High-density stable glasses formed on soft substrates*

Physical vapor deposition can create high-density stable glasses comparable to liquid-quenched glasses aged for millions of years, enabled by surface-mediated equilibration. Deposition is often performed on rigid substrates, at various rates and temperatures, to control glass properties. Here, we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable molecular glasses with densities up to 2.5% higher than liquid-quenched glasses, within 2.5 hours of deposition. To gain these properties on rigid substrates requires 10 million times slower deposition, taking ∼3000 years. Controlling the modulus of the rubbery substrate provides a large degree of control over the glass structure and density at a constant deposition condition. When heated to above the glass transition temperature, the stable glasses on soft substrates transform into supercooled liquid via two growth fronts, the one initiated from the soft interface propagates with a faster initial velocity, persisting up to 100 nm away from the interface, before slowing down to the same velocity as the one initiated from the free surface. This behavior also exhibits a high sensitivity to the modulus of the rubbery substrates. These results underscore the significance of substrate elasticity as a novel factor in manipulating the long-range dynamic perturbations, and thus the structure and properties of vapor-deposited glasses, allowing access to deeper states of the energy landscape, without the need for prohibitively slow deposition rates.