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 poly propylene bulk samples and high-density polyethylene thin films crysta llized isothermally at various temperatures has been studied by polari zed light microscopy. The regions around cavities and multiple boundar y points between spherulites have higher melting temperatures than the other parts of spherulites crystallized in Regime III. The increase i n melting temperature is explained as a result of crystallization unde r negative pressure arising locally in pockets of occluded melt due to density change during spherulitic crystallization. The negative press ure lowers locally the equilibrium melting temperature and therefore d ecreases the undercooling, which results in an increase in lamellar th ickness. Sectioning of bulk samples releases frozen negative pressure and reveals the increase in melting temperature of those parts of sphe rulites that were crystallized at lower undercooling. (C) 1993 John Wi ley & Sons, Inc.