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Merge multiple carbon nanotube fibers into a robust yarn
Carbon 145 (2019) 266e272
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Merge multiple carbon nanotube fibers into a robust yarn Wenya Li a, b, d, 1, Jingna Zhao b, c, 1, Yuan Xue a, **, Xueqin Ren d, Xiaohua Zhang b, c, *, Qingwen Li b, *** a
Key Laboratory of Ecotextiles, Ministry of Education, Jiangnan University, Wuxi, 214122, China Division of Advanced Nano-Materials, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China c Division of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang, Chinese Academy of Sciences, Nanchang, 330200, China d School of Textile Science & Engineering, Xi'an Polytechnic University, Xi'an, 710048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2018 Received in revised form 11 December 2018 Accepted 15 January 2019 Available online 19 January 2019
Carbon nanotube (CNT) fibers are merged into a robust larger-diameter yarn by using an adhesion agent of polyethylenimine (PEI). As compared to poly(vinyl alcohol) and epoxy, PEI molecules can unite neighboring CNT fibers by constructing a strong inter-fiber adhesion, owing to their high mobility to soften the fiber surface. The merged yarn inherits fully the tensile strength and toughness from the single fiber, remains electrically conducting, and can be stably used up to 180 C even in O2 . It also exhibits high mechanical performances under various application conditions, such as bending, pressing, twisting, and rubbing. © 2019 Elsevier Ltd. All rights reserved.
Keywords: carbon nanotube fiber Multi-plied yarn Polyethylenimine Mechanical property
1. Introduction Carbon nanotube (CNT) fiber is an important one-dimensional macroscopic assembly material with comparable mechanical performance and much richer functionalities as compared with carbon fibers [1e6]. It has gained great attention over the past decade. For example, the tensile strength of a pure CNT fiber has been improved up to > 3 GPa [1,6e8], a typical value comparable to the carbon fiber T300, the electrical conductivity can be up to 4 104 e1 105 S=m [9,10], and the thermal conductivity is about 100 W=ðmKÞ [11,12]. In many studies, the fiber diameter was just 10 mm (unsuitable for weaving) in order to maintain a high strength, while a CNT fiber with a diameter larger than 50 mm was difficult to be stronger than 1 GPa [13,14]. Thus for the wearable applications, it is still necessary to multi-ply small-diameter CNT fibers into a largesize CNT yarn (diameter > 100 mm), which is very common in
* Corresponding author. Division of Advanced Nano-Materials, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Xue), [email protected] (X. Zhang), [email protected] (Q. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.01.054 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
textile manufacturing [15]. Unfortunately, after the multi-plying there is always a remarkable decrease in tensile strength, due to the weak inter-fiber adhesion and the loosely compacted yarn structure. For example, unavoidable big voids with a typical dimension of hundreds of nm can be formed after the multi-plying [16,17]. Furthermore, the conventional yarn technique usually introduces into the final yarn a structural parameter of twist, which also hinders the mechanical performance. Therefore, toward a high-strength CNT yarn, a new untwisted strategy should be developed to realize a strong adhesion between the component CNT fibers. Very recently, we reported that a tightly-densified and untwisted one-dimensional assembly of CNTs can induce better mechanical performances [8]. According to such assembly structure, it should be also possible to realize multi-plying without introducing the twist treatment. On the other hand, owing to the assembly feature, the surface of CNT fibers can be well impregnated by polymer molecules to realize ‘sizing’ effect [18,19]. This provides us an opportunity to adhere neighboring CNT fibers by introducing an entangled network of the surface CNTs with polymer chains [20]. It is thus expected that we can multi-ply CNT fibers together with strengthened inter-fiber interactions and, importantly, without loosing the fiber's tensile properties as the conventional yarns do so. In this study, we use the drawing die method to realize the
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merging, and various polymers, e.g., polyethylenimine (PEI), poly(vinyl alcohol) (PVA), and epoxy, to assist the adhesion between the neighboring fibers. The merged multiple fibers can be considered as either a CNT yarn or a big-size fiber (yarn is used here). PEI shows the best merging ability as it can soften the fiber surface owing to its high mobility. Its molecular softness also benefits the toughness of the merged yarn. As expected, the specific strength can be up to 0.69 N/tex at a retention over 100%, and the toughness can also be fully inherited ( > 60 J=g). The merged yarn is very robust upon heating, bending, pressing, twisting, and rubbing. 2. Experimental The die-based multi-plying method to merge CNT fibers was designed by following the methods to densify and align CNTs [21,22]. Fig. 1a shows the schematic and photograph of such method. The diamond wire drawing dies with pore diameters of 90, 100, 120, 140, 160, 180, and 200 mm were used, purchased from http://www.cangzhouzhendong.com/. This treatment can well eliminate the void spaces between and even inside the CNT fibers due to the radial pressing. The CNT fibers were fabricated by an injection chemical vapor deposition (iCVD), also called a floating catalyst CVD [5,7,23,24]. For the growth, a solution of absolute ethyl alcohol with 1.2 vol%
Fig. 1. Multi-plying of CNT fibers. (a) Schematic and photograph of the die-based processing. (b) The component CNTs were mainly 2e4-walled. (c,d) SEM morphology images of a pristine CNT fiber and a PEI-merged 6-plied yarn, whose diameter were 52.8 and 106.1 mm, respectively. (e,f) TGA curves in air and N2 for a pristine fiber and a PEI-merged yarn, respectively.
