TEMPERATURE AND MOLECULAR-WEIGHT DEPENDENCE OF THE VISCOMETRIC PROPERTIES OF MAIN-CHAIN LIQUID-CRYSTAL POLYMERS IN NEMATIC SOLVENTS

The twist and bend viscosities of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (50CB) and in 4'-pentyl-4-cyanobiphenyl (5CB) of the main-chain liquid crystal polymers, TPB10 and TPB13, which have a mesogenic group, 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane, separated, respectively, by flexible decamethylene and tridecamethylene spacers, were determined via dynamic light scattering analysis. A theoretical expression for the increment of twist viscosity, gamma1, derived by Brochard can be applied to interpret the experimental results in terms of changes in the mean square end-to-end distances parallel (R(parallel-to)2 to the and perpendicular (R(perpendicular-to)2 to the nematic director. We find that the hydrodynamic behavior of TPB10 oligomers moves closer to that predicted for a rigid rod as the temperature decreases. By modeling the oligomers as a mixture of configurational isomers, the hairpin probability per repeat unit and the associated activation energy can be found. The latter is approximately twice the energy barrier of a trans to gauche transformation. As the molecular weight is increased from oligomers to polymers a decrease in the exponent characterizing the contour length dependence of the intrinsic twist and bend viscosities, [gamma1] and [eta(bend)], is found, emblematic of a crossover from rigid rod to biased random walk behavior. The effect of absolute temperature on the viscometric properties of TPB13 polymer (GPC determined M(w) = 79100 g/mol) was studied by comparing the viscosity in high and low temperature nematic solvents at equal order parameters, viz. TPB13/5CB at T = 27.3-degrees-C and TPB13/50CB at T = 62.0-degrees-C. A large increase in [gamma1] but negligible change in [eta(bend)] was found on decreasing temperature.