polymers Article
Properties and Fabrication of PA66/Surface-Modified Multi-Walled Nanotubes Composite Fibers by Ball Milling and Melt-Spinning Tian Chen 1,2 , Haihui Liu 1,2 , Xuechen Wang 1,2 , Hua Zhang 1 and Xingxiang Zhang 1,2, * 1
2
*
Tianjin Municipal Key Lab of Advanced Fiber and Energy Storage Technology, Tianjin Polytechnic University, Tianjin 300387, China;
[email protected] (T.C.);
[email protected] (H.L.);
[email protected] (X.W.);
[email protected] (H.Z.) Key Lab of Advanced Textile Composite (Tianjin Polytechnic University), Ministry of Education, Tianjin 300387, China Correspondence:
[email protected]; Tel.: +86-022-8395-5054
Received: 11 April 2018; Accepted: 15 May 2018; Published: 19 May 2018
Abstract: PA66/surface-modified multi-walled carbon nanotubes (MWNTs) composite fibers with a better dispersion and a stronger interfacial interaction between MWNTs and polyamide 66 (PA66) matrix were fabricated via the method of ball milling and melt-spinning. The effects of unmodified (U-MWNTs), acid-modified (MWNTs-COOH) and sodium dodecyl benzenesulfonate-modified MWNTs (MWNTs-SDBS) on the physical mechanical and thermal properties of PA66 were investigated. The results show that, the surface modified nanotube can provide homogeneous dispersion and there is a strong interfacial bonding between PA66 and MWNTs-COOH. A homogeneous dispersion of MWNTs in PA66 matrices without agglomeration is obtained by a facile ball milling method, which can increase the utilization ratio of MWNTs, reduce the required amount of MWNTs and ultimately improve the mechanical properties at a lower filler loading. The tensile strength of composite fibers reaches a maximum which respectively improved by 27% and 24% than that of PA66 fibers, when the mass fraction of MWNTs-SDBS and MWNTs-COOH is 0.1%. It is helpful for decrease the producing cost of the composite fibers. Moreover, the incorporation of MWNTs into PA66 improves the crystallizing temperature, crystallinity and thermal stability. The research shows that a novel facile method is developed for the fabrication of polymer composite fiber. Keywords: polyamide 66; surface modification; multi-walled carbon nanotube; composite fibers; physical mechanical property
1. Introduction Polyamide-6, 6 (PA66) is a commercial semi-crystalline polymer. It is one of the most important high-performance engineering materials and it has been widely used for many applications, that is, sports-wear, carpet, netting twine and so forth, due to its excellent mechanical properties, wear resistant property, high thermal stability, processability and relatively low cost. However, its applications, that is, tire cord, conveyer belt, hambroline and so forth, are limited due to insufficient strength. To further improve its physical mechanical properties and wear resistance, some additives, such as glass fiber [1,2], carbon fiber [3], silica dioxide [4], carbon nanotubes (CNTs) [2,5–12] and graphene [13] and so forth, have been used in the research. Multi-walled carbon nanotubes (MWNTs) is a kind of nanomaterial with excellent physical mechanical, electric and thermal properties. Its tensile strength and Young’s modulus is 11–63 GPa [14] and 270–950 GPa, respectively. It has attracted much attention as a material to heighten the strength of composite [2,5–12]. On the one hand, MWNTs is hard to disperse evenly in the polymer matrix (PM) Polymers 2018, 10, 547; doi:10.3390/polym10050547
www.mdpi.com/journal/polymers
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due to its large ratio of length/diameter, high surface energy, van der Waals force inner and inter a single MWNT, which makes them entanglement seriously. On the other hand, the weak surface binding energy between polymer and CNTs is hard to transfer force from polymer to MWNTs, which lead to the result of MWNTs being pulled out under the shear stress [15]. The key issues to strength the composites are first how to disperse MWNTs evenly inside the PM and next how to enhance the biding energy. Many attempts, that is, in-situ polymerization [7,8], solution blended [16] and melting blended [9,17–19] method and so forth, have been tested recently. Meanwhile, noncovalent functional [20,21] and covalent functional [7,8,19,22] MWNTs were used to enhance the binding energy. However, there is still little information available as how to disperse CNTs evenly, low cost and quickly inside the polymer composite which hinders its industrial application. Sodium dodecyl benzenesulfonate (SDBS) is a widely used ionic surfactant. The benzene ring can form π–π conjugate with the sp2 hybrid C=C bond on the surface of MWNTs and the polar sulfonate is helpful to disperse in polar water. SDBS was used to the liquid phase exfoliation graphene [23]. The exfoliated flakes are stabilized against reaggregation by a relatively large potential barrier, which originates in the Coulomb repulsion between surfactant-coated sheets. The application of SDBS as auxiliaries to help disperse MWNTs in PM has never been reported. Multi-walled carbon nanotubes (U-MWNTs), carboxylic MWNTs (MWNTs-COOH) and dodecyl benzenesulfonate-modified MWNTs (MWNTs-SDBS) were used as additives to modify PA66 composite fiber in this research. Ball milling and melt-spinning were also used to fabricate the composite fiber. The process is no solvent, low cost and suitable for industrial produce. Furthermore, the effects of these three additives and loadings on the properties of the composite fiber were further investigated. 2. Materials and Methods 2.1. Materials PA66 powder was a product of BASF Inc., Ludwigshafen, Germany. It was vacuum-dried at 80 ◦ C for 24 h before application. MWNTs and MWNTs-COOH with a mass purity of more than 95%, diameter less than 8 nm, lengths from 0.5 to 2 µm, were provided by Beijing Deke Daojin Co., Ltd. (Beijing, China). There are about 3.58% of surface carbon atoms of MWNTs are carboxyl groups in MWNTs-COOH which can from hydrogen bonds with PA66 chains. Sodium dodecylbenzenes sulfonate (SDBS, A.R.) and HCOOH (A.R.) were purchased from Tianjin Guangfu Reagents Company (Tianjin, China). 