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Polymer nanocomposites with improved mechanical and thermal properties by magnetically aligned carbon nanotubes
Polymer 166 (2019) 81–87
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Polymer nanocomposites with improved mechanical and thermal properties by magnetically aligned carbon nanotubes
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Mingrui Liua, Hammad Younesa,∗, Haiping Honga,∗∗, G.P. Petersonb a b
Department of Electrical Engineering, South Dakota School of Mines and Technology, Rapid City, SD, 57701, USA Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA
H I GH L IG H T S
oxide NPs were tethered successfully SWNTs in a water solution containing a NaDDBS surfactant. • Iron O NPs were aligned in a parallel configuration in the fluid due to the application of a magnetic field. • SWNTs-Fe the SWNTs-Fe O in a polymer matrix using magnetic field improved the mechanical properties of the composites. • Aligning • The alignment method may open a varying field for the applications both in research and industry. 2
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A R T I C LE I N FO
A B S T R A C T
Keywords: SWNTs Fe2O3 nanoparticles Alignment Polymer composites
A procedure to successfully tether iron oxide nanoparticles to Single Wall Carbon Nanotubes (SWNTs) in a water solution containing a sodium dodecylbenzenesulfonate surfactant was developed. Through the application of a magnetic field, the SWNTs with tethered Fe2O3 particles could then be stretched and aligned in a parallel configuration within the fluid. The experimental results indicated that aligning the magnetically sensitive nanoparticle (Fe2O3) attached SWNTs in a polymer matrix under a magnetic field, may substantially improve the physical and mechanical properties of the composites. In this investigation, the mechanical and thermal properties of SWNTs/epoxy composites were investigated to determine the potential impact of the SWNT alignment. Neat epoxy composites were produced to serve as a baseline and the results compared with composites loaded with magnetically sensitive SWNTs with weight percentages of 1 wt% and 2 wt%, both with and without magnetic field alignment. The results indicate that the alignment has a significant effect on the mechanical properties, with 1 wt% loaded composites under a magnetic field exhibiting a 9.8% enhancement in tensile strength compared to 1 wt% loaded composites in the absence of the magnetic field. Similarly, 2 wt% loaded composites under a magnetic field demonstrated a 9.7% enhancement in tensile strength compared to 2 wt% loaded composites without the magnetic field. The thermal conductivity of 0.1 wt %, 0.2 wt %, 0.3 wt%, 0.4 wt% and 0.5 wt% loaded composites under a magnetic field were also evaluated and indicated a 35% increase over the baseline values. As a result, this new alignment methodology may provide significant opportunities by which varying tensile strength and thermal conductivity can be used in both research and industrial applications.
1. Introduction The one-dimensional structure of Carbon Nanotubes (CNTs), coupled with their low density, high aspect ratio, and exceptional mechanical properties make them particularly attractive for use as a reinforcement mechanism in composite materials [1–4]. To date, hundreds of publications have reported on the various aspects of the mechanical enhancement of different types of polymer systems through
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the addition of CNTs. These studies have been discussed in a number of comprehensive reviews [5–7], which have included the impact of variations in a number of different parameters, such as the CNT type, growth method, chemical pre-treatment, as well as the polymer type and processing strategy. The results of these investigations have provided some encouraging results in the fabrication of stronger and more resilient CNT-polymer composites which are briefly summarized below. Zhao et al. [8] found that amino-functionalized SWNTs can be
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Younes), [email protected] (H. Hong).
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https://doi.org/10.1016/j.polymer.2019.01.031 Received 10 November 2018; Received in revised form 13 January 2019; Accepted 16 January 2019 Available online 21 January 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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magnetic field to the Fe2O3/SWNTs within the polymer, the individual SWNTs can be aligned, thereby optimizing the enhancement of the physically reinforced CNTs polymer composites and the thermal conductivity. Comparing the mechanical properties of the composite material with aligned and random SWNTs provides a mechanism by which the influence of the alignment can be determined and indicates the maximum potential enhancement of the mechanical properties of CNTs polymer composites. In this work, the first step in the process was to demonstrate that the Fe2O3 nanoparticles could be effectively attached to the SWNTs through electrostatic force and then to demonstrate that the Fe2O3/SWNT mixture can be aligned in water and epoxy solutions. Finally, the tensile strength and thermal and conductivity of the SWNTs/epoxy composites were measured both before and after applying the magnetic field to align the SWNTs. The degree of attachment, dispersion, and alignment of the SWNTs, and the mechanical properties of the SWNTs reinforced composites were characterized using optical microscopy, SEM, Raman and VSM analyses.
effectively used to improve the dispersion and adhesion in an epoxy matrix. The tensile strength of a specific neat epoxy composite was found to be 67.5 MPa and increased to 78.3 MPa with a weight percentage of as little as 0.5%. The results indicate that the functionalized CNTs lead to better dispersion and morphology in the polymer matrix at low weight percentages, but make it difficult to form the network structure at relatively higher weight percentages. Kim et al. [9] found that nanocomposites containing modified CNTs (e.g., acid, amine or plasma treated CNTs) exhibited greater mechanical properties than those with the untreated CNTs, in part because the surface treatments provided a more homogeneous dispersion of the CNTs and stronger interactions between the CNTs and the polymer matrix. A tensile strength of 1 wt% modified CNTs was shown to be approximately 47 MPa and exhibited an enhancement of approximately 11% when compared with that of the untreated CNTs. Gojny et al. [10] investigated the tensile strength of low wt%’s of SWNT/epoxy composites and found that the tensile strength of the 0.3 wt% SWNTs/epoxy composite which was 67.3 MPa increased by 2.2% when compared with a 0.05 wt% SWNTs/epoxy composite. Several reviews have recently summarized many of the publications that have reported certain aspects of the mechanical enhancement of different polymer systems by CNTs. A majority of the research on CNTs/epoxy composites were targeted at low content CNTs (not more than 0.5 wt% loading) in some sort of polymer matrix. In these types of composites, it is often difficult to obtain a well-dispersed CNTs composite, particularly at high CNTs loading percentages. This is due to a number of factors, the most important of which are the strong Van der Waals forces present between individual nanotubes, and the interfacial adhesion properties occurring between the CNTs and the polymer matrix, since a sufficient stress transfer from the matrix to the tubes is required to efficiently exploit the potential of CNTs to provide structural reinforcement. Ideally, the higher the content of the CNTs the higher the tensile strength of the composite, but this higher filler content can lead to ever increasing amounts of improperly impregnated agglomerates, which act as imperfections in the composite, thereby inducing premature failure. In addition, the random orientation of the CNTs can substantially degrade both the electrical/thermal conductivities and the mechanical properties. These factors currently limit the level to which these properties can be leveraged in any given composite and hence limit the efficient implementation of CNTs in many potential applications. The alignment of CNTs in polymer composites has been reported in the literature using several different methods such as mechanical stretching [11–13], direct and alternating current electric fields [14–16], vertically aligned growth [17–19] and magnetic field [20–22]. E. Camponeschi et al. aligned CNTs in epoxy polymer using a magnetic field strength of 25 T [23], while Garmestani et al. used a high magnetic field strength of 25 T to achieve the alignment of CNTs in an epoxy polymer [24]. Hone et al. reported results obtained for thick films of aligned single wall carbon nanotubes, produced using the filtration from suspension in strong magnetic fields on the order of 7 T [25,26]. In the method presented here, the SWNTs have been aligned under magnetic field 0.062 T, which is typically considered to be a very low magnetic field and can be generated by a pair of barium-ferrite magnet plates. As previously mentioned, the level of enhancement provided by polymer composites using carbon nanotubes as a filler is strongly dependent upon the degree of dispersion in the polymer matrix and the interfacial interactions between the individual CNTs and the matrix. It is also influenced by the alignment of the individual CNTs within the matrix. It is hypothesized here that the physical properties of the composites can be significantly enhanced by controlling this alignment. To determine the extent to which this is true, it is first necessary to identify a mechanism by which the SWNTs can be aligned. One method by which this can done is to first tether or attach Fe2O3 nanoparticles to the ends of SWNTs within the polymer composites. Then, by applying a
2. Experimental 2.1. Materials and chemicals Single wall carbon nanotubes (SWNTs) from Beijing Boda Green High Tech Company Limited were used in the current investigation. The SWNTs had over 90% purity with a residual amorphous carbon value of less than 5%, less than 2% ash/catalyst and an average diameter of less than 2 nm and a length of less than 20 nm. Sodium dodecylbenzene sulfonate (NaDDBS) with 80% purity based on total alkylbenzenesulfonate content and a molecular weight of 348.48 g/mol. obtained from Sigma-Aldrich were used. When dissolved in a water solvent, NaDDBS becomes an anionic surfactant dodecylbenzene sulfonate, with a cationic sodium counter ion. The hydrocarbon chain attached to the benzene ring acts as the hydrophobic tail, while the negatively charged sulfonate group and benzene ring act as the polar head. Fig. 1 A shows the reaction formula of NaDDBS in H2O. γ – Iron (III) oxide nanoparticles with a molecular weight of 159.69 g/mol. obtained from Sigma-Aldrich. The average diameter of the γ - Iron oxide nanopowder was less than 50 nm and was magnetically sensitive, which assisted the alignment of SWNTs. When applying an external magnetic field, the γ - Iron oxide nanoparticles align, thus forcing SWNTs and NaDDBS to adhere to it. The polymer used to produce the composites was EPON Resin 826 bisphenol A based epoxy resin, with an average molecular weight per epoxide of 178–186 g/eq and a viscosity of 65–95 Ps at 25 °C, obtained from EPON. The curing agent used for the polymer was modified aliphatic polyamine with a bulk density of 8.45 pounds/gallon and a viscosity of 200 cP at 25 °C obtained from West System Inc (West System 206 slow hardener).
