High performance carbon nanotube based composite film from layer-by-layer deposition

CARBON

9 0 ( 2 0 1 5 ) 2 1 5 –2 2 1

Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

High performance carbon nanotube based composite film from layer-by-layer deposition Liang Zhang a, Wei Xu a, Xiao Gang Luo b, Jian Nong Wang

a,*

a

Nanomaterials Research Center, School of Mechanical and Power Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China

A R T I C L E I N F O

A B S T R A C T

Article history:

So far, preparation of strong carbon nanotube (CNT)/polymer composites still faces big

Received 29 January 2015

challenges mainly due to the limited controls of CNT dispersion and alignment in

Accepted 9 April 2015

polymers. Here, a new ‘‘layer-by-layer deposition’’ method is put forward to prepare CNT/

Available online 16 April 2015

polyvinyl alcohol (PVA) composite films. This is based on intermittent deposition of aligned CNT and PVA layers on a paper tape substrate. The in situ deposition allows PVA to infiltrate into the CNT film efficiently, and, as a result, the mechanical property of CNT/PVA composite film has been improved remarkably. For example, the composite film possesses a tensile strength of 1.7 GPa, which is almost one order of magnitude and 20 times higher than those of the pure CNT and PVA films, respectively. The high performance of the composite film could be ascribed to the role of PVA infiltration, which leads to not only the formation of strong interfacial bonding between CNTs and PVA matrix but also the reduction of film thickness. The novel process offers a new research direction for preparing CNT-based composites and future performance maximization.  2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) have attracted tremendous research since the landmark paper published in the early 1990s [1] due to their remarkable mechanical [2], electrical [3] and thermal [4] properties. One of their important applications is their uses as excellent nanofillers to reinforce polymer matrix. The main methods of preparing CNT/polymer composites developed to date include solution processing [5], melt mixing [6] and in situ polymerization [7]. However, the tensile strength and Young’s modulus of the composite materials are far below those expected. This is mainly because it is quite difficult to disperse CNTs with high volume fraction uniformly in polymer matrix due to the entangling

* Corresponding author. E-mail address: [email protected] (J.N. Wang). http://dx.doi.org/10.1016/j.carbon.2015.04.026 0008-6223/ 2015 Elsevier Ltd. All rights reserved.

and agglomeration phenomena of CNTs. Other factors such as poor alignment, residual solvent or monomer and week interfacial integration between CNTs and polymer also hinder the achievement of strength and modulus. Therefore, the excellent property of CNTs has not yet been utilized, which certainly limits their wide applications. Recently, efforts have been made to assemble CNTs into macroscopic fibers or films, such as spinning from CNT solutions [8], arrays [9] and aerogels [10]. The solution and array approaches have also been applied to fabricate CNT/polymer composites. In the solution case, CNTs should be dispersed in a solvent first and then spun into a fiber [11]. The main barrier which has not been well overcome is dispersing CNTs, particularly, at a high volume, in the solvent. Such a barrier,

216

CARBON

9 0 ( 2 0 1 5 ) 2 1 5 –2 2 1

however, appears minimal in other cases as the polymer to be included could penetrate into the empty spaces within the CNT macroscopic material due to the capillary force. A few studies focused on preparing CNT/polymer composites based on the film with aligned CNT arrays. Directly immersing a CNT film or yarn spun from arrays in epoxy matrix was a simple method but the prepared composite did not perform well as it had a low tensile strength (402–467 MPa) [12,13]. The process of so called ‘‘resin transfer molding’’ was also a good method to prepare CNT/polymer composites [14], although the composites showed a low strength of 231.5 MPa due to a limited CNT fraction of 16.5% by weight. Liu et al. [15] spun yarns from CNT arrays and passed them through a polyvinyl alcohol (PVA) solution acting as a shrinking solvent. The composite fibers with 81 wt.% CNTs possessed a tensile strength up to 2 GPa and modulus more than 120 GPa. Liu et al. [16,17] fabricated CNT/PVA composite films by spraying a PVA solution on the film drawn from the aligned CNTs. They found that the walls and diameters of CNTs had a great effect on the mechanical and electrical properties of the composites. The composite films containing 65 wt.% CNTs (6 walls and 8–10 nm in diameter) had the tensile strength and modulus up to 1.8 GPa and 45 GPa, respectively. Although the process of spray winding is an effective way to prepare CNT/polymer composites [16–21], the exact layers of the composite films are hard to

