|Title||Calorimetric Evidence for Two Distinct Molecular Packing Arrangements in Stable Glasses of Indomethacin|
|Publication Type||Journal Article|
|Year of Publication||2009|
|Authors||Kearns, K. L., S. F. Swallen, M. D. Ediger, Y. Sun, and L. Yu|
|Journal||Journal of Physical Chemistry B|
|Keywords||AMORPHOUS PHASE, CRYSTAL-GROWTH, DIFFERENTIAL SCANNING CALORIMETRY, GLACIAL STATE, HEAT-CAPACITY, LIQUID-PHASE, PHOSPHITE, STRUCTURAL RELAXATION, THIN POLYMER-FILMS, TRANSITION TEMPERATURE, TRIPHENYL|
Indomethacin glasses of varying stabilities were prepared by physical vapor deposition onto substrates at 265 K. Enthalpy relaxation and the mobility onset temperature, were assessed with differential scanning calorimetry (DSC). Quasi-isothermal temperature-modulated DSC was used to measure the reversing heat capacity during annealing above the glass transition temperature T(g). At deposition rates near 8 angstrom/s, scanning DSC shows two enthalpy relaxation peaks and quasi-isothermal DSC shows a two-step change in the reversing heat capacity. We attribute these features to two distinct local packing structures in the vapor-deposited glass, and this interpretation is supported by the strong correlation between the two calorimetric signatures of the glassy to liquid transformation. At lower deposition rates, a larger fraction of the sample is prepared in the more stable local packing. The transformation of the vapor-deposited glasses into the supercooled liquid above Tg is exceedingly slow, as much as 4500 times slower than the structural relaxation time of the liquid.
|Alternate Journal||J. Phys. Chem. B|