pH controlled assembly of a polybutadiene-poly(methacrylic acid) copolymer in water: packing considerations and kinetic limitations

The morphologies formed by a polybutadiene-poly(methacrylic acid) block copolymer when directly rehydrated in aqueous solution were studied as a function of pH. A combination of transmission electron microscopy (TEM), dynamic light scattering (DLS) and zeta-potential measurements were used to characterize the assemblies. At low pH very large vesicles were formed. As the pH increased, the vesicles became progressively smaller until a transition to worm-like micelles was observed at pH 8. At pH 10, spherical micelles were formed. The initial transformation between vesicles of very low curvature and higher curvature can be directly related to the globule-coil transition which occurred for poly(methacrylic acid) (PMAA) at between pH 6-8. Subsequent morphological changes were dominated by the increasing repulsion between PMAA chains as the degree of ionisation increased. The phase behaviour of the copolymer in solution can thus be explained in terms of packing factors. It was also observed that the transformation from vesicles to worm-like micelles was rapid as was the change from worm-like micelles to spherical micelles. However, micelles formed at high pH did not readily form worm-like micelles and vesicles on lowering the pH. It was found that the observed behaviour was due to the slow kinetics of the phase transitions from spherical micelle to vesicle but it could be seen that the transformation took place over time i.e. vesicles are the thermodynamically stable species at low pH but ionic repulsion effects between species provide a kinetic barrier to their formation.