High Strength Conductive Polyamide 6 Nanocomposites Reinforced

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High Strength Conductive Polyamide 6 Nanocomposites Reinforced by Prebuilt Three-Dimensional Carbon Nanotube Networks Youdan Zheng, Rui Wang, Xiangyu Dong, Lixin Wu, and Xu ZHANG ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08944 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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ACS Applied Materials & Interfaces

High Strength Conductive Polyamide 6 Nanocomposites Reinforced by Prebuilt Three-Dimensional Carbon Nanotube Networks

Youdan Zheng,1,2 Rui Wang,1,2 Xiangyu Dong,3 Lixin Wu,1,* and Xu Zhang1,*

1

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou,

Fujian 350002, P. R. China 2

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

3

School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou, Fujian 350116, P.

R. China

* Corresponding author. E-mail: [email protected] (L. Wu), [email protected] (X. Zhang) ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The rapidly growing fields of aerospace, energy and electronic devices raise the demand for materials with ever-increasing mechanical properties, electrical and thermal conductivity. However, the combination of unusually high performance is difficult to attain. In this study, using pre-built three-dimensional (3D) closely packed interconnected multi-walled carbon nanotubes (MWNTs) networks as backbone for assembly of polymer matrix, a novel, green (solvent-free) and energysaving method to prepare robust, highly electrically and thermally conductive polyamide nanocomposites is reported. The as-prepared nanocomposites exhibit significant enhancements of 16 orders of magnitude in electrical conductivity and 505% in thermal conductivity, which mainly benefits from the contributions of closely packed 3D networks and conductive pathways of carbon nanotubes (CNTs). In addition, even at a high MWNTs loading of 25 wt%, the as-prepared nanocomposites still possess high tensile strength of 99.4 MPa and Young’s modulus of 5.3 GPa. The performance of the as-prepared nanocomposites exceeds that of most of the composites, which confirms the potential of the pre-built MWNTs network method for fabricating robust and highly conductive nanocomposites and the importance of good interconnectivity of nanofillers-nanofillers and nanofillers-matrix. The special fabrication method could open up a broad range of possibilities for aerospace, conducting elements and structural nanomaterials, as well as electronic components with the requirements of heat dissipation, mechanical strength, thermal repair, corrosion resistance, and so on.

Keywords: highly conductive polyamide, surface functionalization, multi-walled carbon nanotubes, polymer nanocomposites, closely packed 3D networks

ACS Paragon Plus Environment

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1. Introduction The rapid developments of aerospace, electronics, energy, communication and other technologies have spawned the need for materials with excellent mechanical, electrical, thermal and other properties. For example, in aerospace applications, issues like lightning strike dissipation in air vehicles can be solved by employing materials with superior electrical properties, and the extreme service environments also require the materials to be robust, well thermal conductive and corrosion resistant. It is well-known that nanoscale materials exhibit amazing properties which could meet the above requirements simultaneously. However, they are difficult to shape into macroscopic scale by themselves without support materials. With the support of matrix materials, nanoscale materials can be included to form nanocomposites, which has superior combination property compared with the matrix materials. The addition of nanoscale fillers significantly improves the performance of the matrix materials, while the performance of nanocomposites is much worse than that of nanoscale fillers themselves.1-5 The sp2 hybridized carbon network and high aspect ratio endow one-dimensional carbon nanotubes (CNTs) with ultrahigh mechanical properties as well as electrical and thermal conductivity, light weight, corrosion resistance and easy processing make polymers the ideal matrix materials for nanofillers, which results in the hybrid CNTs/polymer nanocomposites have attracted special attention in recent years. A good deal of research shows that introducing CNTs into polymers could reinforce the performance of polymers at low volume fractions, but has a negative effect on the performance at high volume fractions. Up to now, the ability to fabricate macroscopic bulk materials which maintain the exceptional tensile strength, electrical and thermal conductivity, and other properties of nanoscale materials is still a critical challenge in the nanocomposite fabrication. Nanoscale materials possess excellent performance well above that of the polymer matrix, further increasing the volume fraction of fillers can facilitate the formation of percolated networks for much enhanced transport properties and theoretically result in better performance of nanocomposites. However, the properties of polymer based nanomaterials fall far below both the



theoretical and the experimental values of nanoscale materials mainly on account of the difficulty of achieving uniform dispersion of reinforcing nanomaterials at high volume fraction, deficiency in structural control and the inefficiency of transferring load from the polymer matrix to reinforcing nanomaterials. Therefore, researchers have carried out a large number of studies to address these limitations.6-9 It has been demonstrated that the properties of polymer nanocomposites can be much closer to the theoretical value of reinforcing nanomaterials by arranging them in space, controlling their orientation on the nanoscale, and maintaining the arranged order in polymer matrix on the macroscale.10 An attractive and effective way to realize expected dispersion and orientation of CNTs in polymer matrix is that pre-construct a three-dimensional (3D) network structure of CNTs and then infuse the prebuilt CNTs 3D network with polymers.10 The prebuilt 3D network structure of aligned CNTs solves the problem that CNTs tend to aggregate at high volume fractions due to large specific surface and π-π interaction. However, the commonly used 3D network preparation methods such as chemical vapor deposition,11-13 sol-gel polymerization,14-16 freeze-drying,17,18 chemical and hydrothermal reduction10,19-21 are complicated, costly and time-consuming. For example, Ding and co-workers have reported the synthesis of a graphene foam/polyamide 6 (PA6) composite with a remarkable thermal conductivity of 0.847 W m-1 K-1 at 2.0 wt% of graphene foam prepared by one-step hydrothermal method. Although these structures impart high electrical and thermal conductivity to polymers, large-scale manufacture of 3D CNTs networks is still a severe challenge. In addition, solvents or thinners for polymers are frequently used during the process of impregnating a 3D network with polymers, while the subsequent desolvent process is cumbersome, energy intensive and environmentally unfriendly. Furthermore, the fact that solvents cannot be completely removed from polymers would cause damage to the performance of nanocomposites inevitably.22-24 In this study, we report a novel strategy to prepare polymer nanocomposites with preeminent mechanical properties, electrical and thermal conductivity taking advantage of both closely packed


