Thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) nanostructured hydrogels

Here we report the preparation and characterization of nanostructured thermo-responsive poly(acrylamide) (PAM)-based hydrogels. The addition of slightly crosslinked poly(N-isopropylacrylamide) (PNIPA) nanogels to AM reactive aqueous solution produces nanostructured hydrogels that exhibit a volume phase transition temperature (TVPT). Their swelling kinetics, TVPT's and mechanical properties at the equilibrium-swollen state (Heq) are investigated as a function of the concentration of PNIPA nanogels in the nanostructured hydrogels. Nanostructured hydrogels with PNIPA nanogels/AM mass ratios of 20/80 and above exhibit higher Heq and longer time to reach the equilibrium swelling than those of the conventional PAM hydrogels. However, the PNIPA nanogels possess thermo-responsive character missing in conventional PAM hydrogels. The TVPT of nanostructured hydrogels depends on PNIPA nanogel content but their elastic and Young moduli are larger than those of conventional hydrogels at similar swelling ratios. Swelling kinetics, TVPT, and mechanical properties are explained in terms of the controlled in-homogeneities introduced by the PNIPA nanogels during the polymerization.