Preparation of Polyimide Films with Ultralow Dielectric Loss at High Frequency by Reducing Intermolecular Friction

Due to the rapid development of high frequency and high rate communication technology, polyimide (PI) films with ultra-low dielectric loss (Df) D f ) at high frequency, excellent thermal and mechanical properties are urgently needed. A series of PI films with ultra-low D f were prepared by introducing different mesogen units. Among them, one novel PI film with biphenyl liquid-crystal-like units (BAHQ-TFMB) achieve a recorded low D f of 0.00169 (10 GHz) which surpasses previous studies on pure PI bulk films. Importantly, the high degree of collinearity of the friction work (Wf) W f ) and D f of PI films (Pearson's r = 0.992) was confirmed, which indicate that W f among PI macromolecules plays an important role in determining D f of PI films. To reveal underlying reason for reducing the W f and D f of PI films, the contribution of the degree of orientation and crystallinity of PI films with or without liquid-crystal-like structures were comparative studied. Through a two-dimensional orientation determinant, it is found that increasing the crystallinity of liquid-crystal-like PI films is the main contribution to reducing W f and corresponding D f of PI films while the orientation of PI films seems to have weak effect. Moreover, BAHQ-TFMB PI film exhibits excellent mechanical properties. The tensile strengths is 145.5 +/- 12.0 MPa and Young's moduli is 4.76 +/- 0.28 GPa.