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Formation mechanisms and morphological effects on multi-properties of carbon nanotube fibers and their polyimide aerogel-coated composites
Composites Science and Technology 117 (2015) 114e120
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Formation mechanisms and morphological effects on multi-properties of carbon nanotube fibers and their polyimide aerogel-coated composites Peng Liu 1, Thang Q. Tran 1, Zeng Fan, Hai M. Duong* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online 21 June 2015
This paper aims to understand the formation mechanisms and morphological effects of polymer-aerogel reinforcements that guide the production of lightweight and multifunctional carbon nanotube (CNT) fibers. The CNT fibers and the polyimide aerogel-coated CNT fiber (PIA) composites were successfully developed. CNT thin films were fabricated from an aerogel technique; these films were then mechanically rolled to form CNT fibers of various diameters and densities. The CNT fibers thus prepared were dipcoated in the polyamic acid solutions to form gel-coated fibers, which were then dried with supercritical CO2 to obtain CNT/PIA composite fibers. The composite fibers presented as a coreeshell structure in which the CNT fibers were tightly wrapped by porous polyimide aerogels. The relationships between the electrical and mechanical structural properties of the composites were investigated. By controlling the diameter of CNT fibers to 500 mm, we achieved electrical conductivity of 418 S/cm, and strength and stiffness values of 11.6 and 68.1 MPa, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Polyimide aerogels Nano composites Electrical properties Mechanical properties
1. Introduction Carbon nanotubes (CNTs) are rolled-up graphene sheets with cylindrical nanostructures, where sp2 bonded carbon atoms are arranged in a hexagonal lattice [1]. Because of their unique structure, individual CNTs show extraordinary mechanical (Young's modulus of up to 1 TPa [2]), electrical (over 3 104 S/cm [3]), and thermal (up to 3500 W/m$K [4]) properties. However, due to the difficulty in handling and in their dispersion, the extension of these superior properties of individual CNTs onto a macroscopic scale still remains a challenge. Regarding the practical application of CNTs, assembling CNTs into CNT fibers has been considered a significant step. To date, methods to produce CNT fibers generally include: (1) wet-spinning from CNT acid/polymer solutions [5,6], (2) dry-drawing and spinning from vertically aligned CNT arrays [7], and (3) direct-assembly via floating catalyst chemical vapor deposition (FCCVD) [8,9]. Among them, the direct-CVD growth method has gained tremendous interest owing to its straightforward process and capability for
* Corresponding author. E-mail address: [email protected] (H.M. Duong). 1 Equal distribution. http://dx.doi.org/10.1016/j.compscitech.2015.06.009 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
mass production [8e10]. In addition, Ma et al. [11] and Feng et al. [12] have demonstrated an indirect approach to produce CNT fibers, by twisting and/or rolling of the CVD-grown CNT films. Compared to direct spinning methods, the twisting/rolling approach may be unfavorable for the continuous production of CNT fibers, but the control of the resultant fiber diameters could be more conveniently achieved [13]. The CNT fibers, as prepared in this way, commonly possess satisfactory electrical and thermal conductivities [14], and even better flexibility and higher modulus, strength and breaking energy than commercial carbon fibers and polymer fibers [9]. In order to further improve their properties (mechanical performance in particular), polymer infiltration is usually performed on these CNT fibers. Liu et al. [15] reported the preparation of CNT/polyvinyl alcohol (PVA) composite fibers by infiltrating a polyvinyl alcohol (PVA)/dimethyl sulfoxide (DMSO) solution into the raw CNT fibers, and obtained an increase of 400% in fiber strength. By a similar process, Fang et al. [16] fabricated the CNT/polyimide (PI)/poly(amic acid) (PAA) composite fibers, which exhibited a high strength value of 2.06 GPa, superior even to Grafil 34e700 carbon fibers and Kevlar 49 aramid fibers, when considering their individual densities. Such mechanical enhancement of CNT fibers via polymer infiltration is generally considered a result of the adhesion and load
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transfer between CNTs and the infiltrated polymers [17]. However, the resulting composite fibers would unfortunately suffer excess weight and compromised electrical conductivity with the presence of non-conducting polymers. In recent years, polymer aerogels have been developed as a good alternative to polymers for a lightweight application of reinforcement. Polyimide aerogels (PIAs, either in a linear [18] or crosslinked structure [19e21]), inheriting the excellent thermal stability from polyimides and the low densities of aerogels, are especially promising. Nevertheless, the development of a CNT/polymer aerogel composite fiber has not been reported so far. For CNT/polymer composites, the existence of CNTs has been shown to alter the polymer packing, which significantly impacts their multifunctional properties [22]. However, the role of CNTs in a CNT/polymer aerogel composite is still unclear. As the CNT fibers are already tightly packed, how the polymer chains form around CNTs, as well as the resulting morphology of the composite fibers, are also necessary avenues to explore in contrast to the raw CNT fibers. In addition, electrical and mechanical properties of most porous materials depend strongly on their morphologies. In view of a comprehensive study, it is worthwhile to investigate the formation mechanisms and morphological effects on the multifunctional properties of both CNT fibers and their aerogel composites. Additionally, enhancements to PIA properties are also expected, due to the presence of oriented CNTs. In this paper, we first proposed a simple and versatile route to prepare CNT/PIA composite fibers. To have better control over the fiber diameters, CNT fibers were prepared by simply rolling the CVD-grown CNT films. CNT/PIA composite fibers were obtained by dip-coating these CNT fibers in the polyamic acid oligomer, followed by gelation, aging and supercritical drying. Formation mechanisms of both the CNT fibers and CNT/PIA composite fibers were carefully compared and investigated, in relation to their morphologies. The morphological effects on electrical properties and mechanical performance (of both CNT fibers and CNT/PIA composite fibers) were also comprehensively quantified. This research would be meaningful for exploiting the property of CNT fibers, and prompting the development of CNT/aerogel composites. 2. Experiments 2.1. Materials Ferrocene, thiophene, 1emethyle2epyrrolidone (NMP), 4, 40 eoxydianiline (ODA), 3, 30 , 4, 40 ebiphenyltetracarbonxylic dianhydride (BPDA), acetic anhydride, pyridine and ethanol were purchased from SigmaeAldrich Company Ltd. Methane (CH4); hydrogen (H2) and helium (He) were purchased from ChemeGas Pte Ltd.; and Octa(aminophenyl)silsesquioxane (OAPS) was purchased from Gulf Chemical (Singapore) Pte Ltd. All the chemicals above were used in the condition they were received in, without further purification. 2.2. Synthesis of carbon nanotube fibers (CNT fibers) The CNT fibers were fabricated from CNT films, which were synthesized by the FCCVD method [9]. In this process, CH4, ferrocene, and thiophene were applied as the carbon source, catalyst, and reaction promoter respectively. In a reducing hydrogen atmosphere, the nanotubes formed an aerogel in the hot zone of the furnace (1200 C) and stretched into cylindrical hollow socks due to the gas flow. To collect a CNT film, the CNT sock, as it was formed inside the reactor, was pulled out from the furnace by a stainless steel rod, and continuously wound on a roller for 10 min. This film was then cut
