Design, Synthesis and Properties of Semi-Alicyclic Colorless and Transparent Polyimide Films with High Glass Transition Temperatures and Low Retardation for Potential Applications in Flexible Electronics

Abstract

Polyimide (PI) optical films with high glass transition temperatures (high-Tg), high optical transparency, and low optical retardations (low-Rth) are highly desired in advanced optoelectronic applications. However, the standard PI films usually suffer from deep colors, high optical anisotropies and limited Tg values. In the current work, a series of semi-alicyclic colorless and transparent PI (CPI) films were developed from hydrogenated pyromellitic dianhydride stereoisomers, 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride, and fluorene-containing diamines, including 9,9-bis(4-aminophenyl)fluorene and 9,9-bis(3-fluoro-4-aminophenyl)fluorene, respectively. The derived CPI films showed Tg values higher than 420 °C according to differential scanning calorimetry measurements. In addition, the fluorene-based CPI film showed optical transmittances higher than 80% at the wavelength of 400 nm, with yellow indices in the range of 0.60~1.01 and haze values below 3.0%. The CPI films showed average refractive indices from 1.5407 to 1.6309, extremely low birefringence at the level of minus fourth power of ten, and further exhibited quite low optical retardations below 10 nm.

Methyl Side-Group-Regulated Terphenyl Polyimides with High Heat Resistance and Ultralow Thermal Expansion for OLED Applications

“…Additionally, as polyimides need to be combined with glass, copper, or silicon in substrate applications, large differences in the coefficient of thermal expansion (CTE) may lead to warping and cracking problems; − as a consequence, a CTE value lower than 10 ppm·K –1 is necessary. To improve the thermal properties, several strategies have been proposed including the rigid and conjugated structures such as fluorene rings and spirofluorene rings − or heterocyclic conjugated structures, − which facilitate the formation of charge-transfer complexes and enhancing intermolecular forces. However, the introduction of bulky groups as well as asymmetric bending structures (spiro rings) also tends to reduce the molecular chain packing density, thereby leading to a high CTE value.…”