; however, despite the small energetic contribution the force is large. In other words, although the expression that we get is small in energy, it is large in force. This is particularly important for surface force balance measurements, where using the developed expression, some apparent discrepancies between measured and theoretical values may now have a possible explanation, and especially those associated with surfactant-coated surfaces. We apply the new expression to a given system, and compare with the experimental results.
The shear and normal forces between layers of poly(ethylene oxide) (PEO), of molecular weights Mw = 37 kg/mol adsorbed onto smooth, curved solid (mica) surfaces across the good solvent toluene have been determined using a surface force balance (SFB). The equilibrium Fn(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shearing motion causes the removal of polymer from within the intersurface gap during sliding. The recovery of the adsorbing layer has been measured and the amount of adsorption with time was calculated. Our findings showed that there is no migration of polymers on the surfaces and that the recovery of the layer is controlled by the rate of diffusion of polymer chains into the gap.
2001
The fundamental features of friction between two polymer-bearing surfaces in relative sliding motion are investigated by molecular dynamics simulation. Adsorbed and grafted polymers are considered in good and bad solutions. The solvent is not treated explicitly but indirectly in terms of a Langevin thermostat. In both systems, we observe shear thinning that is attributed to an orientation of the radius of gyration along the sliding direction. This effect is particularly strong for surfaces bearing polymer brushes. In this case, the shear stresses are mainly determined by the degree of the interpenetration of brushes.
We have used a new design of the mica surface force balance (SFB), with extreme sensitivity in measuring normal and particularly shear or frictional forces between two surfaces sliding past each other, to measure the forces between two mica surfaces across a confined 4-cyano-4-hexabiphenyl nematogen (6CB). In an earlier study (Langmuir 1997, 13, 4466, paper 1 of the series) we investigated the normal force-distance profiles and the orientation of the confined liquid crystal (LC). Here we extend this to Study the shear forces Fs between the sliding mica surfaces across the 6CB nematogen as a function of orientation of the confined LC, the applied normal load Fn, the separation D of the mica surfaces, the shear velocity vs, and the relative shear direction of the confining surfaces (which may be varied using the new SFB design). Our results, where the shear forces are measured down to levels that are some orders of magnitude more sensitive than in earlier studies, show that the highly confined nematogen (D in the range from 16 to ca. 100 Å) behaves under shear in a quasi-solidlike fashion for all three orientations studied: planar, planar twisted, and homeotropic. There is a linear relation between Fs and Fn for each of the three orientations, with the effective friction coefficient (dFs/dFn) largest for the homeotropic orientation and lowest for the planar twisted one. Intriguingly, for the planar orientation a clear increase in the friction could be observed when the initial shear direction was changed by 90°. We attribute this to the effect of the initial shear in orienting the confined nematogen layer in the initial direction, so that subsequent sliding motion at right angles to this encounters greater resistance. We also find that within a range of some 40-fold in vs, there was little change in the shear force, in line with what is generally observed for solid-solid friction, and consistent with the fact that little relaxation in Fs is observed over macroscopic times on applying a step strain to the confined LC.
The fluidity of water in confined geometries is relevant to processes ranging from tribology to protein folding, and its molecular mobility in pores and slits has been extensively studied using a variety of approaches1-6. Studies in which liquid flow is measured directly suggest that the viscosity of aqueous electrolytes confined to films of thickness greater than about 2-3 nm remains close to that in the bulk7-9; this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers8,10,11. Here we observe that the effective viscosity of water remains within a factor of three of its bulk value, even when it is confined to films in the thickness range 3.5 ± 1 to 0.0 ± 0.4 nm. This contrasts markedly with the behaviour of organic solvents, whose viscosity diverges when confined to films thinner than about 5-8 molecular layers10-15. We attribute this to the fundamentally different mechanisms of solidification in the two cases. For non-associative liquids, confinement promotes solidification by suppressing translational freedom of the molecules11,15-18; however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing1,3.
The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 × 103 (PEO37) and M = 112 × 103 (PEO112) adsorbed onto smooth, curved solid (mica) surfaces across the good solvent toluene have been determined using a surface force balance (SFB). The SFB used is capable of measuring both normal interactions Fn(D) as a function of surface separation D and, with extreme sensitivity, shear or frictional forces Fs(D,νs) between them as they slide past each other at velocity νs. The Fn(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shear or frictional forces between the sliding PEO-bearing surfaces are very low up to moderate compressions of the adsorbed layers (local pressures up to ca. 105 N m-2), corresponding to effective friction coefficients μeff = (Fs/Fn) of order 0.003 or less. This is attributed to the fluid interfacial layer between the adsorbed layers resulting from their weak mutual interpenetration. At higher loads Fs increases markedly, and two forms of behavior are found depending on the PEO molecular weight. For PEO37, a sharp increase in Fs is followed by removal of polymer from within the intersurface gap during sliding, high friction, and adhesion between the surfaces. For the longer PEO112, the initial increase in Fs and in μeff saturates at the highest loads (for the case of μeff even decreasing), indicating that the slip plane has moved from the polymer/polymer midplane to the polymer/ solid interface. The dependence of Fs on the sliding velocity in the high-friction regime is weak, suggesting that at low compressions there is a thinning of the mutual adsorbed-layer-interpenetration region at high νs that offsets the higher viscous dissipation in that region. At the highest loads, when the slip plane has shifted to the mica surface, the weak Fs(νs) dependence is characteristic of sliding friction at solid substrates.
The breakup of binary fluids was systematically studied as a function of their thickness. Results reveal a strongly linear relationship, over some one-and-a-half decades, both between the rupture time of the films and their thickness, and between the characteristic surface rupture wavelengths.
Polymers that are adsorbed on a surface, or grafted to it (so-called polymer brushes), generally adopt extended configurations, For this reason forces between polymer-bearing surfaces are long-ranged, and are largely determined by steric interactions between molecules on the opposing layers. When two polymer-bearing surfaces are compressed and made to slide past each other, the frictional forces between them are determined by several different factors. The reduction of friction by polymer brushes occurs because such brushes can support a large load while maintaining a very fluid interface due to limited mutual interpenetration. At the highest pressures, slip reverts from the mid-plane to the polymer-substrate interface. Functionalized polymers with specifically attractive groups can lead to frictional dissipation on sliding which is modulated by the breaking and reforming of bonds. Detailed understanding of polymer-modulated friction may be obtained by taking due account of the polymer dynamics, surface structure and topology, and their specific surface attachment.
2000
Oleic acid and stearic acid are similar surfactants which, however, lead respectively to stability and to precipitation of ferrofluid suspensions: to understand this, the forces between layers of oleic-like surfactants and between layers of stearic-like surfactants across a hexadecane (HD) medium were measured using a surface force balance (SFB). Separate measurements reveal that only the oleic layers are solvated by HD, while the SFB results reveal that for both surfactants a marked net attraction is present between the surfaces. Simple considerations based on these observations explain why, despite this attraction, ferrofluid dispersions are stabilized by oleic but not by stearic surfactants.
The interactions between two mica surfaces bearing fifth-generation amino acid-modified poly(propyleneimine) dendrimers (the dendritic box that exposes methyl groups at their outer surface) were studied across a toluene medium, using a surface force balance capable of measuring shear as well as normal forces. Normal force measurements indicated that dendrimers adsorb from dilute toluene solution (ca. 5×10-5-3×10-4 w/w) as a monolayer on the surfaces. Two interacting dendrimer monolayer-bearing surfaces experience a van der Waals attraction followed by steric repulsion on compression. The dendrimer bilayer could be compressed reversibly, yielding a measure of the compressibility of the molecules. Frictional force versus normal load profiles were measured at different shear velocities, and reveal both solidlike and liquidlike behavior of the confined dendrimers, consistent with NMR measurements on the dendritic box. The results show that the yield stress increases with compression of the layers. Observation of the relaxation behavior of sheared dendrimer layers - for adsorption from dilute solution - suggest that, within the parameters of our experiments, the relaxation times are insensitive to the compression. For the case of surface interactions after incubation in more concentrated dendrimer/toluene solutions (ca. 10-3 w/w), the results of both normal and shear force measurements suggested aggregation of much thicker, loose dendrimer layers on the mica surfaces. These layers resulted in hysteretic and longer-ranged monotonic repulsion, and much weaker frictional forces - at comparable loads and shear velocities - than in the case of the monolayers adsorbed from dilute solutions.
The interactions between two mica surfaces bearing a fourth-generation carbosilane dendrimer (modified to expose -OH groups on its outer surface) were studied across a toluene medium, using a surface force balance capable of measuring shear as well as normal forces. Normal force measurements indicate that the dendrimers adsorb from dilute toluene solution (ca. 5×10-4 w/w) as a monolayer on each surface. Two such interacting surfaces experience a longer-ranged van der Waals attraction followed by strong short-range adhesion (probably of dipolar origin) as the adsorbed dendrimers come into contact. Within the range of our parameters, the dendrimer layers were incompressible normal to the surfaces. Friction versus load profiles were measured at different shear velocities, revealing marked stick-slip sliding, whereas the magnitude of the yield stress increased with longer times of contact and with normal pressure. This suggests that over time scales comparable with the experimental times the interacting layers rearrange to optimize their interfacial shear strength. The behavior of these -OH-exposing carbosilane dendrimers differs qualitatively from that of CH3-exposing poly(propyleneimine) dendrimers studied earlier, a difference attributable to the much more polar nature of the hydroxyl groups.
We have used composition depth profiling, based on nuclear-reaction analysis and secondary ion mass spectroscopy, to study segregation at the free surface of a partly miscible binary mixture consisting of random olefinic copolymers. The equilibrium surface excess data, analysed within a mean-field Cahn approach, point to a wetting transition. The surface phase diagram obtained was confirmed by the observed dynamics of the segregation from a coexistence composition: the monotonic and halted growth of the surface layer was observed at temperatures above and below the predicted wetting point, respectively.
Using nuclear reaction analysis composition-depth profiling, we investigate the influence of symmetric/asymmetric confining walls on the equilibrium configuration of thin films of phase-separated polymer blends. Depth profiles of samples annealed under symmetric boundary conditions show a laterally averaged concentration, while samples confined by nonsymmetric walls show (as in earlier studies) clear separation into two thin layers of coexisting phases. This suggests that for phase separation under symmetric boundary conditions the interface between the two phases is orthogonal to the sample plane, in line with recent theoretical discussion.
Based on segregation data, determined with profiling techniques, we analyze surface phase diagram of binary liquid mixtures composed of random olefinic copolymers. Blends with extended- and small- critical point wetting regimes are discussed. First observation of wetting transition is presented. Surface enrichment-depletion duality, a phenomenon prerequisite to the 2nd order wetting transition, is described.
