Micelle Formation of Diblock Copolymer in a Thin Film Homopolymer: a Comparison with Polymer Brush-Coated Nanoparticles

We investigated micelle formation of a diblock copolymer polystyrene-$b$-poly(2-vinylpyridine) (PS-$b$-P2VP) of degree of polymerization $N$, in thin films of homopolymer polystyrenes (PS) of different degrees of polymerization $P$, supported on substrates, and compared the results with the phase behavior of PS brush-coated Au nanoparticles in homopolymer PS matrix. PS-$b$-P2VP copolymer chains aggregated to form micelles, composed of an inner P2VP core and an outer PS corona. The size of the micelle cores, $D_{core}$, increased with increasing $P$, and reached a plateau at very large $P$. The transition occurred at a larger $P$/$N$ than expected from brush-melt interaction theories. The organization of micelles at large $P$ regimes suggested attractions between micelles. P2VP block also adsorbed onto the substrate to form a brush layer and the surface adsorption process was affected by micellization of copolymers. We compared the micelle formation of PS-$b$-P2VP in PS with the phase behavior of PS coated Au nanoparticle/PS mixtures: the host chains penetrate into the corona of the micelles more easily than into the PS brush grafted on the particle due to low ``grafting density;'' what's more, micelles can self-adjust their aggregation number as the interaction between host chains and the corona changes.