TRANSIENT RHEOLOGICAL BEHAVIOR OF A THERMOTROPIC LIQUID-CRYSTALLINE POLYMER .2. INTERMITTENT SHEAR-FLOW AND EVOLUTION OF DYNAMIC MODULI AFTER CESSATION OF SHEAR-FLOW

The intermittent shear flow behavior in the nematic state of a well-characterized thermotropic liquid-crystalline polymer, poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylene-bis (4-oxybenzoate)] (PSHQ10), was investigated using a cone-and-plate rheometer. For the study, PSHQ10 was synthesized in our laboratory. In order to erase the thermal history associated with polymerization and sample preparation and control initial morphology, an as-cast specimen was first heated to the isotropic region (190 degrees C), sheared there at a rate of 0.085 s(-1) for 10 min, and then cooled slowly down to a predetermined nematic state (130, 140, 150, or 160 degrees C). A fresh specimen was used for each intermittent shear flow experiment, thus circumventing the effect of previous shear history on subsequent transient theological responses. Upon startup of shear flow, the growths of shear stress sigma(+)(t,gamma) and first normal stress difference N-1(+)(t,gamma) were recorded as functions of applied shear rate, temperature, and rest period. We found that the peak values of sigma(+)(t,gamma) and N-1(+)(t,gamma), respectively, increased with increasing rest period and applied shear rate, and with decreasing temperature. In order to facilitate an analysis of the data obtained under different experimental conditions, the transient stresses were rescaled using the equilibrium value as a reference. The time evolution of dynamic storage and loss moduli (G' and G'') after cessation of shear flow was monitored, by applying small amplitude oscillatory deformations to the specimens. We found that the value of G'' increased with time, the rate of which becoming greater with increasing applied shear rate, while the value of G' remained more or less constant, for a rest period of 150 min.