Nanoscale Phase Behavior of Mixed Polymer Ligands on a Gold Nanoparticle Surface.

The phase behavior of mixed polymer ligands anchored on Au nanoparticle surfaces was investigated using small-angle neutron scattering (SANS). An equimolar mixture of deuterated polystyrene (PS) and normal poly(methyl methacrylate) (PMMA) was attached to Au nanoparticles, and the polymer-grafted nanoparticles were characterized in an isotopic toluene mixture, a good solvent for both homopolymers. Poly(deuterated styrene--methyl methacrylate) (P(S--MMA)) attached to the Au nanoparticles was also characterized as a control case. The results suggest that as the molecular weight increases, the two species of polymers become phase-separated on the nanoparticle surface, resulting in the formation of Janus-type nanoparticles. Monte Carlo simulations for the model polymer-grafted particle system suggest that the effective attraction between the polymers and the particle leads to dense wetting layers of solvophilic polymer blends in the vicinity of the solvophobic particle surface, which plays a decisive role in the formation of the phase-separated morphology.