Department of Physics, Addis Ababa University, Addis Ababa, Ethiopia.
Department of Polymer Science, The University of Akron, Akron, Ohio 44325, USA.
J Chem Phys. 2018 Apr 7;148(13):134705. doi: 10.1063/1.5010276.
We have carried out atomistic (all-atom) molecular dynamics simulations to investigate the effect of tacticity on the structure and glass transition temperature (T) of polystyrene (PS) thin films adsorbed on two distinct types of solid substrates. The systems consist of thin films made of atactic, isotactic, and syndiotactic PS chains supported by graphite or hydroxylated α-quartz substrates, which are known to be atomically flat but chemically and structurally different. We have observed a marked dependence of the film structure on substrate type as well as on tacticity. For instance, rings' orientation near substrate surfaces depends on substrate type for atactic PS and isotactic PS films, while no such dependence is observed for syndiotactic PS films whose interfacial structure seems to result from their propensity to adopt the trans conformation rather than their specific interaction with the substrates. Moreover, our results indicate that glass transition temperatures of substrate supported polystyrene films are higher compared to those of the corresponding free-standing films. More specifically, PS films on graphite exhibit larger T values than those on α-quartz, and we have noticed that syndiotactic PS has the largest T irrespective of the substrate type. Furthermore, the local T in the region of the film in contact with the substrates shows a strong tacticity and substrate dependence, whereas no dependencies were found for the local T in the middle of the film. Substrate-film interaction energy and chains' dynamics near substrate-film interfaces were subsequently investigated in order to substantiate the obtained Ts, and it was found that films with higher Ts are strongly adsorbed on the substrates and/or exhibit smaller interfacial chains' dynamics essentially due to steric hindrance.
我们进行了原子（全原子）分子动力学模拟，以研究立构规整度对吸附在两种不同类型固体基底上的聚苯乙烯（PS）薄膜的结构和玻璃化转变温度（Tg）的影响。这些体系由无规、等规和间规 PS 链组成的薄膜组成，支撑在石墨或羟基化的α-石英基底上，这些基底已知是原子级平坦的，但在化学和结构上是不同的。我们观察到薄膜结构对基底类型以及立构规整度有明显的依赖性。例如，无规和等规 PS 薄膜中环的取向依赖于基底类型，而间规 PS 薄膜则没有这种依赖性，其界面结构似乎是由于它们倾向于采用反式构象，而不是它们与基底的特定相互作用。此外，我们的结果表明，基底支撑的聚苯乙烯薄膜的玻璃化转变温度（Tg）高于相应的自由薄膜。更具体地说，石墨上的 PS 薄膜比α-石英上的 Tg 值更高，我们注意到间规 PS 无论基底类型如何，都具有最大的 Tg。此外，与基底接触的薄膜区域的局部 Tg 表现出强烈的立构规整度和基底依赖性，而在薄膜中部则没有发现这种依赖性。随后研究了基底-薄膜相互作用能和基底-薄膜界面附近链的动力学，以证实所得到的 Tg，发现 Tg 较高的薄膜强烈吸附在基底上，或者表现出较小的界面链动力学，这主要是由于空间位阻。
