Polymer Blend Method

The polymer blend or polymer alloy method has been developed as a way to improve properties of polymers by blending other polymers. In this sense, the polymer blend is a kind

Figure 25. Regularly ordered macroporous carbons by using opal as a template (SEM image). Reprinted with permission from [90], A. A. Zakhilov et al., Science 282, 894 (1998). © 1998, American Association for the Advancement of Science.

of composite material. Polymer blends are divided into two categories depending on mixing levels. One is macroscopic blending and another is microscopic blending. Phase separated polymers are usually included in the first category and blends with compatibilizers, block co-polymers, and grafted co-polymers are included in the second category.

Pyrolysis behaviors of polymers are decided by the competition between scission of the polymer main chain and formation of cross-linking. Polymers with a tendency to main chain scission are unstable against heat treatment and therefore such polymers decompose without leaving carbon residues. Here these kinds of polymers will be referred to as pyrolyzing polymers. On the other hand, polymers with a tendency to form cross-linking can stand with the heat treatment and consequently leave carbon materials. Usually the latter category polymers are used as carbon precursors, and tentatively we call such polymers carbonizing polymers in this chapter.

Let us consider polymer blends between the pyrolyzing and carbonizing polymers. When we heat such a polymer blend for example up to 1273 K, theoretically we will have a porous carbon without further activation process, by leaving pores where the pyrolyzing polymer existed in the carbon matrix from the carbonizing polymers. If we control the distribution of the pyrolyzing polymer in the carbonizing polymer matrix, we will obtain carbon materials with controlled pores in them. This is the principle of the polymer blend method for preparing porous carbon materials as illustrated in Figure 26 [91].

Hatori et al. prepared a carbon from a polymer blend between polyimide and polyethylene glycol [92, 93]. In this carbonizing pyrolyzing polymer polymer b tending stirring spinning a stabilization b tending stirring spinning a stabilization

polymer blend fiber

carbonization 3

pyro lysis carbonization 3

pyro lysis

porous carbon fiber

Figure 26. A concept of polymer-blend method for preparing porous carbons without activation. Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

porous carbon fiber

Figure 26. A concept of polymer-blend method for preparing porous carbons without activation. Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

case, polyimide was the carbonizing polymer and polyethylene glycol was the pyrolyzing polymer. They successfully obtained mesoporous carbon of ca. 5 nm diameter. Takeichi et al. used co-polymers between amide and urethane [94]. In this case the former was a carbonizing and the latter was a pyrolyzing polymer, respectively. The carbon material obtained from this polymer was indeed porous; however, the pore size was as large as 10 /m. These could be classified as macropores.

Ozaki et al. tried to use the polymer blend method for preparing ACFs [91]. In this case, the materials for the blends were limited from the standpoints of spinnabil-ity, compatibility, and difference in the decomposition temperatures between the selected polymers. They chose novolac-type phenol-formaldehyde resin as the carbonizing polymer and poly(vinyl butyral) as the pyrolyzing polymer. Figure 27 shows the weight decrease during heat treatment. The pyrolyzing polymer [poly(vinyl butyral)] shows a steep decrease in weight between 573 and 673 K. The carbonizing polymer (phenol-formaldehyde resin) showed a stepwise weight decrease. The first weight decrease is observed at around 523 K followed by a greater decrease between 673 and 823 K than the former. The final weight loss of this sample was 40%. For a 1:1 polymer blend, the final weight loss was 73%, which is very close to the simple average of the weight losses of both component polymers. Pore size distributions for micropores and mesopores were obtained by N2 adsorption and are presented in Figure 28. As can be seen, the carbon fiber made from the polymer had a suppressed micropore volume and an enhanced mesopore volume. However, the obtained increase in the pore volume was less than they expected. Ozaki et al. analyzed the details of the polymer blend by using Fourier transform infrared and TG-MS techniques [95]. They found that when the

