Enhanced glass dynamics facilitated by soft interfaces

A persistent mystery in glass physics is the origin of the rapid slowdown of relaxations during supercooling toward Tg (glass transition temperature). Theoretical studies suggest that growing elastic barriers are critical in this phenomenon[1, 2]. However, direct experimental measurements remain elusive. In deeply supercooled liquids, relaxation occurs through kinetic facilitation, whereby mobile domains promote relaxation in neighboring regions[3, 4]. While kinetic facilitation is believed to be primarily local, there is no direct experimental verification of the role played by elasticity. Here, we study elasticity-induced kinetic facilitation in bilayer molecular glasses (vapor-deposited stable-glass above liquidquenched form) supported by substrates with varying elasticity. Soft interfaces enhance the relaxation dynamics > 100 nm away, allowing Arrhenius relaxations (activation energy 407±13 kJ/mol) that can be > 5 decades faster than on rigid substrates (for 0.8Tg). On rigid substrates, the non-Arrhenius activation energy increases with decreasing temperature, by ∼ 205−435 kJ/mol (from Tg to 0.96Tg), which is attributed to non-local elastic barriers. The free surface also facilitates relaxation, but differently. A well-defined mobile surface layer with decoupled dynamics forms at the surface below Tg (activation energy 31±3 kJ/mol). This study elucidates that long-range elastic effects and boundary conditions are critical in the structural evolution of supercooled liquids and glasses. A soft interface boundary enhances mobility and prevents vitrification, which can enable tailoring of stabilization and functional properties in glass coatings and flexible devices.