Transformation kinetics of vapor-deposited thin film organic glasses: the role of stability and molecular packing anisotropy

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 T-dep < 250 K. These glasses transform faster with increasing T-f. 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.