Carbon Alloys part 5 ppt

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Tanso, 191: 4941,2000. 129 Chapter 8 Polymer Blend Technique for Designing Carbon Materials Asao Oya Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Abstract: Selection and modification of carbon precursors are important in controlling structure and properties of resultant carbons. A technique using polymer blends as precursors was developed several years ago and could be used widely as a unique method for preparing porous and non-porous carbons with special shapes. Studies using polymer blend techniques are reviewed. Keywords: Carbon precursor, Polymer blend, Carbon structure, Design. 1. Introduction The polymer blend technique is used widely in the polymer materials field to improve properties and to develop functionality [l]. The technique is also used effectively for carbons; in fact, trial studies of the carbon were started several years ago and interesting results are already available, some of which are introduced in Chapter 7. In this chapter, carbons prepared by using the polymer blend technique are intro- duced in more detail and the potential of the technique is elaborated upon. 2. Porous Carbon Materials A polymer blend consisting of two kinds of polymers is used as an example. Suppose two polymers, A and B, which form a graphitizable and a non-graphitizable carbon respectively, are blended and used as the precursor. The resulting carbon has a polymer blend texture as shown in Fig. 1, i.e., consisting of a graphitizable carbon matrix in which non-graphitizable carbon is dispersed. The polymer blend technique is usually used to prepare porous carbon materials as described below. Two kinds of polymers, with and without a carbon residue after heating in an inert atmosphere, are used to prepare a polymer blend. These are called carbonizing and pyrolyzing polymers in Chapter 7 and correspond to Polymers A and B in Fig. 1, 130 Chapter 8 Fig. 1. A schematic illustration for the design of carbon materials by using polymer blend technique. respectively. When such a polymer blend of carbonizing and pyrolyzing polymer is carbonized, pores are left in the carbon matrix of the carbonizing polymer following the volatilization of the pyrolyzing polymer. Pore sizes and volumes in the resultant carbons are controlled by the fineness of polymer blend texture and the blending ratio of the component polymers [l]. It is emphasized that the texture of a polymer blend is changed severely during such moulding processes such as spinning and casting. Consequently, it is the texture after moulding which determines the porous structure in the resultant carbon. Shrink- age of pores, which occurs during carbonization, also has to be considered when designing precise porous structures. As porous carbon films are discussed by Hatori et al. [2,3] and Takeichi et al. [4,5] in Chapter 7, a unique porous carbon fiber, only, is described below. It is important to control the blending ratio between the carbonizing polymer and pyrolyzing polymer, e.g., a polymer blend with carbonizing/pyrolyzing polymer > 1 must be used for the preparation of porous carbon fibers. With a ratio of carbonizing polymer/pyrolyzing polymer e 1 is used carbon nanofibers are obtained. Polystyrene microbeads (PS) of ca. 10pm diameter, as a pyrolyzing polymer, were dispersed uniformly in Novolac-type phenolic polymer (PF), as carbonizing polymer, with a blending ratio of PFPS = 70/30 by weight. The polymer blend was subjected to melt-spinning, stabilization in an acid solution and finally carbonization at 900°C for 1 h [6,7]. Figure 2a is a SEM photograph of the cross-section of a stabilized fiber. The arrowed small spots are thinly extended PS fibers which disappear during carboniza- tion resulting in porous carbon fibers. Figures 2b-d are SEM photographs of a cross-section and a side-view of the carbonized fiber. Round pores, e 1 pm diameter, are seen on the cross-section of the fiber. These pores extend along the fiber axis as seen in the side-view (Fig. 2d). They were formed by the volatilization of the PS component which was elongated in the PF matrix as a result of the spinning process. These pores are isolated from the fiber surface so they are not available for adsorption. As described above, porous structures prepared by using polymer blends depend strongly on the moulding process. A question arises: is it possible to prepare a carbon fiber with open pores without activation? So far, it has not been completed Polymer Blend Technique 131 Fig. 2. SEM photographs of the polymer blend fiber (PFPS = 70/30) before and after carbonization: (a) cross-section of stabilized fiber; (b) and (c) cross-section of carbonized fiber; (d) side-view of carbonized fiber. successfully. However, the preparation of carbon particles with open pores may be prepared by this technique when polymer blend particles prepared by a spray-drying method are carbonized. 