Abstract
While ordinary glasses transform into supercooled liquid via a homogeneous bulk mechanism, thin film glasses of higher stability transform heterogeneously by a front propagating from the surface and/or the interfaces. In this work, we use quasi-adiabatic fast scanning nanocalorimetry to determine the heat capacity of thin glassy layers of indomethacin vapor-deposited in a broad temperature range of 110 K below the glass transition temperature. Their variation in fictive temperature amounts to 40 K. We show that a propagating front is the initial transformation mechanism in all cases. Using an ad hoc surface normalization procedure we determine the corresponding growth front velocity for the whole range of deposition temperatures. Although the transformation rate changes by a factor of 10 between the most and less stable samples, the relation between the mobility of the front and the thermodynamic stability of the glass is not uniquely defined. Glasses grown above 280 K, which are at equilibrium with the supercooled liquid, present a different dependence of the growth front velocity on fictive temperature compared to glasses grown out of equilibrium at Tdep < 250 K. These glasses transform faster with increasing Tf. Our data clarify previous reports and support the evidence that the fictive temperature alone is not an absolute indicator of the properties of the glass, at least when its structure is not completely isotropic. To interpret the data, we propose that the growth front velocity depends on three terms: the mobility of the liquid at a given temperature, the mobility of the glass and the arrangement of the molecules in the glass.
Highlights
Glasses are systems with great interest from the technological and scientific points of view.[1]
Our data expand previous measurements by Dalal et al.[34] and clearly show that the growth front velocity and the fictive temperature of the glass are not correlated and, we identify two different regimes depending on the deposition temperature
We have previously shown that the normalization of the heat capacity in thin film stable glasses should account for the heterogeneous nature of the transformation into the supercooled liquid, since a standard normalization by the mass can yield incorrect conclusions
Summary
Glasses are systems with great interest from the technological and scientific points of view.[1]. Kinetic facilitation models predicted that the front propagation in highly stable glasses depends mainly on the relaxation time of the liquid phase. This is in agreement with the finding that glasses of different stabilities have the same temperature dependence at least in a limited temperature range, i.e. they share common activation energy. Experimental evaluations of the growth front velocities for glasses of different stabilities grown at Tdep o 0.85Tg, where Tg is the glass transition temperature of a conventional glass, are consistent in general with this view, but the dependence on stability is much larger. The role that the glass plays in the transformation rate is not yet clear
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