Colorless PI Structure Design and Evaluation for Achieving Low CTE Target

Abstract To obtain the colorless polyimide (PI) film with low coefficient of thermal expansion (CTE) and high thermal stability, the rigid biphenyltetracarboxylic acid dianhydride (BPDA) and 4,4'-(Hexafluoroisopropylidene) diphthalic anhydride (FDA) were selected and copolymerized with 2,2'-bis (trifluoromethyl)-[1,1'-biphenyl]-4,4'-diamine (TFDB). The physical properties of PIs were effectively regulated and optimized by adjusting the ratio of BPDA to FDA. When the ratio of BPDA to FDA is 6:4, the thermal imidized film (PI-6) performance is the best. It has a glass transition temperature (Tg) of 363 ℃, a 5% weight loss temperature (Td5) of 533 ℃, a CTE of 18.8 ppm/K, and a tensile strength of 140 MPa. The transmittance of PI-6 at 760 nm and 430 nm are up to 89.6% and 83.2% respectively. Further, the PI-6's precursor, PAA-6 were treated by chemical imidization. The obtained film has a transmittance of 92.6% at 760 nm, a decreased CTE of 15.5 ppm/K, and a tensile strength of 227 MPa, which is expected to be used as flexible display substrates. Finally, the effects of molecular and aggregation structure on the properties of PIs were studied.