2.2. Fabrication of MWNTs-SDBS The MWNTs (1 g) and SDBS (1 g) were dispersed in distilled water (200 mL) and then sonicated for 2 h. After that, the dispersed mixture was separated by using a Buchner filter and was washed repeatedly in distilled water. Finally, the mixture was dried in a vacuum oven at 80 ◦ C for 12 h. MWNTs were modified with SDBS and named as MWNTs-SDBS. 2.3. Fabrication of Composite Powders The MWNTs powders were milled with PA66 powders using a planetary ball mill (Chunlong Experimental Equipment Co., Ltd, Lianyungang, China). The diameters of the larger and the smaller quartz balls were approximately 10 and 6 mm, respectively. The quantity proportion of large quartz balls and small quartz balls was 1:6 and ball-to-powder mass ratio (BPR) was 2:1. The mass loadings of every U-MWNTs, MWNTs-COOH and MWNTs-SDBS samples in the composite powders were 0, 0.05, 0.08, 0.1 and 0.3 wt %, respectively. Grinding was performed as following: predetermined mass ratios of U-MWNTs, MWNTs-COOH, MWNTs-SDBS and PA66 powders were premixed in a beaker, then the two types of quartz balls were mixed and the mixture was placed into a cylindrical polyamide jar pot. The powder mixtures were milled for 3 h at a constant rotational speed of 100 rpm [12]. Afterwards, a series of PA66/U-MWNTs, PA66/MWNTs-SDBS and PA66/MWNTs-COOH composite powders
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composite powders were obtained after ball milling. PA66/MWNTs powders captured by balls
were obtained after ball milling. PA66/MWNTs by balls during and MWNTs during milling and MWNTs distributions duringpowders milling atcaptured various period of times andmilling rotation rates distributions during milling at various period of times and rotation rates are illustrated in Figure 1. are illustrated in Figure 1.
Figure 1. Schematic process of fabrication of PA66/multi-walled nanotubes (MWNTs) composite via
Figure 1. Schematic process of fabrication of PA66/multi-walled nanotubes (MWNTs) composite via ball milling and melt spinning process of composite fibers. ball milling and melt spinning process of composite fibers. 2.4. Melt-Spinning of Composite Fibers
2.4. Melt-Spinning of Composite Fibers
All of the composite fibers were fabricated by a melt-spinning process with a homemade piston All of the composite fibers were fabricated melt-spinning process a homemade spinning machine by using the obtained powders.by Alla of the powders were keptwith at 280–285 °C for 10 piston min to melt totallyby and then the meltobtained spun withpowders. a spinneretAll of 0.3 diameterwere and akept winding velocity ◦ C for spinning machine using of mm the in powders at 280–285 of 20to rpm. 10 min melt totally and then melt spun with a spinneret of 0.3 mm in diameter and a winding
velocity of 20 rpm.
2.5. Characterization
2.5. Characterization The dispersibility of MWNTs in the PA66 matrix and surface morphology of the PA66/MWNTs composite fibers were observed by using a field emission scanning electronic microscope (FESEM; The dispersibility of MWNTs in the PA66 matrix and surface morphology of the PA66/MWNTs Hitachi S4800, Tokyo, Japan). composite fibers were observed by using a field emission scanning electronic microscope (FESEM; The FT-IR spectra of U-MWNTs, MWNTs-COOH and MWNTs-SDBS were obtained by a Bruker Hitachi S4800, Tokyo, Japan). Vector 22 spectrometer (Bruker TERSOR37, Karlsruhe, Germany) in the range of 4000 to 500 cm−1. The FT-IR spectra of −1 The resolution is 4 cm . U-MWNTs, MWNTs-COOH and MWNTs-SDBS were obtained by a Bruker −1 Vector 22 spectrometer TERSOR37, Karlsruhe, inAn theaccurate range of 4000 to Apparent density (Bruker test was performed based on GB/T Germany) 12496.1-1999. amount of 500 300 cm . −1 . of the MWNT was weighed after being dried under vacuum at 80 °C for 24 h. The Themg resolution is 4 cmsamples apparent volume was measured by usingbased a cylinder (10 mL) and then the Apparent density test was performed on GB/T 12496.1-1999. Anapparent accurate density amountwas of 300 mg ◦ C for calculated. each apparent volume, thebeing test was repeated times and finally an 24 average of the MWNTFor samples was weighed after dried under 3vacuum at 80 h. Thewas apparent calculated. volume was measured by using a cylinder (10 mL) and then the apparent density was calculated. For To observe the structure of MWNTs after ball milling, the composite powders (3 g) were each apparent volume, the test was repeated 3 times and finally an average was calculated. dissolved into formic acid (200 mL) and then centrifuged at 8000 rpm. The collected MWNTs were To observe the structure of MWNTs after ball milling, the composite powders (3 g) were dissolved re-dispersed in formic acid (200 mL) and the MWNTs were collected again. This procedure was intorepeated formic acid (200 mL) and then 8000 Thewas collected were five times to remove PA66centrifuged completely. at The finalrpm. product dried inMWNTs a vacuum ovenre-dispersed at 80 in formic acid (200 mL) and the MWNTs were collected again. This procedure was repeated °C for 12 h. Transmission electron microscopy (TEM) was performed with the use of a Hitachi H- five
times to remove PA66 completely. The final product was dried in a vacuum oven at 80 ◦ C for 12 h. Transmission electron microscopy (TEM) was performed with the use of a Hitachi H-7650 electron microscope (Hitachi, Japan) which operated at an accelerating voltage of 100 kV. X-ray diffraction (XRD) and the azimuthal patterns were obtained using a D/MAX-2500 (Rigaku, Japan) (λ = 0.15418 nm).
