Micelle formation of poly(acrylic acid)-block-poly(methyl methacrylate) block copolymers in mixtures of water with organic solvents

The micelle formation of poly(acrylic acid)-block-poly(methyl methacrylate) (AA-MMA) block copolymers in mixtures of water with organic solvents was i nvestigated by non-radiative energy transfer (NRET). In the case of block c opolymers with 70 hydrophobic MMA units, which form strongly aggregated mic elles in pure water, the addition of a non-selective organic solvent (metha nol or 1,4-dioxane) induces a micelle-unimer transition within a relatively small range of solvent composition without significantly increasing the ra te of chain exchange between micelles close to this transition region. The addition of 2 vol.-% dimethyl adipate (a solvent with chemical similarity t o the PMMA block and only limited solubility in water) does not speed up th e chain exchange in this system either. In contrast, this solvent promotes the aggregation of smaller block copolymers (20 or 30 MMA units) which are mainly present as single chains in purl water. in the case of the block cop olymer with 40 MMA units the so formed micelles show a very slow chain exch ange extending over many days. These observations prompt us to assume that the rate of the micelle-unimer exchange equilibrium is not kinetically hind ered (i.e., determined by the T-g of the core material of the micelle) but controlled by a strong thermodynamic preference for the aggregated state.