Carbon 40 (2002) 315–320

Designing carbon materials with unique shapes using polymerblending and coating techniques

*Nirav Patel, Keiji Okabe, Asao OyaFaculty of Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan

Received 21 February 2001; accepted 29 March 2001

Abstract

Three carbon materials having unique and precise shapes were prepared using a polymer blend technique and coating asan auxiliary technique. The polymer blend consists essentially of two kinds of polymers, i.e. a carbon precursor polymer anda decomposable polymer which disappears by pyrolysis without leaving a carbon residue. One of the carbon materialsprepared is a carbon fiber including many thin pores, with diameters of sub-mm to 1 mm, elongated along the fiber axis. Thenext is a carbon tube including a large amount of thin carbon fibers aligned densely along the tube surface. The last one is amicroscopic multi-walled carbon tube consisting of three or four walls. The preparation procedures and SEM photographs ofthese materials are presented and possible applications are discussed briefly. 2002 Elsevier Science Ltd. All rightsreserved.

Keywords: A. Carbon fibers; Porous carbon; B. Carbonization; C. Thermal analysis; Scanning electron microscopy

1. Introduction 2. Experimental

Recently the polymer blend technique has become of 2.1. Materialsinterest as a novel method to prepare porous carbonmaterials without an activation process, i.e. the porous The polymers used were as follows; Novolac-typecarbons were prepared by carbonizing a polymer blend phenol–formaldehyde polymer (PF) was produced byconsisting of a carbon precursor polymer and a decompos- Gun-ei chemical Ind. Co. High density polyethyleneable polymer which disappears completely on pyrolysis (HDPE) and maleinic acid modified low density poly-[1–4]. We conventionally call the latter a pore former ethylene (MLDPE) were produced by Idemitsu Petroch-polymer. Later this technique was applied successfully for emicals Ind. Co. Polystyrene (degree of polymerization:the preparation of thin carbon fiber [5]. We considered that 1600–1800; PS) was purchased from Wako Pure Chemi-these successful experiments indicated that the polymer cals Ind. Thin polyethylene /nylon fiber (16–18 mm inblend technique has a high potential as a novel preparation diameter; PE/NY) was kindly supplied by Kuraray Fibermethod of carbon materials with precise and unique Ind. Ltd. These polymers were selected and used byshapes. The present work was planned in order to demon- considering compatibility and spinability of the resultingstrate the validity of these methods for the preparation of polymer blends.carbon materials. In order to accomplish the trials, how-ever, an auxiliary coating technique was required in 2.2. Preparation proceduresaddition to the polymer blend technique. In the presentpaper, three carbon materials with unique shapes are 2.2.1. Porous carbon fiberreported. MLDPE as a pore former and PF as a carbon precursor

were weighed out in a ratio of MLDPE/PF53/7 anddissolved thoroughly in toluene and acetone, respectively.The MLDPE solution was sprayed onto the acetone*Corresponding author. Tel.: 181-277-30-1350; fax: 181-solution of PF to deposit fine MLDPE particles, followed277-30-1353.

E-mail address: oya@chem.gunma-u.ac.jp (A. Oya). by evaporation of the toluene /acetone mixed solvent by

0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 01 )00101-4

316 N. Patel et al. / Carbon 40 (2002) 315 –320

using a vacuum rotary evaporator resulting in a PF/fineMLDPE particles mixture. The mixture was dried undervacuum at 608C for 24 h and finally subjected to ahomogenization treatment with a laboratory mixtruder(Atalas Electric Devices Co.) at 1358C.

The homogenized polymer blend was spun with aconventional spinning apparatus at 120–1258C, stabilizedin an acid solution containing hydrochloric acid andformaldehyde as main components, neutralized with aque-ous ammonia, washed with de-ionized water and dried atroom temperature for 1 day, details of which have beenreported elsewhere [6]. The fiber was finally carbonized at9008C for 2 h under a nitrogen atmosphere.

2.2.2. Carbon tubes including thin carbon fibersA homogeneous polymer blend consisting of PE as a Fig. 1. TG curves of MLDPE, stabilized PF and MLDPE/PF.

pore former and fine PF particles as carbon precursor wasprepared through a similar procedure to that used inSection 2.2.1. Here the mixing ratio was PE/PF57/3 used as a reference resulted in about 57 wt.% carbonwhich is the opposite of the ratio of MLDPE/PF53/7 residue at 9008C. The TG curve of the stabilized MLDPE/used in Section 2.2.1. The blend polymer fiber prepared by PF fiber coincides well with that predicted from thespinning at 125–1308C was passed through a 25 wt.% behaviors of both MLDPE and PF components and thePF–methanol solution at room temperature and dried to mixing ratio between them, which means no interactionleave a thin PF coating on the fiber. The resulting fiber was occurred between MLDPE and PF to change their thermalstabilized, neutralized, washed, dried and finally carbon- behaviors.ized under the same conditions used in Section 2.2.1. A cross section of the stabilized blend fiber is shown in

Fig. 2a. Many fine spots (arrowed) with diameter of sub-2.2.3. Multi-walled micro carbon tubes mm to 1 mm on the photograph indicate MLDPE elongated