We compare segmental interaction parameters χAB in binary polyolefin mixtures extracted from a fit to the binodal-as deduced from composition-depth profiling of thin films-with values extracted from small-angle neutron scattering (SANS), on the self-same blends. We find good quantitative agreement as regards both absolute values and compositional dependence of χAB. We also compare values of χAB obtained from composition-depth profiles of surface segregation in isotopic polystyrene mixtures with values deduced from SANS from these mixtures, and again we find good agreement between them. These results point to the use of thin-film composition-depth profiling, which is potentially a more accessible technique and requires far smaller quantities of material, as a convenient, accurate, and widely applicable alternative to SANS for measuring segmental interactions in binary polymer mixtures.
The abrupt liquid-to-solid transition experienced by simple non-polar liquids with quasi-spherical molecules when compressed to a few molecular layers between smooth, solid surfaces can be drastically modified by the presence of polymeric chains end-attached to the surfaces (polymer brushes). The origin of this is the weak interpenetration of the brushes, which can be strongly compressed and yet have a very fluid interfacial region when sheared past each other, resulting in very low friction. The use of telechelic brushes, which are functionalized at both ends and could form loops rather than tails, should result in an even lower interpenetration, and thus in a better lubrication effect. A simple calculation, however, shows that brush-like dimer or multimer-structures might be more favourable energetically than simple loops in the case where the telechelic end-groups attract each other, leading to a very different form of interactions. Recent measurements of the normal and shear forces between surfaces bearing layers of telechelic brushes are in line with these expectations.
The abrupt liquid-to-solid transition experienced by simple non-polar liquids with quasi-spherical molecules when compressed to a few molecular layers between smooth, solid surfaces can be drastically modified by the presence of polymeric chains end-attached to the surfaces (polymer brushes). The origin of this is the weak interpénétration of the brushes, which can be strongly compressed and yet have a very fluid interfacial region when sheared past each other, resulting in very low friction. The use of telechelic brushes, which are functionalized at both ends and could form loops rather than tails, should result in an even lower interpénétration, and thus in a better lubrication effect. A simple calculation, however, shows that brush-like dimer or multimer-structures might be more favourable energetically than simple loops in the case where the telechelic end-groups attract each other, leading to a very different form of interactions. Recent measurements of the normal and shear forces between surfaces bearing layers of telechelic brushes are in line with these expectations.
1999
An alternative pathway for the initiation of dewetting in thin metastable films of partially miscible liquid mixtures is described. In this pathway, phase separation is followed by a dewetting process at the interface between the two phases. Dewetting proceeds (from the sample edges inward) as holes form. The initially smooth film breaks up into droplets at rates much faster than those allowed by classical rupture mechanisms. Marangoni flow appears to be responsible for the initiation of the flow of the dewetting front, and coupling between the flow in the two phases leads to accelerated hole formation.
According to a common viewpoint the surface of the binary polymer mixture A/B could be enriched only in one component, say A, regardless of the value of its bulk volume fraction φbulk. From recent theoretical analyses and Monte Carlo simulations one can expect however that some mixtures can exhibit surface segregation in the minority blend component, i.e. enrichment in the component A for φbulkbulk > 50%. Using composition-depth profiling techniques, we have observed both types of the segregation with only one- or both-blend component(s) preferred at the surface. These results were obtained for model mixtures composed of partly deuterated (dx) and protonated (hx) random olefinic copolymers of the structure -[C4H8]1-x [C2H3(C2H5)]x(-). A simple mean-field model is presented to explain both situations.
The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w0) is broadened due to capillary waves, which lead to fluctuations, of correlation length ξ∥ of the local interface positions in the directions parallel to the confining walls. We postulate that ξ∥ acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial width w varies as w2 ∝ ln ξ∥. In the limit of large D this yields w2 ∝ D or w2 ∝ ln D respectively for the case of short-or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model.
The forces between two surfaces bearing solvated, telechelic polymer chains (terminated at both ends with a dipolar group) exhibit a clear attractive regime. The range and magnitude of this attraction indicate the presence of multimers on each surface, and the formation of entropically favored bridges between them as the telechelic layers overlap. On sliding the surfaces a shear force close to that expected from the stretching of the polymer bridges is observed, suggesting that the frictional forces can be tuned by varying the interaction strength of the telechelic end groups.
Motivated by the results of recent experiments on the sliding of smooth surfaces across confined monolayers of simple organic materials, we develop a model for friction in such systems based on a shear-melting picture. By analysing the stick-slip motion during sliding we conclude that the effective viscosity of the confined films during the slip regime is low, and that nearly all the frictional dissipation takes place at the instant of stopping when the confined films abruptly solidify. The extension of this model to more general situations is briefly considered.
1998
We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C4H8]1-x[C2H3(C 2H5)x. Based on surface segregation data we have determined a surface energy difference χs between pure blend constituents. In each binary mixture components have different fractions x1, x2 of the group C2H3(C2H5), and one component is labeled by deuterium (dx) while the other is hydrogenous (hx). The mixtures are grouped in four pairs of structurally identical blends with swapped labeled constituent (dx1/hx2, hx1/dx2). For each pair the surface energy parameter χs increases when the component with higher fraction x is deuterated, i.e., χs(dx1/hx2) > χs(hx1/dx2) for x1 > x2. A similar pattern has been found previously for the bulk interaction parameter χ. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. χs has also an additional contribution, insensitive to deuterium swapping effect, and related to entropically driven surface enrichment in a more stiff blend component with a lower fraction x. Both enthalpic and entropic contributions to χs seem to depend on the extent of chemical mismatch between blend components.
The evolution of pre-rupture undulations at the liquid-air interface of thin non-wetting liquid films spread on a solid substrate was monitored in real time by non-perturbative interference microscopy. The spatial distribution of the incipient undulations is non-random and characterized by a typical wavelength, as predicted for van der Waals unstable films, despite the fact that the system is expected to be vdW-stable, and that ultimate dewetting of films appears to take place via a heterogeneous nucleation mechanism.
We have extended earlier work on the reduction of friction by polymer brushes between moderately-compressed sliding surfaces to the case of shear forces between brush-bearing surfaces as they slide past each other at high compressions, against much larger frictional forces. Our results reveal that at compression ratios β = (2L/D) ≥ 8, where L is the unperturbed thickness of each brush and D the surface separation, the shear forces rise rapidly. At sufficiently high compressions (β ≥ 10) we observe stick-slip behavior, as well as clear indications of detachment of the chains from the surfaces for the case of the shortest brushes. Closer examination of the data shows that at lower compressions (8 ≥ β ≥ 10) the sliding occurs within the overlapped polymer layers, and the friction is due to viscous drag on the interpenetrated polymer moieties. At the highest compressions - corresponding to the highest frictional forces - our data suggest that the sliding may take place by slip at the polymer/substrate interface, as the bonds anchoring the chains to the substrate detach and reform.
We have measured the surface segregation towards the vacuum in films of a binary mixture of deuterated E48EE52 (d52) and hydrogenous E38EE62 (h66) - random olefinic copolymers. Here E and EE are the linear ethylene and branched ethyl ethylene groups (C4H8) and [C2H3(C2H5)], respectively. The d52 copolymer is enriched at the surface when d52 is the minority component in the blend. On the contrary, the surface is depleted in the d52 chains, when they constitute a majority of the mixture. We have determined two branches of the segregation isotherm corresponding to the enrichment and the depletion. A mean-field Cahn model describes consistently these two isotherm branches. The observed enrichment-depletion duality modifies a common viewpoint on surface segregation.
Motivated by recent experiments on the sliding of two surfaces separated by ultrathin films of simple organic liquids, we analyse the shear and resulting dynamics of the confined materials. Our results indicate that the effective viscosity of shear-melted thin films of such liquids is low, and can account for several features of the experimental results, including the observation of stick-slip effects up to high shear rates. Our analysis also points to the origin of the frictional dissipation between sliding surfaces separated by thin films of simple liquids.
Using a surface force balance with high sensitivity in measuring shear forces we investigated the mechanical properties of thin layers of cyclohexane and octamethylcyclotetrasiloxane (OMCTS) in the gap between two smooth solid surfaces at discrete thicknesses n= 6-3 molecular layers. At these layer thicknesses the films have undergone solidification due to their confinement (see preceding paper) and are capable of sustaining a finite yield stress upon being sheared. The sliding of the confining surfaces at mean velocity νs across the films is characterized by a shear or frictional force Fs which varies with a characteristic stick-slip pattern. We investigate comprehensively the dependence of Fs on n, νs, and on the applied normal forces F across the films. We find that transitions in film thickness from n→(n-1), with a consequent increase in Fs, may occur spontaneously during sliding with no change in F, corresponding to a multivalued friction force between the surfaces for a given load. The critical yield stress S for sliding at a given film thickness n increases monotonically with applied normal pressure P as S = S0+ CP where S0 is a constant of order 105 Pa (depending on n) and C is roughly constant and of order 1. A simple model for friction across such films which can account semiquantitatively for this behavior is introduced, based on a shear-melting mechanism using the Lindemann criterion. We find that the characteristic stick-slip behavior persists over the range of film thicknesses and the entire (large) range of mean shear velocities studied, and that over most of this range the mean shear forces are independent of νs.
A surface force balance with extremely high resolution in measuring shear forces has been used to study the properties of films of the simple organic solvents cyclohexane, octamethylcyclotetrasiloxane, and toluene, confined in a gap between smooth solid surfaces. We were able to probe in detail the transition between liquidlike and solidlike behavior of the films as the gap thickness decreased. Our results reveal that in such confined layers the liquids are fluid down to a film thickness of few molecular layers (typically seven, depending on the particular liquid examined). On further decreasing the gap thickness by a single molecular layer, the films undergo an abrupt transition to become solidlike in the sense that they are able to sustain a finite shear stress for macroscopic times. At the transition, the effective rigidity of the films, quantified in terms of an effective creep viscosity, increases by at least seven orders of magnitude. This sharp transition is reversible and occurs as a function of the confinement alone: it does not require external applied pressure. Following the transition the confined films behave under shear in a manner resembling ductile solids.
A detailed analysis of the experimental implications of the scaling theory of polymer adsorption in the good solvent regime is presented. A number of new results that simplify the task of comparing theoretical predictions and experimental data are given: these include a new simple approximate expression for the force vs distance profile, which is close to the exact solutions over an appreciable range of the intersurface separations. The theoretical predictions are compared to available experimental data for the forces arising between mica plates exposed to poly(ethylene oxide) - water solutions. Good quantitative agreement between theory and experiment is found, with no need for adjustable parameters.
Wetting properties of thin films of a polymer melt on top of a network formed by irradiation cross-linking of an identical melt were studied. The permeability of the network and its wettability as a function of the extent of cross-linking were characterized by nuclear reaction analysis, atomic force microscopy, contact-mechanics measurements of the adhesion energy, and light microscopy. As the degree of cross-linking increased, we observed a crossover from complete to partial wetting, followed by a second crossover back to complete wetting at higher cross-link densities. We suggest that the first crossover is a manifestation of entropy-induced autophobicity arising from the brush-like nature of the cross-linked surface, while the second originates from surface roughening induced by the cross-linking.
