Hydrogen-bonded fluorinated Poly(amide imide)s with high refractive indices and transparency for optical communications

High refractive index polyimides for optical communications frequently face the challenge of balancing high refractive index with high transparency. In this study, a diamine monomer, 2,2′-bistrifluoromethoxy-1,1′-biphenyl-4,4′-bisaminobenzamide (ABTFMOB), which features both amide bonds and –OCF 3 groups, was designed and synthesized. Based on this monomer, a series of fluorinated poly(amide imide)s (FPAIs) with dense hydrogen bonding networks were synthesized. Among them, FPAI1, obtained through the polycondensation of ABTFMOB and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride, exhibited the most outstanding overall properties, including high thermal stability ( T g  = 370 °C), excellent mechanical performance (tensile strength >200 MPa), high optical transparency (transmittance >80% at 450 nm), and a remarkably high refractive index (1.814 at 1550 nm). The refractive index of FPAI1 significantly exceeds that of most commercial optical polymers, which could be attributed to: (1) the dense hydrogen bonding networks, which effectively reduce the intermolecular chain distance and increase the chain packing density, thereby minimizing free volume; and (2) the rotational freedom of the –OCF 3 groups, which enhances molecular conformational adaptability and further optimizes chain packing. Moreover, by copolymerizing ABTFMOB with 2,2′-bistrifluoromethoxy-biphenyl-4,4′-diamine, a diamine monomer without amide bonds, and adjusting their molar ratio to modulate the hydrogen bonds density, a continuously tunable refractive index (1.699 to 1.814 at 1550 nm) was achieved while maintaining high transparency and thermal stability. This study provides a valuable design strategy for high refractive index polymers for optical communication applications.