Structural Variations of an Organic Glassformer Vapor-Deposited onto a Temperature Gradient Stage

The structural properties of vapor-deposited organic glasses have been shown to depend significantly upon the temperature of the substrate during the deposition. In order to systematically study this effect, a stage was designed to maintain a linear temperature gradient of 80 K across a substrate during vapor deposition. Brillouin light scattering was used to measure the longitudinal sound velocity of glasses of indomethacin at each deposition temperature by performing measurements at many positions along the direction of the temperature gradient. Glasses with exceptionally high longitudinal moduli are formed at deposition temperatures between 0.7 T g and 0.9 T g, where T g is the glass transition temperature. In this regime, the observed dependence of the deposition rate on the longitudinal modulus is consistent with glass formation via enhanced surface mobility. Glasses prepared at low temperatures were observed to have longitudinal moduli up to 20% less than the modulus of ordinary glass of indomethacin.