Dynamical Heterogeneity in a Vapor-Deposited Polymer Glass
Jack F. Douglas, Wengang Zhang, Francis W. Starr
Recently, there has been great interest in 'ultrastable' glasses formed via vapor deposition, both because of emerging applications of these materials (e.g., active layers in light-emitting diodes and photovoltaics) and, theoretically, as materials for probing equilibrium properties of glassy mate-rials below their glass transition, based on the assumption that these materials are equivalent to glassy materials aged over astronomical timescales. We use molecular simulations to contrast thermodynamic, structural, and dynamic properties of vapor-deposited and ordinary quenched glasses. While our simulated ultrastable glasses reproduce the expected trends in the thermodynamic properties of these ﬁlms, such as a higher density and lower potential energy than ordinary glasses, the relaxation times of the ultrastable and ordinary glasses are nearly the same when com-pared at the same temperature. This observation is clearly inconsistent with the proposition that the ultrastable glasses are equivalent to highly-aged glasses. The stability of vapor-deposited ﬁlms results from the polymers being trapped in a highly non-equilibrium state, as in isolated strongly adsorbed polymers on substrates and solution-deposited multi-layer polyelectrolyte ﬁlms.
Journal of Chemical Physics
ultrastable glass, vapor deposition, glass formation, enthalpy, density, relaxation time, non-equilibrium state