We discovered that the free radical polymerization of N,N-dimethylacrylamide in water can lead, above a certain concentration, to gels without any added difunctional crosslinker. These so called “self-crosslinked” hydrogels were prepared and their weak mechanical properties were improved by introducing silica nanoparticles. From swelling experiments performed at equilibrium in aqueous media, it was shown that silica particles behave as adhesive fillers and strongly interact with PDMA chains. These interactions are responsible for the reinforcement of mechanical properties. From initial elastic moduli, determined in the preparation state, we show that the elastic behaviour of these hydrogels mainly originates from entanglements and from physical crosslinks that can be controlled by the polymer concentration and the ratio between silica particles and polymer chains, respectively. The mechanical behaviour was characterized using: monotonic tensile tests, loading-unloading cycles at large strains and stress relaxation experiments in order to investigate long time behaviour. The introduction of silica highly increases the stiffness of the network without greatly reducing its extensibility, implying that strong interactions take place between PDMA chains and silica surfaces. Non-linear behavior was pointed out: softening at small deformations and hardening at high deformations which is related to finite chain extensibility. All these effects have been shown to strongly depend on the silica content. The analysis of hysteresis and residual strains induced by cycles, clearly indicate that contrary to chemical crosslinkers, hybrid interactions increase the dissipative process.

Significance: Tough soft materials usually show strain softening. Avoiding strain softening, although important for practical application, had not been a focus of research. Recently, a series of tough hydrogels made from hyperconnective physical networks and linear polymers as molecular glue show extraordinarily large nonsoftening, quasi-linear but inelastic deformation. Here, we studied the molecular mechanism of this behavior and clarified that it is a direct consequence of the interplay between hyperconnectivity of the skeleton network and the stress transfer through molecular glues. We believe that this work gives important insight on the design and development of tough soft materials without strain softening.