Abstract :
The reversing heat capacity of polystyrene has been measured in the glass transition region as a function of frequency by temperature-modulated DSC using infra-red light for the modulation. The mean temperature was controlled by the calorimeter operating in the standard DSC mode. The additional power transferred to the specimens is obtained by infra-red diodes with their mean power controlled by a pulse-width modulation method. This method allows to synthesize sinusoidal temperature-profiles with negligible distortion with frequencies of up to 0.125 Hz. The experiments were carried out using multi-frequency temperature-profiles composed by superposition of four sinusoidal modulations of periods of 243, 81, 27, and 9 s, which cover the frequency range from the 1st to the 27th harmonic. To study the kinetics in the glass transition region, the thermal history was produced by cooling the samples at different rates. The results and the advantages of the new method are discussed and compared to a series of less-precise separate single-frequency analyses.
