The series of double network (DN) hydrogels, based on poly(N,N′-diethylacrylamide) (PDEAAm), polyacrylamide (PAAm), or poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) were synthesized with various mass ratios of components and their thermal response was investigated. The formation of DN structure in studied hydrogels results in significant increase in Young’s modulus. PDEAAm/PAMPS hydrogels exhibited rather high swelling ratio and consequently their temperature sensitivity was not detected. DN hydrogels PDEAAm/PAAm and PDEAAm/PDEAAm displayed thermo-responsive behavior, and the dependence of transition parameters on the feed molar concentration of acrylamide and N,N′-diethylacrylamide monomers, respectively, was determined through deswelling, NMR, and Differential scanning calorimetry (DSC) experiments. A two-state process model was employed to describe the phase transition of hydrogels. By utilizing a modified van’t Hoff equation with data from deswelling, NMR, and DSC experiments, we were able to obtain thermodynamic parameters of the transition and determine the size of the cooperative domains consisting of polymer units and water molecules.

Swelling experiments and NMR spectroscopy were combined to study effect of various stimuli on the behavior of hydrogels with a single- and double-network (DN) structure composed of poly(N,N′-diethylacrylamide) and polyacrylamide (PAAm). The sensitivity to stimuli in the DN hydrogel was found to be significantly affected by the introduction of the second component and the formation of the double network. The interpenetrating structure in the DN hydrogel causes the units of the component, which is insensitive to the given stimulus in the form of the single network (SN) hydrogel, to be partially formed as globular structures in DN hydrogel. Due to the hydrophilic PAAm groups, temperature- and salt-induced changes in the deswelling of the DN hydrogel are less intensive and gradual compared to those of the SN hydrogel. The swelling ratio of the DN hydrogel shows a significant decrease in the dependence on the acetone content in acetone–water mixtures. A certain portion of the solvent molecules bound in the globular structures was established from the measurements of the 1H NMR spin–spin relaxation times T2 for the studied DN hydrogel. The time-dependent deswelling and reswelling kinetics showed a two-step profile, corresponding to the solvent molecules being released and absorbed during two processes with different characteristic times.