Structure of PAN Fibers

The —CN group has a large dipole moment and can be attractive or repulsive. In the planar zig-zag form, the nitrile groups are parallel and maximum repulsion is exerted. 

For PAN to be formed into a fiber, it must possess some degree of crystallinity, despite the addition of a comonomer decreasing the crystallinity. Hence Fischer and Fakirov [158] and Hinrichsen [159] have suggested that it is highly likely that a limited two-phase morphology exists for PAN fibers, with lamellar crystalline units being separated by less ordered material of chain-folded lamellae. The unordered domains have been suggested to 

Lewis acid compounds SnCU, SnBr4, PbCU, TiCU, ZnCIa, BF3 142-143 

Source Reprinted with permission from Ermolenko IN, Lyubliner IP, Gulko NV, Chemically modified Carbon Fibres and their Applications, (translated by EPTitovets) VCH, Germany, Chapters, 153,1990. Copyright 1990, John Wiley Sons Ltd. 

Although many attempts have been made to produce a more crystalhne polymer with improved fiber tensile properties, success has been limited and, invariably, the teehniques used would not be commercially viable. Similarly, attempts by Allen, Ward and Bashir [162] to produce a high modulus PAN fiber (55GPa) by gel spinning, were not sueeessful and it was concluded that an ultrahigh modulus PAN fiber eaimot be made by any teehnique, due to the intrinsic chain properties, where the strong intramolecular nitrile repulsions cause the PAN to adopt a semi-extended rod-like conformation, thereby not allowing the chain to unravel from its semi-extended form. 

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An X-ray study of the structure of PAN fibers (74) indicated the following sequence structures (Figure 4). These structures suggest that, in isotactic sequences, the nitrile groups are not in a position to undergo the desired conversion. The syndiotactic chain is, however, poised to convert to the ladder structure. 

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