Highly flexible and optical transparent 6F-PI/TiO₂ optical hybrid films with tunable refractive index and excellent thermal stability

In this study, a synthesis route was developed for preparing highly transparent novel polyimide-nanocrystalline-titania hybrid optical films with a relatively high titania content (up to 50 wt%) and thickness (20–30 μm) from a soluble polyimide 6F-poly(p-hydroxy-imide) (6FPI). It has been demonstrated that the introduction of bulky CF₃ groups into polyimide backbones enhanced both solubility and optical transparency. Furthermore, the attachment of hydroxyl groups provided the organic–inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide : hydroxyl groups. AFM, SEM, TEM, and XRD results indicated the formation of well-dispersed nanocrystalline-titania. The flexible hybrid films revealed excellent optical transparency in the visible range, good surface planarity, high thermally dimensional stability, and tunable refractive index. A three-layer anti-reflection coating based on the hybrid films was prepared and revealed a reflectance of less than 0.7% in the visible range, indicating its potential optical applications.