Fig. 1. A. The sulfonic group carries negative charge interacts with any positive charge in the aqueous solution. B. Parallel barium-ferrite magnet plates. C. Mold design (a) Bottom Glass Plate, (b) Telfon Plate. 82
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2.2. Instruments and equipment Ultra-sonication was used to produce well-dispersed, magnetically sensitive SWNTs particles and composite solutions and was performed using a Branson Model 450 Digital Sonifier with a 1/2″ disrupter horn and a Branson Model 450 Digital Sonifier with a 1/8” tip. A magnetic field was produced using parallel equally-spaced Barium-ferrite magnetic plates. The CNTs/polymer composite samples were placed between two parallel magnets for the alignment of the composites. The magnetic intensity was adjusted by the distance between the two magnets and the intensity was recorded using a F.W. Bell Model 5060 Gaussmeter. Fig. 1 B illustrates the arrangement of the two magnetic plates. The tensile strength of the polymer nano-composites was measured using an 810 Material Test System. The tensile test system was equipped with a 100 kN load cell and screw section. The mold for making CNTs/polymer composites samples used in this investigation is shown in Fig. 1C. The bottom glass plate and the top dog-bone shaped Telfon mold were designed for easy demolding. The thermal conductivity data were obtained by a Hot DiskTM thermal constants analyzer using the following parameters: 6 mm measurement depth, room temperature, 0.012 W power, 10 s measurement time, 3.189 mm sensor radius, 0.0471/K TCR, kapton disk type, temperature drift record.
Fig. 2. A. The reaction scheme for the preparation of magnetically sensitive SWNTs. B. The experimental procedure for making magnetically sensitive SWNTs and C. The procedure for making SWNTs/epoxy composites. D. Alignment of the SWNTs/Fe2O3 by the effect of the external magnetic field.
positive charge as it is described in Fig. 1 A. The zeta potential study found that at pH equal to 6.15, Fe2O3 nanoparticles have a positive zeta potential charge [34], therefore, Fe2O3 nanoparticles interact with the negative charge on the NaDDBS sulfonic group and adhere to it through an electrostatic interaction. As it is known NaDDBS wraps the surface of the SWNTs and helps to obtain good dispersion of the SWNTs in fluids. The three materials, the SWNTs, NaDDBS and Fe2O3 nanoparticles will link to each other as one component. Therefore, through the application of an equal magnetic field from both sides the SWNTs with tethered Fe2O3 particles could be stretched and aligned in a parallel configuration within the fluid. Initially, 0.42 g of NaDDBS surfactant was dispersed in 100 mL of de-ionized water, using ultra-sonication for 15 min at 30 W until a clear solution was achieved. Then, 0.042 g SWNTs was added to the solution and sonicated for an additional 15 min at 30 W. A 15 min wait time was necessary to let the solution cool and then the sonication was continued for another 15 min at 30 W. A ratio was regarded optimum when the SWNTs remained dispersed and there was no aggregation after ultrasonication. The degree of dispersion of the SWNTs was analyzed using UV–vis spectroscopy and Transmission Electron Microscopy (TEM). Because only individual carbon nanotubes can be absorbed in the UV–vis region, bundled carbon nanotubes are not active in the UV–vis region. Therefore, the dispersion of carbon nanotubes can be characterized using UV–vis absorption spectroscopy. For the optimum dispersion of SWNTs with NaDDBS, the ratio of SWNTs to NaDDBS has been reported to be 1:10 [27,28]. Finally, 0.042 g of Fe2O3 nanoparticles were added to the mixture and sonicated for 30 min at 30 W with an interval after the first 15 min. For the filtration, the solution was placed into a three-piece filter funnel. The vacuum pump was turned on and a white bubble came out of the funnel. Another 200 mL deionized water was used to wash away the extra NaDDBS surfactant in the solution. After filtration, filter paper was placed into a clean dish and placed in a vacuum oven at 80 °C and a pressure of twenty inches of mercury for 12 h. A spatula was used to scratch the magnetically sensitive SWNTs off the filter paper. Large clumps were broken up using a glass rod and then milled into a small powder. The dry magnetically sensitive SWNT particles were collected into a vial and used to prepare the composites. Fig. 2 B shows the experimental procedure for making the magnetically sensitive SWNTs.
2.3. Material characterization In the current investigation, an optical microscope, a Scanning Electron Microscopy (SEM) and a Raman Microscope were all used to analyze the physical and chemical properties of the SWNTs. The optical microscope images were obtained using a Leica Z16 APO Zoom Microscope and were used to investigate the evidence of SWNTs in the epoxy and the alignment of the SWNTs. The structure of the magnetically sensitive SWNTs was observed using a Scanning Electron Microscopy (SEM) with a backscattered electron detector on a Zeiss Supra 40VP variable pressure system, and the chemical characteristics of SWNTs were determined using a Renishaw Raman microscope with a laser excitation of 633 nm. 2.4. Sample preparation and test methods In the current investigation, several types of nanofluids were utilized to illustrate the attachment and dispersion between the SWNTs and Fe2O3 nanoparticles, and the SWNT alignment. The procedure used to produce the SWNT suspended solution is as follows: 0.17 g NaDDBS was dissolved in the solution and sonicated for 30 min. The 0.017 g SWNTs were then added to 100 g of de-ionized water and treated in an ultrasonicator for 15 min at 30 W. The resulting mixture was then placed in a plastic dish and optical microscopic images were taken to determine the dispersion of the SWNTs. The procedure for the SWNT fluid preparation was as follows: 0.17 g NaDDBS surfactant was dissolved in 100 g de-ionized water using an ultrasonicator for 15 min at 30 Watts until a clear solution was achieved. Then 0.017 g SWNTs was added to the mixture and sonicated for another 15 min. A 5 min period was allotted to let the solution cool. Finally 0.017 gm of Fe2O3 nanoparticles were added to the mixture and sonicated for 30 min at 30 W after an interval of 15 min. 2.4.1. Preparation of magnetically sensitive single wall nanotubes Fig. 2 A illustrates the reaction schemes for the preparation of the magnetically sensitive SWNTs. The magnetically sensitive SWNTs (Fig. 2A) were prepared in water as explained previously (Fig. 2B) then dispersed in the epoxy polymer. As NaDDBS does not dissociate in epoxy, the preparation of the magnetic sensitive SWNTs in the water medium served as a pre-step to prepare the magnetic sensitive nanocomposites. NaDDBS dissociates in water into two parts; the sulfonic group that carries a negative charge and the sodium ion that carries a
2.4.2. Preparation of carbon nanotube composites Neat EPON Resin 826 bisphenol-A-based epoxy resin was used as the standard to which the tensile strength values of the composites would be compared. In the current investigation, polymer composites were produced with 1 and 2 wt% loading of magnetically sensitive 83
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SWNTs. The EPON Resin 826 bisphenol-A-based epoxy resin was incorporated into the polymer solutions using an identical methodology. First, the desired amount of magnetically sensitive SWNTs was dispersed in the EPON Resin 826 bisphenol A based epoxy resin by ultrasonication; for (0.15 g magnetically sensitive single wall nanotubes) 1 wt% was dispersed in 15 g epoxy resin, and (0.30 g magnetically sensitive SWNTs) 2 wt% was then dispersed in 15 g of epoxy resin. The magnetically sensitive SWNTs/epoxy resin solution was sonicated for 1 h at 35 W for three time intervals in order to create a well-dispersed solution. It was important to ensure that the magnetically sensitive SWNTs were well-dispersed in the epoxy resin, as any large agglomerations of magnetically sensitive SWNTs would result in poor quality composites. The solution was then placed into a vacuum oven at room temperature under a pressure of twenty inches of mercury for 12 h in order to remove any air bubbles in the solution caused by the ultrasonication process. The magnetically sensitive SWNTs/epoxy resin was then mixed manually with the curing agent, West System 206 slow hardener, at a ratio of 2.63:1. One composite sample required about 4 g of magnetically sensitive SWNTs/Epoxy resin. The polymer solution was then poured into dog-bone shaped Polytetrafluoroethylene (PTFE)/ Telfon mold and smear cast using a wire wound rod. The polymer composites were allowed to cure for up to 24 h at room temperature (about 25 °C)depending on the temperature range and then removed for analysis. The polymer composites had a thickness of approximately 3 mm. Fig. 2C illustrates the experimental procedure for producing the SWNTs/epoxy composites. Fig. 2 D illustrates how the resulting SWNTs/Fe2O3 strings are aligned by the external magnetic field.
