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Carbon nanotubes prepared from three-layered copolymer microspheres of acrylonitrile and methylmethacrylate
Carbon 43 (2005) 1246–1253 www.elsevier.com/locate/carbon
Carbon nanotubes prepared from three-layered copolymer microspheres of acrylonitrile and methylmethacrylate Denisa Hulicova a
a,*
, Katsuhiko Hosoi b, Shin-ichi Kuroda b, Asao Oya
b
Energy Storage Materials Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan b Graduate School of Engineering, Gunma University, Tenjincho 1-5-1, Kiryu, Gunma 376-8515, Japan Received 29 July 2004; accepted 22 December 2004
Abstract Carbon nanotubes (CNTs) were synthesized from fine three-layered copolymer microspheres using the polymer blend technique. Diameter of PMMA core/Poly(AN-co-MMA) shell-1/PMMA shell-2 microspheres, prepared by a radical soap-free emulsion polymerization of methylmethacrylate (MMA) and acrylonitrile (AN), was between 400 nm and 500 nm. Microspheres were subjected to melt-spinning at 305 °C, stabilizing in oxygen at 220 °C for 4 h, and finally carbonizing at 1000 °C for 30 min. FE-SEM study of carbonized sample revealed the presence of CNTs arrays on carbon blocks. Similar arrays were observed in a comparative CNTs sample prepared from three-layered microspheres with the pure PAN shells-1 layers. HRTEM showed that the CNTs derived from copolymer microspheres had different structure when compared to the control sample, i.e. CNTs often adhered to each other and contained the internal compartments. The insufficient PMMA shell-2 coating of copolymer microspheres is believed to be a reason for CNTs adhesion. The possible mechanisms of the carbon block formation and the adhesion of CNTs are introduced. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Carbonization; Stabilization; Electron microscopy; Texture
1. Introduction Polymer blend technique has been used widely in a polymer field to improve and/or to develop new properties from polymers [1–3]. We already successfully employed this technique in the preparation of various carbon materials, e.g. activated carbon fibers [4–6]; unique porous carbon [7]; thin carbon fibers [8]; and, the most importantly, carbon nanotubes (CNTs) [9–11]. A communication summarizing concepts of the preparation of each material has been published elsewhere [12]. In this report we focus on a possibility to prepare CNTs with different structures using the same principles
*
Corresponding author. Tel.: +81 29 861 8430; fax: +81 29 861 8408. E-mail address: [email protected] (D. Hulicova).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.12.018
of the polymer blending and the spinning techniques. The concept of using these techniques in CNTs preparation has been introduced elsewhere [9–12]. Basically, the CNTs derive from core/shell polymer microspheres, cores and shells of which are made from polymers without and with carbon residue after heating in an inert atmosphere, respectively. Microspheres are dispersed throughout the matrix polymer and elongated extensively during the melt spinning due to the tension of spinning and drawing. Spun fibers are subjected to stabilization of the shell polymer and finally to carbonization. At this stage, the core and the matrix polymers are removed completely leaving only CNTs derived from elongated shell polymer. This work describes the CNTs synthesized from three-layered microspheres PMMA core/Poly(AN-coMMA) shell-1/PMMA shell-2 with copolymer shell-1
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and their structure is compared with the CNTs synthesized previously from PMMA core/PAN shell-1/PMMA shell-2, i.e. the microspheres with the pure PAN shell-1. One of the crucial steps in the CNTs preparation by polymer blend method is the synthesis of an ideal precursor, i.e. the core/shell microsphere with fine size, structure, and propensity for melt spinning. The history of core/shell microspheres is dated from 1958 when they were commercially introduced as PVC impact modifiers [2]. A major distinction between core/shell particles and other type of impact modifiers is that their size is set during the synthesis process and remains the same after they are dispersed in a host matrix. Such core/shell modifiers are produced by emulsion polymerization of free-radical initiated systems. In this experiment, polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) are chosen as a thermally decomposable and a carbon precursor polymer, respectively and potassium persulfate (K2S2O8, abbreviated as KPS), which liberates SO4 free radicals at temperatures in excess of 50 °C, is used as a radical polymerization initiator. Both monomers, MMA and AN, polymerize easily in the presence of KPS at 70–80 °C and therefore the use of surfactant or chain transfer agents is eliminated. As mentioned already, PAN and Poly(AN-co-MMA) are selected as carbon precursor polymers. PAN is recognized as the most important precursor for the manufacture of carbon fibers [13–15]. The advantages are high degree of molecular orientation and higher melting point compare to the other precursors, such as pitch, rayon, cellulosic precursors, etc. Currently, approximately 90 % of all commercial carbon fibers are produced from PAN precursor fibers. The stabilization of PAN is a well known procedure employing the oxidative atmosphere of pure oxygen and temperatures of 220– 250 °C. PAN-based fibers are stabilized under tension in order to retain the shape and the molecular structure during carbonization. In principle, stabilization converts the thermoplastic PAN fibers into a non-plastic compound that is capable of withstanding the carbonization heat treatment. In addition, the oxidation of PAN fibers is required to develop the aromatic structure. During the oxidation of PAN, a number of chemical reactions occur including the cyclization and dehydrogenation. Another well known fact is that the pure PAN is not possible to melt-spin whereas it decomposes before it melts [13,14] and therefore the PAN-based carbon fibers are prepared by wet spinning from solution. Our concept of CNTs preparation, however, took account of a melt spinning the PMMA/PAN and PMMA/Poly(ANco-MMA) blends. We considered several aspects before undergoing the synthesis. First of all, in these blends the abundant polymer is the PMMA, which is well acknowledged as a polymer with good elongation performance often being used in the preparation of carbon-polymer composite materials [16,17]. In addition, Bhanu et al.