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Micro-Raman mapping spectra (Horiba, Japan) was recorded on a Horiba Jobin Yvon Xplora confocal Raman microscope equipped with a 532 nm laser. Thermal properties were measured using a differential scanning calorimeter (DSC, NETZSCH DSC 200 F3, Selb, Gemany). The determination temperature varied from 20 to 300 ◦ C at a heating or cooling rate of 10 ◦ C/min under a nitrogen atmosphere (0.2 MPa) and the retention period at both 20 and 300 ◦ C was 3 min during the testing process. The DSC thermograms in the first cooling and second heating processes were recorded. Thermal stability of the fibers was conducted on the fibers using a Thermogravimetric Analyzer (TG, NETZSCH STA409PC, Selb, Germany) from 20 to 1000 ◦ C at a rate of 10 ◦ C/min under a nitrogen atmosphere (0.2 MPa). The accuracy of mass measurement is 0.01 mg. In addition, the accuracy of temperature measurement is ±0.1 ◦ C. The mechanical properties of the fibers were obtained using a fiber tensile tester (LLY-06, Laizhou, China). The test used a fiber length of 10 mm at a rate of 10 mm/min. The results were obtained from the averaged results of 10 tests. 3. Results and Discussion Morphology and Structural of MWNTs Figure 2 shows the typical SEM micro-images of U-MWNTs, MWNTs-COOH and MWNTs-SDBS. It is observed that three kinds of MWNTs are entangled and curled, however, their stacking morphologies are significantly different. It is observed that MWNTs-SDBS (2c) shows a looser stacking morphology compared with those of U-MWNTs (2a) and MWNTs-COOH (2b), which implies that SDBS treatment on the MWNTs can weaken the interaction force inner- and inter- MWNTs and therefore leads to a loose stacking morphology. However, acid treatment on the MWNTs will cause a strong interaction MWNTs and thus will result in a compact stacking morphology of U-MWNTs5 [22]. Polymers 2018,among 10, x FOR PEER REVIEW of 14
Figure 2. Scanning electron microscopy (SEM) micro-images of (a) U-MWNTs (b) MWNTs-COOH Figure 2. Scanning electron microscopy (SEM) micro-images of (a) U-MWNTs (b) MWNTs-COOH and and (c) MWNTs-SDBS. (c) MWNTs-SDBS.
Figure 3 displays the apparent volume and apparent density of various MWNTs after various modifications. The apparent density is shown by the following sequence, MWNT-COOH > UMWNTs > MWNTs-SDBS. The results indicate that acid treatment makes a more compact stacking of MWNTs, while the further SDBS modification may weaken the interaction force among MWNTs resulting in a loose stacking of MWNTs-SDBS. This is consistent with the results of SEM. The loose stacking morphology was more beneficial for MWNTs to disperse in PM.
Polymers 2018, 10, 547 Figure 2. Scanning electron microscopy (SEM) micro-images of (a) U-MWNTs (b) MWNTs-COOH and (c) MWNTs-SDBS.
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Figure 3 displays the apparent volume and apparent density of various MWNTs after various Figure 3 displays the apparent volume and apparent density of various MWNTs after various modifications. The apparent density is shown by the following sequence, MWNT-COOH > U-MWNTs modifications. The apparent density is shown by the following sequence, MWNT-COOH > U> MWNTs-SDBS. The results indicate that acid treatment makes amakes more acompact stacking of MWNTs, MWNTs > MWNTs-SDBS. The results indicate that acid treatment more compact stacking of whileMWNTs, the further SDBS modification may weaken the interaction force among MWNTs resulting while the further SDBS modification may weaken the interaction force among MWNTs in a loose stacking of MWNTs-SDBS. This is consistent with thewith results of SEM. The loose stacking resulting in a loose stacking of MWNTs-SDBS. This is consistent the results of SEM. The loose morphology more beneficial MWNTs disperse in PM. in PM. stacking was morphology was morefor beneficial forto MWNTs to disperse 5
Apparent volume Apparent density
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Figure 3. Apparent volume andapparent apparentdensity density of MWNTs andand MWNTs-SDBS. Figure 3. Apparent volume and ofMWNTs-COOH, MWNTs-COOH, MWNTs MWNTs-SDBS.
Figure 4 presents the FTIR spectra of U-MWNTs, MWNTs-COOH and MWNTs-SDBS. For the
Figure 4 presents FTIR spectra of U-MWNTs, and MWNTs-SDBS. –1 is attributed to the MWNTs-COOH, thethe absorption band located at 1726 cmMWNTs-COOH stretching vibrationsFor of the –1 is attributed to the stretching vibrations of –1 MWNTs-COOH, the absorption band located at 1726 cm carboxyl groups. Another new band located at 1130 cm is assigned to the C–O stretching vibrations. Polymers 2018, 10, xbetween FOR PEER REVIEW 6 of 14 carboxyl groups. Another new band located peak at 1130 is assigned to the C–O vibrations. The difference the characteristic in cm the –1spectra of U-MWNTs andstretching MWNTs-COOH indicates that the surface of MWNTs haspeak beeninsuccessfully with carboxylic acid The difference between the characteristic the spectrafunctionalized of U-MWNTs and MWNTs-COOH groups. In the spectra of U-MWNTs and MWNTs-SDBS, a new band at 1457 cm–1 is assigned to C=C indicates that the surface of MWNTs has been successfully functionalized with carboxylic acid groups. in benzene rings, and 1155, 1037 cm–1 peaks are assigned to sulfonate group. The difference between –1 In thethe spectra of U-MWNTs andthat MWNTs-SDBS, a new band at 1457 characteristic peak reveals MWNTs was successfully treated withcm SDBS.is assigned to C=C in benzene rings, and 1155, 1037 cm–1 peaks are assigned to sulfonate group. The difference between the characteristic peak reveals that MWNTs was successfully treated with SDBS.
