Preparation of Single-Walled Carbon Nanotube Reinforced

Polyurethane Nanofibers and. Membranes by Electrospinning. Rahul Sen, Bin Zhao, Daniel Perea, Mikhail E. Itkis, Hui Hu, James Love,. Elena Bekyarova, ...
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NANO LETTERS

Preparation of Single-Walled Carbon Nanotube Reinforced Polystyrene and Polyurethane Nanofibers and Membranes by Electrospinning

2004 Vol. 4, No. 3 459-464

Rahul Sen, Bin Zhao, Daniel Perea, Mikhail E. Itkis, Hui Hu, James Love, Elena Bekyarova, and Robert C. Haddon* Center for Nanoscale Science and Engineering, Departments of Chemistry and Chemical & EnVironmental Engineering, UniVersity of California, RiVerside, California 92521-0403 Received December 5, 2003; Revised Manuscript Received December 19, 2003

ABSTRACT Single-walled carbon nanotube (SWNT) reinforced polymer composite membranes have been fabricated using the electrospinning technique. Nanofibers with a diameter in the range 50−100 nm were obtained by electrospinning SWNT-filled polystyrene composites. TEM observations revealed incorporation of small SWNT bundles oriented parallel to the nanofiber axis. As-prepared (AP) and ester (EST) functionalized SWNTs have been electrospun with polyurethane (PU) to demonstrate the effect of the chemical functionalization of SWNTs on the mechanical properties of SWNT-reinforced composites. The tensile strength of EST-SWNT-PU membranes is enhanced by 104% as compared to electrospun pure polyurethane membranes, while an increase of only 46% was achieved incorporating AP−SWNT in the polyurethane matrix. The tangent moduli of AP- and EST-SWNT-PU membranes were found to be respectively 215% and 250% higher than the control polyurethane membranes.

Single-walled carbon nanotubes (SWNTs) have attracted much attention in the past decade due to their unique onedimensional structure and a range of fascinating properties.1,2 SWNTs are considered to be the ideal reinforcing fibers due to their exceptional mechanical properties, low density, and high aspect ratio.3 Theoretical and experimental studies have shown that SWNTs have extremely high Young moduli, similar to that of graphite in-plane (∼1000 GPa).4-6 Therefore, SWNT-reinforced polymer composites have potential applications in defense and aerospace where high strength and lightweight components are of primary importance.7 In addition to their mechanical properties, the electrical properties of SWNTs can be utilized to impart conductivity to nonconducting polymers,8 thereby improving electrostatic charge dissipation and electromagnetic shielding efficiency. The exceptional properties of SWNTs have prompted intensive studies on high-performance composites based on this material.8-20 The incorporation of SWNTs has been demonstrated to significantly improve the mechanical properties of composite fibers, and SWNT-reinforced composite materials with extremely high Young moduli and tensile strengths have been reported.9,14,16,21 The addition of 10 wt. % SWNTs to poly(p-phenylene benzobisoxazole) (PBO) has been demonstrated to increase the tensile strength by about * Corresponding author. E-mail: [email protected]. 10.1021/nl035135s CCC: $27.50 Published on Web 01/30/2004

© 2004 American Chemical Society

50% in comparison to pure PBO and reduce the shrinkage and high-temperature creep.13 An epoxy composite containing 1 wt % of acid-treated and fluorinated SWNTs led to 30% increase in Young’s modulus and 18% increase in tensile strength.20 Loading of 4 wt % fluorinated SWNTs to poly(ethylene oxide) has been reported to enhance the storage modulus at room temperature by 400%.22 These studies demonstrate the potential of SWNTs as reinforcing material for polymers and point to the critical issues in the development of high-performance SWNT composites. It is widely recognized that the fabrication of high-performance SWNT composites depends on the efficient load transfer from the host polymer matrix to SWNTs. The load transfer requires homogeneous dispersion of SWNTs in the host matrix, incorporation of individual tubes and strong interfacial bonding between the nanotubes and polymer. To address these issues several strategies for synthesis of SWNTreinforced composites have been developed. Currently, these strategies involve physical mixing of SWNTs in solutions of preformed polymers,11,18,23 in-situ polymerization in the presence of SWNTs,8,13,24 surfactant-assisted processing of SWNT-polymer composites,9,15,16,21,25 and chemical modification of the incorporated SWNTs.17,20,22,26,27 Although significant progress in the dispersion of SWNTs in polymer matrix has been made, alignment of the nanotubes

