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Studies on novel tri-phthalonitrile phenyl polyhedral oligomeric silsesquioxane and the phthalonitrile–epoxy blends

A novel tri-phthalonitrile phenyl polyhedral oligomeric silsesquioxane (TPnPh-POSS) was synthesized from trisilanol phenyl POSS and 4-nitrophthalonitrile. The structure of the TPnPh-POSS monomer was characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The phthalonitrile monomer was thermally polymerized with various concentrations (3, 4, and 5 wt%) of 4,4′-diaminodiphenyl sulfone (DDS) as the curing agent. The curing behavior of the monomer was investigated by differential scanning calorimetry. The influence of the content of the curing additive on the polymer properties was studied. Blends of TPnPh-POSS and diglycidyl ether of bisphenol A (epoxy resin) were successfully prepared. The thermal and mechanical properties of these blends with DDS as curing agent were evaluated and compared with the neat epoxy resin. The thermal stability of the cured polymers and epoxy/TPnPh-POSS blends was studied by thermogravimetric analysis. Dynamic mechanical analysis and dielectric studies indicate the influence of TPnPh-POSS content on the relaxation behavior of the prepared blends. The blends exhibit high glass transition temperatures, high storage modulus, and increased thermal stability compared with the commercially available neat epoxy resin. Noncrystalline nature of the cured polymer (with 5 wt% of DDS) was confirmed by X-ray diffraction analysis. The cured polymer shows high flame retardancy, which improved with increase in the amount of the curing additive.