Poly(ester imide)s with low coefficients of thermal expansion ( CTEs ) and low water absorption ( VI ): an attempt to reduce the modulus while maintaining low CTEs and other desired properties

Here, a challenging target in the development of high‐temperature dielectric substrates was established: to significantly reduce the modulus of dielectric substrates while maintaining their low coefficients of thermal expansion (CTEs) and other desired properties, to suppress the spring‐back force, which is generated in folded flexible printed circuit boards. An ester‐linked tetracarboxylic dianhydride with an asymmetric and longitudinally extended structure (TA‐HPBAHQ) exhibited good polymerizability with various aromatic diamines and led to the formation of poly(ester imide) (PEsI) precursors with sufficiently high molecular weights. The thermally imidized TA‐HPBAHQ‐based PEsI films obtained using rigid diamines exhibited a very high glass transition temperature (Tg), ultralow CTE, suppressed water uptake and an extremely low coefficient of humidity (hygroscopic) expansion. In particular, the combination of TA‐HPBAHQ and m‐tolidine significantly lowered the modulus and improved the film ductility while maintaining a low CTE close to that of copper foil with other desired properties. In addition, a TA‐HPBAHQ analogue, TA‐HNAHQ, containing a 2,6‐naphthalene unit, was used. TA‐HNAHQ‐based PEsIs exhibited a further decreased modulus and other excellent properties similar to those of the TA‐HPBAHQ‐based systems except for the appearance of a β‐transition in certain cases, depending on the diamines used. Certain selected PEsIs exhibited an extremely low tan δ at 10 GHz and a relatively good or the highest rank of flame retardancy. Thus, certain PEsIs examined in this study are promising candidates as new dielectric substrates required to exhibit a low CTE, suppressed modulus and other target properties for use in next‐generation flexible printed circuit boards.