Insights into Dual-Cure UV-Induced Photopolymerization and Thermal Curing of Phthalonitrile Resins

Phthalonitrile (PN) resins are well recognized for their exceptional thermal stability. However, their practical implementation is often limited by the high temperatures required to initiate thermal curing. This study reports the development of an ionic-liquid-assisted dual-cure strategy to produce processable PN-based thermosets under significantly reduced curing conditions. The thermosets were prepared using photopolymerizable formulations containing an acrylate-functionalized PN monomer in the presence of a crosslinker (poly(ethylene glycol) diacrylate) and the ionic liquid 1-butyl-3-methylimidazolium dicyanamide as a curing promoter. Solvent-free UV polymerization produced a polyacrylate scaffold in which the PN functionality remains chemically intact, enabling subsequent network formation during thermal curing. The relationships between material composition and physical properties, including thermal behavior, thermal stability, and thermomechanical performance, are reported. Combined spectroscopic, calorimetric, computational, and thermomechanical analyses suggest that the ionic liquid enables early-stage activation of the network during thermal treatment and promotes subsequent PN crosslinking into a rigid heteroaromatic thermoset structure. As a result, highly thermally robust PN-containing materials were obtained under substantially milder curing conditions than those typically associated with conventional PN systems. These findings not only contribute to a deeper understanding of PN curing mechanisms but also establish ionic-liquid-assisted dual curing as an effective route to advanced PN thermosets with improved processability and exceptional thermal performance.