The mechanical properties of confined thin films of simple liquids were measured via a surface force balance as a function of the film thickness. We find an abrupt transition from liquid-like to a solid-like behaviour at a confinement corresponding to ca. six molecular layers of the liquid. The effect is due to confinement alone and does not require external applied pressure. At the transition the effective viscosity increases by at least seven orders of magnitude. It is proposed that this is a cooperative effect, reminiscent of a Lindemann-like mechanism, originating in the suppression of the freedom of the confined molecules to move.
The concentration profiles of high molecular weight, isotopic polystyrene blends in the near-surface region were directly measured using real-space composition-depth profiling based on nuclear reaction analysis. The measured profiles are in good agreement with predictions of the Schmidt-Binder mean field theory. In contrast to previous indications, we find no evidence for a flattening of the concentration profile near the surface.
1997
We describe a novel approach based on atomic force microscopy to determine the dihedral contact angle of a liquid polymer on top of a solid substrate. By cross-linking the polymer in an ion beam, the polymer surface becomes amenable to AFM imaging. This sample preparation enables us to take topography scans of the polymer in a solid state. The cross section of the contact area of a polymer drop and the substrate can then be utilized to determine the contact angle between polymer and substrate. Extremely low contact angles in the range 2-8° are reproducibly measured. Control measurements carried out with optical phase modulated interference microscopy gave contact angles well comparable to the AFM results. We use our method in a study of polymer dewetting on top of a network of itself.
Thin liquid oligostyrene films on top of a surface-anchored, polystyrene brush-solvated by the oligostyrene itself - roughen spontaneously. Using optical phase-interference microscopy and nuclear reaction analysis we have characterized the topography of such roughened films. We see that the undulations formed reach down at most to approximately the top of the solvated brush, but not to the underlying solid substrate on which the brush is anchored. Possible origins for this behaviour are discussed in terms of a picture wherein the solvated brush is regarded as a quasi-solid substrate for the liquid film on top.
Using a surface force balance, we have examined the orientation of a nematic liquid crystal (6CB) confined between mica surfaces. By simultaneous measuring of the force and the refractive index profile, we were able to distinguish between planar, planar twisted, and homeotropic orientation of the nematic. When the mica was exposed to air for short periods (
We have investigated the wetting behaviour of a polymer melt on top of a cross-linked network of itself. For substrate films that were not cross-linked at all (or at very low cross-link densities) the melt completely wets the underlying layer. At intermediate cross-linking densities we observe dewetting, which we suggest is due to the brush-like surface of the network. At higher cross-linking densities the melt again completely wets the network, due, we believe, to increased roughening of the surface of the cross-linked substrate.
Direct depth profiling techniques to date have largely lacked the necessary depth resolution to investigate interfacial phenomena of the order of the bulk correlation length (5 -10 nm for a wide range of systems). Here we investigate the optimal spatial resolution and depth of probe that may be attained for composition - depth profiling of polymeric samples via nuclear reaction analysis (NRA) using the 2H(3He,'H)4He reaction. We find that the spatial resolution can be greatly improved by using a grazing incidence geometry of the incident 3He beam on the sample, and analyzing the emitted protons in a backwards direction. This results in spatial resolutions down to about 3 nm at the sample surface, compared to a value of some 7 nm or more previously reported in earlier studies when emitted a-particles were detected in the forward direction. At the same time the depth to which samples can be profiled via the backwards emitted protons may be considerably extended relative to the a-particle detection mode, when the 3He beam impinges on the sample surface at normal incidence (up to about 4 urn into the sample for incident energies of 1.2 MeV in the proton-detection mode compared to only 1 urn for the equivalent a-particle detection mode).
Keywords: Polymer Science
Surface segregation in thin films of binary liquid mixtures consisting of random olefine copolymers was studied by nuclear reaction analysis over a wide temperature and composition range. A divergence of the surface excess Γ was indicated as the binodal of each mixture was approached from the one-phase region, even at temperatures 100 °C below the critical point Tc, and interpreted as the advent of complete wetting behavior. A consistent description of the adsorption isotherms in terms of a mean field approach assuming a short-ranged surface potential fs is feasible, but requires an unexpected temperature dependence of fs. This dependence causes the wetting transition temperature to be located lower than expected on the basis of present models.
Using a surface force balance, the orientation of a nematic liquid crystal (6CB) confined between mica surfaces, and its response to shear under extreme confinement was examined. Slow transitions (over a period of days) from a planar twisted to a planar orientation were observed in certain conditions. We propose that these are associated with anchoring transitions.
We have measured the growth with time t of a wetting layer (of thickness l(t)) at the surface of a thin film of a binary liquid (polymer) mixture. Over a wide range of experimental parameters, our data is well described by a model of diffusion limited wetting which takes into account the finite film thickness. In this model, l(t) is a function of time which sensitively depends on the nature of the interfacial potential: a detailed comparison shows that long range van-der-Waals forces provide the main driving force for the build-up of the wetting layer.
1996
Using nuclear reaction analysis, we have measured the enrichment by one of the components at the surface of a binary mixture of random olefinic copolymers, with components of monomer structure E1-x1EEx1 and E1-x2EEx2. Here E and EE are the linear ethylene and branched ethylethylene groups (C4H8) and [C2H3(C2H5)], respectively, and x represents the fraction of the EE group randomly distributed on the chains. We examined 12 different couples covering a range x=0.38-0.97. The mixtures, whose thermodynamic behavior was established in our earlier paper, were cast in the form of films on both a silicon and on a gold-covered silicon surface, and were investigated in the one-phase region of the binodal in the vicinity of the critical temperature. We find that it is always the more flexible component - the one with a shorter statistical step length, corresponding to the higher ethylethylene fraction (higher x) - that is enriched at the polymer/air surface. Within our resolution neither component is enriched at the polymer/solid interface. These results show clearly that enthalpic rather than entropic factors dominate the surface potential driving the surface enrichment. For two of the mixtures we determined the excess of the surface-preferred species as a function of mixture composition along an isotherm in the one-phase region of the binodal. A consistent description of our data in terms of a mean-field model is provided by including in the surface potential a term in the mixture composition gradient at the polymer surface.
We have used composition depth profiling of polymer bilayers, based on nuclear reaction analysis, to determine miscibility, phase coexistence, and critical temperatures in mixtures of random olefinic copolymers of mean composition E1-x/EEx; here E is the ethylene group -(C4H8)-, EE is the ethylethylene group -[C2H3(C2H5)]-, and one of the copolymers is partially deuterated. The components in each binary mixture have different values x1,x2 of the EE fraction. Using a simple Flory-Huggins mixing model, our results enable us to extract an interaction parameter of the form χ(x1,x2,T)=A(x1,x2)/T, where for given x1,x2, A is a constant. Calculated binodals using this form fit our measured coexistence curves well, while allowing χ a weak composition dependence improves the fit further. Within the range of our parameters, our results suggest that in such binary polyolefin mixtures the interaction parameter increases roughly linearly with the extent of chemical mismatch expressed as the difference in degree of ethyl branching between the two components.
Stability of thin films of non-volatile liquids is a key issue in a variety of applications. Often a film is forced to spread on a substrate which is not wetted by the liquid. The film then ruptures within minutes and dewets. Common methods for achieving stability include the introduction of surface-active low molecular weight agents, or modification of the chemistry of the substrate. We describe here a mechanism for suppressing the rupture of the films by surface-attached polymers together with trace amounts of free polymers in the bulk of the film. The effect may have a kinetic origin, which is related to the entanglement of free chains and surface-attached polymer chains, or it may be due to a modification of the thermodynamic interactions.
We have examined the growth with time t of the thickness l of wetting layers from thin films of binary fluid mixtures, using real-space composition-depth profiling based on nuclear reaction analysis. We find over a wide range of parameters that the observed growth behavior has a power law form l∝tk, where k is the range 0.20.3. Comparison with the predictions of a diffusion-limited wetting model shows that growth of the wetting layers is driven primarily by long-ranged surface fields.
The forces between polymer-bearing surfaces in motion with respect to each other in a liquid medium are very different from their equilibrium interactions and control effects ranging from colloidal rheology to lubrication and wear. Several aspects of these forces are reviewed, both when surfaces are approaching or receding (lubrication forces) or when they slide past each other (shear and frictional forces). Lubrication forces are considered in different solvency conditions and for different types of polymer attachment to the surfaces, particularly in compressed layers, where viscous segmental drag on the flowing liquid dominates. Frictional forces between compressed, rubbing solid surfaces may be dramatically reduced by the attachment of solvated chain layers, which are able to sustain large normal loads while retaining a fluid interfacial layer. The coupling between sliding motion and normal forces is described, and the relation between the shear rates required to stretch chains and the chain relaxation rate is analyzed.
Stability of thin films of non-volatile liquids is a key issue in a variety of applications. Often a film is forced to spread on a substrate which is not wetted by the liquid. The film then ruptures within minutes and dewets. Common methods for achieving stability include the introduction of surface-active low molecular weight agents, or modification of the chemistry of the substrate. We describe here a mechanism for suppressing the rupture of the films by surface-attached polymers together with trace amounts of free polymers in the bulk of the film. The effect may have a kinetic origin, which is related to the entanglement of free chains and surface-attached polymer chains, or it may be due to a modification of the thermodynamic interactions.
When a film of thickness D of a binary mixture on a substrate segregates into two coexisting phases, an interface between the phases parallel to the substrate may form due to preferential surface attraction of one of them. It is argued then that the correlation length [Formula presented]for interfacial fluctuations parallel to this interface [Formula presented] leads to a size-dependent interfacial width [Formula presented]. Nuclear reaction depth profiling experiments on polymer mixtures as well as Ising model simulations both support this prediction.
1995
Nuclear reaction analysis (NRA) was used to study the segregation of polyisoprene (PI)-deuterated polystyrene (dPS) diblock copolymers to the interfaces formed by high molecular weight (M) polystyrene (PS, M = 3 × 106) with vacuum, silicon, and gold. Two types of diblock copolymers, a symmetric PI(M =104)-dPS(M =104) diblock and a strongly asymmetric PI(M=104)-dPS(M=106) diblock, were investigated, and their segregation isotherms were determined as a function of their bulk concentration within the PS homopolymer. Isotherms were analyzed in terms of a mean-field approach due to de Gennes and Leibler and the numerical results of a self-consistent mean-field model. Both approaches were found to provide a consistent description of the data.
