Impact of fragility on enthalpy relaxation in glass

The macroscopic properties of a glass are continually relaxing toward their equilibrium supercooled liquid values. Experimentally, the shape of the relaxation function in a glass is known to depend on the fragility of the supercooled liquid. In this paper, we investigate the impact of fragility on the relaxation behavior of glasses in the enthalpy landscape framework. We show that the fragility of a supercooled liquid is a direct result of the interplay of enthalpic and entropic effects in the enthalpy landscape. Through proper adjustment of the transition barriers in an enthalpy landscape, the fragility of a system can be adjusted while maintaining the same glass transition temperature. By modeling a set of systems with identical glass transition temperatures but varying values of fragility, we show that supercooled liquid fragility has a significant impact on the enthalpy relaxation behavior of a glass. In particular, the magnitude of enthalpy relaxation decreases dramatically with increasing fragility. Finally, we discuss how in the limit of infinite fragility the glass transition becomes an ideal second-order phase transition where no relaxation is possible in the glassy state.