Electrorheological Behavior of Main-Chain Liquid Crystal Polymers Dissolved in Nematic Solvents

The Miesowicz viscosities η<INF>c</INF> and η<INF>b</INF> of dilute nematic solutions of the main-chain liquid crystal polymer (LCP) TPBx, in 4‘-(pentyloxy)-4-cyanobiphenyl (5OCB) or 4‘-pentyl-4-cyanobiphenyl (5CB) as nematic solvents, were measured by cone-and-plate rheometry in the presence and absence, respectively, of an external electric field. TPBx has a mesogenic group, 1-(4-hydroxy-4‘-biphenyl)-2-(4-hydroxyphenyl)butane, separated by flexible n-alkyl spacers of variable length x. Since for these solutions η<INF>c</INF> ≫ η<INF>b</INF>, a pronounced electrorheological effect is observed, the viscosity with the field on being an order of magnitude larger than that with the field off. The intrinsic viscosity, [η<INF>c</INF>], of TPB10 in 5OCB, was found to follow a Mark−Houwink−Sakurada relationship, [η<INF>c</INF>] = KM^α, with α ≈ 1.0. Applying a theoretical description by Brochard, this result suggests that TPB10 behaves hydrodynamically in 5OCB like a free-draining random coil stretched along the director. Comparisons were made of [η<INF>c</INF>] and [η<INF>b</INF>] for TPBx and a hyperbranched LCP, TPD-b-8, based on a similar mesogen. From the ratio [η<INF>c</INF>]/[η<INF>b</INF>], via the Brochard model, the ratio (R<INF>∥</INF>/R<INF>⊥</INF>) of the end-to-end distances of the LCP measured parallel and perpendicular to the nematic director were found to be ~2−2.5 for TPBx and ~1.45 for TPD-b-8, consistent with the expectation that the chain anisotropy of the branched species in the nematic state is smaller.