Fig. 3. SEM micrograph of A and B for pristine SWNTs at different magnifications. E. SEM micrograph for magnetically sensitive SWNTs. and D. TEM micrograph of pristine SWNTs.
sensitive material. As previously discussed, the SWNTs, NaDDBS and Fe2O3 nanoparticles were attached using the electrostatic interaction. Therefore, by evaporating all of the water from the mixture, the magnetically sensitive carbon nanotubes could be obtained and then dispersed in other solvents as required. Fig. 3 A & B illustrate the SEM images for pristine single wall carbon nanotubes. As shown in the figure, the majority of the nanotubes are tangled with each other due to the strong Van der Waals forces that exists between the different strands. Fig. 3C illustrates an SEM image for the magnetically sensitive SWNTs. The magnetically sensitive Fe2O3 nanoparticles were attached (or tethered) to the ends of the tubes through the surfactants and the bright particles (identified by the red arrows) show the iron oxide on the surface of the carbon nanotubes. The SEM images demonstrated that the Fe2O3 nanoparticles attached well on SWNTs. Fig. 3 D illustrates a TEM image for the SWNTs. A Raman Microscope was used to further investigate the effect of coating the SWNTs with the Fe2O3. Fig. 4 A. shows the Raman microscope spectrum of the pristine SWNTs and magnetically sensitive SWNTs. The main Raman features of CNTs are the disordered induced D peak, the tangential G band, and the G' band (disorder overtone of D band). The D-band for the pristine SWN is near 1328 cm−1, and provides information about the defects within the carbon nanotubes. This peak shows the differences between a perfect carbon nanotube (low Intensity peak) and an imperfect carbon nanotube (high intensity peak). The D band of the used SWNTs was found to have a low intensity, which indicates that the SWNTs used in this work had fewer defects [29,30]. The G-band for the pristine SWNTs at 1580 cm−1, sometimes referred to as the graphitic G-band, is related to the arrangement of the hexagonal lattice of carbon atoms in graphite layers of pristine SWNTs [4,31]. The Raman spectrum for pristine SWNTs indicates that the Gband has a high intensity when compared to the D-band with an ID/IG ratio of 0.33. The low value of the ID/IG ratio indicates the high purity of the SWNTs. For the magnetically sensitive SWNTs the D-band was found to occur at 1350 cm−1 and the peak value of the G-band occurred at
2.4.3. Polymer composites tensile sample preparation and test method After a curing process of at least 24 h at room temperature (about 20 °C) the polymer composites were removed from the dog-bone shape telfon molds. The dog-bone polymer composite samples were polished using a Metaseru 250 grinder polisher purchased from Buehler in order to obtain polymer composite samples of uniform thickness for tensile testing. The uniform dimensions of the dog-bone polymer composite samples (shown in Fig. 1C) was 44.20 mm in length and 6.20 mm in thickness. The adjusted width was determined using ASTM Designation D 3039 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. The tensile strength testing method was the default tensile test method provided by the MTS Test Works 4 program used to control the MTS electromechanical test system. The tensile testing instrument used was 810 Material Test System (100 KN MTS Tester). The rate of grip separation was 2 mm/min. Tensile strength values were calculated using the Test Works program. The number of test specimens was determined using ASTM D-3039, which states that at least five isotropic material specimens should be tested, and for anisotropic materials, at least five normal samples and five samples parallel to the principal axis of anisotropy should be tested. For this investigation, normal and aligned samples were defined according to the direction of the alignment when casting the polymer composites. Using the ASTM Designation D-3039 ASTM Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials was used as a guide for tensile testing the polymer composites, the standard was only a guide and not all specifications contained in the standard were followed precisely. 3. Results and discussion 3.1. Characterization SEM micrographs were taken in order to better understand the different mechanical properties exhibited by the composite samples produced with magnetically sensitive SWNTs both with and without the presence of the magnetic field. The magnetically sensitive SWNTs were prepared by first dispersing the carbon nanotubes in water using NaDDBS as a surfactant and then adding Fe2O3 as the magnetically
Fig. 4. A. Raman spectra of SWNTs and magnetically sensitive single wall nanotubes. B. Magnetization (M) versus applied magnetic field (H). 84
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1582 cm−1. The magnetically sensitive SWNTs ID/IG ratio was 0.4. The higher value of the ID/IG ratio reveals that the ultrasonication treatment process may have introduced structural defects into the structure of the SWNTs. The ID/IG value of the pristine SWNTs is small, which indicates that the pristine SWNTs do not have any defects. Following the sonication process of approximately 30 min performed in order to prepare the magnetically sensitive SWNTs, the magnetically sensitive SWNTs ID/IG ratio had increased to only 0.4, this implies that the preparation method using the tip sonicator did not alter the SWNTs structure or introduce any defects. During the preparation of the magnetically sensitive SWNTs, it was noted that if the ultra-sonication time was increased to 60 min, the ID/IG ratio increased to 0.9. Therefore, the time for ultra-sonication should be carefully considered, otherwise it may lead to damage or breaks in the SWNTs, which will result in a shorter average length, which in turn will impact the thermophysical properities being observed. Vibrating sample magnetization (VSM) measurements were taken to illustrate the magnetic properties of the magnetically sensitive SWNTs. The magnetic property represents the inherent impact of the magnetic field through assessing the possibility of the particles being magnetized and/or attracted to a magnet. This property may then also impact the tensile strength of the magnetically sensitive SWNTs/epoxy resin, both with and without the presence of a magnetic field. Fig. 4 B illustrates the magnetization (M) as a function of the applied magnetic field, or (H) plots for the pristine SWNTs and magnetically sensitive SWNTs. The measurements were made at a maximum field of 15 kOe and at minimum field of −15 kOe at room temperature in steps of 500 Oe. The pristine SWNTs had a maximum magnetization value of 1 emu/g and the Fe2O3 nanoparticles had a maximum magnetization value of 25 emu/g. Magnetically sensitive SWNTs have a maximum magnetization value at 5 emu/g. The results indicate that the magnetically sensitive SWNTs have slight magnetic properties, which can, in fact, be used to align the SWNTs in a polymer matrix under a magnetic field.
Fig. 6. Comparison of the five types of epoxy polymer composites.