control, and the length of the film is limited by the small size of the film from which CNT yarns or ribbons are drawn. This would pose a big challenge for continuous and large-scale production. Here, we report a new ‘‘layer-by-layer (LBL) deposition’’ method for preparing CNT/PVA composite films. Unlike the previous LBL assembling method that involved alternating deposition of monolayers of CNTs and polymer with opposite charges [22–24], our method is based on in situ deposition of a layer of CNT film which is coupled with brushing a layer of PVA solution on a paper tape. Such a process is repeated for deposition of more CNT/PVA layers. Our results show that mechanical and electrical properties can be improved significantly in comparison with the pure CNT and PVA counterparts. Furthermore, not only can we control the number of layers and the length of the ribbonlike composite film, but also the process suitable for continuous production.

2.

Experimental

2.1.

Synthesis of hollow cylindrical CNT assembly

The experimental setup is schematically shown in Fig. 1a. A hollow cylinder-like CNT assembly was continuously synthesized at 1150–1300 C in a horizontal furnace using an

Water/PVA solution

H2O/PVA

CNT layer

(c) (b)

(d)

500 µm

Fig. 1 – Experimental set-up and results. (a) Schematic illustration of the LBL deposition process. A paper tape is wetted with water/PVA, supplied to the reactor for deposition of CNT assembly, and finally winded up after drying. (b) A film in a form of tape with 20 layers of deposition on a winder. (c) A rectangular film of 10 cm long and 1.8 cm wide. (d) SEM image of the smooth surface of the film. (A color version of this figure can be viewed online.)

CARBON

alundum tube and nitrogen as the reactor and carrier gas, respectively. The precursor solution consisted of a liquid feedstock of carbon source (typically, ethanol) with dissolved ferrocene and thiophene. This solution was injected into the reactor at a rate of 2–10 mL min1, and carried into the high-temperature zone by N2 at a flow rate of 16–32 L h1.

2.2.

217

9 0 (2 0 1 5) 2 1 5–22 1

Fabrication of CNT/PVA films

A roll of paper tape was continuously brushed with a water solution and then supplied to the exit of the reactor and used as a substrate for the CNT cylinder to shrink and deposit as a film. In order to make a composite film, PVA (molecular weight 100,000–110,000, 98–99% hydrolyzed) was added into the water solution at different concentrations (0.025–0.5 wt.%). The paper tape with CNT and PVA films was dried in a heater before it was winded. The winding rate was controlled to be in the range of 20–30 m min1. For the deposition of a new layer, the winded paper tape with CNT and PVA films was supplied to the reactor and used as a new substrate. By exchanging the supplying roller with the winding roller, CNT + PVA films were deposited for as many layers as needed. In our experiments, 10 or 20 layers were deposited, and the length and width of the composite film were determined by the size of paper tape, being 50 m and 1.8 cm, respectively. The CNT film was removed from the paper tape by soaking it in water. For comparative study, another group of CNT/PVA composite films was prepared by soaking pure CNT films in 0.05 wt.% PVA solution for 15 min. All pure CNT films were dried at 60 C whereas the composite ones at 150 C for 3 h to ensure a full integration between CNTs and PVA.

(a)

2.3.

Characterization

CNTs were characterized by high resolution transmission electron microscopy (HRTEM, JEOL-2010F, accelerating voltage of 200 kV) and Raman spectroscopy (Raman, Senterra R200-L, excitation wavelength of 532 nm). The weight fraction of CNTs in composite films was analyzed by thermo-gravimetric analysis (TGA, Netzsch Model STA 409 PC) at a heating rate of 10 C min1 in pure nitrogen (99.999%). The thickness and surface morphology of the film were characterized by scanning electron microscope (SEM, S3400N) at an accelerating voltage of 15 kV.

2.4.

Tensile and resistance tests

The obtained films were cut into 30 mm (length) · 2 mm (width) strips along the longitudinal direction by a razor blade for tensile testing. The tensile tests were performed on a fiber tensile tester (XS(08)X-15, Shanghai Xusai Co., China), which is equipped with a deformation loading system and a force measuring system with a maximum force of 15 N and precision of 0.01 cN. The tensile testing was performed at a displacement rate of 20 mm min1 and a GL of 10 mm, which corresponds to an engineering strain rate of 3.33 · 102 s1. 5 specimens were tested for each type of film with 20 layers. The electrical resistance of CNT films was measured by a four-point probes meter (SX1944, Suzhou Baishen Technology Co., China). The electrical conductivity j was calculated through j = L/RA, where L, R, and A are the length, resistance, and cross-sectional area of CNT films, respectively. The measurement was also repeated many times to get an average value.

