Hasebe Mariko, Musumeci Daniele, Powell C Travis, Cai Ting, Gunn Erica, Zhu Lei, Yu Lian
School of Pharmacy and ‡Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53705, United States.
J Phys Chem B. 2014 Jul 10;118(27):7638-7646. doi: 10.1021/jp503110g. Epub 2014 Jun 30.
Organic glasses can grow crystals much faster on the free surface than in the interior, a phenomenon important for fabricating stable amorphous materials. This surface process differs from and is faster than the glass-to-crystal (GC) growth mode existing in the bulk of molecular glasses. We report that similar to GC growth, surface crystal growth terminates if glasses are heated to gain fluidity. In their steady growth below the glass transition temperature T, surface crystals rise above the amorphous surface while spreading laterally and are surrounded by depressed grooves. Above T, the growth becomes slower, sometimes unstable. This damage is stronger on segregated needles (α indomethacin, nifedipine, and o-terphenyl) than on crystals growing in compact domains (γ indomethacin). This effect arises because the onset of liquid flow causes the wetting and embedding of upward-growing surface crystals. Segregated needles are at greater risk because their slow-growing flanks appear stationary relative to liquid flow at a low temperature. The disruption of surface crystal growth by fluidity supports the view that the process occurs by surface diffusion, not viscous flow. Compared to the bulk GC mode, surface crystal growth is disrupted less abruptly by fluidity. Nevertheless, to the extent that fluidity damages them, both processes are solid-state phenomena terminated in the liquid state.
有机玻璃在自由表面上生长晶体的速度比在内部快得多，这一现象对于制造稳定的非晶态材料很重要。这种表面过程不同于且快于分子玻璃主体中存在的玻璃到晶体（GC）生长模式。我们报告称，与GC生长类似，如果玻璃被加热以获得流动性，表面晶体生长就会终止。在玻璃化转变温度T以下的稳定生长过程中，表面晶体在横向扩展的同时会高于非晶表面，并被凹陷的沟槽包围。在T以上，生长变得更慢，有时不稳定。这种破坏在分离的针状晶体（α-吲哚美辛、硝苯地平和邻三联苯）上比在紧密区域生长的晶体（γ-吲哚美辛）上更强。这种效应的出现是因为液体流动的开始会导致向上生长的表面晶体被润湿并嵌入其中。分离的针状晶体面临的风险更大，因为它们生长缓慢的侧面在低温下相对于液体流动似乎是静止的。流动性对表面晶体生长的破坏支持了这样一种观点，即该过程是通过表面扩散而非粘性流动发生的。与本体GC模式相比，表面晶体生长受流动性的破坏不那么突然。然而，就流动性对它们造成破坏的程度而言，这两个过程都是在液态中终止的固态现象。
