Influence of a mechanical field on the liquid crystalline to isotropic phase transformation

The influence of external mechanical stress on the liquid crystalline to is otropic phase transformation of polysiloxane networks with mesogenic side g roups is investigated by stress optical measurements. The hypothetical seco nd order phase transformation temperature T* and the critical exponent gamm a, as described by the phenomenological Landau-de Gennes theory, are determ ined. 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 stre ss, this effect is not found for end-on nematic systems. For smectic elasto mers a hypothetical phase transformation temperature T* close to T-lc,T-i i s observed. In contrast to previous experiments, however, no smectic fluctu ation could be identified above T-lc,T-i.