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 calorimet ry (DSC) and optical microscopy: calorimetric results are directly cor related with corresponding morphologies of microtome sections of DSC s amples. 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 ex periments), 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 p hantom pattern of the initial spherulite morphology is maintained. Aft er partial melting in the upper part of the T(s) range the initial mor phology is erased and self-nucleation affects only the total number of nuclei. The present experimental procedures make it possible to defin e, under ''standard'' conditions, the crystallization range of the pol ymer and in particular, the maximum crystallization temperature achiev able when ''ideally'' nucleated. (C) 1993 John Wiley & Sons, Inc.