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Thermal, mechanical and electrical properties of highly loaded CNT-epoxy composites – A model for the electric conductivity
Composites Science and Technology 117 (2015) 183e190
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Thermal, mechanical and electrical properties of highly loaded CNTepoxy composites e A model for the electric conductivity € mann b, Achim Overbeck a, Stefan Linke b, Arno Kwade a Carsten Schilde a, *, Mario Schlo a b
Institute for Particle Technology, TU Braunschweig, Volkmaroder Strasse 5, 38104 Braunschweig, Germany Invent GmbH, Braunschweig, Christian-Pommer-Straße 34, 38112 Braunschweig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 17 June 2015 Accepted 20 June 2015 Available online 25 June 2015
Based on their extraordinary mechanical, electrical and thermal properties carbon nanotubes (CNT) are attributed as high potential filler material for polymer matrices. Besides the CNT material and functionalization, the properties of the polymer matrix, stabilization additives as well as the production process have a significant influence on the maximum CNT content and the resultant composite properties. Adapted from a modification of commonly used solvent based approaches a production process for high loaded epoxy based composites is presented in this study. In this process high loadings of up to 80 wt% CNTs are embedded in the epoxy composite matrix. The resultant mechanical, electrical and thermal properties are investigated for various CNT types, CNT modification, stabilization and loadings. Moreover, a model for the electric conductivity of CNT reinforced composites above the percolation threshold is presented. This model describes the electric conductivity as a function of the composite structure including CNT content as well as a theoretical value for the number of contacts within such a composite network. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Electrical properties Mechanical properties Thermal properties Modeling
1. Introduction Since the early nineties carbon nanotubes are attributed as high potential filler material for polymer matrices [1,2]. Based on their extraordinary mechanical, electrical and thermal properties as well as their high aspect ratio and specific surface, CNTs are highly suited for a significant enhancement or modification of polymers. Comparatively low nanotube contents below 1 wt% lead to a relevant increase of the mechanical properties, e.g. Young's modulus, strength and strain to failure [3]. Apart from low nanotube contents, high loadings of carbon based filler materials have a great potential for an extraordinary modification or enhancement regarding thermal, electrical and mechanical properties of polymers used in electronic and lightweight construction industry. For example, processing different material classes in lightweight, electro mobility or aeronautics applications leads to thermal stresses in component assemblies which cause component failure. Hence, the adaption of the coefficient of thermal expansion via incorporated fillers into the polymer matrix is advantageously to handle complex multi-component assemblies. In addition,
* Corresponding author. E-mail address: [email protected] (C. Schilde). http://dx.doi.org/10.1016/j.compscitech.2015.06.013 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
polymers and polymer coatings with a high electrical and thermal conductivity and thus high CNT loadings are of particular importance in these industries. The electric charge and discharge as well as the cooling or heating of components is ensured without increasing the weight of a component or using complex multicomponent assemblies, e.g. lightning protection or de-icing of aircraft components. However, the high aspect ratio and specific surface as well as strong attractive interactions between nanotubes lead to disadvantages in processability. As consequence fast agglomeration and a strong increase of the viscosity can be observed [4,5]. The effects of nanotube synthesis, surface functionalization and further processing on various composite properties are represented in a huge number of articles [5,6]. The challenge of embedding nanotubes in polymer matrices is to obtain a homogenous and stable suspension of exfoliated CNTs while concurrently ensuring processable suspension viscosities [7]. As a result various production processes were established which are more or less limited in the maximum content of carbon nanotubes, the feasibility of exfoliation and/or the destruction of CNTs. Moreover, aspects regarding the composite production have to be considered. Exfoliated CNTs cause a strong increase in suspension viscosity. Mechanical processes are commonly favored to embed synthesized and optionally
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functionalized CNTs in the polymer matrix during the production of cabon nanotube reinforces composites [8], e.g. shear, kneading or ultrasonic dispersion devices. These dispersion devices are restricted in their exfoliation performance and their ability to process highly viscous systems, i.e. high filler contents [9e13]. Typically, CNT contents of less than 10 wt% can be realized. In general, length reduction takes place by ultrasonic treatment. An increased amount of exfoliated CNTs without length reduction can be obtained by calender based processes [3]. An schematic overview of the common production methods of CNT containing polymer composites is given by Bauhofer and Kovacs [14]. Similar mechanical processes are applicable for dispersing nanotubes in a solvent phase instead of polymer phase followed by a mixing with the polymer matrix and evaporation of the solvent phase [15]. This production process is particularly advantageous for thermoplastic materials. The production process for highly loaded epoxy based composites presented in this study is a modification of the commonly used solvent based approach. A method for the quantitative characterization of the amount of agglomerated and exfoliated CNTs in epoxy polymers, dispersed by various dispersion devices, was developed by Nadler et al. [16]. Except from this study, the characterization of the amount of agglomerated and dispersed nanotubes is typically done qualitatively by TEM or SEM pictures of the cured composite. 2. Experimental The production process for high loaded epoxy based composites presented in this study is shown in Fig. 1 (top). In a first step, the
CNTs as well as the intermixture of epoxy resin HexFlow RTM6 (company Hexcel Corporation) and the corresponding diamine based hardener were dissolved in a large portion of solvent phase using the ultrasonic dispersing device UP400S (company Hielscher, sonotrode H40, power of 150 W). The epoxy resin HexFlow RTM6 system is often used in aerospace and lightweight construction applications. Eventhough no RTM process (Resin Transfer Molding) was applied in this study, HexFlow RTM6 was used for the experiments because of its high quality and applicability in lightweight construction and aerospace industry. The used solvent was butanone. The suspension was tempered to a temperature of 40 C and dispersed for 120 min. The CNT content in the suspension was set up to 1 wt% depending on the target CNT content in the cured composite. The stability of these suspensions against reagglomeration was characterized via transmission measurements as function of the sample height and measurement using the Turbiscan Lab (company Quantachrome, 25 C). Subsequently, the solvent phase was evaporated for 90 min in a lab kiln at 55 C. The remaining suspensions with solids content between 10 wt% and 80 wt% (approved via thermal gravimetric analysis using the TGA/SDTA 851, company Mettler Toledo, oxygen flow, 20e700 C at 20 C min1) are extremely viscous and possess processing properties similar to solid materials. These suspensions were ground in a steel mortar to particle sizes between 80 mm and 200 mm depending on the CNT content, type, modification and stabilization (ground suspensions were measured via laser diffraction, Helos, company Sympatec). In principle an increase in CNT content leads to smaller particle sizes due to a more brittle processing behavior of the epoxy-CNT suspension. However, the particle size of the epoxy-
Fig. 1. Production process for highly loaded epoxy composites (top). Milled CNT-thermosetting resin suspension, produced composite cubes and high-resolution SEM of a composite fragment containing 50 wt% of CNT's (below).
