Dynamics of Ribavirin Glass in the Sub-Tg Temperature Region

Abstract

Fragility and the effect of annealing in the sub-T(g) temperature region on the characteristics of ribavirin glass were investigated using thermal analysis. The onset T(g) of ribavirin glass was determined to be 55.5 °C from modulated differential scanning calorimetry measurements under a heating rate of 2 °C/min. The ribavirin glass was proved as fragile from dynamic analysis using three evaluation methods and as very fragile from thermodynamic analysis based on the heat capacity difference between glassy and crystalline states. Enthalpy recovery was found for even freshly prepared ribavirin glass and was initiated far below T(g) in the thermal analysis, which suggested low cooperativity of the molecular motion. The width of the glass transition region was significantly narrowed, and T(g) was increased by sub-T(g) annealing at 40-45 °C, suggesting that a temperature region exists for the effective enhancement of molecular cooperativity. Quantification of the recovery enthalpy was conducted by integrating the thermal curve through a wide temperature range that included a baseline to determine the relaxation constants because peak integration, which is a frequently used procedure to determine recovery enthalpy, was not adequate due to significant alteration in the shape of the heat flow curves by annealing. Relaxation times obtained by the baseline integration procedure were larger than those from the conventional peak integration approach and exhibited better agreement with the Vogel-Tammann-Fulcher equation, revealing that this method may be notably beneficial for the evaluation of glasses with low molecular cooperativity. The size of the cooperative rearranging region (CRR) was determined from the width of the glass transition region and indicated the existence of an optimum temperature range to increase the size of the CRR just below T(g).