SANS and NMR study on nanostructure of thermoresponsive double network hydrogels

Temperature induced changes of nanostructure of double network (DN) hydrogels were investigated by a combination of small-angle neutron (SANS) and 1H nuclear magnetic resonance (NMR) spectroscopy. The DN hydrogels consist of the first densely crosslinked thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) network swollen in water in which a second loosely crosslinked hydrophilic polyacrylamide (PAAm) network was prepared. In the series, the crosslinking density of the first network is varied while the crosslinking density of the second network is kept constant. At high-q range (q being the magnitude of scattering vector), SANS profiles are governed by a term originating from thermal movement of polymer segments in water. For q > 0.1 Å−1, this term scales as q−α and the scaling exponent α obtains values lower than 5/3 (at T = 25 °C) to ca 2 (at T ≥ 35 °C) in agreement to the change of quality of PNIPAAm-water interaction. At low-q range, an excess scattering due to formation of large loose domains with non-uniform concentration of crosslinks is observed. At temperatures below the deswelling temperature, Tdsw, the excess scattering is relatively week since the domains differ little in the neutron scattering length density. However, NMR results show that mobility of a great part of NIPAAm units in DN hydrogels is significantly reduced even below Tdsw. At temperatures above Tdsw the excess scattering becomes much stronger showing q−4 scaling in Porod's region corresponding to formation of water-poor and water-rich domains separated by well-defined interface. The size of domains grows with increasing temperature and decreasing crosslinking density of the first network. The SANS intensities from DN hydrogels are reduced with respect to SN hydrogels which is attributed to lower scattering contrast due to an increased water content in water-poor phase caused by the presence of the second hydrophilic PAAm network.