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into 4 cm 4 cm pieces, and mechanically rolled from one end to the other to fabricate the CNT fibers. The 3 different diameters (500e770 mm) of CNT fibers were obtained by carefully controlling the magnitude of the applied force (1e5 N). 2.3. Synthesis of carbon nanotube/polyimide aerogel (CNT/PIA) composite fibers A polyamic acid oligomer with a molar ratio of BPDA: ODA ¼ 26: 25 was formulated in NMP [20]. 1.2796 g of BPDA was added to a solution of ODA (0.8376 g) in 19.0 ml of NMP. After the BPDA was fully dissolved, a solution of 0.0481 g OAPS in 3.15 ml NMP was then added, with 10 min of stirring. Next, 3.30 ml acetic anhydride and 2.80 ml pyridine were added to the obtained mixture, and stirred for another 10 min. For CNT/PIA composite fiber synthesis, the CNT fibers were dipcoated in the polyamic acid oligomer for 1 min to form a uniform coating, and then hung vertically in the air for subsequent gelation and aging. The gelation process took place within 20 min, and the gel-coated fibers were aged for 24 h. After aging, the gel-coated CNT fibers were soaked at 24 h intervals, in the respective solutions of 75% NMP in ethanol, 25% NMP in ethanol and 100% ethanol, sequentially. The CNT/PIA composite fibers were eventually obtained by drying these gel-coated CNT fibers, through the supercritical drying method using CO2. 2.4. Characterization The diameters of the CNT fibers and CNT/PIA composite fibers were measured by an optical microscope (Olympus LG-PS2). The cross-sections of the CNT fibers and the CNT/PIA composite fibers were observed using a field emission scanning electron microscope (FE-SEM, Model S-4300, Hitachi). The structures of the CNT fibers were characterized by a transmission electron microscope (TEM, JEOL JEM-3010) and a micro-Raman microscope (Renishaw InVia 514.5 nm Arþ). To determine the pore properties of the CNT/PIA composite fibers, all the composite fibers were degassed under vacuum at 70 C for 12 h, after which the nitrogen adsorption/desorption test was carried out on a Nova 2200e (Quantachrome). Electrical conductivities of the CNT fibers and CNT/PIA composite fibers were measured using a Fluke 73III multimeter. For better electrical contacts, the two ends of the fiber samples were fixed on two separate glass slides, using silver paste. Given the coreeshell structure of the CNT/PIA composite fibers, their electrical conductivities were measured in two ways, as shown in Fig. 1. The conductivity of an entire composite fiber (scomposite) was obtained by connecting entire cross sections of its two ends, while its core electrical conductivity (score) was obtained by only connecting two core regions with CNT fibers. Tensile tests of the CNT fibers and CNT/PIA composite fibers were performed on an Instron Micro Tester. The samples were sandwiched between 2 pieces of cardboard and clamped by micropneumatic grips. All the tensile tests were conducted with a gauge length of 10 mm at an extension rate of 1.5 mm/min [23]. In addition, a video recorder (Canon Handicam) was used to study failure behavior during the tensile tests. 3. Results and discussion 3.1. Morphological characterizations of CNT fibers and CNT/PIA composite fibers 3.1.1. Morphology of CNT fibers As revealed in Fig. 2c, the CNT fibers consist mainly of multi-
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Fig. 1. Measurement methods of electrical conductivities of CNT fibers and CNT/PIA composite fibers.
Fig. 2. Typical SEM images of (a) the CNT films and (b) the cross section of a CNT fibers. (c) TEM image of nanotubes in the CNT fibers. (d) Raman spectrum of the CNT fibers.
walled CNTs, with a diameter of ~15 nm and approximately 15e20 walls. Raman spectroscopy in Fig. 2d shows the intensity ratio of the G peak (IG ~1580 cm1) to the D peak (ID ~1350 cm1) to be ~2.4, indicating fewer defects in the CNT structures [24]. The cross section of a CNT fiber is shown in Fig. 2b. As the CNT fibers were obtained by rolling up the aligned CNT films (Fig. 2a), the resultant CNT fiber appears to have a layer-by-layer structure, with gaps residing in between the different layers. 3.1.2. Morphology of CNT/PIA composite fibers Polyimide aerogels were synthesized by using OAPS to crosslink the polyamic acid oligomer obtained from BPDA and ODA. BPDA and ODA are commonly used as dianhydride and diamine monomers in polyimide resin and polyimide aerogels. Herein BPDA imparts the high temperature resistivity and glass transition temperature [25], while ODA provides a flexible backbone [20]. In addition, OAPS has been regarded as an effective way to enhance the thermal and mechanical properties of the polyimide nanocomposites [19,20]. As shown in Fig. 3b, the CNT/PIA composite fiber exhibits a coreeshell structure. Fig. 4 shows a further SEM and EDX examination on its cross-sectional areas, revealing that the CNT fiber
serves as a core, and that the layer-by-layer structure in the composite fiber is preserved (Fig. 4a). Each of these layers consists of closely-arranged CNT filaments with diameters of approximately 20e100 nm, estimated from Fig. 4a. Outside the core area of CNTs, a combination area exists where CNT filaments are embedded in the polymer matrix, with a relatively compacted porous structure (Fig. 4b). The combination area mainly comprises clusters of nanoparticles. The existence of closely-arranged CNT filaments in the core area may confine the space for complete crosslinking to develop a porous network. The continuous fibrous structure of polyimides is therefore largely disturbed in the combination area, and consequently results in circular particles instead of bundles. The polyimide aerogel shell is formed at the outermost part of the composite fibers. Polyimide aerogels tightly wrap the combination area and form the cylindrical shell of the composite fiber (Fig. 4c). The porous polyimide aerogels are mainly composed of tangled bundles of polymer fibers, where the fiber diameter is approximately 50 nm, estimated from Fig. 4c. The core, combination area, and shell of the CNT/PIA composite fibers were further confirmed by the elemental compositions evaluated by EDX (Fig. 4d, e and f). However, the reaction between OAPS and polyamic acid in the presence of closely packed CNT
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Fig. 3. (a) The CNT/PIA composite fibers. (b) A SEM image of the cross section of a CNT/PIA composite fiber.
