Publications
2016
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(2016) Composites Science and Technology. 137, p. 35-43 Abstract
Multilevel hierarchical structures built up from nanoscale to macroscale are common in nature, but their potential has not been achieved by man-made composites. The presented multilevel structure consists of carbon nanotube fibers (CNTFs) embedded in epoxy matrix. This structure exploits the supreme mechanical properties of individual CNTs together with the manageability of the microscale CNTFs, and has the potential to overcome the implementation difficulties associated with nanocomposites. Using different chemical treatments (ethylene glycol or nitric acid solvents), the CNTFs are densified and the amount of epoxy penetration inside the CNTFs is controlled, creating an interphase between the single CNTs. The strength and adhesion properties of individual CNTFs in epoxy are measured by continuously monitored fragmentation tests and characterized by electron microscopy. A modified Cottrell-Kelly-Tyson model is applied to account for the CNTF unique cross-sectional geometry, comprising millions of individual multiwalled CNTs, and for the effect of matrix penetration. The composite strength and toughness are found to be strongly dependent on and improved by the extent of penetration, suggesting that the composite mechanical properties would be tunable by controlling the interphase. The presented integrative analysis shows that CNTF based composites are an excellent potential choice for strong and tough structures, as well as for bio-engineering. (C) 2016 Elsevier Ltd. All rights reserved.
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(2016) Applied Clay Science. 132-133, p. 105-113 Abstract
A simple method for preparation of hybrid of graphene oxide (GO) and Laponite (Lap), obtained by solvent evaporation from their highly stable aqueous dispersions is reported. The dispersion up to ~ 1 mg/ml of GO in 1% Lap dispersion, i.e., 10:1 of Lap:GO was found to be stable without flocculation for several months; lower mass ratios of Lap to GO than this showed marginal flocculation with time. The electrostatic interaction between cations present in the interlayers of Lap and the functional groups of GO is envisaged to be the cause for the stable dispersion, which was confirmed by the presence of cations; viz., Na+ and small amounts of K+ and Mg2 + in the aqueous filtrate of the hybrid. Their interaction was further confirmed by higher absorption of GO in aqueous Lap dispersion than that in water using UVvis spectroscopy. The resulting hybrid material was found to be partially reduced and self-assembled to form layered structure in its dry state. The hybrids further showed improved electrical conductivity (~ 0.01 S/cm) upon chemical reduction. The present study demonstrates a facile method for preparation of a new hybrid material and greener pathway for GO reduction; though partially. This hybrid has potential as multifunctional filler for clay polymer nanocomposites.
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(2016) Nanocomposites. 2, 4, p. 162-168 Abstract
The development of polymeric coatings for brittle substrates such as ceramics or glass is an important technological goal, the aim of which is to provide protection or enhance the mechanical properties of the substrate. In this work we propose a novel one-step approach, evaporation-driven self-assembly of polymers, to coat Al2O3 substrates with a thin nanocomposite coating. The films consist of polyvinyl butyrate polymer and either multi-walled carbon or tungsten disulfide nanotubes, which are found to improve both the strength and toughness of the brittle alumina substrates via crack bridging and crack propagation inhibition mechanisms.
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(2016) Journal Of Polymer Science Part B-Polymer Physics. 54, 22, p. 2377-2387 Abstract
Functional fillers in multilayered films provide opportunity in tailoring the mechanical properties through chemical cross-linking. In this study, Laponite-graphene oxide co-dispersion was used to incorporate graphene oxide (GO) easily into polyvinyl alcohol (PVA)/Laponite layer-by-layer (LBL) films. The LBL films were found to be uniform and the layer thickness increased linearly with number of depositions. The process was extended to a large number of depositions to investigate the macroscopic mechanical properties of the freestanding films. The LBL films showed remarkable improvements in mechanical properties as compared to neat PVA film. The GO-incorporated LBL films displayed higher enhancements in the tensile strength, ductility, and toughness as compared to that of PVA/Laponite LBL films, upon chemical cross-linking. This suggests the advantageous effects of GO incorporation. Interestingly, cross-linking of LBL films for longer time period (>1 h) and higher temperature (similar to 80 degrees C) was not found to be much beneficial. (C) 2016 Wiley Periodicals, Inc.
