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Stretching-induced crystallinity and orientation to improve the mechanical properties of electrospun PAN nanocomposites
Materials and Design 31 (2010) 1726–1730
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Stretching-induced crystallinity and orientation to improve the mechanical properties of electrospun PAN nanocomposites Xiaoxiao Hou a, Xiaoping Yang a, Liqun Zhang a, E. Waclawik b, Sizhu Wu a,* a
Key Laboratory of Beijing on ‘‘Preparation and Processing of Novel Polymer Materials”, College of Materials Science and Engineering, Beijing University of Chemical Technology, 100029 Beijing, China b Queensland University of Technology, 4001 Queensland, Australia
a r t i c l e
i n f o
Article history: Received 10 September 2008 Accepted 25 January 2009 Available online 3 August 2009
a b s t r a c t Polyacrylonitrile-based carbon fibers, embedded with single-walled carbon nanotubes have been prepared by the electrospinning technique. The as-spun nanofibers were hot-stretched in an oven to enhance the orientation and crystallinity which has been confirmed by X-ray diffraction and DSC etc. With the hot-stretched process and the introduction of SWNTs, the mechanical properties of PAN nanofibers such as the modulus and tensile strength will be enhanced correspondingly. In addition, the electrical conductivities of the PAN/SWNTs nanofiber composites were also enhanced. It was concluded that the hot-stretched nanofibers and the PAN/SWNTs nanofiber composites can be used as a potential precursor to produce high-performance carbon nanocomposites. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrospinning, a fiber spinning technique that relies on electrostatic forces to produce fibers having the diameters in the nanometer to micron diameter range, has been extensively explored as a simple method to prepare fibers from polymer solutions or melts [1,2]. Recently, carbon nanofibers, like other one-dimensional (1D) nanostructures such as nanowires, nanotubes, and molecular wires, have been receiving increasing attention because of their high aspect ratio [3,4]. This is due to their potential uses in composite reinforcing fillers, heat-management materials, high-temperature catalysts and components in nanoelectronics etc. [1,5]. Polyacrylonitrile (PAN) nanofiber is a common precursor of general carbon fibers. However, the applications of PAN nanofibers were hindered by the poor strength, attributed to their small diameters and unoptimized molecular orientation and crystallinity in the fibers [6]. To enhance the strength of the nanofibers, the single-walled carbon nanotubes (SWNTs) has been considered as ideal reinforced materials for modification of polymer by hybrid or nanocomposites due to their excellent mechanical properties, good electrical and thermal conductivity [7]. Also, stretching of the composites can improve the mechanical properties of polymer nanofibers [8]. Some researchers tried hot-stretched to improve the molecular orientation and the crystallinity of the nanofibers [9–12]. Therefore, in
* Corresponding author. Tel.: +86 10 64444923; fax: +86 10 64421186. E-mail address: [email protected] (S. Wu). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.01.051
the work presented here, PAN nanofibers containing SWNTs with concentrations from 0 to 1 wt.% were produced, and then hotstretched which demonstrated good dispersion of SWNTs and with high orientation and crystallinity of PAN molecules. 2. Experiment 2.1. Materials PAN used in this study included PAN/methyl acrylate/itaconic acid (93:5.3:1.7 w/w) (average molecular weight of 100,000 g/ mol) which was purchased from UK Courtaulds Ltd. Since N,Ndimethylformamide (DMF) is the common solvent of PAN [2,5,8,13,14] which can easily evaporate during the electrospinning, so in this study the DMF was selected as solvent. It was purchased from Beijing Chemical Plant Co. To uniformly disperse the SWNTs in the organic polymer matrix, the SWNTs were modified to form an individually polymer-wrapped structure. The SWNTs was wrapped by regioregular poly (3-hexyl thiophene (rrP3HT) which were obtained from the Queensland University of Technology [15]. As we know, polymer wrapped greatly inhibits the Van der Waals attraction between the polymer with solvent and the interactional polymer chains which normally observed between separate SWNTs with small ropes of SWNTs. These effects causes the wrapped nanotubes to be much more readily suspended at high-concentration SWNTs solutions and suspensions, which in turn substantially enables manipulation of SWNTs into bulk materials of many kinds, including films, fibers, solids, and composites of all kinds [16].
