Preparation and characterization of a double filler polymeric nanocomposite

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 895–899 www.elsevier.com/locate/compscitech

Preparation and characterization of a double filler polymeric nanocomposite Noam Geblinger a, Rajagopalan Thiruvengadathan a, Oren Regev b

a,b,*

a Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

Received 31 October 2005; accepted 5 February 2006 Available online 12 October 2006

Abstract Single walled carbon nanotubes (SWNTs) with an anisotropic morphology (rod) are currently being employed as fillers in polymer matrix to produce novel nanocomposites with enhanced properties and performance in a wide variety of applications. We investigate the effect of the addition of second isotropic (spherical) filler, antimony tin oxide (ATO) particles to the anisotropic SWNT-polymer composites. Cryogenic transmission electron microscope (cryo-TEM) and scanning electron microscope (SEM) were employed to image the aqueous dispersions of the SWNTs–ATO–Latex solution and composite thin films respectively. The SEM imaging of these films shows that SWNTs (rods) tend to aggregate in the presence of ATO clusters, indicating depletion interactions between the rods and the spheres. The difference in the value of electrical conductivity of the films measured along the radial and the tangential directions to the spinning lines is probably due to the preferred orientation of the SWNTs in the matrix during the spin coating process. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; B. Hybrid compounds; B. Electrical properties; B. Scanning electron microscopy (SEM); Double filler

1. Introduction The unique electronic and mechanical properties of carbon nanotubes (CNTs) [1–6], as well as their high aspect ratio and low density [7], have shown great promise for many potential applications, such as electromagnetic interference shielding [8] and nano-electronic devices such as thin-film transistors [9]. In recent years, considerable research has been undertaken on the preparation of novel nanocomposites with CNTs as fillers in composite materials. A wide range of host materials have been used, including polymers, [10,11] ceramics [12] and metals [13]. Ajayan and coworkers [14] first reported preparation of CNT-polymer composites by mechanically mixing multiwalled CNTs and epoxy resin. Since then, many efforts * Corresponding author. Address: Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel. Tel.: +972 8 6472145; fax: +972 8 6472916. E-mail address: [email protected] (O. Regev).

0266-3538/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.02.035

have focused on the design and preparation of advanced composites in which CNTs serve as the reinforcing element in a polymer matrix, in order to obtain a new material with better properties. The host materials include a wide spectrum of polymers ranging from conjugated conductive polymers such as polypyrrole [15] and poly-p-phenylenevynilene [16] to insulating polymers (such as polystyrene [16–19], polymethyl metha-acrylate [19] and polyvinyl alcohol [20]). Different processes such as in situ polymerization, [21] solvent method [17] or melt mixing [22] have been adopted for the preparation of nanocomposites. The main bottleneck in the preparation of CNTs-based composites with extremely low percolation threshold is the bundling of the CNTs in the as-prepared state, preventing their uniform dispersion in the matrix. It is well understood that the degree of the filler’s distribution in the matrix determines the properties of the composite material. The as-prepared CNTs tend to stay as bundles due to strong van-der-Waals interactions [23]. Unbundling of the CNTs can be realized either by their direct functionalization [24]

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or through the ultrasound treatment in aqueous solution with dispersion agents, such as surfactants (sodium dodecyl sulfate (SDS)) [19,25] or polysaccharides (Gum Arabic) [26]. We have used the second approach to exfoliate the bundles and obtain homogeneous dispersions of CNTs in aqueous solution. In previous work we employed a simple procedure to prepare SWNT-polymer composites by direct mixing of the SDS-dispersed single-walled carbon nanotubes (SWNTs) and polystyrene latex [19]. Such a polymeric composite with the anisotropic SWNTs as the only filler in the matrix exhibited a very low percolation threshold of 0.28 wt.%. In the present work we study the effects of the addition of second filler with isotropic morphology to the dispersion of anisotropic SWNTs filler in a polymer matrix. It is expected that the addition of such filler, which has a percolation threshold of its own (although higher than that of the SWNT) is likely to reduce the percolation threshold even further. Antimony tin oxide (ATO) nanoparticles with isotropic morphology are used as the second filler in this work. ATO is a well-known optically transparent and electrically conducting oxide, commonly used in flat panel displays, solar cells and electromagnetic interference shielding applications [27–30]. Sun et al. have reported a very low percolation threshold between 0.05 and 0.075 volume fractions of ATO nanoparticles in a poly(vinyl-acetate-acrylic) copolymer matrix [31,32]. Such a low value of percolation threshold has been attributed to the formation of segregated microstructure in which the ATO phases exist in the boundary regions between the polymer-rich regions. It is speculated that the nature of interactions between the ATO nanoparticles and the SWNTs may determine the overall electrical conductivity of the nanocomposites. The experimental procedure in the preparation of the composite is described in detail in the next section. Briefly, the precursor solution was prepared by simply adding appropriate amounts of ATO nanoparticles’ powder to the stock solutions of SWNTs dispersed in latex. Composite thin films were then deposited on cleaned substrates by spin coating technique. The electrical conductivity was then measured for several films with varying concentration of ATO nanoparticles in the composite along the radial and the tangential directions to the spinning lines. 2. Experimental 2.1. Materials SWNTs AP-Grade (purity 50–70%) was purchased from Carbolex Inc. SDS was purchased from Aldrich. An aqueous solution of carboxylated styrene–butadiene latex nanoparticles (Batch P1587) with a total solid content of 53.5 wt% was received as a gift from Synthomer LTD. The diameter of the latex nanoparticles measured by photon correlation spectroscopy is 180 ± 10 nm (verified by cryogenic-TEM imaging) and the glass transition tempera-