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CNTs 3D networks. Multi-walled carbon nanotubes (MWNTs) were used in this study because they are cheaper than single-walled carbon nanotubes (SWNTs) and can maintain excellent properties after surface modifications.25 To improve the interface binding force and transfer load from polyamide 6 matrix to MWNTs more efficiently, the polyethylenimine (PEI), a kind of branched polymer composed of abundant active amine groups, was grafted onto the surface of MWNTs to modify the MWTNs/PA6 nanocomposites because its active amine groups can react with the carboxyl groups of both acid treated MWNTs and PA6. Four 3D CNTs networks obtained through filter pressing of monodispersed MWNTs solution, were firstly impregnated with heated PA6 monomer solution (caprolactam and 6-aminocaproic acid), and then piled up. Finally, the PA6 monomer in-situ polymerized during hot-pressing and MWNTs/polyamide 6 (MWNTs/PA6) nanocomposites came into being. The entire process is simple, inexpensive and solvent-free, and it can be scaled up for industrial manufacturing. The PEI-modified-MWNTs/polyamide 6 (mMWNTs/PA6) demonstrates tension strength of 99.4 MPa and thermal conductivity of 1.515 W m-1 k-1. The electric conductivity of pristine-MWNTs/polyamide 6 (p-MWNTs/PA6) reaches 19.61 S cm-1, which is close to that of the conductor (≥103 S cm-1). These enhancements mainly benefit from the contributions of closely-packed 3D network and conductive pathways of CNTs. These special constructions offer considerable potential for developing polymer nanocomposites with excellent performance approaching to that of nanoscale materials.

2. Experimental Section 2.1. Materials MWNTs (CVD method, purity > 95%, diameter = 10~20 nm, length = 10~30 µm), were obtained from TimesNano Chengdu Organic Chemicals Co., Ltd. (China). Triton X-100 (Mw = 648.85) was obtained from Shanghai Klamar Reagent Co., Ltd (China). Caprolactam (CPL), polyethylenimine (PEI, Mw = 70000), and 6-aminocaproic acid were supplied by Aladdin Industrial Co., Ltd (China). Ethyl alcohol (analytical grade), nitric acid, and sulfuric acid were supplied by Shanghai Titan



Scientific Co., Ltd (China). All the reagents were used without further purification. Distilled water was self-produced in our lab. The nylon microfiltration membrane (diameter = 10 cm) with a pore size of 0.22 µm was purchased from Taoyuan Medical Co., Ltd (China).

2.2. Fabrication of Closely Packed 3D Modified-MWNTs Network MWNTs (0.4 g), nitric acid (40 ml), and sulfuric acid (120 ml) were mixed through the process of magnetic stirring and ultrasonication for 2 h using a sonication ultrasonic cell disruptor followed by stirring of MWNTs solution for one day. Later, the pH of the MWNTs solution was turned into neutral by using distilled water. PEI (1 g) was added into the solution and stirred overnight at 60 °C. PEI-MWNTs monodisperse solution was injected into the filtrator for filtering closely packed 3D MWNTs network from the solution with the assistance of an air compressor. 3D PEI-modifiedMWNTs (m-MWNTs) network was obtained on the nylon microfiltration membrane inside the filter. Ethyl alcohol (100 ml) was injected into the filtrator and filtered to entrain remaining moisture. This 3D m-MWNTs network was then dried in the air overnight. For comparison, 3D pristine-MWNTs (p-MWNTs) network without grafting PEI were prepared according to the following step.

2.3. Fabrication of Closely Packed 3D Pristine-MWNTs Network MWNTs (0.4 g) were dispersed into distilled water (1000 ml) with the addition of Triton X-100 (0.25 g) by the process of magnetic stirring and ultrasonication for 1 h. The solution was kept overnight at room temperature. Then the MWNTs monodispersed solution was injected into the filtrator for filtering closely packed 3D p-MWNTs network from the solution. Ethyl alcohol (100 ml) was used to entrain remaining moisture. This 3D MWNTs network was then dried in the air overnight.

2.4. Fabrication of MWNTs/PA6 Nanocomposites


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CPL (27 g) and 6-aminocaproic acid (3 g) were melted at 180 °C, then the mixed solution was injected into the filtering equipment to filter (the filtrate was preheated to 180 °C), and the filtration was stopped when the volume of filtrate reached 10 ml. The free-standing 3D MWNTs network permeated with PA6 monomer solution was peeled off from the nylon membrane filter. Four 3D MWNTs networks permeated with PA6 monomer solution were piled up together and were hotpressed at 250 °C under 2MPa for 10 hours using a thermocompressor. After cooling to room temperature, the as-prepared nanocomposites were washed in boiling deionized water for 5 h to remove the monomer and the oligomers. This is a new approach to produce thick 3D MWNTs network faster for industrial applications of strong and multifunctional nanocomposites. By using large scale filtering equipment and high pressure, it is possible to produce nanocomposites with desired size and thickness more rapidly, involving precise control of the size of filter screen, the value of pressure, and the concentration of MWNTs solution. It opens up broad potentials for technological large-scale production of such strong and multifunctional nanocomposites based on CNTs.

2.5. Characterization A Scanning Electron Microscope (SEM) (HITACHI, SU8010 FEG-SEM, Japan) was employed to observe the morphologies, the surfaces of p-MWNTs 3D network and m-MWNTs 3D network, and the tensile rupture interfaces of p-MWNTs/PA6 and m-MWNTs/PA6 samples. Fourier transform infrared spectra (FTIR) characterizations were performed with a Nicolet 6700 (Thermo Fisher Scientific, USA) spectrometer at room temperature. X-ray photoelectron spectroscopy (XPS) experiments were carried out on an Escalab 250 Xi (Thermo Fisher Scientific, USA) with Al Kα radiation (hv = 1486.6 eV). The nanoindentation was performed in a nanoindenter instrument (Hysitron Inc., Tribo Indenter 750, USA) with a three-sided pyramid Berkovich diamond indenter (radius of the indenter probe was 50 nm) and normal force of 5000 µN held for 2 seconds. The samples were polished before the test. Tensile test was performed by using a universal testing



machine (AGS-X PLUS, Shimadzu, Japan) with a crosshead speed of 2 mm/min, according to ASTM D638 M. A 5000 N of load cell and a 25 mm of gauge length were used. For each specimen, the data reported here represent the averaged results of at least five successful tests. The electrical conductivity of the samples was tested through the standard four-probe technique (Azar Electrode, Urmia, Iran) at room temperature. All the samples were dried at 80 °C for 24 h before the test. Thermal conductivity at 20 °C was measured on a TC 3000 thermal conductivity tester (Xiatech Instrument Factory, China) using a transient hot-wire method. The dimensions of each sample were Length (30.00 ± 0.02) mm and Width (1.00 ± 0.02) mm.