X. Hou et al. / Materials and Design 31 (2010) 1726–1730
2.2. Formation of electropun PAN nanofibers and PAN/SWNTs nanofiber composites SWNTs were dispersed in DMF at 40 °C with a fixed concentration for 12 h in bath sonication (KQ-250DB, Kunshan Ultrasonic Instrument Co., Ltd). SWNTs concentrations were 0.25%, 0.5%, 0.75%, 1% of the mass of the dissolved PAN, respectively. The PAN at a concentration of 12 wt% was added to the solution while stirring 12 h to obtain the well-dispersed solution. Then relatively aligned PAN nanofibers and PAN/SWNTs nanofiber composites were obtained by electrospinning. The voltage between the electrode and the counter electrode could be controlled by the high voltage power supply which here was set at 14–16 kV. The 0.16m perimeter collector rotated at a surface speed of about 6.6 m/s which the high speed rotating collector could align the nanofibers into the nanofiber sheets. 2.3. Hot-stretched During the electrospun process, however, the whirlpool jet from the pinhead to the collector made it difficult to get unidirectional alignment in a large-area sheet. Therefore, the electrospun nanofiber sheet needed a subsequent hot-stretched to improve the fiber alignment in the sheet. In this study, the PAN nanofibers and PAN/SWNTs nanofiber composites were hot-stretched according to the method proposed by Phillip and Johnson [17,18]. Both ends of the electrospun PAN/SWNTs sheet were clamped with pieces of graphite plates. Then one end was fixed to the ceiling of the oven and the other end was weighted by 75 g of metal poise to give a desired tension and elongation in the temperature-controlled oven at about 135 °C for 5 min. The stretching ratio, k, was calculated from k = L/L0, where L0 and L are the lengths of nanofiber sheets before and after the hot-stretched, respectively.
The percent crystallinity was obtained by extrapolation of the crystalline and amorphous parts of the diffraction pattern. The crystallite size was calculated by using the formula Lc = kk/(bcosh), with k = 0.89 and k = 1.54 Å, respectively, and b is the full wave at half maximum (FWHM) of the corresponding peak in the XRD curve [19]. 2.6. DSC DSC curves of electrospun nanofibers were obtained using a DSC STARe system by heating from 50 to 350 °C in N2 atmosphere at a heating rate of 10 K/min. The exothermic released heat can be achieved. 2.7. Mechanical properties Mechanical property testing was performed by using a LR30 K Electromechanical Universal Testing Machine (LLOYD company). The samples were prepared in 5 mm width and 20 mm length. The cross section areas of the samples were calculated via the weights of the samples and the densities of PAN and SWNTs. The tensile modulus, tensile strength, and elongation at break were obtained from the stress–strain curves. 2.7.1. Electrical conductivities Electrical conductivities of electrospun PAN/SWNTs nanofiber composites were measured using an ZC43 ultra-high resistance measuring machine (Shanghai Meter Plant Co., Ltd.) at room temperature and ambient condition. The electrical conductivities were , where qv is obtained according to the formula of qv ¼ Rv 21:23 t volume resistivity, Rv is resistance, and t is the thickness of the nanofiber.
2.4. Morphologies
3. Results and discussion
The morphologies and diameters of the PAN nanofibers and PAN/SWNTs nanofiber composites were observed by scanning electron microscopy(SEM, HITACHI S-4700 FEG-SEM). The diameters of electrospun nanofibers were analyzed with an image analyzer (Image J).
3.1. Morphologies
2.5. Crystallinities The crystallinities of the as-spun and hot-stretched PAN nanofibers were investigated with X-ray diffractometer (XRD, Rigaku D/ max 2500VB2+/PC), operated at 40 kV and 200 mA to produce CuKa radiation (k = 1.54 Å).
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Fig. 1 shows the SEM of PAN nanofibers and PAN/SWNTs nanofiber composites with different SWNTs concentration. The nanofibers were very long and straight with the diameters ranging from 100 to 300 nm. From Fig. 1, it can be seen that there was no conglutination in the nanofibers, which proved that the SWNTs were dispersed well in the composites. When the concentration of the SWNTs increased, the surface of the composite fibers became rough, which indicated that at high concentration some SWNTs might not completely embedded into the nanofiber matrix [20].
Fig. 1. SEM micrographs: (a) pure PAN, (b) 0.5% SWNTs and (c) 1% SWNTs.
