European Polymer Journal 46 (2010) 1833–1843
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Macromolecular Nanotechnology
Isotactic polypropylene/carbon nanotube composites prepared by latex technology: Electrical conductivity study Nadia Grossiord a,d, Mariëlle E.L. Wouters b, Hans E. Miltner c, Kangbo Lu a,d, Joachim Loos a,d, Bruno Van Mele c, Cor E. Koning a,c,d,* a
Eindhoven University of Technology, Laboratory of Polymer Chemistry, Laboratory of Materials and Interface Chemistry, Laboratory of Polymer Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands TNO Science and Industry, BU Materials Technology, De Rondom 1, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands c Vrije Universiteit Brussel, Physical Chemistry and Polymer Science (H.E.M., B.V.M.) and Physical and Colloidal Chemistry (C.E.K.), Pleinlaan 2, 1050 Brussels, Belgium d Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Received 24 March 2010 Received in revised form 18 June 2010 Accepted 23 June 2010 Available online 1 July 2010 Keywords: Nanocomposite Electrical conductivity Isotactic polypropylene Percolation threshold
a b s t r a c t Several series of nanocomposites were prepared using a latex-based process, the main step of which consisted of mixing an aqueous suspension of exfoliated carbon nanotubes (CNTs) and a polymer latex. In the present work, a systematic study on the electrical properties of fully amorphous (polystyrene – PS) as well as semi-crystalline (isotactic polypropylene – iPP) nanocomposites containing either single-wall (SWCNTs) or multi-wall carbon nanotubes (MWCNTs) has been conducted. Percolation thresholds as low as 0.05 wt.% or 0.1 wt.% were observed for SWCNT/iPP and MWCNT/iPP nanocomposites, respectively. The formation of a conductive percolating network at such a low CNT concentration is favored by the high intrinsic conductivity and the low viscosity of the polymer matrix. The electrical percolation threshold of the iPP-based system was found to be lower than its rheological percolation threshold. Beyond the percolation threshold, MWCNT-based nanocomposites generally exhibited higher conductivity levels than those based on SWCNTs, most probably due to the higher intrinsic conductivity of the MWCNTs as compared to that of the SWCNTs. These excellent electrical properties, associated with the strong nucleating effect of the CNTs reported earlier [1,2], render this type of nanocomposites extremely attractive from a technological point of view. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Over the last two decades, carbon nanotubes (CNTs) have become the subject of intensive research, both fundamental and applied. In particular, much attention has been given to their use in polymeric composite materials to harness their exceptional mechanical and electronic prop-
* Corresponding author at: Eindhoven University of Technology, Laboratory of Polymer Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. Tel.: +31 (0) 40 247 5353; fax: +31 (0) 40 246 3966. E-mail address:
[email protected] (C.E. Koning). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.06.009
erties [3]. When the nanocomposite matrix is semi-crystalline, incorporation of (nano)particles such as CNTs frequently aims at modifying the crystallization behavior of the polymer in order to improve its properties, among which its mechanical performance [4,5], and/or to shorten processing cycle times. This way, high levels of mechanical reinforcement can be achieved at low CNT loadings due to the formation of a highly crystalline layer in the immediate vicinity of the CNT walls, ensuring effective interfacial stress transfer [6]. In addition, dispersion of electrically conductive particles into a semi-crystalline (as well as amorphous) polymer matrix also leads to the production of conductive materials.
