Morphology of <i>n</i>‐alkanes, linear polyethylene, and isotactic polypropylene crystallized from solution

Abstract

Abstract The morphology of two n ‐alkanes ( n ‐C 44 H 90 and n ‐C 82 H 166 , prepared by E. Stenhagen, et al. ) and linear polyethylenes (Ziegler‐type of low and Marlex 50 of high molecular weight, respectively), crystallized from dilute solution by slow cooling, have been studied by electron microscopy and small‐angle x‐ray scattering. A series of solvents were used with cloud points for polyethylene from about 120°C. (poor solvents, e.g., diphenyl ether) to 65–75°C. (good solvents, e.g., tetrachloroethylene). All samples crystallized with basically the same morphology, showing large, thin flakes or lamellac with the chains perpendicular to the plane of the flakes. This involves chain folding for all polyethylenes studied, as proposed by Keller. In most cases, the large flakes were overgrown with or accompanied by diamond‐ or lozenge‐shaped single crystals (as reported by Jaccodine and Till) containing from 2 to about 20 lamellae and developed as pyramids, sometimes as spirals and sometimes as consecutive layers. Isotactic polypropylene crystallized with a similar morphology. The nucleation and growth mechanisms of the large flakes are not clear. They seem to grow without dislocations, while the diamond‐shaped crystals seem to form according to Frank's screw‐dislocation growth concepts. Crystallization of polyethylene at high temperatures give thicker lamellae (165 A. at 120°C.) than crystallization at low temperatures (103 A. at 60°C.), as measured by small‐angle x‐ray scattering, regardless of molecular weight. When heated, the lamellae recrystallized to larger thickness. This would imply that the folding length of the polyethylene chains and the related thickness of stable lamellae are primarily a function of the crystallization temperature. The lamellae of n ‐alkanes showed a thickness which agreed with the extended chain length of each sample within a few angstroms.