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3.2. Crystallinities Fig. 2a and b showed X-ray diffraction (XRD) patterns from the as-spun and the hot-stretched nanofibers, respectively. The diffraction pattern showed two equatorial peaks which one was at 2h = 29.5° corresponding to a spacing of d 3.03 Å from the (1 1 0) reflection and another was at 2h = 17.0° corresponding to a spacing of d 5.3 Å from the (1 0 0) reflection. These equatorial peaks are common to the fiber diffraction pattern of PAN with hexagonal crystal system [2]. The diffraction pattern of the as-spun nanofiber showed the weak peak with the value of 2h at 17.0°. This indicated that electrospinning of the nanofibers onto a rotating drum generates limited crystallinity. In contrast, the hot-stretched nanofibers showed two diffraction peaks indexed with values of 2h
of 17.0° and 29.5°. And it also can be found that the peak at 2h = 29.5° became much bigger after the process of hot-stretched. Table 1 presented the values of the percent crystallinity and the average crystallite sizes for PAN nanofibers. The percent crystallinity of the hot-stretched nanofiber increased about 3 times in comparison with those of as-spun nanofiber. The crystallite size also increased about 162%. 3.3. Thermal behaviors Generally, PAN begins to degrade when being heated near its melting point. The degradation reaction of PAN is so exothermic that it tends to obscure its melting endotherm in ordinary DSC traces. Therefore, the melting endotherm is normally not observed in PAN. When the PAN precursor is heated above 180 °C, reactions such as cyclization, dehydrogenation, and oxidation take place. These reactions are exothermal, hence a sharp peak appears in the DSC curve at 260–290 °C [21]. Fig. 3a and b showed the DSC curves of PAN and PAN/SWNTs nanofiber composites, respectively. From Fig. 3, it can be seen that
Fig. 2. X-ray diffraction patterns for the nanofibers: (a) as-spun, (b) hot-stretched.
Table 1 Percent crystallinity and crystallite size obtained from X-ray diffraction curves. Nanofiber
Crystallinity (%)
Crystallite size (nm)
As-spun Hot-stretched
11.27 38.34
4.14 10.83
Fig. 3. DSC curves of electrospun nanofibers: (a) PAN nanofibers, (b) PAN/SWNTs nanofiber composites.
Fig. 4. DSC curves of PAN nanofiber sheets before and after hot-stretched: (a) asspun, (b) hot-stretched.
Fig. 5. Stress–strain curves of PAN nanofiber sheets before and after hot-stretched: (a) as-spun, (b) hot-stretched.
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X. Hou et al. / Materials and Design 31 (2010) 1726–1730 Table 2 Mechnical properties of PAN nanofiber sheets. Sample
Tensile strength (MPa)
Impro. (%)
Tensile modulus (GPa)
Impro. (%)
Elongation at break (%)
As-spun hot-stretched
51.84 80.52
55.32
1.08 2.77
156.48
22.05 11.61
process of hot-stretched, the PAN molecular chain moved and arranged again along the fiber axis, the orientation and crystallinity were also improved. So the mechanical properties of PAN nanofibers were improved due to the improvement of orientation and crystallinity. Fig. 6 shows the typical stress–strain curves of the PAN nanofibers and PAN/SWNTs nanofiber composites. SWNTs improved the modulus and tensile strength of the nanofiber. The tensile strength 128.76 MPa of the nanocomposites at about 0.75% SWNTs by weight is increased with 58.9%. And also the tensile modulus showed a peak value of 4.62 GPa with 66.8% improvement. The (e) curve in Fig. 6 is departure from the trend, which might be the uncompleted uniform dispersion of SWNTs at high concentration. 3.5. Electrical conductivities
Fig. 6. Stress–strain curves for PAN and PAN/SWNTs Nanofiber: (a) pure PAN, (b) 0.25% SWNTs, (c) 0.5% SWNTs, (d) 0.75% SWNTs, (e) 1% SWNTs.
