Carbonization of Polymers

Polymers are organic materials consisting of macromolecules, composed of many repeating units called mers , from which the word polymer is derived. The major polymers in the production of synthetic graphite are listed in Table 4.1 above and their chemical structures are shown in Fig. 4.4. 

As a rule, polymers have a lower carbon yield than aromatic hydrocarbons since they contain, in addition to hydrogen and carbon, other elemente with higher molecular weight, such as chlorine, oxygen, or nitrogen. These elements must be removed. The carbonization mechanism of polymers is usually more intricate than that of aromatic hydrocarbons since the composition is more complex. The carbon yield is unpredictable in many cases.l i 

One of the oldest polymer precursors is cellulose which has been used for generations in the production of charcoal from wood and, early in the twentieth century, for lamp filaments from cotton or bamboo by Thomas Edison. 

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From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

Figure 2 shows the SEM pictures of the prepared Membranes A, B, and C. Membrane D is also shown. It is obvious that Membranes A, B, and C have finer carbon whiskers, with diameters of approximately 0.6, 0.4, and 0.2 pm, respectively, while the whiskers of membrane D are larger, 2 pm (Li et al, 2001). As the SEM pictures show, after carbonization of polymer latex and CVD of methane, the novel membranes had thin and dense carbon whiskers compared with die conventional ones. 

Dimensionally, porous carbons can also be processed into size-shortened spheres. Commonly, carbon-based spheres are prepared by carbonization of polymer analogs. In this case, polymer precursors are required to be thermally stable and are able to form carbon residue after a high-temperature pyrolysis. Phenolic resins derived from the polymerization of phenols (e.g., phenol, resorcinol, phloroglucinol) with aldehyde (e.g., formaldehyde, furfuraldehyde, hexamethylenetetramine) are attractive due to their excellent performance characteristics such as high-temperature... 

A new form of carbon is the single-walled and multi-walled nanotubes (CNTs) [5]. The CNTs have a high length-to-width aspect ratio, high corrosion resistance, and good electronic conductivity. Another form of carbon is nanofibers that are typically produced by carbonization of polymer fibers. 

Porous carbons (e.g., activated carbons) are an important family of porous solids that have a wide spectrum of applications becanse of their remarkable properties, such as high specific surface area, chemical inertness, abundant repertory of surface functional groups, good thermal stability, and low cost of manufacture. Their chemistry and physics have been reviewed [22-25]. The most common way to produce activated carbons is to carbonize a carbon-containing precursor, followed by activation or posttreatment [26]. Becanse of practical requirements of various applications, techniques for control over the pore size of activated carbons have been the subject of research for several decades [18] for example, high burn-off activation, catalyst-assisted activation, and carbonization of polymer blends with thermally unstable components. For recent progress in the use of hydroxide activation, see Chapter 1 by Linares-Solano and coworkers in this volume [27]. However, none of these synthesis approaches is suitable for very precise control over pore structure, particle size, and morphology [19]. 

FIGURE 7.1 Fragment analysis of a metallocene-based olefin insertion transition state (a = transition metal -I- associated ligand b = olefin -i- a carbon of polymer chain c = remainder of polymer chain). 

Carbonization of polymers, such as poly(vinylidene chloride) (PVDC) (Walker et al., 1966 Dacey and Thomas, 1954 Dubinin et al., 1964 Lamond et al, 1965) Saran (90/10 mixture of vinylidene chloride and vinyl chloride) and cellulose, sugar, and coconut shell. 

It has been demonstrated with numerous examples that many phosphorus flame retardants of the reactive and additive types contribute to the formation and increase of yield of nonvolatile carbonized residues during the pyrolysis and combustion of polymers. The detailed mechanism of this effect is not clear, as is the case with the mechanism for the carbonization of polymers. 

The carbonization of polymer blend will lead to the formation of porous struc-tirre. It is because of the thermally unstable polymer (pyrolysing polymer) that decomposes and pores in the carbon matrix that are formed from the stable polymer (carbonizing polymer). 

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