Structure-property relationship of low dielectric constant polyimide fibers containing fluorine groups

A series of copolyimide (co-PI) fibers with low dielectric constants and excellent comprehensive properties have been successfully prepared via a two-step wet-spinning method by incorporating 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) into rigid polymer backbones of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (p-PDA). The dielectric constant of PI fibers were substantially reduced from 3.0 to 2.48 at the frequency of 10 GHz after the introduction of trifluoromethyl (-CF3) groups. Meanwhile, the prepared PI fibers exhibited high thermal stability as well as excellent mechanical properties. The co-PI fibers possessed the optimum tensile strength of 1.4 GPa and modulus of 83.3 GPa at the p-PDA/TFMB molar ratio of 7/3. The 5% weight loss temperature in nitrogen ranged from 523 to 558 degrees C, and the glass transition temperature decreased from 372 degrees C to 295 degrees C with increasing TFMB contents. Both experimental and molecular simulations method were adopted to discuss the influence of fluorine group on the aggregation structure and properties of PI fibers, suggesting that the improved free volume were the crucial factor responsible for low dielectric constant. The molecular ordered degree and orientation degree of co-PI fibers decreased firstly and further increased with increasing TMFB content, which is due to the combined effects of large free volume of trifluoromethyl and polymer regularity variation caused by copolymerization.