Progressive Freezing-in of the Junctions in Self-Assembled Triblock Copolymer Hydrogels during Aging

The evolution with time was investigated for self-assembled networks formed by triblock copolymers in aqueous solution. The polymers consisted of a central hydrophilic poly(acrylic acid) block and two hydrophobic end-blocks formed by random copolymers of 50% acrylic acid and 50% n-butyl acrylate units. The rheological properties of the systems at steady state were strongly influenced by the degree of ionization (α) and thus by the pH. This allows one to obtain systems ranging from low viscosity solutions to hydrogels just by varying α. However, steady state was not reached instantaneously when α was changed, but proceeded through a slow progressive increase of the viscosity. The rate at which the systems aged was independent of α and of the polymer concentration and is attributed to slow reorganization of the cores formed by the self-assembled hydrophobic blocks.