(b)

5 µm

(c) Intensity (a.u.)

800

(d)

G, 1579 cm-1 IG/ID = 2.72

600 400

D, 1348 cm-1

200

10 nm

0 1200

1300

1400

1500

1600 -1

1700

1800

Raman Shift (cm )

Fig. 2 – CNT characterizations. (a) SEM image of the aligned CNTs in the cylindrical assembly. (b) TEM image of the CNT bundles attached with some Fe particles. (c) HRTEM image showing CNTs with a few-walled structure. (d) Raman spectra of CNT film, showing an intensity ratio of IG/ID of 2.72.

218

9 0 ( 2 0 1 5 ) 2 1 5 –2 2 1

Results and discussion

(a)

140

Tensile Load (cN)

Using the present experimental method, pure CNT or CNT/ PVA composite films in a tape form with a smooth surface were produced with their width and length determined by the size of the paper tape used for deposition (Fig. 1b–d). The cylindrical assembly formed on the inner side wall of the reactor at the downstream, was driven out from the reactor to air atmosphere by the carrier gas, and deposited on a paper tape (see Videos 1 and 2 in Supporting Information for this process). The cylindrical assembly used for the deposition mainly consisted of aligned CNTs with a low amount of Fe particles (10 wt.%). The alignment was quite good due to the pull force induced by fast winding when the hollow cylindrical CNT assembly was condensed on the paper tape. HRTEM showed that the CNTs were few-walled (2–6 walls) with the diameter ranging from 4 to 9 nm. Raman microscopy illustrated that the intensity ratio (IG/ID) of G band peak at 1579 cm1 to D band peak at 1348 cm1 was 2.72, indicating that the CNTs possessed a high degree of graphitization (Fig. 2). The high levels of alignment and graphitization were suggested to be the important factors for improving the tensile strength and modulus of CNT films [18,20,25–27]. When PVA was included in the water solution, it could infiltrate into each layer of CNT sheet efficiently in the process of LBL deposition, which is in favor of improvement of mechanical properties. CNT/PVA composite films of 20 layers were prepared at different PVA concentrations (0.025– 0.5 wt.%), and the results of their ultimate tensile loads are showed in Fig. 3a. The ultimate tensile load of the pure CNT film was 40 cN on average while that of the CNT/PVA composite film prepared at 0.025 and 0.05 wt.% PVA increased significantly, reaching 77 and 107 cN, respectively. However, with further increasing the PVA concentration to 0.5 wt.%, the tensile load increased only slightly to 140 cN. Thus, in the following, we focus our study on the 0.05 wt.% sample unless otherwise noted. Fig. 3b presents the TGA results of the PVA powder and the CNT/PVA composite films prepared at 0.05 wt.% PVA. The CNT mass fractions in the composite films with 10 and 20 layers were approximately 57 and 60 wt.%, respectively, indicating that the composite film was quite uniformly built up during the process of LBL deposition. Moreover, the results showed that CNT was a dominant phase while PVA acted as a filling phase, which is consistent with recent studies showing that a high CNT loading (53–81 wt.%) was essential to preparing high-performance CNT/PVA composites [15–17]. On the other hand, the CNT/PVA composite film with 20 CNT layers made by soaking method contained about 66 wt.% of CNTs. Fig. 4a–d shows the thicknesses of different films based on a number of measurements. All of these films were prepared with 20 layers of CNTs. The pure CNT film had a thickness of 1.1 lm, which was reduced to 0.8 lm by soaking the film in a 0.05 wt.% PVA solution. With LBL deposition using the same PVA solution, the film thickness was reduced to 0.35 lm. But, using a 0.4 wt.% PVA solution, the thickness was as large as 7.5 lm. Dissolving PVA in ethanol/water at a low concentration tended to induce shrinking and condensation of CNT

160

120 100 80 60 40 20 0

0

0.025

0.05

0.10

0.20

0.30

0.40

0.50

PVA Fraction (wt.%) 120

CNT/PVA films Soaking (20 layers) LBL (20 layers) (72.8% residue) (66.6% residue)

100

Mass (%)

3.

