Morphology and mechanical behavior of isotactic polypropylene with different stereo-defect distribution in injection molding

Abstract

Large amount of work has been published on the isotacticity–properties relationship of isotactic polypropylene (iPP). However, the stereo-defect distribution dependence of morphology and mechanical properties of iPP injection molding samples is still not clear. In this study, two different isotactic polypropylene (iPP) resins (PP-A and PP-B) with similar average isotacticity but different stereo-defect distribution were selected to investigate the morphology evolution and mechanical properties (tensile and notching) of their injection molding samples using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), 2D wide angle X-ray diffraction (2D-WAXD), and scanning electron microscope (SEM). The results of DMA showed that the molecular movement ability of PP-A (with less uniform distribution of stereo-defect) was stronger than that of PP-B, meanwhile the analysis of DSC and SEM suggested that after injection molding, smaller spherullites, and crystals with higher perfection had formed in the specimens of PP-A. The resulting of tensile properties of PP-A were found to be better than that of PP-B. The results of morphology evolution by SEM observation and 2D-WAXD showed that PP-A is more likely to occur interspherulite deformation and can disperse the tensile stress more efficiently, and therefore, its crystal structure can withstand a greater force when tensile stress is applied. On the other hand, PP-B has larger spherulites and boundaries, and low perfection of lamellaes, and the intraspherulte deformation tend to take place. It is easier for the crystal of PP-B to be broken up and reoriented along the tensile direction. Copyright © 2014 John Wiley & Sons, Ltd.