the polymer matrix using different methods. The tensile strength was measured for several different types of polymer composites and then an average tensile strength value was determined. 3.2.2.1. Tensile Strength of Neat Epoxy Resin Composites. The tensile strength of the neat epoxy resin composites was measured to provide a standard reference for comparison with the magnetically sensitive SWNTs/epoxy resin polymer composites. Fig. 6 shows the measured tensile strength of the neat Epoxy resin composites. The experimental results obtained in the current investigation exhibit good correlation with the values obtained by Coleman et al. [1] which ranged from 30 to 60 MPa for the same material, validating the experimental procedures used in the current investigation. 3.2.2.2. Tensile Strength of Composites Containing Magnetically Sensitive Single Wall Nanotubes. Ultra-sonication was used to aid in the dispersion of the magnetically sensitive SWNTs in the polymer matrix and the direction of the tensile strength test was along the direction of the magnetic force, and hence along the length of the carbon nanotubes. Fig. 6 illustrates the measured tensile strength of the magnetically sensitive SWNTs/epoxy resin polymer composites with 1 and 2 wt% loading without the application of a magnetic field, and 1 and 2 wt% loading with the magnetic field. As illustrated, the polymer composite with 1 wt% loading with the magnetic field demonstrated the highest tensile strength of the four magnetically sensitive single wall nanotube samples tested. The results indicated a 9% enhancement for 1 wt% loading without alignment when compared to the neat epoxy resin composite, and a 19.7% enhancement for 1 wt% loading for the aligned composite when both were compared to the neat epoxy resin composite. As illustrated, the results indicated a significant enhancement for 1 wt% loaded polymer composites in the presence of the magnetic field [32–34]. A decrease in tensile strength for the 2 wt% loaded, magnetically sensitive SWNTs was observed when compared with the 1 wt% loaded magnetically sensitive SWNTs. The decrease may be attributed to poor dispersion of the carbon nanotubes. Previous studies have indicated that when the loading of single wall nanotubes reaches 1.5 wt% in a polymer matrix [35], there is typically a sharp enhancement in the viscosity of the SWNT polymer. If the carbon nanotubes could be dispersed more completely in the polymer matrix, higher loading of carbon nanotubes could increase the tensile strength of polymer composites. If ultra-sonication was used as the only way to disperse carbon nanotubes, much more aggressive approaches could be used to reach a complete dispersion such as using a higher intensity and longer time duration for ultrasonication. However, these aggressive approaches may damage the structure of the carbon nanotubes and magnetically sensitive SWNTs, which may in turn hinder the ability to reinforce the polymer composites [9,35]. The tensile strength data of the epoxy polymer composites indicates
3.2. Carbon nano materials alignment characterization 3.2.1. SWNTs alignment in epoxy solution The use of a magnetic field to align carbon nanotubes was successful in the polymer matrixes as well. Fig. 5 A illustrates the macro-scaled digital image of the 0.1 wt% magnetically sensitive SWNTs in an epoxy matrix and Fig. 5 B illustrates the optical microscope image. As illustrated, the black particles form long lines along the direction of the magnetic field and as illustrated in Fig. 5 A, B with the aid of magnetic guidance, the SWNTs could be aligned within the epoxy matrix. 3.2.2. Mechanical properties of polymer composites The mechanical strength of the composite films prepared in this investigation was evaluated through tensile strength testing. The tensile test results were used to determine the extent of reinforcement enhancement provided by the magnetically sensitive SWNTs dispersed in
Fig. 5. SWNTs alignment in epoxy solution. (A. taken by digital camera; B taken by optical microscope, internal reference: 200 μm). 85
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that aligning the individual carbon nanotubes using a magnetic field can increase the tensile strength of epoxy polymer composites. Using this approach, the magnetically sensitive SWNTs were aligned within an epoxy resulting in an enhancement in the tensile strength in the SWNTs/epoxy composites. The dispersion of the carbon nanotubes plays an important role in the physical characteristics of the epoxy polymer composites, and the higher loading of the carbon nano material would ideally result in higher mechanical properties of the composites. Several authors [36,37] have reported that increased loading of CNTs in a composite results in lower strength characteristics suggesting an increase in voids or other defects as the loading of the CNTs increases. This is likely due to the difficulty of homogeneously dispersing concentrated CNT/polymer solutions. The method presented here results in composites in which the CNTs are dispersed homogeneously within the thermosetting polymers, without resulting in damage to the CNTs. Some investigations have attempted to align CNTs in polymer composites using extrusion methods in which a shear force was introduced to align CNTs by passing the composite material through a micro channel and shearing the CNT suspension [38,39]. The tensile strength of the 0.3 wt% SWNTs/epoxy composite for composites prepared in this manner increased by 5.5% when compared with neat epoxy samples. Thostenson et al. [40] found that 1 wt% SWNTs/epoxy lead to significant enhancement in fracture toughness and thermal conductivity compared with the neat epoxy.
difficult and therefore, proper alignment is nearly impossible during the epoxy curing process. (ii) at 0.3 wt% of the magnetically sensitive SWNTs, the nanocomposites have reached the percolation threshold, as 0.3 wt% of the SWNTs is sufficient to align and results in a sufficient network structures to enhance the heat transfer. Any additional amount of SWNTs will have minimal effect. Based on the obtained results, a 0.5 wt % was assumed to be the upper limit of enhancement and further tests at higher SWNTs wt% were assumed to not have any impact on the thermal conductivity. 4. Conclusions The results of the current investigation indicate that Fe2O3 nanoparticles can be attached to SWNTs using an assisting surfactant NaDDBS in a water solution. The results confirm the hypothesis that SWNTs will attach onto Fe2O3 nanoparticles due to the electrostatic attractive forces and that the attachment of the Fe2O3 nanoparticles to the SWNTs can significantly enhance the alignment due to the magnetic sensitivity of the Fe2O3 nanoparticles. This magnetic guidance was shown to be capable of aligning the SWNTs. Images of the movement of the SWNTs, Fe2O3, and NaDDBS in water under a magnetic field were created using optical microscopy. Upon application of the magnetic field, the mixture gradually vibrated, stretched out and the nanotubes ultiimately aligned along the magnetic field. This indicates that SWNTs were well attached to the Fe2O3 nanoparticles, and that these mixtures not only aligned, but also formed chains and clusters. Because of the viscosity resistance of the fluid itself, it takes some time for the mixture to become well aligned. Hence, good dispersion of SWNTs and attachment between SWNTs and Fe2O3 is a prerequisite of alignment. The use of magnetic guidance for aligning SWNTs in a fluid may also be applicable in reinforced polymer composites. The current investigation demonstrates that using specially prepared composites and a magnetic field can improve the mechanical properties and the thermal conductivity of polymer composites. In this investigation, 9.8% enhancement in tensile strength was observed for 1 wt% magnetically sensitive SWNTs/epoxy composites, in the presence of a magnetic field, when compared to similar samples without the magnetic field. For 2 wt % loaded composites, a 19.7% increase was observed when the magnetic field was applied as compared with those in which no magnetic field was present. However, when the loading was increased to 2 wt%, the tensile strength of both the aligned composites and unaligned composites decreased. Further, a 2 wt% unaligned sample demonstrated a 3.9% decrease in tensile strength when compared with 1 wt% composites, while the 2 wt% aligned sample demonstrated a 4.0% decrease in tensile strength compared with the 1 wt% aligned composite. These decreases were expected due to the change in the viscosity of the epoxy solution and the high Van der Waals forces between SWNTs. The thermal conductivity of 0.1 wt %, 0.2 wt %, 0.3 wt %, 0.4 wt% and 0.5 wt% loaded composites both with and without the application of a magnetic field and the results indicated that the thermal conductivity increased by 35%. Above a 3.0 wt% value, no increase in the thermal conductivity was observed, which again is attributed to the percolation threshold and an increase in the viscosity of the epoxy solution and the high Van der Waals forces between the SWNTs, which makes the dispersion process as well as the alignment process very difficult. Overall, the results indicated that the use of a magnetic field to align SWNTs in polymers shows great potential in improving the mechanical properties of polymer composites. As the quality of processing techniques increases, the potential of this method can be determined and utilized for a wide variety of commercial applications.
3.2.3. Thermal conductivity properties of polymer composites The thermal conductivity of the magnetically sensitive SWNTs/ epoxy resin polymer composites with 0.1, 0.2, 0.3, 0.4 and 0.5 wt% loading with and without the application of a magnetic field have been tested. As shown in Fig. 7, the thermal conductivity of the neat epoxy is 0.17 W/mK, applying an external magnetic field doesn't have any effect on the thermal conductivity. Using 0.1 wt% of the magnetically sensitive SWNTs has increased the thermal conductivity to 0.23 W/mK, by applying an external magnetic field that the thermal conductivity increased further, as the alignment of the magnetically sensitive SWNTs within the composites acts as a connected wire that helps to transfer heat as illustrated in Fig. 2 D. At 0.2 wt% the effect of adding extra wt% of the magnetically sensitive SWNTs on the thermal conductivity becomes nearly negligible, which can be attributed to the difficulty in getting a homogenous dispersion of the magnetically sensitive SWNTs in the epoxy. However, applying magnetic field has increased the thermal conductivity significantly. The alignment has a significant effect on the debundling the SWNTs. Beyond a 0.3 wt% of the magnetically sensitive SWNTs, the incremental increase in the thermal conductivity of the nanocomposites in the presence of the magnetic field stabilizes, which can be attributed to the following (i) at 0.3 wt% of the magnetically sensitive SWNTs, the process of obtaining a homogenous dispersion of the magnetically sensitive SWNTs in the epoxy is quite
Acknowledgement The financial support of Army Research Laboratory _Cooperative Agreement No. W911NF-08-2-0022 and NASA EPSCoR _Award No.
Fig. 7. Thermal conductivity for different wt% of SWNs nanocomposites. 86
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NNX09AU83A along with IR&D support from the Georgia Institute of Technology are acknowledged.
