Untwisting of a Strained Cholesteric Elastomer by Disclination Loop Nucleation

TL;DR

This paper investigates how Frank elasticity influences the unwinding of cholesteric elastomers under strain, revealing metastability and nucleation mechanisms involving twist disclination loops through theoretical and finite element analyses.

Contribution

It incorporates Frank elasticity into the analysis of cholesteric elastomer unwinding and models the nucleation of disclination loops as a mechanism for texture removal.

Findings

Frank penalty reduces the unwinding strain threshold.

Cholesteric state becomes metastable above the transition.

Disclination loop nucleation is a key mechanism for texture removal.

Abstract

The application of a sufficiently strong strain perpendicular to the pitch axis of a monodomain cholesteric elastomer unwinds the cholesteric helix. Previous theoretical analyses of this transition ignored the effects of Frank elasticity which we include here. We find that the strain needed to unwind the helix is reduced because of the Frank penalty and the cholesteric state becomes metastable above the transition. We consider in detail a previously proposed mechanism by which the topologically stable helical texture is removed in the metastable state, namely by the nucleation of twist disclination loops in the plane perpendicular to the pitch axis. We present an approximate calculation of the barrier energy for this nucleation process which neglects possible spatial variation of the strain fields in the elastomer, as well as a more accurate calculation based on a finite element modeling of the elastomer.