Characterization of intrinsic low-polyimide films studied by positron annihilation

Four kinds of polyimide (PI) films were prepared by polycondensation reaction using the pyromellitic dianhydride (PMDA) and 4, 4′-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA)as the dianhydride monomer, the p-phenylenediamine (PDA), 4-4′-diaminodiphenyl ether (ODA) and 4,4′-(Hexafluoroisopropylidene) bis(p-phenyleneoxy) dianiline (HFBDA) as the diamine monomer respectively. The incorporation of trifluoromethyl groups can effectively inhibit the formation of intermolecular and intramolecular charge-transfer complex (CTC), improve the energy gap and increase the free volume in PI, which leads to an improvement in the optical and dielectric properties in different degree for three fluorinated PI films compared with the Kapton film. The maximum transmittance in visible region reaches 90.5% and the dielectric constant at 104Hz is close to 2.0 for PI6FDA-HFPBDA film. The breakdown strength weakens because of the increase of free volume. The correlation between the dielectric and breakdown characteristics and their free volume has been explained with the help of positron lifetime spectra.