Quantification of elastic barriers to rearrangement in molecular glasses

Abstract

The glass transition is marked by a rapid increase in the barriers for molecular rearrangement, and this leads to the vitrification of supercooled liquids. Recent theories suggested that at low temperature, molecular rearrangements generate elastic stresses that dissipate through the surrounding material and add a non-local contribution to relaxation barriers. However, it is difficult to experimentally measure this elastic contribution. Here we measure the elastic barriers by investigating the transformation of vapour-deposited stable glasses, influenced by distant boundaries of varying elasticity. Rigid boundaries preserve bulk super-Arrhenius dynamics in which both relaxation times and barriers increase on supercooling. By contrast, distant soft boundaries facilitate fast Arrhenius relaxations—even below the glass transition temperature—by allowing faraway stress dissipation. As such, the constant soft-substrate barrier is attributed to local interactions, whereas the differences between barriers on soft and rigid boundaries arise from non-local elastic barriers. Our results show that the rapid dynamical slowdown and vitrification of supercooled liquids are governed by the emergence and growth of elastic barriers, and provide a direct experimental basis for elasticity-based descriptions of the glass transition. Elastic stresses are believed to play a role in the vitrification process in glasses, but measuring them is challenging. Imposing elastic boundary conditions is now shown to provide a way to probe elastic effects within glassy samples.