Influence of a Mechanical Field on the Liquid Crystalline to Isotropic Phase Transformation

Abstract

The influence of external mechanical stress on the liquid crystalline to isotropic phase transformation of polysiloxane networks with mesogenic side groups is investigated by stress optical measurements. The hypothetical second order phase transformation temperature T* and the critical exponent γ, as described by the phenomenological Landau — de Gennes theory, are determined. Different systems having nematic and smectic behaviour, respectively, are investigated. Furthermore, the crosslinking density is varied for the nematic system. While in previous investigations of side-on nematic polymer networks, the phase transformation temperature depends on the applied stress, this effect is not found for end-on nematic systems. For smectic elastomers a hypothetical phase transformation temperature T* close to Tlc,i is observed. In contrast to previous experiments, however, no smectic fluctuation could be identified above Tlc,i.