Dynamic and Transient Flow Behavior of Thermotropic Homopolyester with Nonamethylene Flexible Unit

Abstract

Rheological properties of a semiflexible polyester having nonamethylene flexible spacer were investigated by small amplitude oscillatory shear flow and intermittent forward flow both in liquid crystalline state and in isotropic state. In dynamic measurements, it was found that the slope of the modulus versus, frequency in terminal region was uniquely determined by the intrinsic nature of each phase and the slope was higher in isotropic state than in liquid crystalline state. The viscoelastic data measured in the isotropic state at different temperatures can be drawn on a master curve, which reflects the homogeneous nature of the phase. The complex viscosity showed Arrhenius type decrease with increasing temperature in isotropic phase. In transient experiments, the large stress overshoot and negative N1 undershoot were observed upon start-up of shear flow in the liquid crystalline state. The negative N1 undershoot was significantly affected by the shear history and did not reappear under subsequent flow even after long rest. The stress relaxed almost instantly after cessation of shear for all shear rates, implying that high structural distortion was introduced by shear flow. The large stress overshoot was not observed in isotropic state due to homogeneous structure.