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CNT suspension has no significant influence on further processing and the resulting composite properties. The ground suspension was further processed by hot press molding at a temperature of 150 C and a specific molding pressure of circa 6 kN/cm2. The curing time was set to 180 min. The ground CNT thermosetting resin suspension with granular processing properties as well as two manufactured composite cubes (untreated and laser polished surface) and a high-resolution scanning electron microscope image of a composite fragment containing 50 wt% of CNTs are shown in Fig. 1. The composite surface was polished (surface roughness of around 50 nm measured via atomic force microscopy) for further composite characterization, e.g. measurement of the electrical, mechanical and thermal properties. During composite production multi walled carbon nanotubes (MWCNTs) Baytubes CP150HP with a carbon purity of >99%, a bulk density of 130e150 kg/m3, an outer diameter of 13e16 nm and a length of 1e10 mm were dispersed in the solvent phase. The agglomerate sizes are specified to 0.1e1 mm. Additionally, Baytubes C70P (carbon purity of >95%, bulk density of 45e95 kg/m3, outer diameter of 13e16 nm, length of 1e10 mm), carboxylated and non-modified nanotubes of the company Carbon Future (carbon purity of >90%, bulk density of 28 kg/m3, outer diameter and length not specified [17]) and single walled carbon nanotubes (SWCTs) produced by the Fraunhofer IWS Dresden (50%e70% SWCNTs, 20% catalysts, 10%e30% nano-graphite, outer diameter of 1e1.6 nm [18]) were investigated. The mechanical properties were measured via nanoindentation (Triboindenter, company Hysitron) using a Berkovich tip. These properties describe the composite deformation behavior at the micro scale. In order to increase the confidence level, 40 measurements of each sample surface were performed. Due to the high measurement accuracy of the indentation device, a distribution of the micromechanical properties of the composite surface can be obtained which is characterized by the standard deviation [19e21]. The electric conductivity was measured using a material testing machine (Zwick Z020, company Zwick, 50 N, 150 kPa) and an universal calibrator (Digistant 4422, 20 mA). The cylindrical sample height was 10 mm and the diameter 20 mm. The coefficient of thermal expansion was measured using a Bohlin Gemini 2 (company Malvern instruments, 3N, gap control). The thermal properties in this study were determined by the progression of the surface temperature as function of the measurement time (heat source with a temperature of 160 C on the opposite sample surface of a cylindrical sample with a diameter of 20 mm and a sample height of 10 mm). To avoid thermal radiation and convection an isolating foam glass specimen holder was used enclosing the composite sample. As reference values, samples made of steel, brass and neat epoxy resin were measured. Hence, the measurement results for the thermal conductivity measurements can be compared relatively. 3. Results and discussion Depending on the properties of the solvent or epoxy matrix, carbon nanotubes tend to reagglomerate or flocculate. Particularly at low CNT content, the stability of the carbon nanotubes in the solvent phase has a significant effect on the resultant composite properties. Fig. 2 shows the transmission of an epoxy-solvent-CNT suspension as function of the sample height and measurement time without stabilization (left) and with the stabilizing additive Disperbyk 2150 (right, 0.5 wt% additive) (measured via Turbiscan Lab, company Quantachrome). This additive is a suitable acrylic copolymer for the stabilization of CNTs and inorganic nanoparticles in non-aqueous solvents [16,22]. Typically, stabilized CNT suspensions are nontransparent. Without stabilization, the CNTs form large flocculates/agglomerates which sediment with increasing
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measurement time. Thus, the transparency of non-stable CNT suspension increases. The effect of the suspension stability on the mechanical, electrical and thermal properties is exemplarily demonstrated in the following. For industrial application the reduction of shrinkage of composites, their thermal expansion, electric and heat conductivity as well as mechanical properties, such as Young's modulus or tensile strength, are of special interest [6,8,14]. In this study these properties are compared for thermosetting resin based composites produced by the above mentioned process for high particle loadings. Besides the content of CNTs, the influence of the CNT type, surface functionalization and the stabilization on the composite properties is shown exemplarily. The comparability of the results for mechanical, thermal or electrical properties with other studies is restricted due to different CNT materials and surface functionalization, deviating production processes (dispersion and stabilization of CNTs) and different polymer properties [14]. However, the maximum values of the different properties are compared to maximum values of other production processes described in literature. Fig. 3 shows the effect of the CNT content on the Young's modulus and the ratio of plastic to elastic deformation energies. At carbon nanotube content of up to 20 wt%, the Young's modulus stays constant or even decreases slightly. Due to the excellent mechanical properties of the CNTs, the Young's modulus increases strongly with further increasing solids content, reaching a maximum value of 28.02 kN/mm2 (percentagewise an increase of 330%) at a solids content of 60 wt%. Because of air inclusions caused by an insufficient molding pressure during composite processing, the Young's modulus decreases again for very high solids content of 70 wt% and 80 wt%, reaching values below the Young's modulus of the neat epoxy composite matrix. The strength of the ground suspension epoxy-CNT particles increases strongly with increasing CNT solids content. Hence, the pressure during further processing via hot press molding is not sufficient to remove the air within the composite material. The air inclusions within the composite structure are confirmed by the strong decrease in the composite densities given in Fig. 4. The ratio of plastic to elastic deformation energies shown in Fig. 3 is used to describe viscoplastic properties of composites, coatings or particulate structures [20,21,23]. At low carbon nanotube content, the local composite deformation behavior is only slightly influenced. Due to the excellent mechanical properties of the CNTs [5], the elastic deformation energy increases strongly with increasing solids content, reaching a minimum ratio of plastic to elastic deformation energy at 70 wt%. However, the slight increase in the elastic deformation behavior between 60 wt% and 70 wt% as well as the strong increase in the plastic deformation behavior at 80 wt% is caused by air inclusions generated during composite processing. The corresponding mechanical properties for the various CNT types, surface functionalizations and stabilizations are given in Table 1. Because of the extremly high CNT content, an additional stabilization of the carbon nanotube material in the solvent matrix by additives (Disperbyk 2150) did not lead to an increase in the resultant mechanical properties. The Young's modulus of the stabilized composite is, indeed, decreasing eventhough the percentagewise standard deviation indicates a more homogeneous distribution of the mechanical properties within the composite matrix. Due to a slightly improved stabilization, similar results can be obtained by processing carboxylated carbon nanotubes. For SWCNTs a reduction in the Young's modulus can be observed, by reasons of a higher specific surface area at similar CNT content. Consequently, the strength of the ground epoxy-CNT particles increases and the air inclusions caused by composite processing
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Fig. 2. Effect of stabilization of the carbon nanotube material in the solvent matrix (butanone) by additives (acrylic copolymer Disperbyk 2150 [16,22]). Transmission of epoxysolvent-CNT suspension (40 wt%) as function of the sample height and measurement time without stabilization (left) and with the stabilizing additive Disperbyk 2150 (right, 0.5 wt% additive).
Fig. 3. Effect of the CNT content on Young's modulus and the ratio of plastic to elastic deformation energies.
Fig. 4. Effect of the CNT content on composite density.
dominate the mechanical behavior at lower CNT content. This is confirmed by a slight increase in the plastic deformation behavior of the composite material (ratio of plastic to elastic deformation energy). For comparison, Table 2 shows exemplary the effect of different production processes and CNT content on the minimum and maximum reachable Young's modulus of thermosetting resin based CNT composites according to Loos et al. [6]. In general, the mechanical properties as well as the maximum realized carbon nanotube content of the production process for highly loaded epoxy based composites presented in this study are higher compared to composites produced by other production processes. An exception is the uncommon super aligned process of Cheng et al. [24]. For this process, carbon nanotubes were synthesized on silicon wafers by low pressure vapor deposition and stacked together to generate CNT preforms. These preforms consist of several thousand sheets, which were inserted in the mold of an RTM process. Thus, a high relative increase in mechanical and electrical properties can be obtained using low viscous epoxy matrices. In summary, the mixture of exfoliated and agglomerated CNTs caused by various production processes with different stress mechanisms have a significant effect on thermal, electrical and mechanical composite properties. Regarding the effect of the CNT content on the electric conductivity a similar trend as shown for the mechanical properties can be observed (see Fig. 5). The electric conductivity increases strongly with increasing solids content, reaching a maximum value of 838 S/m at a solids content of 60 wt%. Because of the air inclusions caused by the composite processing, the electric conductivity decreases again with further increased solids content. Similar to the mechanical properties, for SWCNTs a reduction in the electric conductivity can be observed because of a higher specific surface area at similar CNT content, and, thus, an increased amount of air inclusions within the composite structure. An additional stabilization of the carbon nanotube material in the solvent matrix by the additive Disperbyk 2150 did not lead to an increase in the resultant electric conductivity similar to the mechanical composite properties. According to Baumann et al. [14] the maximum conductivities within a composite vary by ten or more orders of magnitude in different matrices, whereas in similar matrices and mass fraction the conductivity using different functionalized CNTs varies between one or two orders. Moreover, the surface treatment via the modification with functional groups such as carboxylic groups leads to significant reduction in the composite conductivity.
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Table 1 Effect of the functionalization, stabilization and MWCNT/SWCNT on Young's modulus as well as the ratio of plastic to elastic deformation energies (cm: percentage by weight).
Neat resin Baytubes C150 HP (cm ¼ 50) Future carbon carboxylated (cm ¼ 50) IWS DredenSWCNTs (cm ¼ 50) Baytubes C150 HP (cm ¼ 0.4) Baytubes C150 HP (cm ¼ 0.4) stabilized Disperbyk 2150
Young's modulus [kN/mm2]
Standard deviation [kN/mm2]
Standard deviation [%]
Ratio Wplast/Welast [e]
6.531 13.092 8.816 8.631 10.098 8.627
1.381 3.429 0.672 1.989 2.559 0.801
21.14 26.19 7.62 23.04 25.35 9.28
0.95 0.83 0.77 1.10 1.02 0.99
Table 2 Effect of different production processes and CNT content on the minimum and maximum reachable Young's modulus [6]. Production process
Young's modulus [kN/mm2]
Percentagewise change [%]
CNT content [wt%]
Literature
Shear and ultrasonic dispersion devices
1.77 0.45 0.75 3.5 4.12 4.2 20.4
10 þ294 29 þ6 þ6 þ24 þ716
1 4 8 0.1 1.5 1 16
[11] [13] [10] [3] [25] [24] [26]
Calender based process Super-aligned process
Moreno Marcelino et al. [27] observed a similar trend with increasing degree of surface functionalization using either, carbonyl and carboxyl groups. Besides the functionalized material itself, a significant reduction in the fiber length during the dispersion step of the weaker functionalized CNTs are a possible explanation for the decrease in the electric conductivity of the resultant composites. However, according to the review of Bauhofer et al. [14] neither the surface treatment nor the type of CNTs (SWCNT or MWCNT) show a clear trend on the composite conductivity. When the CNT content exceeds the percolation thresholds [28e31], the electric conductivity within a composite with a nonconductive matrix can be described by the number of particle contacts and the electric conductivity of the particles. The contact probability, pc, of an infinitesimal small CNT surface element, dS, to another carbon nanotube depends on the particle properties, i.e. carbon nanotube surface, SCNT, and the average coordination number, k [32]:
dS pc ¼ k$ SCNT
(1)
Fig. 5. Effect of the CNT content (type, functionalization and stabilization) on the electric conductivity.
