Effect of responsive graft length on mechanical toughening and transparency in microphase-separated hydrogels.

Effective remote control of mechanical toughening can be achieved by using thermo-responsive grafts such as poly(N-isopropylacrylamide) (PNIPAm) in a hydrophilic covalently cross-linked polymer network. The weight ratio of PNIPAm grafts in the network may impart such a thermo-responsive mechanical reinforcement. Here, we show that the network topology - especially graft length - is likewise crucial. A series of covalently cross-linked poly(N,N-dimethylacrylamide) (PDMA) gels grafted with PNIPAm side-chains of different lengths were designed and studied on both sides of phase separation temperature T, at a fixed overall polymer concentration of 16.7 wt% and constant PDMA/PNIPAm weight ratio. Phase-separated PNIPAm organic micro-domains were expected to act as responsive fillers above T and to generate a purely organic nanocomposite (NC). In contrast to conventional NC gels where dissipative processes take place at the solid nanoparticle/matrix interface, here dissipation originates from the disruption of the filler itself by the unravelling of the PNIPAm grafts embedded in collapsed domains. Results show that PNIPAm graft length is a key parameter to enhance - reversibly and on-demand - the mechanical response. The longer the graft is, the more effective the mechanical toughening is. Interestingly, for long PNIPAm grafts, above T, the hydrogels combine perfect transparency together with both increased stiffness and fracture toughness (up to 150 J m) at constant macroscopic volume. As a proof of concept, stimuli-responsive adhesion and shape-memory properties were designed to probe the inter-chain bridging efficiency (in bulk or bridging the interface).