Low Angle X-Ray Diffraction of Fibrous Polyethylene

Abstract

Highly axially oriented fibers of linear polyethylene are shown to display four orders of well defined, meridionally directed diffraction maxima corresponding to a fundamental spacing of 408 A. An interpretation of the low angle x‐ray pattern is given in terms of the fiber morphology and the Hess‐Kiessig postulate as to the origin of the periodic variation in electron density. The relative macroscopic length of the fibers was systematically altered both by thermal treatment and by crosslinking, melting, and recrystallization. The magnitude of the spacings observed does not bear any direct relation to the change in length incurred, but reflects the change in crystallite size that develops because of annealing, partial melting or the introduction of crosslinks. It is also shown that in a completely shrunken fiber, where wide‐angle x‐ray diffraction shows that the crystallites are randomly arranged relative to one another, discrete diffraction maxima are observed at about 255 A. These maxima are, however, circular in shape. Intermediate types of line shapes are also observed, which depend solely on the crystallite orientation and not on the method by which the orientation is developed.