Phase transitions, structure formation, and morphology in an aromatic polyimide (BPDA-PFMB)/m-cresol system

Aromatic polyimides show excellent mechanical properties, thermal stability, and chemical resistance. Due to their unique properties, they are being widely used for industrial high-temperature applications as films, fibers, coatings, and matrix materials in composites. However, similar to other aromatic rigid-rod polymers, they cannot be processed through their melts because of their high melting temperatures which often exceed the decomposition temperature. Most aromatic rigid-rod polymers are processed only through the dissolved state. However, during processing, especially in coagulation and/or a cooling stage, a liquid to solid phase transition is induced. The structure initially formed via this phase transition is therefore critical to the final structure in the solid state. A better understanding of phase behavior and structures will permit better control of the final structure and properties of polymeric materials.; An organo-soluble, segmented rigid-rod polyimide has been synthesized from 3,3{dollar}spprime{dollar},4,4{dollar}spprime{dollar}-bis(phenyltetracarboxylic) dianhydride (BPDA) and 2,2{dollar}spprime{dollar}-bis(trifluoromethyl)-4,4{dollar}spprime {dollar}-diaminobiphenyl (PFMB). The phase stability diagram in the aromatic polyimide (BPDA-PFMB)/m-cresol system was established by differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide angle X-ray diffraction (WAXD) heating experiments. A crystallosolvate Form I which has a lower melting temperature was found in a relatively low concentration region at low temperatures. When the temperature increases only the isotropic phase is observed in the low concentration region ({dollar}<{dollar}35%). On the other hand, in the relatively high concentration region a transition from the crystallosolvate Form I to the crystallosolvate Form II, which has a higher transition temperature and a different crystal structure, is observed. In a narrow concentration region between the isotropic and the crystallosolvate Form II, a liquid crystalline phase has also been observed. At very high concentrations and temperatures a BPDA-PFMB polymer crystal is found.; The morphologies of the phases found in this system, i.e., crystallosolvate Form I, crystallosolvate Form II, and polymer crystal, have also been investigated by PLM and transmission electron microscopy (TEM). The crystallosolvate Form I shows a negative spherulite consisting of layered thin lamellae. On the other hand, the crystallosolvate Form II exhibits a positive spherulite which consists of fibrous crystals. Finally, a rod-like crystal morphology is observed for the polymer crystal.