Mechanical pretreatment of polymer melts: Critical aspects and new rheological investigations on a linear and a long-chain branched polypropylene

Abstract

It has been known for some time that mechanical pretreatments of melts of long-chain branched polymers can increase the melt index and reduce the extrudate swell. This effect is not due to changes of the molar mass, its distribution, or the degree of branching because it may be reversed by long-term annealing or dissolution of the polymeric material and subsequent precipitation. Furthermore, molecular analysis shows a molecular structure unaltered by the treatment. The effect has not been completely understood up to now, and it is still an open question as to whether it is restricted to long-chain branched polymers, which have been preferably studied. This paper gives an overview of results from the literature and presents comparable measurements on a well characterized long-chain branched and a linear polypropylene. A wide range of linear and nonlinear rheological properties were studied, and it became obvious that long-chain branching is a strong structural precondition for the pronounced pretreatment effect occurring in processing. Based on this result, models from the literature are discussed, and it is found that chain alignment may qualitatively explain the rheological measurements.