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

Abstract

A systematic\nstudy of diffusion-controlled reversible Diels–Alder\n(DA) network formation is performed under both isothermal and nonisothermal\nreaction conditions based on two amorphous furan–maleimide\nthermoset model systems. The experimental evolution of the glass-transition\ntemperature, <i>T</i>_g, with the predicted DA\nconversion, <i>x</i>, simulated by a two-equilibrium kinetic\nmodel for <i>endo</i> and <i>exo</i>cycloadducts\nleads to the <i>T</i>_g–<i>x</i> relationship of these model systems. The heat capacity, <i>c</i>_p, from modulated temperature differential scanning\ncalorimetry enables the characterization of (partial) vitrification\nalong the reaction path. In isothermal DA reactions at <i>T</i>_cure, a stepwise negative drop in Δ<i>c</i>_p at the onset of vitrification is observed, followed\nby a diffusion-controlled reaction at a reduced rate. <i>T</i>_g can exceed <i>T</i>_cure by at least\n15 °C. In nonisothermal DA cure at a sufficiently low heating\nrate, (partial) vitrification is also possible (negative Δ<i>c</i>_p step), followed by diffusion-controlled cure\nuntil devitrification occurs again (positive Δ<i>c</i>_p). Gelation along the reaction path is proven by dynamic\nrheometry, and gelled glasses can always be obtained under ambient\nconditions. This information is of importance in the damage management\nof reversible thermosets by self-repair of microcracks in bulk, as\nevidenced by dynamic mechanical analysis of a compressed powder after\nhealing below <i>T</i>_g.