On the development of polypropylene spherulites

Abstract The organization of lamellae in developing spherulites of i-polypropylene growing from the melt at 155 and 150°C has been investigated in detail using permanganic etching and transmission electron microscopy (TEM). The study forms a link between the simplest development at 160°C, when there is no twinning, and common cross-hatched textures occurring for crystallization down to at least 110°C. Whereas at 160°C growth starts with fan-like splaying of lath-like lamellae, at 155°C and below early objects are quadrites which develop as two families of laths with a common b -axis and a mutual orientation of 80°. This orientation relaxes with distance but is still recognizable with microbeam X-rays to sizes >0.2mm. Randomization in space of the orientation of the growth direction a∗ , and also around this axis, occurs at smaller distances at lower crystallization temperatures. It is inferred that so-called axialites and spherulites are objects which differ in their degree of randomization rather than in kind. The characteristic cross-hatched texture of α-polypropylene develops differently according to the location in a growing spherulite. Whereas cross-hatching is symmetrical at the centre of a quadrite, in the outer regions radial lamellae dominate. Cross-hatching lamellae form later and are restricted in length to the unfilled space available. As such they are in competition with radial subsidiary lamellae. There is an increasing ratio of transverse to parallel subsidiary lamellae with lower crystallization temperatures. Epitaxy between the two cross-hatching components is observed to occur on the prism faces of lamellae in agreement with molecular mechanisms proposed by Binsbergen and de Lange 5 and Lotz and Wittman 21 .