3. Results and Discussion 3.1 Preparation and Characterization of MWNTs/PA6 Nanocomposites Figure 1a shows the synthesis scheme for pristine MWNTs (p-MWNTs), acidized MWNTs, and PEI-modified-MWNTs (m-MWNTs). The acidized MWNTs were prepared via an oxidization reaction of MWNTs in H2SO4/HNO3, and the m-MWNTs were derived by grafting PEI onto acidized-MWNTs. To confirm successful acidification and covalent modification of MWNTs, solid state FTIR was carried out on p-MWNTs, acidized MWNTs and m-MWNTs respectively to show the presence of the peaks which characterize functional groups on the surface of carbon nanotubes. As shown in Figure 1b, no obvious peaks in the FTIR spectrum of p-MWNTs indicates there were no functional groups on the surface of the p-MWNTs. The characteristic peaks of acidized MWNTs at 3430 and 1730 cm-1 can be attributed to broad O-H stretching vibrations and C=O stretching vibrations in carboxylic acid and carbonyl moieties, respectively. After grafting PEI onto acidized MWNTs, several new peaks appear in the FTIR spectrum of m-MWNTs. The characteristic peaks of N-H deformation vibrations at 1635 cm-1 and C=O stretching vibrations indicate the presence of the amide (-CO-NH-). The C-N stretching vibrations at 1206 cm-1 is due to the primary amine groups of PEI. The residual weak peak of O-H stretching vibrations of m-MWNTs is because of the


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moisture inside the MWNTs. These changes in FTIR spectra clearly confirm the successful grafting of PEI chains onto MWNTs. To further illustrate the formation of acidized MWNTs and m-MWNTs, the XPS was further performed to characterize the changes of chemical structures. The only peak observed at 285 eV in the spectrum of p-MWNTs is attributed to C1s. In the spectrum of acidized MWNTs, the O1s peak at 531 eV appears and the intensity of the O1s peak is stronger than that of C1s, indicating a significant oxidation of the p-MWNTs. As regards m-MWNTs, a N1s peak of amino groups at 400 eV was observed, showing the successful introduction of PEI onto m-MWNTs. Moreover, the intensity of the O1s peak is dramatically reduced and much smaller than that of the carbon peak, indicating enormous deoxygenation of acidized MWNTs by PEI. As shown in Figure 1d, the 3D MWNTs network was prepared by filtering the monodisperse MWNTs solution in order that the MWNTs were tightly packed bottom-up one by one, then four as-prepared 3D MWNTs networks were impregnated with heated PA6 monomer solution and then piled up, finally the monomer in situ polymerized during the process of hot-pressing. The microstructures of 3D p-MWNTs network, m-MWNTs network and their polymer nanocomposites were observed by using scanning electron microscopy (SEM), as illustrated in Figure 2. It can be seen from Figure 2a that the pristine MWNTs disperse in the form of agglomerates. Figure 2b shows that the densely packed 3D network is comprised of randomly distributed pristine MWNTs. This 3D network was employed as the skeleton of polymer matrix and played a part in heat and electron conduction and resistance to external load. Owing to the surface treatment of MWNTs, impurities on the MWNTs were removed and the functional groups (-CONH- and -C-N-) were formed on the external surfaces. Besides, there exists the ionic repulsive interactions between the functional groups. All of these mentioned above contribute to better dispersion of m-MWNTs. Therefore, the m-MWNTs did not generate agglomeration (see Figure 2c). As can be seen from Figure 2d, the 3D m-MWNTs network has a more compact morphology than p-MWNTs network (Figure 2b).



To study the reinforcing mechanism of the 3D MWNTs network, we examined the fracturesurface morphologies of the as-prepared nanocomposites, including p-MWNTs/PA6 (Figure 2e and 2f) and m-MWNTs/PA6 (Figure 2g-j). The p-MWNTs/PA6 exhibits flat fracture surfaces, as shown in Figure 2e. The voids left by the pulled-out MWNTs, as well as the clear interfaces formed between MWNTs and the matrix, indicate that the matrix could not infiltrate well into the agglomerates of the p-MWNTs and the interfacial bonding is weak (Figure 2f). In sharp contrast to p-MWNTs/PA6, the m-MWNTs/PA6 exhibits a rather rough and rugged fracture surfaces (Figure 2g). Rougher fracture surfaces mean larger surface area, and the formation of it needs to absorb more fracture energy. As shown in Figure 2h, the m-MWNTs were pulled out from nanocomposites much more slightly than p-MWNTs because the lengths of draw-out part of m-MWNTs were much shorter, and the m-MWNTs were closely surrounded by the matrix. In addition, the drawn mMWNTs, shown in Figure 2i, curved when the electron beam of scanning electron microscope was focused on them, and the bended m-MWNTs were presented in Figure 2j. Note that the electron beam generated considerable heat when it was focused on m-MWNTs. While the m-MWNTs would not deform at high temperature, the appearance of the curved m-MWNTs is because that the mMWNTs were tightly coated with PA6, which melted at high temperature and made m-MWNTs curved. This phenomenon further proves the strong interfacial bonding between m-MWNTs and PA6 matrix.

