Phase transition in hydrogels of thermoresponsive semi-interpenetrating and interpenetrating networks of poly(N,N-diethylacrylamide) and polyacrylamide

Temperature-induced phase transition in hydrogels of semi-interpenetrating (SIPNs) and interpenetrating (IPNs) polymer networks of thermoresponsive poly(N,N-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) was studied by a combination of 1H NMR spectroscopy, small-angle neutron scattering (SANS) and DSC. The effect of various network composition and reverse sequence in the preparation procedure of the network on the volume phase transition was examined. Transition temperature linearly increased with the content of PAAm component in SIPNs and IPNs. SANS results revealed in investigated SIPN and IPN hydrogels two types of regions (aggregates), small and large. At temperature above the phase transition the small and large regions are related to compact multi-chain aggregates with smooth and fuzzy surface, respectively. From NMR results it follows that the individual polymer groups participated in the transition to various extent dependently on the type of hydrogel preparation. We found that parameters of the phase transition are tunable not only by composition of the SIPNs and IPNs but also by the preparation process. A certain portion of water bound in globular structures and a slow exchange regime between bound and free water were established by NMR methods for all investigated SIPNs and IPNs hydrogels. (C) 2016 Elsevier Ltd. All rights reserved.