An x-ray scattering study of liquid crystalline elastomers

Liquid crystalline elastomers are a class of new materials which combine the elasticity of elastomers and the orientational alignments of liquid crystals. They have the ability to undergo spontaneous shape change in the thermotropic phase transitions of liquid crystal mesogens. We have performed X-ray scattering and strain measurements to study the structural and mechanical behaviors of a series of liquid crystalline elastomer samples which were synthesized with a one-step photopolymerization method. We applied an external mechanical field to the free standing liquid crystalline elastomer films. With the external mechanical field parallel with the initial director, we observed a gradual nematic to isotropic (NI) transition with increasing temperature. Our analysis of the nematic order parameter as a function of temperature suggests that the heterogeneity in the samples introduced a distribution of the transition temperature of the NI transition. We were able to extract the orientational distribution function of the liquid crystal mesogens in the real space. We found that the percentage of mesogens in the nematic phase showed a similar trend as the nematic order parameter with increasing temperature. With the external mechanical field perpendicular to the initial director, we observed the director to rotate in two different manners: in alternative directions in each domain or uniform rotation. We suggest that the geometrical relationship between the uniaxial stress and the initial director determines the choice of rotation. We also studied a different type of liquid crystalline elastomer with strong smectic fluctuations. The orientational distribution function and nematic order parameter analysis showed similar results. By analyzing the angular positions of the peak maxima and the characteristic distances, we were able to determine the temperature dependence of the layer tilt angle and molecular tilt angle