Conventional methods fail to measure cp(omega) of glass-forming liquids
Abstract
The specific heat is frequency dependent in highly viscous liquids. By solving the full one-dimensional thermo-viscoelastic problem analytically it is shown that, because of thermal expansion and the fact that mechanical stresses relax on the same time scale as the enthalpy relaxes, the plane thermal-wave method does not measure the isobaric frequency-dependent specific heat cp(omega). This method rather measures a "longitudinal" frequency-dependent specific heat, a quantity defined and detailed here that is in-between cp(omega) and cv(omega). This result means that no wide-frequency measurements of cp(omega) on liquids approaching the calorimetric glass transition exist. We briefly discuss consequences for experiment.
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