Low-Angle X-Ray Diffraction of Crystalline Nonoriented Polyethylene and Its Relation to Crystallization Mechanisms

Abstract

X‐ray diffraction maxima at low angles have been observed in crystalline but nonoriented linear polyethylene, the crystallization process being conducted from the melt of the pure undiluted polymer. Several orders of diffraction are observed in favorable cases, and the spacings corresponding to the first‐order reflections range from 150–850 A. The values of the maxima depend on the mode of crystallization. Previous assertions that in such systems the maxima are limited to 100–200 A are shown to be very restrictive and typical only of crystallization processes conducted at very large undercooling. Major attention is focused on the properties of specimens crystallized isothermally at relatively low values of the undercooling. The spacings are very sensitive to the crystallization temperature in this range. The highest values are observed at low undercooling, and substantial decreases occur as the temperature is lowered. Concomitantly, the density observed after isothermal crystallization significantly decreases with a lowering of the crystallization temperature. The fact that a periodicity can be developed in such systems, the magnitude of the maxima, and their dependence on the crystallization temperature, is explicable by the application of nucleation theory. It is assumed that subsequent to the formation of critical‐size nuclei from a bundle of polymer chains, crystal growth along the chain direction is severely retarded, while in the transverse direction essentially unimpeded crystallization occurs. From the observed temperature coefficient of the low‐angle spacings, the ratio of the excess free energy (resulting from the junction of crystalline and amorphous regions at the crystallite ends) to the bulk enthalpy of fusion is found to be 2.6. The magnitude of this ratio receives confirmation from another type of experiment.