the exothermic released heat increased from 510.53 J/g to 548.45 J/ g. The improvement of crystallinity may result in the improvement of the exothermic released heat. As stated in the literature [22], the effect of carbon nanotubes on crystallization of PAN is the combination of several factors, namely addition of SWNTs can result in the improvement of the crystallinity, so the addition of SWNTs in PAN increased the exothermic released heat. Fig. 4a and b showed the DSC curves of PAN nanofiber sheets before and after hot-stretched, respectively. It can be found that the peak became broader and a little bigger than that of pure PAN nanofiber, the cyclization exothermic released heat increased from 510.53 J/g to 530.42 J/g. As has been stated above, after the process of hot-stretched, the degree of crystallinity improved, so the peak became bigger correspondingly. 3.4. Mechanical properties In this work, we will have a better representation of the nanocomposites characteristics by measuring the macroscopic nanofiber sheet, not just an individual fiber within the composite. The traditional stress–strain curves using stretching test could be used to evaluate the mechanical properties of the electrospun PAN nanofibers and PAN/SWNTs nanofiber composites. Therefore, the mechanical properties measured here will be indicative the application properties of the nanocomposites. Stretching a piece of the nanofiber sheet gives an assessment of the average mechanical properties of the nanofibers rather than measuring an individual segment of a nanofiber composite [8]. Typical stress–strain curves of PAN nanofiber sheets before and after hot-stretched, respectively are presented in Fig. 5. Hotstretched process improved the modulus and tensile strength of PAN nanofiber sheets. The tensile modulus and tensile strength increased by 156.48% and 55.32%, respectively (see Table 2). It can be concluded that the hot-stretched method can improve the mechanical properties of PAN composite nanofibers. During the
The electrical conductivity of the pure PAN nanofiber is 0.2– 0.5 S/cm [5]. Due to the superb electrical properties of SWNTs, a better electrical conductivity in PAN/SWNTs nanofiber composites was expected. Since electrical conductivity requires a percolating network be formed by the SWNTs, it can be concluded that the composite nanofibers, at a concentration of 1 wt.% SWNTs, has formed the percolating network so that the PAN/SWNTs nanofiber composites could possess electrical conductivity of up to 2.5 S/cm. Therefore these conductive PAN/SWNTs nanofiber sheets have potential applications in conductive nanoelectrodes, supercapacitors and nanosensors [5]. 4. Conclusions PAN nanofibers and PAN/SWNTs nanofiber composites were prepared by electrospinning from PAN/N,N-dimethylformamide solution. Hot-stretched method was used to increase the degree of crystallinity and molecular orientation of PAN nanofibers and PAN/SWNTs nanofiber composites. The crystallinity of the stretched sheet confirmed by X-ray diffraction has enhanced about 3 times in comparison with those of as-spun sheets. The improved fiber alignment and crystallinity resulted in the increased modulus and tensile strength. Incorporation of SWNTs into the nanofibers increased the electrical conductivity to 2.5 S/cm for PAN/1% SWNTs nanofiber composites. Thus, the hot-stretched nanofibers and the nanofiber composites with the component of SWNTs can be used as the potential precursor to produce high-performance carbon nanofibers. The mechanical properties of the PAN nanofibers and PAN/SWNTs nanofiber composites can be improved more by extensive studies of electrospinning and hot-stretched conditions. References [1] Hammel E, Tang X, Trampert M, Schmitt T, Mauthner K, Eder A, et al. Nanofibers for composite applications. Carbon 2004;42:1153–8. [2] Fennessey SF, Farris RJ. Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns. Polymer 2004;45:4217–25. [3] Katta P, Alessandro M, Ramsier RD, Chase GG. Continuous electrospinning of aligned polymer nanofibers onto a wire drum collector. Nano Letters 2004;4:2215–8. [4] Lau KT, Gu C, Hui D. A Critical Review on Nanotube and Nanotube/Nanoclay related Polymer Composite Materials. Compos Port B 2006;37:425–36.
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[5] Ge JJ, Hou HQ, Li Q, Graham MJ, Greiner A, Reneker DH, et al. Assembly of wellaligned multi-walled carbon nanotubes in confined polyacrylonitrile environments: electrospun composite nanofiber sheets. J Am Chem Soc 2004;126:15754–61. [6] Ra EJ, An KH, Kim KK, Jeong S, Lee YH. Anisotropic electrical conductivity of MWCNT/PAN nanofiber paper. Chem Phys Lett 2005;413:188–93. [7] Chand S. Review Carbon fibers for composites. J Mater Sci 2000;35:1303–13. [8] Hou HQ, Zeng J, Li Q. Electrospun polyacrylonitrile nanofibers containing a high concentration of well-aligned multiwall carbon nanotubes. Chem Mater 2005;17:967–73. [9] McCann JT, Marquez M, Xia Y. Highly porous fibers by electrospinning into a cryogenic liquid. J Am Chem Soc 2006;128:1436–7. [10] Dror Y, Salalha W, Khalfin RL, Cohen Y, Yarin AL, Zussman E. Carbon nanotubes embedded in oriented polymer nanofibers by electrospinning. Langmuir 2003;19:7012–20. [11] Maensiri S, Nuansing W. Thermoelectric oxide NaCo2 O4 nanofibers fabricated by electrospinning. Mater Chem Phys 2006;99:104–8. [12] Watt W, Perov BV. Handbook of composites. Amsterdam: Elsevier Science; 1985. [13] Zussman E, Chen X, Ding W, Calabri L, Dikin DA, Quintana JP, Ruoff RS. Mechanical and structural characterization of electrospun PAN-derived carbon nanofibers. Carbon 2005;43:2175–85.
