High Performance Polyimide Films with Low <i>D</i>_k and Low <i>D</i>_f at 10 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 (Dk) and dielectric loss (Df). 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 Dk. Additionally, cross-linked networks restrict the deflection of polar imide groups in the polymer chain, leading to a relatively low Df. 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 Dk (2.50) and Df (0.0060) at a high frequency of 10 GHz, a glass transition temperature (Tg) of 384 °C, a 5 wt % decomposition temperature (Td5%) of 566 °C, a low coefficient of thermal expansion (CTE) of 41.5 ppm/°C, a high tensile strength (σmax) of 123.3 MPa, and a low moisture absorption rate (Wa) of 0.11%.