3.2 Mechanical Properties of MWNTs/PA6 Nanocomposites 3.2.1. Nano-Indentation Analysis of MWNTs/PA6 Nanocomposites To get deep insight into the interlamination between 3D MWNTs networks, the interlayer binding force was further examined through nanoindentation test with a normal force of 5000 µN performed on the samples. Figure 3a shows the indents left on intersecting surface of the m-MWNTs/PA6 sample which has experienced nanoindentation test. According to the elastic modulus contour map (Figure 3b), there are four high elastic modulus areas alternating with low elastic modulus areas,


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which is consistent with the sandwich structures comprised of four m-MWNTs 3D networks (Figure 3c). It should be noted that even the lowest elastic modulus appeared on the interfacial transition zones (4.6 GPa) outdistances that of neat PA6 (1.46 GPa). The dramatical improvement appeared in interfacial transition zones can be explained as follows. During thermocompression, some external m-MWNTs stretched out and entangled with each other between two 3D m-MWNTs networks and concatenated four m-MWNTs 3D networks to an organic assembly, so that the load transfer is more efficient and every single MWNT shares the load acted on the nanocomposites. In addition, the loading-unloading curves of neat PA6, p-MWNTs/PA6 and m-MWNTs/PA6 were also presented in Figure 3d. By analyzing the unloading part of these curves with the Oliver and Pharr method,26 the estimated elastic modulus (E) and hardness (H) of the samples were listed in Table 1. For the p-MWNTs/PA6 (or m-MWNTS/PA6), the mean values of E and H are 9.88 (or 12.63) GPa and 0.85 (or 1.23) GPa, respectively. In comparison with the neat PA6, the E and H of the p-MWNTs/PA6 (or m-MWNTs/PA6) were respectively increased by ~577% (or 765%) and ~430% (or 667%). It is obvious that greater increment of E and H appears in the m-MWNTs/PA6. The better micromechanical properties of m-MWNTs/PA6 are attributed to the better dispersion of m-MWNTs in polymer matrix and the strong interfacial bonding force between m-MWNTs and PA6 matrix, as proved in SEM observations (Figure 2).

3.2.2. Tensile Properties of MWNTs/PA6 Nanocomposites It is well known that macro-mechanical properties of materials are vital in their practical applications, hence, the stress-strain curves and relevant tensile properties of the neat PA6, pMWNTs/PA6, and m-MWNTs/PA6, including tensile strength, Young’s modulus, and elongation at break, were given in Figure 4 and Table 2, respectively. Figure 4 displays that the tensile strength and Young’s modulus of PA6 were dramatically improved by the incorporation of both p-MWNTs and m-MWNTs. Especially, the m-MWNTs/PA6 even reveals an improvement of 82% and 119% in tensile strength and Young’s modulus, respectively. Nevertheless, the elongation at break of p-



MWNTs/PA6 and m-MWNTs/PA6 decreases as compared with that of neat PA6, which suggests that the as-prepared nanocomposites become more brittle. Due to the huge viscosity, the heavy addition of nanofillers into polymer matrix through mechanical mixing or in situ polymerization is usually difficult to process and do harm to the mechanical properties of nanocomposites on account of the defects formed during the preparation process. Fortunately, the possible voids resulted from solvent evaporation could be avoided because of the solvent-free infiltration process of impregnating hot PA6 monomer solution into a densely-packed 3D network of MWNTs (including p-MWNTs and m-MWNTs). It should be also convinced that the pre-built 3D network of MWNTs and surficial treatment are of crucial importance to avoid problems arising from heavy addition of nanofillers. The phenomenon that the significantly improved mechanical properties of the as-prepared nanocomposites can be rationalized by considering the effect of the densely packed 3D network structures of MWNTs and the interfacial interactions. As mentioned in Figure 2b and 2d, the MWNTs entangle with each other to form a densely packed 3D network, which transfers external load more efficiently than the disconnected MWNTs, in the as-prepared nanocomposites. The prebuilt MWNTs network further prevents MWNTs from agglomerating, which enlarges the contact area between MWNTs and PA6 matrix. For the m-MWNTs/PA6, the molecular chains of PEI grafted on the MWNTs improve the compatibility between m-MWNTs and PA6 matrix. Moreover, the amide groups of PEI react with PA6 monomers, which further strengthens the interfacial adhesion and endows higher tensile properties to m-MWNTs/PA6. However, the densely packed 3D network structures of the MWNTs, which acted as a robust skeleton, would greatly restrict the physical motion of PA6 chains and reduce the degree of polymerization during the process of in situ polymerization. Therefore, the elongation at break of the as-prepared nanocomposites was reduced as compared with that of neat PA6.

3.3 Electrical Properties of MWNTs/PA6 Nanocomposites


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The electrical conductivity of the as-prepared nanocomposites was further tested by four-probe scheme at room temperature. It was observed that the electrical conductivity of p-MWNTs/PA6 reaches 19.61 S cm-1. Comparing with that of neat PA6 (2×10-15 S cm-1),27 the p-MWNTs/PA6 displays a dramatic increase of around 16 orders of magnitude in electrical conductivity. Large aspect ratio of MWNTs and closely packed structures of 3D MWNTs networks are beneficial to build up efficient consecutive electrical conductive paths and the p-electrons of the delocalized πbonds also contribute to the exceeding electrical conductivity of MWNTs, which turns the PA6 matrix from an insulator into a material approximating to the conductor (≥103 S cm-1). The electrical conductivity and ultimate tensile strength of the as-prepared nanocomposites as well as those of nanocomposites (CNT/CNF and graphene/RGO based) and conducting polymer (PANI based) were elucidated in Figure 5. Among the involved nanocomposites, the graphene/ANF (data point 10) and graphene/cellulose (data point 11) nanocomposites display higher electrical conductivity and ultimate tensile strength than other nanocomposites. In comparison, although the ultimate tensile strengths of the as-prepared nanocomposites are slightly lower than that of the graphene/ANF and graphene/cellulose nanocomposites, the electrical conductivity is 1~2 orders of magnitude higher. For the m-MWNTs/PA6, the electrical conductivity is 13.94 S cm-1, which is a little lower than that of p-MWNTs/PA6 (19.61 S cm-1). The reduction of the electrical conductivity of mMWNTs/PA6 is due to the fact that the delocalized π-bonds on the ektexine of MWNTs were broken after the purification with strong acids and the covalent modification by grafting functional groups onto the MWNTs. However, thanks to the multiwall onion structures of MWNTs, the aromatic bonds inside the multiwall onion structures were not destroyed during the process of surface modification, so that the electrical conductivity was protected from being dramatically cutting down.