The electric conductivity of an infinitesimal surface element, dS, of an carbon nanotube in the measurement direction, z, is, si,z,CNT. The portion of this surface element to the composite conductivity depends on the contact probability, pc
dsz;CNT ¼ si;z; CNT $k$
dS SCNT
(2)
The electric conductivity of an infinitesimal surface element in measurement direction, si,z,CNT, can be derived from the electric conductivity of carbon nanotubes, si,CNT, and projection of infinitesimal surface element, dS, in the intersection plane, dAU (compare Fig. 6):
si;z; CNT ¼ si;CNT $
dAU dS
(3)
The electric conductivity of a single carbon nanotube, si,CNT, or another particle depends on the material itself, the surface functionalization or amount of conductivity additives. In this case the electric conductivity between two CNTs is an averaged value between CNTs within a critical distance in which electron tunneling
Fig. 6. Modeled and measured electric conductivity of the CNT composite.
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occurs [33]. The portion of all particles, i.e. CNTs, in the intersection plane of the composite is proportional to dAU (similar to cV,CNT ¼ 1ε). Where ε is the expected fraction of the matrix material in the cross-sectional area (under the assumption of uniform random packing equal to the expected volume fraction of the matrix material). Thus, the total composite conductivity can be derived from the combination and integration of Eqs. (2) and (3):
si;CNT sz;CNT ¼ ð1 εÞ$k$ SCNT
(4)
For similar carbon nanotube materials, the CNT surface as well as their electric conductivity remains constant (e.g. Baytubes CP150HP). Thus, the influence of the CNT solids content on the electric conductivity of the composite can be described as product of the CNT volume fraction, cV,CNT ¼ 1ε, and the average coordination number, k. According to Klimenko et al. [34] the average coordination number of porous fibrous material is defined as function of the fiber length, lCNT, fiber diameter, dCNT, and the expected volume fraction of the matrix material, 3, as follows
k
4$lCNT ½1 ε$ð1 lnðεÞÞ $ 1ε dCNT
(5)
Since the stress intensity and frequency during the dispersion process using an ultrasonic dispersion device is independent on the solids content [35,36], the reduction in the fiber length due to the dispersion process should be constant for the same type of CNTs. It must be pointed out that the coordination number according to Klimenko et al. is for thick fibrous materials in the size range of several tens of microns in diameter. Thus, the coordination number using this equation is only approximately proportional to the real coordination number between CNTs. In this case the electric conductivity can be described as function of the expected volume fraction of the matrix material by the following equation:
sCNT ¼ A$½1 ε$ð1 lnðεÞÞ
(6)
A
4$lCNT si;CNT $ dCNT SCNT
where the parameter A depends only on the properties of the particulate system. The electric resistance in the contact points of an excellent conductive material is a crucial factor. An increasing number of contact point between particles/fibers in measurement direction due to a decreased particle/fiber size leads to a decrease in the conductivity (due to the electric resistance in the contact points) which has to be taken into account. It must be pointed out, that a theoretical or apparent number of particulate/fiber layers in measurement direction was not taken into account since the fiber length were constant in the experiments using the Baytubes CP150HP. Thus, the effect of the electric resistance in the contact points in measurement direction is constant for all composite samples. The correlation of the electric conductivity calculated by the model Eq. (6) with the measured data is presented in Fig. 6. For the homogeneous composite material with less air inclusions the model fits excellent to the measured data (R2 ¼ 0.9951). As expected, at very high solids content with decreasing composite density, and thus, increasing amount of air inclusions, the actual number of contacts between CNTs deviates strongly from the theoretical considerations. Considering the data of the composite densities and mechanical properties at 60 wt% the increase in the electric conductivity is less than expected. Indeed, electrical properties are much more sensitive in measuring defects or small amounts of air inclusions in composite materials. However, the model seems to be suited for the characterization of the electric conductivity of carbon nanotube composites (assuming a nonconductive matrix), conductive coatings or the electric conductivity of bulk solids. In Figs. 7 and 8, the effect of the CNT content on the linear coefficient of thermal expansion and the thermal conductivity properties of the enhanced composite are presented. The linear coefficient of thermal expansion can be adjusted over a broad range by a reinforcement of the epoxy matrix using high CNT content. Due to the thermal properties of carbon nanotubes, the linear coefficients of thermal expansion of the reinforced composites
Fig. 7. Coefficient of thermal expansion for different CNT content.
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of the functionalized CNTs with the epoxy matrix. Moreover, the interfacial thermal resistance is reduced and the transfer of thermal energy is increased due to stronger CNT-epoxy interactions. 4. Conclusions
Fig. 8. Progression of the surface temperature as function of the measurement time and CNT content (heat source with a temperature of 160 C on the opposite sample surface; cylindrical sample dimensions: 20 mm diameter, 10 mm height).
decrease with increasing CNT content. Thermal expansion properties of metals can be received for very high carbon nanotube content. According to Yuen et al. [37] this effect does not occur at very low CNT content. Besides the linear coefficient of thermal expansion, the thermal conductivity of epoxy resins can be increased significantly using high carbon nanotube loadings (Fig. 8). Similar trends for lower MWCNT loadings below 1 wt% were obtained by Moisala et al. [38] and for SWCNTs by Yu et al. [39]. However, Gojny et al. [40] stated only a slight enhancement of the thermal conductivity by the incorporation of low CNT content into polymer matrices. The surface treatment or stabilization does not show a clear trend regarding the thermal conductivity (see Fig. 9). The addition of SWCNTs or carboxylated CNTs tend to result in higher thermal conductivities at similar carbon nanotube loadings. Yang et al. [41] postulated that this effect is caused by the formation of a heat flow network due to a well dispersion and good compatibility
Besides the surface functionalization and purification, the processing strategy to transfer the carbon nanotubes into the matrix as well as their dispersion and stabilization within the matrix are of major importance. Typically, the maximum carbon nanotube loadings depend strongly on the production process. Based on a modification of a commonly used solvent based approach, a production process for highly loaded epoxy based composites is presented in this study. In this process high loadings between 10 wt% to 80 wt% of CNTs are realized. The resultant mechanical, thermal and electrical properties show a distinct enhancement with increasing CNT content, reaching a maximum Young's modulus of 28.02 kN/mm2 and electric conductivity of 838 S/m at a weight content of 60% wt. For higher CNT content a strong decrease in the mechanical and electrical properties is observed, caused by air inclusions in the composite structure due to the production process. Because of the extremely high CNT content, an additional stabilization of the carbon nanotube material in the solvent matrix by additives did not lead to differences in the resultant composite properties, e.g. mechanical, thermal and electrical properties. In general, this modified process can be adjusted for the embedding of other nano- and microparticle materials in thermoset or thermoplastic materials. Similar approaches for CNT-composite production methods are known for thermoplastic materials [42,43]. A model for the electric conductivity for CNT reinforced composites above the percolation threshold is derived. This model describes the electric conductivity as function of the composite structure including CNT content as well as a theoretical value for the number of contacts within such a composite network with high accuracy. For the linear coefficient of thermal expansion and the thermal conductivity a behavior close to the properties of metals was observed. High loadings of carbon based filler materials have a great potential for an extraordinary modification or enhancement of epoxy resins and other polymers regarding thermal, electrical and mechanical properties. Although the processing of highly loaded composite materials up to 80 wt% is difficult, these composites are relevant for new design and construction concepts, e.g. highly loaded and graduated composites in electronic and lightweight construction industry. Acknowledgment The authors gratefully acknowledge the financial support by the EU within the jointresearch project “NANOKOMP”. Thermal gravimetric analysis was carried out by Mrs. S. Zellmer from the Institute for Particle Technology, TU Braunschweig. Glossary
Fig. 9. Progression on the surface temperature as function of the measurement time, CNT type, stabilization and functionalization (heat source with a temperature of 160 C on the opposite sample surface; cylindrical sample dimensions: 20 mm diameter, 10 mm height).
