LAUSR

Poly(ester‐imide)s (PEIs) are well‐known for their ultralow dissipation factor (Df) of 0.003 at 10 GHz recently. However, the dielectric constant (Dk) and Df seem to be two difficult‐to‐reconcile indexes in PEIs, a relatively high Dk of 3.2 confines the insulating capability of PEIs, owing to the trade‐off between the polymer's free volume and rigidity. To address this difficulty, a cross‐linking strategy of polymer backbones is proposed to increase the free volume and maintain a rigid structure in PEIs. In this research, the crystallinity of a liquid‐crystal‐like PEI is gradually diminished, and the molecular chain spacing is slightly enlarged as the degree of cross‐linking increases. Besides, the degree of the in‐plane orientation of this PEI is obviously reduced, indicating the loose stacking of molecular chains. Consequently, the Dk at 10 GHz of this PEI is decreased from 3.26 to 2.85 after cross‐linking. Furthermore, the Df of the cross‐linked PEI is remained as low as 0.00237. In addition, the tensile strength and elongation at break of this PEI are increased from 219 MPa and 7.4% to 231 MPa and 9.7% after cross‐linking, respectively. And the thermal stabilities of the cross‐linked PEI are also decent as a candidate for low dielectric materials.