ture (Tg) of the polymer is 12 °C. ATO nanoparticles purchased from Nanophase Technologies Corporation (batch number DDBD0902) have a composition of 10 wt% of Sb3O4 and 90 wt%Sn2O3 with the average particle size of 30 nm. The electrical conductivity of ATO nanoparticles and the SWNTs used as fillers in this work are 0.1 S/cm and 104 S/cm respectively [23,31,32] Deionized (DI) water (18.2 MX cm) was used in the preparation of all the samples. 2.2. Composite preparation Stock solutions of 1 wt% SWNTs dispersed with 1 wt% SDS in DI water were prepared by sonicating the mixture for 20 min (50 W, 43 kHz), followed by centrifugation at 4000 rpm for 15 min using a Megafuge 1.0 (Heraues), to remove the precipitates (mostly metal catalysts and graphitic carbon particles and possibly a few big bundles of SWNTs). The supernatant was separated and filtered through a 0.45 lm PTFE filter (Whatman). The 1:1 by weight (SWNTs to SDS) stock solutions were added to the aqueous dispersions of 40–53.5 wt% of the carboxylated styrene–butadiene latex at different weight ratios. The concentration of the polymer is maintained at 20 wt% for all dispersions. The dispersions were homogenized by sonication for several minutes (50 W, 43 kHz). Subsequently, the as-purchased ATO nanoparticles in dry powder form were added to the dispersions. Prior to the preparation of the composite films, a pure polymer was first spin-coated onto pre-cleaned (1 0 0) p-type silicon substrates at a speed of 2000 rpm for 90 s using a 20 wt% latex dispersion in water. Silicon substrates were cleaned in DI water, methanol, acetone and DI water before drying in air. This polymer coating is found to enhance the surface wetting of the composite precursor solutions. Thus the first layer on the silicon substrate is a polymer layer. Then the CNTs–polymer composite films (with or without the ATO nano-particles) were prepared by spin casting the respective precursor solutions at a speed of 1500 rpm for 90 s onto these polymer coated silicon substrates. 2.3. Characterization Low temperature, cryogenic-TEM (cryo-TEM) technique [33] was employed to image the pure latex dispersion and the SWNTs–Latex–ATO precursor solution. Sample preparation was carried out using a vitrobot [34] at room temperature. A drop of the solution was deposited on a TEM grid coated by a holey carbon film (lacey carbon, 300 mesh, Ted Pella, Inc), automatically blotted with a filter paper, and plunged into liquid ethane at its freezing point. The vitrified samples were stored under liquid nitrogen before transfer to a TEM (Technai 12, FEI) operated at 120 kV using a Gatan cryo-holder (626) for imaging at 98 K in low-dose mode and with a few micrometers underfocus to increase phase contrast. Images were recorded on

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a Gatan 794 CCD camera and analyzed using Digital Micrograph 3.6 software. Surface morphologies of these composite films were also imaged using a scanning electron microscopy (JEOL SEM7400 F) in secondary electron mode. For conductivity measurements, metal contacts of 1 mm length and 0.1 mm width were deposited on the top surface of the composite films by evaporating gold (150 nm thickness) through a mask. The spacing between the contacts was 0.1 mm. I–V curves were measured by contacting tungsten coated electrodes to the gold contacts using a Keithley electrometer 6430 in two probe configuration. 3. Results and discussion Cryo-TEM observation of the composite solution (Fig. 1a) confirms that the different components are homogenously dispersed in the aqueous medium. The latex particles are monodispersed and the exfoliated SWNTs are integrated onto it with clusters of the ATO nanoparticles surrounding the SWNT–latex composites as clearly imaged in Fig. 1a. A micrograph of a solution containing only the pure latex dispersion is shown in Fig. 1b. Thin films of the composites with and without ATO particles were imaged by SEM (Fig. 2). The SWNTs are well dispersed within the polymer matrix when no ATO particles are present (Fig. 2a). No aggregates were observed and the SWNTs were homogeneously distributed, with most of them touching each other creating a network in the polymer matrix. Upon addition of ATO nanoparticles (spheres), the clustered spheres and the SWNTs (rods) tend to form aggregates (Fig. 2b). Both SWNTs and ATO nanoparticles are both hydrophobic in nature. The reasons for such aggregation may be either due to the surface chemistry of the fillers or to the attractive forces related to the depletion interaction between the rods and the spheres (vide infra). Since we directly add the appropriate amounts of as-purchased ATO nanopowder (without any surface