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reported on the possibility to melt-spin acrylonitrile termopolymers made of acrylonitrile and methylacrylate monomers [18]. Another strong argument was the propensity of PMMA and PAN to form uniform, fine, and exact core/shell structures by a simple polymerization method. For all these reasons, we were strongly motivated to undergo the synthesis of CNTs from PMMA/PAN polymer blends Our first CNTs prepared by polymer blend method derived from two-layered core/shell microspheres dispersed in a matrix polymer [9,10]. A serious drawback was the low spinnability of this blend due to the poor homogeneity caused by mechanical mixing of the microspheres and the matrix polymer. Therefore not all microspheres were elongated sufficiently at the spinning process what led to the low CNTs yield. Because we believed the method to be applicable to the larger scale production we took a step further and successfully synthesized CNTs from three-layered PMMA core/PAN shell-1/PMMA shell-2 microspheres [11]. A significant improvement was achieved in the CNTs yield, their purity, and in the overall simplification of the preparation procedure. The idea of using three-layered microspheres proved to be fruitful because the outer PMMA shell-2 played the role of a matrix polymer separating the CNTs from each other and preventing their adhesion. Hence the use of additional matrix polymer was eliminated, preparation procedure simplified, and the spinnability increased due to the improved homogeneity. Here we report on a further modification of three-layered microspheres. A copolymer of AN and MMA, Poly(AN-co-MMA), forms the shell-1 that covers the PMMA core and from which, in point of fact, CNT derives. The final composition of microspheres can be described as PMMA core/Poly(AN-co-MMA) shell-1/ PMMA shell-2 and for the convenience sake, microspheres are denoted as C/coS-1/S-2 hereafter. The CNTs derived from C/coS-1/S-2 are compared with the CNTs prepared from PMMA core/ PAN shell-1/PMMA shell2 (abbr. C/S-1/S-2) microspheres, i.e. the precursor with a pure PAN shell-1. The copolymer shells were expected to result in to the formation of thinner CNTs walls due to the MMA fragments thermally decomposed at heattreatment. Moreover, the spinnability of these microspheres was expected to increase due to the presence of Poly(AN-co-MMA) that was proven to exhibit good elongation performance during melt-spinning [18].
2. Experimental 2.1. Synthesis of C/coS-1/S-2 microspheres Three-layered C/coS-1/S-2 microspheres were prepared by three steps soap-free emulsion polymerization
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of MMA and AN. Potassium persulfate (KPS) was used as a radical polymerization initiator. At first, the core PMMA microspheres were synthesized from 35 ml of MMA and 35 mg of KPS in 350 ml of deionized water. Polymerization reactions proceeded in an atmosphere of nitrogen at temperature of 80 °C under the intensive stirring. Further, 90 ml of PMMA emulsion (corresponding to 8 ml of MMA) was used in the second step of polymerization. Emulsion was mixed with 260 ml of deionized water together with 3 ml of AN and 1 ml of MMA. 5 mg of KPS was used to initialize the polymerization reaction. Mixture was subjected to the polymerization of AN and MMA at 80 °C under the nitrogen flow. As a result, PMMA core/AN-co-MMA shell-1 (abbr. C/coS-1) emulsion was obtained after 8 h of polymerization reactions. To prepare three-layered microspheres, the third and the last step of polymerization was proceeded as follows; 24 ml of MMA and 50 mg of KPS were mixed with 350 ml of C/coS-1 emulsion followed by 8 h polymerization of MMA at 80 °C in nitrogen. Emulsion of C/coS-1/S-2 was finally freeze-dried for 72 h using laboratory freeze-dryer (Iwaki, Freeze-dryer FRD-50M). The preparation procedure of C/S-1/S-2 microspheres can be found elsewhere [11]. Basically the first and the last steps of polymerization were identical, the only difference was the second step of polymerization where 2 ml of AN was polymerized on the PMMA core spheres.
Fig. 1. Detailed photograph of a melt-spinning apparatus: (1) spinneret with the outgoing fiber; (2) heating unit; (3) drawn fiber; (4) collected drawn fibers. Schematic image of the C/S-1/S-2 microspheres (a) subjected to melt-spinning, becoming partly elongated (b) and completely elongated (c).
2.2. Synthesis of CNTs 2.2.1. Melt-spinning of C/S-1/S-2 Freeze-dried C/S-1/S-2 microspheres were subjected to melt-spinning at temperature of ca. 310 °C. Spinning apparatus consisted of an alloy tube with a single spinneret hole with diameter D = 1 mm. Tube was heated with an electrical heater without any precise heat treatment controller and therefore the heating rate was controlled by an operator. The optimum spinning conditions, e.g. temperature, heating rate, drawing speed, etc. were first investigated on the control sample. Generally, the heating rate was ca. 5 °C/min and once the temperature of sample reached 310 °C, the temperature was kept for additional 30 min to let the sample soften entirely. Sample was pressurized with argon and the extruded fiber was drawn and collected on the windup spool (diameter / = 70 mm). Fig. 1 shows a detailed photograph of the spinneret and the drawn fiber. A schematic image of C/S-1/S-2 becoming elongated at the spinning process can be also seen in Fig. 1. Outgoing of melted sample from spinneret could be partially controlled through the regulation of argon flow. Melted sample was drawn at ca. 300–400 rpm and a partially continuous spinning was achieved. Diameter D of spun fibers was determined with SEM and the micrograph is shown in Fig. 2a. Fibers are not perfectly uniform as
Fig. 2. SEM photographs of spun fibers derived from C/S-1/S-2 (a) and C/coS-1/S-2 (b) microspheres.
their diameters vary between 15 lm and 25 lm. According to the windup speed and the diameter of the windup spool, at 400 rpm a fiber with the length of 175.8 m is drawn in one minute (assuming the constant volume). Hence the drawn ratio defined as k = (D1/D)2 is as high as 2500 if considering the average fiber diameter to be 20 lm. For example, Haggenmueller et al. reported on k = 3600 for a pure PMMA [16]. 2.2.2. Melt-spinning of C/coS-1/S-2 Freeze-dried C/coS-1/S-2 microspheres were meltspun using the same spinning apparatus as for C/S-1/
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S-2. The optimum spinning conditions were the temperature of 305 °C, the heating rate of 5 °C/min with the 30 min holding period, and the drawing speed of 500– 600 rpm. A significant improvement in spinnability was noticed and a long-time continuous spinning at higher speed could be achieved. Fig. 2b shows the drawn fibers. They have almost constant diameter of 15 lm, which reflects the superior spinnability of C/coS-1/S-2 and the continuous spinning with almost no tearing the fiber. Following the same logic as for the C/S-1/S-2, at 600 rpm 263.8 m long fiber is drawn in one minute of continuous spinning. Accordingly, the drawn ratio for these fibers is 4444. This value clearly reflects the significant improvement in C/coS-1/S2 spinnability if compared to the C/S-1/S-2.