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For obtaining electrically conductive CNT/polymer composites, the highly conductive CNTs are dispersed into a polymer matrix in order to form a three-dimensional conductive network of CNTs throughout the composite. By adjusting the type and amount of CNTs, plastics exhibiting tunable levels of conductivity can be produced for various applications. For electrostatic dissipation, for example, the conductivity level of the nanocomposite should be in the range between 108 and 103 S/m. On the other hand, for electromagnetic shielding applications, an electrical conductivity in excess of 1 S/m should be targeted. When the conductivity is above 103 S/m, the materials are considered (semi-) conductive [7]. A key issue when aiming for conductive CNT-based compounds, however, is the fact that as-produced CNTs either occur in bundles of hexagonally packed tubes in the case of single-wall carbon nanotubes (SWCNTs) [8], or are highly entangled in the case of multi-wall carbon nanotubes (MWCNTs) [9]. These characteristics constitute one of the main bottlenecks when using CNTs as fillers in polymer matrices. Indeed, CNTs tend to remain bundled or entangled, despite many attempts to homogeneously disperse them, for instance by applying high shear conditions during melt compounding. It has also been demonstrated that the presence of CNT bundles rather than individually dispersed particles reduces their effectiveness as reinforcing agents or as conductive fillers in nanocomposites [10]. Furthermore, the achievement of electrical conductivity is not only a geometrical issue depending on CNT length, orientation or dispersion state, but also strongly depends on the amount of inter-tube contacts and on the nature of the CNT–polymer interface. Several methods were developed over the last years in order to incorporate exfoliated individual CNTs, or at least thin CNT bundles, into semi-crystalline polymeric matrices [11,12]. The most straightforward method consists of directly mixing the polymer and the CNTs, either in the melt phase or by using solvent approaches [13]. In these cases, the CNT–polymer interaction can be improved by modification of the CNT surfaces, for example by CNT wall functionalization aiming at creating covalent chemical bonds between the CNTs and the polymer matrix [6]. A more elegant approach to produce nanocomposites consists of immobilizing catalysts on the CNT surface, and to subsequently carry out the polymerization reaction of, for instance, olefins directly from their surface [14,15]. Finally, certain strategies such as the one presented here involve the use of a third component, typically a surfactant, and are based on latex technology [11]. The key issue in these processes remains the proper mixing of an aqueous colloidally-stable surfactant–CNT dispersion with a polymer latex. The only prerequisite for the preparation of conductive CNT/polymer nanocomposites using a ‘‘latex-based concept” is that the polymer used as matrix is available in latex form. This aspect makes the present approach extremely versatile since any polymer that can be prepared by (mini)-emulsion polymerization, or that can be artificially brought into a latex form, can potentially be used as matrix following the described preparation method. Synthesis of polyolefin latexes is not straightforward, however, and requires the use of water-resistant catalysts
in an aqueous medium under well-controlled synthesis conditions [16]. An alternative route to prepare polyolefin latexes consists of first synthesizing the polymer in a ‘‘conventional” way, and to subsequently bring it into latex form under high shear while stabilizing the resulting polyolefin wax emulsion by use of surfactants. This specific preparation route is generally applied for the preparation of commercial products such as the one studied in this work. In the present paper, we report on the use of a commercial maleic anhydride-grafted isotactic polypropylene emulsion (iPP-g-MA) for the preparation of highly conducting CNT/polymer nanocomposites by means of latex technology. This work therefore also constitutes a demonstration of the fact that the approach initially developed for the preparation of fully amorphous conductive nanocomposites can be successfully extended to commercial latex products in general, and to semi-crystalline polymer matrices in particular. A benchmarking with regard to the achieved electrical properties in these semi-crystalline materials is performed against a well-described fully amorphous model system, i.e., latex-based polystyrene nanocomposites extensively reported earlier [17–20]. With a clear focus on electrical properties, finally, the present paper complements a recent series of publications on CNT/iPP nanocomposites, in which the focus was put on the dramatic effect of CNTs on the crystallization behavior of the iPP matrix [1], as well as on the particular morphology that their presence gives rise to [2].
