The gamma-phase of high molecular weight isotactic polypropylene .2. The morphology of the gamma-form crystallized at 200 MPa

The gamma-form of isotactic polypropylene, crystallized at elevated pressures, exhibits three distinct classes of spherulites, categorized according to the sign of the birefringence. The results show that at 200 MPa positively birefringent spherulites are observed for crystallization temperatures below 184 degrees C and above 199 degrees C, negatively birefringent spherulites are observed for crystallization in the range 187-198 degrees C and mixed birefringence in the ranges 182-188 degrees C and 196-200 degrees C. In addition, the complex arrangement of the lamellae within these spherulites has been studied using optical and electron microscopy. The results indicate the presence of a hitherto-unreported form of lamellar aggregation, which is best described as feather-like. The birefringence results indicate that the feather like structure is solely developed by self-epitaxial growth of the gamma-lamellae. Melting studies of 'mixed birefringent' spherulites show that the epitaxially grown featherlike structures have higher melting points than radial lamellae grown concurrently in the same spherulite, leading to the inference that it is possible to grow substantial amounts of two different types of lamellae concurrently, that are not distinguishable using conventional wide and small angle X-ray analyses. (C) 1997 Elsevier Science Ltd.