Emergence of equilibrated liquid regions within the glass

The conventional understanding of the glass transition is that the transition from glass to liquid appears as a dynamic process in which atoms or molecules relax cooperatively into the equilibrium phase. Here we show that—in contrast to this picture—isolated regions of liquid form within the glassy matrix and the nature of the glass transition at a given temperature depends on the ratio between the relaxation time of the glass, τglass, and the alpha relaxation time of the equilibrated liquid, τα. At temperatures at which τglass/τα is large, we observe that high-mobility regions transit directly into the equilibrated liquid and subsequently grow by dynamic facilitation before—or while—cooperative glass relaxation sets into play. On the contrary, at temperatures associated with smaller τglass/τα, the glass transition proceeds by cooperative relaxation dynamics throughout the material. This behaviour is independent of the experimental procedure or protocol used to produce the glass. The transition from a glassy to a liquid phase is normally assumed to take place cooperatively across the whole material. But now, experiments show that, under certain conditions, isolated regions of liquid can form in the glassy matrix first.