Fig. 4. SEM and EDX images of the CNT/PIA composite fibers.
filaments, as well as the development mechanisms of their structures, warrants further investigation. An IUPAC type IV nitrogen adsorption/desorption isotherm was obtained for the CNT/PIA composite fiber at 77 K, indicating its
mesoporous structure. According to the Branuaer-Emmet-Teller (BET) theory, the surface area of the composite fiber is calculated to be ~240 m2/g. The majority of the pore sizes cluster at 35 and 60 nm respectively, with several small peaks in the range of 2e10 nm.
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3.2. Electrical properties of CNT fibers and their CNT/PIA composite fibers 3.2.1. Electrical properties of CNT fibers The densities and the electrical conductivities of the CNT fibers are listed in Table 1. As all the CNT fibers were made from CNT films under the same collection time of 10 min, a CNT fiber of a smaller diameter thus possesses a more compact structure and thinner gaps than those of a larger diameter. The densities of these CNT fibers increased from 0.40 to 0.97 g/cm3 as their diameters decreased from ~770 to ~500 mm. Correspondingly, their electrical conductivity exhibited an increase, from 217 S/cm for CNTF-770, to 419 S/cm for CNTF-500. It can be explained that the smaller CNT fibers have more compact structures, and therefore increased contact within the CNT, which allows for a faster rate of electron transport. 3.2.2. Electrical properties of the CNT/PIA composite fibers The electrical properties of the CNT/PIA composite fibers are listed in Table 1. Two methods were applied to measure the electrical conductivity of composite fibers (Fig. 1). Due to the ultrainsulating property of polyimide aerogels, the shell area increased the diameter of the composite fibers, while making negligible contribution to electrical conductivity. Therefore, the composite electrical conductivity (Fig. 1b) is much lesser than the core electrical conductivity (Fig. 1c). Comparing the CNT/PIA composite fibers to the CNT fibers, it was found that the core electrical conductivities were of the same order, but the CNT/PIA composite fibers had slightly lower values than those of the CNT fibers, due to less CNT contact caused by the embedded polyimide. As the CNT fiber diameter increased, electrical conductivity decreased significantly. For the CNTF-500/PIA composite fiber, its core electrical conductivity was nearly the same as that of its corresponding CNT fiber as prepared; whereas the core electrical conductivities of CNTF-630/PIA and CNTF-770/PIA composite fibers were reduced to 94% and 85% of their corresponding CNT fibers, respectively. This phenomenon might be caused by the looser structure of the larger CNT fibers, leading to increased uptake of polyimide solution occurring during dip-coating, and broader combination areas and poorer tube-to-tube contacts being induced. If a parallel connection is assumed for the core and shell areas, the electrical resistances of the CNT/PIA composite fibers (Rcomposite) can be calculated by Rcomposite ¼ Rcore $Rshell =ðRcore þ Rshell Þ. Using an equal length of the core and shell areas, the electrical conductivities of the composite fibers can then be calculated by scomposite ¼ ðscore $D2core þ sshell $D2shell Þ=D2composite , where D and s refer to the diameter and electrical conductivity of each part respectively. Given the ultra-insulating property of polyimides, sshell is negligible. Finally, scomposite can be estimated according to scomposite zscore $D2core =D2composite ¼ score $fcore , where fcore is the fractional volume of the core area. Fig. 5 shows that the estimated electrical conductivities of the composite fibers are very close to
Fig. 5. Electrical conductivities of the CNT fibers and the CNT/PIA composite fibers.
their measured values. Meanwhile, the CNT/PIA composite fibers prepared in our study demonstrated considerably higher electrical conductivities as compared to other CNT/polyimide composites (Table 2). 3.3. Failure mechanisms and mechanical properties of CNT fibers and the CNT/PIA composite fibers 3.3.1. Failure mechanism of CNT fibers and the CNT/PIA composite fibers The mechanical properties of the CNT fibers were evaluated by tensile tests. The linear stressestrain curve of the CNT fiber in Fig. 6a indicates uniform load-bearing throughout the CNT fiber. The mechanical properties of the CNT/PIA composite fibers were also evaluated by tensile tests, as shown in Fig. 6b. Unlike the stressestrain curve of the CNT fiber, the CNT/PIA composite fiber exhibited 2 distinct portions in its loading curve in Fig. 6b: the first from a strain of 0%e10%, and the second from 10% onwards. In the first portion, stress reaches the peak at 6% strain, before fluctuating downwards to 2 MPa, which corresponds to the failure of the polyimide shell at the first step, as shown in Fig. 6c. After the PIA shell area breaks, the tensile load is then taken over by the CNT fiber in the core area. Therefore, the second portion in Fig. 6b largely follows the stressestrain curve of the CNT fiber in Fig. 6a. Eventually, both the core and shell areas of the CNT/PIA composite fiber fail within its gauge length, in a brittle manner (Fig. 6d). 3.3.2. Mechanical properties of the CNT/PIA composites Fig. 6e shows that the strength of the CNT fibers gradually decreases from 83 to 37 MPa, with an increase of fiber diameter from 500 to 770 mm. The decreased tensile strengths of the bigger CNT fibers should probably only be a result of their looser structures and
Table 1 Electrical properties of CNT fibers and CNT/PIA composite fibers. Sample No.a,b
Density (g/cm3)
Diameter (mm)
Core diameter (mm)
Electrical conductivity (S/cm)
Core electrical conductivity (S/cm)
CNTF-500 CNTF-630 CNTF-770 CNTF-500/PIA CNTF-630/PIA CNTF-770/PIA
0.97 0.56 0.40 0.39 0.39 0.37
501 638 762 1218 1284 1372
e e e 491 628 773
419 273 217 60 59 49
e e e 418 257 184
a b
CNTF-n: CNTF refers to CNT fiber; n refers to the diameter of the CNT fiber. CNTF-n/PIA: CNTF refers to CNT fiber; n refers to the diameter of the CNT fiber to synthesize the composite fiber; PIA refers to polyimide aerogel.
