Diamine architecture effects on glass transitions, relaxation processes and other material properties in organo-soluble aromatic polyimide films

A series of twelve aromatic diamines, 4,4'-diamino-2,2'-disubstitutedbiphenyls has been designed and synthesized. These diamines were reacted with 2,2'-bis(3,3-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) to form polyimides via a one-step polycondensation method. All of the resulting polyimides could be dissolved in common organic solvents and exhibited excellent film forming ability. At the same time, their inherent high thermal and thermo-oxidative stability of these polyimides was retained in the films. Because of the incorporation of disubstituted groups at the 2- and 2'-positions of these biphenyl diamines, their crystallinity was suppressed to the level that they were in complete amorphous state. Further, the conjugation of the phenylene and imide groups in these polyimide films was interrupted, leading to clear blue shifts during light transmission. As this series of polyimides possessed the same backbone, the chain rigidity and linearity changed very little throughout the series. However, the molecular packing was affected by the introduction of different disubstituted pendant groups. Each polyimide film exhibited an ct relaxation process related to the glass transition. This relaxation changed significantly with the size and the shape of the disubstituted pendant groups. In addition to this process, each of these polyimide films displayed a sub-glass transition, the beta relaxation process, which was initiated by motion of the 4,4'-diamino-2,2'-disubstituted biphenyls. This study provided an opportunity to investigate how disubstituted pendant groups affected the alpha and beta relaxation behaviors of these polyimides. With an increase of the sizes and the shape anisotropy of the disubstituted pendant groups at the 2- and 2'-position, the nature of the motion regarding to the beta relaxation was found to evolve from a non-cooperative process to a cooperative one, while the glass transition temperature (the ct relaxation temperature) correspondingly decreased. (C) 1999 Elsevier Science Ltd. All rights reserved.