To verify the elongation performance of C/coS-1/S-2 and C/S-1/S-2 microspheres, the following experiment was conducted. As-spun fibers were poured in tetrahydrofuran (THF) and due to the solubility of PMMA in THF, the outer PMMA shell-2 dissolved leaving the non-dissolved Poly(AN-co-MMA) or PAN shells dispersed in a solvent. In practice, the sample of as-spun fibers was put into the beaker with THF and, applying the ultrasound, dissolved until no fibers were detected with a naked eye. Then a cupper grid for electron microscopy was dipped into the solution and transferred to TEM (JEOL 1200 EXS, acceleration voltage 80 keV).
2.2.3. Stabilization and carbonization Spun fibers were further stabilized in oxygen at 220 °C for 4 h. The heating rate was kept on 5 °C min 1–150 °C and then on 1 °C min 1 to 220 °C. Stabilized sample was finally carbonized using the same conditions as in the preparation of CNTs from C/S-1/S2, i.e. 1000 °C for 30 min in the atmosphere of continuously flowing pure nitrogen (flow 10 mlmin 1) [11]. The carbonization heating rate was 5 °C min 1 and system was let to naturally cool down to room temperature after 30 min holding period. The morphologies of PMMA, C/CoS-1, C/coS-1/S-2, and C/S-1/S-2 after the freeze drying were observed with scanning electron microscope (SEM, JEOL JSM 5300, operated at 25 kV). Carbonized samples were investigated with field emission SEM (FE-SEM, JEOL JSM 6700F). Transmission electron microscopes (TEM, JEOL 1200 EXS, acceleration voltage 100 keV and Hitachi H-900 NA, acceleration voltage of 300 keV) carried out the structural study of CNTs.
SEM photographs of PMMA core microspheres, C/ coS-1, C/coS-1/S-2, and C/S-1/S-2 microspheres are shown in Fig. 3. It is clear that the formation of uniform core PMMA microspheres with the diameter of ca. 280 nm was achieved (Fig. 3a). Same spherical particles were observed in the entire volume of sample. The size uniformity was further maintained after the polymerization of AN and MMA on the PMMA core microspheres (Fig. 3b). Diameters of C/coS-1 microspheres vary between 350 nm and 380 nm and the size increase suggests the successful formation of core/shell-like structures. C/ coS-1/S-2 obtained after the third step of polymerization of MMA on the C/coS-1 are shown in Fig. 3c. These microspheres are not moderately uniform if compared to C/coS-1, as their diameters vary in the wider range between 400 nm and 500 nm. The reason for the microspheresÕ uniformity decrease can be explained on the basis of core/shell structures formation when hydrophilic monomers tend to remain on the outside of the particle, more in contact with the aqueous phase,
3. Results and Discussion
Fig. 3. SEM photographs of: (a) PMMA; (b) C/coS-1; (c) C/coS-1/S-2; and (d ) C/S-1/S-2. See text for explanation.
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whereas hydrophobic monomers tend to penetrate the core [2]. In our experiment, MMA represents a hydrophobic monomer while AN is relatively hydrophilic. Therefore it can be assumed that some part of PMMA, polymerized in the last step, diffused inside the core PMMA through the MMA fragments of shell-1. This thought is supported by the decrease in size uniformity observed with the SEM and also by the presence of nitrogen in the surface of C/coS-1/S-2 estimated with X-ray photoelectron spectroscopy (not shown here). Fig. 3d shows the SEM photograph of C/S-1/S-2. The size uniformity is retained after the third step of polymerization due to the continuous PAN shell-2 coating. This also supports the above theory of the PMMA diffusion in C/coS-1/S-2. Next, the results on the elongation performance of C/ coS-1/S-2 microspheres are presented. Fig. 4a represents the dominant elongated fibrous structure straight in the shape with the diameters of about 100 nm and 20 nm in a thicker central part and at the thinner ends, respectively. Similar pictures are obtained from C/S-1/S-2 sample (Fig. 4b). The presence of these structures proves that the essential requirement for the successful preparation of CNTs by polymer blend method, which is the elongation propensity of polymer microspheres, was achieved, and that the Poly(AN-co-MMA) copolymer shells as well as PAN shells became elongated at the melt-spinning process of C/coS-1/S-2 and C/S-1/S-2 microspheres, respectively. Other structures, however, were also observed. Fig. 4c shows the spherical structures, without any elongation performance, diameters of which vary in a wide range of 30 nm–200 nm. FE-SEM photographs of CNTs derived from C/coS1/S-2 and C/S-1/S-2 are shown in Fig. 5. The arrays of CNTs appear as the fibrous structures pulled out from the carbon block and oriented perpendicularly to the surface (Fig. 5a). Large fragments of the carbon blocks can also be seen. In addition, the spherical particles adhered to the carbon block are presented too (Fig. 5b). These must origin from the non-elongated microspheres at spinning, which retain the original spherical structures through the stabilization and the carbonization process. This result is in good agreement with the TEM study of spun and dissolved fibers that revealed
Fig. 5. FE-SEM photographs of carbonized samples from C/coS-1/S-2 and C/S-1/S-2: (a) CNTs from C/coS-1/S-2; (b) carbon spherical particles observed in sample derived from C/coS-1/S-2; (c) CNTs arrays prepared through the spinning of C/S-1/S-2 microspheres.
the presence of non-elongated spherical microspheres (Fig. 4c). For the comparison reasons, the FE-SEM photograph of CNTs from C/S-1/S-2 is displayed in Fig. 5c. One can clearly see the similarity between these CNTs and the ones derived from C/coS-1/S-2. Similar large arrays of CNTs on carbon blocks are present. CNTs were further observed with the transmission electron microscope and low-magnification TEM photographs are shown in Fig. 6. A typical photograph of
Fig. 4. SEM observation of spun fibers after the dissolution in THF: (a) elongated structures from C/coS-1/S-2; (b) elongated C/S-1/S-2; (c) nonelongated C/coS-1/S-2.