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Polymer nanocomposites with improved mechanical and thermal properties by magnetically aligned carbon nanotubes
T
Mingrui Liua, Hammad Younesa,∗, Haiping Honga,∗∗, G.P. Petersonb a b
Department of Electrical Engineering, South Dakota School of Mines and Technology, Rapid City, SD, 57701, USA Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA
H I GH L IG H T S
oxide NPs were tethered successfully SWNTs in a water solution containing a NaDDBS surfactant. • Iron O NPs were aligned in a parallel configuration in the fluid due to the application of a magnetic field. • SWNTs-Fe the SWNTs-Fe O in a polymer matrix using magnetic field improved the mechanical properties of the composites. • Aligning • The alignment method may open a varying field for the applications both in research and industry. 2
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A R T I C LE I N FO
A B S T R A C T
Keywords: SWNTs Fe2O3 nanoparticles Alignment Polymer composites
A procedure to successfully tether iron oxide nanoparticles to Single Wall Carbon Nanotubes (SWNTs) in a water solution containing a sodium dodecylbenzenesulfonate surfactant was developed. Through the application of a magnetic field, the SWNTs with tethered Fe2O3 particles could then be stretched and aligned in a parallel configuration within the fluid. The experimental results indicated that aligning the magnetically sensitive nanoparticle (Fe2O3) attached SWNTs in a polymer matrix under a magnetic field, may substantially improve the physical and mechanical properties of the composites. In this investigation, the mechanical and thermal properties of SWNTs/epoxy composites were investigated to determine the potential impact of the SWNT alignment. Neat epoxy composites were produced to serve as a baseline and the results compared with composites loaded with magnetically sensitive SWNTs with weight percentages of 1 wt% and 2 wt%, both with and without magnetic field alignment. The results indicate that the alignment has a significant effect on the mechanical properties, with 1 wt% loaded composites under a magnetic field exhibiting a 9.8% enhancement in tensile strength compared to 1 wt% loaded composites in the absence of the magnetic field. Similarly, 2 wt% loaded composites under a magnetic field demonstrated a 9.7% enhancement in tensile strength compared to 2 wt% loaded composites without the magnetic field. The thermal conductivity of 0.1 wt %, 0.2 wt %, 0.3 wt%, 0.4 wt% and 0.5 wt% loaded composites under a magnetic field were also evaluated and indicated a 35% increase over the baseline values. As a result, this new alignment methodology may provide significant opportunities by which varying tensile strength and thermal conductivity can be used in both research and industrial applications.
1. Introduction The one-dimensional structure of Carbon Nanotubes (CNTs), coupled with their low density, high aspect ratio, and exceptional mechanical properties make them particularly attractive for use as a reinforcement mechanism in composite materials [1–4]. To date, hundreds of publications have reported on the various aspects of the mechanical enhancement of different types of polymer systems through
∗
the addition of CNTs. These studies have been discussed in a number of comprehensive reviews [5–7], which have included the impact of variations in a number of different parameters, such as the CNT type, growth method, chemical pre-treatment, as well as the polymer type and processing strategy. The results of these investigations have provided some encouraging results in the fabrication of stronger and more resilient CNT-polymer composites which are briefly summarized below. Zhao et al. [8] found that amino-functionalized SWNTs can be
Corresponding author. Corresponding author. E-mail addresses: [email protected] (H. Younes), [email protected] (H. Hong).
∗∗
https://doi.org/10.1016/j.polymer.2019.01.031 Received 10 November 2018; Received in revised form 13 January 2019; Accepted 16 January 2019 Available online 21 January 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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magnetic field to the Fe2O3/SWNTs within the polymer, the individual SWNTs can be aligned, thereby optimizing the enhancement of the physically reinforced CNTs polymer composites and the thermal conductivity. Comparing the mechanical properties of the composite material with aligned and random SWNTs provides a mechanism by which the influence of the alignment can be determined and indicates the maximum potential enhancement of the mechanical properties of CNTs polymer composites. In this work, the first step in the process was to demonstrate that the Fe2O3 nanoparticles could be effectively attached to the SWNTs through electrostatic force and then to demonstrate that the Fe2O3/SWNT mixture can be aligned in water and epoxy solutions. Finally, the tensile strength and thermal and conductivity of the SWNTs/epoxy composites were measured both before and after applying the magnetic field to align the SWNTs. The degree of attachment, dispersion, and alignment of the SWNTs, and the mechanical properties of the SWNTs reinforced composites were characterized using optical microscopy, SEM, Raman and VSM analyses.
effectively used to improve the dispersion and adhesion in an epoxy matrix. The tensile strength of a specific neat epoxy composite was found to be 67.5 MPa and increased to 78.3 MPa with a weight percentage of as little as 0.5%. The results indicate that the functionalized CNTs lead to better dispersion and morphology in the polymer matrix at low weight percentages, but make it difficult to form the network structure at relatively higher weight percentages. Kim et al. [9] found that nanocomposites containing modified CNTs (e.g., acid, amine or plasma treated CNTs) exhibited greater mechanical properties than those with the untreated CNTs, in part because the surface treatments provided a more homogeneous dispersion of the CNTs and stronger interactions between the CNTs and the polymer matrix. A tensile strength of 1 wt% modified CNTs was shown to be approximately 47 MPa and exhibited an enhancement of approximately 11% when compared with that of the untreated CNTs. Gojny et al. [10] investigated the tensile strength of low wt%’s of SWNT/epoxy composites and found that the tensile strength of the 0.3 wt% SWNTs/epoxy composite which was 67.3 MPa increased by 2.2% when compared with a 0.05 wt% SWNTs/epoxy composite. Several reviews have recently summarized many of the publications that have reported certain aspects of the mechanical enhancement of different polymer systems by CNTs. A majority of the research on CNTs/epoxy composites were targeted at low content CNTs (not more than 0.5 wt% loading) in some sort of polymer matrix. In these types of composites, it is often difficult to obtain a well-dispersed CNTs composite, particularly at high CNTs loading percentages. This is due to a number of factors, the most important of which are the strong Van der Waals forces present between individual nanotubes, and the interfacial adhesion properties occurring between the CNTs and the polymer matrix, since a sufficient stress transfer from the matrix to the tubes is required to efficiently exploit the potential of CNTs to provide structural reinforcement. Ideally, the higher the content of the CNTs the higher the tensile strength of the composite, but this higher filler content can lead to ever increasing amounts of improperly impregnated agglomerates, which act as imperfections in the composite, thereby inducing premature failure. In addition, the random orientation of the CNTs can substantially degrade both the electrical/thermal conductivities and the mechanical properties. These factors currently limit the level to which these properties can be leveraged in any given composite and hence limit the efficient implementation of CNTs in many potential applications. The alignment of CNTs in polymer composites has been reported in the literature using several different methods such as mechanical stretching [11–13], direct and alternating current electric fields [14–16], vertically aligned growth [17–19] and magnetic field [20–22]. E. Camponeschi et al. aligned CNTs in epoxy polymer using a magnetic field strength of 25 T [23], while Garmestani et al. used a high magnetic field strength of 25 T to achieve the alignment of CNTs in an epoxy polymer [24]. Hone et al. reported results obtained for thick films of aligned single wall carbon nanotubes, produced using the filtration from suspension in strong magnetic fields on the order of 7 T [25,26]. In the method presented here, the SWNTs have been aligned under magnetic field 0.062 T, which is typically considered to be a very low magnetic field and can be generated by a pair of barium-ferrite magnet plates. As previously mentioned, the level of enhancement provided by polymer composites using carbon nanotubes as a filler is strongly dependent upon the degree of dispersion in the polymer matrix and the interfacial interactions between the individual CNTs and the matrix. It is also influenced by the alignment of the individual CNTs within the matrix. It is hypothesized here that the physical properties of the composites can be significantly enhanced by controlling this alignment. To determine the extent to which this is true, it is first necessary to identify a mechanism by which the SWNTs can be aligned. One method by which this can done is to first tether or attach Fe2O3 nanoparticles to the ends of SWNTs within the polymer composites. Then, by applying a
2. Experimental 2.1. Materials and chemicals Single wall carbon nanotubes (SWNTs) from Beijing Boda Green High Tech Company Limited were used in the current investigation. The SWNTs had over 90% purity with a residual amorphous carbon value of less than 5%, less than 2% ash/catalyst and an average diameter of less than 2 nm and a length of less than 20 nm. Sodium dodecylbenzene sulfonate (NaDDBS) with 80% purity based on total alkylbenzenesulfonate content and a molecular weight of 348.48 g/mol. obtained from Sigma-Aldrich were used. When dissolved in a water solvent, NaDDBS becomes an anionic surfactant dodecylbenzene sulfonate, with a cationic sodium counter ion. The hydrocarbon chain attached to the benzene ring acts as the hydrophobic tail, while the negatively charged sulfonate group and benzene ring act as the polar head. Fig. 1 A shows the reaction formula of NaDDBS in H2O. γ – Iron (III) oxide nanoparticles with a molecular weight of 159.69 g/mol. obtained from Sigma-Aldrich. The average diameter of the γ - Iron oxide nanopowder was less than 50 nm and was magnetically sensitive, which assisted the alignment of SWNTs. When applying an external magnetic field, the γ - Iron oxide nanoparticles align, thus forcing SWNTs and NaDDBS to adhere to it. The polymer used to produce the composites was EPON Resin 826 bisphenol A based epoxy resin, with an average molecular weight per epoxide of 178–186 g/eq and a viscosity of 65–95 Ps at 25 °C, obtained from EPON. The curing agent used for the polymer was modified aliphatic polyamine with a bulk density of 8.45 pounds/gallon and a viscosity of 200 cP at 25 °C obtained from West System Inc (West System 206 slow hardener).