2. Experimental 2.1. Materials Two types of CNTs produced by a modified gas process based on chemical vapor deposition were studied: thin MWCNTs (Nanocyl-3100, batch 060213) provided by Nanocyl S.A. (Belgium), and HiPCO SWCNTs (Batch PO257) manufactured by Carbon Nanotechnology Inc. (now merged with Unidym, Inc., USA). According to the manufacturer, the impurities in HiPCO SWCNTs are composed of 5 wt.% of small iron catalyst particles, embedded in thin carbon shells distributed throughout the sample, as well as in the CNTs themselves. The used MWCNTs contain less than 5 wt.% of catalyst impurities as well as a low fraction of amorphous carbon. Both SWCNTs and MWCNTs were purified by the suppliers by a mild non-oxidative acid treatment. As opposed to purification procedures involving the use of oxidative acids [21], this mild treatment has been reported to allow the successful removal of transition metal catalyst particles while preserving the CNT wall structure [22,23]. Styrene (99%) and sodium carbonate (Na2CO3, 99%) were purchased from the Aldrich Chemical Co. Sodium dodecyl sulfate (SDS, 90%) and sodium persulfate (SPS) were supplied by the Merck Chemical Co. All experiments described were carried out with demineralized water. Surface energy determination of the nanocomposite films was conducted using ultrapure water (resistivity 18.2 MX) and
diiodomethane (99%) provided by Aldrich Chemical Co. For surface tension determination of the polystyrene melt, polystyrene GPC standards were purchased from Toyo Soda Manufacturing Co. Ltd. (Japan). A maleic anhydride-grafted isotactic polypropylene emulsion (iPP-g-MA), with trade name PriexÒ 801 was provided by Solvay S.A. (Belgium) and was used as the polymer matrix in one series of nanocomposites. The weight average molar mass (Mw) of the iPP-g-MA before emulsification is in the range of 50,000–60,000 g/mol, with a polydispersity index of about 4 (as measured by GPC in trichlorobenzene at DSM Resolve, The Netherlands, using linear polyethylene standards). About 29% of this iPP-g-MA has a molecular weight below 20,000 g/mol. The Priex emulsion has a solid content of 30–35 wt.% and contains a mixture of four different, SDS-like anionic emulsifiers and other sodium salts, part of which is responsible for the intrinsic anti-static behaviour and for the enhanced conductivity of the dried film (the total amount of additives with respect to the dry iPP-g-MA content is in the range of 20 wt.%, according to thermogravimetry experiments carried out on dried samples). 2.2. Instrumentation The low-shear viscosity of the polystyrene and polypropylene matrix materials was determined at 180 °C and 170 °C, respectively, on a TA Instruments AR-G2 rheometer fitted with 25 mm stainless steel parallel plates. Measurements were conducted in oscillatory/steady shear mode in the Newtonian region. Rheological measurements on the CNT composites were performed in oscillatory mode on an Anton Paar MCR301 rheometer at 170 °C under nitrogen atmosphere using a parallel plate geometry. Frequency sweep experiments between 0.6 and 300 rad/s were carried out at amplitudes that were chosen from the linear viscoelastic range. The products were allowed to equilibrate to the temperature of measurement for 10 min before the measurement was started. The determination of surface tension of the polymer melt as a function of temperature was carried out using a conventional contact-angle microscope (G10, Krüss, Hamburg). A drop of the polymer melt (typically 10 mg) was deposited onto a sample holder at elevated temperature in a measurement chamber equipped with a heating element (Linkam, TMS93). The temperature was subsequently further raised at a heating rate of 10 °C/min until flow and equilibration of the polymer. The polymer was kept isothermally for 30 min at each measuring temperature prior to being cooled down to the next temperature at a rate of 5 °C/min. During each isothermal segment, images of the drop were acquired (1 every 30 s) at a fixed time interval using a Charge Coupled Device (CCD) camera. A drop shape analysis program then fitted the profile of the drop contour to provide contact-angle data, drop volume and surface tension. Typical error margins on the surface tension data are between 0.05 and 0.30 mN/m. The surface energy of the compression molded composites was determined using a goniometer (G10, Krüss) interfaced to image capture software (Drop Shape Analysis 1.90
