Fumed silica induces co-continuity across a wide composition range in immiscible polymer blends

We examine ternary blends composed of two immiscible polymers, polyisobutylene (PIB) and polyethylene oxide (PEO), and fumed silica particles which have strong affinity for the PEO. Samples with up to 10 vol% fumed silica and a wide range of PIB:PEO ratios were melt-blended and their morphology characterized. The unusual structure of fumed silica - which comprises 5-50 nm primary particles permanently bonded together into highly porous, fractal-like aggregates - strongly influences the morphology at all compositions. At relatively low PEO loadings, the PEO is absorbed into the pores within the fumed silica aggregates. The morphology consists of a network of interlocked particle aggregates bonded together by PEO. At higher PEO loadings, the PEO and the particles form a combined phase that is solid-like even at low fumed silica loadings. The morphology somewhat resembles a conventional co-continuous morphology where one phase is PIB and the other is a fumed silica-filled PEO. However, the microstructure is very irregular and the interface between the PIB and the PEO phase appears extremely rough on the scale of the primary particles. Morphologies with two percolating phases appear across almost the entire composition range examined. Compared to spherical fused silica particles examined previously, fumed silica affects the morphology of polymer blends at far lower particle loadings, and greatly widens the range of co-continuity.