3.4 Thermal Properties of MWNTs/PA6 Nanocomposites



The thermal conductivity (denoted by λ) of the as-prepared nanocomposites and PA6 matrix was characterized by transient hot-wire technique, as shown in Figure 6 and Table 3. Obviously, the λ of m-MWNTs/PA6 increases by 672% comparing with that of neat PA6 (from 0.196 to 1.515 W m-1 K-1), and the λ of p-MWNTs/PA6 is 1.186 W m-1 K-1. The previous investigations on thermal conductive polymers with frequently-used fillers, such as CNTs, carbon fiber (CF) and graphite, displayed that higher filler content of addition just achieved lower enhancement of the λ.42-47 It is well known that the thermal energy mainly transferred in form of phonons in polymer composites. Several reports have pointed out that the phonon coupling in the vibrational mode of CNTs and polymer matrix dominates the thermal conductive properties of the composites. It is important to reduce the acoustic phonon scattering at the filler-matrix interfaces.48 The phase interfaces between MWNTs and matrix are relatively small due to the multiwall onion structures, which reduces the acoustic phonon scattering phenomenon and increases the λ of the composites.49 In addition, the successful covalent modification of MWNTs with PEI enhances the interface bonding interaction between the m-MWNTs and the polymer matrix, which further increases the λ. Celzard50 and Munson-McGee51 added aspect ratio into the permeation theory as one of the influencing factors because they both believed that the aspect ratio of filler particles favors the formation of thermal conductive pathways, and based on that, the large aspect ratio of the MWNTs (diameter : length = 10 : 30000) and closely packed 3D network structures were beneficial to build up efficient consecutive thermal conductive paths while less thermal gaps. As proved by SEM observations, the MWNTs tangle with each other to form a continuous network path for heat transmission. Nowadays, when the heat dissipation problems in various fields highlight increasingly, the easily obtained MWNTs/PA6 nanocomposites with high λ has wide potential applications in highperformance thermal management systems, connectors, and other thermal interface materials, such as the heat-conducting fins in the heat dissipation of computer, automobile, high-power lightemitting diode (LED), and other electronic products.


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4. Conclusions In summary, the high conductive and robust PA6 based nanocomposites were easily prepared by infiltrating closely packed 3D network of MWNTs with hot PA6 monomer solution and subsequent thermal polymerization. In the as-prepared nanocomposites (p-MWNTs/PA6 and m-MWNTs/PA6), the high-strength MWNTs network serves as backbone for assembly of polymer matrix. The preparation is solvent-free, which avoids the processing difficulties and structural flaws under high filler content. The as-prepared nanocomposites not only possess very high electrical and thermal conductivity, but also show significantly enhanced mechanical properties even under very high MWNTs loading (25 wt%). The electrical conductivity was increased by 16 orders of magnitude and reached 19.16 S cm-1. In comparison with the observation of SWNTs nanocomposites,52,53 the overall performance of the as-prepared nanocomposites were further improved, leaving however some room for further improvement, especially considering that different polymer matrices may get a better purpose using the same preparation method mentioned in this paper.54,55 The fabrication of such nanocomposites opens up a broad range of possibilities for aerospace, conducting element and structural nanomaterials, as well as electronic components with the requirements of high heat dissipation and mechanical strength.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No.: 51403212), the Fujian-CAS STS Foundation (Grant No.: 2016T3034), and the China Postdoctoral Science Foundation (Grant No.: 2018M632590).

References (1) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Exceptionally High Young's Modulus Observed for Individual Carbon Nanotubes. Nature 1996, 381, 678-680. ACS Paragon Plus Environment


(2) Yu, M. F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load. Science 2000, 287, 637640. (3) Breuer, O.; Sundararaj, U. Big Returns from Small Fibers: A Review of Polymer/Carbon Nanotube Composites. Polym. Compos. 2004, 25, 630-645. (4) Yoonessi, M.; Gaier, J. R. Highly Conductive Multifunctional Graphene Polycarbonate Nanocomposites. ACS Nano 2010, 4, 7211-7220. (5) Ding, P.; Su, S.; Song, N.; Tang, S.; Liu, Y.; Shi, L. Highly Thermal Conductive Composites with Polyamide-6 Covalently-Grafted Graphene by an In Situ Polymerization and Thermal Reduction Process. Carbon 2014, 66, 576-584. (6) Chen, M.; Zhang, L.; Duan, S.; Jing, S.; Jiang, H.; Luo, M.; Li, C. Highly Conductive and Flexible Polymer Composites with Improved Mechanical and Electromagnetic Interference Shielding Performances. Nanoscale 2014, 6, 3796-3803. (7) Chen, M.; Tao, T.; Zhang, L.; Gao, W.; Li, C. Highly Conductive and Stretchable Polymer Composites Based on Graphene/MWCNT Network. Chem. Commun. 2013, 49, 1612-1614. (8) Kim, K. H.; Vural, M.; Islam, M. F. Single-Walled Carbon Nanotube Aerogel-Based Elastic Conductors. Adv. Mater. 2011, 23, 2865-2869. (9) Park, J.; Wang, S.; Li, M.; Ahn, C.; Hyun, J. K.; Kim, D. S.; Kim, D. K.; Rogers, J. A.; Huang, Y.; Jeon, S. Three-Dimensional Nanonetworks for Giant Stretchability in Dielectrics and Conductors. Nat. Commun. 2012, 3, 916. (10) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and Stiff Layered Polymer Nanocomposites. Science 2007, 318, 80-83. (11) Xu, Y.; Sheng, K.; Li, C.; Shi, G. Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process. ACS Nano 2010, 4, 4324-4330. (12) Wu, Y.; Wang, Z.; Liu, X.; Shen, X.; Zheng, Q.; Xue, Q.; Kim, J.-K. Ultralight Graphene


