Dalal Shakeel S, Ediger M D
Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.
J Phys Chem B. 2015 Mar 5;119(9):3875-82. doi: 10.1021/jp512905a. Epub 2015 Feb 23.
Stable organic glasses prepared by physical vapor deposition transform into the supercooled liquid via propagating fronts of molecular mobility, a mechanism different from that exhibited by glasses prepared by cooling the liquid. Here we show that spectroscopic ellipsometry can directly observe this front-based mechanism in real time and explore how the velocity of the front depends upon the substrate temperature during deposition. For the model glass former indomethacin, we detect surface-initiated mobility fronts in glasses formed at substrate temperatures between 0.68Tg and 0.94Tg. At each of two annealing temperatures, the substrate temperature during deposition can change the transformation front velocity by a factor of 6, and these changes are imperfectly correlated with the density of the glass. We also observe substrate-initiated fronts at some substrate temperatures. By connecting with theoretical work, we are able to infer the relative mobilities of stable glasses prepared at different substrate temperatures. An understanding of the transformation behavior of vapor-deposited glasses may be relevant for extending the lifetime of organic semiconducting devices.
通过物理气相沉积制备的稳定有机玻璃通过分子迁移率的传播前沿转变为过冷液体，这一机制不同于通过冷却液体制备的玻璃所表现出的机制。在此我们表明，椭圆偏振光谱法可以实时直接观察到这种基于前沿的机制，并探索前沿速度如何取决于沉积过程中的基板温度。对于模型玻璃形成剂吲哚美辛，我们在基板温度介于0.68Tg和0.94Tg之间形成的玻璃中检测到表面引发的迁移前沿。在两个退火温度中的每一个温度下，沉积过程中的基板温度可使转变前沿速度变化6倍，并且这些变化与玻璃的密度不完全相关。我们还在一些基板温度下观察到基板引发的前沿。通过与理论工作相结合，我们能够推断在不同基板温度下制备的稳定玻璃的相对迁移率。对气相沉积玻璃转变行为的理解可能与延长有机半导体器件的寿命相关。
