The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers

This chapter reviews the experimental findings that help determine the critical properties of the isotropic-to-nematic transition in side-chain and main-chain liquid single-crystal elastomers. Special focus is given to heat-capacity and deuteron nuclear magnetic resonance (H-2-NMR) experiments, which can reliably provide information about the local and macroscopic nature of the isotropic-to-nematic conversion. In particular, it is shown that a critical point exists in the generalized temperature-mechanical-stress phase diagram for both side-and main-chain liquid single-crystal elastomers. Here, the internal frozen mechanical stress field was varied by changing a single chemical parameter, i.e. the density of the crosslinkers. A detailed analysis shows that, due to the local heterogeneities, the critical point is actually smeared in both the temperature and the stress axes. It is also shown that by varying some particular chemical and physical parameters (such as crosslinking temperature, the externally applied stress field during crosslinking and the density of crosslinkers), the system can be prepared in a subcritical or supercritical regime, which correspond to the fast (on-off) or to the continuous (slow) thermomechanical response, respectively. This implies that the thermomechanical response can be influenced by controlling the critical properties of the isotropic-to-nematic transition.