P. Liu et al. / Composites Science and Technology 117 (2015) 114e120 Table 2 Comparison of electrical conductivity of CNT/polyimide composites. Composite components
CNT type
Electrical conductivity (S/cm)
SWNT/polyimide [27] MWNT/polyimide [28] MWNT/polyimide [29] MWNT/polyimide [30] MWNT/polyimide [31] MWNT/polyimide aerogel
CNT CNT CNT CNT CNT CNT
3 107 104e101 105 4 103 183 60
powder powder powder powder film fiber
larger cross-sectional areas. Fig. 6f shows that the stiffness of the CNT fibers decreases with an increase in diameter. The 550-mm CNT fiber has the highest stiffness of 450 MPa, over two times that of the 630-mm and 770-mm fibers. This can be explained by the fact that after mechanical densification, fibers with smaller diameters are denser and more compact. The CNT bundles increase in size, with better alignment and more inter-tube contacts formed. They have better load transfer across tubes; therefore leading to higher
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mechanical performance. The results are consistent with findings on the effects of mechanical densification on the strength of CNT fibers produced from a CNT forest [26].
3.3.3. Mechanical properties of the CNT/PIA composites Fig. 6e shows that the tensile strength of the CNT/PIA composite fibers decreases slightly from 11 MPa to 9 MPa as its diameter increases. Although the CNT core of the CNT/PIA composite fibers has the same diameter as the corresponding CNT fibers, the tensile strength of the CNT/PIA composite fibers is much lower than that of the CNT fibers. The reason is that the initial cross-section area of the CNT/PIA composite fibers, which was assumed constant in the calculation of tensile strength, is much larger than that of the CNT fiber. The CNT/PIA composite fibers show a two-stage failure behavior. For the first-stage failure of the CNT/PIA composite fibers, their stiffness is likely determined by the PIA shells. The downward trend in Fig. 6f may be due to the decreasing thickness of the polyimide
Fig. 6. Stressestrain curves of (a) the CNT fiber, (b) the CNT/PIA composite fiber, (c) the PIA fracture and (d) the CNT fiber fracture during the tensile test. (e) tensile strength and (f) stiffness of the CNT fibers and the CNT/PIA composite fibers.
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shell as the diameter of the CNT fibers increases. For the secondstage failure, the core CNT fibers may determine the failure of the CNT/PIA composite fibers. Here the Young's modulus can thus be attributed to the increased diameters of the corresponding CNT fibers. It is interesting to note that the stiffness in the first stage is higher than that of the second stage. This may be because both core and shell parts contribute to the stiffness in the first stage, whereas only the core contributes to stiffness in the later stage. As a result, the load transfer in the first stage is superior to the second stage, which results in higher stiffness. 4. Conclusions The CNT fibers were fabricated by rolling the CNT thin films, which were produced via the floating catalyst CVD method. Gelcoated CNT fibers were synthesized by dip-coating the CNT fibers in the sol solution and the supercritical CO2 drying process. The formation mechanism and morphological effects on the electrical and mechanical properties of the CNT/PIA composite fibers were comprehensively quantified. These composite fibers present a coreeshell structure with light weight, low density and high surface area, and show significant enhancements in tensile strength, stiffness and electrical conductivity, compared with the pure PIA and CNT/polyimide composites. New synthesis approaches are currently under development for better control of the dimensions of CNT-polyimide aerogel composite fibers. To enhance their mechanical properties, cross-linking should be developed within the CNT fibers. Chemical compositions and fabrication conditions of PIA should also be optimized for better polyimide infiltration into the CNT fibers. References [1] M.F.L. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (6119) (2013) 535e539. [2] M. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly, R.S. Ruoff, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science 287 (5453) (2000) 637e640. [3] Q.W. Li, Y. Li, X.F. Zhang, S.B. Chikkannanavar, Y.H. Zhao, A.M. Dangelewicz, et al., Structure-dependent electrical properties of carbon nanotube fibers, Adv. Mater. 19 (20) (2007) 3358e3363. [4] E. Pop, D. Mann, Q. Wang, K. Goodson, H. Dai, Thermal conductance of an individual single-wall carbon nanotube above room temperature, Nano Lett. 6 (1) (2005) 96e100. nicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, et al., [5] B. Vigolo, A. Pe Macroscopic fibers and ribbons of oriented carbon nanotubes, Science 290 (5495) (2000) 1331e1334. [6] N. Behabtu, C.C. Young, D.E. Tsentalovich, O. Kleinerman, X. Wang, A.W.K. Ma, et al., Strong, light, multifunctional fibers of carbon nanotubes with ultrahigh conductivity, Science 339 (6116) (2013) 182e186. [7] M. Zhang, K.R. Atkinson, R.H. Baughman, Multifunctional carbon nanotube yarns by downsizing an ancient technology, Science 306 (5700) (2004) 1358e1361. [8] Y.-L. Li, I.A. Kinloch, A.H. Windle, Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis, Science 304 (5668) (2004) 276e278.
