Surface diffusion and surface crystal growth of tris-naphthyl benzene glasses

Surface self-diffusion coefficients of alpha,alpha,beta-tris-naphthyl benzene (TNB) glasses have been measured using the method of surface grating decay. For 1000 nm wavelength gratings, the decay occurs by viscous flow at temperatures above T-g + 15 K, where T-g is the glass transition temperature (347 K), and by surface diffusion at lower temperatures. Surface diffusion of TNB is vastly faster than bulk diffusion, by a factor of 107 at T-g. Comparing TNB with other molecular glasses, each evaluated at its own T-g, we find that surface diffusion has a greater system-to-system variation than bulk diffusion, slowing down with increasing molecular size and intermolecular hydrogen bonding. Experimentally determined surface diffusion coefficients are in reasonable agreement with those from simulations and theoretical predictions. TNB and other molecular glasses show fast crystal growth on the free surface and the growth velocity is nearly proportional to the surface diffusion coefficient, indicating that the process is supported by surface mobility. Published by AIP Publishing.