RHEOLOGY AND FLOW-INDUCED LIQUID-CRYSTAL PHASE-TRANSITIONS IN THERMOTROPIC POLYETHERS

This work investigates the interdependence of the phase behaviour, vis cosity, temperature, molecular weight and shear rates of thermotropic liquid crystalline polyethers. The viscosity of the isotropic and nema tic phases are quantitatively compared; a positive variation in viscos ity with respect to temperature is found, with the isotropic viscosity being about an order of magnitude higher than the liquid crystalline viscosity. The dependence of viscosity upon molecular weight of well d efined fractions is investigated in both the liquid-crystal and isotro pic phases. In the liquid crystalline state the viscosity scales with M3.5-5. Variations in the viscosity due to temperature changes affect the isotropic phase more than the liquid crystal phase. No evidence fo r a negative first normal stress difference is seen. Finally, it is sh own how the phase diagram of the material can be altered by shearing t he material in the isotropic phase. This is evident by the onset of a shear thinning region at temperatures slightly above T(i), which can b e attributed to the formation of shear induced liquid crystallinity.