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Formation mechanisms and morphological effects on multi-properties of carbon nanotube fibers and their polyimide aerogel-coated composites Peng Liu 1, Thang Q. Tran 1, Zeng Fan, Hai M. Duong* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online 21 June 2015
This paper aims to understand the formation mechanisms and morphological effects of polymer-aerogel reinforcements that guide the production of lightweight and multifunctional carbon nanotube (CNT) fibers. The CNT fibers and the polyimide aerogel-coated CNT fiber (PIA) composites were successfully developed. CNT thin films were fabricated from an aerogel technique; these films were then mechanically rolled to form CNT fibers of various diameters and densities. The CNT fibers thus prepared were dipcoated in the polyamic acid solutions to form gel-coated fibers, which were then dried with supercritical CO2 to obtain CNT/PIA composite fibers. The composite fibers presented as a coreeshell structure in which the CNT fibers were tightly wrapped by porous polyimide aerogels. The relationships between the electrical and mechanical structural properties of the composites were investigated. By controlling the diameter of CNT fibers to 500 mm, we achieved electrical conductivity of 418 S/cm, and strength and stiffness values of 11.6 and 68.1 MPa, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Polyimide aerogels Nano composites Electrical properties Mechanical properties
1. Introduction Carbon nanotubes (CNTs) are rolled-up graphene sheets with cylindrical nanostructures, where sp2 bonded carbon atoms are arranged in a hexagonal lattice [1]. Because of their unique structure, individual CNTs show extraordinary mechanical (Young's modulus of up to 1 TPa [2]), electrical (over 3 104 S/cm [3]), and thermal (up to 3500 W/m$K [4]) properties. However, due to the difficulty in handling and in their dispersion, the extension of these superior properties of individual CNTs onto a macroscopic scale still remains a challenge. Regarding the practical application of CNTs, assembling CNTs into CNT fibers has been considered a significant step. To date, methods to produce CNT fibers generally include: (1) wet-spinning from CNT acid/polymer solutions [5,6], (2) dry-drawing and spinning from vertically aligned CNT arrays [7], and (3) direct-assembly via floating catalyst chemical vapor deposition (FCCVD) [8,9]. Among them, the direct-CVD growth method has gained tremendous interest owing to its straightforward process and capability for
* Corresponding author. E-mail address: [email protected] (H.M. Duong). 1 Equal distribution. http://dx.doi.org/10.1016/j.compscitech.2015.06.009 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
mass production [8e10]. In addition, Ma et al. [11] and Feng et al. [12] have demonstrated an indirect approach to produce CNT fibers, by twisting and/or rolling of the CVD-grown CNT films. Compared to direct spinning methods, the twisting/rolling approach may be unfavorable for the continuous production of CNT fibers, but the control of the resultant fiber diameters could be more conveniently achieved [13]. The CNT fibers, as prepared in this way, commonly possess satisfactory electrical and thermal conductivities [14], and even better flexibility and higher modulus, strength and breaking energy than commercial carbon fibers and polymer fibers [9]. In order to further improve their properties (mechanical performance in particular), polymer infiltration is usually performed on these CNT fibers. Liu et al. [15] reported the preparation of CNT/polyvinyl alcohol (PVA) composite fibers by infiltrating a polyvinyl alcohol (PVA)/dimethyl sulfoxide (DMSO) solution into the raw CNT fibers, and obtained an increase of 400% in fiber strength. By a similar process, Fang et al. [16] fabricated the CNT/polyimide (PI)/poly(amic acid) (PAA) composite fibers, which exhibited a high strength value of 2.06 GPa, superior even to Grafil 34e700 carbon fibers and Kevlar 49 aramid fibers, when considering their individual densities. Such mechanical enhancement of CNT fibers via polymer infiltration is generally considered a result of the adhesion and load
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transfer between CNTs and the infiltrated polymers [17]. However, the resulting composite fibers would unfortunately suffer excess weight and compromised electrical conductivity with the presence of non-conducting polymers. In recent years, polymer aerogels have been developed as a good alternative to polymers for a lightweight application of reinforcement. Polyimide aerogels (PIAs, either in a linear [18] or crosslinked structure [19e21]), inheriting the excellent thermal stability from polyimides and the low densities of aerogels, are especially promising. Nevertheless, the development of a CNT/polymer aerogel composite fiber has not been reported so far. For CNT/polymer composites, the existence of CNTs has been shown to alter the polymer packing, which significantly impacts their multifunctional properties [22]. However, the role of CNTs in a CNT/polymer aerogel composite is still unclear. As the CNT fibers are already tightly packed, how the polymer chains form around CNTs, as well as the resulting morphology of the composite fibers, are also necessary avenues to explore in contrast to the raw CNT fibers. In addition, electrical and mechanical properties of most porous materials depend strongly on their morphologies. In view of a comprehensive study, it is worthwhile to investigate the formation mechanisms and morphological effects on the multifunctional properties of both CNT fibers and their aerogel composites. Additionally, enhancements to PIA properties are also expected, due to the presence of oriented CNTs. In this paper, we first proposed a simple and versatile route to prepare CNT/PIA composite fibers. To have better control over the fiber diameters, CNT fibers were prepared by simply rolling the CVD-grown CNT films. CNT/PIA composite fibers were obtained by dip-coating these CNT fibers in the polyamic acid oligomer, followed by gelation, aging and supercritical drying. Formation mechanisms of both the CNT fibers and CNT/PIA composite fibers were carefully compared and investigated, in relation to their morphologies. The morphological effects on electrical properties and mechanical performance (of both CNT fibers and CNT/PIA composite fibers) were also comprehensively quantified. This research would be meaningful for exploiting the property of CNT fibers, and prompting the development of CNT/aerogel composites. 2. Experiments 2.1. Materials Ferrocene, thiophene, 1emethyle2epyrrolidone (NMP), 4, 40 eoxydianiline (ODA), 3, 30 , 4, 40 ebiphenyltetracarbonxylic dianhydride (BPDA), acetic anhydride, pyridine and ethanol were purchased from SigmaeAldrich Company Ltd. Methane (CH4); hydrogen (H2) and helium (He) were purchased from ChemeGas Pte Ltd.; and Octa(aminophenyl)silsesquioxane (OAPS) was purchased from Gulf Chemical (Singapore) Pte Ltd. All the chemicals above were used in the condition they were received in, without further purification. 2.2. Synthesis of carbon nanotube fibers (CNT fibers) The CNT fibers were fabricated from CNT films, which were synthesized by the FCCVD method [9]. In this process, CH4, ferrocene, and thiophene were applied as the carbon source, catalyst, and reaction promoter respectively. In a reducing hydrogen atmosphere, the nanotubes formed an aerogel in the hot zone of the furnace (1200 C) and stretched into cylindrical hollow socks due to the gas flow. To collect a CNT film, the CNT sock, as it was formed inside the reactor, was pulled out from the furnace by a stainless steel rod, and continuously wound on a roller for 10 min. This film was then cut
