Effect of molecular weight on the rheological behavior of thermotropic liquid-crystalline polymer

The effect of molecular weight on the rheological behavior of a thermotropic liquid-crystalline polymer was investigated. An aromatic polyester, poly\(phenylsulfonyl)-p-phenylene 1,10-decamethylenebis(4-oxybenzoate)\ (PSHQ10) having different molecular weights was synthesized. The PSHQ10s synthesized were found to have (a) a weight-average molecular weight (M sub w ), as determined by gel permeation chromatography, ranging from 33 000-45 000 relative to polystyrene standards, and a polydispersity index of about two, and (b) a melting point ranging from 100-115 deg C and a nematic-to-isotropic transition temperature ranging from 161-176 deg C, depending upon the molecular weight. Using a cone-and-plate rheometer, the steady and oscillatory shear flow properties of the PSHQ10s were measured in both the isotropic and nematic regions. It was found that (a) eta sub 0 proportional to M exp 6.5 and negligible N sub 1 in the isotropic region, and (b) eta sub 0 proportional to M exp 6 and N sub 1 proportional to M exp 6.7 in the nematic region, where M is the molecular weight, eta sub 0 is the zero-shear viscosity and N sub 1 is the first normal stress difference. Further, it was found that preshearing has a profound influence on both the steady and oscillatory shear properties of PSHQ10. Also investigated were the transient start-up shear flow, stress relaxation, and structural recovery after cessation of steady shear flow. Structural recovery was determined by monitoring the variations of dynamic storage and loss moduli, during application of small-amplitude oscillatory deformations to the specimens. The following observations were made: (1) the maximum overshoot in both shear stress and the first normal stress difference increased with increasing molecular weight; (2) the rate of stress relaxation after cessation of shear flow was slower with increasing molecular weight; and (3) the extent of structural recovery after cessation of shear flow was greater for increasing molecular weight.