THERMAL HISTORY AND TEMPERATURE-DEPENDENCE OF VISCOELASTIC PROPERTIESOF POLYMER GLASSES - RELATION TO FREE-VOLUME QUANTITIES

The dependence of physical properties on temperature and thermal histo ry of glassy polycarbonate is examined. The average stress relaxation time increases during isothermal aging and with decreasing cooling rat e. The temperature dependence of the stress relaxation time was compar ed with a free volume function calculated from specific volume data re ported by Zoller by means of the Simha-Somcynsky theory and with a fra ctional free volume measured by ortho-positronium annihilation. We rep ort in addition comparisons between free volume fractions extracted fr om volume data, and creep measurements for polystyrene, and stress rel axation data for poly(vinyl acetate) given by Kovacs et al., Schwarzl and Zahradnik, and Meyer et al. These investigations suggest that in t he glassy state the Doolittle equation is valid only in isothermal agi ng experiments. It follows that in expressing the polymer mobility not only a temperature-dependent free volume but additional temperature-d ependent processes need to be considered. (C) 1995 John Wiley & Sons, Inc.