Abstract
The flow properties of several polymer melts have been investigated in steady shear flow and when shear flow was applied or removed. The results have been compared with the predictions of three rheological equations of state which have been proposed to describe the behaviour of elastic fluids. In steady shear flow the viscosity eta and the normal stress differences alpha1 and alpha2 have been determined for two polyisobutylenes (Vistanex LM-MH and Vistanex LM-MS) and a depolymerised natural rubber (Lorival R25). There is some disagreement about the magnitude of alpha2 and the main objective was to determine if it was zero or not. The data were obtained from total thrust and torque measurements in a Weissenberg Rheogoniometer using cone-plate and parallel plate geometries. For the polyisobutylenes alpha2 was negative but small compared with alpha1; the ratio |alpha[2]/alpha[1]| was less than 0.3. The Lorival R25 data were not inconsistent with these conclusions. Attempts have been made to measure stress relaxation on the cessation of shear flow and stress growth at the onset of shear flow. The measurements were carried out on the Weissenberg Rheogoniometer. For Vistanex LM-MH the form of the relaxation depended on the stiffness of the torque measuring system. This is shown to be due to illdefined boundary conditions for transient measurements with high viscosity fluids. However reliable data could be obtained on the lower viscosity polyisobutylene (Vistanex LM-MS) provided a stiff torsion bar was used. For this material the shear stress relaxation and growth were more rapid as the shear rate increased. The normal stress transient measurements were unreliable but it appeared that relaxation and growth times for p[21] were less than for alpha1. A cone-plate rheometer, which can measure the steady shear viscosity and elastic recovery of polymer melts, is described. The form of the recovery curve can also be obtained. A constant shear stress is applied to the sample and the resultant rotation and recovery when the stress is removed are measured by a capacitance technique. Data obtained on the two polyisobutylenes are reported. It is shown that the Weissenberg and Lodge theories of elastic recovery are not valid. Die swell measurements have been carried out on an elastic fluid (Lorival R25) and a high viscosity Newtonian fluid (Paralac 385, a modified alkyd resin). The measurements were made on a capillary rheometer at shear rates below 1 sec[-1]. Both fluids showed a significant amount of die swell. For the Newtonian fluid the average die swell was 13.5% and independent of the capillary radius, viscosity and volume rate of flow. Die swell increased with shear rate for the elastic fluid but did not depend on the capillary radius, and at low shear rates was asymptotic to the Newtonian value. It is shown that the momentum balance theory of die swell is not appropriate to high viscosity fluids of any type. The data obtained on the two polyisobutylenes have been compared with the predictions of the WJFLMB, OWFS and Kaye integral rheological equations of state. The OWFS theory is not an appropriate model for high viscosity elastic fluids such as polymer melts. It predicts elastic recovery values which are much too high and in stress growth experiments it is incapable of predicting stress overshoot which has been reported in the literature. The agreement between theory and experiment for the WJFLMB and Kaye models is reasonable for steady shear flow, stress relaxation, stress growth and total elastic recovery. However these theories do not describe the form of the recovery curve well. Both the WJFLMB and Kaye models predict stress overshoot in stress growth experiments at high shear rates. There is some evidence that the Kaye theory might be a more useful model than the WJFLMB theory in some applications.