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into 4 cm 4 cm pieces, and mechanically rolled from one end to the other to fabricate the CNT fibers. The 3 different diameters (500e770 mm) of CNT fibers were obtained by carefully controlling the magnitude of the applied force (1e5 N). 2.3. Synthesis of carbon nanotube/polyimide aerogel (CNT/PIA) composite fibers A polyamic acid oligomer with a molar ratio of BPDA: ODA ¼ 26: 25 was formulated in NMP [20]. 1.2796 g of BPDA was added to a solution of ODA (0.8376 g) in 19.0 ml of NMP. After the BPDA was fully dissolved, a solution of 0.0481 g OAPS in 3.15 ml NMP was then added, with 10 min of stirring. Next, 3.30 ml acetic anhydride and 2.80 ml pyridine were added to the obtained mixture, and stirred for another 10 min. For CNT/PIA composite fiber synthesis, the CNT fibers were dipcoated in the polyamic acid oligomer for 1 min to form a uniform coating, and then hung vertically in the air for subsequent gelation and aging. The gelation process took place within 20 min, and the gel-coated fibers were aged for 24 h. After aging, the gel-coated CNT fibers were soaked at 24 h intervals, in the respective solutions of 75% NMP in ethanol, 25% NMP in ethanol and 100% ethanol, sequentially. The CNT/PIA composite fibers were eventually obtained by drying these gel-coated CNT fibers, through the supercritical drying method using CO2. 2.4. Characterization The diameters of the CNT fibers and CNT/PIA composite fibers were measured by an optical microscope (Olympus LG-PS2). The cross-sections of the CNT fibers and the CNT/PIA composite fibers were observed using a field emission scanning electron microscope (FE-SEM, Model S-4300, Hitachi). The structures of the CNT fibers were characterized by a transmission electron microscope (TEM, JEOL JEM-3010) and a micro-Raman microscope (Renishaw InVia 514.5 nm Arþ). To determine the pore properties of the CNT/PIA composite fibers, all the composite fibers were degassed under vacuum at 70 C for 12 h, after which the nitrogen adsorption/desorption test was carried out on a Nova 2200e (Quantachrome). Electrical conductivities of the CNT fibers and CNT/PIA composite fibers were measured using a Fluke 73III multimeter. For better electrical contacts, the two ends of the fiber samples were fixed on two separate glass slides, using silver paste. Given the coreeshell structure of the CNT/PIA composite fibers, their electrical conductivities were measured in two ways, as shown in Fig. 1. The conductivity of an entire composite fiber (scomposite) was obtained by connecting entire cross sections of its two ends, while its core electrical conductivity (score) was obtained by only connecting two core regions with CNT fibers. Tensile tests of the CNT fibers and CNT/PIA composite fibers were performed on an Instron Micro Tester. The samples were sandwiched between 2 pieces of cardboard and clamped by micropneumatic grips. All the tensile tests were conducted with a gauge length of 10 mm at an extension rate of 1.5 mm/min [23]. In addition, a video recorder (Canon Handicam) was used to study failure behavior during the tensile tests. 3. Results and discussion 3.1. Morphological characterizations of CNT fibers and CNT/PIA composite fibers 3.1.1. Morphology of CNT fibers As revealed in Fig. 2c, the CNT fibers consist mainly of multi-
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Fig. 1. Measurement methods of electrical conductivities of CNT fibers and CNT/PIA composite fibers.
Fig. 2. Typical SEM images of (a) the CNT films and (b) the cross section of a CNT fibers. (c) TEM image of nanotubes in the CNT fibers. (d) Raman spectrum of the CNT fibers.
walled CNTs, with a diameter of ~15 nm and approximately 15e20 walls. Raman spectroscopy in Fig. 2d shows the intensity ratio of the G peak (IG ~1580 cm1) to the D peak (ID ~1350 cm1) to be ~2.4, indicating fewer defects in the CNT structures [24]. The cross section of a CNT fiber is shown in Fig. 2b. As the CNT fibers were obtained by rolling up the aligned CNT films (Fig. 2a), the resultant CNT fiber appears to have a layer-by-layer structure, with gaps residing in between the different layers. 3.1.2. Morphology of CNT/PIA composite fibers Polyimide aerogels were synthesized by using OAPS to crosslink the polyamic acid oligomer obtained from BPDA and ODA. BPDA and ODA are commonly used as dianhydride and diamine monomers in polyimide resin and polyimide aerogels. Herein BPDA imparts the high temperature resistivity and glass transition temperature [25], while ODA provides a flexible backbone [20]. In addition, OAPS has been regarded as an effective way to enhance the thermal and mechanical properties of the polyimide nanocomposites [19,20]. As shown in Fig. 3b, the CNT/PIA composite fiber exhibits a coreeshell structure. Fig. 4 shows a further SEM and EDX examination on its cross-sectional areas, revealing that the CNT fiber
serves as a core, and that the layer-by-layer structure in the composite fiber is preserved (Fig. 4a). Each of these layers consists of closely-arranged CNT filaments with diameters of approximately 20e100 nm, estimated from Fig. 4a. Outside the core area of CNTs, a combination area exists where CNT filaments are embedded in the polymer matrix, with a relatively compacted porous structure (Fig. 4b). The combination area mainly comprises clusters of nanoparticles. The existence of closely-arranged CNT filaments in the core area may confine the space for complete crosslinking to develop a porous network. The continuous fibrous structure of polyimides is therefore largely disturbed in the combination area, and consequently results in circular particles instead of bundles. The polyimide aerogel shell is formed at the outermost part of the composite fibers. Polyimide aerogels tightly wrap the combination area and form the cylindrical shell of the composite fiber (Fig. 4c). The porous polyimide aerogels are mainly composed of tangled bundles of polymer fibers, where the fiber diameter is approximately 50 nm, estimated from Fig. 4c. The core, combination area, and shell of the CNT/PIA composite fibers were further confirmed by the elemental compositions evaluated by EDX (Fig. 4d, e and f). However, the reaction between OAPS and polyamic acid in the presence of closely packed CNT
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Fig. 3. (a) The CNT/PIA composite fibers. (b) A SEM image of the cross section of a CNT/PIA composite fiber.
