Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Chem Phys. 2010 Jul 7;133(1):014702. doi: 10.1063/1.3442416.
Highly stable glass films of indomethacin (IMC) with thicknesses ranging from 75 to 2900 nm were prepared by physical vapor deposition. Alternating current (AC) nanocalorimetry was used to evaluate the heat capacity and kinetic stability of the glasses as a function of thickness. Glasses deposited at a substrate temperature of 0.84T(g) displayed heat capacities that were approximately 19 J/(mol K) (4.5%) lower than glasses deposited at T(g) (315 K) or the ordinary glass prepared by cooling the liquid. This difference in heat capacity was observed over the entire thickness range and is significantly larger than the approximately 2 J/(mol K) (0.3%) difference previously observed between aged and ordinary glasses. The vapor-deposited glasses were isothermally transformed into the supercooled liquid above T(g). Glasses with low heat capacities exhibited high kinetic stability. The transformation time increased by an order of magnitude as the film thickness increased from 75 to 600 nm and was independent of film thickness for the thickest films. We interpret these results to indicate that the transformation of stable glass into supercooled liquid can occur by either a surface-initiated or bulk mechanism. In these experiments, the structural relaxation time of the IMC supercooled liquid was observed to be nearly independent of sample thickness.
采用物理气相沉积法制备了厚度在 75 至 2900nm 之间的吲哚美辛（IMC）高稳定性玻璃膜。交流（AC）纳米量热法用于评估玻璃的热容和动力学稳定性随厚度的变化。在 0.84T(g) 衬底温度下沉积的玻璃显示出的热容比在 T(g)（315K）或通过冷却液体制备的普通玻璃低约 19 J/(mol K)（4.5%）。这种热容差异在整个厚度范围内都存在，明显大于之前观察到的老化玻璃和普通玻璃之间约 2 J/(mol K)（0.3%）的差异。蒸汽沉积的玻璃在 T(g)以上等温转变为过冷液体。低热容量的玻璃表现出高动力学稳定性。随着膜厚从 75nm 增加到 600nm，转变时间增加了一个数量级，对于最厚的膜，转变时间与膜厚无关。我们解释这些结果表明，稳定玻璃向过冷液体的转变可以通过表面引发或体相机制发生。在这些实验中，观察到 IMC 过冷液体的结构弛豫时间几乎与样品厚度无关。
