Crystal transformation behavior and structural changes of the planar zigzag form for syndiotactic polypropylene

Crystal transformation behavior and structural changes for the planar zigza g form (form III) of syndiotactic polypropylene, which is spontaneously cry stallized at 0 degrees C from the melt, have been investigated in the heati ng process by DSC, wide-angle X-ray diffractometry, and high-resolution sol id-state C-13 NMR spectroscopy. The DSC curve for the sample containing onl y form III crystallites, which is crystallized at 0 degrees C for 5000 h, s hows an evident endothermic peak at about 20 degrees C and an additional ex othermic peak at about 100 degrees C prior to melting at about 150 degrees C. The endothermic peak, which is shifted up to about 40 degrees C by annea ling at room temperature, is found to be ascribed to the crystal transforma tion from form III with the planar zigzag conformation to form II composed of isochiral helices with the ttgg conformation by wide-angle X-ray diffrac tometry. Moreover, the CH3 resonance lines of the dipolar decoupling/MAS C- 13 NMR spectra measured at different temperatures for the sample with almos t the same structure as for DSC and WAXD measurements are well resolved int o the components with the tt and tg conformations for the CH2-CH(CH3)-CH2 b ond. As a result, it is found that about 16% form III is allowed to be tran sformed into form II, whereas most of form III is melted gradually in the t ransition temperature region and very rapidly above that temperature. Such melting of form III should induce the crystallization of form I above about 60 degrees C, and the final degree of crystallinity of form I attains to a lmost the same level as for the original form III. This process is really r eflected on the appearance of the exothermic peak at about 100 degrees C in the DSC curve described above. An annealing effect of form III crystallite s has also been examined just below the crystal transformation temperature from form III to II.