Jha Kshitij C, Dhinojwala Ali, Tsige Mesfin
Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States.
ACS Macro Lett. 2015 Nov 17;4(11):1234-1238. doi: 10.1021/acsmacrolett.5b00612. Epub 2015 Oct 23.
We have used all-atom molecular dynamics (MD) simulations to calculate the surface tension of melt poly(methyl methacrylate) (PMMA) as a function of tacticity. Computation of surface tension using the Kirkwood-Buff approach required hundreds of nanoseconds of equilibration. The computed slopes of surface tension versus temperature are in very good agreement with reported experimental values. Using a rigorous treatment of the true interface, which takes into account the molecular roughness, we find that isotactic PMMA, in comparison to syndiotactic and atactic PMMA, shows a larger surface concentration of polar ester-methyl and carbonyl groups on the surface versus nonpolar α-methyl groups. A mechanistic hypothesis based on the helical nature of the isotactic PMMA chains, their relative flexibility, and their reported conformational energies is proposed to explain the trends in composition near the surface. We highlight here how surface composition and surface tension are controlled by both polarity and steric constraints imposed by tacticity.
我们使用全原子分子动力学（MD）模拟来计算作为间规度函数的聚甲基丙烯酸甲酯（PMMA）熔体的表面张力。使用柯克伍德-布夫方法计算表面张力需要数百纳秒的平衡时间。计算得到的表面张力与温度的斜率与报道的实验值非常吻合。通过对真实界面进行严格处理，考虑到分子粗糙度，我们发现与间同立构和无规立构PMMA相比，等规立构PMMA在表面上相对于非极性α-甲基显示出更高浓度的极性酯甲基和羰基。基于等规立构PMMA链的螺旋性质、它们的相对柔韧性以及报道的构象能量，提出了一个机理假设来解释表面附近组成的趋势。我们在此强调了表面组成和表面张力是如何由间规度所施加的极性和空间位阻约束共同控制的。
