Multi-frequency heat capacity measured with different types of TMDSC

Abstract

The heat capacities of sapphire (Al2O3) and sodium chloride (NaCl), have been measured to establish the accuracy and precision of two different temperature-modulated differential scanning calorimeters operated in diverse multi-frequency modes. The calorimeters have then been applied to find the apparent, reversing heat capacity of polystyrene as a function of frequency in the glass transition region. The first modulation mode consisted of a series of linear heating and cooling segments and produced four harmonics with practically equal temperature amplitudes (1st, 3rd, 5th, and 7th), one of lower amplitude (9th), and almost negligible higher harmonics. The second modulation mode is a rather sharp step ending in an isotherm or slow temperature-decrease and leads to a controlled spike in the heat-flow rate response which produces Fourier components of similar amplitudes for all harmonics of the rates of changes of temperature. The apparent, reversing heat capacity is evaluated from the amplitudes of the heat-flow rates and the corresponding sample temperatures or heating-rates. A time-constant or calibration constant which accounts for thermal conductivities and resistances within the calorimeters can be evaluated from the different harmonics of each run. Measurements in the glass transition region have a slow response of the sample. They are evaluated by separating the sample effect from the calorimeter response which can be extrapolated from data gained outside the transition. One measurement is thus sufficient for the evaluation of the frequency dependence of the heat capacity in the glass transition region.