V2O5 nanofibre sheet actuators

Vanadium oxides, such as V2O5, are promising for lithium-ion batteries, catalysis, electrochromic devices and sensors. Vanadium oxides were proposed more than a decade ago for another redox-dependent application: the direct conversion of electrical energy to mechanical energy in actuators (artificial muscles). Although related conducting polymer and carbon nanotube actuators have been demonstrated, electromechanical actuators based on vanadium oxides have not be realized. V2O5 nanofibres and nanotubes provide the potential advantages of low-cost synthesis by sol-gel routes and high charging capacity and long cycle life. Here, we demonstrate electromechanical actuation for obtained high modulus V2O5 sheets comprising entangled V2O5 nanofibres. The high surface area of these V2O5 sheets facilitates electrochemical charge injection and intercalation that causes the electromechanical actuation. We show that the V2O5 sheets provide high Young's modulus, high actuator-generated stress, and high actuator stroke at low applied voltage.     

Effects of annealing on the structural and mechanical properties of electrospun polymeric nanofibres

Biodegradable polymeric nanofibres produced by electrospinning have been used as scaffolds for tissue engineering. Before these nanofibrous scaffolds can be implanted into the human body, it is important to know if the individual nanofibres are strong enough to withstand the forces exerted by the cells as they grow and migrate on the scaffold. However, due to the small size of the nanofibres, it is a challenge to characterize the mechanical properties of individual nanofibres. Therefore, we aim to mechanically characterize a single nanofibre using both a tensile test and a nanoscale three-point bend test. As some scaffolds may be heat-treated by annealing to enhance the stiffness and strength of the nanofibres, we also investigate the effects of annealing on the structural and mechanical properties of single nanofibres. The material properties of as-spun and annealed nanofibres were studied using differential scanning calorimetry and atomic force microscopy. Annealing was found to increase the Young's modulus of the nanofibre mainly due to the increase in crystallinity and the change in morphology from a purely fibrillar structure to a mixture of fibrillar and nano-granular structure with enhanced interfibrillar bonding.     

Enhanced mechanical properties of electrospun nano-fibers through NaCl mediation

Electrospun (ES) nano-scale polymer fibers are known to exhibit lower Young's modulus and strength than their bulk counterpart. We have discovered that minute additions of sodium chloride (NaCl) during the preparation stage of ES polymethyl methacrylate (PMMA) fibers raises the fiber mechanical properties in a significant way, nearly up to bulk values, over a range of diameters. NaCl-induced electrical effects leading to enhanced molecular alignment during nano-fiber formation is the most likely explanation for this synergistic effect. Moreover, beyond the now-recognized rise in Young's modulus values, we observed that the strength and tensile toughness of the ES fibers also significantly increase at progressively smaller diameters.     

Layer removal analysis of residual stress

A method is presented for the analysis of residual stresses in parallel-sided polymer mouldings with depth-varying Young's modulus. The experimental procedure is the same as the layer removal technique developed by Treuting and Read and requires removal of uniform layers from the surface and measurement of the resulting curvature when all external tractions are removed. When the Young's modulus varies with depth, certain of the simplifications made by Treuting and Read are no longer valid, but it is shown here that a satisfactory alternative procedure can be used as long as the modulus distribution is known. Examples of the application of the new procedure are given for an injection-moulded nylon 6, 6 bar stored in dry conditions, in which the modulus rose steeply near to the surface, and another bar, also made from nylon 6, 6, in which exposure to water produced a quite different distribution, with a maximum at the centre.     

First-principles studies of the electronic and mechanical properties of ZnO nanobelts with different dominant surfaces

The electronic and elastic properties of [0001] ZnO nanobelts with different lateral dimensions have been studied by employing first-principles approaches. We find that the surface effects are dominant for the energetic stability of the nanobelt, while the quantum confinement effect plays an important role in the band gaps of the nanobelts. More importantly, we show that the different dominant surfaces of nanobelts have important influences on the band gaps, but minimal effects on the size dependence of the Young's modulus. The Young's modulus is larger than the bulk value and decreases with the increase of the square root of the cross-sectional area of the nanobelts. Finally, we find that the continuum-based model proposed for the Young's modulus of nanostructures is applicable for ZnO nanowires of 10-200?nm diameter, but not for ultrathin nanowires and nanobelts.     