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software). Contact-angles were determined from sessile drops (approximately 2 lL) of water (Millipore grade) and diiodomethane. The contact-angle was determined during the first 10 s after application of the droplet. The surface energy was calculated using the contact-angle values of water and diiodomethane according to the Owens– Wendt method [24]. The molar mass of the polystyrene (PS) studied was analyzed at 40 °C by Gel Permeation Chromatography (GPC) using a Waters Model 510 pump. Tetrahydrofuran (THF) was used as an eluent and the elution volumetric flow rate was maintained at 1.0 mL/min. The measurements were carried out with a refractive index detector Waters Model 410, and a Model 486 UV detector operating at 254 nm. Data acquisition and processing were performed using Waters Millennium32 (v3.2 or 4.0) software. Calibration was done using PS standards supplied by Polymer Laboratories, Inc. (USA). Dynamic light scattering (DLS) measurements were performed after sonication under conditions described by Badaire et al. [25], the only difference being that a laser wavelength of 532 nm was used in the present study. The particle sizes of the PS latex and of the MA-g-iPP emulsion were measured using a Malvern 4700 DLS (Dynamic Light Scattering) particle size analyzer. Thermogravimetry experiments were carried out on a TA Instrument Q500 TGA under dry air flow (25 mL/min). Typically, 1 mg of product was placed in a high temperature platinum crucible and heated from room temperature up to 900 °C at a rate of 5 °C/min. Four-point conductivity measurements [26,27] were directly performed with a Keithley 6512 Programmable Electrometer, which was used either alone or in combination with a Keithley 220 Programmable Current Source. Measurements were performed directly on the surface of the compression molded nanocomposite films. The contact between the sample and the measuring device was improved by the use of a colloidal graphite paste provided by Electron Microscopy Sciences (USA). At least ten conductivity measurements were performed on each sample by choosing different points on the surface and on both sides of the nanocomposite films, and the final resistance of each sample was determined by averaging the measured values. 2.3. Procedures 2.3.1. CNT exfoliations Typically, 0.2 wt.% of MWCNTs (resp. 0.5 wt.% SWCNTs) was mixed with 0.4 wt.% of SDS (resp. 1 wt.% SDS) in water. The mixture was sonicated at 20 W (Vibracell VC750) until maximum debundling was reached. This maximum was determined by an experimental technique based on UV– vis spectroscopy, which has already been reported elsewhere [28,29]. The time of sonication and the CNT-tosurfactant mass ratio necessary to achieve maximum exfoliation of the CNTs were also preliminarily determined and optimized using UV–vis spectroscopy [29,30]. Note that the need for equal CNT/SDS ratios for both SWCNTs and MWCNTs may appear counter-intuitive in view of the anticipated difference in their specific surface areas. However, this 1:2 ratio has been carefully optimized using a
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meticulous experimental procedure, highlighting the fact that CNT coverage depends not only on the specific surface area alone, but also on surfactant packing density governed by CNT diameter and chirality [31–33].
2.3.3. Nanocomposite preparation After the sonication-driven debundling of the CNTs, the colloidally-stable surfactant–CNT dispersion obtained was then mixed with the iPP-g-MA emulsion or with the PS latex under slow stirring; visual observation during mixing and after equilibration of the system confirmed the absence of destabilization and agglomeration. These mixtures were then freeze-dried (Chris Alpha 2–4). The resulting powder was processed into films by compression molding (Collin Press 300G). The latter consisted of a first short heating of the powder, without application of any pressure, in order to reach the desired working temperature. This heating was followed by a degassing step and two compressions at 40 bars for 20 s. The system was finally pressed at 100 bars for 2 min. The processing temperature was 170 °C for the iPP-g-MA-based nanocomposites and 180 °C for the PS-based ones.