[14] Lau KT, Lu M, Lam CK, Cheung HY, Sheng FL, Li HL. Thermal and mechanical properties of single-walled carbon nanotube bundle-reinforced epoxy nanocomposites: the role of solvent for nanotube dispersion. Compos Sci Technol 2005;65:719–25. [15] Waclawik ER, John BJ, Goh SGR, Anthony M, Nunzio M. Self-organization in composites of poly(3-hexylthiophene) and single-walled carbon nanotubes designed for use in photovoltaic applications. Proc of SPIE 2006;6036: 603607–14. [16] Smalley RE, Colbert DT, Smith KA, Michael OC. US10755490 2004. [17] Johnson J, Phillips LN, Watt W. Brit Patent 1,110,790 1965. [18] Johnson J, Watt W, Phillips LN, Moreton R., Brit Patent 1,166,251 1966. [19] Norman WH, Cheetham WH, Tao LP. Variation in crystalline type with amylose content in maize starch granules: an X-ray powder diffraction study. Carbohyd Polym 1998;36:277–84. [20] Lau KT. Interfacial bonding characteristics of nanotube/polymer composites. Chem Phys Lett 2003;370:399–405. [21] Gu SY, Ren J, Wu QL. Preparation and structures of electrospun PAN nanofibers as a precursor of carbon nanofibers. Synthetic Metals 2005;155: 157–61. [22] Kanagaraj S, Fa0 tima RV, Varanda VZ, Mo0 nica SAO, Jose0 AOS. Mechanical properties of high density polyethylene/carbon nanotube composites. Compos Sci Technol 2007;67:3071–7.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Stretching-induced crystallinity and orientation to improve the mechanical properties of electrospun PAN nanocomposites Xiaoxiao Hou a, Xiaoping Yang a, Liqun Zhang a, E. Waclawik b, Sizhu Wu a,* a
Key Laboratory of Beijing on ‘‘Preparation and Processing of Novel Polymer Materials”, College of Materials Science and Engineering, Beijing University of Chemical Technology, 100029 Beijing, China b Queensland University of Technology, 4001 Queensland, Australia
a r t i c l e
i n f o
Article history: Received 10 September 2008 Accepted 25 January 2009 Available online 3 August 2009
a b s t r a c t Polyacrylonitrile-based carbon fibers, embedded with single-walled carbon nanotubes have been prepared by the electrospinning technique. The as-spun nanofibers were hot-stretched in an oven to enhance the orientation and crystallinity which has been confirmed by X-ray diffraction and DSC etc. With the hot-stretched process and the introduction of SWNTs, the mechanical properties of PAN nanofibers such as the modulus and tensile strength will be enhanced correspondingly. In addition, the electrical conductivities of the PAN/SWNTs nanofiber composites were also enhanced. It was concluded that the hot-stretched nanofibers and the PAN/SWNTs nanofiber composites can be used as a potential precursor to produce high-performance carbon nanocomposites. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrospinning, a fiber spinning technique that relies on electrostatic forces to produce fibers having the diameters in the nanometer to micron diameter range, has been extensively explored as a simple method to prepare fibers from polymer solutions or melts [1,2]. Recently, carbon nanofibers, like other one-dimensional (1D) nanostructures such as nanowires, nanotubes, and molecular wires, have been receiving increasing attention because of their high aspect ratio [3,4]. This is due to their potential uses in composite reinforcing fillers, heat-management materials, high-temperature catalysts and components in nanoelectronics etc. [1,5]. Polyacrylonitrile (PAN) nanofiber is a common precursor of general carbon fibers. However, the applications of PAN nanofibers were hindered by the poor strength, attributed to their small diameters and unoptimized molecular orientation and crystallinity in the fibers [6]. To enhance the strength of the nanofibers, the single-walled carbon nanotubes (SWNTs) has been considered as ideal reinforced materials for modification of polymer by hybrid or nanocomposites due to their excellent mechanical properties, good electrical and thermal conductivity [7]. Also, stretching of the composites can improve the mechanical properties of polymer nanofibers [8]. Some researchers tried hot-stretched to improve the molecular orientation and the crystallinity of the nanofibers [9–12]. Therefore, in
* Corresponding author. Tel.: +86 10 64444923; fax: +86 10 64421186. E-mail address: [email protected] (S. Wu). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.01.051
the work presented here, PAN nanofibers containing SWNTs with concentrations from 0 to 1 wt.% were produced, and then hotstretched which demonstrated good dispersion of SWNTs and with high orientation and crystallinity of PAN molecules. 2. Experiment 2.1. Materials PAN used in this study included PAN/methyl acrylate/itaconic acid (93:5.3:1.7 w/w) (average molecular weight of 100,000 g/ mol) which was purchased from UK Courtaulds Ltd. Since N,Ndimethylformamide (DMF) is the common solvent of PAN [2,5,8,13,14] which can easily evaporate during the electrospinning, so in this study the DMF was selected as solvent. It was purchased from Beijing Chemical Plant Co. To uniformly disperse the SWNTs in the organic polymer matrix, the SWNTs were modified to form an individually polymer-wrapped structure. The SWNTs was wrapped by regioregular poly (3-hexyl thiophene (rrP3HT) which were obtained from the Queensland University of Technology [15]. As we know, polymer wrapped greatly inhibits the Van der Waals attraction between the polymer with solvent and the interactional polymer chains which normally observed between separate SWNTs with small ropes of SWNTs. These effects causes the wrapped nanotubes to be much more readily suspended at high-concentration SWNTs solutions and suspensions, which in turn substantially enables manipulation of SWNTs into bulk materials of many kinds, including films, fibers, solids, and composites of all kinds [16].