AU [m2] area of the intersection plane within the composite cm [%] CNT percentage by weight cV,CNT [e]CNT contents by volume dCNT [m] CNT diameter dS [m2] infinitesimal small CNT surface element E [kN/mm2] Young's modulus k [e] average coordination number between CNTs lCNT [m] CNT length pc [e] contact probability between CNTs SCNT [m2]carbon nanotube surface
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t [s] measurement time T [ C] sample temperature Wel [J] elastic deformation energy Wpl [J] plastic deformation energy a [1/K] linear coefficient of thermal expansion 3 [e] matrix contents by volume r [g/cm3]composite density re [S/m] electrical composite conductivity si,CNT [S/m] electric conductivity of CNTs si,z,CNT [S/m] electric conductivity of an infinitesimal CNT surface element si,z,CNT [S/m] modeled electric composite conductivity (measurement direction) References [1] J. Abraham, Functionalization of carbon nanotubes. Friedrich-Alexander-Uni€t Erlangen-Nürnberg, Org. Chem. (2005). versita [2] G.C. Psarras, The ongoing impact of carbon (allotropic forms)/polymer composites, eXPRESS Polym. Lett. 8 (2) (2014) 73. €pke, B. Fiedler, K. Schulte, Carbon [3] F.H. Gojny, M.H.G. Wichmann, U. Ko nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content, Compos. Sci. Technol. 64 (15) (2004) 2363e2371. [4] M. Nadler, J. Werner, T. Mahrholz, U. Riedel, W. Hufenbach, Effect of CNT surface functionalisation on the mechanical properties of multi-walled carbon nanotube/epoxy-composites, Compos. Part A Appl. Sci. Manuf. 40 (6e7) (2009) 932e937. [5] E.T. Thostenson, C. Li, T.-W. Chou, Nanocomposites in context, Compos. Sci. Technol. 65 (2005) 491e516. [6] M.R. Loos, K. Schulte, Is it worth the effort to reinforce polymers with carbon nanotubes? Macromol. Theory Simulations 20 (5) (2011) 350e362. [7] S.S. Rahatekar, K.K.K. Koziol, S.A. Butler, J.A. Elliott, M.S.P. Shaffer, M.R. Mackley, et al., Optical microstructure and viscosity enhancement for an epoxy resin matrix containing multiwall carbon nanotubes, J. Rheol. 50 (5) (2006) 599e610. [8] J.N. Coleman, U. Khan, Y.G. Gk, Mechanical reinforcement of polymers using carbon nanotubes, Adv. Mater. 18 (2006) 689e706. [9] A. Allaoui, S. Bai, H.M. Cheng, J.B. Bai, Mechanical and electrical properties of a MWNT/epoxy composite, Compos. Sci. Technol. 62 (15) (2002) 1993e1998. [10] P. Guo, X. Chen, X. Gao, H. Song, H. Shen, Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites, Compos. Sci. Technol. 67 (15e16) (2007) 3331e3337. ndez-Pe rez, F. Avile s, A. May-Pat, A. Valadez-Gonz [11] A. Herna alez, P.J. Herrerarez, Effective properties of multiwalled carbon nanotube/ Franco, P. Bartolo-Pe epoxy composites using two different tubes, Compos. Sci. Technol. 68 (6) (2008) 1422e1431. [12] L. Sun, G.L. Warren, J.Y. O'Reilly, W.N. Everett, S.M. Lee, D. Davis, et al., Mechanical properties of surface-functionalized SWCNT/epoxy composites, Carbon 46 (2) (2008) 320e328. [13] M.-K. Yeh, T.-H. Hsieh, N.-H. Tai, Fabrication and mechanical properties of multi-walled carbon nanotubes/epoxy nanocomposites, Mater. Sci. Eng. A 483e484 (0) (2008) 289e292. [14] W. Bauhofer, J.Z. Kovacs, A review and analysis of electrical percolation in carbon nanotube polymer composites, Compos. Sci. Technol. 69 (10) (2009) 1486e1498. [15] Y.J. Kim, T.S. Shin, H.D. Choi, J.H. Kwon, Y.C. Chung, H.G. Yoon, Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites, Carbon 43 (1) (2005) 23e30. [16] M. Nadler, T. Mahrholz, U. Riedel, C. Schilde, A. Kwade, Preparation of colloidal carbon nanotube dispersions and their characterisation using a disc centrifuge, Carbon 46 (11) (2008) 1384e1392. €tschke, G. Petzold, Dispersability and particle size [17] B. Krause, M. Mende, P. Po distribution of CNTs in an aqueous surfactant dispersion as a function of ultrasonic treatment time, Carbon 48 (10) (2010) 2746e2754. [18] IWS F-IfW-uS, Hochdurchsatz-Synthese von SWCNTs - Qualitativ hochwertige €hren und deren Anwendungen Dresden, einwandige Kohlenstoff-Nanoro 2009. [19] N. Barth, C. Schilde, A. Kwade, Influence of particle size distribution on micromechanical properties of thin nanoparticulate coatings, Phys. Procedia
40 (2013) 9e18. [20] C. Schilde, A. Kwade, Measurement of the micromechanical properties of nanostructured aggregates via nanoindentation, J. Mater. Res. 27 (4) (2012) 672e684. [21] C. Schilde, B. Westphal, A. Kwade, Effect of primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates, J. Nanoparticle Res. 14 (3) (2012) 1e11. [22] H. Nolte, C. Schilde, A. Kwade, Determination of particle size distributions and the degree of dispersion in nanocomposites, Compos. Sci. Technol. 72 (2012) 948e958. [23] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Mater. Sci. Eng. Rep. 44 (2004) 91e149. [24] Q.F. Cheng, J.P. Wang, J.J. Wen, C.H. Liu, K.L. Jiang, Q.Q. Li, et al., Carbon nanotube/epoxy composites fabricated by resin transfer molding, Carbon 48 (1) (2010) 260e266. nez-Su [25] A. Jime areza, M. Campoa, I. Gaztelumendib, N. Markaideb, M. S ancheza, ~ aa, The influence of mechanical dispersion of MWCNT in epoxy matrix A. Uren by calendering method: batch method versus time controlled, Compos. Part B Eng. 48 (2013) 88e94. [26] K. Jiang, J. Wang, Q. Li, L. Liu, C. Liu, S. Fan, Superaligned carbon nanotube arrays, films, and yarns: a road to applications, Adv. Mater. 23 (9) (2011) 1154e1161. ndez [27] J.E. Moreno Marcelino, E. Vigueras Santiago, G. Lopez-Tellez, S. Herna pez, Chemical functionalization of carbon nanotubes and its effects on Lo electrical conductivity, J. Nano Res. 28 (2014) 51e61. [28] B. Kim, J. Lee, I. Yu, Electrical properties of single-wall carbon nanotube and epoxy composites, J. Appl. Phys. 94 (10) (2003) 6724e6728. [29] M.B. Bryning, M.F. Islam, J.M. Kikkawa, A.G. Yodh, Very low conductivity threshold in bulk isotropic single-walled carbon nanotube-epoxy composites, Adv. Mater. 17 (9) (2005) 1186e1191. [30] C.A. Martin, J.K.W. Sandler, M.S.P. Shaffer, M.K. Schwarz, W. Bauhofer, K. Schulte, et al., Formation of percolating networks in multi-wall carbon nanotube-epoxy composites, Compos. Sci. Technol. 64 (15) (2004) 2309e2316. [31] J. Li, P.C. Ma, W.S. Chow, C.K. To, B.Z. Tang, J.K. Kim, Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes, Adv. Funct. Mater. 17 (2007) 3207e3215. [32] H. Rumpf, Zur Theorie der Zugfestigkeit von Agglomeraten bei Kraftübertragung an Kontaktpunkten, Chem. Ing. Tech. 42 (8) (1970) 538e540. [33] J.Z. Kovacs, B.S. Velagala, K. Schulte, W. Bauhofer, Two percolation thresholds in carbon nanotube epoxy composites, Compos. Sci. Technol. 67 (5) (2007) 922e928. [34] V.N. Klimenko, A.G. Kostornov, Y.N. Kryuchkov, Coordination number and bulk porosity of porous fibrous materials, Powder Metall. Metal Ceram. 38 (1999) 202e204. [35] C. Schilde, C. Mages-Sauter, A. Kwade, H.P. Schuchman, Efficiency of different dispersing devices for dispersing nanosized silica and alumina, Powder Technol. 207 (2011) 353e361. [36] H. Nolte, Production of Nanocomposites by Dispersing Nanoparticles in Epoxy Resin Based Polymers, Institute for Particle Technology, TU Braunschweig, 2013. [37] S.M. Yuen, C.C.M. Ma, H.H. Wu, H.C. Kuan, W.J. Chen, S.H. Liao, et al., Preparation and thermal, electrical, and morphological properties of multiwalled carbon nanotube and epoxy composites, J. Appl. Polym. Sci. 103 (2) (2007) 1272e1278. [38] A. Moisala, Q. Li, I.A. Kinloch, A.H. Windle, Thermal and electrical conductivity of single- and multi-walled carbon nanotube-epoxy composites, Compos. Sci. Technol. 66 (10) (2006) 1285e1288. [39] A.P. Yu, M.E. Itkis, E. Bekyarova, R.C. Haddon, Effect of single-walled carbon nanotube purity on the thermal conductivity of carbon nanotube-based composites, Appl. Phys. Lett. 89 (13) (2006) 1e3. [40] F.H. Gojny, M.H.G. Wichmann, B. Fiedler, I.A. Kinloch, W. Bauhofer, A.H. Windle, et al., Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites, Polymer 47 (6) (2006) 2036e2045. [41] S.-Y. Yang, C.-C.M. Ma, C.-C. Teng, Y.-W. Huang, S.-H. Liao, Y.-L. Huang, et al., Effect of functionalized carbon nanotubes on the thermal conductivity of epoxy composites, Carbon 48 (3) (2010) 592e603. [42] H. Aarab, M. Baitoul, J. Wery, R. Almairac, S. Lefrant, E. Faulques, et al., Electrical and optical properties of PPV and single-walled carbon nanotubes composite films, Synth. Met. 155 (1) (2005) 63e67. [43] M.S.P. Shaffer, A.H. Windle, Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites, Adv. Mater. 11 (11) (1999) 937e941.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Thermal, mechanical and electrical properties of highly loaded CNTepoxy composites e A model for the electric conductivity € mann b, Achim Overbeck a, Stefan Linke b, Arno Kwade a Carsten Schilde a, *, Mario Schlo a b
