A Molecular Theory for Liquid Crystal Elastomers: Nematic Ordering, Shape Deformation and Mechanical Response

TL;DR

This paper develops a molecular self-consistent field theory for liquid crystal elastomers, capturing the coupling between nematic order, backbone alignment, and network deformation, and predicts how molecular features influence shape change and mechanical response.

Contribution

It introduces a comprehensive molecular-level model for LCEs that includes various molecular features and predicts their effects on nematic ordering and deformation modes.

Findings

Crosslinking suppresses nematic order due to elastic energy.

Adjusting LC grafting density tunes deformation mode.

Stress can transform the nematic phase transition from first-order to continuous.

Abstract

Modeling liquid crystal elastomers (LCEs) at the molecular level is crucial for the predictable design of energy-conversion and stimuli-responsive materials. Here, we develop a self-consistent field theory for LCEs which captures the coupling between nematic ordering, backbone alignment and network deformation. Molecular features such as density of elastic strands, strength and architecture of local chemical hinge, and LC grafting density are systematically included. Crosslinking suppresses nematic ordering as a result of the elastic energy stored during network deformation. Higher work capacity can be achieved by less crosslinked LCEs. The spontaneous shape change of end-on side-chain LCEs can be either elongation or contraction depending on the competition between the local and global couplings. Adjusting LC grafting density is found to be an effective way to fine-tune the deformation mode. We elucidate a universal scaling relationship between the transition temperature and the shear modulus as $T_{\rm NI,0}-T_{\rm NI}\sim {\mu^*}^{\frac{4}{5}}$. Furthermore, we predict that the first-order nematic phase transition can be degraded into a continuous manner upon applied stress. Coupled with nematic ordering, the mechanical response of LCEs significantly deviates from classical rubber elasticity. A plateau in the stress-deformation curve appears accompanied by the nematic phase transition. Our theoretical predictions are in good agreement with the experimental results reported in the literature.