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'-disubstitutedbiphen yls has been designed and synthesized. These diamines were reacted with 2,2 '-bis(3,3-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) to form pol yimides via a one-step polycondensation method. All of the resulting polyim ides could be dissolved in common organic solvents and exhibited excellent film forming ability. At the same time, their inherent high thermal and the rmo-oxidative stability of these polyimides was retained in the films. Beca use of the incorporation of disubstituted groups at the 2- and 2'-positions of these biphenyl diamines, their crystallinity was suppressed to the leve l that they were in complete amorphous state. Further, the conjugation of t he phenylene and imide groups in these polyimide films was interrupted, lea ding to clear blue shifts during light transmission. As this series of poly imides possessed the same backbone, the chain rigidity and linearity change d very little throughout the series. However, the molecular packing was aff ected by the introduction of different disubstituted pendant groups. Each p olyimide film exhibited an ct relaxation process related to the glass trans ition. This relaxation changed significantly with the size and the shape of the disubstituted pendant groups. In addition to this process, each of the se polyimide films displayed a sub-glass transition, the beta relaxation pr ocess, which was initiated by motion of the 4,4'-diamino-2,2'-disubstituted biphenyls. This study provided an opportunity to investigate how disubstit uted pendant groups affected the alpha and beta relaxation behaviors of the se polyimides. With an increase of the sizes and the shape anisotropy of th e 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-coo perative process to a cooperative one, while the glass transition temperatu re (the ct relaxation temperature) correspondingly decreased. (C) 1999 Else vier Science Ltd. All rights reserved.