Structure of polyacrylonitrile

Figure 11.5. Model of structure of polyacrylonitrile-based carbon fibre (after Johnson 1994).

Polyacrylonitrile, known commercially as Orion, is made by polymerizing acrylonitrile (see Figure 13-3) Orion is used to make fibers for carpeting and clothing. Draw the Lewis structure of polyacrylonitrile, showing at least three repeat units. 

Acrylonitrile polymerizes in the same way as ethylene. Notice that this polymer has the same structure as polyethylene, except that a CN group is attached to every second carbon atom, so the structure is a reasonable one. A line structure of polyacrylonitrile eliminates the clutter caused by the H atoms. A ball-and-stick model of the same polymer segment is included for comparison. 

Structure of Polyacrylonitrile (a) before pyrolysis (b) singly conjugated and (c) doubly conjugated ladder. 

Polymer Science Series A 35, No.3, March 1993, p.311-4 IR SPECTROSCOPIC STUDY OF THE EFFECT OF IONISING IRRADIATION ON THE STRUCTURE OF POLYACRYLONITRILE Platonova N V Klimenko I B Maiburov S P St.Petersburg,Textile Light Industry Institute... 

Sun, J. et al. (2006). Effects of Activation Time on the Properties and Structure of Polyacrylonitrile-Based Activated Carbon Hollow Fiber. 

Acrylonitrile, CH,CHCN, is used in the synthesis of acrylic fibers (polyacrylonitriles), such as Orion. Write the Lewis structure of acrylonitrile and describe the hybrid orbitals on each carbon atom. What are the approximate values of the bond angles ... 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

Metallisation of fibres is not only a physical process determined by absorption capacity of the fibres for the metal and diffusion capacity of the metal in the fibre structure, but also depends on chemical parameters such as chemical structure of the fibres, presence of functional groups, reactivity of the fibre and the metal, oxidation state of the metal and the presence, necessity and reactivity of supporting chemicals (e.g. reducing agent). Therefore, it was necessary first to study metallisation at different types of fibres in order to investigate which structure is most useful for further research. In this respect, viscose, cotton, natural silk and polyacrylonitrile fibres were investigated because of their different structure and properties and their availability in the New Independent States of the former Soviet Union (Uzbekistan, Kazakhstan, Kyrgyzstan). 

It is also possible to prepare all-carbon polymers of closely related structure. For example, pyrolysis of polyacrylonitrile, (-CH2CHCN-)X, first results in cyclization of some of the -CN side chains.61 Prolonged pyrolysis yields very pure graphitic material. It is very strong and has high thermal stability. In the form of fibers, it can be used for reinforcement in high-performance composites. Additional information on pyrolysis is given in Chapter 9. 

In spite of the fact that this classification makes things look quite complicated, it is a dangerously simplified one, and the real situation is much more involved. Therefore it is not surprising that many organic materials have been reported as catalysts for oxidation, but that the explanations given for their activity are often contradictory. Pyrolized polyacrylonitrile and polyphenylacetylene are reported to inhibit the oxidation of cumene 56X This may be connected with the reported quinonic structure of these polymers, which makes them active towards free radicals. 

Usami, T., Itih, T., Ohtani, H., Tsuge, S. (1990) Structural study of polyacrylonitrile libers during oxidative thermal degradation by pyrolysis-gas chromatography, solid state 13C Nuclear magnetic resonance and Fourier transform infrared spectroscopy, Macromolecules 23, 2460-2465. 

An alternative to using commercially available carbon for electrocatalyst carbon substrates is to build a specific carbon structure having controlled properties. Thus, carbons have been prepared by the controlled pyrolysis of polyacrylonitrile (PAN) and contain surface nitrogen groups that act as peroxide decomposing agents.62... 

A.K. Fitzche, A.R. Arevalo, M.D. Moore and C. O Hara, The surface structure and morphology of polyacrylonitrile membranes by atomic force microscopy. J. Membr. Sci., 81 (1993) 109. 

Polyacrylamides are also suitable for a variety of biomedical uses the structure of polyacrylamide is shown in Fig. IG, although the use of acrylamides in copolymers is much more common. Polyvinylpyrrolidinone has also found use as a biocompatible coating material (Fig. IH). Polyacrylonitrile, though not suitable in itself, can be hydrolyzed to form some hydrophilic polymers such as the Hypan (Hymedix Inc.) series of hydrogels. 

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