Swelling behavior of strong polyelectrolyte poly(N-t-butylacrylamide-co-acrylamide) hydrogels

Temperature-sensitive ionic hydrogels based on N-t-butylacrylamide (TBA), acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) and N,N^′-methylenebis(acrylamide) (BAAm) monomers were prepared. The molar ratio of TBA to the monomers AAm and AMPS was fixed at 60/40, while the AMPS content of the hydrogels was varied. The elastic modulus of the hydrogels was in the range of 347-447 Pa, much lower than the modulus of PAAm or poly(N-isopropylacrylamide) hydrogels due to the reduced crosslinking efficiency of BAAm in TBA/AAm copolymerization. The hydrogels exhibited swelling-deswelling transition in water depending on the temperature. Increasing ionic group (AMPS) content resulted in shifting of the transition temperature interval in which the deswelling takes place. The higher the ionic group content, the broader the temperature interval at the phase transition. Ionic hydrogels exhibited first-order reentrant conformational transitions in ethanol-water and in dimethylsulfoxide (DMSO)-water mixtures. The higher the ionic group content of the hydrogels the narrower the ethanol (or DMSO) range in which the reentrant phenomena occur. By taking into account the difference of the solvent mixture composition inside and outside the gel, the equilibrium swelling theory provided a satisfactory agreement to the experimental swelling data of the hydrogels immersed in the solvent mixtures.