pH-Sensitive hydrogels formed by self-assembled amphiphilic triblock copolyelectrolytes

Results are reviewed of a recent extensive investigation of the behavior of self-assembled pH-sensitive triblock copolymers in aqueous solution. The hydrophilic central block was polyacrylic acid and the two hydrophobic end-blocks were statistical copolymers of n-butyl acrylate and acrylic acid containing 50 mol% acrylic acid units. The hydrophobicity of the end blocks could be modified by changing the degree of ionization of the acrylic acid units (α). The relationship between the pH and α was determined. Scattering techniques showed that flower-like micelles are formed that upon increasing concentration connect via bridging into larger aggregates and above a critical concentration into a percolating network. The rheology of the system is controlled by the exchange rate of the end-blocks between micelles and can be fine-tuned by varying the pH. The exchange rate increases exponentially with increasing α. As a consequence the system changes from a quasi-permanent hydrogel at pH < 4.5 to a free flowing liquid at pH > 5.5. The effect of the ionic strength on the structure and the rheology was found to be important only above 0.5 M.