EFFECT OF N-PHENYLAMINOPROPYL POLYHEADRAL OLIGOMERIC SILSESQUIOXANE DISPERSION METHOD ON STRUCTURE-PROPERTY RELATIONSHIPS FOR THERMOSTABLE POLYCYANURATE-BASED NANOCOMPOSITES

In this work, the effect of the method of dispersion of a reactive N-phenylaminopropyl polyhedral oligomeric silsesquioxane (NPAP-POSS, 0.025 wt.%) with eight secondary amino groups, in dicyanate ester of bisphenol E (DCBE) on the chemical processes occurring in reactive DCBE/NPAP-POSS blends during dispersion, as well as on the chemical structure, viscoelastic, thermophysical, and thermal properties of heat-resistant organic-inorganic PCN/NPAP-POSS nanocomposites was investigated. The synthesis of the nanocomposite samples was carried out in two stages. In the first stage, to improve the efficiency of nanofiller dispersion, high-speed mechanical or ultrasonic mixing of NPAP-POSS with DCBE was used at different temperatures (T = 65 °C, T = 165 °C), which ensured the chemical interaction of the components. In the second stage, PCN/NPAP-POSS nanocomposites were synthesized by in situ high-temperature reactive molding by dynamic heating the samples in the temperature range of T = 20–300 °C. Using dynamic mechanical thermal analysis (DMTA) for PCN/NPAP-POSS nanocomposites synthesized by high-speed mechanical or ultrasonic mixing at a temperature of T = 65 °C, an unusually significant increase (by 26.5–28.5 °C compared to PCN) in the glass transition temperature (Tg) of the samples even at ultra-low NPAP-POSS content. This phenomenon demonstrates the so-called nanoscale effect. It was also found that the method of nanofiller dispersion affects the increase (compared to unfilled PCN) of the storage modulus (E’) and other viscoelastic properties, as well as the apparent network density (v) and the apparent average molecular weight (Mc) between crosslinks in the hybrid network matrix of the nanocomposites. Differential scanning calorimetry (DSC) also showed that the dispersion method changes the thermophysical properties of the synthesized PCN/NPAP-POSS nanocomposites. This effect is associated with the formation of additional organic-inorganic crosslinks due to the chemical embedding of NPAP-POSS nanoparticles and the formation of the hybrid PCN/NPAP-POSS network. Fourier transform infrared (FTIR), 1H NMR, and 13C NMR spectroscopy confirmed that during nanofiller dispersion in DCBE/NPAP-POSS reactive blends, a chemical interaction occurs between the –O–C≡N groups of DCBE and the secondary –NH groups of NPAP-POSS. This interaction is confirmed by the appearance of corresponding absorption bands and signals (chemical shifts) in the spectra indicating the formation of intermediate isourea fragments and triazine rings of polycyanurates. It was concluded that ultrasonic dispersion of the nanofiller is the most effective under these synthesis conditions for PCN/NPAP-POSS nanocomposites as it ensures the highest degree of cyanate group conversion in DCBE at the final stages of synthesis (confirmed by DMTA data), thereby extending the range of working temperatures within which the samples retain their mechanical and physical properties. Using thermogravimetric analysis (TGA), it was found that all nanocomposites exhibit high resistance to thermo-oxidative degradation (Td > 440 °C), which is largely unaffected by the method of nanofiller dispersion and is determined by the chemical structure of the densely cross-linked PCN/NPAP-POSS hybrid network.