Transient and steady-state rheological behavior of the thermotropic liquid crystalline polymer Vectra B950

The shear stress and the first normal stress difference in steady and transient shear flows of the commercial thermotropic liquid crystalline polymer (LCP) Vectra B950 are measured at 300 degrees C. The viscosity curve shows the typical three region flow curve of LCPs. The first normal stress difference is everywhere positive and its steady-state value increases linearly with the shear rate throughout regions I and II. The growth curves of the shear and the normal stresses for shear rates in region II show slow oscillations that scale with strain. The stress transients during flow reversal and stepwise increase or decrease of the shear rate also scale with strain. The damping of the transients is slower than what is found in other thermotropic LCPs. The oscillations of N-1 after reversal and after stepwise increase of the shear rate do not have a clear phase difference. The relaxation of the stresses after cessation of flow shows a fast initial part and a long strain scaling tail. The elastic texture contribution to the total stress is lower than in other semiflexible systems. Either the viscous/elastic molecular contribution or an additional viscous contribution of the defects seems to be responsible for this low value. (C) 1996 Society of Rheology.