Ultra-low dielectric constant polyimides: Combined efforts of fluorination and micro-branched crosslink structure

In the previous work, a micro-branched crosslink structure was constructed to molecular chain in the polyimide (PI) film of the 3,3' 4,4'-biphenyltetracarboxylic dianhydride/1,4-phenylenediamine (BPDA/PDA) system by introducing a trifunctional monomer 1,3,5-tri(4-aminophenoxy) benzene (TAPOB). It was found that the micro-branched structure is instrumental in reducing the dielectric constant via increasing the free fraction volume (FFV) while the crosslink can lower the coefficient of the thermal expansion (CTE) through confining the chain movement. This helps to break the design contradiction between low dielectric and CTE, thereby realizing the synchronous optimization of the two. To further reduce the dielectric constant to meet the application requirements of modified PI (MPI) in 5G equipment, PI films were designed and prepared using pyromellitic anhydride (PMDA), 4,4'-(hexafluoroisopropyl) phthalic anhydride (6FDA), 4,4'-diaminobiphenyl ether (ODA), and 2,2'-bis(trifluoromethyl)-4,4'-diaminobipheyl (TFMB) as the monomers while TAPOB as the crosslinking agent, which involved four systems of PMDA/ODA/TAPOB, PMDA/TFMB/TAPOB, 6FDA/ODA/TAPOB, and 6FDA/TFMB/TAPOB. The influence of molecular chain stiffness and flexibility on the micro-branched crosslink structure was studied by means of experiment and molecular dynamic simulation, and PI films with lowered dielectric constant, which are expectably applicable in the electronics industry, were obtained.