Figure 27. Weight loss curves for a polymer blend and its component polymers. Broken line: polymer blend; dash-dot line: pyrolyzing polymer; solid line: carbonizing polymer. The polymers used here were a 1:1 blend of phenol-formaldehyde resin (carbonizing polymer) and poly(vinyl butyral) (pyrolyzing polymer). Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

Figure 28. Mesoporous size distribution of the carbons prepared from a phenol-formaldehyde resin (solid line) and from a poly(vinyl butyral) (broken line). Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

Figure 27. Weight loss curves for a polymer blend and its component polymers. Broken line: polymer blend; dash-dot line: pyrolyzing polymer; solid line: carbonizing polymer. The polymers used here were a 1:1 blend of phenol-formaldehyde resin (carbonizing polymer) and poly(vinyl butyral) (pyrolyzing polymer). Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

Figure 28. Mesoporous size distribution of the carbons prepared from a phenol-formaldehyde resin (solid line) and from a poly(vinyl butyral) (broken line). Reprinted with permission from [91], J. Ozaki et al., Carbon 35, 1031 (1997). © 1997, Elsevier Science.

two components were prepared, they chemically interacted, which inhibited the independent pyrolysis of each polymer resulting in insufficient pore development.

Yang et al. prepared size-controlled mesoporous carbon spheres from polymer blends of novolac-type phenolic resin and two pyrolyzing polymers, poly(ethylene glycol) (PEG) and poly(vinyl butyral) (PVB) [96]. They put only a small amount of pyrolyzing polymers (7.5 units of PVB and 15 units of PEG were blended to 100 units of phenol-formaldehyde resin on weight basis), and finally they subjected the carbonized materials to steam activation. The obtained activated carbons possess mesopores ranging 3-5 nm. Addition of ferrocene to the polymer blend could give additional larger pores (10-90 nm) to the polymer blend derived activated carbons.

If a pyrolyzing polymer includes metal precursor and is dispersed in carbonizing polymer matrix, we can expect a porous carbon material on which pore surfaces are deposited by metal particles. Ozaki et al. prepared platinum loaded carbon fibers with the same polymer combination as described previously [97, 98]. By comparing the platinum loaded carbon fiber with platinum loaded fibers prepared by a simple mixing method, the former gave a higher hydrogen adsorption ratio and showed platinum particles deposited exclusively on the pore wall by electron microscopic observation.

During carbonization unstable parts in carbon precursor materials are eliminated as volatile matters. Devolatilization can introduce voids or pores in the resultant carbons. Such a preparation technique can be included in the polymer blend method, as the raw polymers are designed to have carbonizing parts and pyrolyzing parts. Takakura et al. synthesized a series of triple bonds containing polymers, poly(phenylene butadiyne) derivatives as shown in Figure 29, and studied their pore structures [99]. The micropore volume was around 0.3 ml/g independent of the polymer; however, the mesopore volume varied from 0.01 to 0.47 ml/g. The highest mesopore volume was achieved on P3 structure in Figure 29. Furthermore, the mesopore size distribution was tremendously influenced by the kinds of side chain groups. They considered that mesopores were introduced by the elimination of side chain groups. There is another example of preparing porous carbons by modifying the original polymer structure. Lenghaus et al. prepared phenolic resin from

Figure 29. Structurally modified poly(phenylene butadiyne) polymers for carbonization. Reprinted with permission from [99], T. Takakura et al., in "Proceedings of the 28th Annual Meeting of the Carbon Society of Japan," Kiryu 2001, p. 208 [in Japanese] © 2001, Carbon Society of Japan.

alkyl-substituted phenols and studied the influence of these groups on the formation of porous structures [100]. By comparing the CO2 and N2 surface areas, para-alkyl substituted phenols gave wider micropores. Thus changing the substitution group will be a promising way of controlling the pore size, but we need to know the carbonization mechanisms of the parent polymers for designing the raw materials.

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