Silica nanoparticle dispersions in homopolymer versus block copolymer

Silica nanoparticles (17 +/- 4 nm in diameter) were modified by grafting polystyrene chains to the surfaces using atom transfer radical polymerization (ATRP). The molecular weight of the grafted chains ranged from 8 to 48 kDa. These modified nanoparticles were mixed in solution with poly(styrene) homopolymer (18-120 kDa) and symmetric poly(styrene-b-butadiene) (PS-PB) diblock copolymer (34-465 kDa) and the states of dispersion in the dried composites were characterized by transmission electron microscopy (TEM). In the so-called wet brush limit, when the graft molecular weight equals or exceeds the matrix value, the silica particles form a uniform random dispersion in poly(styrene). Increasing the homopolymer matrix, molecular weight above the graft value results in particle clustering and macroscopic-phase separation. Mixtures of the lamellar forming block copolymer and nanoparticles exhibit a very different trend, with particle clustering at the lower PS-PB molecular weights and dispersion at the highest value. This latter finding is rationalized on the basis of packing constraints associated with lamellar order and the effective particle dimensions, and the degree of solvation at ordering, both of which favor higher molecular weight block copolymers. (c) 2007 Wiley Periodicals, Inc.