Processing preshear and orientation effects on the rheology of an LCP melt

Abstract

Independent LCP melt rheological measurements were performed using both capillary and slit rheometer die equipment. The latter was attached to the end of a laboratory extruder so that the attending rheological measurements were obtained under actual processing conditions. The viscosity–shear rate behavior determined by these two methods for the subject LCP melts was significantly different. For a specified melt temperature and shear rate, the viscosity measured by the slit rheometer was as much as 10× lower than that measured by capillary rheometry. The above measured viscosity differences appear real since corresponding viscosity measurements on conventional polymers such as LLDPE and HDPE by these two methods produced equivalent results. Thus, the rheology of these LCP melts is strongly dependent upon preshear history, die geometry, and residence time. This dependence is attributed to the highly anisotropic nature of the LCP melt, the tendency of the polymer ‘‘domains’’ or chains to orient when processed and a long relaxation time for the oriented structure. Knowledge of LCP rheological behavior under actual processing conditions is of paramount importance to properly design extruder or injection molding screws, as well as corresponding dies and molds.