VISCOELASTIC PROPERTIES OF A MODEL MAIN-CHAIN LIQUID-CRYSTALLINE POLYETHER

We report oscillatory shear and steady-shear measurements on a model m ain-chain thermotropic polyether in its nematic and isotropic phases, and in the nematic-isotropic biphase. The nematic phase is viscoelasti c on all time scales measured. Since these time scales are longer than the molecular time scale of diffusion, we conclude that the nematic v iscoelasticity is controlled by the large-scale defect structure. The isotropic phase approaches the response of a viscoelastic liquid, but there are much longer relaxation times present in the isotropic phase than would be expected for a flexible polymer. Viscoelasticity of the nematic and isotropic phases show very weak temperature dependences, b ut the viscoelastic response of the biphase is extremely temperature s ensitive due to the first-order phase transition. The Cox-Merz rule wa s tested and found to apply in the isotropic phase and fail in the nem atic unless the sample is sheared just prior to the dynamic analysis.