Thermoviscoelasticity of glass-forming liquids

In supercooled liquids structural relaxation shows up as a frequency dependence of mechanical and thermal properties such as the adiabatic bulk modulus, K-s, shear modulus, G, expansion coefficient, alpha(p), and isobaric specific heat, c(p). Whereas K-s and G can be measured directly, 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 described by the heat diffusion equation but by four coupled thermoviscoelastic equations. Thus the effective specific heat obtained by specific heat spectroscopy can differ from c(p). We present specific heat spectroscopy data on 1,2,4-butanetriol obtained in an approximate spherical symmetric geometry. In this geometry the effect of the thermomechanical coupling can be calculated. The thermal wavelength can be varied from much greater to much smaller than the sample size. Thus the novel method 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.