Transient shear rheology and rheo-optical microstructural characterization of a thermotropic liquid crystalline polymer

The shear rheology and the corresponding microstructure evolution during transient flow of a thermotropic liquid crystalline polymer (LCP; Vectran V400P) are reported; the polymer does not display a crystalline melting phenomenon. The steady shear viscosity displays three regions that are typical of LCPs. However, the transient shear stress displays a local stress maximum at a strain of approximate to 2, a local minimum at a strain of approximate to 20, and then a stress overshoot at a strain of approximate to 100 before attaining a steady state value. The transient first normal stress difference exhibits a distinct overshoot at approximate to 100 strain units. The steady-state first normal stress difference is always positive in the tested shear rate range 0.1-10 s(-1). In situ rheo-optical characterization revealed that the melt shows a threaded texture at rest. Upon start up of shear flow, this threaded texture becomes deformed, and the domains initially appear to stretch and align in the shearing direction. Next, the domains break up, and a significant drop in the optical intensity is observed. These microstructural features are used to explain the presence of the first maximum and the minimum observed in the transient shear stress. (C) 2005 Society of Plastics Engineers.