After more than two decades of study, many fundamental questions remain unanswered about the dynamics of glass-forming materials confined to thin films. Experiments and simulations indicate that free interfaces enhance dynamics over length scales larger than molecular sizes, and this effect strengthens at lower temperatures. The nature of the influence of interfaces, however, remains a point of significant debate. In this work, we explore the properties of the nonequilibrium phase transition in dynamics that occurs in trajectory space between high- and low-mobility basins in a set of model polymer freestanding films. In thick films, the film-averaged mobility transition is broader than the bulk mobility transition, while in thin films it is a variant of the bulk result shifted toward a higher bias. Plotting this transition’s local coexistence points against the distance from the films’ surface shows thick films have surface and film-center transitions, while thin films practically have a single transition throughout the film. These observations are reminiscent of thermodynamic capillary condensation of a vapor–liquid phase between parallel plates, suggesting they constitute a demonstration of such an effect in a trajectory phase transition in the dynamics of a structural glass former. Moreover, this transition bears similarities to several experiments exhibiting anomalous behavior in the glass transition upon reducing film thickness below a material-dependent onset, including the broadening of the glass transition and the homogenization of surface and bulk glass transition temperatures.

经过二十多年的研究，有关薄膜中玻璃形成材料动力学的许多基本问题仍未得到解答。实验和模拟表明，自由界面增强了大于分子尺寸的长度尺度上的动力学，并且这种效应在较低温度下增强。然而，界面影响的本质仍然是一个备受争议的问题。在这项工作中，我们探索了一组模型聚合物独立薄膜中高迁移率盆地和低迁移率盆地之间轨迹空间中发生的动力学非平衡相变的特性。在厚膜中，薄膜平均迁移率转变比体迁移率转变更宽，而在薄膜中，它是体结果向更高偏置移动的变体。绘制该转变的局部共存点与距薄膜表面的距离的关系图表明，厚薄膜具有表面和薄膜中心转变，而薄膜实际上在整个薄膜中具有单一转变。这些观察结果让人想起平行板之间汽液相的热力学毛细管冷凝，这表明它们证明了结构玻璃形成体动力学中轨迹相变的这种效应。此外，这种转变与几个实验有相似之处，这些实验表现出在将膜厚度降低到材料相关起始点以下时玻璃化转变的异常行为，包括玻璃化转变的加宽以及表面和本体玻璃化转变温度的均质化。