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.