Nanocalorimetry as a means to explore thin films of vapor-deposited organic glasses

Vapor deposition was used to prepare nanometer thick films of small molecule organic glasses. Films of indomethacin (IMC) and 1,3,5-(tris)naphthylbenzene (TNB) with a range of stabilities and thicknesses were created and characterized using differential nanocalorimetry. The heat capacity-like calorimetric signal was lower for the stable vapor-deposited glass films at temperatures below the glass transition T$_{g}$ than for an ordinary glass prepared by cooling the liquid. A gradual increase in the calorimetric signal was observed during the isothermal transformation above T$_{g}$ from stable to ordinary glass with the fastest transformation taking place in about 200 $\tau _{\alpha }$. The time for this transformation was dependent on film thickness with 350 nm thick films transforming approximately 5 times faster than 40 $\mu $m thick films. Aging experiments on the ordinary glass also showed a thickness dependence with thinner films aging more rapidly. This thickness dependent behavior is consistent with a mechanism where the dynamics at the film interfaces are faster than those in the bulk.