Diffusion- and Mobility-Controlled Self-Healing Polymer Networks with Dynamic Covalent Bonding

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, Tg, with the predicted DA conversion, x, simulated by a two-equilibrium kinetic model for endo and exocycloadducts leads to the Tg–x relationship of these model systems. The heat capacity, cp, from modulated temperature differential scanning calorimetry enables the characterization of (partial) vitrification along the reaction path. In isothermal DA reactions at Tcure, a stepwise negative drop in Δcp at the onset of vitrification is observed, followed by a diffusion-controlled reaction at a reduced rate. Tg can exceed Tcure by at least 15 °C. In nonisothermal DA cure at a sufficiently low heating rate, (partial) vitrification is also possible (negative Δcp step), followed by diffusion-controlled cure until devitrification occurs again (positive Δcp). 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 Tg.