Scheme 1. Ester-Functionalized SWNTs (SWNT-COO(CH2)11CH3, EST-SWNTs)31

Figure 1. Schematic representation of the electrospinning or electrospraying apparatus.

in the fiber and the manufacturing of useful macroscopic structures remain a challenge. Most commonly used techniques for fabrication of SWNT-composite materials rely on solution casting,8,17,19,22 melt processing,28 and wet spinning.9,13,21 Recently, SWNT-filled nanofiber yarns have been produced by the electrospinning technique.18,29 This technique provides a very useful way to bridge the dimensional gap between the SWNTs and engineering materials; however, bulk mechanical properties of the composites prepared in this manner have not been reported. In this paper we report the fabrication of membranes of SWNT-filled polystyrene and polyurethane by the electrospinning/electrospraying technique. As-prepared (AP) SWNTs30 and ester-functionalized SWNTs (SWNT-COO(CH2)11CH3, Scheme 1) were used to fabricate nanocomposites. The use of SWNTs with different functionalities allows us to begin to study the importance of the interfacial interaction between the SWNTs and the polymer matrix in forming high-strength SWNT composites. We report the preparation of nanocomposites with homogeneously dispersed SWNTs and efficient load transfer from the polymer matrix to the SWNTs. SWNT-reinforced nanocomposites with significantly improved mechanical strength have been fabricated using the electrospinning technique. Electrospinning is a technique to fabricate nonwoven fibers with submicron diameters that can find applications in coatings, membranes, fabrics, and scaffolds.32,33 This technique has been used to manufacture man-made fibers since 193434 and is applicable to a wide variety of polymers and composite polymers.35-40 In the electrospinning technique (Figure 1) polymer nanofibers are produced from an electrostatically driven jet of polymer solution (or melt). The discharged polymer solution jet undergoes a whipping process wherein the solvent evaporates and the highly stretched polymer fiber deposits on a grounded target. A number of experimental parameters control the fiber diameter and morphology. The nanofibers can be fabricated into a variety of forms such as membranes, coatings and films. The membranes or coatings can be deposited onto targets of different shapes. 460

The polymers used in this study (polystyrene and polyurethane) are of significant industrial importance. Polystyrene (PS) is a common thermoplastic polymer, which is tough and brittle. Strength and resilience of PS can be improved by graft copolymerization with polybutadiene and polyacrylonitrile. Such copolymeric materials, commonly known as hi-impact polystyrene (HIPS) and poly-acrylonitrile-butadiene-styrene (ABS), have applications in automobile dashboards, computer and telephone housings, and refrigerator lining. Carbon nanotube reinforced PS has been widely studied.10,27,41 Polyurethane (PU) is a resilient elastomer that has a wide range of applications as coatings, sealants, transdermal patches, and catheters. In this work, solutions of the host polymers in DMF with dispersed SWNTs were electrospun to produce nanocomposite materials. The host polymers, PS (average molecular weight 280 000) and poly[4,4′-methylenebis(phenyl isocyanate)-alt-1,4-butanediol/poly(butylene adipate)] (polyurethane, PU) were used as received from Aldrich. The polymers were dissolved in dimethylformamide (DMF) to obtain standard solutions of concentration 10 mg/mL. The polymer solutions were used directly in the electrospinning process to make pure polymer membranes. For composite fabrication, 2.5 mg of AP-SWNTs (Carbon Solutions, Inc., www.carbonsolution.com) or EST-SWNTs were added to 25 mL of the polymer solution in DMF (10 mg/mL) to obtain a SWNT-to-polymer weight ratio of 1:100. The mixture was sonicated in a bath sonicator (VWR, model 50HT, 45 W) for 4-5 h to obtain homogeneous dispersion. The dispersions were directly used in the electrospinning process. The required volume (10 mL) of the solution was loaded into a syringe fitted to a syringe pump. The positive terminal of a Spellmann high voltage DC power supply was connected to the metallic needle of the syringe. A grounded stainless steel sheet placed 3 cm from the tip of the needle was used as the target to deposit the membranes. The syringe pump was set to deliver the solution at a rate of 10 mL/h, and when the first drop appeared at the tip of the needle, the high voltage (15 kV) was applied. Electrospinning was carried out at room temperature in air for 1 h, and the samples were peeled from the target as free-standing membranes. The thickness of the membranes was in the range 50-100 µm. Electrospinning of pure PS results in amorphous nanofibers with a diameter in the range 50-100 nm (Figure 2). Scanning electron microscopy (SEM, Philips XL30-FEG instrument) and transmission electron microscopy (TEM, Technai12 instrument operating at 120 kV), were used to characterize the electrospun materials. Incorporation of SWNTs into PS nanofibers was accomplished by electrospinning a dispersion of AP-SWNT in DMF solution of PS with SWNT-to-PS weight ratio of 1:100. The SEM image in Figure 3a reveals that the SWNTreinforced composite exhibits nanofiborus morphology similar to that of electrospun pure PS fibers (Figure 2). While the pure PS fibers are white, the addition of SWNTs to polystyrene changes the color of the membranes to grayish-black. The incorporation of SWNTs in the PS nanofibers was confirmed by Raman spectroscopy (Figure Nano Lett., Vol. 4, No. 3, 2004

