The differences in reactivity and thermal stability of the stereoisomers define the thermal properties and responsiveness of the reversible polymer network.

The field of self-healing soft robots was initiated a few years ago. A healing ability can be integrated in soft robots by manufacturing their soft membranes out of synthetic self-healing polymers, more specifically elastomeric Diels-Alder (DA) networks. As such they can recover completely from macroscopic damage, including scratches, cuts, and ruptures. Before this research, these robots were manufactured using a technique named ''shapingthrough-folding-and-self-healing.'' This technique requires extensive manual labor, is relatively slow, and does not allow for complex shapes. In this article, an additive manufacturing methodology, fused filament fabrication, is developed for the thermoreversible DA polymers, and the approach is validated on a soft robotic gripper. The reversibility of their network permits manufacturing these flexible self-healing polymers through reactive printing into the complex shapes required in soft robotics. The degree of freedom in the design of soft robotics that this new manufacturing technique offers is illustrated through the construction of adaptive DHAS gripper fingers, based on the design by FESTO. Being constructed out of self-healing soft flexible polymer, the fingers can recover entirely from large cuts, tears, and punctures. This is highlighted through various damage-heal cycles.

A systematic study of diffusion-controlled reversible Diels−Alder (DA) network formation is performed under both isothermal and nonisothermal reaction conditions based on two amorphous furan−maleimide thermoset model systems. The experimental evolution of the glass-transition temperature, T g , with the predicted DA conversion, x, simulated by a two-equilibrium kinetic model for endo and exocycloadducts leads to the T g −x relationship of these model systems. The heat capacity, c p , from modulated temperature differential scanning calorimetry enables the characterization of (partial) vitrification along the reaction path. In isothermal DA reactions at T cure , a stepwise negative drop in Δc p at the onset of vitrification is observed, followed by a diffusion-controlled reaction at a reduced rate. T g can exceed T cure by at least 15 °C. In nonisothermal DA cure at a sufficiently low heating rate, (partial) vitrification is also possible (negative Δc p step), followed by diffusion-controlled cure until devitrification occurs again (positive Δc p ). Gelation along the reaction path is proven by dynamic rheometry, and gelled glasses can always be obtained under ambient conditions. This information is of importance in the damage management of reversible thermosets by self-repair of microcracks in bulk, as evidenced by dynamic mechanical analysis of a compressed powder after healing below T g .

Time-Temperature-Transformation (TTT), Temperature-conversion-Transformation (TxT), and Continuous-Heating-Transformation (CHT) diagrams are studied for a set of reversible 0.0 0.2 2 elastomeric and thermosetting covalent networks, based on Diels-Alder (DA) furan-maleimide cycloaddition reactions, using different concentrations of furan and maleimide functional groups.Microcalorimetry, modulated temperature differential scanning calorimetry and dynamic rheometry are used as experimental tools in combination with kinetic modelling. The DA kinetics, based on two parallel equilibrium reactions for endo and exo cycloadducts, are optimized for the set of reversible networks cured between 20 °C and 90 °C. Each simulated iso-conversion line in TTT and CHT, in contrast with irreversible networks, shows a totally different shape with a horizontal asymptotic limit at the high temperature side, Tcure, corresponding to the DA equilibrium conversion xeq at Tcure. It is also proven that all gelation lines are iso-conversion lines, and that each gel conversion can be predicted by the Flory-Stockmayer equation. Moreover, the slight differences in the endo-exo kinetics and equilibrium constants lead to a predicted superposition and a double asymptotic behavior of the iso-conversion lines in TTT and CHT. As a consequence, two subsequent gelation/de-gelation events can occur during non-isothermal cure, as shown in the CHT diagram. These phenomena are experimentally confirmed for one of the reversible covalent networks.

While thermally reversible polymer network coatings based on the Diels-Alder reaction are widely studied, the mechanisms responsible for the heating-mediated healing of damage is still not well understood. The combination of microscopic evaluation techniques and fundamental insights for the thermoreversible network formation in the bulk and coating shed light on the mechanisms behind the damage healing events. The thermomechanical properties of thermoset and elastomer coatings, crosslinked by the furan-maleimide Diels-Alder cycloaddition reaction, were studied in bulk and compared to the thermal behaviour applied as coatings onto aluminium substrates. The damage sealing of thermoset (Tg = 79 °C) and elastomer (Tg = −49 °C) coatings were studied using nano-lithography and atomic force microscopy (AFM). The sealing event is studied and modelled at multiple temperatures and correlated to the changes in the network structure and corresponding thermomechanical properties.