Nanoparticle Percolation Improves the Mechanical Properties of Polymer Nanocomposite Films

Nanoparticle (NP) percolation governs the mechanical reinforcement of polymer nanocomposite (PNC) films. In this study, we demonstrate that both internal network morphology and near-surface morphology of NPs influence the mechanical properties of PNC films composed of poly(methyl methacrylate)-grafted silica nanoparticles (PMMA-NPs) in a poly(styrene-ran-acrylonitrile) (SAN) matrix. By systematically varying film thickness and annealing conditions, we achieve distinct NP morphologies, including continuous pillars, discrete pillars, clusters, and interconnected networks, each exhibiting different levels of NP percolation. Atomic force microscopy nanoindentation reveals that films with interconnected networks exhibit the highest reduced modulus (7.6 GPa), nearly quadrupling that of as-cast films with uniform NP dispersion (1.8 GPa), along with a substantial increase in hardness (624 MPa compared to 298 MPa). Notably, surface NP structures formed during annealing contribute to an enhanced modulus at low loads. These findings establish a direct structure-property relationship, providing insights into designing mechanically robust PNCs for applications in coatings, electronics, and energy storage.