Thermoresponsive Toughening in LCST-Type Hydrogels with Opposite Topology: From Structure to Fracture Properties

The challenge of this work was to investigate the role of topology in LCST hydrogels that strongly and reversibly thermo-reinforce their mechanical strength under isochoric conditions. To achieve this, two different hydrogels with opposite topologies were designed on the basis of grafted architectures using equal amounts of water-soluble chains (poly(N,N-dimethylacrylamide) = PDMA) and LCST polymer chains (poly(N-isopropylacrylamide) = PNIPA). By working under isochoric conditions, with almost 85 wt % of water in the whole temperature range (20–60 °C), we were able to clearly highlight the impact of the phase transition of PNIPA on the mechanical reinforcement of the gel without any interference of the volume transition. These graft hydrogels, designed with PNIPA in the backbone (GPN-D) or as pendant chains (GPD-N), have been studied more specifically by tensile tests and 2D neutron scattering at rest and under deformation. From these complementary techniques, we show that PNIPA side-chains in GPD-N self-assemble above their transition temperature into a micellar network greatly interfering with the covalent PDMA frame. While the elastic modulus increases reversibly more than ten times throughout the phase transition, other properties like elongation at break and fracture resistance are greatly enhanced with temperature. At high temperature and under extension, SANS data highlight the affine deformation of PNIPA domains. By comparison, the opposite topology with PNIPA forming the cross-linked backbone undergoes a similar phase separation with temperature and gives rise to a bicontinuous structure that aligns under loading. The collapsed phase being topologically defined as the load bearing phase, GPN-D displays remarkable fracture toughening with crack bifurcation at high temperature whereas GPD-N gels fracture in a more conventional way.