The network formation reaction of several covalently cross-linked polymer network systems has been made reversible by introducing thermally reversible covalent bonds into the material's network structure. The reversibility is based on the thermoreversible Diels-Alder (DA) reaction between a diene (e.g. furan) and dienophile (e.g. maleimide). At low temperatures the Diels-Alder equilibrium is shifted towards the formation of the adduct and the reversible covalent bonds are formed, resulting in a densily cross-linked polymer network structure. As the temperature increases the equilibrium is shifted towards the initial building blocks and gradually the reversible bonds are broken. This shift in equilibrium with temperature allows a temporary increase in local mobility, which is essential to seal damage. The recovery of initial properties takes place in a subsequent cooling by recombination of covalent bonds through the exothermic DA reaction. This network reversibility can be combined with irreversible network formation to design the thermomechanical propeties to the user's desire.
The material development is supported by a thorough physicochemical characterization, aiming at building-up fundamental knowledge on the thermoreversible reactions, the kinetics and equilibria of the [4+2] Diels-Alder cycloadditions, the associated dynamics, the reversible gel transition, the sealing and healing process, the recovery of the initial properties and the resulting structure-processing-property relations.
These self-healing materials are applied as thin layers on metallic substrates prone to corrosion to serve as self-healing coatings for the corrosion protection of these substrates. In a collaboration with the Surface Analysis and Electrochemistry (SURF) research group of the Departement of Materials and Chemistry (MACH) the corrosion protection and healing ability of these coatings are studied. Additional corrosion protection mechanisms can be incorporated into these self-healing coatings, e.g. corrosion inhibitors, resulting into multiple-action self-healing coatings.
In collaboration with the Robotics and MultiBody Mechanics (R&MM) research group of the Departement of Mechanical Engineering (MECH) of the VUB the thermoreverisble covalent polymer network systems are also used to create various self-healing robotic actuators.