THERMODYNAMIC AND PHYSICAL PROPERTIES OF CRYSTALS FORMED FROM DILUTE SOLUTIONS

Some of the thermodynamic and physical properties of crystals of linear polyethylene, formed from dilute solution are discussed. These properties include density, enthalpy of fusion, infrared absorption spectrum, selective oxidation and dynamic mechanical measurements. Despite the fact that electron microscopic examination reveals an apparent geometric regularity to these crystals, all these physical measurements yield a consistent interpretation in that they require about 15–20% of the chain units to be in a non‐random conformation, and to form a disordered amorphous overlayer. Detailed studies of the experimentally observed relations between the crystallite thickness, crystallization temperature, and dissolution temperature (melting temperature in a dilute solution) are also presented. A thermodynamic analysis of the dissolution temperature‐size relations, based on independently determined equilibrium solubility temperatures, indicates that the interfacial free‐energy characteristic of the mature crystallites is significantly different from that involved in nucleation. The interfacial structures of the nucleus and the crystallites must, therefore, also be different. Size control by a nucleation process can be demonstrated from very general theoretical considerations. However, the detailed molecular structure characteristic of the nucleus cannot be specified solely from the fact that this process is size controlling. Copyright © 1969 Society of Plastics Engineers, Inc.