Figure 4. FTIR spectra MWNTs-COOH, MWNTs MWNTs and Figure 4. FTIR spectra ofof MWNTs-COOH, andMWNTs-SDBS. MWNTs-SDBS.
Figure 5 shows TEM micro-images of various MWNTs obtained after ball milling and removal of PA66. The length of the U-MWNTs and MWNTs-SDBS are slightly decreased and end tips are gradually opened. The diameters of the U-MWNTs and MWNTs-SDBS show almost no change after ball milling 3 h. However, the diameters of the MWNTs-COOH are significantly increased compared with the performance of U-MWNTs and MWNTs-SDBS, while the lengths of the MWNTs-COOH decreased most after ball milling 3 h, which are due to the strong interfacial interactions between
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Figure 5 shows TEM micro-images of various MWNTs obtained after ball milling and removal of PA66. The length of the U-MWNTs and MWNTs-SDBS are slightly decreased and end tips are gradually opened. The diameters of the U-MWNTs and MWNTs-SDBS show almost no change after ball milling 3 h. However, the diameters of the MWNTs-COOH are significantly increased compared with the performance of U-MWNTs and MWNTs-SDBS, while the lengths of the MWNTs-COOH decreased most after ball milling 3 h, which are due to the strong interfacial interactions between PA66 and MWNTs-COOH and defects on the sidewalls of the MWNTs-COOH. These results are very similar to those results of solid-state shear milling (S3 M) technique which can induce strong interfacial interactions between MWNTs and polyamide 6 [24]. The ball milling method can incorporate large amounts of N-groups onto the surface of CNTs [25]. We also proved that the strong interfacial interactions between PA6 and MWNTs-COOH were induced by the ball milling method in our previous studies [12]. Accordingly, we can infer that there is a strong interfacial interaction between PA66 and MWNTs-COOH, which makes it difficult to remove PA66 from MWNTs-COOH by the employed separation procedure, so the diameters of the MWNTs-COOH are significantly increased. In the process of preparing MWNTs-COOH, strong oxidizing agents (H2 SO4 and HNO3 ) will produce oxygenated sites as defects on the sidewalls of the MWNTs [7] and the fracture of MWNTs generally occurs in the bending, kinking and other structural defects of nanotube walls in the ball milling process [12], so the lengths of the decreased most. Polymers 2018, 10,MWNTs-COOH x FOR PEER REVIEW 7 of 14
Figure microscopy (TEM) micro-images of (a,b) U-MWNTs; (c,d) MWNTs-COOH Figure 5. 5. Transmission Transmissionelectron electron microscopy (TEM) micro-images of (a,b) U-MWNTs; (c,d) MWNTsand (e,f)and MWNTs-SDBS before and afterand ballafter milling 3 h and of PA66.of PA66. COOH (e,f) MWNTs-SDBS before ball for milling for removal 3 h and removal
Figure 66 shows shows Raman Raman spectroscopy spectroscopy of of various various MWNTs obtained after after ball ball milling milling and and the the Figure MWNTs obtained removal of PA66. For these three different kinds of MWNTs, it is possible to see evident bands at 1350 removal of PA66. For these three different kinds of MWNTs, it is possible to see evident bands at 1350 −11 and 1580 1580 cm cm− , ,are to D D band band and areiningood goodagreement agreementwith withthe thestructures structurestypically typicallyassociated, associated, respectively, respectively, to and G band. The ratio of the D band and G band intensity (I D/IG) is usually used to evaluate the defect and G band. The ratio of the D band and G band intensity (ID /IG ) is usually used to evaluate the concentration in carbon material. After ball milling 3 h, the3U-MWNTs and MWNTs-COOH have a defect concentration in carbon material. After ball milling h, the U-MWNTs and MWNTs-COOH D band with increased intensity compared with that of the U-MWNTs and MWNTs-SDBS before ball have a D band with increased intensity compared with that of the U-MWNTs and MWNTs-SDBS milling, which can be attributed to the side defects after ball milling. These defects suggest that ball before ball milling, which can be attributed to the side defects after ball milling. These defects suggest milling of U-MWNTs and MWNTs-COOH. Conversely, the ID/IG that ballprocess millingshould processdestroy shouldstructures destroy structures of U-MWNTs and MWNTs-COOH. Conversely, ratios for MWNTs-SDBS decrease after ball milling 3 h. This is probably because SDBS are adsorbed onto the surface of the MWNTs in the process of preparing MWNTs-SDBS. The side edge or surface defects of MWNTs are increased. However, the desorption of SDBS during ball milling or formic acid washing result in the decrease of the ID/IG ratio of MWNTs-SDBS. Interestingly enough, the ID/IG ratio of MWNTs-SDBS (0.85) after ball milling 3 h is similar to the ID/IG ratio of pristine MWNTs (0.84). Thus, we infer that SDBS can protect the structure of MWNTs to some extent during ball milling
removal of PA66. For these three different kinds of MWNTs, it is possible to see evident bands at 1350 and 1580 cm−1, are in good agreement with the structures typically associated, respectively, to D band and G band. The ratio of the D band and G band intensity (ID/IG) is usually used to evaluate the defect concentration in carbon material. After ball milling 3 h, the U-MWNTs and MWNTs-COOH have a D band with increased intensity compared with that of the U-MWNTs and MWNTs-SDBS before ball Polymers 2018, 10, 547 7 of 13 milling, which can be attributed to the side defects after ball milling. These defects suggest that ball milling process should destroy structures of U-MWNTs and MWNTs-COOH. Conversely, the ID/IG ratios decrease after ball milling 3 h.milling This is3probably SDBS are adsorbed the IDfor /IGMWNTs-SDBS ratios for MWNTs-SDBS decrease after ball h. This isbecause probably because SDBS are onto the surface of the MWNTs the process of process preparing The side edge surface adsorbed onto the surface of theinMWNTs in the of MWNTs-SDBS. preparing MWNTs-SDBS. Theor side edge defects of MWNTs are increased. However, the desorption of SDBS during ball milling or formic acid or surface defects of MWNTs are increased. However, the desorption of SDBS during ball milling or washing result in the result decrease of the ID/IG ratio of MWNTs-SDBS. Interestingly enough, the ID/I G ratio formic acid washing in the decrease of the ID /IG ratio of MWNTs-SDBS. Interestingly enough, of after ball (0.85) milling 3 hball is similar /IG ratiotoofthe pristine (0.84). theMWNTs-SDBS ID /IG ratio of (0.85) MWNTs-SDBS after millingto3 the h isIDsimilar ID /IG MWNTs ratio of pristine Thus, we infer that SDBS can protect the structure of MWNTs to some extent during ball milling MWNTs (0.84). Thus, we infer that SDBS can protect the structure of MWNTs to some extent during process. ball milling process.
