Zhang Wei, Brian Caleb W, Yu Lian
†School of Pharmacy and ‡Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States.
J Phys Chem B. 2015 Apr 16;119(15):5071-8. doi: 10.1021/jp5127464. Epub 2015 Apr 6.
Surface self-diffusion coefficients have been measured for the model molecular glass o-terphenyl (OTP) through surface-grating decay driven by capillarity. The decay mechanism transitions from viscous flow at high temperatures to surface diffusion at low temperatures; for 1000 nm wavelength gratings, the transition occurs at Tg + 11 K. The surface diffusion of OTP is 10(8) times faster than bulk diffusion at Tg and even faster at lower temperatures because of its weaker temperature dependence. At Tg, OTP has approximately the same bulk diffusivity as the previously studied molecular liquid indomethacin, but its surface diffusion is 100 times faster. While the molecular glass-formers exhibit transitions from viscous flow to surface diffusion as the mechanism of capillarity-driven surface flattening, polystyrenes and silicates show no such transition under comparable conditions, suggesting slower surface diffusion on these materials and a general dependence of surface diffusion on intermolecular forces. The velocity of surface crystal growth on molecular glasses is proportional to surface diffusivity, indicating a common kinetic barrier for both processes for temperatures below Tg.
通过毛细作用驱动的表面光栅衰减，测量了模型分子玻璃邻三联苯（OTP）的表面自扩散系数。衰减机制从高温下的粘性流动转变为低温下的表面扩散；对于1000nm波长的光栅，转变发生在玻璃化转变温度（Tg）+11K时。在Tg时，OTP的表面扩散比体相扩散快10⁸倍，在更低温度下甚至更快，因为其对温度的依赖性较弱。在Tg时，OTP的体相扩散系数与之前研究的分子液体吲哚美辛大致相同，但其表面扩散快100倍。虽然分子玻璃形成剂在毛细作用驱动的表面平整机制中表现出从粘性流动到表面扩散的转变，但在可比条件下，聚苯乙烯和硅酸盐没有这种转变，这表明这些材料上的表面扩散较慢，且表面扩散普遍依赖于分子间力。分子玻璃上表面晶体生长的速度与表面扩散系数成正比，这表明在低于Tg的温度下，这两个过程存在共同的动力学障碍。
