Front propagation in ultrastable glasses is dynamically heterogeneous

Abstract

Upon heating, ultrastable glassy films transform into liquids via a propagating equilibration front, resembling the heterogeneous melting of crystals. A microscopic understanding of this robust phenomenology is, however, lacking because experimental resolution is limited. We simulate the heterogeneous transformation kinetics of ultrastable configurations prepared using the swap Monte Carlo algorithm, thus allowing a direct comparison with experiments. We resolve the liquid–glass interface both in space and in time as well as the underlying particle motion responsible for its propagation. We perform a detailed statistical analysis of the interface geometry and kinetics over a broad range of temperatures. We show that the dynamic heterogeneity of the bulk liquid is passed on to the front that propagates heterogeneously in space and intermittently in time. This observation allows us to relate the averaged front velocity to the equilibrium diffusion coefficient of the liquid. We suggest that an experimental characterization of the interface geometry during the heterogeneous devitrification of ultrastable glassy films could provide direct experimental access to the long-sought characteristic length scale of dynamic heterogeneity in bulk supercooled liquids.

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

“…Single-component highly stable glasses transform into the supercool liquid upon heating by a route not observed for other glasses, 20,45,46 and we tested whether this is also true for highly stable two-component glasses. When single-component stable glasses are annealed above T g , the transformation initiates from the free surface because molecular mobility is largest there, and a front is observed because "nucleation" of the supercooled liquid in the bulk of the glass is very slow.…”

“…The calculated activity coefficients are all close to unity, consistent with an ideal solution. Δ H m,i , x i and γ i denote the enthalpy of fusion, molecular fraction, and activity coefficient of component i, respectively, and R is the ideal gas constant. − ln nobreak0em.25em⁡ x normali γ normali = normalΔ H normalm , normali ( 1 T l − 1 T m , i ) 1 R Single-component highly stable glasses transform into the supercool liquid upon heating by a route not observed for other glasses, ,, and we tested whether this is also true for highly stable two-component glasses. When single-component stable glasses are annealed above T g , the transformation initiates from the free surface because molecular mobility is largest there, and a front is observed because “nucleation” of the supercooled liquid in the bulk of the glass is very slow. ,, In order to test the transformation mechanism of a two component PVD glass, we deposited at 50:50 NPD:TPD glass at 315 K (0.91 T g,mix ), and then subsequently isothermally annealed at 360 K ( T g,mix + 14 K).…”