Christopher Viney

Silkworm silk as an engineering material

While silk exhibits high values of tensile strength and stiffness, these properties are compromised by their poor reproducibility. We present the results of experiments aimed at characterizing the variability of tensile properties exhibited by cocoon silk from Bombyx mori silkworms. Scanning electron microscopy is used to measure an average diameter for individual test specimens; the interspecimen variability of diameter is quantified and found to be inadequately represented by standard deviation. When load-extension data are converted into stress-strain curves, a marked improvement in reproducibility is realized if each specimen cross-section is calculated from diameter measurements specific to that specimen. Nevertheless, a significant variability in fracture stress remains; a Weibull analysis reveals that silkworm silk has a failure predictability comparable with that of glass and nonengineering ceramics. Unloading/reloading tests demonstrate that stiffness is not significantly affected by cumulative deformation, and the stress–strain relationship is not sensitive to strain rate.

Natural silks: archetypal supramolecular assembly of polymer fibres

Abstract Major ampullate silk produced by Nephila clavipes spiders exhibits a unique spectrum of desirable mechanical properties. There are useful lessons for materials scientists concerned with the interrelation of synthesis, processing, microstructural design and properties of polymer fibres; lessons learned to date are reviewed briefly. A significant feature of silk spinning in vivo is the evidence that a processable liquid crystalline phase assembles by non-covalent aggregation of polymer (fibroin) having a random coil conformation. It is shown in this paper that the form birefringence calculated for such a liquid crystalline phase is compatible with the measured birefringence of liquid crystalline silk secretion. Also, the linear rate of assembly is estimated for the rod-like aggregates. Comparison with the linear aggregation rate achieved by G-actin (in the well-characterized acrosomal process) confirms that the estimate for fibroin is credible.

Water–mucin phases: conditions for mucus liquid crystallinity

Previous studies of mucin conformation in aqueous media have focussed on the polymer backbone, on dilute concentrations, and on ambient temperatures. Using differential scanning calorimetry and transmitted polarised light microscopy, we demonstrate that commercially available pig gastric mucin forms a liquid crystalline gel at concentrations above approximately 26% w/w in water. Solvated mucin exhibits a glass transition at approximately 24.8°C, and the liquid crystalline phase is only fluid above this temperature. We associate the glass transition with the onset of flexibility in the backbone of glycosylated molecular segments. Consideration of molecular geometry suggests that mucin liquid crystallinity is governed by interactions between side chains, not main chains. For mucin from a given source, neither the conditions required for liquid crystallinity nor the glass transition temperature should be sensitive to the mucin molecular weight, as long as the integrity of the glycosylated segments is maintained. We consider some physiological implications of our results, including the impact of hypothermic conditions on the functionality of mucus, and we emphasise the need to characterise mucin and mucus structure at physiological temperatures.

Electrical resistance curing of carbon-fibre/epoxy composites

Carbon-fibre/epoxy resin composites were cured by the ohmic heating that results from passing an electrical current through the fibres. Additional cured composite was prepared conventionally. All samples consisted of eight-layer stacks of pre-preg, containing 38% fibre by volume. Vickers hardness measurements demonstrate that both processes lead to the same level of cross-linking. In three-point bend tests, the properties of resistance cured composite compare favourably with those of oven-cured samples. Significantly less energy is used in the process of resistance curing small samples, relative to curing in an oven or autoclave.

Spider (Araneus diadematus) cocoon silk: a case of non-periodic lattice crystals with a twist?

Transmission electron microscopy was used to investigate the supramolecular structure of Araneus diadematus (garden spider) cocoon silk. Electron diffraction patterns contain features which are consistent with the presence of non-periodic lattice crystals, i.e. highly frustrated crystalline regions as identified previously in the major ampullate silk (MAS, dragline) of Nephila clavipes spiders. The diffraction patterns further suggest that crystals in A. diadematus cocoon silk may be twisted parallel to the chain direction, offering a potential explanation for the lower tensile stiffness of this fibre relative to MAS.

Molecular order in spider major ampullate silk (dragline): Effects of spinning rate and post‐spin drawing

Optical birefringence measurements are used to characterize how the molecular order of spider (Nephila clavipes) major ampullate silk is affected by linear spinning rate, by the extent of post-spin drawing, and by post-spin drawing rate. Results are interpreted qualitatively in terms of a simple microstructural model, in which birefringence depends on both the overall degree of molecular orientation and the extent to which crystalline regions are present. In contrast to the behavior of conventional, synthetic polymers, birefringence is found to be an unreliable predictor of tensile stiffness: microstructural changes that lead to increased birefringence may leave stiffness unchanged or, in some cases, lower than before. It is unlikely that economic processing of silk-like polymers into fiber that exhibits biomimetic tensile properties can be achieved with spinning followed by drawing, or with a single spinning step. Instead, spinning followed by thermochemical treatment under load may be needed to obtain the critical combination of molecular orientation and crystallinity in commercially satisfactory time scales.

The effect of post-spin drawing on spider silk microstructure: a birefringence model

Measurements of optical birefringence have been used to characterise the effect of mechanical history on supramolecular structure in major ampullate silk from Nephila clavipes (golden orb weaver) spiders. Birefringence modelling is demonstrated to be a powerful technique for obtaining quantitative information on supramolecular rearrangement in response to macroscopic strain. Temporary and permanent birefringence changes measured as a function of strain and strain rate are interpreted in terms of two types of microstructural response: increased molecular alignment in all the microstructural phases present is accompanied by decreased lateral register within crystallographically ordered phases. Significant implications of these studies for the commercial processing of silks and silk-like biopolymers are discussed.