Sterically Hindered Ether Diamines Enable PFAS-Free Polyimides with Ultralow Dielectric Loss

High-speed electronic interconnects require polymer dielectrics that combine ultralow dissipation factor ( D f ) with excellent thermal and mechanical robustness. In our previous work, the polyimide derived from 3,4 ′ -diaminodiphenyl ether (3,4-ODA) and 1,4-phenylene bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) (TAHQ) showed a lower dielectric loss ( D f = 0.0017 at 10 GHz) than the corresponding 4,4-ODA/TAHQ polyimide ( D f = 0.0023), suggesting that diamine geometry strongly influences dielectric performance. To further restrict rotational freedom around the aryl–O–aryl linkage and promote more extended chain conformations, a sterically hindered analogue of 3,4-ODA, 2-methyl-3 ′ ,4-oxydianiline (3,4-MODA), was designed and synthesized via a two-step route. DFT torsional scans of representative TAHQ/diamine/TAHQ trimer models supported this design concept by confirming the increased rotational barrier around the aryl–O–aryl linkage in the 3,4-MODA-based structure. 3,4-MODA was then incorporated into TAHQ-based polyimides by copolymerization with either 3,4-ODA or 4,4-ODA, affording the 34PI- X and 44PI- X series, where X is the mole fraction of 3,4-MODA in the diamine feed. The resulting films exhibit excellent thermal stability ( T g > 300 ° C, T d5% up to ∼500 ° C) and good mechanical properties. Moderate incorporation of 3,4-MODA reduced the in-plane coefficient of thermal expansion and increased birefringence, while wide-angle X-ray diffraction (WAXD) patterns suggested enhanced molecular ordering. These changes were accompanied by improved dielectric performance. In the 34PI- X series, compositions containing 25–37.5 mol% 3,4-MODA achieved an ultralow D f of 0.0011 at 10 GHz. In the 44PI- X series, D f decreased from 0.0023 to 0.0013 at 50 mol% 3,4-MODA. These results suggest that steric modulation near the ether linkage is an effective strategy for developing PFAS-free polyimides with ultralow dielectric loss for high-frequency flexible printed circuit boards (FPCBs).