ON THE LINEARITY OF THE PRESSURE-DROP DURING FLOW OF THERMOTROPIC LCPS IN SLITS AND CAPILLARIES

The flow behavior of a thermotropic liquid-crystalline polymer (LCP) w as measured in a slit rheometer with nine pressure transducers flush m ounted along its axis. Thus it was possible to measure the pressure dr op and its (non)linearity. The rheometer was provided with a restricti on at the end of the flow channel, enabling measurements at elevated p ressures. The shape of the pressure profile was found not to depend on the position in the slit but to be entirely determined by the pressur e. The model of deorientation of a LCP melt as proposed by Wissbrun [J . Rheol. 37, 777-797 (1993)] cannot account for the observed nonlinear ities. The effect of pressure on the viscosity seems a better explanat ion. The value of beta calculated from the slit rheometer measurements ranges between 7.1 x 10(-9) Pa-1 for gamma = 151 s-1 and 5.3 x 10(-9) Pa-1 for gamma = 2262 s-1, which is in the same order of magnitude as the reported values for flexible polymers. The influence of the measu ring geometry on the viscosity was investigated by comparing the visco sities of the material in the slit with the value of the viscosity mea sured in a capillary rheometer. When the pressure dependence of the vi scosities is taken into account, it is found that the viscosities as m easured in both dies have the same values. The value of beta calculate d from the slit rheometer measurements cannot describe the observed no nlinearity of the Bagley plots.