Fig. 4. SEM and EDX images of the CNT/PIA composite fibers.
filaments, as well as the development mechanisms of their structures, warrants further investigation. An IUPAC type IV nitrogen adsorption/desorption isotherm was obtained for the CNT/PIA composite fiber at 77 K, indicating its
mesoporous structure. According to the Branuaer-Emmet-Teller (BET) theory, the surface area of the composite fiber is calculated to be ~240 m2/g. The majority of the pore sizes cluster at 35 and 60 nm respectively, with several small peaks in the range of 2e10 nm.
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3.2. Electrical properties of CNT fibers and their CNT/PIA composite fibers 3.2.1. Electrical properties of CNT fibers The densities and the electrical conductivities of the CNT fibers are listed in Table 1. As all the CNT fibers were made from CNT films under the same collection time of 10 min, a CNT fiber of a smaller diameter thus possesses a more compact structure and thinner gaps than those of a larger diameter. The densities of these CNT fibers increased from 0.40 to 0.97 g/cm3 as their diameters decreased from ~770 to ~500 mm. Correspondingly, their electrical conductivity exhibited an increase, from 217 S/cm for CNTF-770, to 419 S/cm for CNTF-500. It can be explained that the smaller CNT fibers have more compact structures, and therefore increased contact within the CNT, which allows for a faster rate of electron transport. 3.2.2. Electrical properties of the CNT/PIA composite fibers The electrical properties of the CNT/PIA composite fibers are listed in Table 1. Two methods were applied to measure the electrical conductivity of composite fibers (Fig. 1). Due to the ultrainsulating property of polyimide aerogels, the shell area increased the diameter of the composite fibers, while making negligible contribution to electrical conductivity. Therefore, the composite electrical conductivity (Fig. 1b) is much lesser than the core electrical conductivity (Fig. 1c). Comparing the CNT/PIA composite fibers to the CNT fibers, it was found that the core electrical conductivities were of the same order, but the CNT/PIA composite fibers had slightly lower values than those of the CNT fibers, due to less CNT contact caused by the embedded polyimide. As the CNT fiber diameter increased, electrical conductivity decreased significantly. For the CNTF-500/PIA composite fiber, its core electrical conductivity was nearly the same as that of its corresponding CNT fiber as prepared; whereas the core electrical conductivities of CNTF-630/PIA and CNTF-770/PIA composite fibers were reduced to 94% and 85% of their corresponding CNT fibers, respectively. This phenomenon might be caused by the looser structure of the larger CNT fibers, leading to increased uptake of polyimide solution occurring during dip-coating, and broader combination areas and poorer tube-to-tube contacts being induced. If a parallel connection is assumed for the core and shell areas, the electrical resistances of the CNT/PIA composite fibers (Rcomposite) can be calculated by Rcomposite ¼ Rcore $Rshell =ðRcore þ Rshell Þ. Using an equal length of the core and shell areas, the electrical conductivities of the composite fibers can then be calculated by scomposite ¼ ðscore $D2core þ sshell $D2shell Þ=D2composite , where D and s refer to the diameter and electrical conductivity of each part respectively. Given the ultra-insulating property of polyimides, sshell is negligible. Finally, scomposite can be estimated according to scomposite zscore $D2core =D2composite ¼ score $fcore , where fcore is the fractional volume of the core area. Fig. 5 shows that the estimated electrical conductivities of the composite fibers are very close to
Fig. 5. Electrical conductivities of the CNT fibers and the CNT/PIA composite fibers.
their measured values. Meanwhile, the CNT/PIA composite fibers prepared in our study demonstrated considerably higher electrical conductivities as compared to other CNT/polyimide composites (Table 2). 3.3. Failure mechanisms and mechanical properties of CNT fibers and the CNT/PIA composite fibers 3.3.1. Failure mechanism of CNT fibers and the CNT/PIA composite fibers The mechanical properties of the CNT fibers were evaluated by tensile tests. The linear stressestrain curve of the CNT fiber in Fig. 6a indicates uniform load-bearing throughout the CNT fiber. The mechanical properties of the CNT/PIA composite fibers were also evaluated by tensile tests, as shown in Fig. 6b. Unlike the stressestrain curve of the CNT fiber, the CNT/PIA composite fiber exhibited 2 distinct portions in its loading curve in Fig. 6b: the first from a strain of 0%e10%, and the second from 10% onwards. In the first portion, stress reaches the peak at 6% strain, before fluctuating downwards to 2 MPa, which corresponds to the failure of the polyimide shell at the first step, as shown in Fig. 6c. After the PIA shell area breaks, the tensile load is then taken over by the CNT fiber in the core area. Therefore, the second portion in Fig. 6b largely follows the stressestrain curve of the CNT fiber in Fig. 6a. Eventually, both the core and shell areas of the CNT/PIA composite fiber fail within its gauge length, in a brittle manner (Fig. 6d). 3.3.2. Mechanical properties of the CNT/PIA composites Fig. 6e shows that the strength of the CNT fibers gradually decreases from 83 to 37 MPa, with an increase of fiber diameter from 500 to 770 mm. The decreased tensile strengths of the bigger CNT fibers should probably only be a result of their looser structures and
Table 1 Electrical properties of CNT fibers and CNT/PIA composite fibers. Sample No.a,b
Density (g/cm3)
Diameter (mm)
Core diameter (mm)
Electrical conductivity (S/cm)
Core electrical conductivity (S/cm)
CNTF-500 CNTF-630 CNTF-770 CNTF-500/PIA CNTF-630/PIA CNTF-770/PIA
0.97 0.56 0.40 0.39 0.39 0.37
501 638 762 1218 1284 1372
e e e 491 628 773
419 273 217 60 59 49
e e e 418 257 184
a b
CNTF-n: CNTF refers to CNT fiber; n refers to the diameter of the CNT fiber. CNTF-n/PIA: CNTF refers to CNT fiber; n refers to the diameter of the CNT fiber to synthesize the composite fiber; PIA refers to polyimide aerogel.