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ferrocene and 0.4 vol% thiophene carried by Ar/H2 was injected at a rate of 30 mL=h into the furnace (diameter 80 mm) which had been heated up to 1300 C. An entangled CNT network was formed in the furnace and was carried out by the gas flow. Then these CNTs were introduced into water for shrinking and spun into a continuous fiber out of the water. Under these growth parameters, slightly different from our previous study [24], a long CNT fiber with a diameter around 50e60 mm could be stably obtained. The CNTs were mainly 2e4-walled (Fig. 1b) and the nanotube length was at least 100 mm [8]. PEI, PVA, and epoxy resin were used to assist the multi-plying (merging). PEI (branched, average molecular weight 10 kDa, Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in ethanol at a concentration of 5.5 wt%. PVA (type 1788, polymerization degree 1750, Xiya Reagent Company, China) was dissolved in water at 5.0 wt%. Epoxy resins (Araldite LY 1564, Huntsman, USA), mixed with the curing agent (Aradur 3487, Huntsman, USA) at a mass ratio of 3:1, were dissolved in acetone at a total concentration of 5.5 wt%. The molecular weight of PEI was already the optimal choice, as the smaller PEI molecules are different to form entanglement with each other and the larger ones are difficult to be infiltrated into CNT assemblies. During the multi-plying, the polymer solution was injected into the entrance cone of the die, with the cone fulfilled during all the whole processing procedure. Thus the polymer infiltration level was determined just by the multiple CNT fibers themselves, and was finally examined by using thermogravimetric analysis (TGA). For the epoxy assisted multi-plying, a following thermal curing at 90 C for 1 h was applied. A high-precision XP2U analytical balance (resolution 0.1 mg, Mettler-Toledo LLC., Columbus, USA) was used to measure the sample weight. A Quanta 400 FEG scanning electron microscope (SEM, FEI, Hillsboro, USA) and a Tecnai G2 F20 S-Twin transmission electron microscope (TEM, FEI, Hillsboro, USA) were used to characterize the sample's dimensions, assembly morphology, and internal microstructure. The tensile properties were measured by an Instron 3365 universal testing machine (UTM, Instron Corp., Norwood, USA) and an MTS E44 UTM (MTS Systems Corporation, Eden Prairie, USA), equipped a 10-N and 1000-N load cell respectively. The strain rates were in the order of 10-1 s1 (or 0.06 mm=min) and the sample's gauge length was 10 mm. A peel test was also performed on the Instron 3365 UTM to measure the force to peel two neighboring fibers out of the merged yarn. The TGA was performed €tebau with a Netzsch TG 209 F1 Libra apparatus (NETZSCH-Gera GmbH, Selb, Germany) in both air and N2 at a heating rate of 10 C from 30 to 900 C. The electrical resistance was measured by a digital multimeter, and was used to calculate the conductivity. In order to evaluate the performance under different applications, heating, bending, pressing, twisting, and rubbing treatments were applied to the PEI-merged 6-plied yarn. The thermal treatment was performed in O2 and N2 for 2 h, at 180, 200, 300, and 400 C. The bending was applied to a 5-cm long yarn. After placing the yarn on a plastic film, the film was folded to make the yarn's two half segments contact with each other. The bending was repeated by 500 and 1000 times. The yarn was also hot-pressed at 100 C by a HR01 Hot Rolling Machine (MTI Corporation, USA), to cause a circleto-ellipse change of the cross section. For the pressing, a CNT yarn was placed between two 10-mm-thick foils, and the distance between the rollers was set to 60 mm. To apply the twist, one end of a yarn was hung by a weight of 20 g, and then a motor is used to twist the yarn from the top. The twisting speed was 30 rpm and the total twist was characterized by the number of twist per yarn length, namely 220, 860, and 1800 twist per meter (T/m) (the final yarn length was used for the calculation). For the rubbing evaluation, two different tests were used. In one test, the merged yarn was physically rubbed with a sandpaper (very fine P800, extra fine
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P600, and super fine P320 sandpapers were used, whose average particle diameters are 46.2, 25.8, and 21.8 mm, respectively), by following a test on human hair fiber [25]. In the other test, two merged yarns were rubbing against each other in a cross, where the transverse yarn was actively moving while the longitudinal one was fixed. Although the rubbing motion was performed just by hand, the number of rubbing motions was still a controllable parameter. For example, it took about 110 rubbing motions by using a P600 sandpaper to break the merged yarn, and for the rubbing with CNT yarns themselves, it took more than 300 force-and-back motions to induce just 10% loss in yarn strength.
3. Results and discussion 3.1. Strength retention after multi-plying The die-based multi-plying can cause densification to CNT fibers. PEI was used as it can provide efficient interaction or binding with CNTs [26e28]. The fiber diameter was in a range of 42e58 mm, determined by the growth parameters including the furnace diameter, concentrations of the feedstock, and the collection speed. The as-produced CNT fibers were not highly densified, as one can find the fiber surface was not smooth (Fig. 1c). For this fiber with a diameter of z53 mm, after the multi-plying with 5 similar-sized fibers, the diameter of the merged yarn was about 106 mm, see Fig. 1d. In all cases, the cross-sectional area of the merged yarn (diameter 95e115 mm) was much smaller than the sum of those before the treatment, usually decreased by more than 30% due to the densification effect. For the case shown in Fig. 1, the decrease was about 33.3%. As a comparison, the direct twisting of CNT fibers could not produce a compact and strong yarn; the fibers were still separating from each other as shown in Supporting Information Fig. S1a. When the number of plies increased, the densification was still very remarkable; the diameter of a 12-plied yarn was about 160e168 mm, see Supporting Information Fig. S1b, corresponding to a decrease in cross-sectional area of 16e24%. Fig. 1e and f compares the TGA curves for a pristine CNT fiber and a PEI-merged yarn, respectively. The residue mass of 10e15 wt % was ascribed to the catalyst nanoparticles [24]. From the mass loss in air at 500 C, the PEI mass percent was estimated to be about 23 wt%. If PVA or epoxy was used, a similar polymer mass percent was measured, see Supporting Information Fig. S2. To evaluate the strength retention, we used the maximum load to avoid the influence from the large diameter shrink. For a pristine CNT fiber with a diameter of 51 mm, the maximum tensile load was up to 0.79 N (Fig. 2a), corresponding to a strength of 387 MPa and
specific strength of 0.50 N=tex (its linear mass density was 1.58 tex). After being merged, we use the maximum load per ply to describe the strength. Fig. 2a compares the typical loadestrain curves for the 6-, 10-, 12-, and 16-plied yarns. Generally, with increasing the number of plies, the strain at break decreased gradually from > 10% to < 5%, while the maximum load per ply could be over 1 N for the 6- and 10-plied yarns. For example, the 6plied yarn exhibited a maximum load of 1.03 N per ply, corresponding to a specific strength of 0.69 N=tex (linear density 8.90 tex, smaller than 1:58 6 ¼ 9:48 tex due to the stretching). Table 1 shows the typical values of the multi-plied yarns in terms of tensile strength, Young's modulus, strain at break, and toughness, obtained from Supporting Information Fig. S3, among which the 1-, 6-, 10-, 12-, and 16-plied yarns are also shown in Fig. 2a. Due to the densification, the yarn strength could be nearly doubled for the 6-plied yarn, from 387 to 756 MPa. However, due to the decrease in strain at break, the multi-plying caused the decrease in toughness, which became much larger when the number of plies was more than 10. The present results indicated that the optimal number of plies was 6, where the strength and toughness were both high. In yarn industry, the specific strength or more directly the ability to carry a load is preferred to describe the strength, as it might be not easy to calculate the yarn's cross-sectional area. Therefore, here the strength retention after the multi-plying was evaluated by using the maximum load per ply. Fig. 2b plots the strength retention as a function of the number of plies, averaged from at least 10 samples. Clearly, the ply numbers from 4 to 12 could induce strengthening for the merged yarn. This shows the present multiplying treatment could not cause any strength loss, quite different from the conventional yarn plying. However, due to the introduction of non-conducting polymer of PEI, the electrical conductivity became smaller by one order of magnitude after the multi-plying (Fig. 2b), yet as conducting as the forest spun CNT fibers whose conductivity is usually in the order of 104 S=m [29,30].
