Dynamic Polymer Network Design for Self-Healing Robotic Applications

Dynamic polymer network systems based on the reversible Diels-Alder cycloaddition reaction have gained a lot of interest over the past decade. These smart, adaptive materials show distinctly different thermomechanical behaviour than conventional thermosetting polymers. The dynamically thermoreversible character of the Diels-Alder reaction offers a stimuli-responsiveness to the network (de)formation, opening up numerous possibilities for advanced processing, new applications and smarter material systems. The reaction thermodynamics and kinetics are studied extensively and well understood. In combination with intelligent polymer network design, it is possible to purposely alter the stimuli-responsiveness in function of the intended applications. It is shown how the concentration and stoichiometry of the reactive groups, as well as the functionality of the monomers allow precise tuning of the network properties and stimuli-responsiveness for dedicated applications.
The thermoreversible (de)gelation of the polymer network allows remending and reprocessing of covalently crosslinked polymer networks. Processing techniques that are not readilly available for conventional thermosetting materials show promising possibilities for thermal processing of these dynamic network systems. A combination of thermoforming and welding is used to create 3D shapes using the remendability due to the reversible network formation [1]. In a next step it is possible to create filaments of reversible polymer networks using reactive filament extrusion. These filaments can then be fed to a 3D printer to print complex geometries. Scrap material and erroneous printed objects can afterwards be recycled into new filaments.
The dynamic character and resulting remendability also allow healing of damage. Materials that exhibit self-repair mechanisms have greatly expanded over the last decade. This is especially true for polymers and polymer composite materials. This self-healing concept aims at considerably prolonging the service lifetime and performance of material systems and structures. Repair of damage can be established at the microscopic scale, e.g. for coatings used to protect substrates from an aggressive environment [2]; or on the macroscopic scale, e.g. for soft robotic actuators that are able to recover their performance after damage healing [1]. In unpublished research the authors demonstrate the ability of such thermoreversible polymer networks to autonomously heal macroscopic damage without the need for thermal activation or human intervention [3]. The Diels-Alder cycloadduct bonds prove to be fully reversible upon mechanical activation, leaving the functional groups ready for rebonding upon removal of the damaging force.