Carbon-nanofibre-reinforced poly(ether ether ketone) fibres

Nano-reinforced fibres were spun from a semicrystalline high-performance poly(ether ether ketone) containing up to 10 wt% vapour-grown carbon nanofibres using conventional polymer processing equipment. Mechanical tensile testing revealed increases in nanocomposite stiffness, yield stress, and fracture strength for both as-spun and heat-treated fibres. X-ray and differential scanning calorimetry analyses were performed in order to investigate both the orientation of nanofibres within the polymer matrix and the matrix morphology. The carbon nanofibres were found to be well aligned with the direction of flow during processing. Significantly, the degree of crystallinity of the poly(ether ether ketone) matrix was found to increase with the initial addition of nanofibres although the crystal structure was not affected. The measured increase in composite tensile modulus is compared to injection-moulded nanocomposite samples made from the same blends. The results highlight the need to characterise the matrix morphology when evaluating nanocomposite performance and hence deducing the intrinsic properties of the nanoscale reinforcement.     

Morphology and properties of injection-moulded carbon-nanofibre poly(etheretherketone) foams

Poly(ether ether ketone) (PEEK) is a high performance polymer that cannot usually be foamed reliably using conventional injection-moulding processes. Here, vapour-grown carbon nanofibres (CNFs) are introduced to stabilise the foaming process, and the resulting morphology of injection-moulded integral foams is investigated in detail. Different image analysis techniques revealed the positive effect of the nanofiller on the cellular structure. Electron microscopy confirmed a homogeneous dispersion of the nanofibres in the cellular PEEK cores. The mechanical properties of the foam injection-moulded samples, in bending, showed an increase in yield strength and elastic modulus with nanofibre loading fractions up to 15 wt%. Although the compressive properties of the foams were reduced as compared to the solid-polymer, the CNFs clearly offset this reduction in properties. Detailed differential scanning calorimetry (DSC) and dynamic mechanical analysis provide further evidence of an interaction between the matrix and the nanoscale filler.     

Mechanical properties of polycarbonate and poly(methyl methacrylate) films reinforced with surface-functionalized nanodiamonds

Polymer nanocomposites (PNCs) possess highly versatile characteristics, depending on the nanofiller properties such as its chemical composition, particle size, dimension, polydispersity, concentration, or surface functional groups. In comparison with micron-sized materials, the nanofiller having a large surface area facilitates stronger interaction with the matrix. In this work, various surface-functionalized nanodiamonds (sf-NDs) having hydroxyl, carboxyl, amino, and amide group were prepared, and dispersed into polycarbonate (PC) and poly(methyl methacrylate) (PMMA) polymers. The polymer nanocomposites (PNCs) which contain the ND content of 5 wt% were subjected to the measurements of mechanical properties such as hardness and Young's modulus by atomic force microscopy (AFM) nanoindentation. It was observed that the hardness and Young's modulus of the polymer nanocomposites depend on strongly the nature of functional groups. The amine or amide functionalization gives the high mechanical properties for both polymers. The interfacial interaction between sf-NDs and polymer matrices is an important factor determining the mechanical properties of the PNCs.     

Structure and properties of electrospun PLLA single nanofibres

An electrospinning method was used to spin semi-crystalline poly(L-lactide) (PLLA) nanofibres. Processing parameter effects on the internal molecular structure of electrospun PLLA fibres were investigated by x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Take-up velocity was found as a dominant parameter to induce a highly ordered molecular structure in the electrospun PLLA fibres compared to solution conductivity and polymer concentration, although these two parameters played an important role in controlling the fibre diameter. A collecting method of a single nanofibre by an electrospinning process was developed for the tensile tests to investigate structure-property relationships of the polymer nanofibres. The tensile test results indicated that higher take-up velocity caused higher tensile modulus and strength due to the ordered structure developed through the process.     

Measuring True Young's Modulus of a Cantilevered Nanowire: Effect of Clamping on Resonance Frequency

The effect of clamping on resonance frequency and thus measured Young's modulus of nanowires (NWs) is systematically investigated via a combined experimental and simulation approach. ZnO NWs are used in this work as an example. The resonance tests are performed in situ inside a scanning electron microscope and the NWs are cantilevered on a tungsten probe by electron-beam-induced deposition (EBID) of hydrocarbon. EBID is repeated several times to deposit more hydrocarbons at the same location. The resonance frequency increases with the increasing clamp size until approaching that under the "fixed" boundary condition. The critical clamp size is identified as a function of NW diameter and NW Young's modulus. This work: 1) exemplifies the importance of considering the effect of clamping in measurements of Young's modulus using the resonance method, and 2) demonstrates that the true Young's modulus can be measured if the critical clamp size is reached. Design guidelines on the critical clamp size are provided. Such design guidelines can be extended to other one-dimensional nanostructures such as carbon nanotubes.     

