Viscoelastic Properties of Hydrogels Based on Self-Assembled Multisticker Polymers Grafted with pH-Responsive Grafts

Graft copolymers consisting of a poly­(acrylic acid) (PAA) backbone and random grafts of n-butyl acrylate and acrylic acid, P­(nBA_50%-stat-AA_50%)₁₀₀, were synthesized by free radical polymerization of the backbone followed by ATRP polymerization of the grafts via a grafting-from approach. The rheological properties of their aqueous solutions were measured by oscillatory shear measurements at different temperatures, pHs, and concentrations. All graft copolymers formed transient networks above their percolation concentrations with pH-dependent relaxation times. These results implied that incorporation of hydrophilic AA units within the hydrophobic grafts allowed controlling their exchange dynamics in a pH-dependent way, leading to viscoelastic fluids with a pH-tunable terminal relaxation time. Provided that the grafting density remained low, the rheological properties of the graft copolymers were very similar to those of model BAB triblock copolymers consisting of a PAA central block and P­(nBA_50%-stat-AA_50%)₁₀₀ lateral blocks.