Nonlinear Architectures Can Alter the Dynamics of Polymer–Nanoparticle Composites

Abstract

Polymer nanocomposites exhibit remarkable physical properties that are attractive for many applications. These systems have been so far investigated using linear polymer chains; the role of polymer matrix architecture in local dynamics, bulk rheology, and nanoparticle (NP) motion remains unexplored. Here, using quasi-elastic neutron scattering, bulk rheology, and X-ray photon correlation spectroscopy, we investigated nanocomposites with spherical silica nanoparticles well dispersed in poly(ethylene oxide) matrices having different architectures (specifically linear, stars, and hyperbranched). The results reveal that the mechanical reinforcement of the nanocomposites with the nonlinear polymers can be altered by orders of magnitude with respect to the conventional nanocomposite with the linear polymer. Polymer compactness and interpenetrability are found to play crucial roles in determining their bulk rheology. At the microscopic level, average segmental dynamics is remarkably slowed down by the attractive NPs in the matrices of high degree of branching, whereas no significant effect is observed in the linear polymer matrix at the same NP loading. In addition, the nanoscale dynamics of particles in the compact nonlinear matrices exhibits strong decoupling from the bulk viscoelasticity, allowing their fast relaxation even at ≈30% by volume. These results provide an experimental evidence that macromolecular architecture is a powerful new tool for tuning the bulk rheological properties as well as the nanoscale dynamics of polymer nanocomposites (PNCs) without the need for changing polymer molecular weight, nanoparticle size, shape, loading, or dispersion state.