Molecular architecture dependence of mesogen rotation during uniaxial elongation of liquid crystal elastomers

Abstract

Macroscopic deformation of liquid crystal elastomers (LCEs) has been assumed to result from the coupling dynamics of the elastic polymer network and liquid-crystalline mesogenic units. However, the microscopic dynamics of LCE molecules are extremely difficult to directly observe from experiments. In this work, in silico experiments of uniaxial elongation of LCEs were performed by molecular dynamics simulations. The molecular systems of main- and side-chain-type LCEs were constructed using coarse-graining techniques. The stress–strain curves as the response to an external force were obtained, and the orientational order parameter of the mesogens along the elongation axis was observed as evidence of symmetry breaking. For the main-chain LCEs, soft elasticity was clearly observed regardless of the details of the molecular architecture. A plateau region of the stress–strain curves appeared, corresponding to drastic symmetry breaking of the orientational order. In contrast, for the side-chain LCEs, the presence/absence of soft elasticity was highly dependent on the details of the molecular architecture. In some cases, rotation of the mesogens was inhibited by a local steric constraint by the main chain and crosslinked particles, which prevented the onset of soft elasticity.