High PerformancePolyimide Films with Low <i>D</i>_k and Low <i>D</i>_f at10 GHz via Cross-Linking of Bulky Pendant Groups

Herein, an effective design strategy is utilized to simultaneously reduce two difficult-to-reconcile indexes in polyimide (PI): the dielectric constant (<i>D</i>_k) and dielectric loss (<i>D</i>_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 (<i>co</i>-FPIs: <i>co</i>-FPI-10, <i>co</i>-FPI-20, and <i>co</i>-FPI-30) with varying TFVPPDA content. Then, cross-linked PIs (<i>co</i>-FPIs-CL: <i>co</i>-FPI-10-CL, <i>co</i>-FPI-20-CL, and <i>co</i>-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 <i>D</i>_k. Additionally, cross-linked networks restrict the deflection of polar imide groups in the polymer chain, leading to a relatively low <i>D</i>_f. More importantly, this design strategy effectively retains the excellent thermal, mechanical, and low moisture absorption properties of the PIs. Notably, <i>co</i>-FPI-30-CL has a low <i>D</i>_k (2.50) and <i>D</i>_f (0.0060) at a high frequency of 10 GHz, a glass transition temperature (<i>T</i>_g) of 384 °C, a 5 wt % decomposition temperature (<i>T</i>_d5%) of 566 °C, a low coefficient of thermal expansion (<i>CTE</i>) of 41.5 ppm/°C, a high tensile strength (<i>σ</i>_max) of 123.3 MPa, and a low moisture absorption rate (<i>W</i>_a) of 0.11%.