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

摘要

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-chan 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) and #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 viscosites, #eta#_c and #eta#_b,respectively, these results can be interpreted in terms of the LCP chain dimensions parallel and perpendicular to the nematic director,R_|| and R_'(since theory predicts #delta##eta#_c and #delta##eta#_b=R_||~4/R_'~4).The results indicate that the conformation of each LCP is strongly prolate,i.e.R_||R_'. For the rigid main-chain LCP,R_||R_' changes very little with temperature;for the flexible LCPs,R_||R_' decreases strongly with increase of temperature,indicative that the conformation becomes more spherical.The beavior of R_||R_' is compared with recent theoretical models of LCP conformation.