Swelling kinetics and rheological behaviour of microwave synthesized poly(acrylamide-co-acrylic acid) hydrogels

Poly(acrylamide-co-acrylic acid) hydrogels are synthesized by free-radical polymerization in a microwave oven, in a one-step procedure that provides reduction in time, energy and resources in comparison with conventional heating in aqueous solution. Hydrogel microstructures are analysed by scanning electron microscopy (SEM). Dynamic swelling tests are carried out at three pH values (3.0, 10.0 and 7.0) and two temperatures (25 and 37 °C) considering the conditions for hydrogel application. SEM pictures indicate irregular cellular and porous structure of microwave synthesized hydrogels, obtained as a result of simultaneous polymerization and water evaporation. The results of swelling measurements show that the swelling ratio at pH 7.0 and 10.0 increases with increasing in acrylic acid amount, while at pH 3.0 decreases. Swelling rate is higher at physiological temperature, and solvent diffusion is relaxation-controlled. Swelling of hydrogels follows the second-order kinetics. Rheological behaviour of hydrogels has been assessed using the results of frequency and amplitude sweep tests applied at the systems in equilibrium swollen state. Obtained data reveal a dominant elastic character and a long linear viscoelastic region (up to 2000 Pa), as well as a similar three-dimensional internal structure of investigated hydrogels.