Enhancing the Fracture Toughness of Polymer-Infiltrated Nanoparticle Films via Polymer Bridging and Entanglement

The mechanical properties of disordered nanoparticle (NP) packings can be significantly improved through capillary rise infiltration (CaRI) of polymers to enhance the interactions between the NPs. While results from previous nanoindentation-based fracture measurements suggest that polymer infiltration can toughen NP packings, it is challenging to fully understand how the relative size of polymer and nanoparticles and the extent of confinement affect the toughness. In the present work, a thin-film fracture testing method based on the double cantilever beam (DCB) specimen is developed and used to investigate the fracture properties of polymer-infiltrated nanoparticle films. In the DCB specimen, a crack is propagated in NP films over distances of tens of millimeters, allowing for highly accurate measurements of toughness in a mode I (tensile opening) configuration. The fracture toughness of the polymer-infiltrated NP films is found to be strongly dependent on polymer molecular weight (MW) and NP size. Low MW, unentangled polymers, effectively toughen small NP packings; whereas high MW, entangled polymers, show enhanced toughening in large NP packings. Possible toughening mechanisms, including confinement-induced polymer bridging and polymer entanglement, will be discussed.