Mechanical properties of poly( N ‐isopropyl‐acrylamide‐ co ‐itaconic acid) hydrogels

Hydrogels of N ‐isopropylacrylamide and itaconic acid were synthesized with different monomer ratios and with two crosslinking agent concentrations. The different xerogels were immersed in water and the swelling process was conducted up to equilibrium conditions at two temperatures (22 and 37°C). These temperatures are lower and higher than the transition temperature shown by PNIPA hydrogels. The mechanical properties of the different solvated hydrogels were examined by oscillatory shear measurements at 22 and 37°C. The copolymer volume fraction and the elastic storage modulus of the hydrogels decreased as the itaconic acid percentage in the copolymer increased. This behavior was attributed to the higher hydrophilic character of the itaconic acid comonomer. Effective crosslinking density, molar mass between crosslinks, and the polymer–solvent interaction parameter were determined from the experimental values of the elastic storage moduli and the copolymer volume fractions.