Polymer nanocomposites have gathered substantial academic and industrial interest since the first reports in the early 1990s. Observations of large property changes at very low volume fractions of added nanoparticles, and the possibility to integrate nanoparticles with specific properties providing new functions have motivated an increasing number of investigations. Further progress in nanocomposites is crucially limited by the strong tendency of nanoparticles to aggregate in polymeric matrices, a complication that also prevents a clear identification of the underlying nanoscale mechanism that leads to the striking efficiency of nanocomposites.

We discovered that coating nanoparticles with a brush-like polymer layer provides a general and versatile route to fully miscible nanocomposites that show no nanoparticle aggregation over the whole range of nanoparticle volume fractions. [1] In a spherical polymer brush, the polymer chains are attached with one end to the particle surface. The chain ends may be attached covalently, but we found that a more versatile and simpler approach was to attach the polymers via coordinative interactions. We demonstrate the generality of this approach with the preparation of fully miscible metal (Au, Ag), oxide (ZnO, Fe2O3), sulfide (PbS) and selenide nanoparticles (CdSe) having catalytic, magnetic, or semiconducting properties in polymers such as polyethylene, polyisoprene, polystyrene, and polyethylene oxide. Optically transparent materials with nanoparticle weight fractions of more than 40% can be obtained.

Fully miscible nanocomposites enable for the first time to identify the relevant nanoscale mechanism that improves the efficiency of nanocomposites, revealing important parallels to the behavior of colloidal solutions. Our experiments show that the observed improvements of the mechanical properties derive from nanoparticle-induced retardation of polymer chain mobility and a liquid-solid transition induced by the mutual interaction of the nanoparticles. Both effects are mediated by the interfacial polymer layer already at very low volume fractions. In miscible nanocomposites nanoparticles assemble into superlattices with unprecedented order, which allows excellent control of the interparticle distance in polymeric matrices relevant for applications in magnetic and photovoltaic devices.
[1] S. Fischer, A. Salcher, A. Kornowski, H. Weller, S. Förster, Angew. Chem. Int. Ed. 50 (2011) 7811.