Hydrogels of Amphiphilic Triblock Copolymers with Independently Tunable pH and Temperature-Controlled Exchange Dynamics

BAB amphiphilic triblock copolymers comprising a central A-block of poly(dimethyl acrylamide) (PDMAc) and poly(2-methoxyethyl acrylate) (PMEA) B-blocks containing 5 mol % of acrylic acid (AA), P(MEA0.95-co-AA0.05)100-b-PDMAc400-b-P(MEA0.95-co-AA0.05)100, were prepared by RAFT polymerization. The BAB triblocks form transient physical networks in aqueous solution, their rheological properties being primarily controlled by the exchange rate of the B-blocks, which act as physical cross-links bridged by A-blocks. We showed that combining a thermosensitive PMEA block with a low fraction of pH-sensitive AA units resulted in a dual and independent control of the exchange dynamics of the B-blocks with T and pH. The MEA units give rise to thermothickening behavior (viscosity increases with T) that is opposite to conventional Arrhenius behavior, while the presence of only 5 mol % of AA units allows the exchange rate to be tuned over a wide range by varying pH. Overall, the rheological relaxation time could be tuned over 5 orders of magnitude with pH and/or T. The impact of the distribution of AA units within the B-blocks or of the presence of divalent Ca2+ salts on the exchange rate was also investigated, but no significant variation was observed.