The effect of thermal treatment on the decomposition of phthalonitrile polymer and phthalonitrile-polyhedral oligomeric silsesquioxane (POSS) copolymer

Phthalonitrile polymer and phthalonitrile-polyhedral oligomeric silsesquioxane (POSS) copolymer were sintered at 500 °C, 600 °C, or 800 °C. The samples before and after sintering were evaluated by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) coupled to FT-IR (TG-FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results suggest that phthalonitrile-polyhedral oligomeric silsesquioxane copolymers have better thermal stability than phthalonitrile polymers. After sintering at 800 °C, the phthalonitrile polymers decomposed completely while the phthalonitrile-polyhedral oligomeric silsesquioxane copolymers retain their original structure. The presence of the rigid POSS structure in the copolymer may limit the thermal movement of polymer chains at higher temperatures, thereby increasing the intermolecular forces between polymer chains. Moreover, POSS generates spatial steric hindrance, further restraining movement of chains and hindering degradation of labile groups; the overall effect is to improve the thermal stability of the composite under higher temperatures.