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Fig. 6. (a) CNTs derived from C/co/S-1/S-2 microspheres. Diameter of CNTs varies between 7 and 10 SSSnm; (b) CNTs from C/S-1/S-2 with the mean diameter of 10–20 nm.
straight CNTs derived from C/soS-1/S-2, about 7–10 nm in diameter, are displayed in Fig. 6a. No clear difference, except for the diameters, is noticed if compare these CNTs with the CNTs derived from C/S-1/S-2 (Fig. 6b). The CNTs from C/S-1/S-2 are also straight but the diameter larger, about 10–20 nm. However, the detailed HRTEM study revealed the structural differences between the CNTs derived from two different precursors. The most interesting difference is that CNTs from C/coS-1/S-2 often adhere to each other through the amorphous carbon covering their surfaces. Fig. 7 shows two examples. CNTs in Fig. 7a adhere parallel to each other while CNTs in Fig. 7b adhere through their tips in a certain angle. When compare these CNTs with the CNTs from C/S-1/S-2 (Fig. 7c), it is obvious that the graphene sheets are developed and ordered more even though the temperature and the duration of heat-treatment were the same. In addition, these CNTs are thinner as the mean diameter is 10 nm and as thin as 7 nm ones are present too. Differences may be attributed to more serious stretching and elongation at spinning whereas C/coS-1/S-2 exhibit higher spinnability than C/S-1/S-2 microspheres. A severe stretching of microspheres during the melt-spinning results in a preferred orientation of PAN molecules along the fiber axis. Similar mechanical stretching techniques are used practically to prepare PAN-based high-performance carbon fibers [14,15]. Another significant microstructural feature is the presence of internal compartments in CNTs from C/coS-1/S2. Taking into account the composition of C/coS-1/S-2 microspheres, SEM study, XPS analysis, and the above listed results on TEM and HRTEM, the following thought is our proposal for the formation of adherent CNTs with the internal compartments. In the first step of polymerization, uniform PMMA core microspheres result from the polymerization reaction of MMA. Further polymerized AN and MMA form a copolymer shell-1 consisted of AN and MMA fragments. According
to the amount of monomers, AN fragments are predominant. During subsequent third step of polymerization of MMA, when the formation of outer PMMA shell-2 is expected to proceed, a part of polymerized PMMA diffuse inside the core PMMA through the MMA fragments of copolymer shell-1. This behavior can be explained in the light of the principle of core/shell microspheres formation, i.e. the hydrophilic monomers tend to remain on the outside of the particle, more in contact with the aqueous phase, but hydrophobic monomers attempt to enter the hydrophobic core [2]. The relevance of this theory in our PMMA/PAN system was approved practically with the XPS analysis (presence of nitrogen in the outer layer) and SEM observation (decrease of the size uniformity of C/coS-1/S-2). For this reason the resultant microspheres are not covered entirely with the PMMA and therefore C/coS-1/S-2 microspheres might have a propensity to adhere to each other at melt-spinning process through the AN fragments, memory of which remains in the CNTs after stabilization and carbonization. The formation of internal compartments may result from the rearrangement of PAN molecules during the thermal decomposition of MMA fragments in Poly(AN-coMMA) shell-1. One may question why such a precise PMMA shell-2 coating is so important since we adopt a melt-processing method where all components may exist mixed in a molten state. Our argument lies in the basic ideas of polymer blend method and the results we have obtained so far. Through many trials and errors we found that in order to prepare nanotubes, the microspheres must be separated sufficiently by a matrix polymer. The results suggest that even though the polymers exist in a molten state, they retain the shape and the composition during the softening and the melting. Furthermore, the matrix and the core polymers are always chosen among the polymers with the good elongation performance and are preferably the same in order to avoid the lowering in spinnability. In addition, each time they are in
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Weight change[wt%]
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Heat flow [µV]
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200
400
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Fig. 8. TG-DTA profile of PMMA.
Fig. 7. HRTEM images of CNTs derived from C/coS-1/S-2 (a,b); C/S1/S-2 (c).
abundance compared to the carbon precursor polymer. Thus the elongation of matrix as well as core polymer also promotes the elongation performance of shell polymer at spinning. Since the presented concept implies the three-layered microspheres, where the outside PMMA layer plays the role of a matrix polymer, we conclude that the insufficient PMMA coating is responsible for the adhesion of CNTs as explained above. It may be possible to consider a possible usefulness of adhered CNTs in the areas such as gas adsorption, composites materials, etc. Another point to be discussed is the presence of carbon blocks. The reason of their ineligible formation is
not clarified yet, but a possible mechanism can be interpreted as follows: PAN in the inner part of spun fiber may be stabilized less than the PAN of the outer part because of the difficulty of oxygen diffusion for stabilization. The former PAN results in CNTs, but the latter PAN is semi-fused and reacts with PMMA to form carbon blocks. Moreover, PMMA is known as a thermally decomposable polymer which decomposes in not a simply way. TG-DTA profile of PMMA is shown in Fig. 8. It is clear from the TG curve that no carbon residue remains after 1000 °C since the decomposition occurs below 400 °C. Furthermore, DTA curve suggests that the thermal decomposition of PMMA is a complicate process consisting of several steps. Generally it occurs through the radical reaction when the end radical attacks the polymer chain causing the depolymerization of PMMA. Radical depolymerization involves the formation of MMA. PMMA involved in CNTs preparation, however, may behave differently during the thermal depolymerization according to the following thought: PMMA is surrounded by PAN and therefore the end radical of PMMA can show an ambition to attack the reactive nitrile group of PAN molecule. Hence such PMMA does not depolymerize according to the theory but rather forms the back-chains of PAN. It is well known that nitrile groups of PAN chain interact each other leading to the intramolecular polymerization and formation of the hexagonal structure [15]. This structure is a base of the graphene sheets formed at high temperature heat-treatment. Thence, the cycled PAN with the back-chained PMMA should have a different propensity to the graphene sheets development. It is believed, that such PAN resulted in the formation of amorphous carbon during the carbonization. The suppression of carbon blocks formation is required to complete the preparation method successfully.
4. Conclusions The use of co-polymer C/coS-1/S-2 microspheres allowed us to prepare CNTs with the structure different from the CNTs derived from C/S-1/S-2, i.e. the micro-
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spheres with the pure PAN shell-1. It was observed that CNTs often adhered to each other and contained the internal compartments. These differences were attributed to the insufficient PMMA shell-2 coating and the composition of shell-1 from which the CNTs derived. The spinnability of C/coS-1/S-2 was improved and the CNTs as thin as 7 nm could be observed by electron microscopy. More serious stretching of C/coS-1/S-2 during the meltspinning improved the crystallinity of the resulted CNTs. Presented results clearly showed the advantage of polymer blend method which is the possibility to control the structure of CNTs by controlling the composition of microspheres. We believe that the scope for the polymer blend method application in the preparation of various carbon materials is much wider and that this method will become a significant technique in carbon materials field.
