Polyimides. IV. Synthesis and characterization of fluorinated copolyimides

A series of new copolyimides were synthesized by the reaction of pyrometallic dianhydride, benzophenone tetracarboxylic acid dianhydride, and 2,2‐bis(3,4‐dicarboxyphenyl) hexafluoropropane (6FDA) with two diamines, 4,4″‐bis(aminophenoxy)‐3,3″‐trifluoromethyl terphenyl and 4,4′‐diaminodiphenyl ether, at a 50:50 ratio by a thermal imidization route. The complete spectral assignment of the copolymer derived from 6FDA was done with the help of distortionless enhancement of polarization transfer 90° and two‐dimensional heteronuclear (C, H) shift correlation spectroscopy. The various types of couplings in the copolymers were assigned with the help of two‐dimensional correlated spectroscopy (COSY) homonuclear correlation spectroscopy. A long‐range connectivity was also observed by a heteronuclear correlation spectroscopy via long‐range coupling experiment. The copolyimides derived from 6FDA were soluble in common organic solvents such as N , N ‐dimethyl formamide, N , N ‐dimethyl acetamide, dimethyl sulfoxide, N ‐methyl pyrrolidone, and chloroform. Water absorption values, after the samples were boiled in water for 72 h, were in the range 0.3–0.7%. These polyimides had low dielectric constants of 2.79–3.01 at 1 MHz, low refractive indices of 1.655–1.705 at 589.3 nm, and high optical transparencies (>90%). The onset temperature for 5% weight loss for these polyimides, as assessed by thermogravimetric analysis, was up to 551°C in air, indicating a very high thermal stability, and the polyimides showed glass‐transition temperatures as high as 316°C. Transparent thin films of these polyimides exhibited tensile strengths up to 93 MPa, moduli of elasticity up to 3.1 GPa, and elongations at break up to 24% depending on the exact repeat unit structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 235–246, 2004