Fig. 1. A. The sulfonic group carries negative charge interacts with any positive charge in the aqueous solution. B. Parallel barium-ferrite magnet plates. C. Mold design (a) Bottom Glass Plate, (b) Telfon Plate. 82
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2.2. Instruments and equipment Ultra-sonication was used to produce well-dispersed, magnetically sensitive SWNTs particles and composite solutions and was performed using a Branson Model 450 Digital Sonifier with a 1/2″ disrupter horn and a Branson Model 450 Digital Sonifier with a 1/8” tip. A magnetic field was produced using parallel equally-spaced Barium-ferrite magnetic plates. The CNTs/polymer composite samples were placed between two parallel magnets for the alignment of the composites. The magnetic intensity was adjusted by the distance between the two magnets and the intensity was recorded using a F.W. Bell Model 5060 Gaussmeter. Fig. 1 B illustrates the arrangement of the two magnetic plates. The tensile strength of the polymer nano-composites was measured using an 810 Material Test System. The tensile test system was equipped with a 100 kN load cell and screw section. The mold for making CNTs/polymer composites samples used in this investigation is shown in Fig. 1C. The bottom glass plate and the top dog-bone shaped Telfon mold were designed for easy demolding. The thermal conductivity data were obtained by a Hot DiskTM thermal constants analyzer using the following parameters: 6 mm measurement depth, room temperature, 0.012 W power, 10 s measurement time, 3.189 mm sensor radius, 0.0471/K TCR, kapton disk type, temperature drift record.
Fig. 2. A. The reaction scheme for the preparation of magnetically sensitive SWNTs. B. The experimental procedure for making magnetically sensitive SWNTs and C. The procedure for making SWNTs/epoxy composites. D. Alignment of the SWNTs/Fe2O3 by the effect of the external magnetic field.
positive charge as it is described in Fig. 1 A. The zeta potential study found that at pH equal to 6.15, Fe2O3 nanoparticles have a positive zeta potential charge [34], therefore, Fe2O3 nanoparticles interact with the negative charge on the NaDDBS sulfonic group and adhere to it through an electrostatic interaction. As it is known NaDDBS wraps the surface of the SWNTs and helps to obtain good dispersion of the SWNTs in fluids. The three materials, the SWNTs, NaDDBS and Fe2O3 nanoparticles will link to each other as one component. Therefore, through the application of an equal magnetic field from both sides the SWNTs with tethered Fe2O3 particles could be stretched and aligned in a parallel configuration within the fluid. Initially, 0.42 g of NaDDBS surfactant was dispersed in 100 mL of de-ionized water, using ultra-sonication for 15 min at 30 W until a clear solution was achieved. Then, 0.042 g SWNTs was added to the solution and sonicated for an additional 15 min at 30 W. A 15 min wait time was necessary to let the solution cool and then the sonication was continued for another 15 min at 30 W. A ratio was regarded optimum when the SWNTs remained dispersed and there was no aggregation after ultrasonication. The degree of dispersion of the SWNTs was analyzed using UV–vis spectroscopy and Transmission Electron Microscopy (TEM). Because only individual carbon nanotubes can be absorbed in the UV–vis region, bundled carbon nanotubes are not active in the UV–vis region. Therefore, the dispersion of carbon nanotubes can be characterized using UV–vis absorption spectroscopy. For the optimum dispersion of SWNTs with NaDDBS, the ratio of SWNTs to NaDDBS has been reported to be 1:10 [27,28]. Finally, 0.042 g of Fe2O3 nanoparticles were added to the mixture and sonicated for 30 min at 30 W with an interval after the first 15 min. For the filtration, the solution was placed into a three-piece filter funnel. The vacuum pump was turned on and a white bubble came out of the funnel. Another 200 mL deionized water was used to wash away the extra NaDDBS surfactant in the solution. After filtration, filter paper was placed into a clean dish and placed in a vacuum oven at 80 °C and a pressure of twenty inches of mercury for 12 h. A spatula was used to scratch the magnetically sensitive SWNTs off the filter paper. Large clumps were broken up using a glass rod and then milled into a small powder. The dry magnetically sensitive SWNT particles were collected into a vial and used to prepare the composites. Fig. 2 B shows the experimental procedure for making the magnetically sensitive SWNTs.
2.3. Material characterization In the current investigation, an optical microscope, a Scanning Electron Microscopy (SEM) and a Raman Microscope were all used to analyze the physical and chemical properties of the SWNTs. The optical microscope images were obtained using a Leica Z16 APO Zoom Microscope and were used to investigate the evidence of SWNTs in the epoxy and the alignment of the SWNTs. The structure of the magnetically sensitive SWNTs was observed using a Scanning Electron Microscopy (SEM) with a backscattered electron detector on a Zeiss Supra 40VP variable pressure system, and the chemical characteristics of SWNTs were determined using a Renishaw Raman microscope with a laser excitation of 633 nm. 2.4. Sample preparation and test methods In the current investigation, several types of nanofluids were utilized to illustrate the attachment and dispersion between the SWNTs and Fe2O3 nanoparticles, and the SWNT alignment. The procedure used to produce the SWNT suspended solution is as follows: 0.17 g NaDDBS was dissolved in the solution and sonicated for 30 min. The 0.017 g SWNTs were then added to 100 g of de-ionized water and treated in an ultrasonicator for 15 min at 30 W. The resulting mixture was then placed in a plastic dish and optical microscopic images were taken to determine the dispersion of the SWNTs. The procedure for the SWNT fluid preparation was as follows: 0.17 g NaDDBS surfactant was dissolved in 100 g de-ionized water using an ultrasonicator for 15 min at 30 Watts until a clear solution was achieved. Then 0.017 g SWNTs was added to the mixture and sonicated for another 15 min. A 5 min period was allotted to let the solution cool. Finally 0.017 gm of Fe2O3 nanoparticles were added to the mixture and sonicated for 30 min at 30 W after an interval of 15 min. 2.4.1. Preparation of magnetically sensitive single wall nanotubes Fig. 2 A illustrates the reaction schemes for the preparation of the magnetically sensitive SWNTs. The magnetically sensitive SWNTs (Fig. 2A) were prepared in water as explained previously (Fig. 2B) then dispersed in the epoxy polymer. As NaDDBS does not dissociate in epoxy, the preparation of the magnetic sensitive SWNTs in the water medium served as a pre-step to prepare the magnetic sensitive nanocomposites. NaDDBS dissociates in water into two parts; the sulfonic group that carries a negative charge and the sodium ion that carries a
2.4.2. Preparation of carbon nanotube composites Neat EPON Resin 826 bisphenol-A-based epoxy resin was used as the standard to which the tensile strength values of the composites would be compared. In the current investigation, polymer composites were produced with 1 and 2 wt% loading of magnetically sensitive 83
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SWNTs. The EPON Resin 826 bisphenol-A-based epoxy resin was incorporated into the polymer solutions using an identical methodology. First, the desired amount of magnetically sensitive SWNTs was dispersed in the EPON Resin 826 bisphenol A based epoxy resin by ultrasonication; for (0.15 g magnetically sensitive single wall nanotubes) 1 wt% was dispersed in 15 g epoxy resin, and (0.30 g magnetically sensitive SWNTs) 2 wt% was then dispersed in 15 g of epoxy resin. The magnetically sensitive SWNTs/epoxy resin solution was sonicated for 1 h at 35 W for three time intervals in order to create a well-dispersed solution. It was important to ensure that the magnetically sensitive SWNTs were well-dispersed in the epoxy resin, as any large agglomerations of magnetically sensitive SWNTs would result in poor quality composites. The solution was then placed into a vacuum oven at room temperature under a pressure of twenty inches of mercury for 12 h in order to remove any air bubbles in the solution caused by the ultrasonication process. The magnetically sensitive SWNTs/epoxy resin was then mixed manually with the curing agent, West System 206 slow hardener, at a ratio of 2.63:1. One composite sample required about 4 g of magnetically sensitive SWNTs/Epoxy resin. The polymer solution was then poured into dog-bone shaped Polytetrafluoroethylene (PTFE)/ Telfon mold and smear cast using a wire wound rod. The polymer composites were allowed to cure for up to 24 h at room temperature (about 25 °C)depending on the temperature range and then removed for analysis. The polymer composites had a thickness of approximately 3 mm. Fig. 2C illustrates the experimental procedure for producing the SWNTs/epoxy composites. Fig. 2 D illustrates how the resulting SWNTs/Fe2O3 strings are aligned by the external magnetic field.