Institute for Particle Technology, TU Braunschweig, Volkmaroder Strasse 5, 38104 Braunschweig, Germany Invent GmbH, Braunschweig, Christian-Pommer-Straße 34, 38112 Braunschweig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 17 June 2015 Accepted 20 June 2015 Available online 25 June 2015
Based on their extraordinary mechanical, electrical and thermal properties carbon nanotubes (CNT) are attributed as high potential filler material for polymer matrices. Besides the CNT material and functionalization, the properties of the polymer matrix, stabilization additives as well as the production process have a significant influence on the maximum CNT content and the resultant composite properties. Adapted from a modification of commonly used solvent based approaches a production process for high loaded epoxy based composites is presented in this study. In this process high loadings of up to 80 wt% CNTs are embedded in the epoxy composite matrix. The resultant mechanical, electrical and thermal properties are investigated for various CNT types, CNT modification, stabilization and loadings. Moreover, a model for the electric conductivity of CNT reinforced composites above the percolation threshold is presented. This model describes the electric conductivity as a function of the composite structure including CNT content as well as a theoretical value for the number of contacts within such a composite network. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanotubes Electrical properties Mechanical properties Thermal properties Modeling
1. Introduction Since the early nineties carbon nanotubes are attributed as high potential filler material for polymer matrices [1,2]. Based on their extraordinary mechanical, electrical and thermal properties as well as their high aspect ratio and specific surface, CNTs are highly suited for a significant enhancement or modification of polymers. Comparatively low nanotube contents below 1 wt% lead to a relevant increase of the mechanical properties, e.g. Young's modulus, strength and strain to failure [3]. Apart from low nanotube contents, high loadings of carbon based filler materials have a great potential for an extraordinary modification or enhancement regarding thermal, electrical and mechanical properties of polymers used in electronic and lightweight construction industry. For example, processing different material classes in lightweight, electro mobility or aeronautics applications leads to thermal stresses in component assemblies which cause component failure. Hence, the adaption of the coefficient of thermal expansion via incorporated fillers into the polymer matrix is advantageously to handle complex multi-component assemblies. In addition,
* Corresponding author. E-mail address: [email protected] (C. Schilde). http://dx.doi.org/10.1016/j.compscitech.2015.06.013 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
polymers and polymer coatings with a high electrical and thermal conductivity and thus high CNT loadings are of particular importance in these industries. The electric charge and discharge as well as the cooling or heating of components is ensured without increasing the weight of a component or using complex multicomponent assemblies, e.g. lightning protection or de-icing of aircraft components. However, the high aspect ratio and specific surface as well as strong attractive interactions between nanotubes lead to disadvantages in processability. As consequence fast agglomeration and a strong increase of the viscosity can be observed [4,5]. The effects of nanotube synthesis, surface functionalization and further processing on various composite properties are represented in a huge number of articles [5,6]. The challenge of embedding nanotubes in polymer matrices is to obtain a homogenous and stable suspension of exfoliated CNTs while concurrently ensuring processable suspension viscosities [7]. As a result various production processes were established which are more or less limited in the maximum content of carbon nanotubes, the feasibility of exfoliation and/or the destruction of CNTs. Moreover, aspects regarding the composite production have to be considered. Exfoliated CNTs cause a strong increase in suspension viscosity. Mechanical processes are commonly favored to embed synthesized and optionally
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functionalized CNTs in the polymer matrix during the production of cabon nanotube reinforces composites [8], e.g. shear, kneading or ultrasonic dispersion devices. These dispersion devices are restricted in their exfoliation performance and their ability to process highly viscous systems, i.e. high filler contents [9e13]. Typically, CNT contents of less than 10 wt% can be realized. In general, length reduction takes place by ultrasonic treatment. An increased amount of exfoliated CNTs without length reduction can be obtained by calender based processes [3]. An schematic overview of the common production methods of CNT containing polymer composites is given by Bauhofer and Kovacs [14]. Similar mechanical processes are applicable for dispersing nanotubes in a solvent phase instead of polymer phase followed by a mixing with the polymer matrix and evaporation of the solvent phase [15]. This production process is particularly advantageous for thermoplastic materials. The production process for highly loaded epoxy based composites presented in this study is a modification of the commonly used solvent based approach. A method for the quantitative characterization of the amount of agglomerated and exfoliated CNTs in epoxy polymers, dispersed by various dispersion devices, was developed by Nadler et al. [16]. Except from this study, the characterization of the amount of agglomerated and dispersed nanotubes is typically done qualitatively by TEM or SEM pictures of the cured composite. 2. Experimental The production process for high loaded epoxy based composites presented in this study is shown in Fig. 1 (top). In a first step, the
CNTs as well as the intermixture of epoxy resin HexFlow RTM6 (company Hexcel Corporation) and the corresponding diamine based hardener were dissolved in a large portion of solvent phase using the ultrasonic dispersing device UP400S (company Hielscher, sonotrode H40, power of 150 W). The epoxy resin HexFlow RTM6 system is often used in aerospace and lightweight construction applications. Eventhough no RTM process (Resin Transfer Molding) was applied in this study, HexFlow RTM6 was used for the experiments because of its high quality and applicability in lightweight construction and aerospace industry. The used solvent was butanone. The suspension was tempered to a temperature of 40 C and dispersed for 120 min. The CNT content in the suspension was set up to 1 wt% depending on the target CNT content in the cured composite. The stability of these suspensions against reagglomeration was characterized via transmission measurements as function of the sample height and measurement using the Turbiscan Lab (company Quantachrome, 25 C). Subsequently, the solvent phase was evaporated for 90 min in a lab kiln at 55 C. The remaining suspensions with solids content between 10 wt% and 80 wt% (approved via thermal gravimetric analysis using the TGA/SDTA 851, company Mettler Toledo, oxygen flow, 20e700 C at 20 C min1) are extremely viscous and possess processing properties similar to solid materials. These suspensions were ground in a steel mortar to particle sizes between 80 mm and 200 mm depending on the CNT content, type, modification and stabilization (ground suspensions were measured via laser diffraction, Helos, company Sympatec). In principle an increase in CNT content leads to smaller particle sizes due to a more brittle processing behavior of the epoxy-CNT suspension. However, the particle size of the epoxy-
Fig. 1. Production process for highly loaded epoxy composites (top). Milled CNT-thermosetting resin suspension, produced composite cubes and high-resolution SEM of a composite fragment containing 50 wt% of CNT's (below).