U-MWNTs
0h 3h
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ID/IG=1.06 ID/IG=0.84
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Figure 6. 6. Raman Raman spectra obtained before before and and after after being being milled milled for h and and the the removal removal Figure spectra of of MWNTs MWNTs obtained for 33 h of PA66. PA66. of
The XRD obtained after are shown shown in in The XRD patterns patterns of of MWNTs MWNTs obtained after ball ball milling milling and and removal removal of of PA66 PA66 are Figure 7. For the three different MWNTs, (002) diffraction peak of the MWNTs appears at 26° (2θ), ◦ Figure 7. For the three different MWNTs, (002) diffraction peak of the MWNTs appears at 26 (2θ), which is is attributed attributed to to the the interlayer interlayer spacing spacing of of the thecarbon carbonnanotube nanotube(d(d002).). According Bragg which According to to Bragg 002 formula, the crystalline interplanar spacing is 0.34 nm. On the one hand, the peak position of the three formula, the crystalline interplanar spacing is 0.34 nm. On the one hand, the peak position of the different MWNTs shows no significant change after ball milling, thus the crystal structures of three different MWNTs shows no significant change after ball milling, thus the crystal structures of MWNTS are are preserved. preserved.On Onthe theother otherhand, hand,it it notable that diffraction intensity of U-MWNTs MWNTS is is notable that thethe diffraction intensity of U-MWNTs and and MWNTs-COOH decreases after ball milling. However, the diffraction intensity of the MWNTsMWNTs-COOH decreases after ball milling. However, the diffraction intensity of the MWNTs-SDBS SDBS no significant change aftermilling, ball milling, which indicates the crystallization property of has nohas significant change after ball which indicates that that the crystallization property of the the MWNTs-SDBS conserved during the ball milling process, which further confirms MWNTs-SDBS has has beenbeen conserved during the ball milling process, which also also further confirms that that SDBS can effectively protect the integrity of MWNTs during ball milling process. SDBS can effectively protect the integrity of MWNTs during ball milling process. 7 6
U-MWNTs 0 h U-MWNTs 3 h
4 (002)
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Intensity
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MWNTS are preserved. On the other hand, it is notable that the diffraction intensity of U-MWNTs and MWNTs-COOH decreases after ball milling. However, the diffraction intensity of the MWNTsSDBS has no significant change after ball milling, which indicates that the crystallization property of the MWNTs-SDBS has been conserved during the ball milling process, which also further confirms Polymers 2018, 10, 547 8 of 13 that SDBS can effectively protect the integrity of MWNTs during ball milling process. 7 6
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Figure 7. X-Ray diffraction (XRD) patterns of MWNTs obtained before and after being milled for 3 h and the removal of PA66. (XRD) patterns of MWNTs obtained before and after being milled for 3 h Figure 7. X-Ray diffraction
and the removal of PA66.
Figure 8 presents SEM micrographs of fractured surfaces of PA66/U-MWNTs, PA66/MWNTsFigure 8 presents SEM micrographs of fractured surfaces ofMWNTs, PA66/U-MWNTs, PA66/MWNTsCOOH and PA66/MWNTs-SDBS composite fibers with 0.3 wt % which were broken in a liquidand nitrogen bath. The white dots or rods regions represent endswhich of MWNTs that were COOH PA66/MWNTs-SDBS composite fibers with 0.3 wtthe %broken MWNTs, were broken in a stretched out of the The PA66white matrix [17]. MWNTs-COOH and MWNTs-SDBS were uniformly liquid nitrogen bath. dots orBoth rodsthe regions represent the broken ends of MWNTs that were dispersed matrixmatrix after ball milling 3 h;MWNTs-COOH however, for the and PA66/U-MWNT composite fibers, stretched outinofPA66 the PA66 [17]. Both the MWNTs-SDBS were uniformly the U-MWNTs aggregates are dispersed nonuniformly in PA6 matrix and small aggregates can fibers, still dispersed in PA66 matrix after ball milling 3 h; however, for the PA66/U-MWNT composite be found. These data prove that the SDBS can weaken the interaction force among MWNTs and a the U-MWNTs aggregates are dispersed nonuniformly in PA6 matrix and small aggregates can still better dispersion of the MWNTs-SDBS in the PA66 matrix can be achieved; moreover, there is a strong be found. These data prove that the SDBS can weaken the interaction force among MWNTs and a interfacial interaction between PA66 and MWNTs-COOH after millingmoreover, (as provedthere by Figure 4). better dispersion of the MWNTs-SDBS in the PA66 matrix can beball achieved; is a strong Hydrophobicity is improved in carboxylic functionalized MWNTs, so is, their dispersibility. In a word, interfacial interaction between PA66 and MWNTs-COOH after ball milling (as proved by Figure 4). surface-modified MWNTs can achieve a better dispersion in PA66 matrix as compared to U-MWNTs Hydrophobicity is improved in carboxylic functionalized MWNTs, so is, their dispersibility. In a and they can even achieve a strong interfacial bonding with PA66 matrix. word, surface-modified MWNTs can achieve a better dispersion in PA66 matrix as compared to UMWNTs and they can even achieve a strong interfacial bonding with PA66 matrix.