Fig. 3. SEM micrograph of A and B for pristine SWNTs at different magnifications. E. SEM micrograph for magnetically sensitive SWNTs. and D. TEM micrograph of pristine SWNTs.
sensitive material. As previously discussed, the SWNTs, NaDDBS and Fe2O3 nanoparticles were attached using the electrostatic interaction. Therefore, by evaporating all of the water from the mixture, the magnetically sensitive carbon nanotubes could be obtained and then dispersed in other solvents as required. Fig. 3 A & B illustrate the SEM images for pristine single wall carbon nanotubes. As shown in the figure, the majority of the nanotubes are tangled with each other due to the strong Van der Waals forces that exists between the different strands. Fig. 3C illustrates an SEM image for the magnetically sensitive SWNTs. The magnetically sensitive Fe2O3 nanoparticles were attached (or tethered) to the ends of the tubes through the surfactants and the bright particles (identified by the red arrows) show the iron oxide on the surface of the carbon nanotubes. The SEM images demonstrated that the Fe2O3 nanoparticles attached well on SWNTs. Fig. 3 D illustrates a TEM image for the SWNTs. A Raman Microscope was used to further investigate the effect of coating the SWNTs with the Fe2O3. Fig. 4 A. shows the Raman microscope spectrum of the pristine SWNTs and magnetically sensitive SWNTs. The main Raman features of CNTs are the disordered induced D peak, the tangential G band, and the G' band (disorder overtone of D band). The D-band for the pristine SWN is near 1328 cm−1, and provides information about the defects within the carbon nanotubes. This peak shows the differences between a perfect carbon nanotube (low Intensity peak) and an imperfect carbon nanotube (high intensity peak). The D band of the used SWNTs was found to have a low intensity, which indicates that the SWNTs used in this work had fewer defects [29,30]. The G-band for the pristine SWNTs at 1580 cm−1, sometimes referred to as the graphitic G-band, is related to the arrangement of the hexagonal lattice of carbon atoms in graphite layers of pristine SWNTs [4,31]. The Raman spectrum for pristine SWNTs indicates that the Gband has a high intensity when compared to the D-band with an ID/IG ratio of 0.33. The low value of the ID/IG ratio indicates the high purity of the SWNTs. For the magnetically sensitive SWNTs the D-band was found to occur at 1350 cm−1 and the peak value of the G-band occurred at
2.4.3. Polymer composites tensile sample preparation and test method After a curing process of at least 24 h at room temperature (about 20 °C) the polymer composites were removed from the dog-bone shape telfon molds. The dog-bone polymer composite samples were polished using a Metaseru 250 grinder polisher purchased from Buehler in order to obtain polymer composite samples of uniform thickness for tensile testing. The uniform dimensions of the dog-bone polymer composite samples (shown in Fig. 1C) was 44.20 mm in length and 6.20 mm in thickness. The adjusted width was determined using ASTM Designation D 3039 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. The tensile strength testing method was the default tensile test method provided by the MTS Test Works 4 program used to control the MTS electromechanical test system. The tensile testing instrument used was 810 Material Test System (100 KN MTS Tester). The rate of grip separation was 2 mm/min. Tensile strength values were calculated using the Test Works program. The number of test specimens was determined using ASTM D-3039, which states that at least five isotropic material specimens should be tested, and for anisotropic materials, at least five normal samples and five samples parallel to the principal axis of anisotropy should be tested. For this investigation, normal and aligned samples were defined according to the direction of the alignment when casting the polymer composites. Using the ASTM Designation D-3039 ASTM Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials was used as a guide for tensile testing the polymer composites, the standard was only a guide and not all specifications contained in the standard were followed precisely. 3. Results and discussion 3.1. Characterization SEM micrographs were taken in order to better understand the different mechanical properties exhibited by the composite samples produced with magnetically sensitive SWNTs both with and without the presence of the magnetic field. The magnetically sensitive SWNTs were prepared by first dispersing the carbon nanotubes in water using NaDDBS as a surfactant and then adding Fe2O3 as the magnetically
Fig. 4. A. Raman spectra of SWNTs and magnetically sensitive single wall nanotubes. B. Magnetization (M) versus applied magnetic field (H). 84
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1582 cm−1. The magnetically sensitive SWNTs ID/IG ratio was 0.4. The higher value of the ID/IG ratio reveals that the ultrasonication treatment process may have introduced structural defects into the structure of the SWNTs. The ID/IG value of the pristine SWNTs is small, which indicates that the pristine SWNTs do not have any defects. Following the sonication process of approximately 30 min performed in order to prepare the magnetically sensitive SWNTs, the magnetically sensitive SWNTs ID/IG ratio had increased to only 0.4, this implies that the preparation method using the tip sonicator did not alter the SWNTs structure or introduce any defects. During the preparation of the magnetically sensitive SWNTs, it was noted that if the ultra-sonication time was increased to 60 min, the ID/IG ratio increased to 0.9. Therefore, the time for ultra-sonication should be carefully considered, otherwise it may lead to damage or breaks in the SWNTs, which will result in a shorter average length, which in turn will impact the thermophysical properities being observed. Vibrating sample magnetization (VSM) measurements were taken to illustrate the magnetic properties of the magnetically sensitive SWNTs. The magnetic property represents the inherent impact of the magnetic field through assessing the possibility of the particles being magnetized and/or attracted to a magnet. This property may then also impact the tensile strength of the magnetically sensitive SWNTs/epoxy resin, both with and without the presence of a magnetic field. Fig. 4 B illustrates the magnetization (M) as a function of the applied magnetic field, or (H) plots for the pristine SWNTs and magnetically sensitive SWNTs. The measurements were made at a maximum field of 15 kOe and at minimum field of −15 kOe at room temperature in steps of 500 Oe. The pristine SWNTs had a maximum magnetization value of 1 emu/g and the Fe2O3 nanoparticles had a maximum magnetization value of 25 emu/g. Magnetically sensitive SWNTs have a maximum magnetization value at 5 emu/g. The results indicate that the magnetically sensitive SWNTs have slight magnetic properties, which can, in fact, be used to align the SWNTs in a polymer matrix under a magnetic field.
Fig. 6. Comparison of the five types of epoxy polymer composites.