3 Preferential Support of Metal Particles on Pore Surface The procedure for supporting metal particles was briefly described in Chapter 7. Here the validity of the technique is shown through the loading of platinum particles. The materials used were PF and maleinic acid-modified polyethylene (m-PE) as carbonizing and pyrolyzing polymer, respectively; Pt-acetylacetonate (Pt-acac) was used as a precursor of Pt particles. Initially, m-PE containing finely dispersed Pt-acac was prepared, followed by blending with PF with heating (m-PEPF = 20/80 by weight). When a precursor of metal particles has a low affinity for a pyrolyzing polymer but has a high affinity for a carbonizing polymer, the precursor diffuses from the pyrolyzing polymer into the carbonizing polymer during the blending process, using a kneader with heating. The metal particles are found finally in the matrix carbon derived from the carbonizing polymer. This is why Pt-acac was selected as a precursor compound for Pt particles [8]. To test this, a coarse blend texture of m-PE and PF was prepared hopefully to form large pores, as shown in Fig. 3 as a SEM micrograph. Electron probe X-ray microanalysis (EPMA) is not available for observation of the state of distribution of platinum particles in this narrow area. The 132 Chapter 8 Fig. 3. SEM photograph of cross-section of porous carbon fiber supporting platinum particles. polymer blend was subjected to spinning, stabilization and carbonization at 900°C Figure 4 shows SEM photographs and EPMA maps of platinum distributed within a stabilized fiber and the 900°C carbonized fibers. During carbonization the m-PE volatilized leaving pores as seen in Fig. 4, the resulting pore surface being covered with fine platinum particles identified by EPMA analyses. There were no platinum particles in the matrix carbon. Thus, the polymer blend technique can be used not only for pore formation but also for preferential support of metal particles on the pore surface. ~71. I Fig. 4. SEM photographs and platinum distribution maps of the cross-sections of stabilize fiber (left) and carbonized fiber (right). Polymer Blend Technique 133 4 Carbon Nanofibers and Carbon Nanotubes 4.1 Carbon Nanofibers When a polymer blend of carbonizing polymer/pyrolyzing polymer < 1 is used as a precursor, fine carbon fibers are left instead of thin and long pores, as seen in Fig. 2 following the loss of the pyrolyzing polymer matrix. For the preparation of thinner carbon fibers, smaller carbonizing polymer particles with a high elongation ability should be used as a precursor. PF and PE (PFPE = 30/70 by weight) were dissolved in acetone and warmed toluene, respectively. The acetone solution of PF was sprayed into the warmed toluene solution of PE with stirring. Fine PF particles were precipitated in the toluene solution of PE. Most of the precipitated particles were submicrometre. After evaporation of toluene under reduced pressure, PE containing the finely dispersed PF particles was obtained. Mechanical kneading was applied to the PE so obtaining a homogeneous dispersion of the particles. The polymer blend was then subject to spinning, stabilization and carbonization at 600°C for 1 h [9]. Figure 5 (top) shows a cross-section of the stabilized fiber. Thin PF fibers (arrowed) and fine pores are seen in cross-section; the latter supposedly were formed after the thin fibers were pulled out. The stabilized fiber resulted in a bundle of Fig. 5. SEM photograph of a cross-section of polymer blend fiber (PF/PE = 30/70) after stabilization (top) and the resulting bundle of carbon nanofibers (bottom). [...]... a carbon nanofiber cross-section carbon nanofibers after the loss of the PE matrix during carbonization (Fig 5 (bottom)) The bundle was easily separated into individual filaments,just by touching As the blend ratio of P F P E was reduced, so the carbon nanofiber bundle became loose As can be seen from the histogram in Fig 6, the nanofiber diameter ranged from 50 to 50 0 nm with a maximum at 200- 250 ... 333-338,2000 5 T Takeichi, Y Yamazaki, M Zuo, A Ito, A Matsumoto and M Inagaki, Preparation of porous carbon films by the pyrolysis of poly(urethane-imide) Films and their pore characteristics Carbon, 39: 257 -266,2001 142 Chapter 8 6 S Otani, On the carbon fiber from the molten pyrolysis products Carbon, 3: 31-38,19 65 7 A Oya, S Yoshida, Y Abe, T Iizuka and S Makiyama, Antibacterial activated carbon fiber... 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TEM micrographs of carbon nanofibers after heating to 900 and 3000°C The 900°C-fiber has a characteristic irregular surface and consists of fine carbon crystallites After graphitization at 3000”C, the carbon crystallites developed and the surface became - - 25 nm 25 nm Fig 7 TEM photographs of carbon nanofibers heated at 900°C (left) and 3000°C (right) Polymer Blend Technique 1 35 more irregular It... 40 -50 nm, respectively Fig 10.TEM photographof cross-sectionsof core(PMMA)/shell(PAN) particlescoated with Os0,vapor Polymer Blend Technique 137 Fig 11 TEM photograph of carbon nanotubes prepared from core(PMMA)/shell(PAN) particles The water emulsion of the core/shell particles was mixed with a water emulsion of PMMA particles, as a matrix, prepared in advance under sonication for 30 min The PMMA particles... Vol 258 Springer-Verlag, Berlin, 1986 2 M Parrinello and A Rahman, Polymorphic transitions in single crystals: A new molecular dynamics method J Appl Phys., 5 2 7182-7190,1981 3 S NosC, A unified formulation of the constant temperature molecular dynamics methods J Chem Phys., 81: 51 1 -51 9,1984 4 W.G Hoover, Canonical dynamics: Equilibrium phase-space distributions Phys Rev A, 31: 16 95- 1697,19 85 5 P Deuflhard,... textures 5 Other Fibrous Carbon Materials with Unique Shapes In addition to the carbon materials described above, other fibrous carbon materials with unique shapes may be prepared using the polymer blend technique [14] Figure 15 is a schematic of cross-sections of two types of microcapsules First consider carbon materials formed from microcapsules through spinning, stabilizing 140 Chapter 8 CP 0 PP Fig 15. .. for carbon nanofiber preparation The resulting fibers were passed through a methanol solution of PF, dried to coat the fibers with the PF, stabilized again and then finally carbonized at 1000°C The carbon material shown in Fig 16 was thus prepared a Fig 16 SEM photographs of carbon tubes including carbon nanofibers Polymer Blend Technique 141 Fig 17 SEM photograph of multi-walled carbon tube The carbon . area porous car- bons. 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