Time Dependence of Dissipative and Recovery Processes in Nanohybrid Hydrogels

The strain rate effect on large strain dissipation and behavior recovery are presented to understand the toughening effect of silica nanoparticles in nanohybrid hydrogels. Such nanohybrid gels combine a poly(N,N-dimethylacrylamide) (PDMA) covalent network and physical interactions by adsorption of polymer chains at the silica nanoparticle surface. A series of model nanohybrid gels has been designed to obtain a well-controlled architecture. First insights on the structure (SANS) demonstrated that silica nanoparticles were well-dispersed in the gel, including after cyclic mechanical loading. The characteristic times involved in the nanoparticle/polymer association were investigated by large strain mechanical cycling varying the strain rate from 3 x 10(-4) s(-1) to 0.6 s(-1). The mechanical behavior of the hybrid hydrogel varies tremendously over a relatively small range of strain rates, ranging from almost non dissipative (at slow strain rates) to highly dissipative at high strain rates. However, upon cycling over time scales of tens of seconds, the strong physical interactions taking place between nanosilica particles and PDMA network chains enabled the hydrogel to recover its initial mechanical properties. The main feature of this work is the remarkable role played by silica nanoparticles in the network to promote transient and recoverable connectivity by reversible adsorption/desorption processes. The strong strain rate dependence suggests that toughening mechanisms operating at standard strain rates as often reported, maybe quite different at slower or larger strain rates.