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(2016) Proceedings of the National Academy of Sciences of the United States of America. 113, 48, p. 13624-13629 Abstract
The simple process of a liquid wetting a solid surface is controlled by a plethora of factors - surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.
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(2016) Macromolecules. 49, 17, p. 6518-6530 Abstract
The simultaneous sharp rise in stiffness, strength, and toughness of electrospun nanofibers at small diameters is explained here as the result of the molecular orientation induced by the strong stretching of the electrospinning extensional flow. Differing from the common view that this phenomenon is related to the nanofibers size scale, we show by theoretical analysis that it is likely the result of an abrupt transition in polymer chain extension that occurs at high flow strain rates. Consequently, the molecular orientation and mechanical properties experience a matching transition, followed by a linear rise with the strain rate. The model compares well with published experimental data, supporting the assertion that the observed phenomena can be explained as the consequence of electrospinning conditions instead of size dependence. We show how the mechanical properties can be tuned by controlling the process as well as set the goal for future improvement in these properties.
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(2016) Journal of the Mechanical Behavior of Biomedical Materials. 59, p. 527-537 Abstract
Dentin possesses unique hierarchical structure, which has a significant influence on the mechanical properties. Understanding the relationship between structure and mechanical properties of dentin is essential for preventing and curing oral diseases, as well as, potentially for developing man-made engineering materials with superior mechanical performance. In this study, the effect of the two-layered structure, where hard peritubular dentin (PTD) containing dentin tubules are embedded in soft intertubular dentin (ITD), on the fracture behavior of dentin is investigated. A numerical model is developed, in which PTD cracking, ITD cracking and the debonding of the interface between PTD and ITD are all taken into account. Numerical simulations reveal that PTD fracture and interface debonding are the major failure mechanisms, which are consistent with experimental observation. It is identified that the cohesive strength and critical separation of interface are the key parameters controlling which of the mechanisms is active. The low cohesive strength of interface and small critical separation of interface can lead to interface debonding, while the large cohesive strength and critical separation give rise to PTD fracture. In addition, it is found that large volume fraction of dentin tubules and small volume fraction of PTD can enhance the toughness of dentin, which provides a new insight into the degraded mechanical properties of old dentin.
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(2016) Scientific Reports. 6, 26305. Abstract
The development of composite materials that are simultaneously strong and tough is one of the most active topics of current material science. Observations of biological structural materials show that adequate introduction of reinforcements and interfaces, or interphases, at different scales usually improves toughness, without reduction in strength. The prospect of interphase properties tuning may lead to further increases in material toughness. Here we use evaporation-driven self-assembly (EDSA) to deposit a thin network of multi-wall carbon nanotubes on ceramic surfaces, thereby generating an interphase reinforcing layer in a multiscale laminated ceramic composite. Both strength and toughness are improved by up to 90%, while keeping the overall volume fraction of nanotubes in a composite below 0.012%, making it a most effective toughening and reinforcement technique.
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(2016) Composites Science and Technology. 127, p. 47-53 Abstract
Polyvinyl alcohol (PVA) is a biocompatible, semi-crystalline and water soluble polymer with moderate tensile properties. To improve the thermal and mechanical properties of PVA, as well as to reduce water uptake, structural modification by glutaric acid (GA) and nanoparticle reinforcement by tungsten disulphide nanotubes (WSNTs) were used to prepare PVA based composites. We observed a significant drop in the water uptake of GA crosslinked PVA, an indication of the formation of a network. Fourier transform infrared spectroscopy was applied to confirm the presence of covalent bonds formed during the crosslinking. Crosslinked PVA and composites are found to have higher thermal stability and mechanical properties compared to their un-crosslinked counterparts. Tensile tests show that the presence of WSNTs increases the strength (up to 25%), modulus (up to 120%) and toughness (up to 80%) of the pristine as well as the crosslinked PVA. (C) 2016 Elsevier Ltd. All rights reserved.