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Foam/Conductive Polymer Composites for Exceptional Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2017, 9, 9059-9069. (13) Shen, X.; Wang, Z.; Wu, Y.; Liu, X.; He, Y.-B.; Zheng, Q.; Yang, Q.-H.; Kang, F.; Kim, J.-K. A Three-Dimensional Multilayer Graphene Web for Polymer Nanocomposites with Exceptional Transport Properties and Fracture Resistance. Mater. Horiz. 2018, 5, 275-284. (14) Pettes, M. T.; Ji, H.; Ruoff, R. S.; Shi, L. Thermal Transport in Three-Dimensional Foam Architectures of Few-Layer Graphene and Ultrathin Graphite. Nano Lett. 2012, 12, 2959-2964. (15) Worsley, M. A.; Kucheyev, S. O.; Kuntz, J. D.; Hamza, A. V.; Satcher, J. H., Jr.; Baumann, T. F. Stiff and Electrically Conductive Composites of Carbon Nanotube Aerogels and Polymers. J. Mater. Chem. 2009, 19, 3370-3372. (16) Worsley, M. A.; Pauzauskie, P. J.; Olson, T. Y.; Biener, J.; Satcher, J. H., Jr.; Baumann, T. F. Synthesis of Graphene Aerogel with High Electrical Conductivity. J. Am. Chem. Soc. 2010, 132, 14067-14069. (17) Ni, Y.; Chen, L.; Teng, K.; Shi, J.; Qian, X.; Xu, Z.; Tian, X.; Hu, C.; Ma, M. Superior Mechanical Properties of Epoxy Composites Reinforced by 3D Interconnected Graphene Skeleton. ACS Appl. Mater. Interfaces 2015, 7, 11583-11591. (18) Luo, S.; Luo, Y.; Wu, H.; Li, M.; Yan, L.; Jiang, K.; Liu, L.; Li, Q.; Fan, S.; Wang, J. SelfAssembly of 3D Carbon Nanotube Sponges: A Simple and Controllable Way to Build Macroscopic and Ultralight Porous Architectures. Adv. Mater. 2017, 29, 1603549. (19) Worsley, M. A.; Kucheyev, S. O.; Mason, H. E.; Merrill, M. D.; Mayer, B. P.; Lewicki, J.; Valdez, C. A.; Suss, M. E.; Stadermann, M.; Pauzauskie, P. J.; Satcher, J. H., Jr.; Biener, J.; Baumann, T. F. Mechanically Robust 3D Graphene Macroassembly with High Surface Area. Chem. Commun. 2012, 48, 8428-8430. (20) Li, X.; Shao, L.; Song, N.; Shi, L.; Ding, P. Enhanced Thermal-Conductive and Anti-Dripping Properties of Polyamide Composites by 3D Graphene Structures at Low Filler Content. Compos. Part A-Appl. Sci. Manuf. 2016, 88, 305-314.



Page 18 of 29

(21) Chen, L.; Hou, X.; Song, N.; Shi, L.; Ding, P. Cellulose/Graphene Bioplastic for Thermal Management: Enhanced Isotropic Thermally Conductive Property by Three-Dimensional Interconnected Graphene Aerogel. Compos. Part A-Appl. Sci. Manuf. 2018, 107, 189-196. (22) Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J. Ultralight and Highly Compressible Graphene Aerogels. Adv. Mater. 2013, 25, 2219-2223. (23) Wang, S.; Liang, Z.; Pham, G.; Park, Y.-B.; Wang, B.; Zhang, C.; Kramer, L.; Funchess, P. Controlled Nanostructure and High Loading of Single-Walled Carbon Nanotubes Reinforced Polycarbonate Composite. Nanotechnology 2007, 18, 095708. (24) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Carbon Nanotubes - the Route toward Applications. Science 2002, 297, 787-792. (25) Kaleemullah, M.; Khan, S. U.; Kim, J.-K. Effect of Surfactant Treatment on Thermal Stability and Mechanical Properties of CNT/Polybenzoxazine Nanocomposites. Compos. Sci. Technol. 2012, 72, 1968-1976. (26) Oliver, W. C.; Pharr, G. M. An Improved Technique for Determining Hardness and Elastic Modulus Using Load and Displacement Sensing Indentation Experiments. J. Mater. Res. 1992, 7, 1564-1583. (27) Wang, X.; Bradford, P. D.; Liu, W.; Zhao, H.; Inoue, Y.; Maria, J.-P.; Li, Q.; Yuan, F.-G.; Zhu, Y. Mechanical and Electrical Property Improvement in CNT/Nylon Composites through Drawing and Stretching. Compos. Sci Technol. 2011, 71, 1677-1683. (28) Chen, J.; Ramasubramaniam, R.; Xue, C.; Liu, H. A Versatile, Molecular Engineering Approach

to

Simultaneously

Enhanced,

Multifunctional

Carbon-Nanotube-Polymer

Composites. Adv. Funct. Mater. 2006, 16, 114-119. (29) Gorrasi, G.; Sarno, M.; Di Bartolomeo, A.; Sannino, D.; Ciambelli, P.; Vittoria, V. Incorporation of Carbon Nanotubes into Polyethylene by High Energy Ball Milling: Morphology and Physical Properties. J. Polym. Sci. Part B-Polym. Phys. 2007, 45, 597-606. (30) Allaoui, A.; Bai, S.; Cheng, H. M.; Bai, J. B. Mechanical and Electrical Properties of a


Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MWNT/Epoxy Composite. Compos. Sci. Technol. 2002, 62, 1993-1998. (31) McNally, T.; Potschke, P.; Halley, P.; Murphy, M.; Martin, D.; Bell, S. E. J.; Brennan, G. P.; Bein, D.; Lemoine, P.; Quinn, J. P. Polyethylene Multiwalled Carbon Nanotube Composites. Polymer 2005, 46, 8222-8232. (32) Yuen, S.-M.; Ma, C.-C. M.; Lin, Y.-Y.; Kuan, H.-C. Preparation, Morphology and Properties of Acid and Amine Modified Multiwalled Carbon Nanotube/Polylmide Composite. Compos. Sci. Technol. 2007, 67, 2564-2573. (33) Choi, Y. K.; Sugimoto, K.; Song, S. M.; Gotoh, Y.; Ohkoshi, Y.; Endo, M. Mechanical and Physical Properties of Epoxy Composites Reinforced by Vapor Grown Carbon Nanofibers. Carbon 2005, 43, 2199-2208. (34) Vadukumpully,

S.;

Paul,

J.;

Mahanta,

N.;

Valiyaveettil,

S.