P. Liu et al. / Composites Science and Technology 117 (2015) 114e120 Table 2 Comparison of electrical conductivity of CNT/polyimide composites. Composite components
CNT type
Electrical conductivity (S/cm)
SWNT/polyimide [27] MWNT/polyimide [28] MWNT/polyimide [29] MWNT/polyimide [30] MWNT/polyimide [31] MWNT/polyimide aerogel
CNT CNT CNT CNT CNT CNT
3 107 104e101 105 4 103 183 60
powder powder powder powder film fiber
larger cross-sectional areas. Fig. 6f shows that the stiffness of the CNT fibers decreases with an increase in diameter. The 550-mm CNT fiber has the highest stiffness of 450 MPa, over two times that of the 630-mm and 770-mm fibers. This can be explained by the fact that after mechanical densification, fibers with smaller diameters are denser and more compact. The CNT bundles increase in size, with better alignment and more inter-tube contacts formed. They have better load transfer across tubes; therefore leading to higher
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mechanical performance. The results are consistent with findings on the effects of mechanical densification on the strength of CNT fibers produced from a CNT forest [26].
3.3.3. Mechanical properties of the CNT/PIA composites Fig. 6e shows that the tensile strength of the CNT/PIA composite fibers decreases slightly from 11 MPa to 9 MPa as its diameter increases. Although the CNT core of the CNT/PIA composite fibers has the same diameter as the corresponding CNT fibers, the tensile strength of the CNT/PIA composite fibers is much lower than that of the CNT fibers. The reason is that the initial cross-section area of the CNT/PIA composite fibers, which was assumed constant in the calculation of tensile strength, is much larger than that of the CNT fiber. The CNT/PIA composite fibers show a two-stage failure behavior. For the first-stage failure of the CNT/PIA composite fibers, their stiffness is likely determined by the PIA shells. The downward trend in Fig. 6f may be due to the decreasing thickness of the polyimide
Fig. 6. Stressestrain curves of (a) the CNT fiber, (b) the CNT/PIA composite fiber, (c) the PIA fracture and (d) the CNT fiber fracture during the tensile test. (e) tensile strength and (f) stiffness of the CNT fibers and the CNT/PIA composite fibers.
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shell as the diameter of the CNT fibers increases. For the secondstage failure, the core CNT fibers may determine the failure of the CNT/PIA composite fibers. Here the Young's modulus can thus be attributed to the increased diameters of the corresponding CNT fibers. It is interesting to note that the stiffness in the first stage is higher than that of the second stage. This may be because both core and shell parts contribute to the stiffness in the first stage, whereas only the core contributes to stiffness in the later stage. As a result, the load transfer in the first stage is superior to the second stage, which results in higher stiffness. 4. Conclusions The CNT fibers were fabricated by rolling the CNT thin films, which were produced via the floating catalyst CVD method. Gelcoated CNT fibers were synthesized by dip-coating the CNT fibers in the sol solution and the supercritical CO2 drying process. The formation mechanism and morphological effects on the electrical and mechanical properties of the CNT/PIA composite fibers were comprehensively quantified. These composite fibers present a coreeshell structure with light weight, low density and high surface area, and show significant enhancements in tensile strength, stiffness and electrical conductivity, compared with the pure PIA and CNT/polyimide composites. New synthesis approaches are currently under development for better control of the dimensions of CNT-polyimide aerogel composite fibers. To enhance their mechanical properties, cross-linking should be developed within the CNT fibers. Chemical compositions and fabrication conditions of PIA should also be optimized for better polyimide infiltration into the CNT fibers. References [1] M.F.L. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (6119) (2013) 535e539. [2] M. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly, R.S. Ruoff, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science 287 (5453) (2000) 637e640. [3] Q.W. Li, Y. Li, X.F. Zhang, S.B. Chikkannanavar, Y.H. Zhao, A.M. Dangelewicz, et al., Structure-dependent electrical properties of carbon nanotube fibers, Adv. Mater. 19 (20) (2007) 3358e3363. [4] E. Pop, D. Mann, Q. Wang, K. Goodson, H. Dai, Thermal conductance of an individual single-wall carbon nanotube above room temperature, Nano Lett. 6 (1) (2005) 96e100. nicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, et al., [5] B. Vigolo, A. Pe Macroscopic fibers and ribbons of oriented carbon nanotubes, Science 290 (5495) (2000) 1331e1334. [6] N. Behabtu, C.C. Young, D.E. Tsentalovich, O. Kleinerman, X. Wang, A.W.K. Ma, et al., Strong, light, multifunctional fibers of carbon nanotubes with ultrahigh conductivity, Science 339 (6116) (2013) 182e186. [7] M. Zhang, K.R. Atkinson, R.H. Baughman, Multifunctional carbon nanotube yarns by downsizing an ancient technology, Science 306 (5700) (2004) 1358e1361. [8] Y.-L. Li, I.A. Kinloch, A.H. Windle, Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis, Science 304 (5668) (2004) 276e278.
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