Evolution of microstructure during shape memory cycling of a main-chain liquid crystalline elastomer

Abstract

The field of shape memory polymers (SMPs) has been dominated by polymeric systems whose fixing mechanism is based on crystallization or vitrification of the constituent chains, rendering such systems stiff in comparison to elastomers, gels, and living tissues. Previously, we reported the synthesis and characterization of main-chain, segmented liquid crystalline elastomers (LCEs) that exhibit both bulk and surface shape memory effects. These LCEs have excellent shape fixing and recovery characteristics with compositionally-dependent transition temperatures that determine the fixing and recovery critical temperatures. Synthesis of the soft shape memory LCEs proceeded by hydrosilylation-linking of poly(dimethylsiloxane) oligomers with mesogenic dienes of two compositions and a tetravinyl crosslinker. The present report describes microstructural changes during ex situ shape memory deformation and recovery of one such LCE. Wide-angle x-ray scattering showed that, once a critical deformation stress was reached, the microstructure of the fixed, oriented LCEs was independent of the stress applied above the clearing (isotropization) transition. Stepped recovery of the fixed, oriented LCE showed additional intermediate microstructures, however. Recovery was shown to proceed through changes in both the smectic layer thickness and chevron architecture, while mesogen tilt angle remained unchanged. The mechanical and microstructural studies described herein give deeper insight to shape memory fixing and recovery mechanisms of these unique materials, which offer potential for exploitation in areas such as cell and tissue culture, microcontact printing, and microfluidics.