Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Phys Chem B. 2011 Jan 27;115(3):455-63. doi: 10.1021/jp1092916. Epub 2010 Dec 17.
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.
对吲哚美辛的气相沉积玻璃进行了一维和二维广角 X 射线散射（1D 和 2D WAXS）测量。物理气相沉积可用于制备具有高动力学稳定性的有机玻璃，以及其他预期在几千年老化后具有的玻璃特性。此前有报道称，这种稳定玻璃的一维 WAXS 在 q = 0.6 Å（-1）处存在一个额外的峰，这不是普通玻璃的特征，也不是高度老化玻璃所预期的。本文介绍的二维 WAXS 测量表明，额外的 WAXS 峰是由气相沉积玻璃中的各向异性堆积引起的。电子密度沿垂直于膜表面的方向调制，周期大致等于吲哚美辛分子的质心分离。当对这样的样品进行退火时，样品中的堆积变得各向同性。这个过程的转变时间比过冷液体的结构弛豫长得多，并且温度依赖性较弱。观察到的转变时间的温度依赖性与稳定玻璃向过冷液体转变的生长前沿机制一致。