the U-MWNTs aggregates are dispersed nonuniformly in PA6 matrix and small aggregates can still be found. These data prove that the SDBS can weaken the interaction force among MWNTs and a better dispersion of the MWNTs-SDBS in the PA66 matrix can be achieved; moreover, there is a strong interfacial interaction between PA66 and MWNTs-COOH after ball milling (as proved by Figure 4). Hydrophobicity is improved in carboxylic functionalized MWNTs, so is, their dispersibility. In a Polymers 2018, surface-modified 10, 547 word, MWNTs can achieve a better dispersion in PA66 matrix as compared to U-9 of 13 MWNTs and they can even achieve a strong interfacial bonding with PA66 matrix.
Figure 8. SEM micrographs of fractured surfaces of PA66 and the composite fibers ((a)-PA66; (b)-U-
Figure 8. SEM micrographs of fractured surfaces of PA66 and the composite fibers ((a) PA66; MWNTs; (c)-MWNTs-COOH; (d)-MWNTs-SDBS). (b) U-MWNTs; (c) MWNTs-COOH; (d) MWNTs-SDBS). Polymers 2018, 10, x FOR PEER REVIEW
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Figure 9 presents SEM micrographs of the fibers surface and fracture surface of the composite Figure 9 presents SEM micrographs of the fibers surface and fracture surface of the composite fibers containing 0.3 wt % MWNTs-SDBS. The surface of the fiber is smooth. The diameter of the fiber, fibers containing 0.3 wt % MWNTs-SDBS. The surface of the fiber is smooth. The diameter of the which is approximately 120 µm, is even. This performance further proves that the melting fluid of fiber, which is approximately 120 μm, is even. This performance further proves that the melting fluid PA66/ofMWNTs-SDBS can exhibit good fluidity, which reflects a good dispersion of MWNTs-SDBS in PA66/ MWNTs-SDBS can exhibit good fluidity, which reflects a good dispersion of MWNTs-SDBS polyamide 66. in polyamide 66.
Figure 9. SEM micrographs thesurface surface and and fracture composite fibers. Figure 9. SEM micrographs ofofthe fracturesurface surfaceofof composite fibers.
The DSC curves of the composite fibers with 0.3 wt % additives are shown in Figure 10 and the
The DSC curves of the composite fibers with 0.3 wt % additives are shown in Figure 10 and results are also listed in Table 1. There is only one peak both in the heating scanning and cooling the results are The alsomelting listed in Table 1. Therefluctuates is only one peak both in 261 the °C. heating scanning scanning. peak temperature at approximately In contrast, the and ◦ cooling scanning.temperature The melting peak about temperature at of approximately 261 C. In contrast, crystallizing increases 5 °C afterfluctuates the addition U-MWNTs, MWNTs-SDBS and ◦ C after the addition of U-MWNTs, MWNTs-SDBS the crystallizing temperature increases about 5 MWNTs-COOH, respectively. Such a phenomenon can be explained by the function of MWNTs as a and MWNTs-COOH, respectively. Such a phenomenon can be explained by the function ofPA66 MWNTs heterogeneous nucleating agent. The crystallinity of the composite fiber increases from 41.8% of to 47.7% of MWNTs-SDBS. decreases to 45.3% MWNTs-COOH. It is probably due as a heterogeneous nucleatingFurthermore, agent. The itcrystallinity of theofcomposite fiber increases from 41.8% to the function of MWNTs-COOH. It improvesitthe crystallinity a heterogeneous nucleating It is of PA66 to dual 47.7% of MWNTs-SDBS. Furthermore, decreases to as 45.3% of MWNTs-COOH. agent; and bondsofbetween PA66 chainItand MWNTs the crystallization. The probably due to the the chemical dual function MWNTs-COOH. improves thehinder crystallinity as a heterogeneous addition of MWNTs does not change the crystallographic form which is different from that in in-situ nucleating agent; and the chemical bonds between PA66 chain and MWNTs hinder the crystallization. polymerization results of PA66 and MWNTs-COOH [7,8]. The addition of MWNTs does not change the crystallographic form which is different from that in in-situ polymerization results of PA66 and MWNTs-COOH [7,8]. 2.0
2.5 PA66/MWNTs-COOH 2.0 PA66/MWNTs-SDBS 1.5
1.5 low (W/g) Endo
ow (W/g) Endo
3.0
1.0
PA66/MWNTs-COOH PA66/MWNTs-SDBS
0.5
PA66/MWNTs
0.0
PA66
heterogeneous nucleating agent. The crystallinity of the composite fiber increases from 41.8% of PA66 to 47.7% of MWNTs-SDBS. Furthermore, it decreases to 45.3% of MWNTs-COOH. It is probably due to the dual function of MWNTs-COOH. It improves the crystallinity as a heterogeneous nucleating agent; and the chemical bonds between PA66 chain and MWNTs hinder the crystallization. The addition of MWNTs does not change the crystallographic form which is different from that in in-situ Polymers 2018, 10, 547 10 of 13 polymerization results of PA66 and MWNTs-COOH [7,8]. 2.0 1.5
2.5 PA66/MWNTs-COOH
Heat Flow (W/g) Endo
Heat Flow (W/g) Endo
3.0
2.0 PA66/MWNTs-SDBS 1.5 PA66/MWNTs 1.0 0.5
PA66
200
1.0
PA66/MWNTs-COOH PA66/MWNTs-SDBS
0.5
PA66/MWNTs
0.0
PA66
-0.5 -1.0 -1.5
225
250
275
300
175
200
225
Temperature (℃)
250
275
Temperature (℃)
(a)
(b)
Figure 10. Differential scanning calorimeter (DSC) heating scans (a) and cooling scans (b) of the Figure 10. Differential scanning calorimeter (DSC) heating scans (a) and cooling scans (b) of the composite fibers. composite fibers.
