Organo-soluble segmented rigid-rod polyimide films. Part 5.—Effect of orientation

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Abstract
A series of semi-rigid aromatic polyimides have been synthesized via a one-step polymerization in which the poly(amic acid) precursors were not isolated. The polyimides were synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in refluxing m-cresol. Uniaxially oriented films were utilized in order to investigate the chain orientation in the ordered state. Wide-angle X-ray diffraction (WAXD) patterns showed an increase in the crystal orientation and the crystallinity for oriented CPI(100/0) films with increasing draw ratio. Dichroic ratios obtained under polarized Fourier transform infrared spectroscopy (FTIR) represented overall orientation factors attributed to both ordered and non-ordered regions, and they were a function of the incident angle and draw ratio. This indicates that the conformational arrangement and chain packing changed during deformation. The orientation along the molecular chain axis and the lateral orientation of the imide planes became increasingly enhanced with increasing draw ratio, an indication of planar alignment of the imide planes as well as uniaxial orientation of the molecular axis. The surface morphology of unoriented and uniaxially oriented films was also studied via transmission electron microscopy (TEM). The lateral crystal dimension of the ordered regions observed from TEM corresponded well to the apparent crystallite sizes of the (310) crystalline plane in BPDA–PFMB crystals obtained from WAXD experiments.