The evaluation of temperature and pH influences on equilibrium swelling of poly(n-isopropylacrylamide-co-acrylic acid) hydrogels

Hydrogels are synthesized by the method of radical polymerization of monomers: N -iso­propylacrylamide (NIPAM) and acrylic acid (AA). Characterization of poly( N -isopropyl­acryl­amide- co -acrylic acid) hydrogels, p(NIPAM/AA), has been performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and by determination of the swelling behaviour in aqueous solutions at different temperatures (25, 31 and 37 °C) and pH values (2.2, 4.5, 6 and 6.8). After lyophilisation in the solution at pH 6 and temperature of 25 °C, p(NIPAM/AA) hydrogels have rapidly reached equilibrium degree of swelling, α e , in com­parison to non-lyophilized samples. The mechanism of solvent transport within matrix in lyophilized samples corresponds to less Fickian diffusion, whereas Super case II diffusion is characteristic for non-lyophilized samples. p(NIPAM/AA) hydrogel with 1.5 mol% of ethyl­ene glycol dimethacrylate (EGDM) at the temperature of 25 °C and pH 6.8, has reached the highest swelling equilibrium degree, α e = 259.8. The results of swelling studies have shown that p(NIPAM/AA) hydrogels can be classified as superabsorbent polymers (SAPs). For the evaluation of pH and temperature influences on synthesized hydrogels swelling, a full three-level experimental design has been used. Two-factor interaction model (2FI) is the most optimal model of a full three-level experimental design for representing the swelling equilibrium degree of p(NIPAM/AA) hydrogels as a function of investigated parameters, i.e., temperature and pH.