High <i>T</i>_g, Low <scp>CTE</scp> Transparent Poly(Amide‐Imide)s Based on Synergistic Regulation of Lithium Ions Coordination and <i>Ortho</i>‐Position Effect of 9,9‐Bis(4‐Aminophenyl)Fluorene

ABSTRACT Transparent polyimides were expected to be an alternative to glass substrates. To meet the requirements of high transparency, high heat resistance, and low coefficients of thermal expansion (CTE) for substrate materials, a series of poly(amide‐imide)s (PAIs) were synthesized from 2,2′‐bis(trifluoromethyl)benzidine (TFDB), (hexafluoroisopropylidene)diphthalic anhydride (6FDA), cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CBDA), terephthaloyl chloride (TPC) and 9,9‐bis(4‐aminophenyl)fluorene (FDA) with different substituents. And the corresponding films were prepared by adding 0 or 2 pph lithium chloride (LiCl). Through the adjustment of monomer ratio, regulation of the amino ortho ‐position of diamine monomer, and coordination of Li ions, all PAIs exhibited high glass transition temperatures ( T g ) of 390°C–436°C, low CTE of 12.5–60.9 ppm K −1 , excellent optical transmittances of 71%–84% at 400 nm, high tensile strengths of 140–204 MPa, and high tensile moduli of 4.4–7.2 GPa. In particular, PAI‐M‐1‐2‐2Li showed the best comprehensive properties with the transmittance of 83% at 400 nm, T g of 432°C, CTE of 18.7 ppm K −1 , tensile modulus of 6.9 GPa and tensile strength of 204 MPa, which made it competitive in flexible display applications.