MELTING BEHAVIOR OF CONTROLLED RHEOLOGY POLYPROPYLENE

The melting behaviour of reactor-made isotactic polypropylene (PP) and various controlled rheology polypropylene (CR-PP) samples crystallized under both isothermal and non-isothermal conditions were studied by differential scanning calorimetry and the crystal structure of isothermally crystallized samples at 120-degrees-C was investigated using wide angle X-ray diffraction. Multiple endotherms were observed dependent on the molecular weight of the samples, heating rate and cooling rate, and method of crystallization. Recrystallization or reorganization occurred more easily for low molecular weight samples and also for fast cooled samples during the reheating and the melting process. The isothermally crystallized CR-PPs produced by peroxide controlled degradation showed a small melting endotherm peak at approximately 143-degrees-C which is probably connected to the beta-form crystal, while unreacted PP showed the alpha-form crystal only. The small endotherm peak was more prominent in one CR-PP sample of intermediate molecular weight. Attempts were made to explain this behaviour through the observed maximum in the half-time of crystallization versus molecular weight plot.