3.2. Comparison between plying agents: PEI, PVA and epoxy The type of polymer is a key issue in determining the multiplying result. For this purpose, PVA and epoxy resin were also used to assist the fiber merging. Fig. 3a compares the merging results by using these different plying agents. (Notice that, another group of pristine CNT fibers was used here, resulting in slight differences in the tensile load and strain at break from Fig. 2a.) For the strength, PEI and epoxy both showed nice retention; their averaged maximum tensile loads were 5.93 and 5.81 N, respectively.
Fig. 2. Effect of multi-plying on the mechanical and electrical properties. (a) Typical loadestrain curves for a pristine fiber and 6-, 10-, 12-, and 16-plied yarns, respectively. (b) Strength retention (the ratio between the maximum loads of the plied yarn and pristine fiber) and electrical conductivity as functions of the number of plies. (A colour version of this figure can be viewed online.)
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Table 1 Typical values of the mechanical properties for the multi-plied CNT yarns tested in Fig. 2a, including the tensile strength (s), Young's modulus (E), strain at break (x), and tensile toughness UT . The fiber diameter df and specific strength and modulus are also provided. For the two-plied yarn, the cross section was not circular, making it difficult to calculate the strength and modulus. Number of plies
df (mm)
s (MPa)
s (N=tex)
E (GPa)
E (N=tex)
x
UT (J=g)
1 2 4 6 8 10 12 14 16
51 e 100 102 121 146 168 184 185
387 e 491 756 641 586 472 443 397
0.50 0.44 0.64 0.69 0.64 0.58 0.49 0.48 0.42
9.72 e 10.8 16.4 11.5 20.6 19.2 18.4 17.3
12.6 19.5 14.1 15.0 11.5 20.4 19.9 19.9 18.3
16.1% 14.2% 8.8% 11.8% 11.3% 5.3% 4.9% 6.1% 3.2%
64.9 52.0 39.5 60.1 51.0 20.9 16.8 23.3 8.5
Fig. 3. Comparison of different plying agents. (a) For 6-plied yarns, PEI showed a larger strength (plotted by dots) than PVA, and a larger strain at break (by using the average plus and minus one standard deviation) than epoxy. (b) Forceedisplacement curves obtained from a peel test to show the different inter-fiber adhesions, where two component CNT fibers were peeled off. (cee) SEM images of the microstructures after the peel test. (feh) Fracture morphology of different merged yarns. (A colour version of this figure can be viewed online.)
Differently, PVA could just reach an average of 4.08 N. For the ability to resist the length elongation, PEI and PVA resulted in a strain at break of z8.2%, while epoxy made the merged yarn brittle, with a strain at break of just 6.2%. This means that PEI could produce benefit both the strength and toughness. From the load-strain curve for the PEI-assisted 6-plied yarn shown in Fig. 2a, the toughness was up to 60.1 J=g by integrating the area beneath the loadestrain tensile curve and then dividing by its linear mass
density, nearly unchanged as compared to the value of 64.9 J=g for the pristine fiber shown in Fig. 2a. However, for PVA and epoxy, the merged yarns exhibited typical toughness values of 25.2 and 23.8 J= g due to the reduced strength or strain at break. To analyze the underlying mechanism, a peel test was performed as schematically shown in the inset of Fig. 3b, where two CNT fibers were peeled off from the merged yarn. Fig. 3b shows the peeling force as a function of peeling displacement (the half of the
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displacement of load cell). Clearly, it required a large force (z0.3e0.35 N) to peel them off for the PEI- and epoxy-merged yarns, while the force was z0.2e0.25 N for PVA. The microstructures of the peeled fiber showed some evidence of the different inter-fiber adhesions, see Fig. 3cee. Due to the strong adhesion of PEI and epoxy, the CNTs at the inter-fiber contacts were pulled out, like plush to cover the fiber surface (see the dashed red ellipses). However the fiber surface was clean for the PVA-adhered sample, corresponding to a weak inter-fiber adhesion. The different fracture morphology (Fig. 3feh) was also a general result of the different polymers. As compared to PVA, the small molecular weights of PEI and epoxy resin allowed more efficient infiltration into the fiber surface, also providing a certain strengthening to the fiber strength [31]. Thus for PEI and epoxy, a flat fracture was observed (Supporting Information Fig. S1c shows another example of epoxy-merged yarn), while long-distance
sliding between CNT bundles dominated the fracture of PVAadhered sample. The excellent merging ability of PEI was ascribed to two possible reasons: the high mobility of the PEI molecules and the intimate interaction between PEI and CNT. Due to the branched structure, the PEI molecule is not a linear chain, but globule like. Thus there is lack of entanglement for the PEI molecules, resulting in the low viscosity [32,33]. Here the small molecular weight (10 kDa) further reduced such viscosity. As a result, the molecules could exhibit high mobility and act as a lubricant between the CNT bundles. Thus a PEI-infiltrated CNT assembly (here is the fiber surface) could become soft and deform very easily. On the other hand, the high polarity of the N H group, higher than the C H and slightly smaller than the O H, is enough to induce an intimate interaction between PEI and CNT. Thus, under the external pressing (like passing through a die), the surface CNTs of the adjacent fibers could
Fig. 4. Performance evaluation of PEI-merged CNT yarns. (a) Fracture loads of 6-plied yarns after being treated at different temperatures in O2 or N2 for 2 h. (bef) Stress-stain curves after being bent for 500 and 1000 times (b), pressed (c), twisted up to 220, 860, and 1800 T/m (d), rubbed against the P800, P600, and P320 sandpapers with 30 rubbing motions (e), and rubbed against another merged CNT yarn with 300 rubbing motions (f). The insets are the corresponding structural morphologies, cross sections, or the test set-up, as labelled by arrows. (A colour version of this figure can be viewed online.)
