Based on rigid xanthone group and hydrogen bonding to construct polyimide films with low coefficient of thermal expansion, high temperature resistance, and fluorescent property

In recent years, an increasing amount of attention has been paid to polyimide (PI) substrate materials with high glass transition temperature (Tg) and low coefficient of thermal expansion (CTE) due to the rapid development of the flexible OLED field. To improve Tg and CTE, rigid conjugated structure and hydrogen bonds are designed. Two diamines containing amide and rigid xanthone groups were synthesized (p-DAXBA and m-DAXBA). Two series of PI films, PIa and PIb, were prepared by polymerization of DAXBA with different dianhydrides, i.e., pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyl tetracarboxylic diandhydride (BPDA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and 4,4′-oxydiphthalic anhydride (ODPA). PIa films prepared by p-DAXBA have more hydrogen bonding, closer molecular chain packing, and stronger molecular charge transfer (CT) effect, as well as better heat resistance, dimensional stability, and mechanical properties than PIb films prepared by m-DAXBA. However, their absolute fluorescence quantum efficiency and transparency are weaker than those of PIb films. A PIa-1 film prepared by p-DAXBA/PMDA has a Tg of 400 °C, tensile strength of 192 MPa, modulus of 6.14 GPa, and CTE as low as 2.3 ppm·K−1. Both PIa-4 and PIb-4 films prepared by ODPA show fluorescent property with maximum emission wavelengths at 509 and 508 nm, and their absolute fluorescence quantum efficiencies are 1.5% and 2.9%, respectively. The as-prepared PI films exhibit potential application in the flexible OLED field.