3.1. CNT percolation in PS and iPP-g-MA matrices The so-called percolation theory [34] is generally used to describe the insulator-to-conductor transition in composites consisting of a conductive filler in an electrically insulating matrix. At low filler concentrations, the conductivity remains very close to the conductivity of the pure, electrically insulating polymer matrix, since the fillers only occur individually or in small clusters throughout the matrix. When a critical filler volume fraction, the so-called percolation threshold, is reached, the conductivity of the composite drastically increases by many orders of magnitude with very little increase in the conductive filler loading [34]. It coincides with the formation of a conductive, three-dimensional network of filler particles throughout the continuous polymer phase. Upon further increase of the CNT concentration, the conductivity levels off at a certain value, the maximum conductivity of the composite. Evidence for the formation of conductive CNT networks throughout the iPP-g-MA and PS matrices was obtained by conductivity measurements as a function of filler loading, as shown in Figs. 1 and 2. At low filler concentrations, below the percolation threshold for the CNT/iPP-g-MA nanocomposite series, the conductivity is equal to that of the neat polymer, i.e. 105 S/m, which is already high enough for electrostatic dissipation applications [7]. Actually, iPPg-MA as such can be used as coating on a substrate in order to improve the anti-static properties of the latter [35]. The percolation threshold is reached around 0.11 wt.% (resp. 0.04 wt.%) for MWCNT/(resp. SWCNT/) iPP-g-MA nanocomposites. These values were obtained by taking the midpoint between the last point giving the conductivity of the host materials and the first conductivity having a value above 3 105 S/m due to percolation [18] of MWCNTs (resp. SWCNTs), see Fig. 1 (resp. Fig. 2). Upon further CNT addition, the conductivity levels off around 70 S/m (resp.
10 2
10 0
Conductivity (S/m)
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2.3.2. Emulsion polymerization The emulsion polymerization run was carried out in an oxygen free atmosphere. Prior to polymerization, the styrene was distilled in order to remove the inhibitor. Two hundred and fifty-two grams of styrene was mixed under vigorous stirring with 712 g water in the presence of 26 g sodium dodecyl sulfate (SDS) surfactant and 0.7 g sodium carbonate (Na2CO3) buffer. The reaction was initiated by 0.7 g sodium persulfate (SPS) dissolved in 5 g of demineralized water. The polymerization took place at a constant temperature of 50 °C. The polystyrene obtained possessed mainly high molecular weight polymeric chains (peak molecular weight of about 1000,000 g/mol), with about 20 wt.% of PS chains with a molecular weight lower than 20,000 g/mol.
3. Results and discussion
10 -2 10 -4
10 -6
10 -8
10 -10 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
MWCNT concentration (wt%) Fig. 1. Conductivity (four-point measurements) as a function of MWCNT concentration for: (d) MWCNT/iPP-g-MA and (h) MWCNT/PS nanocomposites. The dashed lines are guides for the eye and do not correspond to any theoretical fitting of the data.
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10 2
Conductivity (S/m)
10 0
10 -2
10 -4
10 -6
10 -8
10 -10 0
0.2
0. 4
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1.0
1. 2
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SWCNT concentration (wt%)
7 S/m) at 2 wt.% of MWCNTs (resp. SWCNTs), irrespective of the type of matrix. Note however that the same CNTs dispersed in a PS matrix following the same experimental procedure exhibit significantly higher percolation threshold values than in the CNT/iPP-g-MA series, namely of the order of 0.55 wt.% for SWCNT/PS systems and 0.81 wt.% for MWCNT/PS nanocomposites. It is worth mentioning at this point that, in the absence of polymer flow, i.e. when the continuous phase of the composite consists of compacted rigid polymer particles [36,37], the fillers are forced to organize into the interstitial space between the polymer particles. The volume of the latter is governed by the size of the polymer particles, yet in all cases will the formation of a segregated network of CNTs reduce the percolation threshold as compared to a more homogeneous CNT dispersion. On the contrary, in the composites described in the present paper, the freezedried polymer particles and CNT powder were processed at temperatures well-above the flow temperatures of the polymer for a sufficiently long time, allowing the integrity of the latex particles to be disrupted and the CNTs to reorganize into an equilibrated system [18], as was previously visually demonstrated using a powerful SEM method [20]. As a consequence, even if the particle size and the particle size distributions of the PS and iPP-g-MA latexes are somewhat different (85 nm vs. 100 nm), this factor is expected to have a negligible impact on the conductivity of the final nanocomposites. Even if conductivity results of CNT/polymer nanocomposites are commonly reported in terms of wt.