Synthesis and properties of polyimides containing fluorinated aromatic amines

Abstract

Polyimides utilizing fluorinated aromatic amines and some unfluorinated counterparts were synthesized. In cases where fluorinated amines were used two stages of polymerization were necessary to obtain high molecular weight; a standard solution polycondensation followed by a high temperature solid state chain extension. The condensation reaction resulted in low molecular weight oligomers leaving amine and anhydride end groups available for further reaction. Dried films of the first stage material were heated to high temperature to affect linear chain extension and imidization. Two materials which formed relatively tough polymers were 2,2′hexafluoro isopropylidine bis-phthalic anhydride-tetrafluorophenylene diamine (HFDA-TFPDA), and 2,2′hexafluoro isopropylidine bis-phthalic anhydride-octaflurobenzidine (HFDA-OFB). It appears that chain flexibility is essential for the synthesis of tough polymers utilizing these rigid amines of low reactivity. Polymers from the anhydrides PMDA, BPDA, and BTDA, which are less flexible than HFDA were extremely brittle and were unable to form continuous films. A comparison of a pair of polymers utilizing a fluorinated and unfluorinated amine, HFDA-TFPDA and HFDA-PDA, illustrate some interesting property differences that can be attributed to the substitution of fluorine for hydrogen in the ring. HFDA-TFPDA has a dry dielectric constant of 2.75 at 100 khz, a Tg of 380 °C, and a Beta transition at around 230°C (1hz). The 6FDA-PDA has a dielectric constant of 2.90, a Tg of 360°C, and a B transition of around 170°C.