Influence of Shear Flow on the Specific Volume and the Crystalline Morphology of Isotactic Polypropylene

Abstract

The influence of shear flow on the temperature evolution of the specific volume and the crystalline morphology of two iPP's, differing in weight-averaged molar mass M̄w, was investigated at nonisothermal conditions and elevated pressures, using a custom-designed dilatometer. These conditions are typically in the range of conditions as experienced during polymer processing. A pronounced influence of flow on the temperature marking the transition in specific volume and the rate of transition could be observed. In general, this influence increased with increasing shear rate, decreasing temperature where flow was applied, increasing pressure, and increasing weight-averaged molar mass M̄w of the polymer. Although the degree of orientation and the overall structure of the resulting crystalline morphology were greatly affected by the flow, the resulting specific volume was only little affected by the thermomechanical conditions presently investigated. Finally, when flow was applied at sufficiently high temperature, in some cases the polymer was able to fully erase the effect of flow. This is attributed to remelting of the flow-induced crystalline structures.