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European Journal of Scientific Research ISSN 1450-216X Vol.29 No.1 (2009), pp.13-21 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm

Electrical Conductivity Behaviour of Chemical Functionalized MWCNTs Epoxy Nanocomposites Abu Bakar Sulong Dept. of Mechanic and Materials Engineering Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia E-mail: [email protected] Tel: +603-8921-7029; Fax: +603-8925-9569 Nurhamidi Muhamad Dept. of Mechanic and Materials Engineering Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia Jaafar Sahari Dept. of Mechanic and Materials Engineering Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia Rizauddin Ramli Dept. of Mechanic and Materials Engineering Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia Baba Md Deros Dept. of Mechanic and Materials Engineering Faculty of Engineering & Built Enviroment Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia Joohyuk Park Dept. of Mechanical Engineering, School of Engineering Sejong University 98 Gunja dong, Gwanjin gu 143-747 Seoul South Korea E-mail: [email protected] Tel: +822-3408-3771; Fax: +822-3408-3333 Abstract Few attempt to study the effects of chemical functionalized carbon nanotubes (CNTs) on the electrical conductivity of nanocomposites, instead to increase dispersion and interfacial bonding strength between CNTs and polymer matrix for improvement of mechanical properties. Therefore, in this study the electrical conductivity of two types’ chemical functionalized (Carboxylated and Octadecylated) multi-walled carbon nanotubes (MWCNTs), non ionic surfactant additive MWCNTs and as produced MWCNTs epoxy

Electrical Conductivity Behaviour of Chemical Functionalized MWCNTs Epoxy Nanocomposites

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nanocomposites are investigated as function of MWCNTs concentration. As produced MWCNTs and surfactant additive MWCNTs nanocomposites gave higher electrical conductivity than functionalized MWCNTs nanocomposites. However, chemical functionalization of MWCNTs significantly decreased the electrical conductivity of nanocomposites. This behaviour is further investigated through image analysis of powder CNTs and formation electrical conductive pathway network under applied electric field. It can be deduced that the chemically functionalized MWCNTs through severe acidic treatment are not suitable for electrical applications.

Keywords: Nano material filler, functionalizations

conductive

polymer

composite,

chemical

1. Introduction Single walled carbon nanotubes (SWCNTs) consisting of a single graphite plane (so-called graphene) rolled up into a cylinder behave as either a metal or a semiconductor depending on their wrapping angle of graphene sheet and its diameter, while MWCNTs are reported to be always electrically conductive and to have an electrical conductivity approximately 1.85 x 103 S/cm [1-4]. CNTs polymeric composites are aimed at the exploitation of the high electrical conductivity of CNTs coupled to high mechanical properties, thermal properties and others unique properties [5-7]. Due to the low filler loading fractions required, the mechanical properties and the surface finish of the composite matrix can be maintained. In contrast, conventional surface coating such as Indium Tin Oxide (ITO) coating offer a high electrical conductivity but very high production cost and low quality of surface finish. Therefore, alternate approach using CNTs as conductive fillers in polymeric matrix promises a unique combination of low cost production, ease processing with high electrical conductivity. Indeed, these materials are increasingly used in the electronics sector, as well as for automotive and aerospace applications, where an electrical conductivity exceeding 10-8 S/cm is required for dissipation of electrostatic charges. Recently, many researchers have been interested in chemical functionalization of CNTs to achieve the better dispersion of CNTs in polymer matrix and increase the interfacial bonding strength between CNTs and polymer matrix [8-13]. From our previous study, incorporated chemical functionalized MWCNTs improve dispersion quality MWCNTs in epoxy matrix and increase the tribology properties of epoxy nanocomposites than As produced MWCNTs epoxy nanocomposites [14,15]. Moreover, Scanning Electron Microscopic (SEM) image analysis of fracture surfaces indicated that chemically functionalized MWCNTs have higher interfacial bonding strength with epoxy matrix than As produced MWCNTs. A high electrical conductivity at low loading concentration of As produced SWCNTs and As produced MWCNTs have been reported [16-19]. However, there were few attentions to study the effects of the chemical functionalization of CNTs on the electrical conductivity of epoxy nanocomposites. Therefore, the electrical conductivity of two types chemical functionalization (Carboxylated and Octadecylated MWCNTs), non ionic surfactant additive MWCNTs and As produced MWCNTs epoxy nanocomposites are investigated in variation of CNTs loading concentration.

2. Experimental The epoxy resin is a bisphenol-A-based epoxy resin (KER 215), which contains mono-epoxidized alcohol as a reactive diluent (Kumho P&B Chemical). The non-MDA aromatic amine curing agent (Amicure 101) is supplied by Daemyung Chemical Tech. The viscosity of the epoxy resin is reported as 0.7~1.1 Pa s and the curing agent as 0.2 Pa s. A lower viscosity of epoxy matrix is selected for better wetting conditions with reinforcement fillers. The reinforcement filler used in this study is MWCNTs.

