NON-EXPONENTIALITY OF THE ENTHALPY RELAXATION UNDER CONSTANT-TEMPERATURE CONDITIONS IN THE TIME-DOMAIN IN SUPERCOOLED LIQUIDS AND GLASSES

Enthalpy relaxations of glycerol and propylene glycol were observed wi th an adiabatic calorimeter. The experiments were carried out by the t emperature-jump method in the time domain under constant-temperature c ondition for the first time. The relaxation processes were analyzed wi th a stretched exponential function with a non-exponentiality paramete r, beta. The beta values extrapolated from positive and negative sides in the temperature jump Delta(j)T to 0 K, 0.67 +/- 0.03 for glycerol and 0.62 +/- 0.02 for propylene glycol, agree with those obtained in t he frequency domain, respectively. The non-linear effect of the relaxa tion function was observed at finite Delta(j)T and was more remarkable under the constant-temperature condition than that under the adiabati c condition. The importance of the relaxation study under the constant -temperature condition is emphasized for characterization of the irrev ersible structural relaxation in liquid glasses. The origin of the non -linearity in the relaxation is inferred to be connected with the non- Arrhenius property of the average relaxation times.