Fig. 2. SEM micrographs of composite of thin films consisting of polymer and SWNT (a) without ATO—polymer:SWNTs = 100:2, and (b) with ATO – polymer:SWNTs:ATO = 100:2:1.35. The white arrows point at representative SWNTs. Bar = 1 lm.

Fig. 1. Cryo-TEM micrographs of solution of (a) polymer:SWNTs:ATO—100:1.5:0.15 by weight, Solid content: 2.89 wt%. The round dot (black arrow) at the head of the nanotube is Ni–Y catalyst particle from which the nanotube is grown. An ATO particle is indicated by a white arrow. (b) Polymer latex nanoparticles dispersed in water. Bar = 200 nm.

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Resistivity (log(ohm cm))

modification) to the dispersion containing SWNTs and latex, it is more reasonable to believe that the observed aggregation is induced by the depletion interaction between the isotropic ATO nanoparticles and anisotropic SWNTs than the surface chemistry of the fillers. The phenomenon of aggregation in colloidal suspensions containing mixtures of particles with spherical and rod-like morphology is a topic of deep interest to contemporary scientific community as reflected in a wealth of theoretical and experimental works reported in recent years [35–39]. Aggregation and phase behaviour in a mixed colloidal system are mainly affected by the size (diameter) and shape (aspect ratio) of the constituent particles and their concentration [37,38,40]. This could originate from the high aspect ratio of SWNTs (in the range of L/ D = 2000–3500, L—NT length and D—diameter) and its resultant interaction with very small diameter, r, of the ATO nanoparticles (r = 30 nm). To prevent the aggregation of the fillers and achieve low percolation threshold in a double-filler composite, in-depth studies are essential to probe the nature of attractive interactions between the extremely long SWNTs and the spherical ATO nanoparticles. Both L/D and L/r values in our system are much higher and have not yet been explored in the theoretical calculations reported in the literature. The aggregation of the SWNTs and hence the absence of net formation of SWNTs is expected to influence the macroscopic properties of the composites, such as electrical conductivity. Electrical conductivity of three samples having the same concentration of SWNTs (1 wt%) but different concentrations of ATO nanoparticles was measured (Fig. 3). The resistivity is found to increase with increasing ATO concentration from 0 to 1 wt%. These results are in line with the SEM micrographs showing aggregation of

the SWNTs around the ATOs clusters. The aggregation results in an increase in the percolation threshold and decrease in the conductivity. Another interesting result is that the resistivity in the direction that is tangent to the spin coating lines (between contacts that are radial to the spinning lines) is half order of magnitude lower than that in the radial direction. This result is nearly the same, independent of the ATO concentration used. This may probably indicate that the SWNTs tend to rearrange in rings around the spinning axis. Earlier works reported in literature also demonstrate such a preferential orientation upon spin coating or drop casting of the suspension of SWNTS in aqueous solutions [25,41–43]. 4. Conclusion Preparation of SWNTs-based polymeric composites has been accomplished by a simple method involving direct mixing of latex dispersion with aqueous dispersion of SWNTs in appropriate concentrations. The effects of the incorporation of second filler, ATO nanoparticles, on the electrical conductivity of the composite are investigated. The ATO nanoparticles are found to induce aggregation with the SWNTs resulting in enhanced resistivity of the composite films. Aggregation is probably due to the attractive forces related to depletion interaction between the rods and the spheres. It is pertinent to carry out thorough theoretical and experimental investigations on the interaction between isotropic ATO particles and anisotropic SWNTs having very high aspect ratio. Optimization of the concentration of SWNTs and ATO nanoparticles is found to tune the electrical resistivity of the composite films. The consistent difference between the resistivity in the tangential and radial direction indicate preferred orientation of the SWNTs in the matrix along the flow during the spin coating process.

4.8

Acknowledgement

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Dr. Ayelet Vilan is kindly acknowledged for running the electrical conductivity measurements. References

4.0

3.6

3.2 0.0

0.2

0.4 0.6 ATO (weight %)

0.8

1.0

Fig. 3. Conductivity measurements results on three thin films containing the same SWNTs concentration (1 wt%) with different ATO nanoparticles concentrations; from 0 to 1 wt%. The squares represent resistivity in the tangent (T) direction while the triangles represent resistivity in the radial (R) direction. The letters refers to the conduction direction and not the contacts orientation.

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