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(2016) Science advances. 2, 2, e1500969. Abstract
Defects in crystalline structure are commonly believed to degrade the ideal strength of carbon nanotubes. However, the fracture mechanisms induced by such defects, as well as the validity of solid mechanics theories at the nanoscale, are still under debate. We show that the fracture toughness of single-walled nanotubes (SWNTs) conforms to the classic theory of fracture mechanics, even for the smallest possible vacancy defect (∼2). By simulating tension of SWNTs containing common types of defects, we demonstrate how stress concentration at the defect boundary leads to brittle (unstable) fracturing at a relatively low strain, degrading the ideal strength of SWNTs by up to 60%. We find that, owing to the SWNT's truss-like structure, defects at this scale are not sharp and stress concentrations are finite and low. Moreover, stress concentration, a geometric property at the macroscale, is interrelated with the SWNT fracture toughness, a material property. The resulting SWNT fracture toughness is 2.7 MPa m0.5, typical of moderately brittle materials and applicable also to graphene.
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(2016) Regenerative Medicine - from Protocol to Patient. Steinhoff G.(eds.). 3 ed. p. 1-55 Abstract
Extracellular matrix (ECM) is an important component of stem cell niche areas, which provide residence, regulate stem cell pool size and control stem cell mobilization. ECM is a complex interlinked composite of collagenous molecules, non-collagenous molecules and water-rich mucopolysaccharide ground substance. Cells are integrated to their matrix via integrin and non-integrin receptors, which control adhesion, migration, division, growth, anoikis, transdifferentiation and other cellular behaviour. ECM safeguard cells and tissue architecture and strength, but also growth factor deposits, which proteinases as signalling scissors can release in a site- and process-specific manner. Selected processes, like wound healing, cartilage and heart ECM, and tumor growth are used to exemplify participation of ECM in tissue regenerative processes.
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(2016) Journal of Biomaterials Applications. 30, 6, p. 873-885 Abstract
The aim was to develop a hybrid three-dimensional-tissue engineering construct for chondrogenesis. The hypothesis was that they support chondrogenesis. A biodegradable, highly porous polycaprolactone-grate was produced by solid freeform fabrication. The polycaprolactone support was coated with a chitosan/polyethylene oxide nanofibre sheet produced by electrospinning. Transforming growth factor-β3-induced chondrogenesis was followed using the following markers: sex determining region Y/-box 9, runt-related transcription factor 2 and collagen II and X in quantitative real-time polymerase chain reaction, histology and immunostaining. A polycaprolactone-grate and an optimized chitosan/polyethylene oxide nanofibre sheet supported cellular aggregation, chondrogenesis and matrix formation. In tissue engineering constructs, the sheets were seeded first with mesenchymal stem cells and then piled up according to the lasagne principle. The advantages of such a construct are (1) the cells do not need to migrate to the tissue engineering construct and therefore pore size and interconnectivity problems are omitted and (2) the cell-tight nanofibre sheet and collagen-fibre network mimic a cell culture platform for mesenchymal stem cells/chondrocytes (preventing escape) and hinders in-growth of fibroblasts and fibrous scarring (preventing capture). This allows time for the slowly progressing, multiphase true cartilage regeneration.
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(2016) CrystEngComm. 18, 2, p. 230-239 Abstract
Onion-like carbon spheres (OLCSs) were synthesized by heating a mixture of naphthalene and graphite powder inside a kitchen microwave oven for ∼1 min under atmospheric conditions. Naphthalene, a hydrocarbon, provides carbon for growth, while graphite plays the crucial role of microwave absorber. The size of the OLCS particles was distributed over a wide range from a few 10s of nm up to a few μm. The OLCS particles self-assembled in a long range chain-like structure. The duration of microwave heating and the ratio of the precursor components were found to be important factors affecting the size and density of the particles. Alternative hydrocarbon sources and microwave absorbers were also examined for OLCS growth. Addition of ferrocene as a catalyst to the precursor mixture of naphthalene and graphite resulted in the formation of highly crystalline carbon-encapsulated iron (in the form of oxide/carbide) nanoparticles with a core-shell structure. The as-synthesized OLCS mixture containing graphite as the precursor was mixed with poly-alpha olefin oil (PAO4) as an additive. A significantly lower friction coefficient and wear rate were obtained with this mixture as compared to the neat PAO4 and PAO4+graphite mixture. Microwave heating was also employed to coat thin films of OLCSs on alumina/glass substrates. The conductivity of these films was measured using the four probe method. The microwave-assisted method used to produce OLCSs has several advantages like cost and energy efficiency, minimal preprocessing, etc. This technique can be of industrial importance for bulk productions of OLCSs and graphitic shell-encapsulated metal nanoparticles.