Correlation of Stress and Optical Properties in Highly Transparent Polyimides for Future Flexible Display

Abstract: High transmittance and low birefringence are desirable optical properties in polyimide films which are promising flexible substrates in next generation display devices. However, thermal processes for the fabrication of polyimide films can cause unpredictable changes in the optical properties due to thermal imidization and consequent charge transfer complex formation. In this study, we examined the changes in optical retardation with sub-nanometer resolution and dimensional stability induced by the deformation of transparent polyimide substrates. We prepared fluorinated polyimide films containing 2,2‘-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 2,2‘-bis(trifluoromethyl)-4,4‘-diaminobiphenyl diamine through a solvent-casting process. During film deformation, the optical retardation for the highly transparent fluorinated polyimide films is changed markedly and the driving forces of birefringence introduced greatly enhanced thermo-dimensional stability. In addition, we quantified the degree of change in the optical properties to derive a stress-optical coefficient of the fluorinated polyimide films. The experimental findings suggest that optimized process conditions should be determined to address both the optical and thermal stabilities. Also, the results provided useful information in regards to large-scale film processing to be further utilized in the fabrication of the transparent polyimide films.