A combined measurement of thermal and mechanical relaxation

Abstract

In order to describe relaxation the thermodynamic coefficient 1 β S = ( ∂ V ∂ S ) p can be generalized into a complex frequency-dependent cross response function. We explore theoretically the possibility of measuring 1 β S ( ω ) for a supercooled liquid near the glass transition. This is done by placing a thermistor in the middle of the liquid which itself is contained in a spherical piezoelectric shell. The piezoelectric voltage response to a thermal power generated in the thermistor is found to be proportional to 1 β S ( ω ) but factors pertaining to heat diffusion and adiabatic compressibility κ S ( ω) do also intervene. We estimate a measurable piezoelectric voltage of 1 mV to be generated at 1 Hz for a heating power of 0.3 mW. Together with κ S ( ω) and the longitudinal specific heat c l ( ω) which may also be found in the same setup a complete triple of thermoviscoelastic response functions may be determined when supplemented with shear modulus data.