Interfacial adsorption from a binary mixture of short and long polystyrenepolyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 × 106) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 104)-PI(M = 104) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 106)PI(M = 104) (L) were used in the binary mixtures. We found that the shorter diblocks adsorb preferentially at the interfaces: the isotopic contrast between dS and hS in the mixtures with L enables us to extract detailed information on the surface segregation of the short and long species. Our data are analyzed in terms of a modified Flory-type mean field model due to de Gennes and Leibler. Interfacial segregation from the binary diblock mixture is well predicted by this model with no adjustable parameters, using the adsorption characteristics determined earlier for systems with a single diblock component.
We have used nuclear reaction analysis to measure diffusion coefficients D in couples consisting of hydrogenated polybutadienes of structure (C2H3(C2H5))x(C4H8)1−x and their partly deuterated counterparts. The 1,2 and 1,4olefinic isomers are randomly distributed along the chains and the mean vinyl fraction x varies between 0.38 and 0.94. We find that the effective monomeric mobility D0 [defined by D = D0(Ne/N2) for each copolymer, where N is the backbone length and Ne the entanglement spacing] decreases monotonically with increasing vinyl content x. Over the range of microstructures and temperatures T (−14−40°C) investigated we find log(D0/T) varies smoothly with (T − Tg), where Tg is the glass transition temperature of the respective melts. An analysis of our data in terms of a simple activated rate process model suggests that D0 is controlled by thermally activated hopping of segments whose effective volume is close to that of the respective statistical segment lengths of the copolymeric chains. ©1995 John Wiley & Sons, Inc.
The liquid-to-solid transition of a simple model liquid confined between two surfaces was studied as a function of surface separation. From large surface separations (more than 1000 angstroms) down to a separation corresponding to seven molecular layers, the confined films displayed a liquid-like shear viscosity. When the surface separation was further decreased by a single molecular spacing, the films underwent an abrupt, reversible transition to a solid. At the transition, the rigidity of the confined films (quantified in terms of an "effective viscosity") increased reversibly by at least seven orders of magnitude.
Thin films of a low molecular weight, nonvolatile liquid which are forced to spread on silicon wafers, rupture within minutes and dewet. Addition of long polystyrene chains (for which the liquid is a good solvent) up to a concentration of 10% does not change this behavior qualitatively. In contrast we find that the unbroken uniformity of the films may be preserved for many months or longer by a polystyrene brush attached to the silicon, together with some unattached polystyrene in the liquid. By varying the molecular weights and concentrations of the unattached chains within the film, we weee able to establish a stability diagram in this system which shows that suppression of rupture is only observed at free-polymer concentrations which exceed the overlap concentration. This suggests that the effect may arise from the formation of an entanglement network between the free chains (within the liquid film) and the surface-tethered brush molecules.
We have used a recently developed surface force balance to measure, with extreme sensitivity, both lateral and normal forces between interacting surfaces, for the case of simple liquids and particularly with surfaceattached polymers. The presence of polymers on the surfaces reduces drastically the force required to maintain them in sliding motion, under a given normal load, relative to the bare surface case. We believe this is due to the long range steric repulsion which can sustain a large normal load while maintaining a very fluid interfacial layer. The effect is much more marked for endtethered chains in a good solvent than for adsorbed chains in a θsolvent. This is attributed to the different extents of interpenetration of the compressed polymer layers.
Using nuclear reaction analysis, we have measured the rate of surface segregation of an asymmetric diblock polyisoprenepolystyrene (PIPS) copolymer to the free surface of polystyrene (PS) homopolymers within which it is incorporated at low bulk concentrations. We have also measured the tracer diffusion coefficient of PIPS in PS under similar conditions, and shown it to equal, within error, the selfdiffusion coefficient of PS of a similar length. The equilibrium surface excess Γ of the diblock is low (
The way in which flexible polymers can modify the structure of a coated surface has been studied by measuring the interactions between polymer-bearing surfaces in liquids. It is observed that interactions induced by polymers attached to surfaces can also lead to striking changes in the effective 'wetting' properties of thin liquid films. A film is said to 'wet' a surface if it tends to spread uniformly on a solid surface. The smoothness and long-term stability of such films depends on how well they wet the surface, and this is essential for the electronic and electro-optical devices, for example, to work properly. If the films are to break up or 'dewet' the surface, the device would be of little use.
1994
It has been shown by surface force studies that the properties of grafted polymer layers formed from functionalized polystyrene (polystyrene terminated by a zwitterionic group at the end) can be changed dramatically by surface exchange: the replacement of longer molecules attached to the surface via the end group by the shorter amphiphile terminated with the same group - behaviour opposite to that shown by adsorbed homopolymers. This exchange requires a certain minimum concentration of the shorter polymer in the solution, but the substantial replacement of long chains by short chains can occur even when the monomer concentration of the shorter polymer is relatively very small, i.e. only 1/500 that of the longer polymers.
The diffusion process in dPS (Mw = 1.06 × 106 g mol-1)/hPS (Mw = 2.89 × 106 g mol-1) mixtures at constant temperature below the critical temperature slows down approaching the coexistence curve from the homogeneous single fluid phase region of the phase diagram. This effect can be understood in terms of the composition dependence of the thermodynamic factor (∂μdPS/∂ΦdPS)T,P in combination with the Flory-Huggins theory of polymer melts. The theoretically expected decrease of the mutual diffusion coefficient with increasing temperature along the coexistence curve approaching the critical temperature is not observed in the temperature range studied (5 K c - T)
We have measured the normal, lateral and hydrodynamic forces that act between polymer-brush-bearing surfaces immersed in a good-solvent medium, both when stationary and when in motion with respect to each other. At low sliding velocities we find a very marked decrease in the frictional force between the surfaces, an effect interpreted in terms of a fluid interfacial layer resulting from the non-interpenetration of the opposing brush layers even when they are compressed against each other. In a different study we measured the lubrication forces between the brush-bearing surfaces, due to hydrodynamic interactions resulting from normal mutual motion. These reveal that, when the surfaces are far apart, the fluid flowing past them penetrates only very slightly into the brush layers. At high compressions, fluid is forced to flow through the brushes; the magnitude of the hydrodynamic forces may then be compared with predictions based on a model for fluid flow through porous media. This is used to discriminate between scaling and mean-field exponents of the composition-dependent hydrodynamic screening length in the semidilute polymer brushes, and shows the results to be much closer to those expected using scaling exponents.
Stabilization against the rupture and breakup of thin, nonwetting liquid films spread on surfaces is generally sought by modification of equilibrium interfacial properties. A mechanism for suppressing rupture in such films that uses surface-attached polymers together with free chains in the bulk of the film is reported. Films of an oligostyrene liquid, which rupture within several minutes when spread on a silicon wafer, may be stabilized for many months by a polystyrene brush attached to the substrate, together with some free polystyrene in the liquid. The effect may arise from entanglements of the free chains with the immobilized brush.
Prof. Klein opened the discussion: In adsorbed polymer layers the polymer segment concentration is generally in the range 0%, at the very tip of the tails, to ca. lo%, at theadsorbing surface. This spans the semi-dilute regime, for which mean-field models do not hold too well in the bulk, while scaling results do. Do you feel that scaling results foradsorbed layers are likely to be a good approximation, at least in the limit of very long chains? ...Prof. Klein asked: Would you expect annealing of your samples to result in the enrichment of shorter chains from your (relatively polydispersed) samples at the interfacial regions (where they might influence, for example, chain pull-out)? ...Prof. Klein said: Work carried out in Cambridge' investigating waves of detachment during the sliding of PDMS rubber on a solid substrate showed that such waves can occur at tangential surface stresses of more than a few KPa. Since this is the range of surface stresses in your experiments, do you feel such waves (which would be difficult toobserve due to their small amplitudes and contrast) might be playing a role in your experiments? ...Prof. Klein made a general point: It seems to me that the structure of what you call a pseudo-brush is very similar to that of an absorbed chain, and the use of that term(pseudo-brush) can lead to misunderstanding and debasement of a commonly accepted currency (i.e. a real brush). A real molecular brush consists only of tails, with noloops. What you call a pseudobrush consists only of loops, with no tails. Thus the two types of surface structures have very little to do with each other. This is not a majorpoint, but can, as we have heard from some of the questions today, lead to confusion.
We have measured simultaneously both the normal forces F⊥(D) and the shear forces F∥(D) that act between compressed polymer-bearing mica surfaces, a distance D apart, as they slide past each other. We find that for surface-attached polystyrene (PS) brushes in the good solvent toluene the shear forces are extremely weak, over a wide range of shear velocities and at normal loads that correspond to 2L ∥/F⊥) by two-three orders of magnitude. For mica surfaces bearing adsorbed PS layers in the near-θ-solvent cyclopentane, the forces required to slide the surfaces are very much larger than for the PS brushes in toluene, at similar normal loads and shear velocities. The very different behaviour in the two cases is attributed to the different extents of mutual interpenetration and entanglement of the compressed polymer layers.
THE use of lubricants to reduce friction and wear between rubbing surfaces has been documented since antiquity1-3. Recent approaches have focused on boundary lubrication by surfactant-like species coating the surfaces, whereby the friction between them is replaced by the weaker forces required for shear of adhesive contacts between the surfactant layers3,4. An alternative approach is to tether polymer chains to the surfaces by one end which, when swollen by a solvent, then act as molecular 'brushes' that may facilitate sliding. The normal forces between sliding brush-bearing surfaces have been previously investigated5,6, but the lateral forces, which are the most important from the point of view of lubrication, are harder to measure. Here we report the measurement of lateral forces in such a system. We find a striking reduction in the effective friction coefficients μb between the surfaces to below our detection limit (μb-1. We believe that this effect is due to the long-ranged repulsion, of entropic origin, between the brushes, which acts to keep the surfaces apart while maintaining a relatively fluid layer at the interface between them.
Critical point wetting generally occurs whenever a binary fluid system in the demixed region of phase space, in contact with a surface, approaches its critical temperature. For the case of polymeric mixtures the segmental interaction parameter, which determines the interfacial tension between the coexisting phases, can be very low even far from the critical point. This suggests that polymer blends should be likely candidates to exhibit complete wetting even far below the critical point. Using nuclear reaction analysis, we have now observed such complete wetting behaviour from two different classes of binary polymer mixtures. These are mixtures of statistical olefinic copolymers, with slightly different chemical microstructures; and an isotopic pair of deuterated and protonated polymers. In the former case we have been able to follow the growth with time t of the wetting layer thickness l; our results indicate l approx. log t.
Keywords: Crystallography; Polymer Science
Keywords: Chemistry, Physical; Physics, Condensed Matter; Polymer Science
A thin bilayer of two coexisting polymer phases, in contact with a surface which favors and is partially wetted by one of them, was studied by nuclear reaction analysis and phase contrast microscopy. We find that when the less-favored phase is initially in contact with this surface, it is entirely replaced by the partially wetting phase. This inversion evolves with time very differently than in the case of complete wetting. The results suggest that a microscopic surface layer enriched in the surface-preferred component may not be a stable signature of partial wetting in small binary mixtures.