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Table 1. DSC melting and crystallization properties for PA66/MWNTs composite fibers. Table 1. DSC melting and crystallization properties for PA66/MWNTs composite fibers. Sample T m /◦TC m /J/g Sample m/°C ∆H ΔH m/J/g PA66 260.3 81.93 PA66 260.3 81.93 PA66/U-MWNTs 261.0 261.0 90.48 90.48 PA66/U-MWNTs PA66/MWNTs-SDBS PA66/MWNTs-SDBS 260.8 260.8 93.49 93.49 PA66/MWNTs-COOH PA66/MWNTs-COOH262.0 262.0 88.76 88.76
T cT/◦c/°C C
c /% XXc/% 41.8 41.8 46.2 46.2 47.7 47.7 45.3 45.3
231.8 231.8 236.8 236.8 236.7 236.7 236.6 236.6
The thermal thermalstability stabilityofofPA66 PA66fiber fiberand andthe thecomposite composite fibers with additives shown The fibers with 0.30.3 wtwt %% additives areare shown in in Figure 11 and the results are listed in Table 2. The addition of MWNTs enhances the thermal stable Figure 11 and the results are listed in Table 2. The addition of MWNTs enhances the thermal stable temperaturefor for5.5–7.4 5.5–7.4◦ C. °C.The The thermal stable temperature of the sample 0.3%wt % MWNTstemperature thermal stable temperature of the sample withwith 0.3 wt MWNTs-SDBS SDBS is the highest which is assigned to the best dispersion of MWNTs in the PM. Such a is the highest which is assigned to the best dispersion of MWNTs in the PM. Such a phenomenon phenomenon is significantly different from that in-situ polymerization resultant by using MWNTsis significantly different from that in-situ polymerization resultant by using MWNTs-COOH and COOH and amino-MWNTs 2) as additives, respectively, in which the ◦ Ti is 14–28.8 °C, amino-MWNTs (MWNTs-NH(MWNTs-NH 2 ) as additives, respectively, in which the Ti is 14–28.8 C, which is lower which is lower than that of PA66 [7,8]. It is attributed to the PA66 chain length, which is bonded on than that of PA66 [7,8]. It is attributed to the PA66 chain length, which is bonded on MWNTs-COOH MWNTs-COOH and MWNTs-NH 2, is short. It implies that the PA66 chain length is not shortened and MWNTs-NH2 , is short. It implies that the PA66 chain length is not shortened seriously by the mill seriously by the mill balling process. balling process. 100
100
95 90
Mass fraction (%)
80
85 80
60
75
380
400
420
20
40 20 0 350
15
PA66 MWNTs-SDBS MWNTs-COOH U-MWNTs 400
450
10
5
460
500
470
480
490
500
550
600
Temperature (℃)
Figure Figure 11. 11. Thermogravimetric Thermogravimetric (TG) (TG) plots plots of of the the composite composite fibers. fibers.
Table 2. TG data of PA66/MWNTs composite fibers. Sample PA66 PA66/MWNTs PA66/MWNTs-SDBS PA66/MWNTs-COOH
Ti/°C 408.9 414.4 416.3 414.5
△T/°C* 0 5.5 7.4 5.6
T10%/°C 403.3 408.1 409.9 406.8
Tmax/°C 440.9 443.1 445.4 446.2
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Table 2. TG data of PA66/MWNTs composite fibers. Sample
T i /◦ C
4T/◦ C*
T 10% /◦ C
T max /◦ C
PA66 PA66/MWNTs PA66/MWNTs-SDBS PA66/MWNTs-COOH
408.9 414.4 416.3 414.5
0 5.5 7.4 5.6
403.3 408.1 409.9 406.8
440.9 443.1 445.4 446.2
* 4T is the difference of the initial mass loss temperature of the composite fiber and the initial mass loss temperature of PA66 fiber.
