Isotropic-Cholesteric Transition of a Weakly Chiral Elastomer Cylinder

TL;DR

This paper investigates the isotropic-cholesteric transition in weakly chiral elastomer cylinders, comparing helical and double twist configurations, and finds the double twist state minimizes free energy more effectively.

Contribution

It introduces an analysis of the isotropic-cholesteric transition in weakly chiral elastomers, highlighting the efficiency of the double twist state over the helical state.

Findings

Double twist state minimizes elastic and chiral free energies.

Helical state pitch is strongly influenced by nemato-elastic coupling.

Double twist state is more energetically favorable in weak chirality limit.

Abstract

When a chiral isotropic elastomer is brought to low temperature cholesteric phase, the nematic degree of freedom tends to order and form a helix. Due to the nemato-elastic coupling, this also leads to elastic deformation of the polymer network that is locally coaxial with the nematic order. However, the helical structure of nematic order is incompatible with the energetically preferred elastic deformation. The system is therefore frustrated and appropriate compromise has to be achieved between the nematic ordering and the elastic deformation. For a strongly chiral elastomer whose pitch is much smaller than the system size, this problem has been studied by Pelcotivs and Meyer, as well as by Warner. In this work, we study the isotropic-cholesteric transition in the weak chirality limit, where the pitch is comparable or much larger than system size. We compare two possible solutions: a helical state as well as a double twist state. We find that the double twist state very efficiently minimizes both the elastic free energy and the chiral nematic free energy. On the other hand, the pitch of the helical state is strongly affected by the nemato-elastic coupling. As a result this state is not efficient in minimizing the chiral nematic free energy.