STUDIES ON THE alpha AND beta FORMS OF ISOTACTIC POLYPROPYLENE BY CRYSTALLIZATION IN A TEMPERATURE GRADIENT.

Abstract

In the reported experiments samples of isotactic PP were zone-solidified in temperature gradients up to 300 degree C/cm at growth rates down to 3 mu m/min. Oriented alpha -type spherulites were obtained only by nucleation. While beta nucleation is extremely rare, the beta phase is easily initiated by growth transformations along the oriented alpha front. Since the beta phase was found to grow considerably faster than the alpha phase, the alpha -to- beta transformation points diverge across the sample, interrupting growth of the oriented alpha fibrils. This causes subsequent nucleation to yield teardrop-shaped alpha spherulites. Differential scanning calorimetry (DSC) studies of zone-solidified PP show the beta -phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt - all of which tend to suppress nucleation. Differential thermograms of largely beta -PP obtained at a heating rate of 1 omicron C/Min show the actual melting and recrystallization of the beta -spherulites into the alpha form.