CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE AND HIGH-DENSITY POLYETHYLENE UNDER NEGATIVE-PRESSURE RESULTING FROM UNCOMPENSATED VOLUME CHANGE

The melting behavior of spherulites in thin sections of isotactic polypropylene bulk samples and high-density polyethylene thin films crystallized isothermally at various temperatures has been studied by polarized light microscopy. The regions around cavities and multiple boundary points between spherulites have higher melting temperatures than the other parts of spherulites crystallized in Regime III. The increase in melting temperature is explained as a result of crystallization under negative pressure arising locally in pockets of occluded melt due to density change during spherulitic crystallization. The negative pressure lowers locally the equilibrium melting temperature and therefore decreases the undercooling, which results in an increase in lamellar thickness. Sectioning of bulk samples releases frozen negative pressure and reveals the increase in melting temperature of those parts of spherulites that were crystallized at lower undercooling. (C) 1993 John Wiley & Sons, Inc.