DETERMINATION OF THE AXIAL CORRELATION LENGTHS AND PARACRYSTALLINE DISTORTION FOR AROMATIC COPOLYIMIDES OF RANDOM MONOMER SEQUENCE

X-ray methods have been used to investigate the structure of aromatic copolyimide fibers prepared from 3,3',4,4'-biphenyltetracarboxylic dia nhydride (BPDA), o-tolidine (OTOL), and p-phenylenediamine (PPD). The X-ray fiber diagrams of these copolymers contain nonperiodic layer lin es that are indicative of a random comonomer sequence. The meridional peak positions are reproduced accurately in the predicted scattering f or an infinite chain with random sequence and highly extended chain co nformation. However, there is a less than adequate match between the o bserved and calculated peak profiles, which are predicted to be very m uch sharper than those observed. It is shown that the latter match is improved by consideration of chains of finite length with significant distortions from linearity. The results favor a sinuous chain conforma tion consisting of highly but not completely extended segments separat ed by less ordered regions or discontinuities. From analysis of the pe ak profiles, we have derived the correlation length in the chain axis direction, analogous to the axial crystallite size for homopolymers. T his correlation length is 180 Angstrom for homopoly(BPDA-OTOL) but inc reases steadily with the introduction of PPD comonomer, reaching appro ximately 1000 Angstrom for the 60/40 BPDA-OTOL/BPDA-PPD copolymer at a x 12 draw ratio. The results are compared to those for Kevlar 149 pol y(p-phenylene terephthalamide) fibers, for which the observed axial cr ystallite width is 395 Angstrom, much less than for the copolyimides. The correlation length and sinuosity are independent of OTOL/PPD ratio at constant draw ratio. Hence the increasing linearity obtained at hi gher PPD content and higher draw ratio is simply due to the presence o f the more flexible BPDA-PPD units.