Molecular engineering of step-growth liquid crystal elastomers

Abstract

Liquid crystal elastomers (LCEs) are shape-responsive materials that combine the elastic properties of a polymer network with the molecular ordering and responsiveness of liquid crystals. Recent work has relied on step-growth chemistries to produce LCEs with remarkable diversity and functionality. However, the connection between molecular structure and macroscopic properties such as shape responsiveness and phase behavior are poorly understood. Here, we demonstrate general molecular design principles for increasing shape-responsiveness and reducing the glass-transition temperature of LCEs produced through step-growth chemistries. We systematically investigate the phase behavior, liquid crystal ordering, mechanical properties and shape-responsiveness of a series of model polyester LCE networks using a combination of two-dimensional X-ray diffraction and dynamic mechanical analysis. We demonstrate that tailoring the length and composition of the linking group can reduce the glass-transition temperature with little impact on the liquid crystal order parameter, resulting in LCEs with a large shape-response near room temperature. Furthermore, the incorporation of a chain extending unit can significantly increase shape-responsiveness, from 35 up to 78% reversible strain, and the network crosslink density can be controlled by variation of the network stoichiometry. This work establishes generally applicable molecular design principles applicable to LCEs prepared through step-growth chemistries.