Quasi-Static and Dynamic Density Fluctuations in the Glassy State IV. Dynamic Thermal Density Fluctuation and Isothermal Compressibility of Glass

Abstract

The temperature dependence of isothermal compressibility {β(T) dilato} in the glassy state was measured by dilatometry at temperatures ranging from room temperature to around the glass transition temperature (T g) for polymethyl methacrylate (PMMA) and polycarbonate (PC) glasses. The results were compared with those of β(T) light obtained from isotropic light scattering caused by dynamic thermal density fluctuation (Vv iso dyn), not Vv iso (=Vv-( 4/3)Hv;Hv is anisotropic scattering), in order to test the validity of Einstein's fluctuation theory for the glassy state. Consequently, it was experimentally demonstrated for the first time that β(T) light is in very good agreement with β(T) dilato at all temperatures below T g for PMMA and PC. These experimental findings indicate that the isothermal compressibility of glass in a nonequilibrium system is thermodynamically dominated by the dynamic thermal density fluctuation having relaxation times much shorter than the experimental observation time scale. Furthermore, the isothermal compressibility of glass, in general, increases drastically as the temperature increases in the glass transition region. However, it has been suggested that this drastic increase is the result of a dramatic increase of the dynamic thermal density fluctuation, with thawing of quasi-static thermal density fluctuation.