Silica nanoparticle dispersions in aqueous media
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Amphiphilic block copolymers of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) family self-assemble in water (selective solvent for PEO) into (core-shell spherical) micelles. This nanostructure could be utilized for the spatial organization of nanoparticles to form hybrid nanocomposite materials. Fundamental questions pertaining to the nanoparticle-polymer interfacial interactions (as affected by the nanoparticle surface chemistry and size) remain open and formulate the objectives of the present investigation. We are exploring the effects of various parameters on the particle-polymer interfacial interaction. In particular, we are interested in the use of small polar organic solvent molecules in controlling nanoparticle-polymer interactions. We discuss here, systems of silica nanoparticles dispersed in the micellar phases formed by PEO-PPO-PEO block copolymers (Pluronic P105) in water and its mixtures with polar organic solvents. PEO block of Pluronic P105 adsorbs on the silica surface under mild acidic condition and adsorbed P105 molecules start to self-assemble into micelle like surface aggregates above some critical concentration (called critical surface micelle concentration (csmc)). We use the csmc as a measure of particle-polymer interfacial interactions. The csmc is found to be lower than the cmc of Pluronic P105 in plain water and is found to decrease with increasing particle concentration and decreasing particle size. Small organic solvent molecules, Glycerol, DMSO, and DMF act as displacers below critical displacer concentration (cdc). Below cdc, the csmc increases with increasing concentration of glycerol, DMSO, or DMF. Above cdc, Pluronic P105 does not adsorb on the silica nanoparticle surface and no surface micelle are formed, and also glycerol, DMSO, and DMF act as cosolvents rather than displacers. Solvent-regulated block copolymer-nanoparticle interactions have interesting repercussions on the development of solvent based processing of block copolymer nanoparticle composite materials.