Synthesis and characterization of semi-alicyclic transparent polyimide films containing benzanilide and fluorene units with improved high-temperature dimensional stability and low-optical retardations

Two solution-processable semi-alicyclic transparent polyimide (PI) films with improved high-temperature dimensional stability and low-optical retardations were prepared from the stereo-isomeric alicyclic dianhydrides, 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (ccHPMDA) and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride (ctHPMDA) and an aromatic diamine, 9,9-bis[4-(4-amino-benzamide)phenyl]fluorene (FDAADA), respectively. The alicyclic cyclohexane units in the dianhydride moiety endowed the derived PI-1 (ccHPMDA-FDAADA) and PI-2 (ctHPMDA-FDAADA) films good optical transparency, the rigid-rod benzanilide units provided the PI films low coefficients of thermal expansion (CTE), and the fluorene units resulted in good solution processability and low-optical retardations (Rth) for the PI films. The PI films exhibited the glass transition temperatures over 400 degrees C and the CTE value of 33.2 x 10(-6)/K for PI-1 and 29.0 x 10(-6)/K for PI-2 in the temperature range of 50 similar to 250 degrees C, respectively. In addition, the PI films showed good optical transparency in the ultraviolet-visible light region with the optical transmittances higher than 80% for PI-1 film at the wavelength of 500 nm. The PI films exhibited the average refractive indices of 1.6505 for PI-1 and 1.6485 for PI-2 film, respectively. The birefringence values of the PI films were all lower than 0.01, resulting in the low-optical anisotropies with the optical retardation (Rth) of 77 nm for PI-1 film and 84 nm for PI-2 film.