Viscoelastic behavior of nematic monodomains containing liquid crystal polymers

Abstract

Current understanding of the relationship between molecular structure, chain configuration and the viscoelastic behavior of liquid crystal polymers (LCPs) in the nematic state is reviewed. Three classes of nematic materials are considered: thermotropic side-chain and main-chain LCPs (a) dissolved in nematic solvents and (b) in the bulk state, and (c) lyotropic solutions of rigid-rod and disc-like polymers. With regard to the first class, we discuss the effect of the addition of LCPs on the Frank elastic constants and on the nematic viscosities. The changes in viscosity are very sensitive to the molecular structure and configuration and can be interpreted via the theoretical description of Brochard. The relative magnitude and molecular weight dependence of the intrinsic twist and intrinsic bend viscosities of main-chain and side-chain LCPs are consistent with the idea that the chain configuration of the former is highly prolate (with the major axis parallel to the director) whereas the former are quasi-spherical and often slightly oblate (with the minor axis along the director). Additional viscometric evidence for this conclusion is afforded by the observation that addition of a sufficient quantity of a main-chain LCP to a ‘director-tumbling’ solvent produces a flow-aligning mixture whereas the corresponding addition of a side-chain LCP to a flow-aligning solvent produces a tumbling mixture. These results are qualitatively consistent with direct measurement of chain dimensions using small-angle scattering techniques. The twist viscosity of lyotropic solutions of the rod-like polymer, poly(benzyl glutamate), is reported to increase linearly with molecular weight whereas that of thermotropic main-chain LCPs increases as the sixth power of molecular weight, and that of thermotropic side-chain LCPs increases as the third power of molecular weight. These results are consistent with theoretical analyses which, in the first case, assume a nematic of rigid cylinders with excluded-volume interactions, and in the last two assume semi-flexible chains with entanglement dynamics. Extensive rheological and rheo-optical studies of lyotropic solutions of rod-like polymers have been reported. These experimental studies and associated theoretical modeling suggest that the director dynamics in flow depend on the initial orientation. At lowest shear rates director tumbling occurs and the director tends to orbit close to its initial orientation. At shear rates high enough that the polymer chains do not have time to relax to their equilibrium configuration, molecular elasticity has a strong influence on director dynamics in flow. At intermediate shear rates, the director is driven towards the shear plane or the vorticity axis depending on the initial orientation, leading respectively to ‘wagging’ and ‘log-rolling’ or ‘kayaking’ motions of the director. At very high shear rate the director exhibits flow-aligning behavior in the shear plane. The domain size in flow is determined by the balance between Frank gradient and viscous stresses and decreases inversely as the square root of the shear rate. The rheology of thermotropic LCPs shows a strong dependence on mechanical history. Recent observations on pre-sheared specimens of main-chain LCPs show a Newtonian region at low shear rates and a crossover to a shear-thinning region at high shear rates, similar to what is observed in lyotropics. A discrete increase in steady shear viscosity is observed at the nematic-to-isotropic transition temperature, T NI. In contrast, similar studies on a thermotropic side-chain LCP show no significant change in rheology at T NI.