Rate and Pressure Effects in Polymers and Other Glass-Forming Substances

Abstract

A simplified normal coordinate computation for the thermal vibrations of molecules in a liquid has been carried out. From the result, the size distribution of voids or holes in the liquid is computed, and this distribution is then used to compute the diffusion constant. The result for low temperatures is a modified Doolittle equation. An analog to the WLF rate-process equation is also obtained. It is shown that the concept of free volume arises quite naturally in this formalism. Thermal expansion and compressibility constants are also calculated. A treatment of the rate of thermal contraction at low temperatures leads to a precise meaning for the glass temperature in terms of the parameters of the molecular system. The effects of applied pressure on glass temperature and rate processes are derived. Results of the theory are compared with experiment and, in general, are found to be fairly satisfactory.