Construction of furan-maleimide Diels-Alder reversible network cure diagrams: modelling and experimental validation

In recent years, there has been significant research on thermoreversible networks utilizing Diels-Alder (DA) cycloadditions, particularly as self-healing materials. The DA reaction establishes two equilibria forming endo and exo cycloadducts, with covalent bond opening favored at high temperatures and the cycloadducts (re)formation preferred at low temperatures.1 These dynamic bonds not only confer self-healing properties but also contribute to prolonged material lifetimes, heightened stability, reliability, and sustainability. Moreover, these enhancements address constraints common in classical network-forming materials, offering increased recyclability, reprocessability, and reshapeability,2 making them appealing for applications requiring mechanical robustness and thermomechanical stability. This implies the necessity of a (partially) vitrified network with a sufficiently high glass transition temperature (Tg). Self-healing as well as forward and retro-DA reaction will thus occur, at least partially, in diffusion-controlled conditions for most application temperatures.3–6
This study concentrates on investigating the impact of vitrification on DA reaction kinetics within a reversible thermosetting network based on furan-maleimide chemistry. A novel mechanistic model for this vitrifying system is derived from the traditional two equilibria model, incorporating a diffusion-controlled encounter pair formation as an intermediary step.7 Through optimization of kinetic, thermodynamic, and diffusion parameters using calorimetric data and long-term Tg evolution, a set of parameters is obtained, capable of describing the system under both kinetically-controlled and diffusion-controlled conditions. These parameters are then used to simulate Time-Temperature-Transformation and Continuous-Heating-Transformation diagrams. These cure diagrams were then experimentally confirmed with Modulated Temperature Differential Scanning Calorimetry (for vitrification/de-vitrification phenomena) and dynamic rheometry (for gelation/de-gelation phenomena). The observed unique shape of these diagrams provide a visual representation of the differences in the cure process between these reversible networks and classical irreversible thermosets. This holds particular relevance in the context of material design and processing, especially concerning their potential applications in self-healing technologies.