CARBON

80 60 LBL (10 layers) (63.7% residue)

40 20 0

PVA powder (6.8% residue)

(b) 200

400

600

800

Temperature (oC) Fig. 3 – Characterizations of composite films. (a) Comparison of the ultimate tensile load among films prepared by LBL method at different PVA concentrations. (b) TGA curves of PVA powder and CNT/PVA composite films prepared at 0.05 wt.% PVA concentration by either LBL or soaking method. (A color version of this figure can be viewed online.)

films at least at the thickness direction, and the LBL deposition induced much higher condensation than the soaking. Nevertheless, dissolving PVA at a high concentration had a large thickening effect on the CNT film, indicating deposition of PVA as the dominant phase in the composite. Fig. 4e presents the representative stress–strain curves of the pure CNT and CNT/PVA composite films. At least five specimens were tested for each film. The average tensile strength and Young’s modulus of the pure CNT film were 182 MPa and 3.3 GPa, respectively. The strength of the soaked film increased to 369 MPa, 2 times higher than the pure film. By comparison, the tensile strength of the composite film prepared by LBL deposition (0.05 wt.% PVA) increased up to 1673 MPa, which is about one order of magnitude higher than the pure CNT film. Such a large improvement is apparently a result of the load increment (40 ! 107 cN) and thickness reduction (1.1 ! 0.35 lm) with the inclusion of a low PVA concentration (Fig. 4a, c and f). However, with the inclusion of a high PVA concentration of 0.4 wt.%, the tensile strength was only 92 MPa due to a large increase of thickness (1.1 ! 7.5 lm) (Fig. 4d). In addition to strength improvement, an improvement of electrical conductivity from 279 to 804 S cm1 was also measured with the inclusion of a low concentration of PVA (0.05 wt.%). This conductivity is higher

CARBON

219

9 0 (2 0 1 5) 2 1 5–22 1

(b)

(a)

5 µm

10 µm

(d)

(c)

1 µm 140

(e)

1800

CNT/PVA film (by LBL deposition)

120

1400 1200

CNT/PVA film (by soaking)

1000 800

Pure CNT film

600 400 200 0

0

2

4

6

8

10

12

14

Strain (%)

16

Tensile Load (cN)

Stress (MPa)

1600

(f)

Tensile Load Thickness

1200 1000

100

800

80

600

60 40

400

20

200

0

Pure CNT film

CNT/PVA film CNT/PVA film (soaking) (LBL)

Thickness (nm)

2000

10 µm

0

Fig. 4 – Characterizations of microstructures and mechanical properties. (a–d) SEM images of the cross sections of films of pure CNTs, CNT/PVA by soaking, CNT/PVA by LBL at 0.05 wt.% PVA, and CNT/PVA by LBL at 0.4 wt.% PVA, showing a thickness of 1.1, 0.8, 0.35, and 7.5 lm, respectively. (e) Typical stress vs. strain curves of the films of pure CNTs, CNT/PVA composite by soaking, and CNT/PVA composite by LBL deposition at 0.05 wt.% PVA. (f) Comparisons of ultimate tensile load and thickness among different films. (A color version of this figure can be viewed online.)

than those for the CNT/PVA composites fabricated by spray winding [17] and conventional solution methods [28,29]. The above results clearly show that inclusion of PVA in CNTs benefits strength improvement, and the LBL deposition is advantageous over the conventional soaking for making composite films. This may be because in conventional soaking PVA infiltration into the CNT film is limited, and in the LBL deposition process, a proper concentration of PVA (0.05 wt.%) favored the infiltration of PVA into the whole film. The complete infiltration might have resulted in not only significant improvement of interfacial bonding between CNTs and thus tensile load but also large condensation and thus thickness reduction. The observation of widespread Y-type junctions linking neighboring CNT bundles at the fracture surface is indicative of the improved interfacial bonding with the infiltration of PVA (Fig. 5). As a matter of fact, during the deposition of each CNT layer, PVA molecules dissolved in an ethanol/water solution were introduced. This ensured complete mixing and infiltration between PVA and CNTs. Since individual CNTs can be considered as molecules, the mixing

and consequent bonding between PVA and CNTs might have taken place at the molecular level, which is essential to the formation of high performance composites as already suggested before [30]. However, excessive PVA might not provide additional increase of interfacial bonding but extra thickness and weak PVA layers intercalated between CNT layers, all of which could lower the overall strength of the composite. The present LBL technique does not involve CNT dispersion in a solvent and can be easily applied with aligned CNTs, thus leading to composites with higher mechanical properties than the conventional solution approach reported before [31,32]. Composite fiber and film made from aligned CNT arrays and PVA introduced from a solution or by spraying also had a high strength up to 2.0 GPa [15], perhaps due to the infiltration of PVA into individual CNT layers as well. The present approach has advantages over this CNT arrays based one. For example, the present composite tape-like film can be made to be very long, decided by the length of the substrate paper tape (usually 50 m), but that from the CNT arrays is very limited owing to the small size of the spinnable CNT film.

