Effect of Bulkiness of Pendent Side Groups on the Rheology of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers

Abstract

The effect of bulkiness of pendent side groups on the rheology of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) was investigated. The TLCPs investigated have aromatic triads with 10 methylene groups as flexible spacer and varying pendent side groups: ethoxy group, tert-butyl group, and phenylsulfonyl group. The TLCP having ethoxy pendent side groups (PEHQ10) undergoes smectic−nematic transition at 148 °C and nematic−isotropic (N−I) transition at 239 °C, the TLCP having tert-butyl pendent side groups (PTHQ10) undergoes only N−I transition at 193 °C, and the TLCP having phenylsulfonyl pendent side groups (PSHQ10) undergoes only N−I transition at 179 °C. Comparison of the rheological behavior of the three TLCPs was made at 40 °C below the N−I transition temperature (TNI) and at 15 °C above the TNI. The following observations were made. Upon startup of shear flow all three TLCPs exhibited a large overshoot in shear stress (σ) followed by monotonic decay and a very large overshoot in first normal stress difference (N1) followed by oscillatory decay. The peak values of overshoot in both σ and N1 increased with increasing bulkiness of pendent side groups, and only positive N1 was observed over the entire range of shear rates and temperatures investigated. Upon cessation of steady-state shear flow the relaxation rate of both σ and N1 decreased with increasing bulkiness of pendent side groups, and during rest after cessation of steady-state shear flow the rate of increase in reduced dynamic loss modulus was fastest in PTHQ10 and slowest in PSHQ10. In the nematic state the differences in bulkiness of pendent side groups played an important role, while in the isotropic state the differences in molecular weight played an important role in determining the viscosities of the TLCPs.