Theory of Polymers and Soft Matter, Department of Applied Physics, Technische Universiteit Eindhoven, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
J Chem Phys. 2013 Jul 14;139(2):024906. doi: 10.1063/1.4811237.
Molecular-dynamics simulations have been carried out for a coarse-grained model of a random AB-copolymer confined between two crystalline substrates. The strength of substrate-polymer interactions, and the distance between the two substrates have been varied in a wide range. For thick films the film-averaged segmental mobility decreases for intermediate adsorption strengths, but start to increase for very high substrate-polymer attraction strength. We saw that this non-monotonic behavior is caused by a very strong heterogeneity of the segmental dynamics above the glass-transition temperature: the segmental mobility slows down drastically close to adsorbing substrates, but strongly increases in the middle part of the film. This effect, and its sensitive dependence on film thickness, are explained by finite-size effects in confinement, in combination with glassy boundary layers. It is demonstrated that film-averaged mobility as often measured cannot be understood without resolving local mobility in space and time.
已针对受限于两个结晶基底之间的随机 AB 共聚物的粗粒度模型进行了分子动力学模拟。在很宽的范围内改变了基底-聚合物相互作用的强度和两个基底之间的距离。对于较厚的薄膜，在中等吸附强度下，薄膜平均片段迁移率降低，但对于非常高的基底-聚合物吸引力强度开始增加。我们发现这种非单调行为是由玻璃化转变温度以上片段动力学的强烈非均质性引起的：片段迁移率在靠近吸附基底的地方急剧减慢，但在薄膜的中间部分强烈增加。这种效应及其对薄膜厚度的敏感依赖性，是由受限中的有限尺寸效应与玻璃状边界层相结合引起的。结果表明，如果不解析空间和时间上的局部迁移率，就无法理解通常测量的薄膜平均迁移率。