220

CARBON

9 0 ( 2 0 1 5 ) 2 1 5 –2 2 1

Nanoscience and Nanotechnology Promotion Center (project #: 12nm0503300) are greatly acknowledged.

(a)

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2015.04.026.

10 µm R E F E R E N C E S

(b)

2 µm Fig. 5 – SEM images of the fracture surface of CNT/PVA composite film. (a) Observation of individual CNTs and bundles (left region) after the dense surface (right region) had been pulled apart. (b) Y-type junctions linking neighboring CNTs and bundles.

Other advantages include one-step process (in situ deposition of film from CNTs directly from the reactor) and good controllability of PVA percentage. On a final note, it is possible to make CNT-based composites with any other materials as long as they can be dissolved or suspended in alcohol/water solution. All of these are essential to large scale production of various composite materials.

4.

Conclusions

In this work, a new approach involving LBL deposition is demonstrated to prepare CNT/PVA composite films. The in-situ deposition of a CNT film on a PVA coated tape substrate is beneficial to full infiltration of PVA within CNT film, leading to enhanced interfacial bonding and reduced film thickness when compared with the pure CNT film and PVA soaked one under otherwise identical conditions. The overall effect is a strength improvement by one order of magnitude (from 182 to 1673 MPa). The advantages of the present approach for making high strength composites include molecular LBL composite formation, good controllability of PVA percentage, flexibility for different composites, and one-step continuous production. Thus, our study demonstrates a new strategy for future performance maximization and wide applications of CNT-based composites.

Acknowledgements Financial supports from National Natural Science Foundation of China (project #: 51271077, U1362104) and Shanghai

[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 2000;287(5453):637–40. [3] Yao Z, Kane CL, Dekker C. High-field electrical transport in single-wall carbon nanotubes. Phys Rev Lett 2000;84(13):2941–4. [4] Hone J, Whitney M, Piskoti C, Zettl A. Thermal conductivity of single-walled carbon nanotubes. Phys Rev B 1999;59(4):2514–6. [5] Cadek M, Coleman JN, Barron V, Hedicke K, Blau WJ. Morphological and mechanical properties of carbonnanotube-reinforced semicrystalline and amorphous polymer composites. Appl Phys Lett 2002;81(27):5123–5. [6] Haggenmueller R, Gommans HH, Rinzler AG, Fischer JE, Winey KI. Aligned single-wall carbon nanotubes in composites by melt processing methods. Chem Phys Lett 2000;330(3–4):219–25. [7] Jia ZJ, Wang ZY, Xu CL, Liang J, Wei BQ, Wu DH, et al. Study on poly(methyl methacrylate)/carbon nanotube composites. Mater Sci Eng A 1999;271(1–2):395–400. [8] Vigolo B, Pe´nicaud A, Coulon C, Sauder C, Pailler R, Journet C, et al. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 2000;290(5495):1331–4. [9] Zhang M, Atkinson KR, Baughman RH. Multifunctional carbon nanotube yarns by downsizing an ancient technology. Science 2004;306(5700):1358–61. [10] Li YL, Kinloch IA, Windle AH. Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis. Science 2004;304(5668):276–8. [11] Dalton AB, Collins S, Munoz E, Razal JM, Ebron VH, Ferraris JP, et al. Super-tough carbon-nanotube fibres. Nature 2003;423(6941):703. [12] Bradford PD, Wang X, Zhao HB, Maria JP, Jia QX, Zhu YT. A novel approach to fabricate high volume fraction nanocomposites with long aligned carbon nanotubes. Compos Sci Technol 2010;70(13):1980–5. [13] Shimamura Y, Oshima K, Tohgo K, Fujii T, Shirasu K, Yamamoto G, et al. Tensile mechanical properties of carbon nanotube/epoxy composite fabricated by pultrusion of carbon nanotube spun yarn preform. Compos Part A Appl Sci Manuf 2014;62:32–8. [14] Cheng QF, Wang JP, Wen JJ, Liu CH, Jiang KL, Li QQ, et al. Carbon nanotube/epoxy composites fabricated by resin transfer molding. Carbon 2010;48(1):260–6. [15] Liu K, Sun YH, Lin XY, Zhou RF, Wang JP, Fan SS, et al. Scratch-resistant, highly conductive, and high-strength carbon nanotube-based composite yarns. ACS Nano 2010;4(10):5827–34. [16] Liu W, Zhang XH, Xu G, Bradford PD, Wang X, Zhao HB, et al. Producing superior composites by winding carbon nanotubes

