Tunable diblock copolymer micelles-adapting behaviour via subtle chemical modifications

Selectively quaternising the PDMA block of poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) copolymers modifies both their solution and adsorption behaviour. These copolymers exist as free unimers in aqueous solution at low pH and form micelles, with PDMA coronas, at high pH. The critical micellisation pH, the hydrodynamic micelle diameter and electrophoretic behaviour are all affected by the degree of quaternisation of the PDMA block. Highly quaternised copolymers form smaller, more highly charged micelles at lower pH than weakly or non-quaternised copolymers. The adsorption of the copolymer micelles onto muscovite mica is studied by in situ atomic force microscopy. The adsorbed micelle monolayer becomes increasingly disordered as the degree of quaternisation increases. No micelle desorption occurs on removal of the bulk copolymer solution. Addition of acid to the overlying solution leads to different responses from the surface-adsorbed micelles. Unquaternised micelles undergo a reversible change in morphology due to the formation of localised polymer brushes, whereas lightly quaternised micelles are characterised by irreversible changes. Highly quaternised micelle monolayers are disrupted by the addition of acid. Such differences can be rationalised by simple electrostatic arguments. This behaviour has been confirmed by quartz crystal microbalance studies, which show that the adsorbed mass decreases with increasing degrees of quaternisation.