High Performance Polyimide Films with Low <i>D</i><sub>k</sub> and Low <i>D</i><sub>f</sub> at 10 GHz via Cross-Linking of Bulky Pendant Groups
Shiying Qi, Shuo Han, Jianhao He, Jianchao Jiang, Chi Zhang, Jun Wang, Fei Xiao, Yuanrong Cheng
Abstract
Herein, an effective design strategy is utilized to simultaneously reduce two difficult-to-reconcile indexes in polyimide (PI): the dielectric constant ( D k ) and dielectric loss ( D f ). It is realized by the synthesis of cross-linked PIs using a bulky and cross-linkable diamine, (4,4′-(4-(4-(2,3,5,6-tetrafluoro-4-vinylphenoxy)phenyl)pyridine-2,6-diyl)dianiline) (TFVPPDA), featuring bulky triphenylpyridine and cross-linkable tetrafluorostyrol pendant groups. And copolymerized with 2,2′-bis(trifluoromethyl)benzidine (TFDB) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to synthesize cross-linkable PIs ( co -FPIs: co -FPI-10, co -FPI-20, and co -FPI-30) with varying TFVPPDA content. Then, cross-linked PIs ( co -FPIs-CL: co -FPI-10-CL, co -FPI-20-CL, and co -FPI-30-CL) were obtained by thermal cross-linking reactions. The cross-linked PIs with cross-linked networks possess more free volume compared with the corresponding uncured PIs, thus further reducing the D k . Additionally, cross-linked networks restrict the deflection of polar imide groups in the polymer chain, leading to a relatively low D f . More importantly, this design strategy effectively retains the excellent thermal, mechanical, and low moisture absorption properties of the PIs. Notably, co -FPI-30-CL has a low D k (2.50) and D f (0.0060) at a high frequency of 10 GHz, a glass transition temperature ( T g ) of 384 °C, a 5 wt % decomposition temperature ( T d5% ) of 566 °C, a low coefficient of thermal expansion ( CTE ) of 41.5 ppm/°C, a high tensile strength ( σ max ) of 123.3 MPa, and a low moisture absorption rate ( W a ) of 0.11%.