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be forced to move due to the high mobility of PEI, and the two surfaces could be adhered together without forming voids or gaps between the fibers. On the contrary, when the linear polymer PVA was used, the adhesion between the adjacent fibers was less sufficient, resulting in the clean morphology in the peel test. For epoxy, the curing certainly could cause a polymeric cross-linking network to bind the CNTs from two neighboring fibers. As a result, the inter-fiber adhesion became strong and the peeling could cause the CNT pulling-out. However, the inter-fiber contact was brittle due to the thermosetting feature of epoxy. 3.3. Performance evaluation in different applications Branched PEI molecules are highly thermally stable; they start to decompose in air at 250 C and in N2 at 300 C [34]. Therefore, the PEI-plied CNT yarns could be used in high temperatures. To evaluate this, PEI-merged yarns were treated at different temperatures for 2 h before being tensile-tested. Fig. 4a compares the final fracture loads for 6-ply yarns. The treatment in O2 at 180 C did not show any degradation in fracture load, while a slight load loss of less than 10% was observed for the 200- C treatment, where PEI just started to decompose. With further increasing the temperature, the loss reduce became more and more. Nevertheless, the merged yarn could still exhibit more than 50% retention of fracture load after being treated at 400 C. (Such treatment increased the brittleness as also reflected by the fracture morphology shown in Supporting Information Fig. S1d.) Different from the strong oxidation ability of O2 , the treatment in N2 would not cause such large load loss. After the treatment in 300 C, the merged yarn could still carry a load up to 5.19 N. Clearly, just the removal of PEI in N2 could well remain the inter-fiber contact where a new CNT network had been formed during the multi-plying, while the strong oxidation could burn a certain CNTs at the contacts, strongly affecting the load transfer within the merged yarn. When CNT fibers are merged together, a large-diameter yarn is formed. Upon bending, there will be a large compression stress on one side, and a large tension one on the other side. The softness of PEI molecules, different from the rigid thermosetting ones, could benefit the stress relaxation by allowing slight displacement between the CNTs. As shown in Fig. 4b, after being bent for 500 and 1000 times, there was no clear decrease in the fracture load. The inset also shows that the component 6 fibers did not detach from each other. The softness of PEI also helped avoiding the fiber detachment upon pressing the merged yarn. After being hot-pressed, the circular yarn cross section turned into elliptical, as shown in the insets in Fig. 4c. The morphologies of a slight pressing and a local indentation were shown in Supporting Information Figs. S1e and f. However, several small voids appeared at the ellipse vertices, and the fracture load decreased down to 4.3 N, by 24%. The merged yarn could still carry a high load after introducing high levels of twist. Fig. 4d shows that with increasing the level of twist, the yarn strength decreased with the strain at break increased. This is a general phenomenon of the effect of twist [35]. For a twisted structure, the helical angle is highest at the yarn surface, while the interior is less or even not twisted. Therefore, the yarn surface is densified and becomes the major part to carry the load. This is why the large decrease in strength, more than 50%, was observed in the merged yarn with a diameter of > 100 mm after the twisting, while the decrease is no more than 30% for a small CNT fiber with a diameter of just z10 mm [8]. As a large-size yarn, rubbing and chafing are very common during the application. To evaluate the rubbing performance, fine sandpapers were used. The number of rubbing motions to break the
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yarn can be an important parameter. For example, it took about 110 rubbing motions to break the yarn with a P600 sandpaper. For the strength retention, the merged yarns were rubbed with different sandpapers for 30 times (Fig. 4e). After the rubbing, the retention was about 70%, 30%, and 27% for P800, P600, and P320 sandpapers; the corresponding fracture loads were 3.5, 1.7, and 1.5 N, respectively. The friction between yarns is also important in the textile application. Therefore, the rubbing between the merged yarns was also applied, by moving a transverse yarn against a longitudinal one, see Fig. 4f. Due to the small friction, the wear was not much after hundreds of rubbing motions. For example, after 300 motions, the two yarns could still carry loads up to 4.8e5.4 N. Most importantly, no fiber detachment was observed in the yarn after the rubbing, which was the main reason for the high strength retention. 4. Conclusion CNT fibers can be merged into a robust yarn by using a branched polymer of PEI. Owing to the entanglement between the polymer and CNTs, the merging caused the disappearance of the inter-fiber interface and densification of the CNTs. The merged yarn can exhibit over 100% strength retention from the component fibers, remain high electrical conductivity, and show good stability against heating, bending, pressing, twisting, and rubbing. Acknowledgement The authors thank financial supports from the National Natural Science Foundation of China (51561145008, 21503267, 51862036, 21473238), Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015256), Suzhou Science and Technology Plan Projects (SYG201741), Outstanding Youth Fund of Jiangxi Province (2018ACB21023), and Science and Technology Project of Nanchang (2017-SJSYS-008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.01.054. References [1] K. Koziol, J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, A. Windle, High-performance carbon nanotube fiber, Science 318 (2007) 1892e1895. [2] J.J. Vilatela, A.H. Windle, Yarn-like carbon nanotube fibers, Adv. Mater. 22 (2010) 4959e4963. [3] T.-W. Chou, L. Gao, E.T. Thostenson, Z. Zhang, J.-H. Byun, An assessment of the science and technology of carbon nanotube-based fibers and composites, Compos. Sci. Technol. 70 (2010) 1e19. [4] M. Miao, Yarn spun from carbon nanotube forests: production, structure, properties and applications, Particuology 11 (2013) 378e393. [5] D. Janas, K.K. Koziol, Carbon nanotube fibers and films: synthesis, applications and perspectives of the direct-spinning method, Nanoscale 8 (2016) 19475e19490. [6] J. Di, X. Zhang, Z. Yong, Y. Zhang, D. Li, R. Li, Q. Li, Carbon-nanotube fibers for wearable devices and smart textiles, Adv. Mater. 28 (2016) 10529e10538. [7] J.N. Wang, X.G. Luo, T. Wu, Y. Chen, High-strength carbon nanotube fibre-like ribbon with high ductility and high electrical conductivity, Nat. Commun. 5 (2014) 3848. [8] J. Zhao, X. Zhang, Y. Huang, J. Zou, T. Liu, N. Liang, F. Yu, Z. Pan, Y. Zhu, M. Miao, Q. Li, A comparison of the twisted and untwisted structures for onedimensional carbon nanotube assemblies, Mater. Des. 146 (2018) 20e27. [9] A. Lekawa-Raus, J. Patmore, L. Kurzepa, J. Bulmer, K. Koziol, Electrical properties of carbon nanotube based fibers and their future use in electrical wiring, Adv. Funct. Mater. 24 (2014) 3661e3682. [10] P. Wang, D. Liu, J. Zou, Y. Ye, L. Hou, J. Zhao, C. Men, X. Zhang, Q. Li, Gas infiltration of bromine to enhance the electrical conductivity of carbon nanotube fibers, Mater. Des. 159 (2018) 138e144. [11] L. Qiu, X. Wang, D. Tang, X. Zheng, P.M. Norris, D. Wen, J. Zhao, X. Zhang, Q. Li,
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Carbon journal homepage: www.elsevier.com/locate/carbon
Merge multiple carbon nanotube fibers into a robust yarn Wenya Li a, b, d, 1, Jingna Zhao b, c, 1, Yuan Xue a, **, Xueqin Ren d, Xiaohua Zhang b, c, *, Qingwen Li b, *** a
Key Laboratory of Ecotextiles, Ministry of Education, Jiangnan University, Wuxi, 214122, China Division of Advanced Nano-Materials, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China c Division of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang, Chinese Academy of Sciences, Nanchang, 330200, China d School of Textile Science & Engineering, Xi'an Polytechnic University, Xi'an, 710048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2018 Received in revised form 11 December 2018 Accepted 15 January 2019 Available online 19 January 2019
Carbon nanotube (CNT) fibers are merged into a robust larger-diameter yarn by using an adhesion agent of polyethylenimine (PEI). As compared to poly(vinyl alcohol) and epoxy, PEI molecules can unite neighboring CNT fibers by constructing a strong inter-fiber adhesion, owing to their high mobility to soften the fiber surface. The merged yarn inherits fully the tensile strength and toughness from the single fiber, remains electrically conducting, and can be stably used up to 180 C even in O2 . It also exhibits high mechanical performances under various application conditions, such as bending, pressing, twisting, and rubbing. © 2019 Elsevier Ltd. All rights reserved.
