Photopolymerization uses light, instead of heat, to initiate reactions that convert monomer to polymer. The technique is increasingly used in industry due to its high levels of spatial and temporal control, low reaction temperatures, fast curing speeds, and virtually no emission of volatile organic compounds. Free-radical and cationic photopolymerizations have been implemented in many applications, such as coatings, electronics, biomedicine, and adhesives. Although free-radical polymerizations of acrylates are most common, inhibition by oxygen remains a problem. Oxygen reacts with the excited-state photoinitiators and the free-radical active centers to prevent chain propagation. Cationic ring-opening reactions of epoxides, while not inhibited by oxygen, generally exhibit low reaction rates and are sensitive to moisture and alcohol. Epoxide-acrylate hybrid systems promise to reduce the sensitivity of the reaction to oxygen and moisture by combining the advantages inherent to each reaction. However, the much faster acrylate reaction interferes with epoxide conversion because the epoxide chain growth takes place via the relatively slow active chain end (ACE) mechanism. In order to increase epoxide conversion, the faster activated monomer (AM) mechanism, a chain transfer reaction, is promoted in this research by pairing hydroxyl-containing acrylates with a diepoxide monomer.
Hybrid formulations with varying concentrations of 2-hydroxyethyl acrylate (HEA, acrylate) and 3,4-epoxycyclohexane carboxylate (EEC, epoxide) were photopolymerized using the cationic photoinitiator diaryliodonium hexafluoroantimonate (DAI). During photolysis, DAI also produces free radicals capable of initiating the acrylate polymerization, thereby removing the need for a separate free-radical photoinitiator. Conversions of acrylate and epoxide functional groups were obtained using real-time Raman spectroscopy. As the acrylate monomer concentration increased from 50 to 80%, the epoxide conversion increased from 55 to 85%, indicating that the hydroxyl group did indeed facilitate the AM mechanism. In a control experiment to verify that the increased epoxide conversion was not due to a dilution effect, HEA was replaced with a similar non-hydroxyl-containing acrylate, ethylene glycol methyl ether acrylate (EGMEA). The maximum EGMEA and EEC conversion obtained for an 80:20 mixture after 5-minutes illuminations was 86% and 35%, respectively. These results further confirm that the hydroxyl group in HEA promotes the AM mechanism. Physical and mechanical properties (e.g., glass transition temperature, modulus, and phase separation) for these systems are being investigated using dynamic mechanical analysis to determine how the AM mechanism affects the final hybrid polymer properties.