Acknowledgment Authors are grateful to Mitsubishi Chemical Corp. for HRTEM.
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Carbon nanotubes prepared from three-layered copolymer microspheres of acrylonitrile and methylmethacrylate Denisa Hulicova a
a,*
, Katsuhiko Hosoi b, Shin-ichi Kuroda b, Asao Oya
b
Energy Storage Materials Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan b Graduate School of Engineering, Gunma University, Tenjincho 1-5-1, Kiryu, Gunma 376-8515, Japan Received 29 July 2004; accepted 22 December 2004
Abstract Carbon nanotubes (CNTs) were synthesized from fine three-layered copolymer microspheres using the polymer blend technique. Diameter of PMMA core/Poly(AN-co-MMA) shell-1/PMMA shell-2 microspheres, prepared by a radical soap-free emulsion polymerization of methylmethacrylate (MMA) and acrylonitrile (AN), was between 400 nm and 500 nm. Microspheres were subjected to melt-spinning at 305 °C, stabilizing in oxygen at 220 °C for 4 h, and finally carbonizing at 1000 °C for 30 min. FE-SEM study of carbonized sample revealed the presence of CNTs arrays on carbon blocks. Similar arrays were observed in a comparative CNTs sample prepared from three-layered microspheres with the pure PAN shells-1 layers. HRTEM showed that the CNTs derived from copolymer microspheres had different structure when compared to the control sample, i.e. CNTs often adhered to each other and contained the internal compartments. The insufficient PMMA shell-2 coating of copolymer microspheres is believed to be a reason for CNTs adhesion. The possible mechanisms of the carbon block formation and the adhesion of CNTs are introduced. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Carbonization; Stabilization; Electron microscopy; Texture
1. Introduction Polymer blend technique has been used widely in a polymer field to improve and/or to develop new properties from polymers [1–3]. We already successfully employed this technique in the preparation of various carbon materials, e.g. activated carbon fibers [4–6]; unique porous carbon [7]; thin carbon fibers [8]; and, the most importantly, carbon nanotubes (CNTs) [9–11]. A communication summarizing concepts of the preparation of each material has been published elsewhere [12]. In this report we focus on a possibility to prepare CNTs with different structures using the same principles
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Corresponding author. Tel.: +81 29 861 8430; fax: +81 29 861 8408. E-mail address: [email protected] (D. Hulicova).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.12.018
of the polymer blending and the spinning techniques. The concept of using these techniques in CNTs preparation has been introduced elsewhere [9–12]. Basically, the CNTs derive from core/shell polymer microspheres, cores and shells of which are made from polymers without and with carbon residue after heating in an inert atmosphere, respectively. Microspheres are dispersed throughout the matrix polymer and elongated extensively during the melt spinning due to the tension of spinning and drawing. Spun fibers are subjected to stabilization of the shell polymer and finally to carbonization. At this stage, the core and the matrix polymers are removed completely leaving only CNTs derived from elongated shell polymer. This work describes the CNTs synthesized from three-layered microspheres PMMA core/Poly(AN-coMMA) shell-1/PMMA shell-2 with copolymer shell-1
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and their structure is compared with the CNTs synthesized previously from PMMA core/PAN shell-1/PMMA shell-2, i.e. the microspheres with the pure PAN shell-1. One of the crucial steps in the CNTs preparation by polymer blend method is the synthesis of an ideal precursor, i.e. the core/shell microsphere with fine size, structure, and propensity for melt spinning. The history of core/shell microspheres is dated from 1958 when they were commercially introduced as PVC impact modifiers [2]. A major distinction between core/shell particles and other type of impact modifiers is that their size is set during the synthesis process and remains the same after they are dispersed in a host matrix. Such core/shell modifiers are produced by emulsion polymerization of free-radical initiated systems. In this experiment, polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) are chosen as a thermally decomposable and a carbon precursor polymer, respectively and potassium persulfate (K2S2O8, abbreviated as KPS), which liberates SO4 free radicals at temperatures in excess of 50 °C, is used as a radical polymerization initiator. Both monomers, MMA and AN, polymerize easily in the presence of KPS at 70–80 °C and therefore the use of surfactant or chain transfer agents is eliminated. As mentioned already, PAN and Poly(AN-co-MMA) are selected as carbon precursor polymers. PAN is recognized as the most important precursor for the manufacture of carbon fibers [13–15]. The advantages are high degree of molecular orientation and higher melting point compare to the other precursors, such as pitch, rayon, cellulosic precursors, etc. Currently, approximately 90 % of all commercial carbon fibers are produced from PAN precursor fibers. The stabilization of PAN is a well known procedure employing the oxidative atmosphere of pure oxygen and temperatures of 220– 250 °C. PAN-based fibers are stabilized under tension in order to retain the shape and the molecular structure during carbonization. In principle, stabilization converts the thermoplastic PAN fibers into a non-plastic compound that is capable of withstanding the carbonization heat treatment. In addition, the oxidation of PAN fibers is required to develop the aromatic structure. During the oxidation of PAN, a number of chemical reactions occur including the cyclization and dehydrogenation. Another well known fact is that the pure PAN is not possible to melt-spin whereas it decomposes before it melts [13,14] and therefore the PAN-based carbon fibers are prepared by wet spinning from solution. Our concept of CNTs preparation, however, took account of a melt spinning the PMMA/PAN and PMMA/Poly(ANco-MMA) blends. We considered several aspects before undergoing the synthesis. First of all, in these blends the abundant polymer is the PMMA, which is well acknowledged as a polymer with good elongation performance often being used in the preparation of carbon-polymer composite materials [16,17]. In addition, Bhanu et al.
