Phase structure, morphology and phase boundary diagram in an aromatic polyimide (BPDA–PFMB)/m‐cresol system

An organo-soluble aromatic polyimide has been synthesized from 3,3', 4,4'-bis(phenyltetracarboxylic) dianhydride (BPDA) and 2,2'-bis (trifluoromethyl)-4,4'-diaminobiphenyl (PFMB) via a one-step polymerization in m-cresol. The phase boundary diagram for this system has been established by differential scanning calorimetry, polarized light microscopy (PLM) and wide angle X-ray diffraction (WAXD) experiments. A crystallosolvate form I has been found over the entire concentration region at low temperatures. When the temperature is increased, an isotropic phase has been observed below concentrations of about 40%. In the relatively high concentration region between 45 and 95%, a transition from the crystallosolvate form I to a crystallosolvate form II has been observed. Form II exhibits a different WAXD pattern. In a narrow concentration region between the isotropic and the crystallosolvate form II (40-45%), a biphase behavior has been found. The birefringence of this region may be an indication of a liquid crystalline phase. The BPDA-PFMB polymer crystal has been found in the very high concentration region (>95%) for temperatures >250°C. The morphologies of these phases have been investigated via PLM and transmission electron microscopy. Above the gel/sol transition temperature, the form I shows negative birefringent spherulites consisting of thin lamellae. The form II exhibits a tendency of positive birefringent spherulites. Possible mechanisms of the formation of the metastable phase morphology and their associations with the mechanical gel/sol transition are also discussed.