Effect of molecular weight on the electrorheological 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 side-chain liquid crystal polymer (SCLCP) with an 11-methylene spacer. We observe a weak enhancement of the ER response, Δη[equals]δηon−δηoff, where δηon is the viscosity increment on dissolving polymer in the presence of a strong electric field, applied transverse to the flow, and δηoff is the corresponding increment in the absence of the field. Equating ηon and ηoff, respectively, to the Miesowicz viscosities ηc and ηb, and using the theoretical prediction that δηc/δηb[equals]R∥4/R⊥4, where R∥ 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, τr, 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 τr. The difference is ascribed to dissimilar chain conformations of the two SCLCPs.