Effects of stereo-defect distribution on the crystalline morphology and tensile behavior of isotactic polypropylene prepared by compression molding process

Abstract

ABSTRACTIn this study, the aggregation morphology, tensile behavior, and morphology evolution during the tensile test of two isotactic polypropylene (iPP) samples with similar molecular weight and average isotacticity but different uniformities of stereo-defect distribution are investigated by differential scanning calorimetry (DSC), two-dimensional wide angle X-ray diffraction (2D-WAXD), and scanning electronic microscopy (SEM). The results revealed that the uniformity of stereo-defect distribution of iPP determines the crystalline structure and aggregation morphology, and further influences the tensile behavior and morphology evolution during the tensile test. For PP-A with less uniform stereo-defect distribution, its ability of crystallization is stronger compared with PP-B, resulting in smaller spherulite sizes, higher melting point and degree of crystallinity, and narrower distribution of lamellar thickness of the compression molding specimens. During the tensile test, mainly the inter-spherulite deformation takes place at the early stage for deformation, which further results in drastic deformation of lamellar and high degree of reorientation at the strain increases, exhibiting higher yield strength and elastic modulus, and lower elongation at break compared with PP-B; for PP-B with more uniform stereo-defect distribution, larger spherulite sizes, lower melting point and degree of crystallinity in its compression molding sample are observed. During the tensile test, intra-spherulite deformation mainly takes place, which can disperse the tensile stress more uniformly. As the strain increases, lower degree of crystalline destruction and reorientation of the crystallites take place. The yield strength and elastic modulus of PP-B is lower than PP-A, and its elongation at break is higher.