Influence of Bulky Substituents on Optical Transparency and Dimensional Stability of Polyamide-imide

Aiming to circumvent the conflict between strong intra/interchain interactions and the charge transfer effect in designing colorless polyimide, amide-functionalized diamine monomers incorporating sterically hindered side groups were designed and subsequently copolymerized with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) through a two-step polycondensation process. Systematic investigation was conducted to elucidate the structure–property relationship, particularly focusing on how the spatial arrangement and density of the bulky moieties influence the optical characteristics and thermomechanical stability of the resulting polymeric films. Polyamide imide films derived from N,N′-(2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diyl)bis(4-amino-3-methylbenzamide) (3-AMB-TFMB) and 6FDA own higher optical transparency (λcutoff = 337 nm, Ttot = 89.0%, Haze = 0.7) and lighter color (L* = 91.57, a* = −3.77, b* = 0.72, and YI = 0.73) as compared with polyimide containing amide bonds but without methyl substituents. The film exhibits excellent thermal stability and demonstrated exceptional thermomechanical properties, exhibiting a remarkably low in-plane coefficient of thermal expansion (CTE) of 39.6 ppm·K–1 while maintaining an elevated glass transition temperature (Tg) reaching 356 °C. In addition, these polyamide-imides have good solubility in common solvents and are easy to process. By introducing bulky substituents near the neighborhood of the imide group in the polyamide-imide system, the deterioration of the optical properties of the film due to the introduction of amide bonds can be effectively reduced. Current work provides a promising strategy to improve the comprehensive performance of colorless polyamide-imide film for flexible displays and other photovoltaic fields.