Anisotropic structure and transformation kinetics of vapor-deposited indomethacin glasses.

One- and two-dimensional wide-angle X-ray scattering (1D and 2D WAXS) measurements were performed on vapor-deposited glasses of indomethacin. Physical vapor deposition can be used to prepare organic glasses with high kinetic stability and other properties that are expected for glasses that have been aged for thousands of years. It was previously reported that 1D WAXS from such stable glasses contains an extra peak at q = 0.6 Å(-1) that is not characteristic of the ordinary glass or expected for a highly aged glass. 2D WAXS measurements presented here show that the extra WAXS peak is caused by anisotropic packing in the vapor-deposited glass. The electron density is modulated normal to the film surface with a period roughly equal to the center of mass separation of indomethacin molecules. When such samples are annealed, the packing in the sample becomes isotropic. The transformation time for this process is much longer than the structural relaxation of the supercooled liquid and has a weaker temperature dependence. The observed temperature dependence of the transformation time is consistent with a growth front mechanism for the conversion of the stable glass into the supercooled liquid.