1993
The surfaces of thin, liquid films can be unstable due to thinning van der Waals interactions, leading to the formation of holes in the initially uniform film. These instabilities can be greatly retarded in viscoelastic materials (and completely inhibited in elastic materials) even when the finite frequency shear modulus, E, is small compared to the infinite frequency modulus, G. This occurs when E/G much greater than (a/h0)5 much less than 1 where a is a molecular size and h0 is the film thickness. We relate the growth rate of the instability to the dynamic viscosity, eta (omega), with examples for the cases of a polymer brush, an elastic fluid (gel), and a transient polymer network, described by reptation dynamics.
A model is developed which explains a recently-observed asymmetry in the phase-equilibrium behavior of mixtures of statistical ethylene (E)/ethylethylene (EE) copolymers [formula omited], with differing microstructure and isotopic substitution levels. A coupling between the isotope and the microstructure contributions to the overall effective segmental interaction parameter χ is predicted, which could enhance or reduce χ depending on the ethylene content of the protonated component relative to that of the deuterated one. From the overall χ, the theory permits the extraction of the bare isotope interaction parameter χh/d between hydrogenated and deuterated species and of the parameter χE/EE for the E/EE interaction. This is done for the recent data set on coexistence in E/EE mixtures obtained by Eiser et al. The parameters thus estimated are compared with available experimental data on similar polyolefinic mixtures.
Using surface force measurements, we have determined the structure of polymer brushes formed onto mica, from dilute solutions of zwitterion-terminated polystyrene chains of molecular weights 26 500 and 375 000 in the good solvent toluene. We find that brushes of the longer amphiphiles, formed from a solution of these polymers alone, are rapidly replaced by shorter chains when the latter are added to the solution. This requires a certain minimum concentration of the shorter amphiphiles, but substantial replacement of long-by-short chains can occur even when the monomer concentrations of the shorter polymers is only (1/500) that of the longer ones. By analyzing the force profiles for the mixed brushes thus formed, we obtain a quantitative measure of their structural details. We evaluate these observations in terms of a simple scaling model for the free energy of brushes that are in equilibrium with free end-functionalized chains in solution.
We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a zwitterionic group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance LH from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L ≈ LH. When the layers are compressed (D
Nuclear reaction analysis (NRA) was used to study the segregation of an asymmetric diblock copolymer, consisting of polyisoprene (PI, molecular weight M = 104)/deuterated polystyrene (dPS, M = 105) blocks, to the interfaces formed by polystyrene (PS) homopolymer with various phases. PS/vacuum, PS/silicon, and PS/PI homopolymer interfaces were investigated for different M values of the PS matrix (M = 1.7 × 103−330 × 103), and segregation isotherms were established as a function of temperature and of diblock concentration within the PS homopolymer. In all cases the diblocks attach to the interfaces by their PI moieties alone, to form brushlike structures of end-attached PS tails. The high spatial resolution of the NRA technique enabled studies of the brush conformation as a function of the attachment density at the PS/vacuum surface and was used to characterize the extent of penetration of the PS matrix chains into the diblock brushes. Detailed analysis of the brush conformation and of the segregation isotherms, mainly in terms of a Flory-type mean field model based on those due to de Gennes and to Leibler, provided a consistent description of our data; it enabled the extraction of the PI/PS segmental interaction parameter χPIPS, yielding values in accord with scattering studies, and of the attachment energies of the PI diblock moiety to the PS/air and PS/silicon interfaces. The values of χPIPS extracted from our data using this Flory-type model were found to increase at lower M values of the PS matrix, in qualitative accord with previous results.
1992
We have measured, using nuclear-reaction analysis, the concentration depth profiles of polymer brushes consisting of end-tethered deuterated polystyrene tails within a polystyrene homopolymer, as a function of the surface coverage σ of tails and of the degree of polymerisation P of the polymer matrix. We find that the onset of brush swelling is shifted to higher c values at higher P as expected from theory. Within the range of our parameters the L(σ) and L(P) variations are consistent with predictions of scaling and mean-field models, where L is the effective brush thickness.
Coexisting polymer phases are characterized by very small interfacial energies, even well below their critical solution temperature. This situation should readily lead to the exclusion of one of the phases from any interface that favors the other. Such complete wetting behavior from a binary mixture of statistical olefinic copolymers is reported. By means of a self-regulating geometry, it is found that the thickness of a wetting layer of one of the phases at the polymer-air interface, growing from the other coexisting phase, attains macroscopic dimensions, increasing logarithmically with time. These results indicate that binary polymer mixtures could be attractive models for the study of wetting phenomena.
The structure of polymer chains tethered at the solid-liquid interface, and their response to shear, is briefly reviewed. Experiments to date on static structure support both scaling and self-consistent field calculations. The forces between polymer-bearing surfaces undergoing shear can yield information on the dynamics of the tethered chains, and we may expect extension of this approach in the future.
The use of optical second harmonic generation (SHG) to probe, in situ and in real time, the physical grafting of polymers from dilute toluene solution onto mica crystal surfaces is reported. Despite using polymers with no hyperpolarizable groups, and the very low surface density of end-attached species, we show how changes in the SHG signal of the substrate may be monitored during polymer grafting. Characteristic attachment rates are obtained by this method.
We have measured directly the phase coexistence characteristics in a high-molecular-weight binary isotopic polymer mixture over a range of temperatures around the critical point, using nuclear-reaction analysis. Our results reveal an equilibrium phase diagram with an upper critical solution temperature, in fair quantitative accord with the simplest form of the Flory-Huggins mean-field model. Deviations may be due to a mild compositional dependence of the monomeric interaction parameter.
We have measured the composition-distance profile across a film consisting of two thin layers (200-600 nm) of a model binary isotopic mixture of deuterated polystyrene (dPS) and protonated polystyrene (hPS), coexisting with each other near their equilibrium compositions below the critical temperature for phase demixing for this pair. Profiles were determined normal to the silicon wafer on which the bilayer is mounted using nuclear reaction analysis, both for an uncoated silicon surface and for one coated with a gold layer. Measurements reveal that when both layers are thick relative to the characteristic width w (∼100 nm) of the interfacial region between them, the coexisting compositions about the interface are close to their bulk values as determined earlier for this system. When the dimensions of the layers are made comparable with w, however, interactions with the confining surfaces may significantly modify the composition profile of the coexisting layers about the interface. This effect is marked at the polymer/silicon interface as a result of its interactions with one of the components (dPS), but is absent for a gold-coated surface in an identical geometry due to the much weaker influence of the surface. Our results are discussed in detail in terms of mean-field models of mixing in polymeric mixtures, and enable quantitative determination (using a Cahn construction approach) of the interaction parameters both at the polymer-air and polymer-silicon interfaces. Though we are not able to calculate in a completely a priori fashion the coexistence profiles as a function of the film thickness, we propose an approximate approach which provides good agreement of calculated composition profiles with those determined experimentally over the range of parameters in our experiments.
Polymers that attach at solid-liquid interfaces can considerably modify the long-ranged forces acting between surfaces. Over the past decade or so these forces have been studied directly using a surface-force-balance approach, and the equilibrium force laws between two polymer-bearing surface interacting across a liquid medium are reasonably well understood.Recently we have extended these studies to the case of surfaces with adsorbed and with grafted layers as they shear part each other. Our results suggest that the lateral forces dependon the extent of interpenetration of the opposing layers and on their relative shear velocities; they reveal a marked coupling between the normal forces and the shear velocity, beyond a certain critical value of the latter which is probably related to relaxation rates of the end-tethered chains. We find that while non-adsorbing but end-anchored polymer layers are stable with respect to shear and compression, they can be rapidly replaced by addition of shorter polymers with the same adhering end-group.
1991
The mutual mixing, and hence the dvelopment with time of the interface between two chemically different polymeric species, may be dominated by enthalpic rather than entropic considerations. Two extreme cases are described. The first is accelerated interdiffusion, that occurs when the polymers have a large mutual attraction, and results in remarkable concentration profiles across the interface. At the oxtreme is suppressed interdiffusion, where two partially miscible polymers are in contact. The interface in this case tends to an equilibrium width, that varies with time quite differently than for the case of free interdiffusion.
We describe a method for directly determining the composition profile of deuterated polymer chains in polymer mixtures. Our technique, nuclear reaction analysis, is based on the 2H(3He, 4He)1H nuclear reaction. By detecting 4He in a forward geometry, we achieve a spatial resolution of 14 nm (FWHM). We use this technique to probe the broadening of the interface between two partially miscible polymers. We found that such a system attains a finite interfacial width in equilibrium. For short times, we monitor the dynamics of interface formation. We found that the interfacial width increases significantly slower than in the case of free diffusion.
The interface between two partly miscible polymers is characterized by a finite interfacial region of width w comparable to the polymer size. The composition variation and in particular the time development of this restricted zone have recently been studied experimentally using a new depthprofiling approach based on nuclear reaction analysis. The results show w to vary quantitatively in a way consistent with meanfield models, while its time development appears to follow a power law w ∝ tα, with the mean value of the exponent α = 0.34 ± 0.06 being significantly lower than its free interdiffusion value of 0.5.
ADSORBED or grafted polymers are used to control phenomena such as colloidal stability, fluid flow and the tribological properties of surfaces. Forces between polymer-bearing surfaces have been studied comprehensively over the past decade1-10, but little is known about such forces in shear. These are intimately related to the dynamic as well as the equilibrium properties of the surface-attached chains. We have constructed a device that measures directly the forces that act between surfaces bearing polymer layers in a liquid medium as they slide past each other. For the case of mica sheets bearing end-grafted chains of polystyrene in toluene (a good solvent), we find that, as the surfaces move parallel to each other, there is a marked change in the normal forces between them. These become increasingly repulsive at higher velocities. The effect occurs only above certain critical shear rates, probably related to relaxation dynamics of the end-grafted chains themselves. Our findings have direct implications for the properties of polymeric lubricants, and for the rheological behaviour both of stabilized dispersions and of multi-phase polymeric systems.
1990
In this lecture we briefly review our understanding of surface forces with adsorbed, neutral flexible polymer chains, and describe some recent studies of surface forces with a range of end-grafted chains. In the latter case we find the polymer layers to be approximately twice the thickness of adsorbed layers of comparable size and surface-exess; we find that the interactions are monotonically repulsive and rate-independent at all surface coverages. Our results are compared with both scaling and meanfield models. and are in very good qualitative and quantitative agreement with both theoretical approaches.
In contrast to interdiffusion in simple liquids, interdiffusion of polymeric chains is dominated by their intertanglement and their large size. These properties profoundly reduce both the molecular mobilities and the role of entropy in driving the mixing. The resulting diffiisional processes have only recently been studied. Such studies reveal a wide spectrum of behavior ranging from accelerated interdiffusion (for strongly compatible chains) to its suppression below the critical point for phase separation. Effects that are still poorly understood include the initial disposition at interfaces of the chains' ends (through which diffusion proceeds by reptation) and the need for cooperative motion, which can strongly magnify local friction.
