A structural view on viscosity deviations from the Einstein–Batchelor prediction in polymer nanoparticle composites

Incorporating nanoparticles (NPs) into polymers greatly expands the accessible property space for polymers. One property that can be extensively tuned by NP incorporation is a polymer's viscosity. While some polymers become more viscous upon NP incorporation, as predicted by the conventional Einstein–Batchelor model, others are more viscous than predicted, and still others even become less viscous. Mechanisms proposed so far are experimentally and computationally disputed and cannot explain the viscosity overshoot and reduction within a single framework. To address the issue, we examine a series of all-polystyrene polymer nanoparticle composites, which can display the NP-induced viscosity overshoot or reduction. We show that the existing mechanisms cannot explain the observed viscosity modulations, whereas small- and wide-angle x-ray scattering elucidates the structural origin of both the viscosity overshoot and reduction, providing the first structural picture to explain both of the off-prediction viscosity modulations.