Electro-rheological behavior of liquid crystal polymers (LCPs) dissolved in a nematic solvent: dependence on temperature and LCP structure

The viscosity increments delta eta(on) and delta eta(off), upon dissolving a liquid crystal polymer (LCP) in a thermotropic nematic solvent, were determined by rheometry in the presence and absence, respectively. of a saturation electric field. Four species of molecular architectures were studied. a rigid main-chain LCP, two types of flexible main chain LCP, one with mesogens oriented perpendicular to the backbone, the other with mesogens parallel to the backbone, and a flexible side-chain LCP with mesogens attached side-on to the backbone. For all mixtures, delta eta(on) and delta eta(off) exhibit a similar Arrhenius dependence on temperature, determined by the viscous activation energy of the nematic solvent. However, the temperature dependence of the ratio delta eta(on)/delta eta(off) and of the intrinsic viscosities, [eta(on)] and [eta(off)], is different for each LCP, and is strongly dependent on the flexibility of the LCP. By equating eta(on) and eta(off) to the Miesowicz viscosities, eta(c) and eta(b), respectively, these results can be interpreted in terms of the ratio of the LCP chain dimensions parallel and perpendicular to the nematic director, R-parallel to and R-perpendicular to (since theory predicts delta eta(c)/delta eta(b) = R-parallel to(4)/R-perpendicular to(4)). The results indicate that the conformation of each LCP is strongly prolate, i.e. R-parallel to much greater than R-perpendicular to. For the rigid main-chain LCP, R-parallel to/R-perpendicular to, changes very little with temperature; for the flexible LCPs, R-parallel to/R-perpendicular to, decreases strongly with increase of temperature, indicative that the conformation becomes more spherical. The behavior of R-parallel to/R-perpendicular to, is compared with recent theoretical models of LCP conformation. (C) 2000 Elsevier Science Ltd. All rights reserved.