Structure of PAn

PAn s formed by both the chemical and electrochemical processes have been extensively studied to establish structure-property relationships. In this section, the structural studies of PAn are reviewed the influence of structure on properties is considered in Chapter 5. The description of PAn structure is complicated by its complexity. As described earlier, PAn can exist in six different forms (the salt or base forms of leucoemeraldine, emeraldine, and pemigraniline). In addition, the proton-ated forms of PAn also have counteranions intimately associated with the positively charged PAn chains. Finally, it has also been observed that PAn s may contain considerable amounts of solvent molecules. 

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Because of the repulsion of the cyanide groups the polymer backbone assumes a rod-like conformation. The fibers derive their basic properties from this stiff structure of PAN where the nitrile groups are randomly distributed about the backbone rod. Because of strong bonding between the chains, they tend to form bundles. Most acrylic fibers actually contain small amounts of other monomers, such as methyl acrylate and methyl methacrylate. As they are difficult to dye, small amounts of ionic monomers, such as sodium styrene sulfonate, are often added to improve their dyeability. Other monomers are also employed to improve dyeability. These include small amounts (about 4%) of more hydrophilic monomers, such as -vinyl-2-pyrrolidone (Equation 6.69), methacrylic add, or 2-vinylpyridine (Equation 6.70). 

Figure 8.8 (c) three-dimensional structure of PAN-based carbon fiber (after Bennett et al, 1983). 

Ko, T.H., Chiranairadul, P., Lu, C.K., et al. (1992). The effects of activation by carbon-dioxide on the mechanical-properties and structure of pan-based activated carbon-fiben. Carbon, 30, 647-55. 

More recent studies have shown the consistency of the crystal structure of PAn/ CSA to be variable and highly dependent on the conditions of film preparation. Sara-vanan and coworkers197 have reported a crystallinity as high as 56% in CSA-doped PAn. Djurado and coworkers198 have speculated that the influence of chain branching during polymerization may play a role in the nature of the crystal structure formed. 

Broadhead and Tresco studied the effects of fabrication conditions on the structures and performances of membranes formed from poly(acrylonitrile-vinylchloride) (PAN-PVC) by using the phase inversion process [85]. They reported the relationship of the fine-surface structure of PAN-PVC membranes to the membrane performance and membrane fabrication method. The fine-surface structure of nodular elements and the size of these elements could be altered by changing the precipitation conditions. Membranes were prepared at 22 on 55 mm diameter polished silicon wafers by spinning at 1500 rpm for 20 s with a spin coater [86]. The film was immediately precipitated in one of the four different precipitation media. The first three media consisted of deionized water at 4,22, and 54 °C. These membranes were referred to as Type 1 , Type 2 , and Type 3 , respectively. The fourth medium was a 50/50 mixture of deionized water and N,iV-dimethylformamide (DMF) at 54 °C and coded as Type 4 . Figure 4.53 shows the histograms of the nodule size distributions observed at the skinned surface of the membranes made under four different precipitation conditions. The sizes of these nodular elements became smaller and more uniform with milder precipitation conditions, which supports the theory that nodules are formed through spinodal decomposition under these conditions. In addition, the size of these nodules could be related to water permeability. Hence, water transport occurred through the interstitial spaces where the pores could be situated. 

Recent developments of solid-state NMR techniques have allowed a direct and detailed analysis of the local structure of PAN in the solid state using homonuclear two-dimensional (2D) double quantum (DQ) (DOQSY) [187, 188] and H- C 2D heteronuclear multiple quantum (MQ) correlation (HMQC) [189] solid-state NMR spectroscopy. This method was applied to... 

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. 

Brandrup and Peebles (1968) [165] studyied model compounds which contained certain aspects of the structure of PAN and found that the initial attack of O2 on the polymeric chain was at the methylene hydrogens. The hydroperoxide was formed, whieh subsequently broke down to yield a j6-ketonitrile, which initiated the color forming polymerization of nitrile groups. The conjugated structure then reacted with atmospheric O2 to partially form a polynitrone (Scheme XII), the final chromophore. 

Watt W, Chemistry and physics of the conversion of polyacrylonitrile fibres into high modulus carbon fibres. Watt W and Perov BV eds., Vol, Strong Fibres, Elsevier, Amsterdam, 327-388,1985. Johnson W, The structure of PAN-based carbon fibres and relationship to physical properties. Watt W and Perov BV eds., Vol 1, Strong Fibres, Elsevier, Amsterdam, 389-444, 1985. 

Figure 12.7 Structure of carbon fiber, (a) A schematic iiiustralion of tree trunk or onion skin structure (ieft) and radial structure (right), (b) A typicai opticai micrograph of carbon fiber cross sections under polarized light in crossed nicols condition showing maltose cross patterns. Polarizer and analyzers are parallel to picture edges. Source Reprinted from Nyo H, Heckler AJ, Hoemschemeyer DL, Characterizing the structures of PAN based carbon fibers, 24 Nat Symposium, San Francisco, 179, 51-60, May 8-10.

Rgure 5. Proposed structure of PAN-based and carbon fibers showing layers undulating in and out of crystalline regions is the width and the turbostratic crystals and ii the length of the crystal. (Reprinted with permission from Ref. 27. Copyright 197( International Union of Crystallography). 

Similarly to carbon microfibers, it is profitable to increase the final graphitization temperature because carbon nanofibers become more graphitic and structurally ordered [164], which is reflected by improved tensile properties. The graphitic structure of PAN-derived carbon nanofibers can be seen in Figure 10.39 [165]. 

Structure. The structure of PAN-based carbon fibers is still conjectural to some degree. Yet, thanks to the recent advances in analytical techniques just mentioned, an accurate picture is beginning to emerge. 

Unlike the well-ordered parallel planes of pyrolytic graphite which closely match the structure of the graphite crystal, the structure of PAN-based carbon fibers is essentially turbostratic and is composed of small two-dimensional fibrils or ribbons. These are already present in the precursor and are preferentially aligned parallel to the axis of the fiber. The structure may also include lamellas (small, flat plates) and is probably a combination of both fibrils and lamellas. ... 

To summeirize, small ciystallite size, high interlayer spacing, and general structural disorder are the factors that contribute to the unique and stable turbostratic structure of PAN-based carbon fibers (which is likely to include both sp and sp hybrid bonds), and explain their inability to form a graphitic structure even after high-temperature heat-treatment (i.e., 3000°C). 

The two-phase concept for the structure of PAN is supported by the observation that the absorption curve (measured as a weight gain of PAN in an aqueous solution of iodine/ potassium iodide) shows several steps. This is interpreted as the penetration of the solution into domains of different order. X-ray diffraction patterns of PAN differ from those of other fibers [594,605]. In the patterns of PAN, distinct off-equatorial reflections are absent. This indicates a lack of order along the chain axis. Equatorial reflections are present, and they indicate high order parallel to the fiber axis. These observations support the view of the structure of PAN given above. The mechanical properties of PAN fibers can be improved considerably by spinning from metal ion containing solutions [606]. 

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