ABSTRACT
Recent work has shown that diffusion and crystal growth can be much faster on the surface of molecular glasses than in the interior and that the enhancement effect varies with molecular size and intermolecular hydrogen bonds (HBs). In a related phenomenon, some molecules form highly stable glasses when vapor-deposited, while others (notably those forming extensive HBs) do not. Here we examine all available data on these phenomena for quantitative structure-property relations. For the systems that form no HBs, the surface diffusion coefficient D s decreases with increasing molecular size d (d = Ω1/3, where Ω is the molecular volume); when evaluated at the glass transition temperature T g, D s decreases â¼5 orders of magnitude for 1 nm of increase in d. Assuming that center-of-mass diffusion is limited by the deepest part of the molecule in the surface-mobility gradient, these data indicate a mobility gradient in reasonable agreement with the Elastically Collective Nonlinear Langevin Equation theory prediction for polystyrene as disjointed Kuhn monomers. For systems of similar d, the D s value decreases with the extent of intermolecular HB, x (HB), defined as the fraction of vaporization enthalpy due to HB. For both groups together (hydrogen-bonded and otherwise), the D s data collapse when plotted against d/[1 - x(HB)]; this argues that the HB effect on D s can be described as a narrowing of the surface mobility layer by a factor [1 - x(HB)] relative to the van der Waals systems. Essentially the same picture holds for the surface crystal growth rate u s. The kinetic stability of a vapor-deposited glass decreases with x(HB) but is not better organized by the combined variable d/[1 - x(HB)]. These results indicate that surface crystal growth depends strongly on surface diffusion, whereas the formation of stable glasses by vapor deposition may depend on other factors.
