Scaling of domain spacing in concentrated solutions of block copolymers in selective solvents

The dependence of block copolymer interdomain spacing d on polymer volume fraction φ_P in solution, d ∼ φ_P^-β, is examined through a series of styrene-isoprene dibiock copolymers in four solvents, three of them strongly selective (nonsolvents for polystyrene). The exponent β is found to depend on solvent selectivity (both temperature and chemical identity of solvent; β increases with selectivity), on mesophase (β is largest when the insoluble blocks form lamellae, then cylinders, then spheres), and on diblock composition within a given mesophase (β is largest when the soluble blocks are most highly diluted). Values of β as large as unity are observed, indicating that selective solvents can swell d quite substantially over the value for the bulk diblock. In one example, a 2.5-fold increase in d is obtained when a cylinder-forming polystyrene-rich diblock is diluted into its mirror-image polystyrene-poor cylindrical mesophase.