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

Abstract

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