Superabsorbent
polymer (SAP) hydrogels function by absorbing and
retaining water. Facile removal of the absorbed water would make it
easier to recycle used SAP hydrogels. However, the hydrophilic interior
of traditional SAP hydrogels inhibits the desorption of the absorbed
fluid. Herein, we report the synthesis and properties of CO2-responsive SAP hydrogels capable of switching from being relatively
hydrophobic to hydrophilic in the presence of CO2, and
vice versa when the CO2 is removed, accompanied by the
simultaneous absorption and expulsion of large volumes of water respectively,
mimicking a CO2-directed sponge. The hydrogels studied
are synthesized by the free-radical copolymerization of either N-[3-(dimethylamino)propyl]methacrylamide (DMAPMAm) or 2-N-morpholinoethyl methacrylate (MEMA) with N,N′-dimethylacrylamide (DMAAm), which acts
as both a monomer and a self-cross-linker. In the presence of CO2, both p(DMAAm-co-DMAPMAm) and p(DMAAm-co-MEMA) gels were able to achieve a maximum swelling ratio
(SR) of ∼800, demonstrating that they are superabsorbent. These
gels release more than 70% of the absorbed water if they are immersed
in noncarbonated water. Repeated swelling/deswelling of the hydrogels
(with recovery of the original swelling ratio) in this manner over
four cycles demonstrates the reusability of these materials and their
potential use in a variety of applications.
Small angle neutron scattering (SANS) studies of a model pharmaceutical formulation reveal how formulation stability depends on the compatibility of individual components. Solutions of two common protein formulation excipients, polysorbate 80 (PS80), a nonionic surfactant that prevents aggregation, and m-cresol, an antimicrobial agent for multi-dose injectable formulations, are investigated. The addition of m-cresol to PS80 solutions leads to solution turbidity and irreversibly alters PS80 micelle morphology. This slow preservative-induced destabilization of PS80 micelles progresses over days or even weeks, which highlights the essential role that aggregation kinetics plays in preservative-surfactant interactions. The temperature-dependence of PS80 micelle growth kinetics is quantified by SANS in the presence of m-cresol. Aggregation is a two-step process, where initial formation of small aggregates is followed by a period of monotonic power-law growth, providing evidence for the mechanism. Total aggregate mass stays constant after initial aggregate formation, and addition of a pH-regulating citrate buffer dramatically accelerates aggregation kinetics.
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