Organosoluble, Segmented Rigid-Rod Polyimide Films. 1. Structure Formation

A segmented rigid-rod polyimide has been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in refluxing m-cresol. The polymer is soluble in hot phenolic solvents and can be solution cast into thin films. When the films are highly oriented, the polymer crystallizes in a monoclinic lattice with a = 15.40 Å, b = 9.918 Å, c = 20.21 Å, and γ = 56.2°. When unoriented films are annealed at 425 and 450 °C, an in-plane orientation process of the crystals and chain molecules takes place, so that the crystals’ c axes end up parallel to the films’ surfaces. This is followed by further structure development where the apparent crystal size increases with annealing time and temperature. It is postulated that the long-range molecular motion required for orientation is facilitated by residual solvent in the film. Avrami exponents for the structure development support a molecular-motion-controlled process.