Recyclable Epoxies Through Depolymerization Using Photothermal Nanoparticles

Epoxy are an important class of thermosetting polymers for many long-term applications such as adhesives, structural material, and protective coatings. While they provide durable and robust mechanical properties, conventional epoxy products are typically non-recyclable and extremely difficult to remove at the end of their life. To address these limitations, recyclable epoxies through depolymerization are of interest. In this talk, synthesis and characterization of recyclable epoxies crosslinked by the Diels-Alder (DA) reaction will be presented. The recyclable epoxies were synthesized by reactions between two precursors containing maleimide and furan functional groups to form cyclo-adducts. The retro-Diels–Alder (rDA) reaction at elevated temperature can serve as a recycle process through depolymerization of the epoxy network. Various precursors were synthesized by manipulating molecular weights, architecture (e.g., 4-arm or 6-arm), and spacers between functional groups. Different stoichiometric ratios between maleimide and furan precursors were examined to control a crosslinking density of the recyclable epoxy. Thermomechanical behavior of these recyclable epoxies was characterized by calorimetry and rheometry. The epoxies demonstrated a high-temperature (~120 °C) endotherm corresponding to rDA. Moduli of the epoxies dropped at high temperature which indicates depolymerization of the epoxy network through rDA. Plasmonic titanium nitride (TiN) nanoparticles were introduced to trigger the depolymerization through photothermal mechanism. Plasmonic nanoparticles permit localized heating by converting incident light into heat. The generated heat initiates a thermally reversible reaction. The photothermal mechanism offers several benefits compared to the bulk heating because the light allows onset of depolymerization remotely, instantly, and exclusively at a targeted area. Various contents of TiN nanoparticles were loaded to the recyclable epoxy. Removal and recycle processes of these nanocomposite samples were examined by multiple cycles of adhesive tests. Two glass slides were attached using the reversible epoxy/TiN nanocomposites. The sample was then placed under a high-intensity light source. The recyclable epoxy turned soft by depolymerization, consequently, the slides separated under stress. After reattachment of these two glass slides, it could undergo the same stress as before the photothermal test without failing.