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CNT suspension has no significant influence on further processing and the resulting composite properties. The ground suspension was further processed by hot press molding at a temperature of 150 C and a specific molding pressure of circa 6 kN/cm2. The curing time was set to 180 min. The ground CNT thermosetting resin suspension with granular processing properties as well as two manufactured composite cubes (untreated and laser polished surface) and a high-resolution scanning electron microscope image of a composite fragment containing 50 wt% of CNTs are shown in Fig. 1. The composite surface was polished (surface roughness of around 50 nm measured via atomic force microscopy) for further composite characterization, e.g. measurement of the electrical, mechanical and thermal properties. During composite production multi walled carbon nanotubes (MWCNTs) Baytubes CP150HP with a carbon purity of >99%, a bulk density of 130e150 kg/m3, an outer diameter of 13e16 nm and a length of 1e10 mm were dispersed in the solvent phase. The agglomerate sizes are specified to 0.1e1 mm. Additionally, Baytubes C70P (carbon purity of >95%, bulk density of 45e95 kg/m3, outer diameter of 13e16 nm, length of 1e10 mm), carboxylated and non-modified nanotubes of the company Carbon Future (carbon purity of >90%, bulk density of 28 kg/m3, outer diameter and length not specified [17]) and single walled carbon nanotubes (SWCTs) produced by the Fraunhofer IWS Dresden (50%e70% SWCNTs, 20% catalysts, 10%e30% nano-graphite, outer diameter of 1e1.6 nm [18]) were investigated. The mechanical properties were measured via nanoindentation (Triboindenter, company Hysitron) using a Berkovich tip. These properties describe the composite deformation behavior at the micro scale. In order to increase the confidence level, 40 measurements of each sample surface were performed. Due to the high measurement accuracy of the indentation device, a distribution of the micromechanical properties of the composite surface can be obtained which is characterized by the standard deviation [19e21]. The electric conductivity was measured using a material testing machine (Zwick Z020, company Zwick, 50 N, 150 kPa) and an universal calibrator (Digistant 4422, 20 mA). The cylindrical sample height was 10 mm and the diameter 20 mm. The coefficient of thermal expansion was measured using a Bohlin Gemini 2 (company Malvern instruments, 3N, gap control). The thermal properties in this study were determined by the progression of the surface temperature as function of the measurement time (heat source with a temperature of 160 C on the opposite sample surface of a cylindrical sample with a diameter of 20 mm and a sample height of 10 mm). To avoid thermal radiation and convection an isolating foam glass specimen holder was used enclosing the composite sample. As reference values, samples made of steel, brass and neat epoxy resin were measured. Hence, the measurement results for the thermal conductivity measurements can be compared relatively. 3. Results and discussion Depending on the properties of the solvent or epoxy matrix, carbon nanotubes tend to reagglomerate or flocculate. Particularly at low CNT content, the stability of the carbon nanotubes in the solvent phase has a significant effect on the resultant composite properties. Fig. 2 shows the transmission of an epoxy-solvent-CNT suspension as function of the sample height and measurement time without stabilization (left) and with the stabilizing additive Disperbyk 2150 (right, 0.5 wt% additive) (measured via Turbiscan Lab, company Quantachrome). This additive is a suitable acrylic copolymer for the stabilization of CNTs and inorganic nanoparticles in non-aqueous solvents [16,22]. Typically, stabilized CNT suspensions are nontransparent. Without stabilization, the CNTs form large flocculates/agglomerates which sediment with increasing
185
measurement time. Thus, the transparency of non-stable CNT suspension increases. The effect of the suspension stability on the mechanical, electrical and thermal properties is exemplarily demonstrated in the following. For industrial application the reduction of shrinkage of composites, their thermal expansion, electric and heat conductivity as well as mechanical properties, such as Young's modulus or tensile strength, are of special interest [6,8,14]. In this study these properties are compared for thermosetting resin based composites produced by the above mentioned process for high particle loadings. Besides the content of CNTs, the influence of the CNT type, surface functionalization and the stabilization on the composite properties is shown exemplarily. The comparability of the results for mechanical, thermal or electrical properties with other studies is restricted due to different CNT materials and surface functionalization, deviating production processes (dispersion and stabilization of CNTs) and different polymer properties [14]. However, the maximum values of the different properties are compared to maximum values of other production processes described in literature. Fig. 3 shows the effect of the CNT content on the Young's modulus and the ratio of plastic to elastic deformation energies. At carbon nanotube content of up to 20 wt%, the Young's modulus stays constant or even decreases slightly. Due to the excellent mechanical properties of the CNTs, the Young's modulus increases strongly with further increasing solids content, reaching a maximum value of 28.02 kN/mm2 (percentagewise an increase of 330%) at a solids content of 60 wt%. Because of air inclusions caused by an insufficient molding pressure during composite processing, the Young's modulus decreases again for very high solids content of 70 wt% and 80 wt%, reaching values below the Young's modulus of the neat epoxy composite matrix. The strength of the ground suspension epoxy-CNT particles increases strongly with increasing CNT solids content. Hence, the pressure during further processing via hot press molding is not sufficient to remove the air within the composite material. The air inclusions within the composite structure are confirmed by the strong decrease in the composite densities given in Fig. 4. The ratio of plastic to elastic deformation energies shown in Fig. 3 is used to describe viscoplastic properties of composites, coatings or particulate structures [20,21,23]. At low carbon nanotube content, the local composite deformation behavior is only slightly influenced. Due to the excellent mechanical properties of the CNTs [5], the elastic deformation energy increases strongly with increasing solids content, reaching a minimum ratio of plastic to elastic deformation energy at 70 wt%. However, the slight increase in the elastic deformation behavior between 60 wt% and 70 wt% as well as the strong increase in the plastic deformation behavior at 80 wt% is caused by air inclusions generated during composite processing. The corresponding mechanical properties for the various CNT types, surface functionalizations and stabilizations are given in Table 1. Because of the extremly high CNT content, an additional stabilization of the carbon nanotube material in the solvent matrix by additives (Disperbyk 2150) did not lead to an increase in the resultant mechanical properties. The Young's modulus of the stabilized composite is, indeed, decreasing eventhough the percentagewise standard deviation indicates a more homogeneous distribution of the mechanical properties within the composite matrix. Due to a slightly improved stabilization, similar results can be obtained by processing carboxylated carbon nanotubes. For SWCNTs a reduction in the Young's modulus can be observed, by reasons of a higher specific surface area at similar CNT content. Consequently, the strength of the ground epoxy-CNT particles increases and the air inclusions caused by composite processing
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Fig. 2. Effect of stabilization of the carbon nanotube material in the solvent matrix (butanone) by additives (acrylic copolymer Disperbyk 2150 [16,22]). Transmission of epoxysolvent-CNT suspension (40 wt%) as function of the sample height and measurement time without stabilization (left) and with the stabilizing additive Disperbyk 2150 (right, 0.5 wt% additive).
Fig. 3. Effect of the CNT content on Young's modulus and the ratio of plastic to elastic deformation energies.
Fig. 4. Effect of the CNT content on composite density.
dominate the mechanical behavior at lower CNT content. This is confirmed by a slight increase in the plastic deformation behavior of the composite material (ratio of plastic to elastic deformation energy). For comparison, Table 2 shows exemplary the effect of different production processes and CNT content on the minimum and maximum reachable Young's modulus of thermosetting resin based CNT composites according to Loos et al. [6]. In general, the mechanical properties as well as the maximum realized carbon nanotube content of the production process for highly loaded epoxy based composites presented in this study are higher compared to composites produced by other production processes. An exception is the uncommon super aligned process of Cheng et al. [24]. For this process, carbon nanotubes were synthesized on silicon wafers by low pressure vapor deposition and stacked together to generate CNT preforms. These preforms consist of several thousand sheets, which were inserted in the mold of an RTM process. Thus, a high relative increase in mechanical and electrical properties can be obtained using low viscous epoxy matrices. In summary, the mixture of exfoliated and agglomerated CNTs caused by various production processes with different stress mechanisms have a significant effect on thermal, electrical and mechanical composite properties. Regarding the effect of the CNT content on the electric conductivity a similar trend as shown for the mechanical properties can be observed (see Fig. 5). The electric conductivity increases strongly with increasing solids content, reaching a maximum value of 838 S/m at a solids content of 60 wt%. Because of the air inclusions caused by the composite processing, the electric conductivity decreases again with further increased solids content. Similar to the mechanical properties, for SWCNTs a reduction in the electric conductivity can be observed because of a higher specific surface area at similar CNT content, and, thus, an increased amount of air inclusions within the composite structure. An additional stabilization of the carbon nanotube material in the solvent matrix by the additive Disperbyk 2150 did not lead to an increase in the resultant electric conductivity similar to the mechanical composite properties. According to Baumann et al. [14] the maximum conductivities within a composite vary by ten or more orders of magnitude in different matrices, whereas in similar matrices and mass fraction the conductivity using different functionalized CNTs varies between one or two orders. Moreover, the surface treatment via the modification with functional groups such as carboxylic groups leads to significant reduction in the composite conductivity.
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Table 1 Effect of the functionalization, stabilization and MWCNT/SWCNT on Young's modulus as well as the ratio of plastic to elastic deformation energies (cm: percentage by weight).
Neat resin Baytubes C150 HP (cm ¼ 50) Future carbon carboxylated (cm ¼ 50) IWS DredenSWCNTs (cm ¼ 50) Baytubes C150 HP (cm ¼ 0.4) Baytubes C150 HP (cm ¼ 0.4) stabilized Disperbyk 2150
Young's modulus [kN/mm2]
Standard deviation [kN/mm2]
Standard deviation [%]
Ratio Wplast/Welast [e]
6.531 13.092 8.816 8.631 10.098 8.627
1.381 3.429 0.672 1.989 2.559 0.801
21.14 26.19 7.62 23.04 25.35 9.28
0.95 0.83 0.77 1.10 1.02 0.99
Table 2 Effect of different production processes and CNT content on the minimum and maximum reachable Young's modulus [6]. Production process
Young's modulus [kN/mm2]
Percentagewise change [%]
CNT content [wt%]
Literature
Shear and ultrasonic dispersion devices
1.77 0.45 0.75 3.5 4.12 4.2 20.4
10 þ294 29 þ6 þ6 þ24 þ716
1 4 8 0.1 1.5 1 16
[11] [13] [10] [3] [25] [24] [26]
Calender based process Super-aligned process
Moreno Marcelino et al. [27] observed a similar trend with increasing degree of surface functionalization using either, carbonyl and carboxyl groups. Besides the functionalized material itself, a significant reduction in the fiber length during the dispersion step of the weaker functionalized CNTs are a possible explanation for the decrease in the electric conductivity of the resultant composites. However, according to the review of Bauhofer et al. [14] neither the surface treatment nor the type of CNTs (SWCNT or MWCNT) show a clear trend on the composite conductivity. When the CNT content exceeds the percolation thresholds [28e31], the electric conductivity within a composite with a nonconductive matrix can be described by the number of particle contacts and the electric conductivity of the particles. The contact probability, pc, of an infinitesimal small CNT surface element, dS, to another carbon nanotube depends on the particle properties, i.e. carbon nanotube surface, SCNT, and the average coordination number, k [32]:
dS pc ¼ k$ SCNT
(1)
Fig. 5. Effect of the CNT content (type, functionalization and stabilization) on the electric conductivity.
