Effect of polymer ligand molecular weight on polymer-coated nanoparticle location in block copolymers

Gold nanoparticles (Au-PS) coated with an areal chain density Sigma of end-attached polystyrene (PS) chains of different molecular weights (M-n) are added to a lamellar diblock copolymer of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) to determine the critical areal chain density Sigma(C) below which the Au-PS nanoparticles adsorb to the PS-b-P2VP interface. Gold nanoparticles coated by thiol end functional polystyrene homopolymers (PS-SH) with M-n ranging from 1.5 to 13 kg/mol are synthesized with various areal densities of PS-SH chains on the nanoparticle surface. The particles are incorporated into a PS-b-P2VP diblock copolymer with M-n = 196 kg/mol. The P2VP block has a more favorable interaction with a bare gold particle surface than does the PS block, and this interaction becomes important as the coverage of gold particles by PS-SH decreases, leading to adsorption of the Au-PS to the interface below a critical areal chain density Sigma(C). Sigma(C) decreases from 3.1 to 0.9 chains/nm(2) as the M-n of PS-SH chains increases from 1.5 to 13 kg/mol, leading to a scaling relation, Sigma(C) similar to Rg(-1) similar to M-n(-0.6), which is very different from the behavior expected for polymer chains tethered to a flat surface, Sigma c similar to R-g(-2) similar to M-n(-1), where R-g is the radius of gyration of the end-attached chains. A simple scaling relationship, Sigma(C) similar to ((R + R-g)IRRg)(2) that takes into account the high curvature of the Au nanoparticle core of radius R, is derived and is shown to describe the data very well. For Sigma approximate to Sigma(C), there is a slight tendency for the larger particles within the particle size distribution to adsorb preferentially to the PS/P2VP interface, a tendency that can be qualitatively understood by the fact that the absorption free energy per particle is predicted to scale roughly as R-2.