Methacrylate-Epoxy Photopolymer Networks via Orthogonal Free-Radical/Epoxy-Acid Reactions in Neat Conditions

We report a series of photopolymer networks fabricated by free-radical polymerizations of methacrylic acid (MAA) followed by epoxy-acid reactions. The free-radical polymerization was accelerated by hydrogen bonding between MAA and multifunctional epoxide. The subsequent epoxy-acid reaction eliminated free acids, resulting in hydrophobic, covalently cross-linked network polymers. These hybrid MAA-epoxy resins showed low viscosities (<36 cP @ 23 °C) while maintaining fast reaction kinetics. Using 405 nm UV light (5 mW/cm2), 50% MAA conversion was achieved within 13 s; quantitative conversion was within 2 min. Macroscopic phase separation was observed after the MAA polymerization, which was mitigated by the addition of other monofunctional methacrylate monomers as “reactive diluents.” When the reactive diluents were included, we obtained glassy materials with robust mechanical properties, including Tg values ranging from 81 to 136 °C and Young’s modulus ranging from 860 to 1410 MPa. These hybrid MAA-epoxy resins enable a platform for the design of photopolymers beyond the typical multifunctional polyester-based methacrylate oligomers.