The electric conductivity of an infinitesimal surface element, dS, of an carbon nanotube in the measurement direction, z, is, si,z,CNT. The portion of this surface element to the composite conductivity depends on the contact probability, pc
dsz;CNT ¼ si;z; CNT $k$
dS SCNT
(2)
The electric conductivity of an infinitesimal surface element in measurement direction, si,z,CNT, can be derived from the electric conductivity of carbon nanotubes, si,CNT, and projection of infinitesimal surface element, dS, in the intersection plane, dAU (compare Fig. 6):
si;z; CNT ¼ si;CNT $
dAU dS
(3)
The electric conductivity of a single carbon nanotube, si,CNT, or another particle depends on the material itself, the surface functionalization or amount of conductivity additives. In this case the electric conductivity between two CNTs is an averaged value between CNTs within a critical distance in which electron tunneling
Fig. 6. Modeled and measured electric conductivity of the CNT composite.
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occurs [33]. The portion of all particles, i.e. CNTs, in the intersection plane of the composite is proportional to dAU (similar to cV,CNT ¼ 1ε). Where ε is the expected fraction of the matrix material in the cross-sectional area (under the assumption of uniform random packing equal to the expected volume fraction of the matrix material). Thus, the total composite conductivity can be derived from the combination and integration of Eqs. (2) and (3):
si;CNT sz;CNT ¼ ð1 εÞ$k$ SCNT
(4)
For similar carbon nanotube materials, the CNT surface as well as their electric conductivity remains constant (e.g. Baytubes CP150HP). Thus, the influence of the CNT solids content on the electric conductivity of the composite can be described as product of the CNT volume fraction, cV,CNT ¼ 1ε, and the average coordination number, k. According to Klimenko et al. [34] the average coordination number of porous fibrous material is defined as function of the fiber length, lCNT, fiber diameter, dCNT, and the expected volume fraction of the matrix material, 3, as follows
k
4$lCNT ½1 ε$ð1 lnðεÞÞ $ 1ε dCNT
(5)
Since the stress intensity and frequency during the dispersion process using an ultrasonic dispersion device is independent on the solids content [35,36], the reduction in the fiber length due to the dispersion process should be constant for the same type of CNTs. It must be pointed out that the coordination number according to Klimenko et al. is for thick fibrous materials in the size range of several tens of microns in diameter. Thus, the coordination number using this equation is only approximately proportional to the real coordination number between CNTs. In this case the electric conductivity can be described as function of the expected volume fraction of the matrix material by the following equation:
sCNT ¼ A$½1 ε$ð1 lnðεÞÞ
(6)
A
4$lCNT si;CNT $ dCNT SCNT
where the parameter A depends only on the properties of the particulate system. The electric resistance in the contact points of an excellent conductive material is a crucial factor. An increasing number of contact point between particles/fibers in measurement direction due to a decreased particle/fiber size leads to a decrease in the conductivity (due to the electric resistance in the contact points) which has to be taken into account. It must be pointed out, that a theoretical or apparent number of particulate/fiber layers in measurement direction was not taken into account since the fiber length were constant in the experiments using the Baytubes CP150HP. Thus, the effect of the electric resistance in the contact points in measurement direction is constant for all composite samples. The correlation of the electric conductivity calculated by the model Eq. (6) with the measured data is presented in Fig. 6. For the homogeneous composite material with less air inclusions the model fits excellent to the measured data (R2 ¼ 0.9951). As expected, at very high solids content with decreasing composite density, and thus, increasing amount of air inclusions, the actual number of contacts between CNTs deviates strongly from the theoretical considerations. Considering the data of the composite densities and mechanical properties at 60 wt% the increase in the electric conductivity is less than expected. Indeed, electrical properties are much more sensitive in measuring defects or small amounts of air inclusions in composite materials. However, the model seems to be suited for the characterization of the electric conductivity of carbon nanotube composites (assuming a nonconductive matrix), conductive coatings or the electric conductivity of bulk solids. In Figs. 7 and 8, the effect of the CNT content on the linear coefficient of thermal expansion and the thermal conductivity properties of the enhanced composite are presented. The linear coefficient of thermal expansion can be adjusted over a broad range by a reinforcement of the epoxy matrix using high CNT content. Due to the thermal properties of carbon nanotubes, the linear coefficients of thermal expansion of the reinforced composites
Fig. 7. Coefficient of thermal expansion for different CNT content.
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of the functionalized CNTs with the epoxy matrix. Moreover, the interfacial thermal resistance is reduced and the transfer of thermal energy is increased due to stronger CNT-epoxy interactions. 4. Conclusions
Fig. 8. Progression of the surface temperature as function of the measurement time and CNT content (heat source with a temperature of 160 C on the opposite sample surface; cylindrical sample dimensions: 20 mm diameter, 10 mm height).
decrease with increasing CNT content. Thermal expansion properties of metals can be received for very high carbon nanotube content. According to Yuen et al. [37] this effect does not occur at very low CNT content. Besides the linear coefficient of thermal expansion, the thermal conductivity of epoxy resins can be increased significantly using high carbon nanotube loadings (Fig. 8). Similar trends for lower MWCNT loadings below 1 wt% were obtained by Moisala et al. [38] and for SWCNTs by Yu et al. [39]. However, Gojny et al. [40] stated only a slight enhancement of the thermal conductivity by the incorporation of low CNT content into polymer matrices. The surface treatment or stabilization does not show a clear trend regarding the thermal conductivity (see Fig. 9). The addition of SWCNTs or carboxylated CNTs tend to result in higher thermal conductivities at similar carbon nanotube loadings. Yang et al. [41] postulated that this effect is caused by the formation of a heat flow network due to a well dispersion and good compatibility
Besides the surface functionalization and purification, the processing strategy to transfer the carbon nanotubes into the matrix as well as their dispersion and stabilization within the matrix are of major importance. Typically, the maximum carbon nanotube loadings depend strongly on the production process. Based on a modification of a commonly used solvent based approach, a production process for highly loaded epoxy based composites is presented in this study. In this process high loadings between 10 wt% to 80 wt% of CNTs are realized. The resultant mechanical, thermal and electrical properties show a distinct enhancement with increasing CNT content, reaching a maximum Young's modulus of 28.02 kN/mm2 and electric conductivity of 838 S/m at a weight content of 60% wt. For higher CNT content a strong decrease in the mechanical and electrical properties is observed, caused by air inclusions in the composite structure due to the production process. Because of the extremely high CNT content, an additional stabilization of the carbon nanotube material in the solvent matrix by additives did not lead to differences in the resultant composite properties, e.g. mechanical, thermal and electrical properties. In general, this modified process can be adjusted for the embedding of other nano- and microparticle materials in thermoset or thermoplastic materials. Similar approaches for CNT-composite production methods are known for thermoplastic materials [42,43]. A model for the electric conductivity for CNT reinforced composites above the percolation threshold is derived. This model describes the electric conductivity as function of the composite structure including CNT content as well as a theoretical value for the number of contacts within such a composite network with high accuracy. For the linear coefficient of thermal expansion and the thermal conductivity a behavior close to the properties of metals was observed. High loadings of carbon based filler materials have a great potential for an extraordinary modification or enhancement of epoxy resins and other polymers regarding thermal, electrical and mechanical properties. Although the processing of highly loaded composite materials up to 80 wt% is difficult, these composites are relevant for new design and construction concepts, e.g. highly loaded and graduated composites in electronic and lightweight construction industry. Acknowledgment The authors gratefully acknowledge the financial support by the EU within the jointresearch project “NANOKOMP”. Thermal gravimetric analysis was carried out by Mrs. S. Zellmer from the Institute for Particle Technology, TU Braunschweig. Glossary
Fig. 9. Progression on the surface temperature as function of the measurement time, CNT type, stabilization and functionalization (heat source with a temperature of 160 C on the opposite sample surface; cylindrical sample dimensions: 20 mm diameter, 10 mm height).
