Fabrication of semi-alicyclic colorless polyimide films with high optical transparency and low CTE for flexible AMOLED displays

The rapid development of cover sheets and backplanes in flexible active matrix organic light emitting diode (AMOLED) displays has intensified the demand for colorless polyimide (CPI) films with superior optical transparency, remarkable dimensional stability and great thermal resistance. In this study, three semi-alicyclic dianhydride monomers (HAMDA, HIMDA, TCDA) were successfully fabricated, followed by the separate polymerization with 2,2’-bis(trifluoromethyl)benzidine (TFDB) and trans-1,4-diaminocyclohexane (t-CHDA) to afford a series of colorless polyimide films. In terms of overall performance, TCDA/t-CHDA outperformed all other as-synthesized CPI films, featuring an excellent optical transmittance of 88% at 550 nm, a low haze value of 0.98 and a modest yellowness index at 2.95, alongside a reduced CTE of 17.7 ppm K − 1 and an elevated T g value of 334 ℃. Besides, TCDA/t-CHDA exhibited a robust tensile strength of 109 MPa and a remarkable elongation at break of 24%. The best overall performance of TCDA/t-CHDA was primarily attributed to the incorporation of the linear and rigid alicyclic structure as well as the formation of hydrogen bond originating from amide groups. The findings in this study provide valuable guidance for the design and fabrication of advanced colorless polyimide films targeting high-end applications.