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reported on the possibility to melt-spin acrylonitrile termopolymers made of acrylonitrile and methylacrylate monomers [18]. Another strong argument was the propensity of PMMA and PAN to form uniform, fine, and exact core/shell structures by a simple polymerization method. For all these reasons, we were strongly motivated to undergo the synthesis of CNTs from PMMA/PAN polymer blends Our first CNTs prepared by polymer blend method derived from two-layered core/shell microspheres dispersed in a matrix polymer [9,10]. A serious drawback was the low spinnability of this blend due to the poor homogeneity caused by mechanical mixing of the microspheres and the matrix polymer. Therefore not all microspheres were elongated sufficiently at the spinning process what led to the low CNTs yield. Because we believed the method to be applicable to the larger scale production we took a step further and successfully synthesized CNTs from three-layered PMMA core/PAN shell-1/PMMA shell-2 microspheres [11]. A significant improvement was achieved in the CNTs yield, their purity, and in the overall simplification of the preparation procedure. The idea of using three-layered microspheres proved to be fruitful because the outer PMMA shell-2 played the role of a matrix polymer separating the CNTs from each other and preventing their adhesion. Hence the use of additional matrix polymer was eliminated, preparation procedure simplified, and the spinnability increased due to the improved homogeneity. Here we report on a further modification of three-layered microspheres. A copolymer of AN and MMA, Poly(AN-co-MMA), forms the shell-1 that covers the PMMA core and from which, in point of fact, CNT derives. The final composition of microspheres can be described as PMMA core/Poly(AN-co-MMA) shell-1/ PMMA shell-2 and for the convenience sake, microspheres are denoted as C/coS-1/S-2 hereafter. The CNTs derived from C/coS-1/S-2 are compared with the CNTs prepared from PMMA core/ PAN shell-1/PMMA shell2 (abbr. C/S-1/S-2) microspheres, i.e. the precursor with a pure PAN shell-1. The copolymer shells were expected to result in to the formation of thinner CNTs walls due to the MMA fragments thermally decomposed at heattreatment. Moreover, the spinnability of these microspheres was expected to increase due to the presence of Poly(AN-co-MMA) that was proven to exhibit good elongation performance during melt-spinning [18].
2. Experimental 2.1. Synthesis of C/coS-1/S-2 microspheres Three-layered C/coS-1/S-2 microspheres were prepared by three steps soap-free emulsion polymerization
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of MMA and AN. Potassium persulfate (KPS) was used as a radical polymerization initiator. At first, the core PMMA microspheres were synthesized from 35 ml of MMA and 35 mg of KPS in 350 ml of deionized water. Polymerization reactions proceeded in an atmosphere of nitrogen at temperature of 80 °C under the intensive stirring. Further, 90 ml of PMMA emulsion (corresponding to 8 ml of MMA) was used in the second step of polymerization. Emulsion was mixed with 260 ml of deionized water together with 3 ml of AN and 1 ml of MMA. 5 mg of KPS was used to initialize the polymerization reaction. Mixture was subjected to the polymerization of AN and MMA at 80 °C under the nitrogen flow. As a result, PMMA core/AN-co-MMA shell-1 (abbr. C/coS-1) emulsion was obtained after 8 h of polymerization reactions. To prepare three-layered microspheres, the third and the last step of polymerization was proceeded as follows; 24 ml of MMA and 50 mg of KPS were mixed with 350 ml of C/coS-1 emulsion followed by 8 h polymerization of MMA at 80 °C in nitrogen. Emulsion of C/coS-1/S-2 was finally freeze-dried for 72 h using laboratory freeze-dryer (Iwaki, Freeze-dryer FRD-50M). The preparation procedure of C/S-1/S-2 microspheres can be found elsewhere [11]. Basically the first and the last steps of polymerization were identical, the only difference was the second step of polymerization where 2 ml of AN was polymerized on the PMMA core spheres.
Fig. 1. Detailed photograph of a melt-spinning apparatus: (1) spinneret with the outgoing fiber; (2) heating unit; (3) drawn fiber; (4) collected drawn fibers. Schematic image of the C/S-1/S-2 microspheres (a) subjected to melt-spinning, becoming partly elongated (b) and completely elongated (c).
2.2. Synthesis of CNTs 2.2.1. Melt-spinning of C/S-1/S-2 Freeze-dried C/S-1/S-2 microspheres were subjected to melt-spinning at temperature of ca. 310 °C. Spinning apparatus consisted of an alloy tube with a single spinneret hole with diameter D = 1 mm. Tube was heated with an electrical heater without any precise heat treatment controller and therefore the heating rate was controlled by an operator. The optimum spinning conditions, e.g. temperature, heating rate, drawing speed, etc. were first investigated on the control sample. Generally, the heating rate was ca. 5 °C/min and once the temperature of sample reached 310 °C, the temperature was kept for additional 30 min to let the sample soften entirely. Sample was pressurized with argon and the extruded fiber was drawn and collected on the windup spool (diameter / = 70 mm). Fig. 1 shows a detailed photograph of the spinneret and the drawn fiber. A schematic image of C/S-1/S-2 becoming elongated at the spinning process can be also seen in Fig. 1. Outgoing of melted sample from spinneret could be partially controlled through the regulation of argon flow. Melted sample was drawn at ca. 300–400 rpm and a partially continuous spinning was achieved. Diameter D of spun fibers was determined with SEM and the micrograph is shown in Fig. 2a. Fibers are not perfectly uniform as
Fig. 2. SEM photographs of spun fibers derived from C/S-1/S-2 (a) and C/coS-1/S-2 (b) microspheres.
their diameters vary between 15 lm and 25 lm. According to the windup speed and the diameter of the windup spool, at 400 rpm a fiber with the length of 175.8 m is drawn in one minute (assuming the constant volume). Hence the drawn ratio defined as k = (D1/D)2 is as high as 2500 if considering the average fiber diameter to be 20 lm. For example, Haggenmueller et al. reported on k = 3600 for a pure PMMA [16]. 2.2.2. Melt-spinning of C/coS-1/S-2 Freeze-dried C/coS-1/S-2 microspheres were meltspun using the same spinning apparatus as for C/S-1/
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S-2. The optimum spinning conditions were the temperature of 305 °C, the heating rate of 5 °C/min with the 30 min holding period, and the drawing speed of 500– 600 rpm. A significant improvement in spinnability was noticed and a long-time continuous spinning at higher speed could be achieved. Fig. 2b shows the drawn fibers. They have almost constant diameter of 15 lm, which reflects the superior spinnability of C/coS-1/S-2 and the continuous spinning with almost no tearing the fiber. Following the same logic as for the C/S-1/S-2, at 600 rpm 263.8 m long fiber is drawn in one minute of continuous spinning. Accordingly, the drawn ratio for these fibers is 4444. This value clearly reflects the significant improvement in C/coS-1/S2 spinnability if compared to the C/S-1/S-2.