%, mainly because of lack of precise CNT density values, percolation is considered to be a volumetric phenomenon. In an attempt to fairly compare conductivity results obtained for systems containing either SWCNTs or MWCNTs dispersed in a given polymer matrix, we aim at calculating the percolation threshold values of the various nanocomposites series in terms of vol.% CNT. Reported values of SWCNT density are about 1.5 g/cm3 [12,38,39]. The percolation threshold value at which PS-based (resp. iPP-g-MA-based)
SWCNT nanocomposites start being conductive can thus be found in the range of 38 102 vol.% (resp. 2 102 vol.%). MWCNT densities are more difficult to evaluate since the MWCNTs used in the present study consist of tubes with a variable number of layers. By assuming that the graphitic shell of the MWCNT layers has the density of graphite, one can calculate the density of a MWCNT by using the following formula [40]:
qCNT ¼
qg ðd2 d2i Þ 2
d
ð1Þ
with qg the density of fully dense graphite, which is equal to 2.25 g/cm3, d the outside diameter of the MWCNT, and di its inner diameter. The external diameter of the MWCNTs used in this study can vary from 10 to 30 nm. Similarly, the variation of their inner diameter is roughly of the same order of magnitude, i.e. from 3 to 10 nm. As a result, the density of this batch of MWCNTs is calculated to be comprised in the range 1.7–2.1 g/cm3, as confirmed by the manufacturer, [41] meaning that the MWCNT/PS (resp. MWCNT/ iPP-g-MA) nanocomposites start being conductive at about 40–50 102 vol.% (resp. 4–5 102 vol.%) of MWCNTs. For the sake of clarity and in order to compare the various systems, Table 1 lists the main results obtained with the different series of nanocomposites prepared. 3.2. Conduction mechanism in CNT/polymer nanocomposites: iPP-g-MA vs. PS matrices For both SWCNT- and MWCNT-based nanocomposites, the percolation threshold is indisputably much lower (by about one order of magnitude) in the iPP-g-MA matrix as compared to the PS matrix, see Figs. 1 and 2. In this respect, it is worth noting that the polymer latex used to prepare the CNT/iPP-g-MA nanocomposites is a commercial formulation containing conductivity-enhancing additives. It was specifically developed for anti-static purposes through the
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Fig. 2. Conductivity (four-point measurements) as a function of SWCNT concentration for: ( ) SWCNT/iPP-g-MA and (h) SWCNT/PS nanocomposites. The dashed lines are guides for the eye and do not correspond to any theoretical fitting of the data.
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N. Grossiord et al. / European Polymer Journal 46 (2010) 1833–1843 Table 1 Comparison of the percolation threshold values and of the conductivity levels measured for the different systems studied. Systems studied
Percolation threshold wt.%
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SWCNT/PS SWCNT/iPP-g-MA MWCNT/PS MWCNT/iPP-g-MA
0.55 ± 0.05 0.04 ± 0.004 0.81 ± 0.08 0.11 ± 0.01
Measured conductivity (S/m) at: vol.% 38 2 40–50 4–5
choice of the surfactant system; the conductivity of the original polypropylene material before emulsification, i.e. without conductivity-enhancing additives or emulsifiers, was measured to be lower than 109 S/m, i.e. much lower than the 105 S/m measured for the unfilled PriexÒ 801 (i.e. the commercial iPP-g-MA anti-static formulation), see Fig. 1. The resulting relatively high intrinsic conductivity of the latter – when used as a matrix in CNT composites – obviously allows a larger average inter-tube distance through which electrons can tunnel or hop between CNTs at the junctions of the percolation network [42]. This view is further supported when this result is compared to more insulating polymeric matrices such as PS [43], where a comparatively higher ‘contact resistance’ exists due to the presence of a thin, yet insulating, polymer layer that prevents direct contact between conductive fillers at the network junctions [44–46]. Therefore, at comparable filler dispersion quality as provided by our reproducible latexbased dispersion approach, a first conductive path would be observed at lower filler concentrations for the system based on the intrinsically more conductive matrix polymer. In this respect, the lower percolation thresholds recorded for the CNT/iPP-g-MA systems as compared to those of the CNT/PS