Oriented crystallization in polymers

Abstract

This paper deals with various aspects of crystallization leading to oriented textures. In the first place it attempts to clarify the problem of melt-extruded polyethylene sheet, which leads to the more general problem of crystallization under stress. Lamellar crystal growth, normal to the stress direction nucleated along lines parallel to the stress, is found to be the underlying morphology of the resulting texture. The alignment and twisting of the lamellae and the density of the nucleating lines are the principal stress-dependent variables. Fortuitous nucleating sources may exist, but nucleation intrinsic to the polymer is strongly indicated. In analogy to stirrer-induced crystallization (which we have investigated following its discovery by Pennings), the existence of a fibrous backbone serving to nucleate the lamellae is suggested. The fibrils are thought to be essentially of an extended chain character and the lamellae chain-folded. The implications of these results are manifold. They show promise of unifying effects observed in materials as different as thermoplastics and elastomers, and hitherto disconnected effects found during solution crystallization, stress-induced crystallization, melt extrusion, and in some aspects of the drawing and annealing of fibers. Finally, they may help to reconcile the concepts of fibrous and lamellar polymer crystals.