Thermoresponsive Toughening in LCST-Type Hydrogels: Comparison between Semi-Interpenetrated and Grafted Networks

Hydrophilic and LCST polymer chains, poly(N,N-dimethylacrylamide) (PDMA) and poly(N-isopropylarchitectures to investigate their responsive properties in acrylamide) (PNIPA), were combined in semi-interpenetrated swollen isochoric conditions comparatively to grafted network structures. Using equal weight fractions of PDMA and PNIPA, semi-IPN designed with opposite topologies have demonstrated a thermoresponsive behavior with very different structure/properties relationships as investigated by calorimetry, swelling experiments, tensile tests, and 2D neutron scattering at rest and under deformation. In the case of the PDMA network interpenetrated by linear PNIPA chains, the phase transition of PNIPA gives rise to the formation of large microdomains, loosely percolating the PDMA network. Above the transition, the enhancement of the mechanical properties remains low in terms of elastic modulus and fracture energy. Conversely, the opposite topology, with PDMA chains interpenetrating the cross-linked PNIPA network, brings a large improvement of the mechanical properties at high temperature with a 10-fold increase of the modulus and very high fracture energy. By comparison with grafted hydrogels of similar composition and different topologies, the impact of the primary structure over the phase-separated morphology and the resulting mechanical properties was clearly highlighted.