We describe a method based on nuclear reaction analysis, using the reaction 2H(3He, 4He)1H, (Q=18.352 MeV) to determine composition profiles of deuterated polymer chains in thin films. By detecting the emitted α particles (4He) at forward angles (30°) we are able to achieve a spatial resolution of 7 nm half width at half maximum (HWHM) at the deuterated sample surface, and 15 nm HWHM at a depth of some 130 nm. We use our method to probe initial diffusional broadening at the interface between deuterated and protonated polystyrene films. Our measured profiles are in close agreement with earlier measurements (over larger spatial scales) and with mean field models for the diffusional process in this system.
In the last decade there has been considerable advancement in the understanding of the role played by adsorbed polymers in modifying the forces between surfaces. This development has been both theoretical and experimental and the two have advanced largely in parallel. In this paper we report the forces observed between adsorbed layers of poly(ethylene oxide) of molecular weight 1.2 × 106 at both partial coverages of adsorbed polymer, where an initial attractive force is measured, followed by a shorter-range repulsion and at full surface coverages, where a long-range, essentially osmotic, repulsive force is noted. The attractive forces observed at low surface coverages are attributed to polymer bridging and are qualitatively similar to the predictions of both mean-field and scaling treatments. The force-distance profiles on initial interaction between the fully covered surfaces resemble those predicted and observed for dangling, end-anchored chains.
We have measured the forces between mica surfaces immersed in cyclopentane (CP) in the presence of linear (LPS) and cyclic polystyrene (CPS) chains of similar molecular weights, a few degrees above the Θ-point of the LPS/CP system: while LPS adsorbs (in accordance with earlier studies), CPS does not adsorb under the same conditions. This is likely to be due to differences in the thermodynamics induced by the different topologies of the two types of polystyrene chains.
We have measured the force-distance profiles between two curved mica sheets immersed in toluene and in xylene, both in the pure solvents and following addition of polystyrene (PS), end-functionalized polystyrene of different molecular weights M, PS-X(M), and polystyrene-poly(ethylene oxide) diblock copolymers (PS-PEO) with a short PEO block. Our results show that PS does not adsorb from the (good solvents) toluene and xylene but that once PS-X or PS-PEO are added to the solution the surface is rapidly covered, showing that the nonadsorbing PS tails are anchored at one end only. The force profiles following surface coverage are monotonically repulsive, and the range 2L0 for onset of interaction is roughly twice that of the corresponding adsorbed chains. There is no evidence of bridging attraction at low surface coverage or of finite relaxation times following strong compression, both of which are characteristic of adsorbed chains. The qualitative behavior with both PS-X and PS-PEO is very similar. For the PS- X(M), we find L0 α M0.6, and we are also able to estimate the mean interanchor spacing s and find it to increase markedly with M. These features are in accord with equilibrium expectations for a fixed anchoring energy of the polystyrene on the mica. We find that our data are well fitted quantitatively both by scaling and by mean-field models.
The author discusses the nature of the initial interactions as two polymer-bearing surfaces in a good solvent approach each other, as measured directly by the mica force apparatus. It is suggested that, while for grafted chains and for adsorbed chains at low surface coverages these initial interactions are probably well described by constrained-equilibrium models, this may not be the case for adsorbed chains at high surface coverages.
The adsorption of polymers onto a surface from a θ solvent is treated in terms of a mean- field model based on a Cahn-de Gennes approach. In particular, we calculate the forces between two such polymer bearing surfaces and compare our results in some detail with available experimental data on force profiles between mica surfaces bearing adsorbed polymer near the θ temperature. All parameters in our model may be estimated from experiment. The predicted force profiles are in very good qualitative agreement with the experimental data, though the absolute spatial and energy scales are smaller for the calculated profiles by factors of ca. 4 and 2.5 relative to experiment. The causes of this discrepancy and possible improvements to the model are considered.
We have measured the time dependence of interface formation between two polymers, below their critical temperature for phase separation, using the H2(3He,4He)1H nuclear reaction as a direct profiling technique. We find that the width of the interface initially increases with time as t, where the exponent =0.340.06 is significantly lower than its value of 0.5 for free diffusion.
1989
The interfacial composition profile bwtween two polymer phases at temperatures below the upper critical solution point has been measured directly, using an ion-beam method based on nuclear reaction analysis. The interface width grows with time to a finite limiting value. The variation of the limiting interfacial width as the critical temperature is approached from below is in quantitative accord with mean-field theories.
1988
We have directly measured the interface formation and diffusion between two partially miscible polymers, SCPE and PMMA, using Rutherford backscattering spectrometry, as the temperature was varied from the glass transition into the two-phase region. As the temperature was increased, the interfacial width for a given diffusion time increased sharply, attaining a maximum value approximately 25 °C above the lower critical solution temperature and then dropped abruptly. The results are briefly discussed within the framework of the extended Flory-Huggins theory wherein nonlocal terms are included in the free energy.
The adsorption of high polymers modifies the short-range forces between surfaces in liquid media and is widely used to stabilize or flocculate colloidal dispersions1. Previous studies have shown that low surface-coverage by the polymer leads to long-range bridging attraction between the surfaces 2, whereas repulsion sets in at high adsorbance in good solvents 3,4. In addition, marked hysteresis has been observed for both adsorbed homopolymers and co-polymers on approach and rapid separation of the surfaces3-5. Here we report the direct measurement of forces between terminally anchored, non-adsorbing polymer chains (polystyrene) in good solvents (toluene and xylene). The polymer layers are found to extend some 6R g (where Rg is the unperturbed radius of gyration of the polymer) out from each surface, substantially further out than adsorbing homopolymers. Monotonically increasing repulsive forces are observed as the surfaces approach each other and there is no evidence of bridging attraction at low-surface coverage. In addition we find that the force law is reproducible and independent of compression/decompression rates, in marked contrast with adsorbed homopolymers.
We have used infrared microdensitometry to study the mutual diffusion across an originally sharp interface between two saturated polybutadienes of degrees of polymerization NA and NB(»Ne, the entanglement length) for three different ratios NA/NB. The extent ofdiffusion broadening suggests that the mutual diffusion coefficient is controlled by the mobility of the faster moving (shorter) polymer chains, indicating that convective flow may beimportant insuch mixing phenomena.
1987
The force as a function of the surface separation, D, between two mica surfaces bearing adsorbed layers of nonionic surfactant Triton X-405 was measured in 0.1 M KNO3. The force commenced at D {reversed tilde equals} 30 nm and was monotonically repulsive at all surface separations D
Interactions between mica surfaces immersed in 10-4-10-1 mol dm-3 aqueous KNO3 have been measured: in contrast to earlier studies in this system, at the higher concentration range (> 4 × 10-2mol dm-3) the potential of the diffuse double-layer drops to values ψd-3 KNO3 only attraction is observed down to the range of 'hydration forces' (ca. 2 nm). The discrepancy with the earlier results is attributed to adsorption on the mica of surfactant originating in the Millipore filter used in our previous studies.
1986
Surface interactions, as revealed by surface force studies between mica sheets in 0.1-0.2 M KNO3 aqueous electrolyte, are critically examined for three different types of macromolecules adsorbed on the surfaces: flexible uncharged polymer, flexible charged polyelectrolyte, and rigid rod protein. Aspects of specific and nonspecific effects, such as surface conformation, the origin of repulsive interactions and the possibility of attractive bridging, as well as the question of equilibrium and reversibility for such adsorbed macromolecular layers, are compared and contrasted.
Self-diffusion in polymer melts has been extensively studied and is reasonably well understood in terms of the reptation model1-4; the related phenomenon of mutual diffusion in miscible blends of chemically different polymers has received little attention, despite its practical relevance and implications for physical processes, such as phase separation kinetics. In such blends, attractive interactions between the monomers, when summed over a polymer chain, may lead to large enthalpic driving forces favouring the mixing; this in turn results in a mutual diffusion rate which is rapid compared with the entropically driven self-diffusion5, and which is strongly composition-dependent6. We have measured mutual diffusion as a function of composition in one such binary blend, polyvinyl chloride (PVC)/polycaprolactone (PCL), and report here that the mutual diffusion coefficient is strongly enhanced in the middle of the composition range. This result is qualitatively, though not quantitatively, in accord with the results of some recent theoretical treatments5-8.
The variation of the effective adsorbed layer thickness δeffwith molecular weight M for a polymer adsorbed at a mica-solvent interface under good solvent conditions, as revealed by force measurements, is analyzed. The observed variation, δeff∝M0-43, is shown to be consistent with a scaling form for the extension of the polymer from the surface. The corresponding effective thickness of adsorbed layers in poor solvents is briefly considered.
We evaluate the translational diffusion coefficients and longest relaxation times for entangled linear n-mers, for f-arm star branched polymers (nb-mers/arm), and for cyclic (closed ring) polymers (nR-mers) in a fixed entanglement network and also in a melt of entangled linear p-mers. \u201cTube-renewal\u201d effects for the latter case are reexamined, taking into account both hydrodynamic interactions and especially the interdependence of p-mer constraints about a diffusing n-mer; the characteristic tube-renewal time in this case becomes τtube~ n2p5/2, in contrast with earlier proposals. At very high n values one expects an unscreened form for the tube-renewal time, τtube~ n3/2p3, equivalent to the \u201chydrodynamic sphere\u201d diffusion of the n-mer in the p-mer melt. The diffusion coefficients and longest relaxation times for stars in a fixed entanglement lattice are calculated with a diffusion equation approach as Ds ~ nb-1 exp(-αnb/nc) and τ8 ~ nbexp(αnb/nc), with a a constant and nc the entanglement degree of polymerization, a form similar (in the dominant exponential term) to earlier calculations. For rings the corresponding expressions are DR~ exp(-βnR/nc) and rR~ exp(βnR/nc), where β ≳ α, though these are limiting forms valid for high nR(≳20nc); at lower nRvalues ring polymers may exhibit a quasi-linear behavior. When incorporated in melts of linear p-mers, the diffusion and relaxation of both stars and rings is dominatedfor nband nR greater than some crossover value of order 10ncby tube-renewal effects mediated by reptation of the linear melt matrix.
Analytical expressions, based on a mean-field model and derived in a previous paper (part 1: Klein, J.; Pincus, P. A. Macromolecules 1982,15,1129), for the interaction between two parallel plates bearing adsorbed polymer in poor-solvent medium, are solved numerically to yield (a) segmental density profiles of the adsorbed polymer and (b) interaction-energy vs. plate-separation profiles. All parameters appearing in these calculations are obtainable from bulk data such as the polymer-solvent phase diagram and adsorbance measurements. The calculated results are critically compared with model experiments on forces between smooth mica surfaces bearing polystyrene in cyclohexane in poor-solvent conditions: the agreement is qualitatively very good and quantitatively fair. Some improvements to the model are suggested.