Flexible

Conductive

Graphene/Poly(vinyl chloride) Composite Thin Films with High Mechanical Strength and Thermal Stability. Carbon 2011, 49, 198-205. (35) Song, P.; Cao, Z.; Cai, Y.; Zhao, L.; Fang, Z.; Fu, S. Fabrication of Exfoliated Graphene-Based Polypropylene Nanocomposites with Enhanced Mechanical and Thermal Properties. Polymer 2011, 52, 4001-4010. (36) Nguyen Dang, L.; Hippi, U.; Korhonen, J. T.; Soininen, A. J.; Ruokolainen, J.; Johansson, L.-S.; Nam, J.-D.; Sinh, L. H.; Seppala, J. Enhanced Mechanical and Electrical Properties of Polyimide Film by Graphene Sheets via In Situ Polymerization. Polymer 2011, 52, 5237-5242. (37) Fan, J.; Shi, Z.; Tian, M.; Yin, J. Graphene-Aramid Nanofiber Nanocomposite Paper with High Mechanical and Electrical Performance. RSC Adv. 2013, 3, 17664-17667. (38) Nguyen Dang, L.; Pahimanolis, N.; Hippi, U.; Korhonen, J. T.; Ruokolainen, J.; Johansson, L.S.; Nam, J.-D.; Seppala, J. Graphene/Cellulose Nanocomposite Paper with High Electrical and Mechanical Performances. J. Mater. Chem. 2011, 21, 13991-13998. (39) He, L.; Tjong, S. C. Facile Synthesis of Silver-Decorated Reduced Graphene Oxide as a Hybrid Filler Material for Electrically Conductive Polymer Composites. RSC Adv. 2015, 5,



15070-15076. (40) Wang, X.; Bai, H.; Yao, Z.; Liu, A.; Shi, G. Electrically Conductive and Mechanically Strong Biomimetic Chitosan/Reduced Graphene Oxide Composite Films. J. Mater. Chem. 2010, 20, 9032-9036. (41) Hu, W.; Chen, S.; Yang, Z.; Liu, L.; Wang, H. Flexible Electrically Conductive Nanocomposite Membrane Based on Bacterial Cellulose and Polyaniline. J. Phys. Chem. B 2011, 115, 84538457. (42) Zhou, H.; Zhang, S.; Yang, M. The Thermal Conductivity of Nylon 6/Clay Nanocomposites. J. Appl. Polym. Sci. 2008, 108, 3822-3827. (43) Ji, L.; Stevens, M. M.; Zhu, Y.; Gong, Q.; Wu, J.; Liang, J. Preparation and Properties of MultiWalled Carbon Nanotube/Carbon/Polystyrene Composites. Carbon 2009, 47, 2733-2741. (44) Yan, X.; Imai, Y.; Shimamoto, D.; Hotta, Y. Relationship Study between Crystal Structure and Thermal/Mechanical Properties of Polyamide 6 Reinforced and Unreinforced by Carbon Fiber from Macro and Local View. Polymer 2014, 55, 6186-6194. (45) Peters, J. E.; Papavassiliou, D. V.; Grady, B. P. Unique Thermal Conductivity Behavior of Single-Walled Carbon Nanotube-Polystyrene Composites. Macromolecules 2008, 41, 72747277. (46) Keith, J. M.; Hingst, C. D.; Miller, M. G.; King, J. A.; Hauser, R. A. Measuring and Predicting In-Plane Thermal Conductivity of Carbon-Filled Nylon 6,6 Polymer Composites. Polym. Compos. 2006, 27, 1-7. (47) Weidenfeller, B.; Hofer, M.; Schilling, F. Thermal and Electrical Properties of Magnetite Filled Polymers. Compos. Part A-Appl. Sci. Manuf. 2002, 33, 1041-1053. (48) Martin-Gallego, M.; Verdejo, R.; Khayet, M.; Maria Ortiz de Zarate, J.; Essalhi, M.; Angel Lopez-Manchado, M. Thermal Conductivity of Carbon Nanotubes and Graphene in Epoxy Nanofluids and Nanocomposites. Nanoscale Res. Lett. 2011, 6, 610. (49) Gojny, F. H.; Wichmann, M. H. G.; Fiedler, B.; Kinloch, I. A.; Bauhofer, W.; Windle, A. H.;


Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Schulte, K. Evaluation and Identification of Electrical and Thermal Conduction Mechanisms in Carbon Nanotube/Epoxy Composites. Polymer 2006, 47, 2036-2045. (50) Celzard, A.; McRae, E.; Deleuze, C.; Dufort, M.; Furdin, G.; Mareche, J. F. Critical Concentration in Percolating Systems Containing a High-Aspect-Ratio Filler. Phys. Rev. B 1996, 53, 6209-6214. (51) Munson-McGee, S. H. Estimation OF the Critical Concentration in an Anisotropic Percolation Network. Phys. Rev. B 1991, 43, 3331-3336. (52) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Molecular Design of Strong Single-Wall Carbon Nanotube/Polyelectrolyte Multilayer Composites. Nat. Mater. 2002, 1, 190-194. (53) Rouse, J. H.; Lillehei, P. T. Electrostatic Assembly of Polymer/Single Walled Carbon Nanotube Multilayer Films. Nano Lett. 2003, 3, 59-62. (54) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Super-Tough Carbon-Nanotube Fibres - These Extraordinary Composite Fibres can be Woven into Electronic Textiles. Nature 2003, 423, 703-703. (55) Geng, H. Z.; Rosen, R.; Zheng, B.; Shimoda, H.; Fleming, L.; Liu, J.; Zhou, O. Fabrication and Properties of Composites of Poly(ethylene oxide) and Functionalized Carbon Nanotubes. Adv. Mater. 2002, 14, 1387-1390.



Figures

a MWNTs

Acidized MWNTs

b

PEI-modified-MWNTs

c C=O

N-H

O-H

Intensity (a.u.)