To verify the elongation performance of C/coS-1/S-2 and C/S-1/S-2 microspheres, the following experiment was conducted. As-spun fibers were poured in tetrahydrofuran (THF) and due to the solubility of PMMA in THF, the outer PMMA shell-2 dissolved leaving the non-dissolved Poly(AN-co-MMA) or PAN shells dispersed in a solvent. In practice, the sample of as-spun fibers was put into the beaker with THF and, applying the ultrasound, dissolved until no fibers were detected with a naked eye. Then a cupper grid for electron microscopy was dipped into the solution and transferred to TEM (JEOL 1200 EXS, acceleration voltage 80 keV).
2.2.3. Stabilization and carbonization Spun fibers were further stabilized in oxygen at 220 °C for 4 h. The heating rate was kept on 5 °C min 1–150 °C and then on 1 °C min 1 to 220 °C. Stabilized sample was finally carbonized using the same conditions as in the preparation of CNTs from C/S-1/S2, i.e. 1000 °C for 30 min in the atmosphere of continuously flowing pure nitrogen (flow 10 mlmin 1) [11]. The carbonization heating rate was 5 °C min 1 and system was let to naturally cool down to room temperature after 30 min holding period. The morphologies of PMMA, C/CoS-1, C/coS-1/S-2, and C/S-1/S-2 after the freeze drying were observed with scanning electron microscope (SEM, JEOL JSM 5300, operated at 25 kV). Carbonized samples were investigated with field emission SEM (FE-SEM, JEOL JSM 6700F). Transmission electron microscopes (TEM, JEOL 1200 EXS, acceleration voltage 100 keV and Hitachi H-900 NA, acceleration voltage of 300 keV) carried out the structural study of CNTs.
SEM photographs of PMMA core microspheres, C/ coS-1, C/coS-1/S-2, and C/S-1/S-2 microspheres are shown in Fig. 3. It is clear that the formation of uniform core PMMA microspheres with the diameter of ca. 280 nm was achieved (Fig. 3a). Same spherical particles were observed in the entire volume of sample. The size uniformity was further maintained after the polymerization of AN and MMA on the PMMA core microspheres (Fig. 3b). Diameters of C/coS-1 microspheres vary between 350 nm and 380 nm and the size increase suggests the successful formation of core/shell-like structures. C/ coS-1/S-2 obtained after the third step of polymerization of MMA on the C/coS-1 are shown in Fig. 3c. These microspheres are not moderately uniform if compared to C/coS-1, as their diameters vary in the wider range between 400 nm and 500 nm. The reason for the microspheresÕ uniformity decrease can be explained on the basis of core/shell structures formation when hydrophilic monomers tend to remain on the outside of the particle, more in contact with the aqueous phase,
3. Results and Discussion
Fig. 3. SEM photographs of: (a) PMMA; (b) C/coS-1; (c) C/coS-1/S-2; and (d ) C/S-1/S-2. See text for explanation.
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whereas hydrophobic monomers tend to penetrate the core [2]. In our experiment, MMA represents a hydrophobic monomer while AN is relatively hydrophilic. Therefore it can be assumed that some part of PMMA, polymerized in the last step, diffused inside the core PMMA through the MMA fragments of shell-1. This thought is supported by the decrease in size uniformity observed with the SEM and also by the presence of nitrogen in the surface of C/coS-1/S-2 estimated with X-ray photoelectron spectroscopy (not shown here). Fig. 3d shows the SEM photograph of C/S-1/S-2. The size uniformity is retained after the third step of polymerization due to the continuous PAN shell-2 coating. This also supports the above theory of the PMMA diffusion in C/coS-1/S-2. Next, the results on the elongation performance of C/ coS-1/S-2 microspheres are presented. Fig. 4a represents the dominant elongated fibrous structure straight in the shape with the diameters of about 100 nm and 20 nm in a thicker central part and at the thinner ends, respectively. Similar pictures are obtained from C/S-1/S-2 sample (Fig. 4b). The presence of these structures proves that the essential requirement for the successful preparation of CNTs by polymer blend method, which is the elongation propensity of polymer microspheres, was achieved, and that the Poly(AN-co-MMA) copolymer shells as well as PAN shells became elongated at the melt-spinning process of C/coS-1/S-2 and C/S-1/S-2 microspheres, respectively. Other structures, however, were also observed. Fig. 4c shows the spherical structures, without any elongation performance, diameters of which vary in a wide range of 30 nm–200 nm. FE-SEM photographs of CNTs derived from C/coS1/S-2 and C/S-1/S-2 are shown in Fig. 5. The arrays of CNTs appear as the fibrous structures pulled out from the carbon block and oriented perpendicularly to the surface (Fig. 5a). Large fragments of the carbon blocks can also be seen. In addition, the spherical particles adhered to the carbon block are presented too (Fig. 5b). These must origin from the non-elongated microspheres at spinning, which retain the original spherical structures through the stabilization and the carbonization process. This result is in good agreement with the TEM study of spun and dissolved fibers that revealed
Fig. 5. FE-SEM photographs of carbonized samples from C/coS-1/S-2 and C/S-1/S-2: (a) CNTs from C/coS-1/S-2; (b) carbon spherical particles observed in sample derived from C/coS-1/S-2; (c) CNTs arrays prepared through the spinning of C/S-1/S-2 microspheres.
the presence of non-elongated spherical microspheres (Fig. 4c). For the comparison reasons, the FE-SEM photograph of CNTs from C/S-1/S-2 is displayed in Fig. 5c. One can clearly see the similarity between these CNTs and the ones derived from C/coS-1/S-2. Similar large arrays of CNTs on carbon blocks are present. CNTs were further observed with the transmission electron microscope and low-magnification TEM photographs are shown in Fig. 6. A typical photograph of
Fig. 4. SEM observation of spun fibers after the dissolution in THF: (a) elongated structures from C/coS-1/S-2; (b) elongated C/S-1/S-2; (c) nonelongated C/coS-1/S-2.