1985
The forces F(D) between smooth mica surfaces immersed in cyclopentane have been measured as a function of their separation D both in the absence and in the presence of polystyrene (of two molar masses, 6 × 105 and 2 × 106) adsorbed onto the mica from the solvent, at a temperature (23 ± 1°C) slightly above the θ-temperature for this system (19.6°C). The interactions between the bare mica surfaces in the cyclopentane were short-ranged (≲10 nm) and attractive. After addition of polymer to the solvent the surfaces were incubated in the solution at small surface separation to limit the rate of polymer adsorption, and interactions were measured following progressively longer incubation periods. The following trends were noted for both molar masses: (i) Following initial adsorption of polymer, a long-ranged attraction between the surfaces was followed by an ultimate repulsion as the surfaces were compressed close together. (ii) At longer incubation times in the solution, the magnitude of the attraction decreased while the range of interaction and the ultimate repulsive regime moved to larger D values. (iii) After overnight incubation at large surface separation the interaction became monotonically repulsive, starting at a surface separation D ∼- 1.7Rg (unperturbed radius of gyration) for both molar masses used. This behavior did not change on further incubation. (iv) When the solution was replaced by pure solvent after overnight incubation, the monotonic repulsion changed to an initial weak attraction followed by an ultimate repulsion, very similar to the behavior following partial adsorption. Overall, the results indicate that attraction between the polymer covered mica surfaces is strongly correlated with extent of polymer adsorption, and disappears following full adsorption. The origin of the attraction is probably due to "bridging" effects, while the ultimate repulsion most likely occurs because of the dominance of osmotic interactions.
Direct measurements of the interaction forces F(D) between two atomically smooth solid (mica) surfaces immersed in toluene have been carried out as a function of surface separation D; interaction profiles were also measured following introduction of (i) poly(ethylene oxide) (PEO) and (ii) polystyrene (PS) into the toluene (at low concentrations) and incubating the surfaces overnight in the solutions. Toluene is a good solvent for both polymers. In the pure toluene the interactions were short-ranged (≤10 nm) and attractive. Following full overnight adsorbance of PEO a quasi-equilibrium forcedistance law was indicated on compression/decompression of the surfaces, with repulsion commencing at D ⋍ (8.5 ± l)Rg(unperturbed radii of gyration for the three molar mass polymers studied) and increasing monotonically at lower D. Rapid compression/decompression resulted in general in lower F(D) values for a given D. This behavior qualitatively resembles that between adsorbed PEO layers in aqueous 0.1 M KNO3electrolyte, as reported earlier. In the PS/toluene solution, no adsorbance of polymer was indicated even after overnight incubation of the surfaces: the forcedistance profiles were unchanged (within error) relative to those in the polymer-free solvent. A consideration of the magnitude of \u201cdepletion layer\u201d forces suggests these (if any) would be undetectable at the polymer concentrations (104 (w/w)) used in our experiments.
1984
We have measured the forces F(D) between two smooth curved mica surfaces a distance D apart immersed in 0.1 M KNO3, both in the absence and in the presence of adsorbed layers of monodispersed polyethylene oxide), of two molar masses M (40000 and 160000). Interactions between the bare surfaces are characteristic of electrostatic double-layer overlap, and strongly repulsive for D ≲ 8 nm. Following adsorption of the polymer, the main features of the interactions are as follows: (i) On a first approach of the surfaces following adsorption and on subsequent compression/decompression cycles that are sufficiently slow to allow relaxation of the polymer on the surfaces (≳1 h), a quasi-equilibrium F(D) profile is indicated. The onset of repulsive interactions occurs at D ≃ 6 ± 1Rg (the respective radii of gyration for the two molar masses), and F(D) is increasingly repulsive at lower D values, (ii) More rapid (~5 min) compression/decompression cycles reveal time-dependent effects. The general observation in these cases is that F(D) is lower for any given D than the corresponding quasi-equilibrium value, (iii) The adsorbance of polymer is 4 ± 1.5 mg m-2, and adsorption is irreversible over the times of our experiments, (iv) There is no evidence for attraction or adhesion between the surfaces once the equilibrium adsorption has been attained. Our results contrast with an earlier report for a similar system but utilizing a commercial-grade polymer (Israelachvili et al. J. Colloid Interface Sci., 78,432 (1980)), in particular as regards our observations of an equilibrium force-distance profile.
Forces between polystyrene layers absorbed on mica and immersed in cyclohexane have been measured. The measurements were made on two different molecular weights (6 X 105 and 9 X 105) in two different laboratories and therefore support the quantitative reliability of the results. We have reproduced previous results of this type of measurement below the θ temperature and extended the force measurement to (a) adsorbed layers at lower surface coverage and (b) temperatures above the bulk polystyrene-cyclohexane θ temperature. At a coverage of about 1.1 mg/m2 of polystyrene, which is on the order of 20-30% of saturation, we found strongly attractive forces below Tθ, detectable at separations of about 300 Å between the bare mica surfaces. The forces can be measured very accurately and precisely in this situation. The force reaches a minimum at 46 ± 2 Å and becomes strongly repulsive closer in. The long-range attractive portion of the force curve is very nearly exactly exponential, with a decay length of 45 ± 2 Å. For saturated surfaces with about 4.5 ± 1 mg/m2 the force is detectably attractive at larger distances (600-1200 Å) both below (23 and 26 °C) and above (37 ± 2 °C) the temperature (34.5 °C). For each molecular weight the positions of the minimum in the F(D) curve and of the short-range repulsive barrier are at smaller separations at T > Tθ. The magnitudes of the minima are smaller at T > Tθ as well. Both of these new results suggest strongly that the forces, especially the attractive components, between the polymer surfaces are influenced by effects in addition to the usual segment-segment interactions which determine bulk thermodynamic properties.
The forces acting between atomically smooth mica surfaces immersed in both organic and aqueous liquid media have been determined in the range of surface separations 0 - 300 nm, both in the absence and the presence of adsorbed polymer layers. In this way the interactions between the adsorbed macromolecular layers themselves were determined. We present results for the following cases: i) Poor solvent, ii) θ - solvent, iii) good solvent, iv) polyelectrolytes in aqueous media. Our results show that interactions between the adsorbed layers may be either attractive or repulsive, depending on the nature of the solvent and the extent of adsorption. For the polyelectrolyte case the interactions are a combination of field-type (electrostatic) and steric forces.
We have measured the forces F as a function of surface separation D acting between two smooth curve mica sheets, immersed in aqueous electrolyte medium (potassium nitrate), both in the absence and in the presence of polyelectrolyte (poly-L-lysine, M = 90 000) adsorbed onto the surfaces from solution. Forces between the bare surfaces are typical of double-layer electrostatic interactions and in reasonable accord with DLVO theory. Following adsorption of the polyelectrolyte we find a monotonically increasing, long-range repulsion on a first approach (or compression) of the surfaces, commencing at D ≈ 120 nm. On decompression, as well as on subsequent compression/decompression cycles, we find the long-range repulsion has irreversibly disappeared, to be replaced by a reversible much shorter-range repulsion (D ≲ 20 and 70 nm, for 10-1 and 10-3 mol dm-3 KNO3 electrolyte, respectively). An analysis of the F against D profiles suggests that a combination of DLVO-type electrostatic repulsion and steric factors is primarily responsible for the observed interactions. The adsorbance of poly-L-lysine onto the mica surfaces has also been determined, via refractive-index measurements, and is 2 ± 0.5 mg m-2 of mica surface.
High polymers are widely used as flocculants in aqueous colloidal dispersions1,2. All direct investigations to date of the forces between surfaces bearing adsorbed polymers in good solvent media, however, have indicated a monotonically increasing repulsion, rather than attraction, as the surfaces approached3-8. We have now measured the interactions between two smooth mica surfaces immersed in an aqueous solution of polyethylene oxide (a good solvent system) in the range 0-300 nm apart, and find that at low adsorbance of the polymer on the mica there is a reversible, time-independent, long-range (∼2.5 Rg, the unperturbed radius of gyration of the polymer) attraction as the surfaces approach. On permitting equilibrium adsorption of the polymer to take place, the attraction disappears, to be replaced by monotonically increasing, long-range repulsion.
The forces acting between two atomically smooth mica surfaces immersed in aqueous electrolyte have been measured, in the range of surface separations D, 0 ∼ 1000 nm, both in the absence and in the presence of macromolecules adsorbed onto the surfaces from solution. In this way the interactions between the adsorbed macromolecular layers themselves were determined. Interactions between the bare mica surfaces were short-range and in good accord with DLVO (electrostatic-double layer) theory for both electrolyte concentrations (10-1 and 10-3 M KNO3). Two macromolecular systems were used as adsorbing species; (i) soluble calf-skin collagen and (ii) synthetic poly-L-lysine (MW = 77,000). The forces between adsorbed layers of collagen were monotonically repulsive, with interactions starting at D ∼ 650 nm. This indicates an adsorbed layer extending some 300 nm from each surface, and suggests the rod-like collagen monomers (of length 300 nm and diameter 1.5 nm) are adsorbed end-on normal to the mica substrate. This finding is in accord with recent independent viscometric and radiometric studies of monomeric collagen adsorbed onto glass substrates. For the case of the adsorbed polylysine layers, the forces are again monotonically repulsive, and onset of interactions occurs at D ∼ 120-140 nm. As the surfaces approach, irreversible changes in the surface structure of the adsorbed polyelectrolyte take place and the final quasi-equilibrium adsorption configuration is that of a dense, flat surface layer. A close analysis of the force-distance profiles reveals that the interaction is a combination of both reversible, repulsive field-type (DLVO) forces, and a repulsive steric component which is largely irreversible.
1983
Diffusion of endogenous hyaluronic acid and 125I-labelled albumin, monitored by desorption from umbilical cord (Wharton's jelly) slices, was studied in relation to tissue structure. Diffusion of hyaluronic acid was Fickian and some two orders of magnitude slower than that in free solution. After treatment of tissue with trypsin which removes proteoglycan(s) and degrades glycoprotein microfibrils, hyaluronic acid mobility through the collagen fibril network that remains is increased by an order of magnitude. These findings indicate that the mobility of hyaluronic acid in tissue is reduced both by the collagen network and by the presence of proteoglycan(s) and/or microfibrils. Estimates of the reduction in mobility due to physical entanglements with the fibrillar networks show that these play a major role The mobility of hyaluronic acid found for intact tissue is sufficient for it to permeate the extracellular space within its metabolic turnover. time. Labelled albumin diffusion is intact tissue, on the other hand, is reduced by only some 30% relative to free solution. This is consistent with the approximate 10% reduction found for the polysaccharide-free tissue (given by the excluded volume fraction) and the approximate 20% reduction expected for the polysaccharides in the interstitial fluid. Similar effects appear to be involved in the mobility of endogenous diffusible proteins in tissue.
