Influence of the sample mass on the study of the glass transition and the structural relaxation by differential scanning calorimetry

Differential scanning calorimetry has been widely used in the description of the glass transition dynamics and the structural relaxation in glass-forming systems. However, the effect of the mass (or thickness) of the sample may influence the result of the fitting procedure to experimental data as the temperature gradient within it may change both the location and the broadness of the response in the temperature axis. In this work, a procedure is proposed that allows one to determine the temperature profile along the sample thickness by performing conventional calibration experiments with metallic standards positioned at different locations within layers of a polymeric material. Significant temperature gradients (amplitudes higher than 2.5 degreesC for a heating rate of 20 degreesC/ min) were found for samples with mass higher than 16 mg. Simulations suggested that such gradients shifts the trace in the temperature axis but the effect in the response broadening is small. Experimental results on the structural relaxation in polycarbonate with different masses were analyzed using a phenomenological model. It was found that, for this particular system and for data obtained at 10 degreesC/min, the fitted parameters slightly depend on the sample's mass, that reflects, for example, a (small) apparent enlargement of the distribution of characteristic times with increasing mass. (C) 2004 Published by Elsevier B.V.