Figure 2. (a) SEM image of PS nanofibers made by the electrospinning process. (b) TEM image of one of the nanofiber showing amorphous nature of the polymer matrix.

3b). The Raman spectra were recorded on Bruker FT Raman spectrometer (RFS 100/S) with excitation wavelength of 1064 nm and 20 mW laser power. The Raman spectrum from a PS nanofibrous membrane is featureless in the range of 100 to 1800 cm-1, whereas the spectrum of the SWNT-PS composite nanofibers shows peaks in the low-frequency range centered at 165 cm-1, associated with the radial breathing mode (RBM) and in the high-frequency range of 1500-1600 cm-1 (tangential mode). The SWNT diameter estimated from the RBM peak is 1.36 nm,42 which is typical for electric arc-produced SWNTs used in this study.43 The distribution of SWNTs within the polymer matrix was studied by TEM. No agglomeration of SWNTs in the

polystyrene fibers was observed. The incorporated nanotubes were straight and aligned along the fiber axis. Specific alignment of the SWNT bundles with respect to each other was not discerned. A typical TEM image of a SWNT-PS composite nanofiber is illustrated in Figure 3c. While the TEM observations showed homogeneous dispersion of SWNTs within the PS matrix, initial attempts to measure the mechanical properties of these composites were not successful as the composites were very brittle. To examine the effect of the SWNT dispersion and the nature of interaction with the polymer matrix on the mechanical properties of the composites, SWNT-polyurethane (SWNT-PU) composites were studied. In contrast to the fibrous morphology of the SWNT-PS composites, the SWNT-PU composite membranes prepared by the electrospinning process have a flat fibrous morphology, typical for wet fibers or beads hitting the surface of the substrate.32,37 Wet coating elements coalesce while drying on the surface. This results in the ribbon-like or flat fiber morphology as shown in Figure 4a. Wet coating elements also promote intraand interlayer bonding. The surface morphology of the membrane within the flat ribbons is very uniform, as shown in the higher resolution SEM image in Figure 4b. Figure 4c shows the TEM image of a SWNT-PU membrane which was torn into small pieces by a pair of tweezers and then loaded onto carbon-coated Cu TEM grids after dispersal in ethanol. The TEM image shows SWNT bundles protruding from the torn surface. The inset in Figure 4c shows a higher resolution TEM image from another region with a small SWNT bundle protruding from the polymer membrane. The TEM observations showed the incorporation of homogeneously distributed small bundles of SWNTs. Although alignment of the nanotubes is difficult to discern, TEM study revealed that SWNTs retain their straight shape in the polymer matrix. This observation is important as the nanotubes in the electrospun SWNT-poly(lactic acid) composites have been found to be highly tangled, forming spherical agglomerates.18 Also, in many cases poor alignment and

Figure 3. (a) SEM image of SWNT-PS nanofibrous membrane obtained by the electrospinning technique. (b) Raman spectra of PS and SWNT-PS composite with SWNT-to-polymer weight ratio of 1:100. (c) TEM image of one such composite nanofiber showing a SWNT bundle (white arrow) embedded in the PS nanofiber. The black particle in image (c) is a metal catalyst from SWNTs. Nano Lett., Vol. 4, No. 3, 2004

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Figure 6. Stress-strain curves for various membranes. The SWNT-to-PU weight ratio is 1:100 in the composite membranes (b and c).

