"Gel-like" Mechanical Reinforcement in Polymer Nanocomposite Melts

We critically explore the role of particle dispersion oil the melt state mechanical properties of nanocomposites formed by mixing polystyrene homopolymers with polystyrene grafted silica nanoparticles. We selected this system since we previously showed that nanoparticle spatial distribution can be controlled through judicious choices of the brush and matrix parameters. Here we focus oil the temporal evolution of the nanoparticle self-assembly dispersion state and its effect on mechanical reinforcement using rheology, electron microscopy, and the measurement of nanoscale particle dynamics using X-ray photon correlation spectroscopy. Nanoscale and macroscopic experiments show that it composite with percolating sheets of particles displays "gel-like" or solid-like mechanical behavior at lower particle loadings than one with uniform particle dispersion. This conclusion allows us to conjecture that mechanical reinforcement is primarily controlled by interparticle interactions (including those facilitated by the grafted chains) and that the matrix plays a relatively minor role. This statement has far-reaching consequences oil the design of polymer nanocomposites with desired properties.