A model for equilibrium swelling of the upper critical solution temperature type thermoresponsive hydrogels

Thermoresponsive gels of the upper critical solution temperature (UCST) type shrink below their critical temperature and swell above it. Changes in water uptake by these gels are driven by thermally induced dissociation of hydrogen or ionic bonds between chains. A simple model is developed for the description of the equilibrium swelling of thermophilic gels with hydrogen bonds. To confirm its ability to describe observations, equilibrium swelling diagrams are fitted on poly(acrylamide-acrylic acid) and poly(acrylamide-acrylonitrile) macroscopic gels and microgels with various structures (copolymer gels, gels with interpenetrating networks, nanocomposite gels), as well as on biocompatible poly(N-acryloyl-glycinamide) and poly(allylurea-co-allylamine) gels. Numerical simulation reveals that material parameters evolve consistently with molar fractions of crosslinker, hydrophobic and hydrophilic comonomers in the feed, degree of ionization of functional groups, and concentration of nanofiller. It is shown that the model can also be applied to describe observations on thermophilic zwitterionic gels.