Keywords: carbon nanotube fiber Multi-plied yarn Polyethylenimine Mechanical property
1. Introduction Carbon nanotube (CNT) fiber is an important one-dimensional macroscopic assembly material with comparable mechanical performance and much richer functionalities as compared with carbon fibers [1e6]. It has gained great attention over the past decade. For example, the tensile strength of a pure CNT fiber has been improved up to > 3 GPa [1,6e8], a typical value comparable to the carbon fiber T300, the electrical conductivity can be up to 4 104 e1 105 S=m [9,10], and the thermal conductivity is about 100 W=ðmKÞ [11,12]. In many studies, the fiber diameter was just 10 mm (unsuitable for weaving) in order to maintain a high strength, while a CNT fiber with a diameter larger than 50 mm was difficult to be stronger than 1 GPa [13,14]. Thus for the wearable applications, it is still necessary to multi-ply small-diameter CNT fibers into a largesize CNT yarn (diameter > 100 mm), which is very common in
* Corresponding author. Division of Advanced Nano-Materials, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Xue), [email protected] (X. Zhang), [email protected] (Q. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.01.054 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
textile manufacturing [15]. Unfortunately, after the multi-plying there is always a remarkable decrease in tensile strength, due to the weak inter-fiber adhesion and the loosely compacted yarn structure. For example, unavoidable big voids with a typical dimension of hundreds of nm can be formed after the multi-plying [16,17]. Furthermore, the conventional yarn technique usually introduces into the final yarn a structural parameter of twist, which also hinders the mechanical performance. Therefore, toward a high-strength CNT yarn, a new untwisted strategy should be developed to realize a strong adhesion between the component CNT fibers. Very recently, we reported that a tightly-densified and untwisted one-dimensional assembly of CNTs can induce better mechanical performances [8]. According to such assembly structure, it should be also possible to realize multi-plying without introducing the twist treatment. On the other hand, owing to the assembly feature, the surface of CNT fibers can be well impregnated by polymer molecules to realize ‘sizing’ effect [18,19]. This provides us an opportunity to adhere neighboring CNT fibers by introducing an entangled network of the surface CNTs with polymer chains [20]. It is thus expected that we can multi-ply CNT fibers together with strengthened inter-fiber interactions and, importantly, without loosing the fiber's tensile properties as the conventional yarns do so. In this study, we use the drawing die method to realize the
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merging, and various polymers, e.g., polyethylenimine (PEI), poly(vinyl alcohol) (PVA), and epoxy, to assist the adhesion between the neighboring fibers. The merged multiple fibers can be considered as either a CNT yarn or a big-size fiber (yarn is used here). PEI shows the best merging ability as it can soften the fiber surface owing to its high mobility. Its molecular softness also benefits the toughness of the merged yarn. As expected, the specific strength can be up to 0.69 N/tex at a retention over 100%, and the toughness can also be fully inherited ( > 60 J=g). The merged yarn is very robust upon heating, bending, pressing, twisting, and rubbing. 2. Experimental The die-based multi-plying method to merge CNT fibers was designed by following the methods to densify and align CNTs [21,22]. Fig. 1a shows the schematic and photograph of such method. The diamond wire drawing dies with pore diameters of 90, 100, 120, 140, 160, 180, and 200 mm were used, purchased from http://www.cangzhouzhendong.com/. This treatment can well eliminate the void spaces between and even inside the CNT fibers due to the radial pressing. The CNT fibers were fabricated by an injection chemical vapor deposition (iCVD), also called a floating catalyst CVD [5,7,23,24]. For the growth, a solution of absolute ethyl alcohol with 1.2 vol%
Fig. 1. Multi-plying of CNT fibers. (a) Schematic and photograph of the die-based processing. (b) The component CNTs were mainly 2e4-walled. (c,d) SEM morphology images of a pristine CNT fiber and a PEI-merged 6-plied yarn, whose diameter were 52.8 and 106.1 mm, respectively. (e,f) TGA curves in air and N2 for a pristine fiber and a PEI-merged yarn, respectively.
