ISOTACTICITY EFFECT ON CRYSTALLIZATION AND MELTING IN POLY(PROPYLENE) FRACTIONS .2. LINEAR CRYSTAL-GROWTH RATE AND MORPHOLOGY STUDY

Linear crystal growth rate and crystal morphology changes in a set of poly(propylene) (PP) fractions with similar molecular masses and distributions but different isotacticities have been studied via polarized light microscopy (PLM), transmission electron microscopy (TEM), and wide-angle X-ray diffraction (WAXD) methods. All but one of these PP fractions exhibit a regime III/II transition in their linear crystal growth rate data at a supercooling of about 48 K. At high supercoolings, two new crystal growth regions are observed for these PP fractions with low isotacticities. These crystallization regions are isotacticity dependent, which could be due to a disordered alpha-crystal form and cross-hatched branching. At a constant supercooling, the linear crystal growth rate data differ by about 3 orders of magnitude because of the isotacticity difference. Crystal morphology changes, in particular, the cross-hatching phenomenon, with crystallization conditions and isotacticity have been observed. At low crystallization temperatures the cross-hatched lamellae cannot be fully developed. Decreasing the isotacticity, on the other hand, reduces the cross-hatching density. Crystal growth behavior and its corresponding morphological changes are also discussed.