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

Abstract

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‐aminobenzamide)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°C and the CTE value of 33.2 × 10−6/K for PI‐1 and 29.0 × 10−6/K for PI‐2 in the temperature range of 50 ~ 250°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.