Effect of aging on glass transformation measurements by temperature modulated DSC

Differential scanning calorimetry (DSC) is a well established technique for studying the glass transformation behavior of glasses and for measuring the glass transition temperature Tg and the relaxation enthalpy ΔHg. However, Tg and ΔHg measurements on glasses from DSC heating scans depend on the thermal history and aging (annealing time at a given temperature below Tg) which makes it difficult to compare measurements on a given glass from different laboratories and also examine small changes in Tg and ΔHg due to small changes in the glass composition. We have performed both temperature modulated DSC (TMDSC) and conventional DSC experiments to study the dependence Tg, complex heat capacity Cp and relaxation enthalpy ΔHg on aging for vitreous Se alloyed with 0.5% As (a-Se:0.5%As). We have studied the behavior of the TMDSC total, reversing and nonreversing heat flows and the DSC heat flow as the glass is heated from 20 °C to a temperature above the glass transition temperature. We observe that TMDSC Tg has a much smaller dependence on the aging time than DSC Tg measurements and, therefore, TMDSC results are particularly useful for Tg comparisons of glasses with unknown thermal history and aging characteristics. The change in TMDSC Tg was almost ten times smaller than the DSC Tg change over an aging time of 100 h. We observe that the apparent relaxation enthalpy ΔHg as measured by conventional DSC, reversing and total heat flows in TMDSC increases with the aging time. If we analyze the long time relaxation in terms of stretched exponential behavior we find β values in the range 0.39–0.43.