Competing Effects of Network Architecture and Composition on Polydomain Liquid Crystal Elastomers

Main-chain liquid crystal elastomers (LCEs) are synthesized to investigate the interplay of the composition and network structure on LCE nematic-to-isotropic (N-I) transitions. We focus on networks synthesized from liquid crystalline oligomers reacted with tri- or tetrafunctional nonmesogenic cross-linker molecules. We find that coupling between mesogens and the polymer backbone increases with the degree of cross-linking. However, this enhanced coupling competes with mesogenic dilution arising from the cross-linker molecules to determine the N-I transition temperature (<i>T</i> _NI). When cross-linker molecules are dilute, the degree of cross-linking directly correlates to the change in <i>T</i> _NI from the oligomer to LCE (Δ<i>T</i> _NI) through mesogen-backbone coupling. In this regime, Δ<i>T</i> _NI ranges from 2.9 to 12.2 °C and 2.9-13.9 °C for tri- and tetrafunctional cross-linkers, respectively. At high cross-linker concentrations, deviations from this linear relationship appear. Further, the fractional mesogen content within an oligomer chain induces molecular weight-dependent mesogenic dilution effects arising from the flexible spacer molecules. Analysis of the N-I transition peak reveals a maximum latent heat per gram of mesogen (Δ<i>H</i> _NI,mes) for this system.