Poly(ester imide)s possessing low coefficient of thermal expansion and low water absorption

The film properties of polyimides (PIs) containing para-ester linkages in the main chains, poly(ester imide)s (PEsI) are reported in this paper. An ester-containing dianhydride monomer (TAHQ) was synthesized from hydroquinone and trimellitic anhydride chloride. PEsIs derived from TAHQ and various diamines with stiff/linear structures, p-phenylenediamine (PDA), trans- 1,4-cyclohexanediamine (CHDA), 2,2'-bis(trifluoromethyl)benzidine (TFMB), 4-aminophenyl-4'-aminophenylbenzoate (APAB), and 4,4'-daminobenzanilide (DABA), exhibited extremely low linear coefficient of thermal expansion (CTE) values, for example, CTE = 3.2 ppm K-1 for TAHQ/PDA and 3.3 ppm K-1 for TAHQ/APAB. The results revealed that the para-ester linkages behave as a rod-like segment favorable for thermal imidization-induced in-plane orientation. Copolymerization with flexible 4,4'-oxydianiline made precise CTE matching possible between PEsI/copper substrate and the significant improvement of film toughness at the same time. Wide-angle X-ray diffraction measurements showed that some of the PEsIs examined are semi-crystal line. The present work proposes novel high temperature insulation materials suitable for the substrates for flexible printed circuit, possessing not only a copper-level low CTE but also high film toughness, high dimensional stability, and low water absorption. PEsIs were compared with the corresponding amide-containing PIs. Highly transparent PEsIs are also reported.