Fast equilibration mechanisms in disordered materials mediated by slow liquid dynamics.

The rate at which a nonequilibrium system decreases its free energy is commonly ascribed to molecular relaxation processes, arising from spontaneous rearrangements at the microscopic scale. While equilibration of liquids usually requires density fluctuations at time scales quickly diverging upon cooling, growing experimental evidence indicates the presence of a different, alternative pathway of weaker temperature dependence. Such equilibration processes exhibit a temperature-invariant activation energy, on the order of 100 kJ mol.

Density of Obstacles Affects Diffusion in Adsorbed Polymer Layers.

The translational diffusion of molecules dispersed into polymer matrices slows down tremendously when approaching a nonrepulsive interface. To unravel the origin of this phenomenon, we investigated the diffusion of molecular probes in the direction normal to an adsorbing wall. Using adsorbed polymer layers as matrices, we were able to decouple interfacial and finite size effects and determined the relation between the diffusion time and the area available at the polymer/solid interface.

Substrate Roughness Speeds Up Segmental Dynamics of Thin Polymer Films.

We show that the segmental mobility of thin films of poly(4-chlorostyrene) prepared under nonequilibrium conditions gets enhanced in the proximity of rough substrates. This trend is in contrast to existing treatments of roughness which conclude it is a source of slower dynamics, and to measurements of thin films of poly(2-vinylpiridine), whose dynamics is roughness invariant. Our experimental evidence indicates the faster interfacial dynamics originate from a reduction in interfacial density, due to the noncomplete filling of substrate asperities.

Surface-Bulk Interplay in Vapor-Deposited Glasses: Crossover Length and the Origin of Front Transformation.

Thin film stable glasses transform into a liquid by a moving front that propagates from surfaces or interfaces with higher mobility. We use calorimetric data of vapor-deposited glasses of different thicknesses and stabilities to identify the role of glassy and liquid dynamics on the transformation process. By invoking the existence of an ultrathin intermediate layer whose transformation strongly depends on the properties of both the liquid and the glass, we show that the recovery to equilibrium is driven by the mismatch in the dynamics between glass and liquid.

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses.

Physical vapour deposition has emerged as the technique to obtain glasses of unbeatable stability. However, vapour deposited glasses exhibit a different transformation mechanism to ordinary glasses produced from liquid. Vapour deposited glasses of different thermodynamic stability, from ultrastable to those similar to ordinary glasses, transform into the liquid state via front propagation starting at the most mobile surfaces/interfaces, at least for the first stages of the transformation, eventually dynamiting the high thermal stability achieved for some of these glasses.