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.

Newtonian Viscosity of Dilute, Semidilute, and Concentrated Polymer Solutions

“…Subsequently, more detailed studies were conducted on nematic solutions of a MCLCP [Chiang et al, 1997b[Chiang et al, , 2000 and an end-on SCLCP [Yao and Jamieson, 1997;Chiang et al, 2002]. First, we note that Eqs.…”

“…(1.82) and (1.83) apply strictly only in the limit of infinite dilution; hence, the experimental values of δη on /c and δη on /c must be extrapolated to c = 0. With this modification, ER experiments were conducted [Chiang et al, 1997b[Chiang et al, , 2000] on a MCLCP designated TPBn which has mesogenic groups, 1-(4-hydroxy-4-O-bisphenyl)-2-(4-hydroxyl phenyl)butane, linearly linked by flexible oligomethylene (n-mer) spacers. For TPB10 dissolved in 5OCB at several temperatures in the nematic range, the Mark-Houwink relation [η c ] = kL a w , where L w is the weight-average contour length of the polymer, was determined.…”

Electrorheological Fluids

“…Those liquid-crystal polymers include 4-n-pentyl-4¢-cyanophenyl, [100] poly(n-hexyl isocyanate), [101,102] poly(1-(4-hydroxy-4¢-biphenyl)-2-(4-hydroxyphenyl)-butane, [103] and poly(c-glutamate). [104] Many other polymers can be found in the literature.…”

Influence of shear flow on the linear response of a nematic liquid crystal to external electric fields

“…The shear viscosity can be easily controlled by applying external fields such as electric and magnetic fields, which change the orientation of the director (the average molecular direction) with respect to the flow velocity and its gradient [1][2][3][4][5][6]. For example, some nematic liquid crystals with positive dielectric anisotropy (where the dielectric constant parallel to the director is larger than that perpendicular to it) show a substantial increase in apparent viscosity upon application of an external electric field owing to the alignment of the director along the field.…”