| Abstract: | A series of poly(ester imide)s (PEsIs) were prepared using longitudinally extended structures of ester‐linked tetracarboxylic dianhydrides with different numbers of aromatic rings (NAr = 4‐6). In the PEsIs obtained using p‐phenylenediamine (p‐PDA), a clear trend was observed: the water absorption (WA) decreased with increasing NAr. In contrast, no clear decrease in the Tg with increasing NAr was observed for the PEsIs obtained using 4,4′‐oxydianiline (4,4′‐ODA). The PEsIs obtained using a methyl‐substituted tetracarboxylic dianhydride (NAr = 5) showed more suppressed WA and higher elongation at break (εb) values than those of the nonsubstituted counterparts. The former result is probably closely related to the enhanced crystallinity. However, methyl substitution caused an appreciable reduction in the thermal stability. Thus, the methyl‐substituted PEsIs did not meet the V‐0 standard in the UL‐94V test, unlike the substituent‐free counterparts. The PEsI copolymer obtained using the substituent‐free tetracarboxylic dianhydride (NAr = 6) with p‐PDA (75 mol%) and 4,4′‐ODA (25 mol%) had excellent combined properties, ie, a very high Tg (361°C), an ultralow coefficient of thermal expansion (2.2 ppm K?1), an extremely low coefficient of hygroscopic expansion (3.3 ppm/RH%), moderate film ductility (εbmax = 23%), a moderate dielectric constant (3.22), and a low tan δ (2.76 × 10?3) at 10 GHz in 50% relative humidity. Thus, this PEsI is a promising novel dielectric substrate material for use in the next generation of high‐performance flexible printed circuit boards. |
