Yang Entao, Pressly James F, Natarajan Bharath, Colby Robert, Winey Karen I, Riggleman Robert A
Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
Department of Materials Science & Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
Soft Matter. 2023 Oct 11;19(39):7580-7590. doi: 10.1039/d3sm00898c.
While recent efforts have shown how local structure plays an essential role in the dynamic heterogeneity of homogeneous glass-forming materials, systems containing interfaces such as thin films or composite materials remain poorly understood. It is known that interfaces perturb the molecular packing nearby, however, numerous studies show the dynamics are modified over a much larger range. Here, we examine the dynamics in polymer nanocomposites (PNCs) using a combination of simulations and experiments and quantitatively separate the role of polymer packing from other effects on the dynamics, as a function of distance from the nanoparticle surfaces. After showing good qualitative agreement between the simulations and experiments in glassy structure and creep compliance, we use a machine-learned structure indicator, softness, to decompose polymer dynamics in our simulated PNCs into structure-dependent and structure-independent processes. With this decomposition, the free energy barrier for polymer rearrangement can be described as a combination of packing-dependent and packing-independent barriers. We find both barriers are higher near nanoparticles and decrease with applied stress, quantitatively demonstrating that the slow interfacial dynamics is not solely due to polymer packing differences, but also the change of structure-dynamics relationships. Finally, we present how this decomposition can be used to accurately predict strain-time creep curves for PNCs from their static configuration, providing additional insights into the effects of polymer-nanoparticle interfaces on creep suppression in PNCs.
尽管最近的研究表明局部结构在均匀玻璃形成材料的动态非均质性中起着至关重要的作用，但对于包含界面的系统，如薄膜或复合材料，人们仍然了解甚少。众所周知，界面会扰乱附近的分子堆积，然而，大量研究表明动力学在更大范围内会发生改变。在这里，我们结合模拟和实验研究了聚合物纳米复合材料（PNC）中的动力学，并根据与纳米颗粒表面的距离，定量地分离出聚合物堆积对动力学的影响与其他影响因素。在玻璃态结构和蠕变柔量方面，模拟和实验结果显示出良好的定性一致性之后，我们使用机器学习的结构指标——柔软度，将模拟的PNC中的聚合物动力学分解为与结构相关和与结构无关的过程。通过这种分解，聚合物重排的自由能垒可以描述为与堆积相关和与堆积无关的垒的组合。我们发现，在纳米颗粒附近这两种垒都更高，并且会随着施加的应力而降低，这定量地表明界面处缓慢的动力学不仅仅是由于聚合物堆积差异，还包括结构 - 动力学关系的变化。最后，我们展示了这种分解如何能够根据PNC的静态构型准确预测其应变 - 时间蠕变曲线，从而为聚合物 - 纳米颗粒界面在PNC中对蠕变抑制的影响提供了更多见解。