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Abu Bakar Sulong, Nurhamidi Muhamad, Jaafar Sahari, Rizauddin Ramli, Baba Md Deros and Joohyuk Park

It has been synthesized by thermo-chemical vapor decomposition of hydrocarbon gases. Two types of functionalized MWCNTs were used, Carboxylated MWCNTs (MWCNTs-COOH) and Octadecylated MWCNTs (MWCNTs-CONHCH3(CH2)17) [8,14]. Characterization of As produced MWCNTs and chemical functionalized MWCNTs were conducted by Thermogravimetric (TGA), Fourier Transforminfrared Spectroscopy (FT-IR), Raman Spectrometer and Transmission Electron Microscopic (TEM) [14]. Moreover, the optimization of dispersion time and process had been also discussed elsewhere [14]. The dispersed MWCNTs in epoxy mixture was injected to Teflon mold and cured under pressure with the hot press to minimize existing voids in epoxy nanocomposites. Finally, the bulk MWCNTs epoxy nanocomposites were machined to specimen with dimension of 13 mm (width) x 25 mm (length) x 2 mm (thickness) for the electrical conductivity measurement as shown in Figure 1. The measured surfaces area of specimen as given in Figure 1 were polished with sand paper, and were pasted with uniform thin thickness of high conductive silver paste to minimize contact resistance during measurement. The electrical measurement system used in this study is shown in Figure 2. Figure 1: Specimen for electrical conductivity (a) Pure epoxy polymer, and (b) Epoxy Matrix with MWCNTs (note: upper and bottom surface pasted with silver paste)

Figure 2: Electrical conductivity measurement system specimen for electrical conductivity

3. Results and Discussion Figure 3 is representative obtained results from electrical measurement system given in Figure 2. Figure 3 shown curves of the variation of current intensity as function of voltage. Linear fit for resulted curves had been applied to obtained constant tangent value of incline, represent as resistance (R) which follow Ohm’s Law [16].

Electrical Conductivity Behaviour of Chemical Functionalized MWCNTs Epoxy Nanocomposites

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Figure 3: Representative of variation Current Intensity as function of Voltage results and it’s linear fit lines

The variation of electrical conductivity of epoxy nanocomposites as function of MWCNTs types and loading concentration in weight percentage (wt%) are plotted in Figure 4. Non-conductor material of pure epoxy polymer become conductive as incorporated CNTs. At low loading concentration As produced MWCNTs gave a high electrical conductivity, which similar results were reported elsewhere [16-19]. The electrical conductivity of epoxy nanocomposites are increased with increasing of CNTs loading concentration. In our previous study reported that chemical functionalized MWCNTs improve the dispersion quality and strengthen of interfacial bonding strength with polymer matrix [14,15]. However in this study, incorporated chemical functionalized MWCNTs in epoxy matrix resulted in significantly decreasing of the electrical conductivity of bulk epoxy nanocomposite. For dissipation of electrostatic charge, it is necessary to load more than 1.0 wt% concentration MWCNTs of surfactant additive MWCNTs, As produced MWCNTs and Octadecylated MWCNTs. However, for Carboxylated MWCNTs, it required loading more than 3.0 wt% for dissipation of electrostatic charge. Fractured surface SEM images of As produced and Carboxylated MWCNTs epoxy nanocomposite are given in Figure 5. Even though there are fiber pull-out phenomenon on fractured surface of As produced MWCNTs epoxy nanocomposite, dispersion level for both types of CNTs is in similar manner. Significant CNTs agglomeration also cannot be observed on both types of CNTs, which may play role as electrical conductive pathway in polymer matrix. Therefore, influences of chemical functionalized CNTs on the electrical conductivity behavior are further investigated. For visualize this behavior, an electric field was applied to 0.01wt% of As produced MWCNTs and Carboxylated MWCNTs in ionic alcohol (Isopropyl Alcohol) as electrolyte through 1 mm distance between of two Cupper electrodes, respectively. The movements of CNTs under given applied electric field were observed by an optical microscope.

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Abu Bakar Sulong, Nurhamidi Muhamad, Jaafar Sahari, Rizauddin Ramli, Baba Md Deros and Joohyuk Park

Figure 4: Variation of electrical conductivity of epoxy nanocomposites as function of MWCNTs loading concentration and MWCNTs types.

Figure 5: SEM fractured surface images of (a) As produced, and (b) Carboxylated MWCNTs epoxy nanocomposites.

r Theoretical consideration states that in the presence of an electric field E , each conductive r nanotube experiences a polarization P . This polarization can be divided into two contributing r r component, i.e. one parallel to the tube axis PΠ and one in radial direction P⊥ . The magnitudes of both components depend on the polarizability tensor of the nanotube. For single-wall carbon nanotubes, it has been already been suggested that the static polarizability in the direction of the tube axis is much larger than across the diameter [20]. This polarization leads to a torque N E acting on the nanotube. Under given condition, this torque aligns the nanotube against the viscous drag of the surrounding medium in the direction of the electric filed. An illustration of the behavior of a cylindrical particle exposed to a homogeneous electric field is given in Figure 6.

