Thermoviscoelasticity of glass-forming liquids

In supercooled liquids structural relaxation shows up as a frequency dependence of mechanical and thermal prop-erties such as the adiabatic bulk modulus, Ks, shear modulus, G, expansion coecient, ap, and isobaric specific heat, cp. Whereas Ks and G can be measured directly, we claim this is dicult for cp. Near the glass transition where cp diers significantly from the isochoric specific heat, cv, and G approaches Ks, heat transport is no longer adequately described by the heat diusion equation but by four coupled thermoviscoelastic equations. Thus the eective specific heat ob-tained by specific heat spectroscopy can dier from cp. We present specific heat spectroscopy data on 1,2,4-butanetriol obtained in an approximate spherical symmetric geometry. In this geometry the eect of the thermomechanical cou-pling 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 cp. Ó 1998 Elsevier Science B.V. All rights reserved. 1