RHEOLOGICAL PROPERTIES OF A HOMOGENEOUS THERMOTROPIC LIQUID-CRYSTALLINE POLYESTER - DYNAMIC VISCOELASTIC AND INTERRUPTED-FLOW MEASUREMENTS

Dynamic viscoelastic and shear stress properties at various shear rates with intermediate rest periods of varying length were measured for a thermotropic nematic liquid-crystalline polyester of homogeneous backbone structure, with a long flexible spacer. Data obtained in the liquid-crystalline and isotropic states were found to independently superimpose when shifted along the omega axis. In the low-frequency region of the liquid-crystalline state, data exhibit log G' and log G'' values less-than-or-equal-to 1 when plotted vs log omega. Properties for the isotropic state correspond to other wide molecular weight distribution melts except here, and in the liquid-crystalline state, eta(gamma) < \eta*(omega)\. While interrupted-flow (I-F) results in the isotropic state were similar to other isotropic materials, in the liquid-crystalline state at 145-degrees-C and similarly at 160-degrees-C, previously unsheared samples sheared at omega = 0.1 s-1 showed a stress maximum, accompanied by a transient negative first normal stress difference peak and later by a positive N1 peak. At 160-degrees-C, omega = 0.5 s-1, a second stress overshoot appeared, as did another positive N1 peak. When compared with optical micrographs of sheared samples, the first peak may be related to initial domain structure deformation, not reappearing in subsequent shear events after short rest periods. The second stress overshoot that appears with higher shear rates appears to correspond to the complete breakdown of these domains, producing a birefringent yet featureless appearance. Structure reappears with longer rest periods but differs from initial texture. If sheared, this structure is capable of producing an initial stress overshoot.