MOLECULAR WEIGHT-DEPENDENT BEHAVIOR OF THE TWIST DISTORTION IN A NEMATIC MONODOMAIN CONTAINING A MAIN-CHAIN LIQUID-CRYSTAL POLYMER

The twist viscoelastic coefficients of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (5OCB) of a main-chain liquid crystal polymer, TPB-10, which has a mesogenic group, 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)butane, separated by flexible decamethylene spacers, were determined over a wide range of molecular weight using electric field-dependent dynamic light scattering. A Mark-Houwink-Sakurada relationship [eta] = KM(v)alpha was found for the intrinsic twist viscosity, with different exponents for oligomeric and polymeric chains. The oligomer behavior is interpreted as comparable to that of a free-draining rigid rod, while the polymer chains appear to approach the free-draining random coil limit. By invoking the wormlike chain model, the chain persistence length is determined. In addition, the configurational anisotropies of the polymers can be estimated. The apparent axial ratio of the chain decreases with molecular weight and then becomes constant. The results are thus consistent with the theoretical prediction for the crossover from rigid rod to a biased random coil. Finally, a model calculation for the dimer, trimer, and tetramer leads to an estimate of the probability of a hairpin turn per repeat unit.