<scp>CF</scp> ‐Reinforced Advanced Composites Using Polyamide‐Imide (PAI) Matrices Synthesized via the Ester‐Acid Method

ABSTRACT In this study, carbon fiber (CF)‐reinforced polyamide–imide (PAI) matrices with diisocyanate monomers were synthesized for the first time using the ester–acid method. 4,4′‐methylene diphenyl diisocyanate (m‐MDI) was reacted with trimellitic anhydride mono‐methyl ester–diacid (TMA ester–acid) and pyromellitic dianhydride di‐methyl ester–diacid (PMDA ester–acid). This method enabled homogeneous synthesis by avoiding precipitation and solubility issues typical of direct isocyanate–anhydride reactions. Continuous fiber‐reinforced PAI composites exhibited excellent mechanical and thermal properties, with a high storage modulus at 250°C–300°C and a Tan δ Tg reaching 335°C. Owing to strong fiber–matrix adhesion, TMA100 showed the highest flexural strength (1201.3 MPa) and modulus (71.96 GPa). Although increasing PMDA content reduced the properties, composites with 90%–60% TMA maintained high performance. While Tg decreased from 335°C (TMA100) to 300°C (TMA50), it still supported high‐temperature applications. Lower TMA levels (≤ TMA50) caused delamination and poor fiber dispersion, whereas higher TMA levels (TMA100, TMA90) demonstrated strong potential for aerospace and defense applications. These findings highlight the effectiveness of the ester–acid method in producing high‐performance CF composites.