Non-uniform deformations in liquid crystalline elastomers

TL;DR

This paper develops a universal molecular model for non-uniform deformations in liquid crystalline elastomers, revealing that certain elastic constants can dominate and lead to highly distorted textures under large strains.

Contribution

It introduces a general expression for the free energy of non-uniform deformations, incorporating effects of large strains and director field gradients, with novel negative elastic constants.

Findings

Elastic free energy is universal and insensitive to molecular details.

Derived elastic constants depend on polymer anisotropy and can be negative.

Large deformations may favor highly distorted polydomain textures.

Abstract

We develop a molecular model of non-uniform deformations within the framework of liquid crystal rubber elasticity. We show that, similarly to the uniform case, the theory is not sensitive to the molecular details of polymer liquid crystal involved and the resulting elastic free energy is quite universal. The result of this work is the general expression for the free energy of deformations, which combines the effects of large non-symmetric affine strains in the rubbery network and gradients of curvature deformations of the director field, $F = \lambda^T (\nabla {n})^2 \lambda $. We derive the molecular expressions for the elastic constants governing non uniform deformations in the presence of elastic strains. They also depend on the polymer step length anisotropy and - most strikingly - have an overall {negative} sense. We therefore predict that in some circumstances, especially at large elastic deformations $\lambda$, these new terms may overpower the usual, positive Frank elastic moduli of the underlying nematic structure, as well as the coupling in nematic elastomers of uniform relative rotations of the director and the elastic matrix. In this event highly distorted polydomain textures n(r) would be favoured.