No abstract

Temperature profiles of the first and the second moment of the nematic order parameter distribution function, as determined from the deuteron nuclear magnetic resonance line shapes, as well as heat capacity response, provide support for the supercritical scenario of the nematic-paranematic phase transition in liquid single crystal elastomers. The relative strength of the locked-in internal mechanical field with respect to the critical field can be decreased by swelling the elastomer samples with low molecular mass nematogen. By increasing the concentration of the dopant, critical and below-critical behavior is promoted.

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.

We investigated the paranematic-to-nematic phase transition of nematic main-chain liquid singlecrystal elastomers by means of high-resolution ac calorimetry and deuteron quadrupole-perturbed nuclear magnetic resonance. We determined that the cross-linking density strongly affects the thermodynamic response of the system, driving the transition from first order to supercritical via critical point of liquid-vapor type. This result together with recent findings in side-chain liquid crystal elastomers support the universality of such thermodynamic behavior in liquid crystal elastomers. Furthermore, the impact of frozen-in mechanical field during the preparation of samples has been systematically explored in this work. It is shown that the critical behavior can be controlled by both cross-linkers-induced internal random mechanical fields and frozen-in uniform mechanical field applied externally.