Modeling nematic phase main-chain liquid crystal elastomer synthesis, mechanics, and thermal actuation via coarse-grained molecular dynamics

Abstract

This paper presents a coarse-grained molecular dynamics simulation study of the synthesis, mechanics, and thermal actuation of nematic phase main-chain liquid crystal elastomers (LCEs), a type of soft, temperature-responsive, polymeric actuating material. The simulations herein model the crosslinking, mechanical stretching, and additional crosslinking synthesis process, following which, the simulated LCE exhibits a direction-dependent thermal actuation and mechanical response. The thermal actuation response shows good qualitative agreement with experimental results, including the variation of a global order parameter that describes the orientation of the mesogen domains comprising the LCE. The mechanical response due to applied deformation shows less agreement, but manifests the key features observed in experiments on LCEs, namely soft strain and hyperelasticity that is present when loaded perpendicularly and in-line, respectively, to the mesogen alignment direction. We also present a topological analysis of the simulated LCEs, which, in conjunction with the simulated thermomechanical responses, allows us to infer the relative contribution of entanglements and chemical crosslinks on those responses. We suggest that the model proposed herein will help enable improved LCE formulations via mechanistic insights that can be gained via the use of such a relatively computationally inexpensive coarse-grained molecular dynamics model, which may be of further value to application areas including soft robotics, bio-mimicking devices, artificial muscles, and adaptive materials.