Softening of rapidly heated nanoscale glassy films of methylbenzene

Using Fast Scanning Calorimetry (FSC), we investigated the impact of thin film confinement on softening and vaporization of glassy films of methylbenzene in the limit of high heating rates. The glassy films of distinct thicknesses ranging from 2 to 1800 nm were grown by vapor deposition and subjected to heating with rates in excess of 10⁵ K/s. For the films with thicknesses above 20 nm, the resulting softening kinetics strongly implied a surface facilitated mechanism: the softening originated at surfaces of a sample and progresses into its bulk via a transformation front. Yet, according to our analysis, the kinetic parameters for the softening front propagation differed significantly for the films with thicknesses below and above 300 nm. For example, the apparent activation energy barrier for the front propagation almost doubled for thicker films. We interpret these results in the framework modified Wilson-Frenkel model of softening front propagation and discuss their implications for past FSC studies of nanoscale vapor-deposited glassy films.