Effect of Stereodefects Distribution on Rheological Behavior of Isotactic Polypropylene

Abstract

A series of five isotactic polypropylenes (iPP) used commercially for producing biaxial oriented polypropylene film (BOPP), which had similar average degrees of isotacticity but different stereodefects distribution, was studied. Xylene solubles (XS) at different temperatures and high temperature solution 13C NMR were used to characterize the stereodefects distribution of the five samples. These samples gradually became more random following the order of sample A to E. The plateau modulus (G 0 N ≈ 0.2 MPa at 190°C) of these samples was lower than those of metallocene-catalyzed polypropylenes with very narrow molecular weight distributions (MWD) (G 0 N ≈ 0.4 MPa at 190°C), indicating that the influence of MWD on G 0 N cannot be neglected. The storage modulus and complex viscosity of the five samples decreased gradually with the widened stereodefects distribution, due to the formation of a less constrained network that finally caused the reduction of G 0 N and the increase of the flow activation energy. The rheological behavior of the BOPP resin samples could be well related to the results of XS and 13C NMR, implying that the rheological measurement can be adopted as a simple and efficient method to characterize the stereodefects distribution of isotactic polypropylene.