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Fig. 6. (a) CNTs derived from C/co/S-1/S-2 microspheres. Diameter of CNTs varies between 7 and 10 SSSnm; (b) CNTs from C/S-1/S-2 with the mean diameter of 10–20 nm.
straight CNTs derived from C/soS-1/S-2, about 7–10 nm in diameter, are displayed in Fig. 6a. No clear difference, except for the diameters, is noticed if compare these CNTs with the CNTs derived from C/S-1/S-2 (Fig. 6b). The CNTs from C/S-1/S-2 are also straight but the diameter larger, about 10–20 nm. However, the detailed HRTEM study revealed the structural differences between the CNTs derived from two different precursors. The most interesting difference is that CNTs from C/coS-1/S-2 often adhere to each other through the amorphous carbon covering their surfaces. Fig. 7 shows two examples. CNTs in Fig. 7a adhere parallel to each other while CNTs in Fig. 7b adhere through their tips in a certain angle. When compare these CNTs with the CNTs from C/S-1/S-2 (Fig. 7c), it is obvious that the graphene sheets are developed and ordered more even though the temperature and the duration of heat-treatment were the same. In addition, these CNTs are thinner as the mean diameter is 10 nm and as thin as 7 nm ones are present too. Differences may be attributed to more serious stretching and elongation at spinning whereas C/coS-1/S-2 exhibit higher spinnability than C/S-1/S-2 microspheres. A severe stretching of microspheres during the melt-spinning results in a preferred orientation of PAN molecules along the fiber axis. Similar mechanical stretching techniques are used practically to prepare PAN-based high-performance carbon fibers [14,15]. Another significant microstructural feature is the presence of internal compartments in CNTs from C/coS-1/S2. Taking into account the composition of C/coS-1/S-2 microspheres, SEM study, XPS analysis, and the above listed results on TEM and HRTEM, the following thought is our proposal for the formation of adherent CNTs with the internal compartments. In the first step of polymerization, uniform PMMA core microspheres result from the polymerization reaction of MMA. Further polymerized AN and MMA form a copolymer shell-1 consisted of AN and MMA fragments. According
to the amount of monomers, AN fragments are predominant. During subsequent third step of polymerization of MMA, when the formation of outer PMMA shell-2 is expected to proceed, a part of polymerized PMMA diffuse inside the core PMMA through the MMA fragments of copolymer shell-1. This behavior can be explained in the light of the principle of core/shell microspheres formation, i.e. the hydrophilic monomers tend to remain on the outside of the particle, more in contact with the aqueous phase, but hydrophobic monomers attempt to enter the hydrophobic core [2]. The relevance of this theory in our PMMA/PAN system was approved practically with the XPS analysis (presence of nitrogen in the outer layer) and SEM observation (decrease of the size uniformity of C/coS-1/S-2). For this reason the resultant microspheres are not covered entirely with the PMMA and therefore C/coS-1/S-2 microspheres might have a propensity to adhere to each other at melt-spinning process through the AN fragments, memory of which remains in the CNTs after stabilization and carbonization. The formation of internal compartments may result from the rearrangement of PAN molecules during the thermal decomposition of MMA fragments in Poly(AN-coMMA) shell-1. One may question why such a precise PMMA shell-2 coating is so important since we adopt a melt-processing method where all components may exist mixed in a molten state. Our argument lies in the basic ideas of polymer blend method and the results we have obtained so far. Through many trials and errors we found that in order to prepare nanotubes, the microspheres must be separated sufficiently by a matrix polymer. The results suggest that even though the polymers exist in a molten state, they retain the shape and the composition during the softening and the melting. Furthermore, the matrix and the core polymers are always chosen among the polymers with the good elongation performance and are preferably the same in order to avoid the lowering in spinnability. In addition, each time they are in
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Weight change[wt%]
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DTA
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TG -85
Heat flow [µV]
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200
400
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Fig. 8. TG-DTA profile of PMMA.
Fig. 7. HRTEM images of CNTs derived from C/coS-1/S-2 (a,b); C/S1/S-2 (c).
abundance compared to the carbon precursor polymer. Thus the elongation of matrix as well as core polymer also promotes the elongation performance of shell polymer at spinning. Since the presented concept implies the three-layered microspheres, where the outside PMMA layer plays the role of a matrix polymer, we conclude that the insufficient PMMA coating is responsible for the adhesion of CNTs as explained above. It may be possible to consider a possible usefulness of adhered CNTs in the areas such as gas adsorption, composites materials, etc. Another point to be discussed is the presence of carbon blocks. The reason of their ineligible formation is
not clarified yet, but a possible mechanism can be interpreted as follows: PAN in the inner part of spun fiber may be stabilized less than the PAN of the outer part because of the difficulty of oxygen diffusion for stabilization. The former PAN results in CNTs, but the latter PAN is semi-fused and reacts with PMMA to form carbon blocks. Moreover, PMMA is known as a thermally decomposable polymer which decomposes in not a simply way. TG-DTA profile of PMMA is shown in Fig. 8. It is clear from the TG curve that no carbon residue remains after 1000 °C since the decomposition occurs below 400 °C. Furthermore, DTA curve suggests that the thermal decomposition of PMMA is a complicate process consisting of several steps. Generally it occurs through the radical reaction when the end radical attacks the polymer chain causing the depolymerization of PMMA. Radical depolymerization involves the formation of MMA. PMMA involved in CNTs preparation, however, may behave differently during the thermal depolymerization according to the following thought: PMMA is surrounded by PAN and therefore the end radical of PMMA can show an ambition to attack the reactive nitrile group of PAN molecule. Hence such PMMA does not depolymerize according to the theory but rather forms the back-chains of PAN. It is well known that nitrile groups of PAN chain interact each other leading to the intramolecular polymerization and formation of the hexagonal structure [15]. This structure is a base of the graphene sheets formed at high temperature heat-treatment. Thence, the cycled PAN with the back-chained PMMA should have a different propensity to the graphene sheets development. It is believed, that such PAN resulted in the formation of amorphous carbon during the carbonization. The suppression of carbon blocks formation is required to complete the preparation method successfully.
4. Conclusions The use of co-polymer C/coS-1/S-2 microspheres allowed us to prepare CNTs with the structure different from the CNTs derived from C/S-1/S-2, i.e. the micro-
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spheres with the pure PAN shell-1. It was observed that CNTs often adhered to each other and contained the internal compartments. These differences were attributed to the insufficient PMMA shell-2 coating and the composition of shell-1 from which the CNTs derived. The spinnability of C/coS-1/S-2 was improved and the CNTs as thin as 7 nm could be observed by electron microscopy. More serious stretching of C/coS-1/S-2 during the meltspinning improved the crystallinity of the resulted CNTs. Presented results clearly showed the advantage of polymer blend method which is the possibility to control the structure of CNTs by controlling the composition of microspheres. We believe that the scope for the polymer blend method application in the preparation of various carbon materials is much wider and that this method will become a significant technique in carbon materials field.
Acknowledgment Authors are grateful to Mitsubishi Chemical Corp. for HRTEM.
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