THERMOVISCOELASTICITY OF GLASS-FORMING LIQUIDS

In supercooled liquids structural relaxation shows up as a frequency d ependence of mechanical and thermal properties such as the adiabatic b ulk modulus, K-s, shear modulus, G, expansion coefficient, alpha(p), a nd isobaric specific heat, c(p). Whereas K-s and G can be measured dir ectly, we claim this is difficult for c(p). Near the glass transition where c(p) differs significantly from the isochoric specific heat, c(v ), and G approaches K-s, heat transport is no longer adequately descri bed by the heat diffusion equation but by four coupled thermoviscoelas tic equations. Thus the effective specific heat obtained by specific h eat spectroscopy can differ from c(p). We present specific heat spectr oscopy data on 1,2,4-butanetriol obtained in an approximate spherical symmetric geometry. In this geometry the effect of the thermomechanica l coupling can be calculated. The thermal wavelength can be varied fro m much greater to much smaller than the sample size. Thus the novel me thod comprises both the homogeneous and inhomogeneous methods. In the homogeneous limit, we, as a good approximation, obtain the pure c(p). (C) 1998 Elsevier Science B.V. All rights reserved.