PEI-modified-MWNTs

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

C-N

Acidized MWNTs

O-H

C=O

PEI-modified-MWNTs

Acidized MWNTs

Pristine MWNTs Pristine MWNTs

4000

3500

2000

1500

1000

500

0

-1

200

400

600

800

1000

Binding Energy (eV)

Wavenumber (cm )

d (1)

MWNTs

(2)

m-MWNTs

Acidized MWNTs

(3)

(4)

(5) m-MWNTs/PA6

Infiltration of PA6

m-MWNTs network

Figure 1. (a) Synthesis scheme, (b) FTIR spectra, and (c) XPS survey spectra of pristine MWNTs (p-MWNTs), acidized MWNTs and PEI-modified-MWNTs (m-MWNTs), respectively. (d) Schematic of the fabrication process for m-MWNTs/PA6 nanocomposites: (1) acidification, (2) surface functional modification of acidized-MWNTs with PEI, (3) pressure filtration, (4) permeation with PA6 monomer solution, and (5) pilling up four 3D m-MWNTs networks followed by thermocompression.


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

3 µm

c

300 nm

d

3 µm

e

300 nm

f

3 µm

g

300 nm

h

3 µm

i

300 nm

j

300 nm

300 nm

Figure 2. SEM images of (a, b) p-MWNTs closely packed 3D network, (c, d) closely packed 3D mMWNTs network, (e, f) the tensile rupture interface of p-MWNTs/PA6, (g, h) the tensile rupture interface of m-MWNTs/PA6, and (i, j) the pulled out m-MWNTs coated with PA6 on the surfaces.



a

b

Elastic modulus (GPa) 4.700 10.70 16.70 22.70 28.70

NTs/P A6

c tP A6

3000

p-MW

d

4000

Ne a

5000

m- M WN Ts/P A6

34.70

Load (µ N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

2000 1000 0

0

500

1000 Depth (nm)

1500

Figure 3. (a) The image of the sample which has experienced the nanoindentation test. (b) The elastic modulus contour map of the cross section of m-MWNTs/PA6 composites. (c) The sketch of the internal sandwich structure of m-MWNTs/PA6 which comprised four m-MWNTs 3D networks. (d) The loading-unloading curves of Neat PA6, p-MWNTs/PA6, and m-MWNTs/PA6.


Page 25 of 29

100 Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Neat PA6 p-MWNTs/PA6 m-MWNTs/PA6

80 60 40 20 0

0

10

20

30

Strain (%) Figure 4. The stress-strain curves of neat PA6, p-MWNTs/PA6, and m-MWNTs/PA6.


-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electrical Conductivity (S cm )


30

15

CNT/CNF based Graphene/RGO based PANI based This work 15: p-MWNTs/PA6 16: m-MWNTs/PA6

10 2 0.5

3

Page 26 of 29

16

10 11 1 7

14 13

12

0.01 2 4

1E-4

6 5 8

9

1E-6 8

16

32

64

128

256

Ultimate Tensile Strength (MPa) Figure 5. 1: CNT/PPE,28 2: CNT/PE,29 3: MWNT/epoxy,30 4: MWNT/PE,31 5: MWNT/PI,32 6: CNF/epoxy,33 7: graphene/PVC,34 8: graphene/PP,35 9: graphene/PI,36 10: graphene/ANF,37 11: graphene/cellulose,38 12: RGO/PVDF,39 13: RGO/chitosan,40 14: PANI/bacterial cellulose,41 15: pMWNTs/PA6, and 16: m-MWNTs/PA6.


-1

1.5

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thermal Conductivity (W m k )

Page 27 of 29

nanocomposites matrix

1.0

0.5

0.0

6 6 6 6 6 6 S S /PATs/PA T/PA am/P F/PANTs/P e/PA6 O 4/PA s T C M t o i f N N e3 W M h MWm-MW0% O NTs/C 30%0% S Grap 47% F % 3 2 5 2 25% MW 40% 20%

Figure 6. Comparison of the thermal conductivity of different polymer-based nanocomposites comprising of various thermal conductive fillers at roughly the same weight fraction. The data of 20% OMMT/PA6, 20% MWNTs/C foam/PS, 30% CF/PA6, 30% SWNTs/PS, 40% Graphite/PA66, and 47% Fe3O4/PA6 were obtained from the existed experiments.42-47



Page 28 of 29

Tables

Table 1. Micromechanical Properties of Samples. Elastic modulus [GPa]

Hardness [GPa]

Contact depth [nm]

Neat PA6

1.46±0.09

0.16±0.01

1796±9

p-MWNTs/PA6

9.88±0.61

0.85±0.05

940±5

m-MWNTs/ PA6

12.63±0.74

1.23±0.09

781±4

Table 2. Tensile Properties of Neat PA6, p-MWNTs/PA6 and m-MWNTs/PA6. Tensile strength [MPa]

Young’s modulus [MPa]

Elongation at break [%]

Neat PA6

54.6±3.8

2410.2±60.2

258.14±14.84

p-MWNTs/PA6

77.0±5.3

3675.6±125.3

4.56±0.24

m-MWNTs/ PA6

99.4±8.1

5278.8±112.9

2.96±0.17

Table 3. Thermal Conductivity of Different Nanocomposites with Various Types of Fillers. Matrix

Filler content [%wt]

λ of matrix [W/(m K)]

λ of nanocomposites [W/(m K)]

Enhancement [%]

Ref.

PA6

25 wt% p-MWNTs

0.196±0.015

1.186±0.046

505

--

PA6

25 wt% m-MWNTs

0.196±0.015

1.515±0.028

672

--

PA6

20 wt% OMMT

0.280±0.028

0.450±0.045

61

42

PS

20 wt% MWNTs/C

0.12

0.32

167

43

PA6

30 wt% CF

0.210±0.024

0.320±0.027

52

44

PS

30 wt% SWNTs

0.133±0.024

0.622±0.038

366

45

PA66

40 wt% Graphite

0.278±0.015

1.219±0.057

338

46

PA6

47 wt% Fe3O4

0.22

0.93

323

47


Page 29 of 29

40

Ne a

20 0

Modulus

A6

60

tP

80

closely-packed 3D networks

WN Ts/P A6 pM W N Ts /P A 6

100

m-M

Tensile Stress (MPa)

Table of Contents Entry

Elastic modulus (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

p-MW NTs/PA6: 19.61 S cm-1

5

Strain (%)

10


15

High Strength Conductive Polyamide 6 Nanocomposites Reinforced

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