Synthesis and comparative study of high T_g and colorless polyamide-imides with different amide-to-imide ratio

Colorless polyimide (CPI), a heat-resistant polymer being widely used in optoelectrical devices, has attracted worldwide attention. Polyamide-imides due to the incorporation of amide groups into the CPI chains enhance the polymer performance by improving the thermal dimensional stability and excellent mechanical properties without sacrificing the optical transparency. In this article, CPI films PI-(1-3) are synthesized from one-step polymerization of 4,4 '-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and cyclopentanone bis-spironorbornane tetracarboxylic dianhydride (CpODA) with amide-containing diamine N,N '-(2,2 '-bis(trifluoromethyl)-[1,1 '-biphenyl]-4,4 '-diyl)bis(4-aminobenzamide) (AB-TFDB) and bis(trifluoromethyl)benzidine (TFDB). In comparison, by adjusting the ratio of imide and amide groups in the repeating unit, PI-(4-5) are prepared from copolymerization of terephthaloyl chloride (TPC), 6FDA, and CpODA with TFDB diamine. PI-(1-5) with 90% transmittance in the visible region tailored the excellent thermal and mechanical properties. Particularly, PI-3 exhibited a high glass transition temperature of 412 degrees C, a coefficient of thermal expansion of 20.8 ppm/K, a tensile strength of 128 MPa, and an elastic modulus of 5.4 GPa. The results also concluded that irrespective of the polyamide-imide synthesis strategy, CPI films with similar amide-to-imide ratio in polymer chain demonstrated comparable optical, thermal, and mechanical properties.