Anomalous enhanced swelling of charged copolymer gels

Copolymer gels are a class of hydrogels that possess at least two different monomers within the same polymer chain. A common method for synthesizing copolymer gels is the one-pot free radical polymerization reaction. In charged copolymer gels, however, this fabrication method has previously led to anomalous, and often unexplained, material properties. In this paper, we report on the anomalous, enhanced swelling of anionic poly(3-sulfopropylmethacrylate-co-2-hydroxyethylmethacrylate) (P(SPMA-co-HEMA)) hydrogels, whereby we provide experimental results and a theoretical framework to explain this swelling behavior. The swelling ratio of our copolymer gels exhibits a local optimum at $\sim$$\sim$ 25 mol% SPMA before transiting into a linear trend between charge density and gel swelling. A kinetic study of P(SPMA-co-HEMA) copolymerization reactions revealed that a transition in copolymer architecture is taking place, from a gradient (0–50 mol%) to a random (50–100 mol%) chain character with increasing SPMA content. With Flory-Rehner’s theoretical framework, we investigate why this change in copolymer architecture leads to enhanced swelling. For gradient copolymer gels, the difference in charge distribution likely enhances the local ionic driving force, and herewith the swelling. With the ability to tune the swelling behavior by controlling the copolymer architecture, we open further possibilities to study architecture-dependent properties, such as visco-elastic and stimuli-responsive behavior, or pH- and salt-dependent swelling.