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 t he method is to use the information about the phase and amplitude of t he temperature wave transmitted through a plate-like sample for simult aneous determination of the sample's heat capacity and thermal conduct ivity. For the correct application of the method in an experimental se t up, it was necessary to study the problem of plane temperature wave transmission through a plate-like multilayered system. This problem wa s solved analytically for a wide range of experimental conditions. It was shown that the AC calorimeter can be used at relatively high frequ encies, when the temperature oscillations in the sample are not quasi- static. Therefore, the width of the appropriate frequency range of cla ssical 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 capaci ty and thermal conductivity of polymers can be measured in real time a nd in a broad frequency region. The method was applied for polystyrene near glass transition and for 4.4'-n-octyloxycyanobiphenyl (8OCB) liq uid crystal in melting region, smectic-nematic and nematic-isotropic t ransitions. The possibility of dynamic heat capacity measurements is d emonstrated. (C) 1998 Elsevier Science B.V.