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Figure 6: Schematic illustration of a polarized cylindrical particle in an electric field.

During the application of a direct current (DC) electric field, a fraction of the CNTs are observed to move forward to anode, under electrophoresis, verifying the presence of negative surface charges. As soon as these CNTs are close enough to the electrode to allow charge transfer, the CNT is discharged and adsorbed onto anode. Tips of CNTs are connected to the electrode then become sources of very high field strength and the location for adsorption of further fillers particles. As a result, ramified CNTs network structures extend away from the anode, eventually reaching the cathode and providing electrical conductive pathways throughout the sample. Above theoretical behavior had been observed at As produced MWCNTs under electrical field as given in Figure 7. A comparable behavior also has been observed for MWCNTs in different solvents [21,22]. Figure 7: Electrical conductivity behavior of As produced MWCNTs in IPA while applied (a) 200V/cm, (b) 400V/cm, and (c) 600V/cm electrical field.

(a) 200V/cm

(b) 400V/cm

(-)

(+)

(-)

(+)

(a) 600V/cm (-)

(+)

The results of CNTs electrical conductive pathways formation while increasing electric field intensity from 200 V/cm to 600 V/cm are given in Figure 8. However, CNTs network structures which work as electrical conductive pathways are not occurred at Carboxylated MWCNTs. It clearly can see that most of Carboxylated CNTs are attached to the anode electrode, where thicker CNTs agglomerations are observed at anode electrode rather than cathode electrode. It assumes that carboxylic groups give unbalance polarization on CNTs walls, where it contributes more negative ion on CNTs surface. This lead most of the Carboxylated CNTs have more tendencies attracted and attached to the anode rather than formation of electrical conductive pathways as observed in Figure 7. Without formation electrical conductive pathways, it wills results on no electric current transfer

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Abu Bakar Sulong, Nurhamidi Muhamad, Jaafar Sahari, Rizauddin Ramli, Baba Md Deros and Joohyuk Park

between electrodes. During introduction of carboxylic group (COOH-) through chemical reaction by refluxing inside concentrated acidic solution may cause the physical structure damage to the wall of CNTs. These carboxylic groups are then chemically reacted with octadecyl-amine to form Octadecylated MWCNTs, which able to reduce the polarization effect by resulted in slightly improvement of the electrical conductivity bulk epoxy nanocomposite. Non-ionic surfactant was used as dispersion agent for As produced CNTs give the best electrical conductivity at low concentration than other types of CNTs. Only surfactant additives MWCNTs epoxy nanocomposite become conductor at 0.1 wt%. This behavior is not influence from the surfactant properties because TRITON X-100 which used in this study, is non ionic surfactant where it can’t work as electrolyte for allow electric current flow through it. It assumes that by addition of surfactant to the epoxy matrix give better wetting condition with CNTs than As produced CNTs. Finally, powder As produced and Carboxylated MWCNTs were observed by SEM as given in Figure 9. Figure 9 (b) shown that after chemical functionalization through acidic treatment, length of individual CNTs have been shortened. It assumes that physical structure of As produced CNTs had been damages, where these damages may interrupt the chirality of CNTs. The chirality of the CNT has significant implications on its material properties. It has a strong impact on the electrical properties of the CNT. Graphite is considered to be semi-metal, but it has been shown that CNT can be either metallic or semi conducting, depending in the tube chirality. All the armchair type CNTs are metallic, whilst this is only in the case for chiral or zigzag CNTs if (n-m)/3 is whole integer, otherwise, they are semi conducting [2]. It assumes that longer tubes of As produced CNTs in Figure 9 (a) give more advantages in the formation of electrical conductive pathways, where it ease to connected between individual CNT. Figure 8: Electrical conductivity behavior of Carboxylated MWCNTs in IPA while applied (a) 200V/cm, (b) 400V/cm, and (c) 600V/cm

(a) 200V/cm

(b) 400V/cm

(-)

(+)

(-)

(+)

(a) 600V/cm

(-)

(+)

Electrical Conductivity Behaviour of Chemical Functionalized MWCNTs Epoxy Nanocomposites

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Figure 9: SEM images of powder: (a) As produced, and (b) Carboxylated MWCNTs

4. Conclusion This study’s results indicated that chemical functionalized CNTs significantly decreased the electrical conductivity of epoxy nanocomposites due to unbalance polarization effect and physical structure defects due to severe condition during acidic treatment process. However, As produced CNTs and As produced CNTs with additional surfactant give the higher electric conductivity of epoxy nanocomposites than incorporated with chemical functionalized CNTs. Therefore it can deduce that non chemical functionalized CNTs are more suitable for the electrical applications. But, chemical functionalization of CNT is still necessary for increase dispersion quality and strengthens the interfacial bonding strength with polymer matrix, which more important in structural applications.

5. Acknowledgement The authors are thankful to ILJIN Nanotech for materials. This work was supported by grant No. R012003-000-10-72-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Sulong, A. B. acknowledges a fellowship support from Sejong University, Universiti Kebangsaan Malaysia and Ministry of Science, Technology and Innovation (Malaysia).

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