Anthony E. English

Equilibrium swelling properties of polyampholytic hydrogels

The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects of ion dissociation equilibria and counter ion shielding in polyampholytic hydrogels near their balance point where both polyelectrolyte and polyampholyte effects are present. The theory considers a Flory type swelling model where the Coulombic interactions between fixed ions in the hydrogel resemble those of an ionic solid with a Debye screening factor. Theoretical predictions from this model are in qualitative agreement with our experimental results.

Equilibrium and non-equilibrium phase transitions in copolymer polyelectrolyte hydrogels

A continuum model of polyelectrolyte hydrogel phase transitions has been developed and used to analyze the equilibrium and non-equilibrium swelling transitions observed in copolymer N-isopropylacrylamide (NIPA), acrylic acid (AAc), and acrylamide (AAm) hydrogels. The contribution of the dominant forces to the overall swelling response has been illustrated experimentally by systematically varying both the hydrogel and bath composition. We have interpreted the equilibrium swelling pattern theoretically by separately examining the role of charge, acid dissociation, and solvent mediated effects. The non-equilibrium swelling response to changes in bath ionic strength has been examined in terms of competing coupled ion, diffusion reaction, polymer network mechanisms, and the potential role of film boundary effects. We have obtained qualitative agreement between our theory and measurements and have critically examined both the strengths and limitations of our model.

Polymer gels that can recognize and recover molecules

The design and development of a polymer gel that can selectively absorb and release metal ions is investigated; the recovery of metal ions has a cooperative switching effect. A synthetic method for producing polymers that can thermodynamically memorize a conformation and selectively absorb a target molecule is proposed.

Polyelectrolyte hydrogel instabilities in ionic solutions

The phase behavior of polyelectrolyte hydrogels has been examined as a function of relative charge composition, bath salt concentration, and solvent quality. Nonlinear swelling instabilities of 2‐hydroxyethyl methacrylate (HEMA) and methacrylic acid (MAAc) copolymer hydrogels manifested themselves as discontinuous first order swelling transitions as a function of bath salt concentration. A modified Flory–Huggins model was used to describe the regions of instability when bath salt concentration and solvent quality are considered as control variables. The role of ion dissociation equilibrium in the change from local or smooth transitions to nonlocal or discontinuous swelling transitions is illustrated within the framework of our model.