Enthalpy relaxation of an epoxy-anhydride resin by temperature-modulated differential scanning calorimetry

The enthalpy relaxation of an epoxy-anhydride resin was studied by physical aging and frequency-dependence experiments with alternating differential s canning calorimetry (ADSC), which is a temperature-modulated differential s canning calorimetry technique. The samples were aged at 80 degrees C, about 26 K below the glass-transition temperature, for periods up to 3800 h and then scanned under the following modulation conditions: underlying heating rate of 1 K min(-1), amplitude of 0.5 K, and period of 1 min. The enthalpy loss was calculated by the total heat-flow signal, and its variation with t he log (aging time) gives a relaxation rate (per decade), this value being in good agreement with that calculated by conventional DSC. The enthalpy lo ss was also analyzed in terms of the nonreversing heat flow, revealing that this property is not suitable for calculating enthalpy loss. The effect of aging on the modulus of the complex heat capacity, \Cp*\, is shown by a sh arper variation on the low side of the glass transition and an increase in the inflexional slope of \Cp*\. Likewise, the phase angle also becomes shar per in the low-temperature side of the relaxation. The area under the corre cted out-phase heat capacity remains fairly constant with aging. The depend ence of the dynamic glass transition, measured at the midpoint of the varia tion of \Cp*\, on 1n(frequency) allows one to determine an apparent activat ion energy, Delta h*, which gives information about the temperature depende nce of the relaxation times in equilibrium over a range close to the glass transition. The values of Delta h*, determined from ADSC experiments in a r ange of frequencies between 4.2 and 33 mHz and at an amplitude of 0.5 K, an d an underlying heating rate of 1 K min(-1), were analyzed and compared wit h that obtained by conventional DSC from the dependence of the fictive temp erature on the cooling rate. (C) 2000 John Wiley & Sons, Inc.