Development of High-Curing-Reactivity Phthalonitrile Resins Inspired by Inverse Vulcanization

Phthalonitrile (PN) resins exhibit broad application prospects in aerospace and high-temperature industries due to their excellent thermal resistance and interfacial compatibility with reinforcing fibers. However, their practical use is hindered by high curing temperatures, low curing reactivity, and high melting/softening points. This study proposes a sulfur–allyl synergistic strategy inspired by inverse vulcanization, blending elemental sulfur with a biobased eugenol-functionalized PN monomer (EPN) containing allyl groups to significantly enhance curing reactivity and reduce melting/softening points and processing temperatures. Experimental results demonstrate that the EPN system with 5 mol % sulfur achieves a softening point below room temperature after prepolymerization at 180 °C. The resin cured at 275 °C exhibits a glass transition temperature (Tg) > 360 °C and a residual carbon yield >69% at 800 °C. In situ Fourier-transform infrared spectroscopy (FTIR), 2DCS, and XPS analyses reveal that elemental sulfur ring-opening mediates nitrile cross-linking, wherein inverse vulcanization generates thiol active sites and polysulfide segments to drive stepwise curing. This work provides an effective and simple approach for the optimization of the curing process of high-performance PN resins and expands pathways for the high-value utilization of sulfur resources.