Molecular, polymeric, colloidal, and other classes of liquids can exhibit very large, spatially heterogeneous alterations of their dynamics and glass transition temperature when confined to nanoscale domains. Considerable progress has been made in understanding the related problem of near-interface relaxation and diffusion in thick films. However, the origin of “nanoconfinement effects” on the glassy dynamics of thin films, where gradients from different interfaces interact and genuine collective finite size effects may emerge, remains a longstanding open question. Here, we combine molecular dynamics simulations, probing 5 decades of relaxation, and the Elastically Cooperative Nonlinear Langevin Equation (ECNLE) theory, addressing 14 decades in timescale, to establish a microscopic and mechanistic understanding of the key features of altered dynamics in freestanding films spanning the full range from ultrathin to thick films. Simulations and theory are in qualitative and near-quantitative agreement without use of any adjustable parameters. For films of intermediate thickness, the dynamical behavior is well predicted to leading order using a simple linear superposition of thick-film exponential barrier gradients, including a remarkable suppression and flattening of various dynamical gradients in thin films. However, in sufficiently thin films the superposition approximation breaks down due to the emergence of genuine finite size confinement effects. ECNLE theory extended to treat thin films captures the phenomenology found in simulation, without invocation of any critical-like phenomena, on the basis of interface-nucleated gradients of local caging constraints, combined with interfacial and finite size-induced alterations of the collective elastic component of the structural relaxation process.

当限制在纳米尺度范围内时，分子、聚合物、胶体和其他类别的液体可以表现出非常大的、空间异质的动力学和玻璃化转变温度变化。在理解厚膜中近界面弛豫和扩散的相关问题方面已经取得了相当大的进展。然而，薄膜玻璃动力学的“纳米限制效应”的起源仍然是一个长期悬而未决的问题，其中来自不同界面的梯度相互作用，并且可能出现真正的集体有限尺寸效应。在这里，我们结合分子动力学模拟（探索 5 个十年的弛豫）和弹性协作非线性朗之万方程（ECNLE）理论（解决 14 个十年的时间尺度），建立对跨度独立薄膜中改变动力学的关键特征的微观和机械理解。从超薄到厚膜的全系列产品。在不使用任何可调参数的情况下，模拟和理论在定性和近乎定量上一致。对于中等厚度的薄膜，使用厚膜指数势垒梯度的简单线性叠加可以很好地预测动态行为，包括薄膜中各种动态梯度的显着抑制和平坦化。然而，在足够薄的薄膜中，由于真正的有限尺寸限制效应的出现，叠加近似失效。 扩展到处理薄膜的 ECNLE 理论捕获了模拟中发现的现象，无需调用任何类似临界的现象，基于局部笼约束的界面成核梯度，结合界面和有限尺寸引起的集体弹性组件的变化的结构弛豫过程。