Crystal Transformation Behavior and Structural Changes of the Planar Zigzag Form for Syndiotactic Polypropylene

Abstract

Crystal transformation behavior and structural changes for the planar zigzag form (form III) of syndiotactic polypropylene, which is spontaneously crystallized at 0 °C from the melt, have been investigated in the heating process by DSC, wide-angle X-ray diffractometry, and high-resolution solid-state 13C NMR spectroscopy. The DSC curve for the sample containing only form III crystallites, which is crystallized at 0 °C for 5000 h, shows an evident endothermic peak at about 20 °C and an additional exothermic peak at about 100 °C prior to melting at about 150 °C. The endothermic peak, which is shifted up to about 40 °C by annealing at room temperature, is found to be ascribed to the crystal transformation from form III with the planar zigzag conformation to form II composed of isochiral helices with the ttgg conformation by wide-angle X-ray diffractometry. Moreover, the CH3 resonance lines of the dipolar decoupling/MAS 13C NMR spectra measured at different temperatures for the sample with almost the same str...