Micellization of poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) copolymers in aqueous solution

A range of water-soluble hydrophilic-hydrophobic diblock copolymers of 2-(dimethylamino)ethyl methacrylate and methyl methacrylate were shown to form micelles if first dissolved in a nonselective solvent (THF or methanol) and subsequently diluted with water. Critical micelle concentrations were found to increase with the length of the hydrophobic block if the the hydrophilic block length is fixed. Increasing the overall molar mass of the copolymer also increased the cmc for a given composition. The hydrodynamic diameters and approximate association numbers of these micelles were obtained by dynamic light scattering and analytical ultracentrifugation. Micellar diameters and association numbers were in the range 10-18 nm and 10-33 chains per micelle, respectively, depending on copolymer composition. Increasing the overall molar mass of the copolymer for a given composition produces larger micelles with lower association number. Both micelle size and association number decrease with the length of the hydrophobic block if the hydrophilic block length is fixed. The results suggest that the solubility of the hydrophilic block is at least as important as that of the hydrophobic block, although nonequilibrium effects cannot be excluded. Both the size and the association number of the micelles can be varied in predictable ways by changing the temperature, the pH, or the ionic strength of the solution.