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ferrocene and 0.4 vol% thiophene carried by Ar/H2 was injected at a rate of 30 mL=h into the furnace (diameter 80 mm) which had been heated up to 1300 C. An entangled CNT network was formed in the furnace and was carried out by the gas flow. Then these CNTs were introduced into water for shrinking and spun into a continuous fiber out of the water. Under these growth parameters, slightly different from our previous study [24], a long CNT fiber with a diameter around 50e60 mm could be stably obtained. The CNTs were mainly 2e4-walled (Fig. 1b) and the nanotube length was at least 100 mm [8]. PEI, PVA, and epoxy resin were used to assist the multi-plying (merging). PEI (branched, average molecular weight 10 kDa, Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in ethanol at a concentration of 5.5 wt%. PVA (type 1788, polymerization degree 1750, Xiya Reagent Company, China) was dissolved in water at 5.0 wt%. Epoxy resins (Araldite LY 1564, Huntsman, USA), mixed with the curing agent (Aradur 3487, Huntsman, USA) at a mass ratio of 3:1, were dissolved in acetone at a total concentration of 5.5 wt%. The molecular weight of PEI was already the optimal choice, as the smaller PEI molecules are different to form entanglement with each other and the larger ones are difficult to be infiltrated into CNT assemblies. During the multi-plying, the polymer solution was injected into the entrance cone of the die, with the cone fulfilled during all the whole processing procedure. Thus the polymer infiltration level was determined just by the multiple CNT fibers themselves, and was finally examined by using thermogravimetric analysis (TGA). For the epoxy assisted multi-plying, a following thermal curing at 90 C for 1 h was applied. A high-precision XP2U analytical balance (resolution 0.1 mg, Mettler-Toledo LLC., Columbus, USA) was used to measure the sample weight. A Quanta 400 FEG scanning electron microscope (SEM, FEI, Hillsboro, USA) and a Tecnai G2 F20 S-Twin transmission electron microscope (TEM, FEI, Hillsboro, USA) were used to characterize the sample's dimensions, assembly morphology, and internal microstructure. The tensile properties were measured by an Instron 3365 universal testing machine (UTM, Instron Corp., Norwood, USA) and an MTS E44 UTM (MTS Systems Corporation, Eden Prairie, USA), equipped a 10-N and 1000-N load cell respectively. The strain rates were in the order of 10-1 s1 (or 0.06 mm=min) and the sample's gauge length was 10 mm. A peel test was also performed on the Instron 3365 UTM to measure the force to peel two neighboring fibers out of the merged yarn. The TGA was performed €tebau with a Netzsch TG 209 F1 Libra apparatus (NETZSCH-Gera GmbH, Selb, Germany) in both air and N2 at a heating rate of 10 C from 30 to 900 C. The electrical resistance was measured by a digital multimeter, and was used to calculate the conductivity. In order to evaluate the performance under different applications, heating, bending, pressing, twisting, and rubbing treatments were applied to the PEI-merged 6-plied yarn. The thermal treatment was performed in O2 and N2 for 2 h, at 180, 200, 300, and 400 C. The bending was applied to a 5-cm long yarn. After placing the yarn on a plastic film, the film was folded to make the yarn's two half segments contact with each other. The bending was repeated by 500 and 1000 times. The yarn was also hot-pressed at 100 C by a HR01 Hot Rolling Machine (MTI Corporation, USA), to cause a circleto-ellipse change of the cross section. For the pressing, a CNT yarn was placed between two 10-mm-thick foils, and the distance between the rollers was set to 60 mm. To apply the twist, one end of a yarn was hung by a weight of 20 g, and then a motor is used to twist the yarn from the top. The twisting speed was 30 rpm and the total twist was characterized by the number of twist per yarn length, namely 220, 860, and 1800 twist per meter (T/m) (the final yarn length was used for the calculation). For the rubbing evaluation, two different tests were used. In one test, the merged yarn was physically rubbed with a sandpaper (very fine P800, extra fine
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P600, and super fine P320 sandpapers were used, whose average particle diameters are 46.2, 25.8, and 21.8 mm, respectively), by following a test on human hair fiber [25]. In the other test, two merged yarns were rubbing against each other in a cross, where the transverse yarn was actively moving while the longitudinal one was fixed. Although the rubbing motion was performed just by hand, the number of rubbing motions was still a controllable parameter. For example, it took about 110 rubbing motions by using a P600 sandpaper to break the merged yarn, and for the rubbing with CNT yarns themselves, it took more than 300 force-and-back motions to induce just 10% loss in yarn strength.
3. Results and discussion 3.1. Strength retention after multi-plying The die-based multi-plying can cause densification to CNT fibers. PEI was used as it can provide efficient interaction or binding with CNTs [26e28]. The fiber diameter was in a range of 42e58 mm, determined by the growth parameters including the furnace diameter, concentrations of the feedstock, and the collection speed. The as-produced CNT fibers were not highly densified, as one can find the fiber surface was not smooth (Fig. 1c). For this fiber with a diameter of z53 mm, after the multi-plying with 5 similar-sized fibers, the diameter of the merged yarn was about 106 mm, see Fig. 1d. In all cases, the cross-sectional area of the merged yarn (diameter 95e115 mm) was much smaller than the sum of those before the treatment, usually decreased by more than 30% due to the densification effect. For the case shown in Fig. 1, the decrease was about 33.3%. As a comparison, the direct twisting of CNT fibers could not produce a compact and strong yarn; the fibers were still separating from each other as shown in Supporting Information Fig. S1a. When the number of plies increased, the densification was still very remarkable; the diameter of a 12-plied yarn was about 160e168 mm, see Supporting Information Fig. S1b, corresponding to a decrease in cross-sectional area of 16e24%. Fig. 1e and f compares the TGA curves for a pristine CNT fiber and a PEI-merged yarn, respectively. The residue mass of 10e15 wt % was ascribed to the catalyst nanoparticles [24]. From the mass loss in air at 500 C, the PEI mass percent was estimated to be about 23 wt%. If PVA or epoxy was used, a similar polymer mass percent was measured, see Supporting Information Fig. S2. To evaluate the strength retention, we used the maximum load to avoid the influence from the large diameter shrink. For a pristine CNT fiber with a diameter of 51 mm, the maximum tensile load was up to 0.79 N (Fig. 2a), corresponding to a strength of 387 MPa and
specific strength of 0.50 N=tex (its linear mass density was 1.58 tex). After being merged, we use the maximum load per ply to describe the strength. Fig. 2a compares the typical loadestrain curves for the 6-, 10-, 12-, and 16-plied yarns. Generally, with increasing the number of plies, the strain at break decreased gradually from > 10% to < 5%, while the maximum load per ply could be over 1 N for the 6- and 10-plied yarns. For example, the 6plied yarn exhibited a maximum load of 1.03 N per ply, corresponding to a specific strength of 0.69 N=tex (linear density 8.90 tex, smaller than 1:58 6 ¼ 9:48 tex due to the stretching). Table 1 shows the typical values of the multi-plied yarns in terms of tensile strength, Young's modulus, strain at break, and toughness, obtained from Supporting Information Fig. S3, among which the 1-, 6-, 10-, 12-, and 16-plied yarns are also shown in Fig. 2a. Due to the densification, the yarn strength could be nearly doubled for the 6-plied yarn, from 387 to 756 MPa. However, due to the decrease in strain at break, the multi-plying caused the decrease in toughness, which became much larger when the number of plies was more than 10. The present results indicated that the optimal number of plies was 6, where the strength and toughness were both high. In yarn industry, the specific strength or more directly the ability to carry a load is preferred to describe the strength, as it might be not easy to calculate the yarn's cross-sectional area. Therefore, here the strength retention after the multi-plying was evaluated by using the maximum load per ply. Fig. 2b plots the strength retention as a function of the number of plies, averaged from at least 10 samples. Clearly, the ply numbers from 4 to 12 could induce strengthening for the merged yarn. This shows the present multiplying treatment could not cause any strength loss, quite different from the conventional yarn plying. However, due to the introduction of non-conducting polymer of PEI, the electrical conductivity became smaller by one order of magnitude after the multi-plying (Fig. 2b), yet as conducting as the forest spun CNT fibers whose conductivity is usually in the order of 104 S=m [29,30].
