Rheology and morphology of model immiscible polymer blends with monodisperse spherical particles at the interface

Abstract

We show that the addition of solid particles to droplet–matrix blends of immiscible polymers induces massive changes in the rheology and the flow-induced structure even at loadings as low as 0.1 vol. %. Experiments were conducted using blends of polyethylene oxide (PEO, dispersed phase), polyisobutylene (PIB, continuous phase), and 470 nm monodisperse silica particles with two different surface wettabilities. Rheological experiments were conducted under molten conditions, while the morphology was characterized at room temperature using scanning electron microscopy. We are able to image the morphology at both lengthscales: The >20 μm lengthscale of the dispersed phase, as well as the submicron lengthscale of the particles. Rheological experiments along different trajectories in the ternary particle/PEO/PIB composition diagram reveal that addition of ∼1 vol. % particles that are preferentially wetted by the PIB induces a large increase in steady shear viscosity, severe shear-thinning, and yield-like behavior. However if the particles are equally wetted by PEO and PIB, these effects are greatly diminished. Remarkably, addition of very low loadings (∼0.1 vol. %) of particles reduces the viscosity under some conditions regardless of wettability. These rheological changes are interpreted in terms of three observations from morphological studies: That particles greatly enhance coalescence at low volume loadings, that particles jam the interface at higher loadings, and that particles bridge across drops and glue them together into large clusters. The first two of these effects occur regardless of particle wettability, whereas the last occurs only with particles that are preferentially wetted by the continuous phase.