Publications

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.

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.

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.

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.

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 years 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 3 orders of magnitude. Here we measure the viscoelectric effect in water directly using a sur-face force balance (SFB), 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 find a value for this mag-nitude which differs respectively by 1 and by 2 orders of magnitude from its highest and lowest previ-ously estimated values.

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.

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 MW were studied using a surface force balance (SFB), and atomic force microscopy (AFM) . Gelatin (gel)-covered mica surfaces were coated with either 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 head-groups exposed by the HA-attached lipids, with friction coefficients (COF) as low as l0-3–10-4 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 is the HHA-HSPC complex which is strongly adhered to the 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.

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.

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.

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.

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

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.

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 v_s. While for the linear brushes friction is only very weakly v_s-dependent (over 3 orders of magnitude in v_s), due to a self-regulating brush interpenetration, for the cross-linked brushes friction increases markedly with v_s (∼v_s^1/2), an effect attributed to suppression of interpenetration by the cross-links.

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.

Extending earlier studies where phosphocholinated brushes and phosphocholine‐exposing phosphatidylcholine lipid bilayer vesicles (PC‐liposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highly‐hydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pc‐PS‐NPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PS‐NP, 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 > 9 atm) the pc‐PS‐NPs 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 PC‐liposomes (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 PS‐NP surfaces. In particular, at higher compressions, sliding results in energy‐dissipating 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 pc‐PS‐NPs.

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.

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 μ_Li+ > μ_Na+ μ_K+. 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 μ_M+ 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.

Jee et al. assert (1) that the effect we measure—what happens to a lubricant film thickness in individual slip events during intermittent stick–slip 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 stick–slip friction, with its characteristic saw-tooth pattern, not shown or measured, but their inability to measure what happens during the individual, fleeting slip events—the key finding of our own study (2)—is explicitly stated in the papers themselves (below).

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.

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.

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.

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.

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.

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, F_n) and shear (frictional, F_s*) 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. F_n 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. F_s* was essentially proportional to F_n 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.

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 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.

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 (F_n) and friction forces (F_s*) between statherin-coated mica substrata. Readings were taken both in the presence of statherin solution (HP and HB mica) and after rinsing (HP mica). F_n 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 F_n observations, were markedly different on the two different substrata: friction coefficients, μ ≡ ∂ F_s*/∂F_n, 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.

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.

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

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 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)

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.

We use a surface force balance to study shear interactions between adsorbed poly(ethylene oxide) layers (M_w = 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 σ_s 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).

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.

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.

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.

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.

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.

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.

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 μ_eff = 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.

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.

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.

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 (M_w = 29.9 kg/mol) end-functionalized with a zwitterionic group were deposited onto freshly cleaved mica (areal density ≈ 1 chain/(170 Ų)), 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.

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 approaches^1-6. 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 bulk^7-9; this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers^8,10,11. 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 layers^10-15. 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 molecules^11,15-18; however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing^1,3.

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 F_n(D) as a function of surface separation D and, with extreme sensitivity, shear or frictional forces F_s(D,ν_s) between them as they slide past each other at velocity ν_s. The F_n(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 μ_eff = (F_s/F_n) 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 F_s increases markedly, and two forms of behavior are found depending on the PEO molecular weight. For PEO37, a sharp increase in F_s 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 F_s and in μ_eff saturates at the highest loads (for the case of μ_eff even decreasing), indicating that the slip plane has moved from the polymer/polymer midplane to the polymer/ solid interface. The dependence of F_s 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 ν_s 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 F_s(ν_s) dependence is characteristic of sliding friction at solid substrates.

We have confirmed the existence of the BeC62- dianion produced in a Cs-sputter negative ion source with a cathode of mixed Be metal and graphite powder. Using our laser-accelerator mass spectrometry (AMS) system, we have studied the photoelectric interaction of the dianion and show that it is bound by more than 1.165 eV relative to any photodetachment or dissociation channel. The total cross-section leading to either of the channels for 2.33 eV photons is 6 Mb. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

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.

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 φ_bulk. 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 φ_bulk bulk > 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.

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.

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.

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.

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.

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.

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.

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 T_c, 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 f_s is feasible, but requires an unexpected temperature dependence of f_s. 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 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 E_1-x/EE_x; 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.

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∝t^k, where k is the range 0.2–0.3. 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.

Interfacial adsorption from a binary mixture of short and long polystyrene—polyisoprene 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 1,4‐olefinic 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(N_e/N²) for each copolymer, where N is the backbone length and N_e 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 − T_g), where T_g 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.

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 surface‐attached 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 end‐tethered chains in a good solvent than for adsorbed chains in a θ‐solvent. 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 polyisoprene-polystyrene (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 self-diffusion coefficient of PS of a similar length. The equilibrium surface excess Gamma of the diblock is low (

The diffusion process in dPS (M_w = 1.06 × 10⁶ g mol^-1)/hPS (M_w = 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 (∂μ_dPS/∂Φ_dPS)_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)

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.

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: Chemistry, Physical; Physics, Condensed Matter; Polymer Science

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 L_H from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L ≈ L_H. When the layers are compressed (D

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.

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 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 depth‐profiling approach based on nuclear reaction analysis. The results show w to vary quantitatively in a way consistent with mean‐field models, while its time development appears to follow a power law w ∝ t^α, with the mean value of the exponent α = 0.34 ± 0.06 being significantly lower than its free interdiffusion value of 0.5.

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.

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 M^0.6, 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.

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 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 surfaces^3-5. 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 R_g 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.

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.

We evaluate the translational diffusion coefficients and longest relaxation times for entangled linear n-mers, for f-arm star branched polymers (n_b-mers/arm), and for cyclic (closed ring) polymers (n_R-mers) in a fixed entanglement network and also in a melt of entangled linear p-mers. “Tube-renewal” 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 τ_tube~ n²p^5/2, in contrast with earlier proposals. At very high n values one expects an unscreened form for the tube-renewal time, τ_tube~ n^3/2p³, equivalent to the “hydrodynamic sphere” 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 D_s ~ n_b^-1 exp(-αn_b/n_c) and τ₈ ~ n_bexp(αn_b/n_c), with a a constant and n_c the entanglement degree of polymerization, a form similar (in the dominant exponential term) to earlier calculations. For rings the corresponding expressions are D_R~ exp(-βn_R/n_c) and r_R~ exp(βn_R/n_c), where β ≳ α, though these are limiting forms valid for high n_R(≳20n_c); at lower n_Rvalues 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 dominated—for n_band n_R greater than some crossover value of order 10n_c—by tube-renewal effects mediated by reptation of the linear melt matrix.

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 T_θ, 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 > T_θ. The magnitudes of the minima are smaller at T > T_θ 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.

High polymers are widely used as flocculants in aqueous colloidal dispersions^1,2. 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 approached^3-8. 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 R_g, 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 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 neighbours^1-3. 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 proposed^4-6. 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 motion^1,7-9. 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.