Thermogelling of Double Hydrophilic Multiblock and Triblock Copolymers of N,N-Dimethylacrylamide and N-Isopropylacrylamide: Chain Architectural and Hofmeister Effects

A series of thermoresponsive double hydrophilic (AB)(n) multiblock and ABA triblock copolymers of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) with varying sequence lengths were synthesized via successive reversible addition fragmentation chain transfer (RAFT) polymerizations by employing polytrithiocarbonate as the chain transfer agent. Previously, we reported that multiblock copolymers in dilute aqueous solutions can form either unimolecular or multimolecular micelles at elevated temperatures depending on the relative chain lengths of PDMA and PNIPAM sequences (Zhou et al. Langmuir 2007, 23, 13076-13084). In this follow-up work, we further explored and compared the chain architectural (multiblock vs triblock) and Hofmeister effects (addition of various sodium salts) on the gelation behavior of multiblock and ABA triblock copolymers at high concentrations and attempted to establish a correlation between the aggregation behavior and gelation properties of multiblock copolymers at low and high polymer concentrations, respectively. It was found that only m-PDMA(p)-PNIPAM(q) multiblock copolymers with PDMA and PNIPAM sequence lengths located within a specific range can form physical gels at elevated temperatures. Rheology measurements revealed that multiblock copolymers possess considerably lower critical gelation temperatures (COT) and higher gel storage modulus, G(gel)', as compared to those of PNIPAM-b-PDMA-b-PNIPAM triblock copolymers possessing comparable sequence lengths. The addition of inorganic sodium salts can effectively facilitate thermogelling for multiblock and triblock copolymers, resulting in decreasing CGTs and critical gelation concentrations (CGCs) in the order of Hofmeister series with increasing hydration capabilities. The unique thermogelling behavior of aqueous multiblock copolymer solutions in the absence and presence of inorganic salts, as compared to that of ABA triblock copolymers, augurs well for their potential applications in various fields such as biomaterials and biomedicines.