The effect of pressure and temperature on the specific volume of polyethylene

Author:
Publisher website
Google Scholar
Abstract
A spherulitic crystallization process can be divided into three successive stages: the formation of spherulite centers, the growth by means of crystalline fibrils with many branches extending into the melt, and finally crystallization of the intervening regions between these already crystallized fibrils. The three stages were identified with parts of a typical dilatometric isotherm for crystallization from the melt. The last stage of the isotherms was analyzed as crystal growth accompanied by the predetermined nucleation of one‐dimensional character. Isothermal annealing was also studied dilatometrically and was found to be similar in mechanism to the last stage of crystallization from the melt. The influence of the thermal history, including the conditions of crystallization, on the degree of crystallinity were summarized as a kinetic factor, and was distinguished from the thermodynamic consequence of branches on the equilibrium degree of crystallinity. Specific volumes of the crystal (estimated) and the melt were determined at various temperatures and pressures. The relationship between the entropy of fusion and pressure was determined from the volume of fusion by utilizing Clausius‐Clapeyron's equation. The entropy of fusion at atmospheric pressure thus obtained was found in agreement with values obtained elsewhere by other methods. The compressibility of the melt and the true volume of fusion decreased substantially under high pressure, while the entropy of fusion was essentially unchanged. This was ascribed to the change of the melt property from the familiar free‐volume type of behavior to a more solidlike one without an increase in structural order.

This publication has 21 references indexed in Scilit: