Thermoresponsive double network hydrogels composed of poly(N-isopropylacrylamide) and polyacrylamide

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) and hydrophilic polyacrylamide (PAAm) was studied by a combination of swelling and mechanical measurements, differential scanning calorimetry (DSC) and 1H nuclear magnetic resonance (NMR) spectroscopy. A series of DN hydrogels with tightly crosslinked first network of PNIPAAm and loosely crosslinked second network of PAAm was prepared. In the series, crosslinking density of the first PNIPAAm network was varied while crosslinking density of the second PAAm network was kept constant. Presence of the second hydrophilic PAAm component in DN hydrogels strongly affects their behavior. Both, degree of deswelling and specific enthalpy of demixing in DN hydrogels are strongly reduced relative to corresponding single network (SN) hydrogels. Presence of the second network shows a reinforcing effect on DN hydrogels. From NMR results it follows that mobility of a great part of PNIPAAm in DN hydrogels is significantly reduced due to agglomeration of PNIPAAm induced by formation of the second network already at low temperature. A certain portion of water confined in agglomerated structures and a slow exchange regime between ''confined" and ''free" water were established by NMR methods for all investigated SN and DN hydrogels.