X. Hou et al. / Materials and Design 31 (2010) 1726–1730
2.2. Formation of electropun PAN nanofibers and PAN/SWNTs nanofiber composites SWNTs were dispersed in DMF at 40 °C with a fixed concentration for 12 h in bath sonication (KQ-250DB, Kunshan Ultrasonic Instrument Co., Ltd). SWNTs concentrations were 0.25%, 0.5%, 0.75%, 1% of the mass of the dissolved PAN, respectively. The PAN at a concentration of 12 wt% was added to the solution while stirring 12 h to obtain the well-dispersed solution. Then relatively aligned PAN nanofibers and PAN/SWNTs nanofiber composites were obtained by electrospinning. The voltage between the electrode and the counter electrode could be controlled by the high voltage power supply which here was set at 14–16 kV. The 0.16m perimeter collector rotated at a surface speed of about 6.6 m/s which the high speed rotating collector could align the nanofibers into the nanofiber sheets. 2.3. Hot-stretched During the electrospun process, however, the whirlpool jet from the pinhead to the collector made it difficult to get unidirectional alignment in a large-area sheet. Therefore, the electrospun nanofiber sheet needed a subsequent hot-stretched to improve the fiber alignment in the sheet. In this study, the PAN nanofibers and PAN/SWNTs nanofiber composites were hot-stretched according to the method proposed by Phillip and Johnson [17,18]. Both ends of the electrospun PAN/SWNTs sheet were clamped with pieces of graphite plates. Then one end was fixed to the ceiling of the oven and the other end was weighted by 75 g of metal poise to give a desired tension and elongation in the temperature-controlled oven at about 135 °C for 5 min. The stretching ratio, k, was calculated from k = L/L0, where L0 and L are the lengths of nanofiber sheets before and after the hot-stretched, respectively.
The percent crystallinity was obtained by extrapolation of the crystalline and amorphous parts of the diffraction pattern. The crystallite size was calculated by using the formula Lc = kk/(bcosh), with k = 0.89 and k = 1.54 Å, respectively, and b is the full wave at half maximum (FWHM) of the corresponding peak in the XRD curve [19]. 2.6. DSC DSC curves of electrospun nanofibers were obtained using a DSC STARe system by heating from 50 to 350 °C in N2 atmosphere at a heating rate of 10 K/min. The exothermic released heat can be achieved. 2.7. Mechanical properties Mechanical property testing was performed by using a LR30 K Electromechanical Universal Testing Machine (LLOYD company). The samples were prepared in 5 mm width and 20 mm length. The cross section areas of the samples were calculated via the weights of the samples and the densities of PAN and SWNTs. The tensile modulus, tensile strength, and elongation at break were obtained from the stress–strain curves. 2.7.1. Electrical conductivities Electrical conductivities of electrospun PAN/SWNTs nanofiber composites were measured using an ZC43 ultra-high resistance measuring machine (Shanghai Meter Plant Co., Ltd.) at room temperature and ambient condition. The electrical conductivities were , where qv is obtained according to the formula of qv ¼ Rv 21:23 t volume resistivity, Rv is resistance, and t is the thickness of the nanofiber.
