Mechanical Properties of Ultrathin Polymer Nanocomposites

The mechanical properties of polymer nanocomposites have been widely studied; however, very few studies have investigated how these properties change for films with thickness approaching the average size of a polymer molecule. Polymer nanocomposites can provide tunable mechanical properties that are essential for advanced applications that require ultrathin glassy polymer films, including filtration membranes and coatings. Herein, we directly measured the stress–strain response of ultrathin polymethylmethacrylate (PMMA)/silica nanocomposite films on a water surface as a function of film thickness and nanoparticle loading. We found that the elastic modulus was independent of nanoparticle loading and thickness for films as thin as 19 nm. The maximum stress prior to failure decreased as nanoparticle loading increased, and the rate of decrease with nanoparticle loading was dependent strongly upon film thickness. We hypothesize that as the volume fraction increases, the interparticle distance decreases, leading to chain confinement between the nanoparticles. This chain confinement limits the extent of interchain entanglement, which has deleterious effects on strength for this weakly interacting polymer–nanoparticle system. We relate these dimensionally controlled effects to the measured changes in the mechanical response of neat PMMA films with film thickness near the average configurational size scale. Our findings suggest that free surface–nanoparticle interactions, film thickness, and interparticle distance all can impact the mechanical response of ultrathin nanocomposites.