The effects of pH and ionic strength on the volume phase transition temperature of thermo-responsive anionic copolymer gels

Applications of thermo-responsive (TR) gels in biomedicine and biotechnoligy require modulation of their volume phase transition temperature T_c in a rather wide interval. This is conventionally performed by incorporation of hydrophilic or hydrophobic monomers in the feed. The most pronounced growth of T_c occurs when TR monomers are copolymerized with anionic, cationic or ampholyte monomers. The effect of ionic monomers on the volume phase transition temperature of copolymer gels is traditionally analyzed in equilibrium swelling tests in distilled water. To evaluate how pH and ionic strength of aqueous solutions affect T_c, a constitutive model is developed. Adjustable parameters are found by fitting observations on poly(N-isopropylacrylamide-co-acrylic acid) and poly(N-isopropylacrylamide-co-itaconic acid) macro- and microgels below and above their critical temperature. The ability of the model to predict T_c is validated by comparison of experimental data with results of simulation. An analytical formula is derived that describes the influence of molar fraction of comonomers, pH and molar fraction of salt in aqueous solutions on the volume phase transition temperature of TR anionic gels.