Elasticity of Spider dragline Silks Viewed as Nematics: Yielding Induced by Isotropic-Nematic Phase Transition

TL;DR

This paper models spider dragline silk as a nematic liquid crystal elastomer, explaining its sigmoidal stress-strain behavior and yielding through an isotropic-nematic phase transition induced by force.

Contribution

It extends Maier-Saupe theory to describe silk elasticity, revealing the phase transition mechanism behind its yielding behavior and predicting humidity effects.

Findings

The model fits experimental stress-strain data well.

Silk behaves as a liquid crystal elastomer with phase transition.

Humidity reduces stress in supercontracted silk.

Abstract

Spider dragline silk shows well-known outstanding mechanical properties. However, its sigmoidal shape of the measured stress-strain curves (i.e. the yield) can not be described by classical polymer theories and recent hierarchical chain model. To solve the long lasting problem, we generalized the Maier-Saupe theory of nematics to construct an elastic model for the polypeptide chain network of the dragline silk. The comprehensive agreement between theory and experiments on the stress-strain curve strongly indicates the dragline silks to belong to liquid crystal elastomers. Especially, the remarkable yielding elasticity of the silk is understood for the first time as the force-induced isotropic-nematic phase transition of the chain network. Our theory also predicts a drop of the stress in supercontracted dragline silk, an early found effect of humidity on the mechanical property in many silks.