Physical vapor deposition of organic glasses: Indomethacin and tris-naphthylbenzene

Physical vapor deposition of the organic glass-formers indomethacin (IMC) and tris - naphthylbenzene (TNB) was studied. The effect of substrate temperature and deposition rate was systematically interrogated with differential scanning calorimetry (DSC). Temperatures near 0.85 Tg and slow deposition rates resulted in glasses with low enthalpy and high kinetic stability. It was estimated that it would have taken 1000 years of aging to prepare the vapor-deposited film. The ability to deposit glasses with these unusual properties was attributed to a layer of enhanced mobility at the surface of the glass estimated to be 107 times faster than the dynamics in the bulk. Configurations can be efficiently sampled at the surface and low positions on the potential energy landscape can be reached. For substrate temperatures far below 0.85 Tg, glasses with high enthalpy and low kinetic stability resulted due to a lack of enhanced mobility at the surface. Prior to this work, this low temperature regime was standard for physical vapor deposition.;Properties of these unique glasses were further studied using other techniques. Quasi-isothermal calorimetry experiments on 30 mum thick IMC films showed that the transformation time from the as-deposited glass into the supercooled liquid was 104 times longer than the structural relaxation time of the liquid. Additional quasi-isothermal experiments using nanocalorimetry on IMC displayed a thickness dependence in the transformation time for films up to 600 nm thick, indicating a surface-initiated transformation. For films between 1.4 and 30 mum, the transformation time is constant, designating a distinct bulk transformation pathway. A 1 microm length scale describes the average distance between transformation initiation sites in the bulk low-energy glass; this length scale is much longer than the few nm length scales typically used to describe glasses and supercooled liquids. Finally, Brillouin light scattering experiments determined that the moduli of the as-deposited glasses were up to 19% higher than for the ordinary liquid-quenched glass.