Surface Confinement Effect on the Elastic Properties of Cross-Linked Polymer Thin Films

Cross-linked polymer thin films have been widely used in energy storage devices and sensors. However, much less is understood about the surface confinement effect on the elastic properties of cross-linked polymer thin films. Here, we systematically investigated how different factors (i.e., cross-link density, temperature, and film thickness) influence the surface confinement effect of cross-linked polymer thin films using coarse-grained molecular dynamics. The free surface exhibits a significant confinement effect on the local structure, dynamics, and elastic properties of the cross-linked polymer thin films. The estimation of interfacial thickness within which the local properties deviate from its interior zone indicates that there is a decoupling phenomenon among the local density, molecular mobility (⟨ u 2 ⟩), and nanoindentation modulus near the free surface, manifested by the distinct interfacial length scale estimated from different properties. In addition, the glassy elastic modulus exhibits a linear scaling law with the local molecular stiffness. The distribution of local molecular stiffness indicates that the degree of elastic heterogeneity in the cross-linked polymer thin film is greater than that in the bulk polymer. Moreover, the effect of film thickness on the surface confinement was also examined. The elastic modulus exhibits the same temperature dependency for cross-linked polymer thin films with thickness larger than a critical value, whereas it decays slower for the ultrathin film, indicating a broadening of glass transition. Our findings provide novel physical insights into how different factors influence the surface confinement effect of cross-linked polymer thin films and shed light on the design of high-performance cross-linked polymer thin films at the nanoscale.