Synthesis, Processing, and Properties of Silicon-Containing Phthalonitrile Resins

Hybrid inorganic-organic resins may provide higher temperature performance in oxidizing environments than their organic counterparts. Phthalonitrile (PN) polymers are excellent candidates for hybridization due to their high thermal stability and glass transition temperatures and their need for improved long-term oxidative stability and toughness. In this work phenyl-substituted organosilicon linkages were incorporated into PN monomers to investigate their effect on the processing, thermo-mechanical properties, and thermal and oxidative stability. Three hybrid silicon-containing phthalonitrile monomers were synthesized incorporating diphenoxydiphenylsilane, tetraphenylsilane, and hexaphenyldisiloxane moieties. Processability of the polymers was highly dependent on catalyst content and an ideal concentration was determined. The impact on glass transition, coefficient of thermal expansion, stability in TGA, and long-term oxidative stability at 250 °C was evaluated. As-synthesized materials performed significantly better than polymers produced from purified monomers. Degradation of the tetraphenylsilane phthalonitrile monomer was examined in detail via IR-TGA and analysis of aged samples. Multiple degradations were identified involving both the organic and hybrid sections of the polymer. Synthesized materials are compared with commercial phthalonitrile reference materials and to other silicon-phthalonitriles in recent literature. Explanations of behavior and suggestions for future improvements are provided.