ISOTACTICITY DEPENDENCE OF SPHERULITIC MORPHOLOGY OF ISOTACTIC POLYPROPYLENE

The isotacticity dependence of the spherulitic morphology of isotactic polypropylene (iPP) with high isotacticity between 98.9% and 99.8% in isotactic pentad fraction ([mmmm]%) was studied. Conventional iPP wit h low isotacticity, [mmmm] = 93.5%, was used as a reference. Isotherma l crystallization was carried out at 125 degrees C-150 degrees C. Morp hological observations were carried out by polarizing optical microsco py and transmission electron microscopy. Nearly all the spherulites sh owed an a-form crystal structure and negative birefringence (Delta n). The radial lamellar (R-lamellar) fraction, f(R), estimated from an in creased with increase in isotacticity and crystallization temperature. This result indicates that the degree of cross-hatching composed of R -lamellae and tangential lamellae (T-lamellae) decreased with increase in isotacticity and crystallization temperature. It was shown for the first time that the spherulite fully occupied by R-lamella, i.e.,f(R) approximate to 1, was obtained when the sample with the highest isota cticity was crystallized at 150 degrees C. This means that there are f ew cross-hatched lamellae. The origin of cross hatching is discussed, focusing on the role of configurational defects within a molecular cha in and the mobility of molecules in the crystallization process. (C) 1 998 Elsevier Science Ltd. All rights reserved.