Synthesis and properties of high heat-resistant transparent polyimides containing terphenyl skeleton

Colorless polyimide (CPI) films have good application prospects in flexible optoelectronic devices due to their excellent heat resistance and transparency. In this study, a diamine monomer containing a terphenyl skeleton and CF 3 group substituted at ortho -position of amino group, 1,4-bis(3-(trifluoromethyl)-4-aminophenyl)benzene (6FTD), was successfully designed and synthesized, and a series of corresponding polyimides (PIs) were obtained by copolymerization of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 6FTD and 2,2′-bis(trifluoromethyl)benzidine (TFDB). The introduction of rigid and linear terphenyl structure in 6FTD was beneficial to improve the heat resistance and mechanical properties of PI films. In addition, the CF 3 group derived from 6FTD effectively inhibited the rotation between the imide ring and the benzene, so that the 6FTD-containing PI film had a high glass transition temperature ( T g ). At the same time, the CF 3 groups caused a large free volume between molecular chains, which ensured the excellent optical transparency and solubility. In particular, PI-50 exhibited high T g of 399°C, transmittance of 82 % at 450 nm, tensile strength of 146 MPa and tensile modulus of 3.9 GPa. These results indicate that the design of rigid skeleton and side group provides an effective method for developing heat-resistant transparent PI.