Figure 4. (a) SEM image of SWNT-PU composite membrane obtained by the electrospinning process. The image shows a flat fiber or ribbon-like morphology. (b) Higher resolution SEM image of the same composite showing uniform surface morphology within the flat ribbons. (c) TEM image of a torn edge of SWNT-PU composite membrane showing a SWNT bundle protruding from the edge. Inset shows another region with a small SWNT bundle protruding; the scale bar in the inset is 20 nm. The black particles in image (c) are metal catalysts from SWNTs.

Figure 5. Raman spectra of PU membrane and SWNT-PU composite membrane with SWNT-to-polymer weight ratio of 1:100.

presence of twisted and bent nanotubes have been observed in electrospun poly(ethylene oxide) nanofibers in which multi-wall carbon nanotubes are embedded.29 The incorporation of SWNTs in the PU membranes was confirmed by Raman spectroscopy (Figure 5). To study the effect of the interfacial interaction between SWNTs and the polymer matrix, ester-functionalized SWNTs, SWNT-COO(CH2)11CH3, were synthesized as described 462

elsewhere31 and electrospun with PU. The chemical functionalization is an effective approach to improve the processability of SWNTs.2,44-47 In addition to improving the chemical compatibility of the nanotubes with the polymer matrix, the functionalization is expected to exfoliate the SWNT bundles.41,46,48-50 Thus, the ester form of SWNTs prepared by attaching 1-octadecanol to carbon nanotubes has been shown to be easily dispersed in organic solvent as both individual nanotubes and small bundles of 2 to 5 nanotubes.31 As the bundle size is known to significantly affect the mechanical properties of the composites, the incorporation of small bundles and ultimately individual SWNTs is desired. Measurements of the elastic and shear moduli of SWNT ropes using AFM have demonstrated that the reduced modulus depends strongly on the diameter of the ropes, decreasing by more than an order of magnitude as the bundle size increases from 3 to 20 nm.5 Improvement of the dispersion of SWNTs into the polymer matrix via surfactantassisted processing15,25 or chemical functionalization17,20 has been reported to significantly affect the properties of the composites. Stress-strain analysis has shown improved mechanical characteristics of EST-SWNTs in comparison to the APSWNT-reinforced composites. Stress-strain analysis was carried out at room temperature in a material testing station (Instron 5543) using standard procedures (ASTM standard 882 for thin films and membranes). Membranes (6.35 mm wide) were fixed into the testing station and subjected to an extension rate of 10 mm/min using a gauge length of 12.7 mm. Information about tensile strength (TS), tensile modulus (TM), and elongation at break point (EB) were derived from the stress-strain curves. Five samples were tested in each case. Typical stress-strain curves for pure PU, AP-SWNTPU composite and EST-SWNT-PU composite membranes are given in Figure 6. All the membranes show a nonlinear elastic behavior in the low stress region (0-2 MPa) and plastic deformation at higher stress. The maximum stress at break is the tensile strength, and Figure 6 shows that the tensile strength increases for PU membranes on SWNT incorporation. Compared to pure PU membranes, the tensile strength of AP-SWNT-PU composites increases by 46% from 7.02 Nano Lett., Vol. 4, No. 3, 2004

Figure 7. (a) Tensile strength (TS), (b) tangent modulus (TM), and (c) elongation to break (EB) for various membranes.

to 10.26 MPa. Composites fabricated with ester functionalized SWNT (EST-SWNTs) show an increase of 104% in tensile strength when compared to electrospun pure PU. This enhancement in the mechanical properties is evidence of efficient load transfer to the SWNTs in the composite membranes. Better mechanical properties for the ESTSWNT composites could be due to improved dispersion of the SWNTs, but could also be a response to the polar functionalities in the SWNT ester groups to the opportunities offered by hydrogen bonding sites between the polymer and matrix or to amidation reactions between free amine in the polyurethane and the ester functionality in the SWNTs. A comparison of the tensile strength for three different types of membranes is shown in Figure 7a. Mean values were obtained from five membranes for each case, and the standard deviation is represented as error bars in the figure. For materials that exhibit a nonlinear elastic behavior, the modulus of elasticity is determined by taking the slope of the stress-strain curve at some specified level of stress and this modulus of elasticity is termed the tangent modulus.51 Tangent modulus was calculated from the stress-strain curves at 1 MPa stress for all the membranes. The tangent modulus values for different types of membranes are shown in Figure 7b. The tangent modulus for AP-SWNT-PU composite membrane and EST-SWNT-PU composite membrane is enhanced by ∼215% and ∼250% respectively, when compared to pure PU membrane. The elongation-tobreak values (Figure 7c) do not show much change from the pure polyurethane to the composite membranes. The improvement in the mechanical properties of these composite materials is significant compared to that of other reported carbon nanotube composites and suggests the success of our approach.13,52 This study indicates that electrospinning is a useful method for producing composite materials and that chemically functionalized SWNTs offer considerable scope for improving the mechanical properties of composites through improvements in the carbon nanotube Nano Lett., Vol. 4, No. 3, 2004

