SELF-NUCLEATION AND RECRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE (ALPHA-PHASE) INVESTIGATED BY DIFFERENTIAL SCANNING CALORIMETRY

The crystallization behavior after partial or complete melting of the a phase of iPP is examined by combined differential scanning calorimetry (DSC) and optical microscopy: calorimetric results are directly correlated with corresponding morphologies of microtome sections of DSC samples. On partial melting at various temperatures (hereafter referred to as T(s)) located in a narrow range (4-degrees-C) below and near T(m), the number of nuclei increases (as in classical self-nucleation experiments), by several orders of magnitude; on subsequent cooling, the crystallization peak is shifted by up to 25-degrees-C. After partial melting in the lower part of the T(s) range and recrystallization, the polymers display a prominent morphology ''memory effect'' whereby a phantom pattern of the initial spherulite morphology is maintained. After partial melting in the upper part of the T(s) range the initial morphology is erased and self-nucleation affects only the total number of nuclei. The present experimental procedures make it possible to define, under ''standard'' conditions, the crystallization range of the polymer and in particular, the maximum crystallization temperature achievable when ''ideally'' nucleated. (C) 1993 John Wiley & Sons, Inc.