Equilibrium swelling of a polyampholytic pH-sensitive hydrogel

Immersed in an ionic solution, a network of polyampholytic polyelectrolyte imbibes the solution and swells, resulting in a polyampholytic pH-sensitive hydrogel, which can respond to changes in the surrounding environmental pH. This paper formulates a continuum field theory for polyampholytic pH-sensitive hydrogels by considering the reaction of hydrogen ions with hydroxide ions, which has been ignored in our previous paper (H. X. Yan, B. Jin, Eur. Phys. J. E 35, 36 (2012)). Comparison with experimental data shows that the proposed continuum field theory, by considering that the reaction of hydrogen ions with hydroxide ions would be more reasonable, can not only give a good qualitative but also a good quantitative prediction of the dependence of swelling on pH and crosslinker. The theory is then applied to study the influence of chain entanglements, salt concentration, uniaxial tension and geometric constraint on mechanical behavior of polyampholytic pH-sensitive hydrogels.