Influence of Composition and Network Formation Sequence on the Responsive Behavior of Double-Network Hydrogels

This study investigates how the composition and synthesis sequence affect the structure and responsive behavior of single-network (SN) and double-network (DN) hydrogels composed of poly(<i>N,N'</i>-diethylacrylamide) (PDEAAm) and polyacrylamide (PAAm). DN hydrogels were prepared in two configurations, PDEAAm/PAAm and PAAm/PDEAAm, and compared with SN copolymer hydrogels of varying DEAAm/AAm ratios. ¹H NMR spectroscopy revealed that DN hydrogels exhibit significant heterogeneity due to polymer-rich domains, impacting the accuracy of compositional determination and leading to broad NMR signals. Temperature-dependent NMR and gravimetric swelling analyses were used to quantify thermoresponsive behavior, showing that SN copolymer hydrogels exhibit composition-dependent phase transition parameters, while DN hydrogels show relatively constant transition parameters due to heterogeneous structures. NMR relaxation studies of water molecules identified "free" and "bound" molecules whose dynamics differ markedly above the transition temperature, especially in DN systems. Finally, the swelling behavior in water-acetone mixtures was examined, revealing distinct responses depending on hydrogel composition and thermal state. PAAm-rich hydrogels showed abrupt deswelling near 40 vol% acetone, while PDEAAm-based hydrogels responded more gradually. The findings demonstrate that both composition and network formation order critically influence the structural, thermal, and solvent-responsive properties of hydrogels, offering insights for the design of stimuli-responsive materials.