2.4. Morphologies
3. Results and discussion
The morphologies and diameters of the PAN nanofibers and PAN/SWNTs nanofiber composites were observed by scanning electron microscopy(SEM, HITACHI S-4700 FEG-SEM). The diameters of electrospun nanofibers were analyzed with an image analyzer (Image J).
3.1. Morphologies
2.5. Crystallinities The crystallinities of the as-spun and hot-stretched PAN nanofibers were investigated with X-ray diffractometer (XRD, Rigaku D/ max 2500VB2+/PC), operated at 40 kV and 200 mA to produce CuKa radiation (k = 1.54 Å).
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Fig. 1 shows the SEM of PAN nanofibers and PAN/SWNTs nanofiber composites with different SWNTs concentration. The nanofibers were very long and straight with the diameters ranging from 100 to 300 nm. From Fig. 1, it can be seen that there was no conglutination in the nanofibers, which proved that the SWNTs were dispersed well in the composites. When the concentration of the SWNTs increased, the surface of the composite fibers became rough, which indicated that at high concentration some SWNTs might not completely embedded into the nanofiber matrix [20].
Fig. 1. SEM micrographs: (a) pure PAN, (b) 0.5% SWNTs and (c) 1% SWNTs.
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3.2. Crystallinities Fig. 2a and b showed X-ray diffraction (XRD) patterns from the as-spun and the hot-stretched nanofibers, respectively. The diffraction pattern showed two equatorial peaks which one was at 2h = 29.5° corresponding to a spacing of d 3.03 Å from the (1 1 0) reflection and another was at 2h = 17.0° corresponding to a spacing of d 5.3 Å from the (1 0 0) reflection. These equatorial peaks are common to the fiber diffraction pattern of PAN with hexagonal crystal system [2]. The diffraction pattern of the as-spun nanofiber showed the weak peak with the value of 2h at 17.0°. This indicated that electrospinning of the nanofibers onto a rotating drum generates limited crystallinity. In contrast, the hot-stretched nanofibers showed two diffraction peaks indexed with values of 2h
of 17.0° and 29.5°. And it also can be found that the peak at 2h = 29.5° became much bigger after the process of hot-stretched. Table 1 presented the values of the percent crystallinity and the average crystallite sizes for PAN nanofibers. The percent crystallinity of the hot-stretched nanofiber increased about 3 times in comparison with those of as-spun nanofiber. The crystallite size also increased about 162%. 3.3. Thermal behaviors Generally, PAN begins to degrade when being heated near its melting point. The degradation reaction of PAN is so exothermic that it tends to obscure its melting endotherm in ordinary DSC traces. Therefore, the melting endotherm is normally not observed in PAN. When the PAN precursor is heated above 180 °C, reactions such as cyclization, dehydrogenation, and oxidation take place. These reactions are exothermal, hence a sharp peak appears in the DSC curve at 260–290 °C [21]. Fig. 3a and b showed the DSC curves of PAN and PAN/SWNTs nanofiber composites, respectively. From Fig. 3, it can be seen that
Fig. 2. X-ray diffraction patterns for the nanofibers: (a) as-spun, (b) hot-stretched.
Table 1 Percent crystallinity and crystallite size obtained from X-ray diffraction curves. Nanofiber
Crystallinity (%)
Crystallite size (nm)
As-spun Hot-stretched
11.27 38.34
4.14 10.83
Fig. 3. DSC curves of electrospun nanofibers: (a) PAN nanofibers, (b) PAN/SWNTs nanofiber composites.
Fig. 4. DSC curves of PAN nanofiber sheets before and after hot-stretched: (a) asspun, (b) hot-stretched.
Fig. 5. Stress–strain curves of PAN nanofiber sheets before and after hot-stretched: (a) as-spun, (b) hot-stretched.
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X. Hou et al. / Materials and Design 31 (2010) 1726–1730 Table 2 Mechnical properties of PAN nanofiber sheets. Sample
Tensile strength (MPa)
Impro. (%)
Tensile modulus (GPa)
Impro. (%)
Elongation at break (%)
As-spun hot-stretched
51.84 80.52
55.32
1.08 2.77
156.48
22.05 11.61
process of hot-stretched, the PAN molecular chain moved and arranged again along the fiber axis, the orientation and crystallinity were also improved. So the mechanical properties of PAN nanofibers were improved due to the improvement of orientation and crystallinity. Fig. 6 shows the typical stress–strain curves of the PAN nanofibers and PAN/SWNTs nanofiber composites. SWNTs improved the modulus and tensile strength of the nanofiber. The tensile strength 128.76 MPa of the nanocomposites at about 0.75% SWNTs by weight is increased with 58.9%. And also the tensile modulus showed a peak value of 4.62 GPa with 66.8% improvement. The (e) curve in Fig. 6 is departure from the trend, which might be the uncompleted uniform dispersion of SWNTs at high concentration. 3.5. Electrical conductivities
Fig. 6. Stress–strain curves for PAN and PAN/SWNTs Nanofiber: (a) pure PAN, (b) 0.25% SWNTs, (c) 0.5% SWNTs, (d) 0.75% SWNTs, (e) 1% SWNTs.