Mechanical behavior and interphase structure in a silica-polystyrene nanocomposite under uniaxial deformation

The mechanical behavior of polystyrene and a silica-polystyrene nanocomposite under uniaxial elongation has been studied using a coarse-grained molecular dynamics technique. The Young's modulus, the Poisson ratio and the stress-strain curve of polystyrene have been computed for a range of temperatures, below and above the glass transition temperature. The predicted temperature dependence of the Young's modulus of polystyrene is compared to experimental data and predictions from atomistic simulations. The observed mechanical behavior of the nanocomposite is related to the local structure of the polymer matrix around the nanoparticles. Local segmental orientational and structural parameters of the deforming matrix have been calculated as a function of distance from nanoparticle's surface. A thorough analysis of these parameters reveals that the segments close to the silica nanoparticle's surface are stiffer than those in the bulk. The thickness of the nanoparticle-matrix interphase layer is estimated. The Young's modulus of the nanocomposite has been obtained for several nanoparticle volume fractions. The addition of nanoparticles results in an enhanced Young's modulus. A linear relation describes adequately the dependence of Young's modulus on the nanoparticle volume fraction.     

Structure-mechanical property of individual cobalt oxide nanowires

We present a comprehensive approach to address the correlation between mechanical properties of nanowires (NWs) with their characteristic size, microstructure, and chemical composition. Using this technique, the Young's modulus of Co3O4 NWs with different sizes was evaluated. Thermal annealing in inert atmosphere was found to induce chemical reduction of as-grown Co3O4 NWs into CoO NWs without modifying their geometrical shape. Both Co3O4 and CoO NWs exhibited a size-dependent variation in Young's modulus.     

Effective reinforcement in carbon nanotube-polymer composites

Carbon nanotubes have mechanical properties that are far in excess of conventional fibrous materials used in engineering polymer composites. Effective reinforcement of polymers using carbon nanotubes is difficult due to poor dispersion and alignment of the nanotubes along the same axis as the applied force during composite loading. This paper reviews the mechanical properties of carbon nanotubes and their polymer composites to highlight how many previously prepared composites do not effectively use the excellent mechanical behaviour of the reinforcement. Nanomechanical tests using atomic force microscopy are carried out on simple uniaxially aligned carbon nanotube-reinforced polyvinyl alcohol (PVA) fibres prepared using electrospinning processes. Dispersion of the carbon nanotubes within the polymer is achieved using a surfactant. Young's modulus of these simple composites is shown to approach theoretically predicted values, indicating that the carbon nanotubes are effective reinforcements. However, the use of dispersant is also shown to lower Young's modulus of the electrospun PVA fibres.     

Elastic membranes of close-packed nanoparticle arrays

Nanoparticle superlattices are hybrid materials composed of close-packed inorganic particles separated by short organic spacers. Most work so far has concentrated on the unique electronic, optical and magnetic behaviour of these systems. Here, we demonstrate that they also possess remarkable mechanical properties. We focus on two-dimensional arrays of close-packed nanoparticles and show that they can be stretched across micrometre-size holes. The resulting free-standing monolayer membranes extend over hundreds of particle diameters without crosslinking of the ligands or further embedding in polymer. To characterize the membranes we measured elastic properties with force microscopy and determined the array structure using transmission electron microscopy. For dodecanethiol-ligated 6-nm-diameter gold nanocrystal monolayers, we find a Young's modulus of the order of several GPa. This remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370 K. The purely elastic response of these ultrathin membranes, coupled with exceptional robustness and resilience at high temperatures should make them excellent candidates for a wide range of sensor applications.     

Elasticity size effects in ZnO nanowires--a combined experimental-computational approach

Understanding the mechanical properties of nanowires made of semiconducting materials is central to their application in nano devices. This work presents an experimental and computational approach to unambiguously quantify size effects on the Young's modulus, E, of ZnO nanowires and interpret the origin of the scaling. A micromechanical system (MEMS) based nanoscale material testing system is used in situ a transmission electron microscope to measure the Young's modulus of [0001] oriented ZnO nanowires as a function of wire diameter. It is found that E increases from approximately 140 to 160 GPa as the nanowire diameter decreases from 80 to 20 nm. For larger wires, a Young's modulus of approximately 140 GPa, consistent with the modulus of bulk ZnO, is observed. Molecular dynamics simulations are carried out to model ZnO nanowires of diameters up to 20 nm. The computational results demonstrate similar size dependence, complementing the experimental findings, and reveal that the observed size effect is an outcome of surface reconstruction together with long-range ionic interactions.     

Force-deflection spectroscopy: a new method to determine the Young's modulus of nanofilaments

We demonstrate the determination of Young's modulus of nanowires or nanotubes via a new approach, that is, force-deflection spectroscopy (FDS). An atomic force microscope is used to measure force versus deflection (F-D) curves of nanofilaments that bridge a trench patterned in a Si substrate. The FD data are then fit to the Euler-Bernoulli equation to determine Young's modulus. Our approach provides a generic platform from which to study the mechanical and piezoelectric properties of a variety of materials at the nanoscale level. Young's modulus measurements on ZnS (wurtzite) nanowires are presented to demonstrate this technique. We find that the Young's modulus for rectangular cross section ZnS nanobelts is 52 +/- 7.0 GPa, about 30% smaller than that reported for the bulk.     

