Molecular Mechanics of Point Defects in Polyethylene

Abstract

Certain properties of crystalline polymers can be associated with the presence of local defects, e.g., deviations of short segments of the chain from the predominant conformation allied with the crystal structure. General methods are developed by which such chain defects may be simulated by computer techniques. The application of these methods to the polyethylene system gives detailed information concerning: stable defect structures, low-energy conversion paths between stable structures (molecular motions), interactions in defect assemblies, and intermolecular interactions between chains and crystalline environments. Such an investigation of all fold structures containing seven (or less) dihedral angles indicates the existence of a small number of stable but immobile structures. The intramolecular energy of these structures is somewhat high and they do not pack efficiently. Expansion of fold structures to include eight and nine dihedral angles and slight relaxation of orientation constraints provides lower-energy structures which are mobile, more efficient packing, and a low-energy path for conversion between diagonal and b-axis folds. In addition, stable structures and motions of other local defects (i.e., kinks and jogs) are identified, rigid segment rotation is found to be questionable, and certain mechanical properties are interpreted in terms of the over-all results.