Phthalonitrile (PN) resins exhibit outstanding thermal and mechanical properties but suffer from intrinsically high melting temperatures and sluggish curing kinetics, which severely restrict their processability and manufacturability. In this work, a eutectic blending strategy based on ternary PN monomers was employed to markedly depress the melting temperature, enabling mild and controllable melt prepolymerization with a diamine curing agent. This approach produced a low-melting phthalonitrile prepolymer (MPPh) exhibiting a viscosity below 1 Pa·s at 88°C, providing a stable melt-processing window. DSC and FTIR analyses revealed that the prepolymerization predominantly proceeded through linear chain growth via amino-nitrile addition reactions. To address the inherently low curing reactivity of MPPh, 3-aminophenylacetylene (APA) was introduced as a reactive diluent to activate the curing process and further tailor the rheological behavior. The modified resin (MP-m) demonstrated significantly enhanced curing efficiency and a reduced viscosity below 1 Pa·s at 66°C, excellent isothermal stability, and pronounced shear-thinning behavior. The cured MP-m resin retained excellent thermal stability and mechanical performance. These findings provide a simple and solvent-free strategy for the preparation of low-melting-point phthalonitrile, broadening the practical application potential of PN resins in high-temperature composite manufacturing.
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