Preparation of biocatalytic nanofibres with high activity and stability via enzyme aggregate coating on polymer nanofibres

We have developed a unique approach for the fabrication of enzyme aggregate coatings on the surfaces of electrospun polymer nanofibres. This approach employs covalent attachment of seed enzymes onto nanofibres consisting of a mixture of polystyrene and poly(styrene-co-maleic anhydride), followed by a glutaraldehyde (GA) treatment that cross-links additional enzyme molecules and aggregates from the solution onto the covalently attached seed enzyme molecules. These cross-linked enzyme aggregates, covalently attached to the nanofibres via the linkers of seed enzyme molecules, are expected to improve the enzyme activity due to increased enzyme loading, and also the enzyme stability. To demonstrate the principle, we coated α-chymotrypsin (CT) on nanofibres electrospun from a mixture of polystyrene and poly(styrene-co-maleic anhydride). The initial activity of CT-aggregate-coated nanofibres was nine times higher than nanofibres with just a layer of covalently attached CT molecules. The enzyme stability of CT-aggregate-coated nanofibres was greatly improved with essentially no measurable loss of activity over a month of observation under rigorous shaking conditions. This new approach of enzyme coating on nanofibres, yielding high activity and stability, creates a useful new biocatalytic immobilized enzyme system with potential applications in bioconversion, bioremediation, and biosensors.     

Characterisation and modelling of the elastic properties of poly(lactic acid) nanofibre scaffolds

The aim of this study is to predict the elastic response of poly(lactic acid) (PLA) electrospun nanofibre scaffolds through mathematical models based on homogenisation and the differential replacement method (DRM). These models principally seek to determine and analyse the effects of the internal morphology of the nanofibres on the effective Young’s modulus of polymer nanofibre scaffolds. The microstructure of the nanofibres was first characterised by SEM, XRD, DSC, AFM, and TEM techniques. From this characterisation, strong evidence of a hierarchical core–shell structure was found. With the experimental data, it was possible to design and validate better models than those currently used. In addition, the effects of the electrospinning parameters, such as take-up velocity and thermal treatment, were analysed and correlated with the morphology and the elastic properties of the nanofibres and their scaffolds. To validate the models’ results, we conducted a series of uniaxial tensile tests on the PLA nanofibre scaffolds. Using the data from the nanofibre measurements, the homogenisation approximations and the model based on the DRM predicted an effective Young’s modulus of 667 and 835 MPa, respectively. The predicted data were in excellent agreement with the experimental results (685–880 MPa). These models will be useful in understanding and evaluating the structure–property relationships of oriented nanofibre scaffolds for medical or biological applications.     

Modelling the Young''s modulus of platelet reinforced thermoplastic sheet composites

Particulate reinforced polymers is a mature field and many models are available to predict the Young's modulus of such composites. However, most existing models have a common flaw; they all predict that the composite modulus equals that of the reinforcing agent when the polymer content approaches zero. This implies, in this limit, a monolithic reinforcement whereas, in fact, it is composed of discrete particles with very little interaction. This is a serious drawback and therefore this study focussed on deriving an improved model for the prediction of the Young's modulus. The porosity of the present samples was correlated with the volume fraction binder and the maximum packing density of the pure reinforcement. A theoretical model for Young's modulus was derived along the lines of the Padawer and Beecher modified Cox model. However, it includes the effect of composite porosity on the composite's mechanical properties. In contrast to other available models, it correctly predicts the loss of material stiffness and strength in the limit of zero binder content. Good agreement was found between the predictions of this model and experimental measurements.     

Studies of Size Effects on Carbon Nanotubes' Mechanical Properties by Using Different Potential Functions

We use molecular mechanics calculations to study size effects on mechanical properties of carbon nanotubes. Both single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) are considered. The size-dependent Young's modulus decreases with the increasing tube diameter for a reactive empirical bond order (REBO) potential function. However, we observe a contrary trend if we use other potential functions such as the modified Morse potential function and the universal force field (UFF). Such confliction is only obtained for small tubes within cutoff diameters (3 nm for REBO and 1.5 nm for others). In light of these predictions, Young's moduli of large nanotubes concur with experimental results for all the potential functions. No matter which potential function is used, the Poisson's ratio decreases with the increasing tube diameter. We also study the chirality effects on mechanical properties of SWNTs. We find that the Young's moduli are insensitive to the chirality of nanotubes. The chirality effect on the Poisson's ratio is significant for the UFF but not the REBO or modified Morse potential functions.