Mechanical and rheological behavior of pNIPAAM crosslinked macrohydrogel

In this study for the first time we investigate the most common reticulated N-isopropylacrylamide (pNIPAAM) macrohydrogel for both its mechanical response and shear rheological behavior in time and frequency domains. Hydrogels are characterized by water content volume and weight measurements, FT-IR spectroscopy, scanning electron microscopy and reflecting index. Compressive uniaxial tests on equilibrated hydrogels individuate a hookean response within a 30% strain range with Ec modulus of 12.2 kPa, and a neo-hookean response within a 79% strain range which upper limit corresponds to material rupture with Gc modulus of 3.8 kPa. Tensile experiments performed for the first time on the pure material evidence a rupture limit for a strain around 30% with hookean modulus Et of 24.8 kPa and neo-hookean modulus Gt of 7.3 kPa. Rheological studies, carried out in linear response regime around the hydrogel swelling-deswelling transition, report relaxation times of the kinetics towards the equilibrium at different temperatures. The phase transition of pNIPAAM is monitored and the transition temperature is determined following the temperature dependence of the shear modulus. We apply different literature models to the rheological response and to the swelling-deswelling transition of the hydrogel. Finally, we analyze the results providing values for microscopic material parameters such as crosslink density and mesh size.