The Deep Effect of Topology on Glass Relaxation

The problems posed by glass are not only practical but also conceptual: despite the progress afforded by advances in glass science, we still lack a complete understanding of how a liquid becomes a glass. Indeed, in contrast to most phase transitions, many measureable physical and chemical glass properties change smoothly across the liquid-to-glass transition at the glass transition temperature Tg. And in the liquid, the origin of the huge increase of viscosity (1012 pascal-seconds at Tg) remains poorly understood. A similar increase is observed for the relaxation time τ of the system towards thermal equilibrium following an imposed stress (Fig. 1). This relaxation time is still measurable at Tg because it is on the order of the timescale of an experiment, but it grows by several orders of magnitude as temperature continues to decrease below Tg. As a result, measuring the relaxation of a glass far below its transition temperature remains a true challenge; thus most studies have focused on the supercooled liquid where the relaxation time of the system is less than, or of the order of, the experimental time scale.