3.2. Comparison between plying agents: PEI, PVA and epoxy The type of polymer is a key issue in determining the multiplying result. For this purpose, PVA and epoxy resin were also used to assist the fiber merging. Fig. 3a compares the merging results by using these different plying agents. (Notice that, another group of pristine CNT fibers was used here, resulting in slight differences in the tensile load and strain at break from Fig. 2a.) For the strength, PEI and epoxy both showed nice retention; their averaged maximum tensile loads were 5.93 and 5.81 N, respectively.
Fig. 2. Effect of multi-plying on the mechanical and electrical properties. (a) Typical loadestrain curves for a pristine fiber and 6-, 10-, 12-, and 16-plied yarns, respectively. (b) Strength retention (the ratio between the maximum loads of the plied yarn and pristine fiber) and electrical conductivity as functions of the number of plies. (A colour version of this figure can be viewed online.)
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Table 1 Typical values of the mechanical properties for the multi-plied CNT yarns tested in Fig. 2a, including the tensile strength (s), Young's modulus (E), strain at break (x), and tensile toughness UT . The fiber diameter df and specific strength and modulus are also provided. For the two-plied yarn, the cross section was not circular, making it difficult to calculate the strength and modulus. Number of plies
df (mm)
s (MPa)
s (N=tex)
E (GPa)
E (N=tex)
x
UT (J=g)
1 2 4 6 8 10 12 14 16
51 e 100 102 121 146 168 184 185
387 e 491 756 641 586 472 443 397
0.50 0.44 0.64 0.69 0.64 0.58 0.49 0.48 0.42
9.72 e 10.8 16.4 11.5 20.6 19.2 18.4 17.3
12.6 19.5 14.1 15.0 11.5 20.4 19.9 19.9 18.3
16.1% 14.2% 8.8% 11.8% 11.3% 5.3% 4.9% 6.1% 3.2%
64.9 52.0 39.5 60.1 51.0 20.9 16.8 23.3 8.5
Fig. 3. Comparison of different plying agents. (a) For 6-plied yarns, PEI showed a larger strength (plotted by dots) than PVA, and a larger strain at break (by using the average plus and minus one standard deviation) than epoxy. (b) Forceedisplacement curves obtained from a peel test to show the different inter-fiber adhesions, where two component CNT fibers were peeled off. (cee) SEM images of the microstructures after the peel test. (feh) Fracture morphology of different merged yarns. (A colour version of this figure can be viewed online.)
Differently, PVA could just reach an average of 4.08 N. For the ability to resist the length elongation, PEI and PVA resulted in a strain at break of z8.2%, while epoxy made the merged yarn brittle, with a strain at break of just 6.2%. This means that PEI could produce benefit both the strength and toughness. From the load-strain curve for the PEI-assisted 6-plied yarn shown in Fig. 2a, the toughness was up to 60.1 J=g by integrating the area beneath the loadestrain tensile curve and then dividing by its linear mass
density, nearly unchanged as compared to the value of 64.9 J=g for the pristine fiber shown in Fig. 2a. However, for PVA and epoxy, the merged yarns exhibited typical toughness values of 25.2 and 23.8 J= g due to the reduced strength or strain at break. To analyze the underlying mechanism, a peel test was performed as schematically shown in the inset of Fig. 3b, where two CNT fibers were peeled off from the merged yarn. Fig. 3b shows the peeling force as a function of peeling displacement (the half of the
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displacement of load cell). Clearly, it required a large force (z0.3e0.35 N) to peel them off for the PEI- and epoxy-merged yarns, while the force was z0.2e0.25 N for PVA. The microstructures of the peeled fiber showed some evidence of the different inter-fiber adhesions, see Fig. 3cee. Due to the strong adhesion of PEI and epoxy, the CNTs at the inter-fiber contacts were pulled out, like plush to cover the fiber surface (see the dashed red ellipses). However the fiber surface was clean for the PVA-adhered sample, corresponding to a weak inter-fiber adhesion. The different fracture morphology (Fig. 3feh) was also a general result of the different polymers. As compared to PVA, the small molecular weights of PEI and epoxy resin allowed more efficient infiltration into the fiber surface, also providing a certain strengthening to the fiber strength [31]. Thus for PEI and epoxy, a flat fracture was observed (Supporting Information Fig. S1c shows another example of epoxy-merged yarn), while long-distance
sliding between CNT bundles dominated the fracture of PVAadhered sample. The excellent merging ability of PEI was ascribed to two possible reasons: the high mobility of the PEI molecules and the intimate interaction between PEI and CNT. Due to the branched structure, the PEI molecule is not a linear chain, but globule like. Thus there is lack of entanglement for the PEI molecules, resulting in the low viscosity [32,33]. Here the small molecular weight (10 kDa) further reduced such viscosity. As a result, the molecules could exhibit high mobility and act as a lubricant between the CNT bundles. Thus a PEI-infiltrated CNT assembly (here is the fiber surface) could become soft and deform very easily. On the other hand, the high polarity of the N H group, higher than the C H and slightly smaller than the O H, is enough to induce an intimate interaction between PEI and CNT. Thus, under the external pressing (like passing through a die), the surface CNTs of the adjacent fibers could
Fig. 4. Performance evaluation of PEI-merged CNT yarns. (a) Fracture loads of 6-plied yarns after being treated at different temperatures in O2 or N2 for 2 h. (bef) Stress-stain curves after being bent for 500 and 1000 times (b), pressed (c), twisted up to 220, 860, and 1800 T/m (d), rubbed against the P800, P600, and P320 sandpapers with 30 rubbing motions (e), and rubbed against another merged CNT yarn with 300 rubbing motions (f). The insets are the corresponding structural morphologies, cross sections, or the test set-up, as labelled by arrows. (A colour version of this figure can be viewed online.)