The dynamic properties of concentrated solutions and melts of linear polymers are reasonably well understood in terms of the reptation concept, whereby molecules are constrained to curvilinear motion within 'tubes' formed by entanglements with their neighbours1-3. Understanding of dynamics of entangled nonlinear polymers, however, is still rather limited. Reptation in such cases is expected to be strongly suppressed, but there is little direct evidence for this, and conflicting models have been proposed4-6. We report here a critical study of the diffusion coefficient D of a series of model linear and three-arm star-branched polymers, diffusing in an entangled linear polymer melt matrix, designed to elucidate dynamic mechanisms for the entangled stars. Measuring D as a function of diffusant degree of polymerization N we find, for the linear molecules, D ∝ N-2, as expected for purely reptative motion1,7-9. For the case of moderately short stars, however, D falls considerably faster than an inverse square power law, and is well fitted by an exponential type relation D ∝ e-αN. There is evidence that for the longest stars used in the present study it is the release of entanglements by reptation of the linear matrix molecules which dominates the dynamics. This is the first direct indication of such 'tube-renewal' effects in a system of entangled polymers8.
The diffusion and longest-relaxation process of star molecules entangled in a fixed-obstacle matrix and in melts of a linear polymer are considered. The dynamics in a fixed matrix are considered in terms of a general diffusive motion in an 'entropic' potential field. The resulting diffusion coefficient Ds and longest relaxation time τs are calculated, and scale as Ds ∝ (1/Nb) exp (-αNb), τs ∝ Nb exp (αNb) (for a 3-arm star with Nb monomers per arm, where α is a constant). For the case of a linear melt matrix it is argued that 'tube-renewal' effects will dominate the dynamic behaviour above a certain Nb. We report the first experimental study of the diffusion coefficient D(N) of 3-arm deuterated polybutadiene N-mer stars diffusing in a highly entangled melt of linear polyethylene. Our results provide strong support for the calculated form of the diffusion coefficient, at low values of N, and suggest that at high N values 'tube' renewal effects become important.
The forces between two molecularly smooth curved mica surfaces a distance D apart immersed in a liquid have been measured, both in the absence and in the presence of macromolecules adsorbed from the liquid. The forces between bare mica surfaces in 0.2 mol dm-3 KNO3 aqueous solution are repulsive for D ≲ 5 nm and correspond reasonably well to electrostatic double-layer (DLVO) theory. Following adsorption of soluble monomeric collagen onto the surfaces from the solution, repulsion commences at D ≲ 600-700 nm and increases monotonically with decreasing D, suggesting that the collagen monomers are adsorbed in an upright configuration normal to each surface. The forces between bare mica surfaces in cyclohexane are monotonically attractive for D ≲ 10 nm, and appear to obey a van der Waals-like potential acting across the non-polar medium. Following adsorption of polystyrene (of two molecular weights) onto the mica from the cyclohexane, interactions between the surfaces are found to commence at D ≲ 2.5 Rg (unperturbed radius of gyration of the respective polymers) and are initially attractive. For D ≲ Rg the attraction decreases, and on further reducing D strong repulsion between the surfaces is observed. These results may be understood in terms of the phase equilibrium of the polystyrene-cyclohexane system (which under the experimental conditions is below its critical temperature), and are in good accord with the predictions of a recent theory for interaction between adsorbed polymer layers in a poor solvent.
1982
The forces between mica surfaces a distance D apart immersed in cyclohexane, bearing adsorbed layers of polystyrene, have been measured at temperatures lower than the corresponding critical temperatures. Polymers of two different molecular weights were used. The results show that these forces are attractive for 3Rg > D >. Rg (where Rg is the polymer radius of gyration) and repulsive at lower D, and may be understood in terms of the phase equilibrium of the polystyrene-cyclohexane system. The adsorption of the polymer under these conditions appears to be effectively irreversible over the time of our experiments. The adsorbance of polymer and the thickness of the adsorbed layers are comparable with values measured in other studies using entirely different techniques.
Recent measurements of the forces acting between two polystyrene layers adsorbed onto mica surfaces, immersed in a poor solvent for the polymer, show strong initial attraction as the surfaces approach, followed by ultimate repulsion1. This has been attributed to attractive osmotic interactions between the adsorbed layers, and possibly to the effect of bridging2,3. We have extended these measurements to the case of polyethylene oxide (PEO) layers adsorbed on mica, immersed in an aqueous 0.1 M KNO3 medium at pH 6 (a good solvent for PEO). Using monodispersed polymer of two molecular weights, we find that an equilibrium force-distance profile is indicated. As the surfaces bearing the adsorbed PEO approach, repulsive forces commence at a surface separation D ≃ 6±1R s (unperturbed radius of gyration of the respective polymers) and increase monotonically on approach; on subsequent separation the forces decrease monotonically to zero. We find no evidence for attraction or adhesion between the adsorbed layers in this system.
1981
The diffusion coefficient D(M. Mm) of deuterated polyethylene fractions of M.W. = M in polyethylene melts of M.W. = M m, has been measured as a function of M m. The results show D(M, M m) to be essentially independent of M m for M m > 23Mc, M c being the critical M.W. for onset of entangled behaviour. This result supports the suggestion that in entangled melts molecules diffuse through effectively fixed surroundings.
Two methods are applied to determine the thickness of an adsorbed soluble collagen layer on glass. One involves time of flow measurements in a glass capillary. Effects due to changes in contact angle and drainage, as the meniscus advances down the instrument, are eliminated. The results are compared with a direct force/distance curve determination for soluble collagen adsorbed to mica. At low pH layer thicknesses in excess of 325 nm are found, indicating that the collagen triple helices are standing end up on the surface. The amount of collagen adsorbed is only about 1 mg/m2. It can, therefore, be calculated that some 12 nm separate the soluble collagen monomers. The surface layer is very diffuse. Raising the pH reduces layer thickness. Surface heat denaturation gives 70 nm thick layers.
1980
Direct measurements of the forces acting between polymer surface phases adsorbed at interfaces have only recently been reported1-3. These were carried out in good solvents and showed that repulsive forces were acting between the adsorbed layers; these forces increased monotonically as the surfaces approached each other. In certain conditions, however, for example, those leading to the flocculation of sterically stabilized colloidal systems, one expects the surfaces bearing the adsorbed phases to attract before repelling each other4,5. I have measured the forces acting between two curved mica surfaces immersed in cyclohexane at 24 °C (a worse than θ solvent at this temperature), each bearing a surface layer of adsorbed polystyrene (Mw = 6 × 105). No forces are observed at surface separations larger than about three radii of gyration of the polymer; on closer approach a strong attraction develops between the surfaces, changing to an ultimate repulsion as the surfaces approach closer than about one radius of gyration. Between times of a few minutes and several hours the forces are stable, well behaved and reproducible.
1979
The rates at which molecular sequences may deposit onto growing lamellae in a crystallising polyethylene melt are examined on the basis of the results of recent self-diffusion experiments in such melts, in conjunction with theories of molecular dynamics in polymer melts based on the reptation picture. The results are compared with rates of sequence deposition in polyethylene melts as estimated from experimentally measured growth rates. This comparison indicates that, purely from a kinetic point of view, some regular folding of molecular trajectories at lamellar surfaces is not incompatible with experiment.
1978
It is proposed that the onset of entangled behavior with increasing N (degree of polymerization) in polymer solutions of concentration c>c* (overlap concentration) corresponds to the point where molecular diffusion becomes restricted to reptation alone. An internally self-consistent model for reptation in such solutions, based on a reptation and \u201ctube\u201d-reorganization concept, is developed. By treating the relaxation of molecules as a cooperative phenomenon the onset of reptation is identified as a second-order, ferromagnetic-like transition occurring at a critical N and entanglement density. It is possible to compare some predictions of our treatment with experiments on the variation of Nc (critical N for onset of entangled behavior) with c, by incorporating results of previous studies on static-correlation properties in semidilute polymer solutions. The agreement is surprisingly good.
ENTANGLEMENTS, their nature and their role in the dynamic properties of concentrated polymer solutions and melts are not well understood1,2. The classical molecular view of entanglements has been one of rope-like intermolecular couplings at a number of points along the length of a molecule; molecules in motion would drag past these couplings, the essential effect being one of enhanced friction1,3. There has been a growing realisation that this model is inadequate2,4,5. The essence of the problem, rather, seems to be that of the topological restrictions imposed on the motion of each polymer molecule by its neighbours: movement of a given polymer chain is constrained at the points of entanglement or intersection with adjacent chains2. Theoretical treatment of the topological problem is difficult6, and has met only with limited success5. An interesting proposal regarding the motion of molecules within entangled polymer systems has been put forward by De Gennes4,7: according to this, the motion of a given polymer molecule is confined within a virtual 'tube' defined by the locus of its intersections (or points of 'entanglement') with adjacent molecules (Fig. 1). The molecule is constrained to wriggle, snake-like, along its own length, by curvilinear propagation of length defects such as kinks or 'twists'8 along the tube; this mode of motion has been termed reptation4 (from reptile). Reptative motion clearly satisfies the central requirement of entangled systems: that of the non-crossability by a given chain of the contours of its adjacent neighbours. In a real polymer melt the topological environment of any given molecule (that is, the virtual 'tube' surrounding it) will itself change with time. This is because the adjacent molecules defining it are themselves mobile. If this reorganisation is sufficiently slow then the translational motion of the enclosed molecule will be effectively curvilinear (reptative). Consideration of the problem9 suggests that this will be the case in an entangled system. One then expects translational diffusion to be dominated by reptation. There is no direct experimental evidence supporting the physical reality of curvilinear motion in entangled polymer systems. I report here the results of experiments on diffusion within a polyethylene melt critically designed to test the reptation concept.
1977
In the reported experiments, the diffusion coefficient for two diffusants of the type n-(CH//2)//NX was measured, where N congruent 25, 45 and X is a suitable label; the matrix was linear polyethylene cooled at widely differing rates from the melt. The measuring technique used is a recently developed one based on infrared microdensitometry. The results indicate that these diffusants diffuse faster in the slowly cooled matrix, in marked contrast to the behavior of gaseous diffusants. They are in agreement with the predictions of the model developed in a previous paper (part I), in which the constraints imposed by interlamellar tie molecules on long diffusants were shown to be important.
It is proposed that the mobility of sufficiently long molecules undergoing translational diffusion through solid polyethylene is significantly reduced by tie molecules and fixed entanglements within the interlamellar regions of the matrix. The effect on the diffusion coefficient both of diffusant molecular length and of the host matrix morphology is considered, and a semiquantitative relation between them obtained. This indicates that the mobility of long diffusants is higher in polyethylene which has been cooled slowly from the melt than it is in quenched polyethylene, in marked contrast to the behavior of gaseous and other small diffusants.