THE GAMMA-PHASE OF HIGH-MOLECULAR-WEIGHT POLYPROPYLENE .1. MORPHOLOGICAL ASPECTS

A high-molecular-weight polypropylene (M(w) = 83 000, M(w)/M(n) = 3.0) has been crystallized isothermally as a function of pressure at a constant supercooling of 50-degrees-C. Wide-angle X-ray diffraction (WAXD) shows that the X-ray diffraction spectrum of the gamma phase is identical to that reported for low-molecular-weight polymers, with the exception that the peaks are broader. Calculations of the gamma content using WAXD show the nearly 100% gamma phase is formed at 2 kbar, but a mixture of gamma and alpha crystals are formed at lower crystallization pressures. Small-angle X-ray scattering (SAXS) studies show that gamma lamellae are approximately one-half the thickness of alpha lamellae produced at an equivalent supercooling. Morphological studies conducted using transmission electron microscopy of carbon replicas of permanganically etched surfaces confirm the thin nature of the lamellae and demonstrate their inherent waviness. Unlike alpha crystals, gamma lamellae do not form cross-hatched textures. A model is proposed, involving gamma/alpha lamellae and epitaxy during the growth process, to account for all the observable morphological data.