the polymer matrix using different methods. The tensile strength was measured for several different types of polymer composites and then an average tensile strength value was determined. 3.2.2.1. Tensile Strength of Neat Epoxy Resin Composites. The tensile strength of the neat epoxy resin composites was measured to provide a standard reference for comparison with the magnetically sensitive SWNTs/epoxy resin polymer composites. Fig. 6 shows the measured tensile strength of the neat Epoxy resin composites. The experimental results obtained in the current investigation exhibit good correlation with the values obtained by Coleman et al. [1] which ranged from 30 to 60 MPa for the same material, validating the experimental procedures used in the current investigation. 3.2.2.2. Tensile Strength of Composites Containing Magnetically Sensitive Single Wall Nanotubes. Ultra-sonication was used to aid in the dispersion of the magnetically sensitive SWNTs in the polymer matrix and the direction of the tensile strength test was along the direction of the magnetic force, and hence along the length of the carbon nanotubes. Fig. 6 illustrates the measured tensile strength of the magnetically sensitive SWNTs/epoxy resin polymer composites with 1 and 2 wt% loading without the application of a magnetic field, and 1 and 2 wt% loading with the magnetic field. As illustrated, the polymer composite with 1 wt% loading with the magnetic field demonstrated the highest tensile strength of the four magnetically sensitive single wall nanotube samples tested. The results indicated a 9% enhancement for 1 wt% loading without alignment when compared to the neat epoxy resin composite, and a 19.7% enhancement for 1 wt% loading for the aligned composite when both were compared to the neat epoxy resin composite. As illustrated, the results indicated a significant enhancement for 1 wt% loaded polymer composites in the presence of the magnetic field [32–34]. A decrease in tensile strength for the 2 wt% loaded, magnetically sensitive SWNTs was observed when compared with the 1 wt% loaded magnetically sensitive SWNTs. The decrease may be attributed to poor dispersion of the carbon nanotubes. Previous studies have indicated that when the loading of single wall nanotubes reaches 1.5 wt% in a polymer matrix [35], there is typically a sharp enhancement in the viscosity of the SWNT polymer. If the carbon nanotubes could be dispersed more completely in the polymer matrix, higher loading of carbon nanotubes could increase the tensile strength of polymer composites. If ultra-sonication was used as the only way to disperse carbon nanotubes, much more aggressive approaches could be used to reach a complete dispersion such as using a higher intensity and longer time duration for ultrasonication. However, these aggressive approaches may damage the structure of the carbon nanotubes and magnetically sensitive SWNTs, which may in turn hinder the ability to reinforce the polymer composites [9,35]. The tensile strength data of the epoxy polymer composites indicates
3.2. Carbon nano materials alignment characterization 3.2.1. SWNTs alignment in epoxy solution The use of a magnetic field to align carbon nanotubes was successful in the polymer matrixes as well. Fig. 5 A illustrates the macro-scaled digital image of the 0.1 wt% magnetically sensitive SWNTs in an epoxy matrix and Fig. 5 B illustrates the optical microscope image. As illustrated, the black particles form long lines along the direction of the magnetic field and as illustrated in Fig. 5 A, B with the aid of magnetic guidance, the SWNTs could be aligned within the epoxy matrix. 3.2.2. Mechanical properties of polymer composites The mechanical strength of the composite films prepared in this investigation was evaluated through tensile strength testing. The tensile test results were used to determine the extent of reinforcement enhancement provided by the magnetically sensitive SWNTs dispersed in
Fig. 5. SWNTs alignment in epoxy solution. (A. taken by digital camera; B taken by optical microscope, internal reference: 200 μm). 85
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that aligning the individual carbon nanotubes using a magnetic field can increase the tensile strength of epoxy polymer composites. Using this approach, the magnetically sensitive SWNTs were aligned within an epoxy resulting in an enhancement in the tensile strength in the SWNTs/epoxy composites. The dispersion of the carbon nanotubes plays an important role in the physical characteristics of the epoxy polymer composites, and the higher loading of the carbon nano material would ideally result in higher mechanical properties of the composites. Several authors [36,37] have reported that increased loading of CNTs in a composite results in lower strength characteristics suggesting an increase in voids or other defects as the loading of the CNTs increases. This is likely due to the difficulty of homogeneously dispersing concentrated CNT/polymer solutions. The method presented here results in composites in which the CNTs are dispersed homogeneously within the thermosetting polymers, without resulting in damage to the CNTs. Some investigations have attempted to align CNTs in polymer composites using extrusion methods in which a shear force was introduced to align CNTs by passing the composite material through a micro channel and shearing the CNT suspension [38,39]. The tensile strength of the 0.3 wt% SWNTs/epoxy composite for composites prepared in this manner increased by 5.5% when compared with neat epoxy samples. Thostenson et al. [40] found that 1 wt% SWNTs/epoxy lead to significant enhancement in fracture toughness and thermal conductivity compared with the neat epoxy.
difficult and therefore, proper alignment is nearly impossible during the epoxy curing process. (ii) at 0.3 wt% of the magnetically sensitive SWNTs, the nanocomposites have reached the percolation threshold, as 0.3 wt% of the SWNTs is sufficient to align and results in a sufficient network structures to enhance the heat transfer. Any additional amount of SWNTs will have minimal effect. Based on the obtained results, a 0.5 wt % was assumed to be the upper limit of enhancement and further tests at higher SWNTs wt% were assumed to not have any impact on the thermal conductivity. 4. Conclusions The results of the current investigation indicate that Fe2O3 nanoparticles can be attached to SWNTs using an assisting surfactant NaDDBS in a water solution. The results confirm the hypothesis that SWNTs will attach onto Fe2O3 nanoparticles due to the electrostatic attractive forces and that the attachment of the Fe2O3 nanoparticles to the SWNTs can significantly enhance the alignment due to the magnetic sensitivity of the Fe2O3 nanoparticles. This magnetic guidance was shown to be capable of aligning the SWNTs. Images of the movement of the SWNTs, Fe2O3, and NaDDBS in water under a magnetic field were created using optical microscopy. Upon application of the magnetic field, the mixture gradually vibrated, stretched out and the nanotubes ultiimately aligned along the magnetic field. This indicates that SWNTs were well attached to the Fe2O3 nanoparticles, and that these mixtures not only aligned, but also formed chains and clusters. Because of the viscosity resistance of the fluid itself, it takes some time for the mixture to become well aligned. Hence, good dispersion of SWNTs and attachment between SWNTs and Fe2O3 is a prerequisite of alignment. The use of magnetic guidance for aligning SWNTs in a fluid may also be applicable in reinforced polymer composites. The current investigation demonstrates that using specially prepared composites and a magnetic field can improve the mechanical properties and the thermal conductivity of polymer composites. In this investigation, 9.8% enhancement in tensile strength was observed for 1 wt% magnetically sensitive SWNTs/epoxy composites, in the presence of a magnetic field, when compared to similar samples without the magnetic field. For 2 wt % loaded composites, a 19.7% increase was observed when the magnetic field was applied as compared with those in which no magnetic field was present. However, when the loading was increased to 2 wt%, the tensile strength of both the aligned composites and unaligned composites decreased. Further, a 2 wt% unaligned sample demonstrated a 3.9% decrease in tensile strength when compared with 1 wt% composites, while the 2 wt% aligned sample demonstrated a 4.0% decrease in tensile strength compared with the 1 wt% aligned composite. These decreases were expected due to the change in the viscosity of the epoxy solution and the high Van der Waals forces between SWNTs. The thermal conductivity of 0.1 wt %, 0.2 wt %, 0.3 wt %, 0.4 wt% and 0.5 wt% loaded composites both with and without the application of a magnetic field and the results indicated that the thermal conductivity increased by 35%. Above a 3.0 wt% value, no increase in the thermal conductivity was observed, which again is attributed to the percolation threshold and an increase in the viscosity of the epoxy solution and the high Van der Waals forces between the SWNTs, which makes the dispersion process as well as the alignment process very difficult. Overall, the results indicated that the use of a magnetic field to align SWNTs in polymers shows great potential in improving the mechanical properties of polymer composites. As the quality of processing techniques increases, the potential of this method can be determined and utilized for a wide variety of commercial applications.
3.2.3. Thermal conductivity properties of polymer composites The thermal conductivity of the magnetically sensitive SWNTs/ epoxy resin polymer composites with 0.1, 0.2, 0.3, 0.4 and 0.5 wt% loading with and without the application of a magnetic field have been tested. As shown in Fig. 7, the thermal conductivity of the neat epoxy is 0.17 W/mK, applying an external magnetic field doesn't have any effect on the thermal conductivity. Using 0.1 wt% of the magnetically sensitive SWNTs has increased the thermal conductivity to 0.23 W/mK, by applying an external magnetic field that the thermal conductivity increased further, as the alignment of the magnetically sensitive SWNTs within the composites acts as a connected wire that helps to transfer heat as illustrated in Fig. 2 D. At 0.2 wt% the effect of adding extra wt% of the magnetically sensitive SWNTs on the thermal conductivity becomes nearly negligible, which can be attributed to the difficulty in getting a homogenous dispersion of the magnetically sensitive SWNTs in the epoxy. However, applying magnetic field has increased the thermal conductivity significantly. The alignment has a significant effect on the debundling the SWNTs. Beyond a 0.3 wt% of the magnetically sensitive SWNTs, the incremental increase in the thermal conductivity of the nanocomposites in the presence of the magnetic field stabilizes, which can be attributed to the following (i) at 0.3 wt% of the magnetically sensitive SWNTs, the process of obtaining a homogenous dispersion of the magnetically sensitive SWNTs in the epoxy is quite
Acknowledgement The financial support of Army Research Laboratory _Cooperative Agreement No. W911NF-08-2-0022 and NASA EPSCoR _Award No.
Fig. 7. Thermal conductivity for different wt% of SWNs nanocomposites. 86
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NNX09AU83A along with IR&D support from the Georgia Institute of Technology are acknowledged.
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