High thermal stable polyimide resins derived from phenylethynyl-endcapped fluorenyl oligoimides with low melt viscosities

To improve the processability and thermal stability of polyimide, a series of novel phenylethynyl-endcapped oligoimides (PEPA-oligoimides) with calculated molecular weights (M _nC) were successfully prepared from thermal imidization of 4,4’-(9-fluorenylidene) dianiline (BAFL) as fluorenyl diamine, 4,4′-oxy-diphthalic anhydride (ODPA) as aromatic dianhydride and 4-phenylethynylphthalic anhydride (4-PEPA) acted as reactive end-capping reagent at elevated temperatures. Experiment results indicated that the oligoimides were the mixtures of PEPA-endcapped oligomers with different degrees of polymerization characterized by MALDI-TOF mass spectra. The influence of chemical structures on the melt processabilities of the oligoimides, the thermal, dielectric and mechanical properties of the thermoset resins was studied. The typical oligoimide resin owned minimum melt viscosity of 0.2 Pa·s at around 310 °C and wide melting processing window, suitable for resin transfer molding (RTM). Besides, its corresponding thermal-cured polyimide resin possessed glass transition temperature (T _g) as high as 514 °C. The dielectric constants of polyimide resins decreased from 3.15 to 2.80 by reducing the M _nC. The mechanical properties of the polyimide neat resins were improved gradually with increasing M _nC. Finally, the carbon fiber/polyimide (C_f/PI) composite laminates showed excellent mechanical strength retention rate at 350 °C, might be long-term served at extremely high temperature in aerospace and aviation field.