The structure design of poly(ester imide)s with low dielectric loss and high mechanical properties

Abstract

Polyimides with ester groups (PEsIs) are ideal dielectric materials for high-frequency communications due to their low dielectric loss (D_f) and low thermal expansion coefficient (CTE). However, the rigidity of the ester segments leads to poor mechanical properties and processability of PEsIs films. To improve the toughness of this kind of polyimide, we proposed a strategy named ether-bond flexible equilibrium, that is, to prepare poly(ether-ester imide) (PEtEsI) by introducing flexible ether bonds into PEsIs, and improve their comprehensive properties by adjusting the ratio of ester groups to ether bonds. The polyimide retained the characteristics of low dielectric loss (D_f < 2 × 10⁻³), low moisture absorption (W_A < 0.7%), high thermal stability (T_g > 420 °C) and low coefficient of thermal expansion (CTE < 20 ppm K⁻¹), similar to PEsIs, while exhibiting high mechanical strength (σ = 216 MPa), and high elongation at break (ε = 18%) when the ether content in diamine was 25%. Additionally, when the ester dianhydride monomer with naphthalene structures was used, superior performance was further exhibited due to the rigidity of naphthalene rings and the “side-step” effect. This new strategy can also be extended to other polymers for high-performance dielectric materials.