Relationship between crystallization regimes and melting phenomena in isotactic polypropylene

A study has been made of the relationship between crystallization regimes and multiple peaks in the melting endotherms of isotactic polypropylene (iPP). Some samples of this polymer can show multiple melting peaks in differential scanning calorimetry (DSC) scans: this phenomenon has been studied and associated with a regime III of crystallization, giving rise to a phase which appears disordered from a crystallographic point of view. During melting, this phase recrystallizes, giving a more ordered phase which melts at a higher temperature. On the contrary, crystallization in regime II produces a crystalline phase which melts with a single endotherm. These phenomena have been studied by DSC, by optical microscopy in polarized light and by X-ray diffraction; the results are discussed in terms of the proposed interpretation.