Improvement of AC calorimetry for simultaneous measurements of heat capacity and thermal conductivity of polymers

The method of Temperature Waves Transmission Spectroscopy is developed on the basis of the classical AC calorimetry technique. The idea of the method is to use the information about the phase and amplitude of the temperature wave transmitted through a plate-like sample for simultaneous determination of the sample's heat capacity and thermal conductivity. For the correct application of the method in an experimental set up, it was necessary to study the problem of plane temperature wave transmission through a plate-like multilayered system. This problem was solved analytically for a wide range of experimental conditions. It was shown that the AC calorimeter can be used at relatively high frequencies, when the temperature oscillations in the sample are not quasi-static. Therefore, the width of the appropriate frequency range of classical AC calorimetry can be enlarged. This is of great importance for the applicability of classical AC calorimetry for low heat-conductive materials, such as polymers. It is shown that the dynamic heat capacity and thermal conductivity of polymers can be measured in real time and in a broad frequency region. The method was applied for polystyrene near glass transition and for 4.4'-n-octyloxycyanobiphenyl (8OCB) liquid crystal in melting region, smectic-nematic and nematic-isotropic transitions. The possibility of dynamic heat capacity measurements is demonstrated. (C) 1998 Elsevier Science B.V.