Synthesis and properties of modified thermoplastic polyimide films with good dielectric properties at high frequency and enhanced thermal stability via incorporation of rigid ester and biphenyl structural units

A series of ester- and fluoro-containing thermoplastic polyimide (PI) films, PEsI-1 similar to PEsI-4 have been prepared by the two-step thermal imidization procedure via the soluble poly(amic acid) (PEsAA) precursors from the copolymerization of four monomers, including the dianhydrides of bisphenol A dibenzoate-3,3 ',4,4 '-tetracarboxylic acid dianhydride (TMBPA) and 3,3 ', 4,4 '-biphenyl tetracarboxylic acid dianhydride (BPDA) and the diamine monomers of 4-aminophenyl-4 '-aminobenzoate (APAB) and 2,2 '-bis(trifluoromethyl)-4,4 '-diamino biphenyl (TFMB) in the polar aprotic solvent of N-methyl-2-pyrrolidinone (NMP). The PI films obtained from the high-temperature dehydration of the PEsAA solutions exhibited the good thermal stabilities with the glass transition temperatures (T-g) over 269 degrees C, the 5% weight loss temperatures (T-5%) higher than 480 degrees C, and the linear coefficients of thermal expansion (CTE) values in the range of 17.7 x 10(-6)/K-33.6 x 10(-6)/K in the temperature range of 50-250 degrees C. Meanwhile, the PI films showed good thermoplasticity in the dynamic mechanical analysis (DMA) tests, in which the storage modulus of the PI films dropped sharply in the temperature range of glass transition of the polymers. At last, the PI films exhibited the good dielectric properties with the breakdown voltages (V-b) over 5200 V, the dielectric strength (D-s) higher than 210 V/mu m, the dielectric constants (D-k) in the range of 3.26 similar to 3.32 at the frequency of 10 GHz, and the dielectric dissipation factors (D-f) of 0.0038 similar to 0.0047. Compared with the commonly used PEsI-ref film for high-frequency flexible copper clad laminates (FCCL), the currently developed PI films exhibited the obviously increased T-g values, decreased CTE values, and comparable dielectric properties.