Effect of Molecular Weight on the Electrorheological (ER) Behavior of Side-Chain Liquid Crystal Polymers In Nematic Solvents

We investigate the change in the electrorheological (ER) properties of a nematic solvent on dissolution of an end-on polyacrylate sidechain liquid crystal polymer (SCLCP) with an 11-methylene spacer. We observe a weak enhancement of the ER response, Dh ¼ dhon 2 dhoff; where dhon is the viscosity increment on dissolving polymer in the presence of a strong electric field, applied transverse to the flow, and dhoff is the corresponding increment in the absence of the field. Equating hon and hoff, respectively, to the Miesowicz viscosities hc and hb; and using the theoretical prediction that dhc=dhb ¼ R 4=R 4; where Rk and R’ are the rms end-to-end distances of the chain parallel and perpendicular to the director, this indicates that the chain conformation of the SCLCP is weakly prolate. The molecular weight dependence of the conformational relaxation time, tr, also extracted via the hydrodynamic theory, is identical to that deduced from the GPC hydrodynamic volume in tetrahydrofuran as solvent. The hydrodynamic volume in the nematic state is larger, however, and increases with decrease in temperature. An earlier study of a polysiloxane SCLCP observed similar behavior, but with a smaller molecular weight dependence of tr. The difference is ascribed to dissimilar chain conformations of the two SCLCPs. q 2002 Elsevier Science Ltd. All rights reserved.