Colorless polyimides derived from 1S,2S,4R,5R-cyclohexanetetra carboxylic dianhydride, self-orientation behavior during solution casting, and their optoelectronic applications

A novel cycloaliphatic monomer for polyimides (PI), 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H'-PMDA) is proposed in this work. H'-PMDA shows high polymerizability with various diamines in contrast to its isomer, i.e., conventional hydrogenated pyromellitic dianhydride (H-PMDA) and leads to highly flexible and colorless PI films with very high T-g's. In particular, the combinations with rigid structures of diamines give rise to Pis with significantly decreased coefficients of thermal expansion (CTE) owing to high extents of in-plane chain orientation induced by thermal imidization, whereas the H-PMDA-based counterparts do not. The decreased CTE reflects structural rigidity/linearity of the H'-PMDA-based diimide units as supported by liquid crystallinity observed in the corresponding model compound. Solution casting of a chemically imidized PI derived from H'-PMDA and 2,2'-bis(trifluoromethyl)benzidine (TFMB) results in a lower GTE than that of the thermally imidized counterpart, suggesting the presence of a self-orientation phenomenon during solvent evaporation. The mechanism is proposed in this work. H'-PMDA/TFMB and its copolymer systems can be useful as plastic substrates in image display devices and/or novel coating-type optical compensation films. (C) 2014 Elsevier Ltd. All rights reserved.