Tailoring thermoresponsive nanostructured poly(N-isopropylacrylamide) hydrogels made with poly(acrylamide) nanoparticles

The thermoresponsive behavior and mechanical properties of nanostructured hydrogels, which consist of poly(acrylamide) nanoparticles embedded in a cross-linked poly(N-isopropylacrylamide) hydrogel matrix, are reported here. Nanostructured hydrogels exhibit a tuned volume phase transition temperature (T (VPT)), which varies with nanoparticle content in the range from 32 up to 39-40 A degrees C. Moreover, larger equilibrium water uptake, faster swelling and de-swelling rates, and larger equilibrium swelling at 25 A degrees C were obtained with nanostructured hydrogels compared with those of conventional ones. Elastic and Young's moduli were larger than those of conventional hydrogels at similar swelling ratios. The tuned T (VPT) and the de-swelling rate were predicted with a modified Flory-Rehner equation coupled with a mixing rule that considers the contribution of both polymers. These behaviors are explained by a combination of hydrophilic/hydrophobic interactions and by the controlled inhomogeneities (nanoparticles) introduced by the method of synthesis.