Some macroscopic properties of stirring-induced crystals of polyethylene

Fibrous crystals of polyethylene are produced by crystallization from agitated xylene solutions. In this paper, the following properties of macroscopic aggregates of such crystals, grown under carefully specified conditions, and having well-defined morphologies, are investigated: (a) regularity and magnitude of long periods, (b) crystal densities, (c) melting and annealing behavior, and (d) response of crystals to attack with fuming nitric acid. Electron microscopy reveals two distinct fiber types-a compact, high-temperature form, comprising both extended-chain and folded-chain regions (designated micro shish kebabs in this paper) and a low-temperature (crystallized below 97[ddot]C) form, in which lamellar crystals are deposited upon micro-shish-kebab fiber backbones (referred to in this paper as shish kebabs). Shish kebabs display properties closely similar to those of single crystals, as would be expected from the electron microscopy results, except for some novel features in their annealing behavior. Micro shish kebabs exhibit considerably different properties from conventionally crystallized material. They are extremely superheatable and show only a slight tendency to stabilize thermally on annealing. Their response to attack with fuming nitric acid is quite unlike that of single crystals, and the distribution of fold lengths within their chain-folded regions is more irregular than in normal bulk or solution-crystallized material.