X-ray analysis and molecular modeling of the structure of aromatic polyimide fibers

 
 
 
 

X-ray analysis and molecular modeling of the structure of aromatic polyimide fibers

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Title: X-ray analysis and molecular modeling of the structure of aromatic polyimide fibers
Author: Wu, Tzong-Ming
Description: X-ray diffraction techniques have been used to investigate the structures of three groups of aromatic copolyimides. The first of these is synthesized by the reaction of 2,2′-bis (trifluoromethyl)4,4′-diaminobiphenyl (PFMB) with a mixture of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA); the second from BPDA and a mixture of o-tolidine (OTOL), p-phenylene diaznine (PPD); and the third from BPDA and a mixture of PPD and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). The X-ray fiber diagram of 70/30 copoly(BPDA-PFMB/PMDA-PFMB) is very similar to that of homopoly(BPDA-PFMB), except that it is more diffuse. It appears that the structure is highly blocky, and the main effect of introduction of comonomer is to reduce the crystallinity of the poly(BPDA-PFMB) blocks. In contrast, the X-ray data for the copoly(BPDA-OTOL/BPDA-PPD) and copoly(BPDA-DMB /BPDA-PPD) are non-periodic along the fiber direction, and the layer line positions are reproduced by models consisting of parallel arrays of extended chains of completely random sequence. Interestingly, the correlation lengths for the extended chain of copoly(BPDA-OTOL/BPDA-PPD) at three different comonomer ratios are larger than that determined for homop oly(BPDA-OTOL). It is suggested that this is due to the fact that "kink" distortions are less disruptive in copoly(BPDA-OTOL/BPDA-PPD). That is probably the reason that copoly(BPDA-OTOL/BPDA-PPD) has the better drawability and mechanical properties than those for homopolymer. A new crystal unit cell for homopoly(BPDA-OTOL) has been determined: the cell is triclinic with dimensions a = 17.49 A, b = 19.94 A, c = 20.43 A, α = 40.7°, β = 74.5° and γ = 79.4°. We have observed that some of the non-periodic meridional maxima of copoly(BPDA-OTOL/BPDA-PPD) sharpen with increasing draw ratio and also shift to slightly lower scattering angles (i.e. higher Bragg d-spacings). It is shown that these observations are to be understood primarily in terms of an increase in the extended chain correlation length for the random sequences. This is analogous to an increase in crystallite size for homopolymers, which leads to a sharpening of the Bragg reflections, and can also result in small changes in d-spacing. In chapter three we showed that part of the line broadening for the copolymer is due to the random microstructure, but that there is additional broadening due to the finite correlation length for the extended chain, which has allowed us to estimate the latter dimension for different comonomer ratios and draw ratios. The results point to a conformation consisting of highly extended rod-like segments separated by less ordered regions or "discontinuities". The length of the rod-like segments increases with PPD content, and it is suggested that the absence of the methyl substituent groups lowers the barrier for torsional rotation about the N-phenyl bond, facilitating adoption of a more linear conformation during the drawing process. (Abstract shortened by UMI.
Permanent Link: http://rave.ohiolink.edu/etdc/view?acc_num=case1062601845
http://hdl.handle.net/2374.OX/16968
Date: 1995

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