Thermodynamics of the Densification Process for Polymer Glasses

A quantitative description is given for the densification process of glasses resulting from glass formation at elevated pressures. Phenomenologieal relations are derived, or justified, which allow estimation of the densification rate κ′ (with respect to formation pressure) from various thermodynamic quantities and glass transition behavior. In addition, the estimation of K′ may be facilitated by the application of the hole theory of Simha and Somcynsky. Using these relations κ′ is estimated, and the results from the different methods are compared for data from 23 different organic polymers with glass transition temperatures ranging from 150 to 455 K. The amount of densification appears to be limited by the apparent convergence of the glass temperature and effective decomposition temperature with increasing pressure. Some estimates of limiting values are presented. Finally, changes of refractive index resulting from densification are estimated from the observed, or predicted, densification rates.