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Hybrid hydrogels as model nanocomposites : reinforcement mechanisms by analogy with filled rubbers

Abstract : Hydrogels are soft and elastic solid materials mainly composed of water. Owing to their ability to interact with their environment through drastic volume change, hydrogels already find a wide range of applications, as superabsorbants, in pharmaceutics as drug delivery systems and more recently, as sensors and actuators, widening even more the possible use of soft materials as structural or load-bearing materials. Hence, providing gels with high mechanical performances is of major importance to meet these demanding applications. Efficient toughening can be achieved by combining inorganic and organic materials linked by physical and/or chemical interactions. In such nanocomposite materials, the reinforcement depends on the interactions between the polymer and the filler and of the dispersion state of the fillers, allowing to reach the full extent of reinforcement. Hybrid hydrogels based on poly(N-alkylacrylamide) and nanosilica demonstrating great mechanical reinforcement at large strain as well as self-healing capabilities were designed by Hourdet and Marcellan . The drastic improvement of the mechanical behavior is attributed to the reversible adsorption of the polymer onto silica surface. Viscoelastic properties of such hybrids gels have been extensively studied but few studies were carried out to understand and quantify the interactions at the solid/liquid interface. Likewise, the behavior beyond viscoelastic regime of such materials remains scarcely investigated. A first part of this study focused on the synthesis and the adsorption behavior of linear polymer chains of polyacrylamides, more precisely poly(acrylamide) (PAAm), poly(N,N-dimethylacrylamide) (PDMA) onto silica nanoparticles with controlled surface chemistry. It evidenced the non-interacting behavior of PAAm towards silica surface, justifying its choice as a non-interacting polymer. A second part dealt with the modulation of particle/polymer interactions in hybrid gels either by substituting the interacting monomer (DMA) with a non-interacting one (AAm) or by tuning the surface chemistry of the nanoparticles. Then, the study of their structure and mechanical properties was conducted with a special focus on the non-linear behavior. The role of polymer adsorption for gel reinforcement was evidenced and more specifically the importance of the silanol groups at the silica surface for PDMA/silica interactions. The contribution of the dispersion state of silica on the structure and the mechanical response of hydrogels was then addressed and the non-linear domain of the resulting hydrogels was investigated using Large Amplitude Oscillatory Shear experiments. The impact of the chosen monomer and of the dispersion state of the fillers was investigated at large strain, allowing a more precise analysis of the reinforcement mechanisms, by analogy with filled elastomers
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Anne-Charlotte Le Gulluche. Hybrid hydrogels as model nanocomposites : reinforcement mechanisms by analogy with filled rubbers. Material chemistry. Université de Lyon, 2019. English. ⟨NNT : 2019LYSE1107⟩. ⟨tel-02355626⟩

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