Department of Physics, Wesleyan University, Middletown, CT 06459.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899.
Proc Natl Acad Sci U S A. 2018 May 29;115(22):5641-5646. doi: 10.1073/pnas.1722024115. Epub 2018 May 14.
There is significant variation in the reported magnitude and even the sign of [Formula: see text] shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate [Formula: see text] in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that [Formula: see text] based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer-substrate interaction strength ε. In contrast, we find that [Formula: see text] based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but [Formula: see text] increases when ε exceeds the polymer-polymer interaction strength. We show that these qualitatively different trends in [Formula: see text] reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of [Formula: see text] in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of [Formula: see text] Our results emphasize the limitations of using [Formula: see text] to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of [Formula: see text] can be combined to predict local changes in [Formula: see text] near the substrate, providing a simple method to infer information about the mobility gradient.
在具有相同化学性质、薄膜厚度和支撑基底的名义上相同的薄聚合物薄膜中，[Formula: see text]的报道幅度甚至符号都存在显著差异。隐含的假设是，用于估计本体材料中[Formula: see text]的方法与推断薄膜中动态变化相关。为了检验这一假设的有效性，我们对支撑在有吸引力基底上的粗粒化聚合物熔体进行了分子模拟。与许多实验观察到的情况一样，我们发现，基于热力学标准（薄膜高度或焓的温度依赖性）的[Formula: see text]随着薄膜厚度的减小而减小，与聚合物-基底相互作用强度 ε 无关。相比之下，我们发现，基于动力学标准（动态结构因子的弛豫）的[Formula: see text]在 ε 相对较弱时也随厚度减小而减小，但在 ε 超过聚合物-聚合物相互作用强度时[Formula: see text]增加。我们表明，[Formula: see text]中这些定性不同的趋势反映了对薄膜中迁移率梯度的不同敏感性。显然，在动态测量中，基底区域中缓慢弛豫的聚合物链段对[Formula: see text]的偏移贡献最大，但薄膜的这一部分对[Formula: see text]的热力学估计贡献较小。我们的结果强调了使用[Formula: see text]推断聚合物薄膜动力学变化的局限性。然而，我们表明，[Formula: see text]的热力学和动力学估计可以结合起来预测基底附近[Formula: see text]的局部变化，为推断迁移率梯度的信息提供了一种简单方法。