dispersion or by modulating the interfacial interaction with the polymer matrix. Our approach to increasing the solubility of SWNTs involves attachment of long chain molecules to SWNTs. Compared to AP-SWNTs, the ester-functionalized SWNTs have higher solubility in organic solvents such as DMF. This provided a homogeneous suspension of SWNTs in the polymer matrix. Moreover, the attached long chain on ester-functionalized SWNTs may enhance the interaction between SWNTs and the polymer matrix. The availability of polar groups in the ester functionality provides opportunities for hydrogen bonding with the polymer and amidation reactions with free amines in PU. In conclusion, we have demonstrated the feasibility of using the electrospinning process to fabricate SWNTreinforced composite nanofibers and membranes. SWNTfilled polystyrene nanofibers have been prepared wherein the SWNT bundles are oriented parallel to the nanofiber axis. SWNT-PU composite membranes have been fabricated in the free-standing form using different types of SWNTs. The composite membranes showed a significant enhancement in the mechanical properties when compared to the pure polymer membrane. The mechanical properties depend on the type of SWNTs used; composites with ester functionalized SWNTs exhibited better mechanical properties than those fabricated with AP-SWNTs. Acknowledgment. This work was supported by DOD/ DARPA/DMEA under Award No. DMEA90-02-2-0216. Carbon Solutions, Inc. acknowledges DARPA Phase I award. References (1) Carbon Nanotubes: Synthesis, Structure, Properties and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; SpringerVerlag: Berlin, 2001; Vol. 80. (2) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105-1113. (3) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324-337. (4) Overney, G.; Zhong, W.; Tomanek, D. Z. Phys. D 1993, 27, 93-96. (5) Salvetat, J. P.; Briggs, G. A. D.; Bonard, J. M.; Basca, R. R.; Kulik, A. J.; Stockli, T.; Burnham, N. A.; Forro, L. Phys. ReV. Lett. 1999, 82, 944-947. (6) Yu, M. F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. ReV. Lett. 2000, 84, 5552-5555. (7) Calvert, P. Nature 1999, 399, 210-211. (8) Park, C.; Ounaies, Z.; Watson, K. A.; Crooks, R. E.; Smith, J. J.; Lowther, S. E.; Connell, J. W.; Siochi, E. J.; Harrison, J. S.; St. Clair, T. L. Chem. Phys. Lett. 2002, 364, 303-308. (9) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (10) Qian, D.; Dickey, E. C.; Andrews, R.; Rantell, T. Appl. Phys. Lett. 2000, 76, 2868-2870. (11) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. AdV. Mater. 2000, 12, 750-753. (12) Haggenmueller, R.; Gommans, H. H.; Rinzler, A. G.; Fischer, J. E.; Winey, K. I. Chem. Phys. Lett. 2000, 330, 219. (13) Kumar, S.; Dang, T. D.; Arnold, F. E.; Bhattacharyya, A. R.; Min, B. G.; Zhang, X.; Vaia, R. A.; Park, C.; Adams, W. W.; Hauge, R. H.; Smalley, R. E.; Ramesh, S.; Willis, P. A. Macromolecules 2002, 35, 9039-9043. (14) Mamedov, A. F.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nature Materials 2002, 1, 190-194. (15) Barraza, H. J.; Pompeo, F.; O’Rear, E. A.; Resasco, D. E. Nano Lett. 2002, 2, 797-802. (16) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (17) Velasco-Santos, C.; Martinez-Hernandez, A. L.; Fisher, F. T.; Ruoff, R.; Castano, V. M. Chem. Mater. 2003, 1049-1052. 463

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Nano Lett., Vol. 4, No. 3, 2004