Polyimide Films with Liquid-Crystal-Like Structures: Toward Ultralow Dielectric Loss at High Frequencies

Abstract

Polyimide films with low dielectric loss (D_f) at high frequencies are urgently needed owing to the rapid development of high-frequency communication technologies. Herein, various polyimide films with rigid mesogenic units are prepared to address this issue. Improving molecular flexibility has been found to promote crystallization during thermal imidization and, consequently, the formation of liquid-crystal-like polyimide (LCPI) films. The optimal LCPI film exhibits an ultralow D_f value of 0.00178 at 10 GHz owing to the limited intermolecular friction. Furthermore, this LCPI film absorbs significantly less water (0.59%), which endows it with favorable durability of low D_f value in a humid environment. The D_f value of the LCPI film at 10 GHz is only 0.00288 after 24 h of exposure to 60% relative humidity. In addition, the LCPI film exhibits a glass transition temperature, coefficient of thermal expansion, and tensile strength of 371 °C, 21.8 ppm K⁻¹, and 240 MPa respectively, and is therefore a suitable low-dielectric-material candidate.