The PE/NY fiber was coated alternatively with PF as a in the PF matrix along the fiber axis at spinning process.carbon precursor and PS as a pore former according to the The pores formed after the elongated MLDPE were pulledprocedure as described in Section 2.2.2. A 35 wt.% PF– out. The thin MLDPE fibers were removed during themethanol solution and a 30 wt.% PS–toluene solution were carbonization process to leave fine pores as shown in Fig.used for the coating. The fiber was subjected to the 2b. As seen on an enlarged photograph of the cross-sectionprocedures used in Section 2.2.1 and finally carbonized at (Fig. 2c), the pore diameter is nearly equal to that of the9008C for 1 h. elongated MLDPE in Fig. 2a. Fig. 2d shows a part of a

broken side surface of the fiber. Many thin pores of2.3. Measurements sub-mm diameter extend parallel with the fiber axis. It is

certain that such a unique pore structure originated fromThe carbon materials were observed with a scanning the stretching of the MLDPE particles in the PF matrix at

electron microscope (SEM, JEOL JSM-5300). The poly- spinning.mers and the polymer blends were analyzed thermogravi- The porous carbon fiber showed a specific surface area

2 3metrically with a Rigaku, Thermoplus TG 8120 at a of 595 m /g, a mesopore volume of 115 mm /g and a2heating rate of 108C/min in a nitrogen gas flow. The mesopore surface area of 118 m /g, respectively. There-

porous structure was characterized by N -adsorption /de- fore the fiber can be characterized by including a relatively2

sorption isotherm obtained with a Belsorp 28SA (BEL large amount of mesopores. The characteristic morphologyJapan Inc.,) at 77 K. Mesoporous structure was determined in the resulting porous carbon fiber, we expect, may giveby pore size distribution which was calculated from the an unusual adsorption /desorption behavior on which appli-adsorption branch using the D–H method [7]. cation fields may be developed.

3.2. Carbon tube including thin carbon fibers3. Results and discussion

Some photographs of a carbon tube including thin3.1. Porous carbon fiber carbon fibers are shown in Fig. 3. Fig. 3a shows thin

carbon fibers broken out from the crack of the carbon tube.Fig. 1 shows TG curves. MLDPE disappeared below The diameter of the thin carbon fiber was sub-mm which

5008C completely after fusing and the stabilized PF fiber was nearly equal to that reported previously [5]. Thin

N. Patel et al. / Carbon 40 (2002) 315 –320 317

Fig. 2. SEM photographs of (a) cross-section of stabilized MLDPE/PF fiber, (b) and (c) cross-sections of carbonized MLDPE/PF fiber, (d)broken side surface of carbonized MLDPE/PF fiber.

carbon fibers were packed densely in the tube and aligned matrix material into the tube is developed, and theparallel to the tube axis as seen in Fig. 3b. Fig. 3c shows a resulting composite may exhibit unique mechanical prop-broken carbon tube edge. The thin carbon fiber bundle erties.came loose and the fibers seem to be soft. When a moreloose carbon fiber bundle in the tube is required, the ratio 3.3. Multi-walled micro carbon tubes (MWMCT)of PE/PF must be increased. Rigid thin carbon fibers asshown in Fig. 3d were rarely observed. We have no TG curves of the materials used in the MWMCTexplanation for the occurrence of the different kinds of preparation are shown in Fig. 5. PS showed a major weightfibers. loss from 3808C and disappeared at ca. 4408C completely.

As stated above the unique carbon material was pre- The PE/NY core fiber resulted in just 5% of carbonpared by using polymer blend and coating techniques. residue after heating to 5008C. The carbon yield of theHowever, if a microcapsule A as shown in Fig. 4a is stabilized PF fiber is shown in Fig. 1. The thermalprepared, such a unique carbon material will be prepared behavior of the multi-coated sample coincided with thateasily by elongating the capsule in the PE matrix by supposed from those of constituent materials, which meansspinning, although the preparation of such material has still no interaction occurred between PE and PS.not been achieved. A general view of the carbonized sample is shown in

How is the unique carbon material used in practice? We Fig. 6a. The fibers wound and entangled each other. Fig.suppose that a carbon fiber reinforced nanocomposite may 6b and c show the cross sections of MWMCTs consistingbe derived if an effective impregnation technique of a of three and four carbon layers, respectively. The core pore

318 N. Patel et al. / Carbon 40 (2002) 315 –320

Fig. 3. SEM photographs of carbon tubes containing thin carbon fibers.

Fig. 4. Schematic models of cross-sections of microcapsules.

N. Patel et al. / Carbon 40 (2002) 315 –320 319

inner carbon layer is seen through the fracture surface ofthe most outer layer as shown by an arrow. The emptyspaces of the tube core and between carbon tubes wereformed after removing PE/NY core fiber and the PScoatings, respectively, during carbonization. The carbonwall thickness and the space between the walls are easilycontrolled by changing the concentration of PF and PSsolutions. If a microcapsule B as shown in Fig. 4b isprepared, MWMCT will be prepared more easily byelongating it in PE matrix by spinning, although such amicrocapsule has yet to be successfully prepared.

Three carbon materials with unique and precise shapeswere prepared by using polymer blend and coating tech-niques. If a more skillful preparation technique of mi-crocapsules is developed, the coating step will be removed

Fig. 5. TG curves of PS, PF, PE/NY core fiber, stabilized PF and from the preparation processes of the carbon materialsmulti-walled fiber consisting of PS/PF/core PE/NY. described in Sections 3.2 and 3.3. Carbon materials, in

future, will be used in higher technological fields wherecarbon materials with precise and unique shapes will be

diameter is nearly equal to that of the PE/NY fiber. Fig. 6d required. Polymer blend techniques may play an importantis an SEM photograph of the side surface of MWMCT. An role in these fields.

Fig. 6. SEM photographs of (a) an overview of MWMCT, (b) and (c) cross-sections of MWMCT consisting of three and four carbon layers,(d) side surface of MWMCT.

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