Many measurements have indicated that thin polymer films in their glass state exhibit a mobile interfacial layer that grows in thickness upon heating, while some measurements indicate the opposite trend. Moreover, simulations and limited measurements on glass-forming liquids at temperatures above the glass transition temperature T_g exhibit a growing interfacial mobility scale ξ upon cooling. To better understand these seemingly contradictory trends, we perform molecular dynamics simulations over a temperature regime for which our simulated polymer film enters a non-equilibrium glassy state and find that the relaxation time τ_α within the film interior, relative to the polymer–air interfacial layer, exhibits a maximum near the computational T_g. Correspondingly, we also observe that the interfacial mobility length scale exhibits a maximum near T_g, explaining the apparent reversal in the temperature dependence of this scale between the glass and liquid states. We show that the non-monotonic variation of ξ and the relative interfacial mobility to the film interior arise qualitatively from a non-monotonic variation of the gradient of the effective activation free energy of the film; we then obtain a quantitative description of this phenomenon by introducing a phenomenological model that describes the relaxation time layer-by-layer in the film for a temperature range both above and below T_g of the film as a whole. This analysis reveals that the non-monotonic trend in the relative interfacial mobility and ξ both arise primarily from the distinctive temperature dependence of relaxation in the interfacial layer, which apparently remains in local equilibrium over the whole temperature range investigated.

中文翻译：

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调和聚合物膜界面迁移率的温度依赖性的计算和实验趋势

许多测量结果表明，处于玻璃态的聚合物薄膜表现出流动的界面层，该界面层在加热时厚度会增加，而一些测量结果表明趋势相反。而且，在高于玻璃化转变温度T _g的温度下对玻璃形成液的模拟和有限的测量在冷却时显示出增大的界面迁移率尺度ξ。为了更好地理解这些看似矛盾的趋势，我们在针对我们的模拟聚合物膜进入非平衡玻璃态的温度制度进行分子动力学模拟，发现弛豫时间τ _α膜内部，相对于所述聚合物-空气界面内层，表现出最大接近计算的T _g。相应地，我们还观察到界面迁移长度标度在T _g附近显示出最大值，解释了该标度在玻璃态和液态之间的温度依赖性的明显逆转。我们发现ξ的非单调变化和相对于膜内部的界面迁移率定性地是由膜的有效活化自由能的梯度的非单调变化引起的。然后，我们通过引入现象学模型来定量地描述这种现象，该模型描述了温度在T _g之上和之下的薄膜中层的松弛时间。电影的整体。该分析表明，相对界面迁移率和ξ的非单调趋势主要是由于界面层弛豫的独特温度依赖性引起的，显然，在整个研究温度范围内，该弛豫与温度的关系仍处于局部平衡状态。