Interfacial Dynamics Governs the Mechanical Properties of Nanoconfined Glassy Polymers

Understanding the mechanical properties of nanoconfined polymers is essential in design of nanostructured soft materials. Here, we investigate the mechanical properties of free-standing polymer thin films by employing an atomistically informed coarse-grained (CG) modeling approach. By examining three representative CG polymer models having distinct segmental structure, PS, PMMA, and PECPMA, our results show that the film elastic moduli are substantially reduced with decreasing film thickness compared to their bulk values at their glassy state. Specifically, the PS and PMMA films exhibit similar size-dependent elastic responses and their film moduli are reduced compared to bulk values at a thickness of less than 40 nm, which agrees well with previous experimental measurements. However, in a model methacrylate-based polymer useful in photolithography, PECPMA, the length scale where elastic modulus deviates from the bulk value is much larger. The local molecular stiffness within the films further reveals a gradient a softer interfacial layer having a size of only a few nanometers. Our simulations uncover the size scaling relationship that universally holds for all three polymers and highlights the importance of interfacial dynamics in the mechanical properties of polymer films.