Design and synthesis of liquid crystal copolyesters with high-frequency low dielectric loss and inherent flame retardancy

Abstract

Ultra-low dielectric loss (Df) and low dielectric constant (Dk) materials are urgently required in high-speed and large-capacity transmission, in which the wholly aromatic liquid crystal polymer (LCP) has gained attention due to its excellent dielectric properties. However, the relationship between molecular structure and dielectric properties is still not clear. In this study, two copolyesters containing phenyl or naphthyl structures are synthesized, as well as the effects of benzene and naphthalene mesogens on dielectric properties are investigated. The synthesized copolyesters containing naphthalene structure have good comprehensive properties with high thermal stability (T5% = 479 °C and Tg = 195–216 °C), inherent flame retardance (LOI = 33.0–35.0 and UL-94 V-0 level at 0.8 mm), low Dk (2.9–3.0@10 GHz) and low Df (0.0027–0.0047@10 GHz). Naphthalene mesogen can reduce the dielectric loss more significantly than benzene at high frequency by reducing the density and mobility of polarizable groups, which leads to the effectively limited dipole polarization in copolyesters. Consequently, we proposed a new strategy for designing low Dk and low Df materials.