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

Abstract

The Miesowicz viscosities ηc and ηb 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 ηc ≫ ηb, 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, [ηc], of TPB10 in 5OCB, was found to follow a Mark−Houwink−Sakurada relationship, [ηc] = KMα, 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 [ηc] and [ηb] for TPBx and a hyperbranched LCP, TPD-b-8, based on a similar mesogen. From the ratio [ηc]/[ηb], via the Brochard model, the ratio (R∥/R⊥) 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.