Overentrant swelling behaviour of poly(Potassium, 3-sulfopropyl acrylate-acrylic acid) gels

Rarely observed in hydrogels, overentrant is referred to an unusual swelling behaviour in which swelling in mixed solvents is higher than in the pure solvent with a higher dielectric constant. Hydrogels based on acrylic acid (AA) and 3-sulfopropyl acrylate, potassium salt (KSPA) were prepared to track this behaviour. The poly(KSPA-AA) products were then treated with HCl to prepare poly(SPA-AA) hydrogels. The overentrant behaviour of poly(SPA-AA) was examined in binary mixture of water and various polar solvents including dimethyl sulfoxide (DMSO), mono- and poly-hydric alcohols as well as amide solvents. It was observed that poly(SPA-AA) showed overentrant swelling behaviour in aqueous solvents such as methanol, ethanol, glycerol, etc. To characterize non-treated and treated samples, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and dynamic mechanical thermal analysis (DMTA) analyses were utilized for the dried samples. Rheometrical testing was also employed to evaluate the modulus of the swollen samples. The structure-property relationship was preliminarily discussed. Dielectric constant and hydrogen bonding formation potential of the solvents were expected to be of the main causes of the extraordinary swelling behaviour.