CARBON

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

9 0 (2 0 1 5) 2 1 5–22 1

onto a mandrel under a poly(vinyl alcohol) spray. Carbon 2011;49(14):4786–91. Liu W, Zhao HB, Inoue Y, Wang X, Bradford PD, Kim H, et al. Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding. Compos Part A Appl Sci Manuf 2012;43(4):587–92. Wang X, Yong Z, Li Q, Bradford PD, Liu W, Tucker DS, et al. Ultrastrong, stiff and multifunctional carbon nanotube composites. Mater Res Lett 2013;1(1):19–25. Jiang Q, Wang X, Zhu YT, Hui D, Qiu YP. Mechanical, electrical and thermal properties of aligned carbon nanotube/ polyimide composites. Compos Part B Eng 2014;56:408–12. Wang X, Bradford PD, Liu W, Zhao HB, Inoue Y, Maria JP, et al. Mechanical and electrical property improvement in CNT/ Nylon composites through drawing and stretching. Compos Sci Technol 2011;71(14):1677–83. Wang X, Jiang Q, Xu WZ, Cai W, Inoue Y, Zhu YT. Effect of carbon nanotube length on thermal, electrical and mechanical properties of CNT/bismaleimide composites. Carbon 2013;53:145–52. Mamedov AA, Kotov NA, Prato M, Guldi DM, Wicksted JP, Hirsch A. Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites. Nat Mater 2002;1(3):190–4. Olek M, Ostrander J, Jurga S, Mohwald H, Kotov N, Kempa K, et al. Layer-by-layer assembled composites from multiwall carbon nanotubes with different morphologies. Nano Lett 2004;4(10):1889–95. Shim BS, Zhu J, Jan E, Critchley K, Ho SS, Podsiadlo P, et al. Multiparameter structural optimization of single-walled carbon nanotube composites: toward record strength, stiffness, and toughness. ACS Nano 2009;3(7):1711–22.

221

[25] Cheng QF, Bao JW, Park J, Liang ZY, Zhang C, Wang B. High mechanical performance composite conductor: multi-walled carbon nanotube sheet/bismaleimide nanocomposites. Adv Funct Mater 2009;19(20):3219–25. [26] Cheng QF, Wang B, Zhang C, Liang ZY. Functionalized carbonnanotube sheet/bismaleimide nanocomposites: mechanical and electrical performance beyond carbon-fiber composites. Small 2010;6(6):763–7. [27] Liu QL, Li M, Gu YZ, Zhang YY, Wang SK, Li QW, et al. Highly aligned dense carbon nanotube sheets induced by multiple stretching and pressing. Nanoscale 2014;6(8):4338–44. [28] Shaffer MSP, Windle AH. Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites. Adv Mater 1999;11(11):937–41. [29] Hernandez YR, Gryson A, Blighe FM, Cadek M, Nicolosi V, Blau WJ, et al. Comparison of carbon nanotubes and nanodisks as percolative fillers in electrically conductive composites. Scripta Mater 2008;58(1):69–72. [30] Ma WJ, Liu LQ, Zhang Z, Yang R, Liu G, Zhang TH, et al. Highstrength composite fibers: realizing true potential of carbon nanotubes in polymer matrix through continuous reticulate architecture and molecular level couplings. Nano Lett 2009;9(8):2855–61. [31] Zhang XF, Liu T, Sreekumar TV, Kumar S, Moore VC, Hauge RH, et al. Poly(vinyl alcohol)/SWNT composite film. Nano Lett 2003;3(9):1285–8. [32] Hou Y, Tang J, Zhang HB, Qian C, Feng YY, Liu J. Functionalized few-walled carbon nanotubes for mechanical reinforcement of polymeric composites. ACS Nano 2009;3(5):1057–62.