Effects of Solvent Composition on the Assembly and Relaxation of Triblock Copolymer-Based Polyelectrolyte Gels

The role of solvent selectivity has been explored extensively with regard to its role in the phase behavior of block copolymer assemblies. Traditionally, thermally induced phase separation is employed for generating micelles upon cooling a block copolymer dissolved in a selective solvent. However few amphiphilic, polyelectrolyte-containing block copolymers demonstrate a thermally accessible route of micellization, and solvent exchange routes are frequently employed instead. Here, we describe the use of mixed solvents for obtaining thermoreversible gelation behavior of poly(methyl methacrylate)-poly(methacrylic acid)-poly(methyl methacrylate) (PMMA–PMAA–PMMA) triblock copolymers. One solvent component (dimethyl sulfoxide) is a good solvent for both blocks, and the second solvent component (water) is a selective solvent for the polymer midblock. Rheological frequency sweeps at variable solvent compositions and temperatures demonstrate an adherence to time–temperature–composition superposition, so that changes in the solvent composition are analogous to changes in the Flory–Huggins interaction parameter between end block and solvent. Shift factors used for this superposition are related to the effective activation energy describing the viscosity and stress relaxation response of the triblock copolymer gels. The effectiveness of solvent exchange processes for producing hydrogels with this system is shown to originate from the ability of a small amount of added water to greatly increase the relaxation times of the self-assembled polymer gels that are formed by this process.