Observation of low heat capacities for vapor-deposited glasses of indomethacin as determined by AC nanocalorimetry.

Highly stable glass films of indomethacin (IMC) with thicknesses ranging from 75 to 2900 nm were prepared by physical vapor deposition. Alternating current (AC) nanocalorimetry was used to evaluate the heat capacity and kinetic stability of the glasses as a function of thickness. Glasses deposited at a substrate temperature of 0.84T(g) displayed heat capacities that were approximately 19 J/(mol K) (4.5%) lower than glasses deposited at T(g) (315 K) or the ordinary glass prepared by cooling the liquid. This difference in heat capacity was observed over the entire thickness range and is significantly larger than the approximately 2 J/(mol K) (0.3%) difference previously observed between aged and ordinary glasses. The vapor-deposited glasses were isothermally transformed into the supercooled liquid above T(g). Glasses with low heat capacities exhibited high kinetic stability. The transformation time increased by an order of magnitude as the film thickness increased from 75 to 600 nm and was independent of film thickness for the thickest films. We interpret these results to indicate that the transformation of stable glass into supercooled liquid can occur by either a surface-initiated or bulk mechanism. In these experiments, the structural relaxation time of the IMC supercooled liquid was observed to be nearly independent of sample thickness.