Interfacial stiffening of polymer thin films under nanoconfinement

Changes in the mechanical properties of polymer thin films due to substrate and free surfaces determine the performance of soft nanomaterials. Here we systematically investigate how these interfaces influence the elasticity of supported polymer thin films. Our simulation results demonstrate that the substrate–film interfacial interaction has a strong effect on the elastic response of the films, in which the modulus increases with the strength of interfacial interaction with a nonlinear scaling relationship. The investigation of the local stiffness within the film reveals that there exists an interfacial stiffened region near the substrate and a soft region with reduced stiffness near the free surface. Both regions exhibit similar length scales on the order of nanometers. Our analyses explain the molecular origins of recently observed interfacial stiffening and softening phenomena in polymer thin films, and provide a high-resolution description of the local stiffness gradients in polymer thin films.