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be forced to move due to the high mobility of PEI, and the two surfaces could be adhered together without forming voids or gaps between the fibers. On the contrary, when the linear polymer PVA was used, the adhesion between the adjacent fibers was less sufficient, resulting in the clean morphology in the peel test. For epoxy, the curing certainly could cause a polymeric cross-linking network to bind the CNTs from two neighboring fibers. As a result, the inter-fiber adhesion became strong and the peeling could cause the CNT pulling-out. However, the inter-fiber contact was brittle due to the thermosetting feature of epoxy. 3.3. Performance evaluation in different applications Branched PEI molecules are highly thermally stable; they start to decompose in air at 250 C and in N2 at 300 C [34]. Therefore, the PEI-plied CNT yarns could be used in high temperatures. To evaluate this, PEI-merged yarns were treated at different temperatures for 2 h before being tensile-tested. Fig. 4a compares the final fracture loads for 6-ply yarns. The treatment in O2 at 180 C did not show any degradation in fracture load, while a slight load loss of less than 10% was observed for the 200- C treatment, where PEI just started to decompose. With further increasing the temperature, the loss reduce became more and more. Nevertheless, the merged yarn could still exhibit more than 50% retention of fracture load after being treated at 400 C. (Such treatment increased the brittleness as also reflected by the fracture morphology shown in Supporting Information Fig. S1d.) Different from the strong oxidation ability of O2 , the treatment in N2 would not cause such large load loss. After the treatment in 300 C, the merged yarn could still carry a load up to 5.19 N. Clearly, just the removal of PEI in N2 could well remain the inter-fiber contact where a new CNT network had been formed during the multi-plying, while the strong oxidation could burn a certain CNTs at the contacts, strongly affecting the load transfer within the merged yarn. When CNT fibers are merged together, a large-diameter yarn is formed. Upon bending, there will be a large compression stress on one side, and a large tension one on the other side. The softness of PEI molecules, different from the rigid thermosetting ones, could benefit the stress relaxation by allowing slight displacement between the CNTs. As shown in Fig. 4b, after being bent for 500 and 1000 times, there was no clear decrease in the fracture load. The inset also shows that the component 6 fibers did not detach from each other. The softness of PEI also helped avoiding the fiber detachment upon pressing the merged yarn. After being hot-pressed, the circular yarn cross section turned into elliptical, as shown in the insets in Fig. 4c. The morphologies of a slight pressing and a local indentation were shown in Supporting Information Figs. S1e and f. However, several small voids appeared at the ellipse vertices, and the fracture load decreased down to 4.3 N, by 24%. The merged yarn could still carry a high load after introducing high levels of twist. Fig. 4d shows that with increasing the level of twist, the yarn strength decreased with the strain at break increased. This is a general phenomenon of the effect of twist [35]. For a twisted structure, the helical angle is highest at the yarn surface, while the interior is less or even not twisted. Therefore, the yarn surface is densified and becomes the major part to carry the load. This is why the large decrease in strength, more than 50%, was observed in the merged yarn with a diameter of > 100 mm after the twisting, while the decrease is no more than 30% for a small CNT fiber with a diameter of just z10 mm [8]. As a large-size yarn, rubbing and chafing are very common during the application. To evaluate the rubbing performance, fine sandpapers were used. The number of rubbing motions to break the
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yarn can be an important parameter. For example, it took about 110 rubbing motions to break the yarn with a P600 sandpaper. For the strength retention, the merged yarns were rubbed with different sandpapers for 30 times (Fig. 4e). After the rubbing, the retention was about 70%, 30%, and 27% for P800, P600, and P320 sandpapers; the corresponding fracture loads were 3.5, 1.7, and 1.5 N, respectively. The friction between yarns is also important in the textile application. Therefore, the rubbing between the merged yarns was also applied, by moving a transverse yarn against a longitudinal one, see Fig. 4f. Due to the small friction, the wear was not much after hundreds of rubbing motions. For example, after 300 motions, the two yarns could still carry loads up to 4.8e5.4 N. Most importantly, no fiber detachment was observed in the yarn after the rubbing, which was the main reason for the high strength retention. 4. Conclusion CNT fibers can be merged into a robust yarn by using a branched polymer of PEI. Owing to the entanglement between the polymer and CNTs, the merging caused the disappearance of the inter-fiber interface and densification of the CNTs. The merged yarn can exhibit over 100% strength retention from the component fibers, remain high electrical conductivity, and show good stability against heating, bending, pressing, twisting, and rubbing. Acknowledgement The authors thank financial supports from the National Natural Science Foundation of China (51561145008, 21503267, 51862036, 21473238), Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015256), Suzhou Science and Technology Plan Projects (SYG201741), Outstanding Youth Fund of Jiangxi Province (2018ACB21023), and Science and Technology Project of Nanchang (2017-SJSYS-008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.01.054. References [1] K. Koziol, J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, A. Windle, High-performance carbon nanotube fiber, Science 318 (2007) 1892e1895. [2] J.J. Vilatela, A.H. Windle, Yarn-like carbon nanotube fibers, Adv. Mater. 22 (2010) 4959e4963. [3] T.-W. Chou, L. Gao, E.T. Thostenson, Z. Zhang, J.-H. Byun, An assessment of the science and technology of carbon nanotube-based fibers and composites, Compos. Sci. Technol. 70 (2010) 1e19. [4] M. Miao, Yarn spun from carbon nanotube forests: production, structure, properties and applications, Particuology 11 (2013) 378e393. [5] D. Janas, K.K. Koziol, Carbon nanotube fibers and films: synthesis, applications and perspectives of the direct-spinning method, Nanoscale 8 (2016) 19475e19490. [6] J. Di, X. Zhang, Z. Yong, Y. Zhang, D. Li, R. Li, Q. Li, Carbon-nanotube fibers for wearable devices and smart textiles, Adv. Mater. 28 (2016) 10529e10538. [7] J.N. Wang, X.G. Luo, T. Wu, Y. Chen, High-strength carbon nanotube fibre-like ribbon with high ductility and high electrical conductivity, Nat. Commun. 5 (2014) 3848. [8] J. Zhao, X. Zhang, Y. Huang, J. Zou, T. Liu, N. Liang, F. Yu, Z. Pan, Y. Zhu, M. Miao, Q. Li, A comparison of the twisted and untwisted structures for onedimensional carbon nanotube assemblies, Mater. Des. 146 (2018) 20e27. [9] A. Lekawa-Raus, J. Patmore, L. Kurzepa, J. Bulmer, K. Koziol, Electrical properties of carbon nanotube based fibers and their future use in electrical wiring, Adv. Funct. Mater. 24 (2014) 3661e3682. [10] P. Wang, D. Liu, J. Zou, Y. Ye, L. Hou, J. Zhao, C. Men, X. Zhang, Q. Li, Gas infiltration of bromine to enhance the electrical conductivity of carbon nanotube fibers, Mater. Des. 159 (2018) 138e144. [11] L. Qiu, X. Wang, D. Tang, X. Zheng, P.M. Norris, D. Wen, J. Zhao, X. Zhang, Q. Li,
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