Concept of symmetric tert-butyl pendant groups toward record-low dissipation factors of polyimides at high frequency

Abstract

Materials with low dielectric constant (D_k) and low dissipation factor (D_f) values at a high-frequency are highly demanded to achieve low signal transmission loss. D_f shows significantly important influence on signal propagation loss rate in comparison with D_k. However, the structure-D_f relationships are still poorly understood. We discuss the influence on D_f by polymer chain packing and polar group content. Two series of polyimides (PIs) were synthesized from the two diamines with symmetric or asymmetric tert-butyl pendant groups. The 3 series with symmetric tert-butyl pendant groups revealed a significantly decreased dissipation factor in comparison with that of the corresponding analogues of the 4 series with asymmetric tert-butyl pendant groups. It is evident that the introduction of symmetric tert-butyl pendant groups is demonstrated an effective strategy to restrict polymer chain motion, leading to low D_f. For the PIs with good packing ability like 3 series, the predominant influence on their D_f is polymer chain packing ability to effectively restrict polymer chain motion. For the PIs without good packing ability like 4 series, the predominant influence on their D_f is polar group content. Herein, the concept of symmetric tert-butyl pendant groups and an ester group by designing new diamine monomer (2) was successfully incorporated into copolyimide 6, and demonstrated the remarkable low D_f (0.0036 at 10 GHz). The record-low D_f is ascribed to the strong intermolecular interaction and good packing ability to effectively restrict molecular motion. PI 6 demonstrates well-balanced properties, including very high Tg, low D_f, and excellent flame retardancy, making it a promising candidate for new dielectric substrate materials in the next generation of 5G-compatible high-performance flexible printed circuit boards (FPCBs).