Molecular Structure Modification and Performance Regulation of Highly Flexible Poly(ester-imide)s with Low Dielectric Constant and Low Dissipation Factor

Inspired by the excellent dielectric properties and low moisture uptake of liquid-crystalline polymers (LCPs), ester units are introduced into polyimide (PI) backbones to develop poly(ester-imide)s (PEIs) for high-frequency communication dielectrics. Though PEIs show reduced dissipation factor (Df) and hygroscopicity relative to traditional PIs, the intrinsic rigidity and polarity of ester groups lead to relatively high dielectric constant (Dk) and low ductility, restricting their practical use in 5G/6G complex service environments. Herein, three dual-ester-functionalized diamine monomers are rationally designed and synthesized, which are polymerized with terephthaloyl bis(trimellitate) dianhydride (TAHQ) to yield four structurally modified MPEI films: ABHQ-TAHQ, Me-ABHQ-TAHQ, 6H-ABHQ-TAHQ, and 6F-ABHQ-TAHQ. XRD, POM, and AFM demonstrate that molecular-structure regulation can tailor the condensed-state and aggregation structures of polymer chains and improve overall performances synergistically. At 10 GHz, 6F-ABHQ-TAHQ exhibits ultralow Dk (2.93), ultralow Df (0.001 56), and excellent humidity resistance. Me-ABHQ-TAHQ and 6H-ABHQ-TAHQ achieve moderate Dk reduction with slightly elevated dissipation factors. All substituents greatly enhance the low elongation at break of pristine PEI in the order 6F (28%) > 6H (20%) > Me (11%) > PEI (6%). Meanwhile, all modified films maintain prominent thermal resistance (Tg > 435 °C) and thermal stability (Td,5% > 463 °C). This work proves that molecular modification of PEI is an effective strategy to acquire high-frequency dielectrics integrating low-dielectric, heat-resistant, and high-toughness features for promising 5G/6G applications.