Promoteur de thèse: Monsieur Jean-Marie Raquez et co-promoteur Monsieur Pascal Gerbaux et Monsieur Philippe Dubois

Résumé de la dissertation

The development of self-healable and recyclable polymer networks emerged as an elegant solution to improve the reliability, the longevity and the quality of synthetic materials. Consequently, this concept provides great opportunities for finding potential practical applications in sustainable technologies. Such properties of synthetic polymers can usually be obtained by inserting moieties or crosslinks containing reversible bonds into the materials, where the dynamic behavior can be triggered by an external stimulus. This work proposes the utilization of boroxines as dynamic organoboron derivatives to crosslink polymer chains and obtain dual stimuli-responsive networks to water and heat. The objective is to develop new boroxine-based approaches enabling self-healing under mild conditions, using water or humidity, and fast recycling using moderate thermal treatment. In the first chapter, the development of a novel family of iminoboronate-based boroxine adduct is described. The reversibility of the nitrogen-coordinated B-O boroxine bonds was verified on a model system before being inserted into an amorphous polymer matrix via a very straightforward process. This approach is easy to implement and allows the obtention of humidity-sensitive self-healing networks by playing on the boroxine/boronic acid equilibrium. However, the rapid rate of boroxine hydrolysis enabling the efficient healing at room temperature yields a material that creeps under these same moderate conditions (i.e., permanent de-crosslinking under humidity exposure). Further strategies are therefore required to get a better balance between healability and mechanical stability/creep resistance. In this context, the second chapter proposes the addition of irreversible epoxy crosslinks into the PPG-boroxine network designed in the first study. This unique combination of boroxine and epoxy chemistries allowed the development of high modulus polymer networks with self-healing properties. Although the healing efficiency was reduced compared to our first work, a much higher mechanical stability of the network against humidity could be observed, as a result of the presence of static hydrophobic crosslinks. The mechanical properties of the double crosslink network can be reversibly tuned from a stiff thermoset to a soft rubber by water stimulus. Diverse potential fields of application were also envisioned for the thermoset, with or without the addition of fillers. The third chapter proposes an original combination between the nitrogen-coordinated boroxine chemistry and cyclocarbonate derivatives as hydroxyurethane moieties precursors for the design of hydrogen-bonding reinforced polymer networks. This approach allows to optimize the balance between healing (dynamics) and mechanical performance (stability) for the designed networks, using this time supramolecular chemistry to ensure the mechanical integrity. The resulting stiff polymer could be healed efficiently at room temperature by using controlled level of humidity to hydrolyze the boroxines crosslinks and silica gel to re-crosslink the material. Furthermore, the temperature-sensitivity of the iminoboronate-based boroxine network was demonstrated, allowing efficient recycling properties through B-O boroxine and C=N iminoboronate bond reshuffling under mild thermal treatment. Application of the polymer for the design of self-healable tri-dimensional objects was also envisioned. In the final chapter, the vitrimer properties of boroxine-based materials were evaluated through single B-O bond rearrangement by developing polyurethane networks containing exclusively non-coordinated boroxine crosslinks. A model study was realized to demonstrate the temperature-sensitivity of the boroxine derivatives as well as probe the boroxine exchange under heat treatment. Efficient recycling properties of the material could be obtained when extra boronic acid groups were generated into the network before heat treatment. The viscoelastic behavior of the network at different temperatures was studied, providing insight on the B-O bond rearrangement mechanism during the recycling of the material