Both the effect of various species of MWNTs and MWNTs loading on the tensile strength of composite fibers are plotted in Figure 12. Compared to the pure PA66 fibers, the tensile strength of composite fibers is greatly improved by the incorporation of MWNTs-SDBS and MWNTs-COOH, especially for loading with MWNTs-SDBS. From these facts, we further confirm that surface-modified MWNTs can achieve high dispersion and strong interfacial bonding with PA66 matrix. However, the addition of U-MWNTs has almost no reinforcing effect on PA66 and the tensile strength of PA66/U-MWNTs composite fibers was even lower than that of pure PA66 fibers with the addition of 0.05 and 0.3 wt % U-MWNTs. This is probably due to the not fully stretch which can be seen from the high strain. A maximum tensile strength for three kinds of composite fibers is achieved at Polymers 2018, 10, FOR REVIEW in PA66 and in general, using conventional wet and melt spinning, 12 of 14 the loading of x0.1 wtPEER % MWNTs the maximum tensile strength and Young’s modulus of fibers were achieved at a loading of 0.5–1.0 wt % MWNTs inPA66 PA66[7,8]. [7,8]. believe surface-modified MWNTs andmilling ball milling can MWNTs in WeWe believe thatthat surface-modified MWNTs and ball methodmethod can achieve achieve high dispersion and even stronger interfacial bonding with PA66 matrix, which can increase high dispersion and even stronger interfacial bonding with PA66 matrix, which can increase the the utilization ratio of MWNTs, reduce amountofofMWNTs MWNTsrequired requiredand andultimately ultimately improve improve the utilization ratio of MWNTs, reduce thethe amount the mechanical properties at a low filler loading. Such a discovery is helpful for decrease the producing mechanical properties at a low filler loading. Such a discovery is helpful for decrease the producing cost cost of of PA66/MWNTs PA66/MWNTs composite composite fibers. fibers. The The tensile tensile properties properties of of PA66 PA66 and and PA66/MWNTs PA66/MWNTs composite composite fibers are summarized in Table 3. Obviously, the most effectively reinforced fibers are summarized in Table 3. Obviously, the most effectively reinforced effect effect is is shown shown in in the the following sequence, MWNTs-SDBS > MWNTs-COOH > U-MWNTs, which can be explained by the following sequence, MWNTs-SDBS > MWNTs-COOH > U-MWNTs, which can be explained by best dispersion of MWNTs-SDBS in in PA66 the best dispersion of MWNTs-SDBS PA66matrix matrixand andthe thestrong strong interfacial interfacial bonding bonding between between MWNTs-COOH and PA66. However, for U-MWNTs, small aggregates can still be found MWNTs-COOH and PA66. However, for U-MWNTs, small aggregates can still be found in in PA66 PA66 matrix. These also confirm that, compared with U-MWNTs, surface-modified MWNTs exhibit matrix. These also confirm that, compared with U-MWNTs, surface-modified MWNTs exhibit more more excellent excellent modification. modification. 160
PA66 PA66/MWNTs-COOH PA66/MWNTs-SDBS PA66/U-MWNTs
Tensile strength/MPa
140 120 100 80 60 40 20 0
0
0
0.05
0.08
0
0.1
0
0.3
MWNTs weight content/wt%
Figure 12. 12. Tensile Tensile strength strength of of the the composite composite fibers. fibers. Figure
Table 3. Mechanical properties of PA66/MWNTs composite fibers. Sample PA66 PA66/MWNTs-SDBS
MWNTs loading (%) 0 0.05 0.08 0.1 0.3
Young’s modulus/GPa (Increase ratio) 0.23 0.30(26%) 0.37(40%) 0.33(32%) 0.30(26%)
Tensile strength/MPa (Increase ratio) 73.60 82.40(11%) 90.00(18%) 100.70(27%) 86.70(15%)
Strain/% 33 27 24 30 28
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Table 3. Mechanical properties of PA66/MWNTs composite fibers. Sample
MWNTs loading (%)
Young’s modulus/GPa (Increase ratio)
Tensile strength/MPa (Increase ratio)
Strain/%
PA66
0
0.23
73.60
33
PA66/MWNTs-SDBS
0.05 0.08 0.1 0.3
0.30(26%) 0.37(40%) 0.33(32%) 0.30(26%)
82.40(11%) 90.00(18%) 100.70(27%) 86.70(15%)
27 24 30 28
PA66/MWNTs-COOH
0.05 0.08 0.1 0.3
0.37(39%) 0.38(40%) 0.41(45%) 0.47(52%)
81.10(9%) 93.40(21%) 96.20(24%) 93.30(21%)
22 25 23 20
PA66/U-MWNTs
0.05 0.08 0.1 0.3
0.13(−68%) 0.20(−13%) 0.36(38%) 0.27(15%)
62.80(−17%) 84.20(13%) 94.10(22%) 63.90(−15%)
47 42 26 24
Ref 7(PA66/MWNT-COOH)
0.5
(65%)
(48%)
Ref 16(PA66/MWNT-COOH)
1
(33%)
(49%)
4. Conclusions The PA66/surface-modified MWNTs composite fibers were fabricated via ball milling and melt-spinning. The surface modified nanotube can provide homogeneous dispersion and there is a strong interfacial bonding between PA66 and MWNTs-COOH. A homogeneous dispersion of MWNTs in PA66 matrices without agglomeration is obtained by an easy ball milling method, which can increase the utilization ratio of MWNTs, reduce the required amount of MWNTs and ultimately improve the mechanical properties at a lower filler loading. The tensile strength of composite fibers reaches a maximum which is improved by 27% and 24% than that of PA66 fibers, respectively, when the mass fraction of MWNTs-SDBS and MWNTs-COOH are 0.1%. Moreover, the incorporation of MWNTs into PA66 improves the crystallizing temperature, crystallinity and thermal stability. The research shows that a novel facile method is developed for the fabrication of polymer composite fiber. Author Contributions: T.C. and X.Z. conceived and designed the experiments; H.L. and X.W. contributed to the characterization; H.Z. contributed to the analysis. X.Z. revised the manuscript. Funding: This research was funded by [National Key Research and Development Plan] grant number [2016YFB0303000] and [Tianjin Municipal New Material Major Projects] grant number [16ZXCLGX00090]. Conflicts of Interest: The authors declare no conflict of interest.
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