the exothermic released heat increased from 510.53 J/g to 548.45 J/ g. The improvement of crystallinity may result in the improvement of the exothermic released heat. As stated in the literature [22], the effect of carbon nanotubes on crystallization of PAN is the combination of several factors, namely addition of SWNTs can result in the improvement of the crystallinity, so the addition of SWNTs in PAN increased the exothermic released heat. Fig. 4a and b showed the DSC curves of PAN nanofiber sheets before and after hot-stretched, respectively. It can be found that the peak became broader and a little bigger than that of pure PAN nanofiber, the cyclization exothermic released heat increased from 510.53 J/g to 530.42 J/g. As has been stated above, after the process of hot-stretched, the degree of crystallinity improved, so the peak became bigger correspondingly. 3.4. Mechanical properties In this work, we will have a better representation of the nanocomposites characteristics by measuring the macroscopic nanofiber sheet, not just an individual fiber within the composite. The traditional stress–strain curves using stretching test could be used to evaluate the mechanical properties of the electrospun PAN nanofibers and PAN/SWNTs nanofiber composites. Therefore, the mechanical properties measured here will be indicative the application properties of the nanocomposites. Stretching a piece of the nanofiber sheet gives an assessment of the average mechanical properties of the nanofibers rather than measuring an individual segment of a nanofiber composite [8]. Typical stress–strain curves of PAN nanofiber sheets before and after hot-stretched, respectively are presented in Fig. 5. Hotstretched process improved the modulus and tensile strength of PAN nanofiber sheets. The tensile modulus and tensile strength increased by 156.48% and 55.32%, respectively (see Table 2). It can be concluded that the hot-stretched method can improve the mechanical properties of PAN composite nanofibers. During the
The electrical conductivity of the pure PAN nanofiber is 0.2– 0.5 S/cm [5]. Due to the superb electrical properties of SWNTs, a better electrical conductivity in PAN/SWNTs nanofiber composites was expected. Since electrical conductivity requires a percolating network be formed by the SWNTs, it can be concluded that the composite nanofibers, at a concentration of 1 wt.% SWNTs, has formed the percolating network so that the PAN/SWNTs nanofiber composites could possess electrical conductivity of up to 2.5 S/cm. Therefore these conductive PAN/SWNTs nanofiber sheets have potential applications in conductive nanoelectrodes, supercapacitors and nanosensors [5]. 4. Conclusions PAN nanofibers and PAN/SWNTs nanofiber composites were prepared by electrospinning from PAN/N,N-dimethylformamide solution. Hot-stretched method was used to increase the degree of crystallinity and molecular orientation of PAN nanofibers and PAN/SWNTs nanofiber composites. The crystallinity of the stretched sheet confirmed by X-ray diffraction has enhanced about 3 times in comparison with those of as-spun sheets. The improved fiber alignment and crystallinity resulted in the increased modulus and tensile strength. Incorporation of SWNTs into the nanofibers increased the electrical conductivity to 2.5 S/cm for PAN/1% SWNTs nanofiber composites. Thus, the hot-stretched nanofibers and the nanofiber composites with the component of SWNTs can be used as the potential precursor to produce high-performance carbon nanofibers. The mechanical properties of the PAN nanofibers and PAN/SWNTs nanofiber composites can be improved more by extensive studies of electrospinning and hot-stretched conditions. References [1] Hammel E, Tang X, Trampert M, Schmitt T, Mauthner K, Eder A, et al. Nanofibers for composite applications. Carbon 2004;42:1153–8. [2] Fennessey SF, Farris RJ. Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns. Polymer 2004;45:4217–25. [3] Katta P, Alessandro M, Ramsier RD, Chase GG. Continuous electrospinning of aligned polymer nanofibers onto a wire drum collector. Nano Letters 2004;4:2215–8. [4] Lau KT, Gu C, Hui D. A Critical Review on Nanotube and Nanotube/Nanoclay related Polymer Composite Materials. Compos Port B 2006;37:425–36.
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