Pyrolysis of polyacrylonitrile

Figure 1.77 Pyrolysis of polyacrylonitrile (PAN) to form carbon fibers.

Polyaniline has been formed in the pores of Cu- or Fe-exchanged MCM-41 by adsorption of aniline vapour and subsequent oxidative polymerization (Figure 7.23), and these molecular wires demonstrate significant electronic conduction, although less than that of bulk polyaniline. Pyrolysis of polyacrylonitrile in the pores produces a graphitelike carbon chain, which exhibits microwave conductivity ten times that of bulk carbonized polyacrylonitrile. Such materials have potential for use in information processing as storage capacitors. 

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. 

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. 

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... 

M. Nielsen, P. Jurasek, J. Hayashi and E. Furimsky, Formation of toxic gases during pyrolysis of polyacrylonitrile and nylons, J. Anal Appl Pyrol, 35, 43-51 (1995). 

Recently, carbon molecular sieves have been fabricated in the form of planar membranes and hollow tubes by the pyrolysis of polyacrylonitrile in suitable forms (12-16). Very high separation selectivities have been reported with these materials. Their pore sizes are in the range from 3 to 5.2A. Selectivities of greater than 100 1 are observed between molecules which differ by as little as 0.2A in their critical dimensions. Kinetics of adsorption on these materials have been determined (2.,ii,l ) -... 

These fibers, obtained by pyrolysis of polyacrylonitrile, have a tensile strength of 2.41 GPa and tensile modulus of 391 GPa. Tows containing several thousand THORNEL 50 filaments were woven into three dimensional, orthogonal preforms in one of two manners, illustrated in Figure 1. Weave 2 (W2) was relatively coarser than weave 1 (Wl), although fiber volume in the preforms was similar (30.0% for W1 and 31.2% for W2). 

By vacuum pyrolysis of polyacrylonitrile fibres, black heat-resistant fibres of cyclic structure ( black orlon ) is formed through the following supposed mechanism ... 

Carbon-carbon composites can be produced with a multitude of structures. The simplest have two-dimensional order and consist of stacked plies of carbon fabric held together by a carbon matrix. The fabric fibers may be any of those described previously, prepared from the pyrolysis of polyacrylonitrile and the like. The matrix could be derived from petroleum pitch or be infiltrated pyrolytic carbon or even silicon carbide. The latter are generally referred to as SiC/C composites. From two-dimensional, the next progression in structure is three-dimensional on to n-dimensional. This terminology refers to fiber orientation within the matrix. 

Grassie N, McGuchan R, Pyrolysis of polyacrylonitrile and related polymers, Europ Poly J, 8, 257, 1972. 

Watt W, Green J, The pyrolysis of polyacrylonitrile. International conference on Carbon Fibres, their Composites and Applications, The Plastics Institute, London, 1971. 

Although elimination reactions are clearly degradation, leading to color and char formation, they can have advantages. Conjugated sequences and ladder structures confer thermal stability on the polymers, forming stable chars. This is the basis of the preparation of carbon fibers by pyrolysis of polyacrylonitrile. It is also one reason for the use of plasticized PVC in electrical insulation in a... 

Another approach to achieve thermal stability is to synthesize ladder polymers, so called because of their ladderUke structure ( ). For example, pyrolysis of polyacrylonitrile gives a ladder... 

Protonation reactions, 370 [Pt(dddt)2]FeCl4, 191 P-type conductors, 648 PVK-bonded dinitrophthalocyanine, 566 PVK-graft-polyisoprene, 566 Pyrazino-fused TCNQs, 246 Pyrazinotetrathiafulvalene, 170 Pyrazoline, 541 Pyridine-TCNQ, 238 Pyrolysis of polyacrylonitrile, 624 Pyromellitic dianhydride (PMDA), 92, 93 Pyropolymers, 762... 

Nevertheless, the promise of a thermally stable matoial prompts continued synthetic endeavor, including pyrolysis of polyacrylonitrile and polycyanoacetylene " (figure 23), explorations of electroidiemical doping reactions on ladder polymers, and work on the synthesis of ladder polymers with alkyl substituents to enhance solubility. ... 

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