AU [m2] area of the intersection plane within the composite cm [%] CNT percentage by weight cV,CNT [e]CNT contents by volume dCNT [m] CNT diameter dS [m2] infinitesimal small CNT surface element E [kN/mm2] Young's modulus k [e] average coordination number between CNTs lCNT [m] CNT length pc [e] contact probability between CNTs SCNT [m2]carbon nanotube surface
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t [s] measurement time T [ C] sample temperature Wel [J] elastic deformation energy Wpl [J] plastic deformation energy a [1/K] linear coefficient of thermal expansion 3 [e] matrix contents by volume r [g/cm3]composite density re [S/m] electrical composite conductivity si,CNT [S/m] electric conductivity of CNTs si,z,CNT [S/m] electric conductivity of an infinitesimal CNT surface element si,z,CNT [S/m] modeled electric composite conductivity (measurement direction) References [1] J. Abraham, Functionalization of carbon nanotubes. Friedrich-Alexander-Uni€t Erlangen-Nürnberg, Org. Chem. (2005). versita [2] G.C. Psarras, The ongoing impact of carbon (allotropic forms)/polymer composites, eXPRESS Polym. Lett. 8 (2) (2014) 73. €pke, B. Fiedler, K. Schulte, Carbon [3] F.H. Gojny, M.H.G. Wichmann, U. Ko nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content, Compos. Sci. Technol. 64 (15) (2004) 2363e2371. [4] M. Nadler, J. Werner, T. Mahrholz, U. Riedel, W. Hufenbach, Effect of CNT surface functionalisation on the mechanical properties of multi-walled carbon nanotube/epoxy-composites, Compos. Part A Appl. Sci. Manuf. 40 (6e7) (2009) 932e937. [5] E.T. Thostenson, C. Li, T.-W. Chou, Nanocomposites in context, Compos. Sci. Technol. 65 (2005) 491e516. [6] M.R. Loos, K. Schulte, Is it worth the effort to reinforce polymers with carbon nanotubes? Macromol. Theory Simulations 20 (5) (2011) 350e362. [7] S.S. Rahatekar, K.K.K. Koziol, S.A. Butler, J.A. Elliott, M.S.P. Shaffer, M.R. Mackley, et al., Optical microstructure and viscosity enhancement for an epoxy resin matrix containing multiwall carbon nanotubes, J. Rheol. 50 (5) (2006) 599e610. [8] J.N. Coleman, U. Khan, Y.G. Gk, Mechanical reinforcement of polymers using carbon nanotubes, Adv. Mater. 18 (2006) 689e706. [9] A. Allaoui, S. Bai, H.M. Cheng, J.B. Bai, Mechanical and electrical properties of a MWNT/epoxy composite, Compos. Sci. Technol. 62 (15) (2002) 1993e1998. [10] P. Guo, X. Chen, X. Gao, H. Song, H. Shen, Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites, Compos. Sci. Technol. 67 (15e16) (2007) 3331e3337. ndez-Pe rez, F. Avile s, A. May-Pat, A. Valadez-Gonz [11] A. Herna alez, P.J. Herrerarez, Effective properties of multiwalled carbon nanotube/ Franco, P. Bartolo-Pe epoxy composites using two different tubes, Compos. Sci. Technol. 68 (6) (2008) 1422e1431. [12] L. Sun, G.L. Warren, J.Y. O'Reilly, W.N. Everett, S.M. Lee, D. Davis, et al., Mechanical properties of surface-functionalized SWCNT/epoxy composites, Carbon 46 (2) (2008) 320e328. [13] M.-K. Yeh, T.-H. Hsieh, N.-H. Tai, Fabrication and mechanical properties of multi-walled carbon nanotubes/epoxy nanocomposites, Mater. Sci. Eng. A 483e484 (0) (2008) 289e292. [14] W. Bauhofer, J.Z. Kovacs, A review and analysis of electrical percolation in carbon nanotube polymer composites, Compos. Sci. Technol. 69 (10) (2009) 1486e1498. [15] Y.J. Kim, T.S. Shin, H.D. Choi, J.H. Kwon, Y.C. Chung, H.G. Yoon, Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites, Carbon 43 (1) (2005) 23e30. [16] M. Nadler, T. Mahrholz, U. Riedel, C. Schilde, A. Kwade, Preparation of colloidal carbon nanotube dispersions and their characterisation using a disc centrifuge, Carbon 46 (11) (2008) 1384e1392. €tschke, G. Petzold, Dispersability and particle size [17] B. Krause, M. Mende, P. Po distribution of CNTs in an aqueous surfactant dispersion as a function of ultrasonic treatment time, Carbon 48 (10) (2010) 2746e2754. [18] IWS F-IfW-uS, Hochdurchsatz-Synthese von SWCNTs - Qualitativ hochwertige €hren und deren Anwendungen Dresden, einwandige Kohlenstoff-Nanoro 2009. [19] N. Barth, C. Schilde, A. Kwade, Influence of particle size distribution on micromechanical properties of thin nanoparticulate coatings, Phys. Procedia
40 (2013) 9e18. [20] C. Schilde, A. Kwade, Measurement of the micromechanical properties of nanostructured aggregates via nanoindentation, J. Mater. Res. 27 (4) (2012) 672e684. [21] C. Schilde, B. Westphal, A. Kwade, Effect of primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates, J. Nanoparticle Res. 14 (3) (2012) 1e11. [22] H. Nolte, C. Schilde, A. Kwade, Determination of particle size distributions and the degree of dispersion in nanocomposites, Compos. Sci. Technol. 72 (2012) 948e958. [23] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Mater. Sci. Eng. Rep. 44 (2004) 91e149. [24] Q.F. Cheng, J.P. Wang, J.J. Wen, C.H. Liu, K.L. Jiang, Q.Q. Li, et al., Carbon nanotube/epoxy composites fabricated by resin transfer molding, Carbon 48 (1) (2010) 260e266. nez-Su [25] A. Jime areza, M. Campoa, I. Gaztelumendib, N. Markaideb, M. S ancheza, ~ aa, The influence of mechanical dispersion of MWCNT in epoxy matrix A. Uren by calendering method: batch method versus time controlled, Compos. Part B Eng. 48 (2013) 88e94. [26] K. Jiang, J. Wang, Q. Li, L. Liu, C. Liu, S. Fan, Superaligned carbon nanotube arrays, films, and yarns: a road to applications, Adv. Mater. 23 (9) (2011) 1154e1161. ndez [27] J.E. Moreno Marcelino, E. Vigueras Santiago, G. Lopez-Tellez, S. Herna pez, Chemical functionalization of carbon nanotubes and its effects on Lo electrical conductivity, J. Nano Res. 28 (2014) 51e61. [28] B. Kim, J. Lee, I. Yu, Electrical properties of single-wall carbon nanotube and epoxy composites, J. Appl. Phys. 94 (10) (2003) 6724e6728. [29] M.B. Bryning, M.F. Islam, J.M. Kikkawa, A.G. Yodh, Very low conductivity threshold in bulk isotropic single-walled carbon nanotube-epoxy composites, Adv. Mater. 17 (9) (2005) 1186e1191. [30] C.A. Martin, J.K.W. Sandler, M.S.P. Shaffer, M.K. Schwarz, W. Bauhofer, K. Schulte, et al., Formation of percolating networks in multi-wall carbon nanotube-epoxy composites, Compos. Sci. Technol. 64 (15) (2004) 2309e2316. [31] J. Li, P.C. Ma, W.S. Chow, C.K. To, B.Z. Tang, J.K. Kim, Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes, Adv. Funct. Mater. 17 (2007) 3207e3215. [32] H. Rumpf, Zur Theorie der Zugfestigkeit von Agglomeraten bei Kraftübertragung an Kontaktpunkten, Chem. Ing. Tech. 42 (8) (1970) 538e540. [33] J.Z. Kovacs, B.S. Velagala, K. Schulte, W. Bauhofer, Two percolation thresholds in carbon nanotube epoxy composites, Compos. Sci. Technol. 67 (5) (2007) 922e928. [34] V.N. Klimenko, A.G. Kostornov, Y.N. Kryuchkov, Coordination number and bulk porosity of porous fibrous materials, Powder Metall. Metal Ceram. 38 (1999) 202e204. [35] C. Schilde, C. Mages-Sauter, A. Kwade, H.P. Schuchman, Efficiency of different dispersing devices for dispersing nanosized silica and alumina, Powder Technol. 207 (2011) 353e361. [36] H. Nolte, Production of Nanocomposites by Dispersing Nanoparticles in Epoxy Resin Based Polymers, Institute for Particle Technology, TU Braunschweig, 2013. [37] S.M. Yuen, C.C.M. Ma, H.H. Wu, H.C. Kuan, W.J. Chen, S.H. Liao, et al., Preparation and thermal, electrical, and morphological properties of multiwalled carbon nanotube and epoxy composites, J. Appl. Polym. Sci. 103 (2) (2007) 1272e1278. [38] A. Moisala, Q. Li, I.A. Kinloch, A.H. Windle, Thermal and electrical conductivity of single- and multi-walled carbon nanotube-epoxy composites, Compos. Sci. Technol. 66 (10) (2006) 1285e1288. [39] A.P. Yu, M.E. Itkis, E. Bekyarova, R.C. Haddon, Effect of single-walled carbon nanotube purity on the thermal conductivity of carbon nanotube-based composites, Appl. Phys. Lett. 89 (13) (2006) 1e3. [40] F.H. Gojny, M.H.G. Wichmann, B. Fiedler, I.A. Kinloch, W. Bauhofer, A.H. Windle, et al., Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites, Polymer 47 (6) (2006) 2036e2045. [41] S.-Y. Yang, C.-C.M. Ma, C.-C. Teng, Y.-W. Huang, S.-H. Liao, Y.-L. Huang, et al., Effect of functionalized carbon nanotubes on the thermal conductivity of epoxy composites, Carbon 48 (3) (2010) 592e603. [42] H. Aarab, M. Baitoul, J. Wery, R. Almairac, S. Lefrant, E. Faulques, et al., Electrical and optical properties of PPV and single-walled carbon nanotubes composite films, Synth. Met. 155 (1) (2005) 63e67. [43] M.S.P. Shaffer, A.H. Windle, Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites, Adv. Mater. 11 (11) (1999) 937e941.