Effect of annealing in the nematic phase on the transient shear response of thermotropic liquid-crystalline polymers

Abstract

The effect of isothermal annealing, prior to start-up of shearing flow, in the nematic region on transient shear responses was investigated using a glassy thermotropic liquid-crystalline polymers (TLCP), PSHQ9, and a crystalline TLCP, PSHQ10. It was observed that the peak value of first normal stress difference (N1,peak+) of PSHQ9 remains constant, independent of the duration of annealing prior to start-up of shearing flow, whereas the N1,peak+ of PSHQ10 increases steadily with increasing period of annealing in the nematic phase prior to start-up of shearing flow. On the other hand, it was observed that the peak value of shear stress (σpeak+) remains constant in both PSHQ9 and PSHQ10, independent of the duration of annealing in the nematic phase prior to start-up of shearing flow. The observed dependence of N1,peak+ for PSHQ10 on the duration of isothermal annealing in the nematic phase is found, via differential scanning calorimetry, to be attributable to the perfection (or growth) of the crystals in PSHQ10 during annealing prior to start-up of shearing flow. The experimental observations made from this study suggest that glassy TLCPs be used to obtain reproducible transient shear responses.