(167ae) Control of Thermomechanical Properties of a Reversible Epoxy Using Diels-Alder Chemistry

Epoxies are frequently used for long term applications because of their versatility, but they cannot be recycled or be easily removed at the end of their lives. To introduce recyclability, the Diels–Alder (DA) reaction using maleimide and furan functional groups was used to create an epoxy capable of depolymerization via the retro-Diels–Alder (rDA) reaction at high temperature. Plasmonic titanium nitride (TiN) nanoparticles were added to the DA epoxies to introduce efficient photothermal conversion that stimulates quick rDA reactions under mild conditions. To explore applications such as additive manufacturing, thermomechanical properties of the DA epoxies need to be controlled because those determine processing condition (e.g., printing temperature, printing speed, viscosity at a printing temperature, etc.) and quality of printed parts (e.g., surface smoothness, adhesion between printed layers, etc.). In this presentation, crosslinking density of the DA epoxies was controlled, and their relationship to thermomechanical properties were analyzed by calorimetry and rheometry. Various DA epoxy precursors were synthesized by manipulating architectures, molecular weight, and number of functional groups per precursor. The crosslinking density was further controlled by the feed ratio of the furan to maleimide functional groups on the DA epoxy. The rDA reaction at high temperature was confirmed by the endothermic reaction at ~120°C and the moduli drop representing the depolymerization of the epoxy network. The FTIR data also indicated the dissociation of the DA adduct at high temperatures. Reduction of crosslinking density of the DA epoxy allowed lower glass transition temperature and lower modulus, in turn, entered into the flowy region at lower temperature. The recyclability of the epoxies and the effectiveness of TiN nanoparticles were tested in a cycle of adhesive tests using a high-intensity light source.