Optically transparent aromatic poly(ester imide)s with low coefficients of thermal expansion (1). Self-orientation behavior during solution casting process and substituent effect

Abstract

A series of ester-linked tetracarboxylic dianhydrides were synthesized from trimellitic anhydride chloride and hydroquinone (HQ) analogs including various alkyl substituents. The properties of aromatic poly(ester imide)s (PEsI) prepared from the tetracarboxylic dianhydrides and 2,2′-bis(trifluoromethyl)benzidine (TFMB) were investigated for applications as novel plastic substrate materials in image display devices. The chemically imidized PEsI from trimethyl-substituted HQ was soluble even in less hygroscopic solvents such as cyclopentanone (CPN). Simple casting from the CPN solution provided a flexible and less colored film with a considerably low coefficient of thermal expansion (CTE = 11.5 ppm K−1), a high Tg of 276 °C, high thermal stability (5% weight loss temperature in N2: 453 °C), and very suppressed water uptake (WA = 0.13%). The low CTE results from highly aligned main chains along the X–Y direction, which occurred during solvent evaporation after the PEsI solution was coated on a substrate. A mechanism is also proposed for such self-orientation behavior in this work. On the other hand, the thermally imidized counterpart was worthless in terms of low CTE and high transparency. Some conventional rigid tetracarboxylic dianhydrides were copolymerized with a minor content to di-tert-butyl- and diamyl-substituted PEsI systems. This approach was very effective not only for further improving some thermal properties (low CTE and high Tg) but also for suppressing film yellowness while keeping excellent solution-processability. Thus, some of the PEsIs developed in this work can be promising candidates as novel plastic substrate materials.