systems could be fully anticipated. As an additional factor, the lower melt viscosity of the iPP-g-MA matrix in comparison with the PS system (about 50 Pa s for the former in non-emulsified form vs. wellabove 105 Pa s for the latter; these low-shear viscosities were measured at the compression molding temperature of the nanocomposites, which were equal to 170 °C and 180 °C, respectively) certainly favors the lowering of the experimental percolation threshold, both for SWCNT and MWCNT nanocomposites (see Table 1). A lower melt viscosity of the matrix indeed reduces the average CNT–CNT distance and was reported to lower the percolation threshold of both CNT/and carbon black/polymer composites [18,47,48]. It should be mentioned at this point that the above results are in line with the conclusions drawn in two earlier studies on the performance of MWCNT-PS nanocomposites prepared by latex technology. In the latter, it was indeed shown that increasing the processing temperature [18] or increasing the amount of low molecular weight polymer in the polymer matrix [48], hence decreasing the polymer melt viscosity in both cases, lowers the percolation threshold of the nanocomposites. A further effect that may also in part be responsible for the observed differences between iPP-g-MA and PS nanocomposite systems is related to the extent of CNT wetting achieved in both cases. The surface tension of PS at the compression molding temperature of 180 °C was measured to be around 32.5 mN/m, whereas that of iPP-g-MA at
2
(±3.8) 10 (±0.2) 102 (±5) 102 (±0.5) 102
2 wt.% CNTs
1 vol.% CNTs
9±1 7±3 46 ± 10 68 ± 19
2±1 5±1 46 ± 10 68 ± 19
170 °C was found to be only 21.4 mN/m. MWCNTs and SWCNTs, on the other hand, have reported surface tensions in the range 40–80 mN/m [49–51], but those values are expected to be lowered by the presence of adsorbed surfactants in the present systems, thus promoting CNT wetting by the matrix polymer [52]. However, whatever the true surface tension of the CNTs in the investigated systems, it is clear that a better wetting of the CNTs can be expected with a iPP-g-MA matrix than in the case a PS matrix is used. It has been reported by Miyasaka et al. for carbon/ polymer composites that the lower the surface tension of the polymer matrix is, the lower the critical concentration at which the composite becomes conductive [53]. According to an article written by Miyazaka, the surface tension of the polymer matrix determines the CNT network formation. This paper shows that the percolation threshold increases with increasing adhesion strength. So, with increasing affinity of the matrix for the filler (i.e. increasing difference in surface tension) the percolation threshold is shifted to higher values for Miyazaka’s composites. This result is in apparent contradiction with our observations since, according to these data, one would expect a higher percolation threshold value with iPP-g-MA-based nanocomposites. However, Miyazaka does not take into account the viscosity of the polymer matrix, which also significantly influences the dispersion state of the conductive fillers into the polymer matrix, and thus the value of the percolation threshold [48]. As a result, in the system studied in the present paper, it appears that the viscosity of the melted polymer (at the temperature at which the nanocomposite is processed) has a larger influence on the percolation threshold than the surface tension of the polymer matrix. Finally, it is worth stressing that the observed differences in conductivity behavior of the PS and iPP-g-MA systems do not originate from the amorphous or semicrystalline nature of the matrix polymer. The presence of crystals in the iPP-g-MA system, with a growth direction perpendicular to the individual CNTs [2], is indeed not expected to affect the formation of the electrically conductive CNT network. As a matter of fact, MWCNT nanocomposites based on a maleic anhydride-grafted ethylene–propylene copolymer emulsion (trade name PriexÒ 701) display similar conductivity levels, despite a substantially lower degree of crystallinity (results not shown). This result is in full agreement with earlier data on SWCNT/polyethylene nanocomposites displaying either 33% or 78% crystallinity, [54] (values unaffected by the CNT loading [55]) unambiguously evidencing that there is no preferential accumulation of CNTs in the amorphous phase, which would affect the percolation threshold.
3.3. Conductivity mechanism in CNT/polymer nanocomposites: SWCNTs vs. MWCNTs A SWCNT (and, by extension, each shell of a MWCNT) can be visualized as a sheet of graphene that has been rolled-up; it may be either metallic or semi-conducting [56]. Individual defect-free SWCNTs are seen as ideal model systems for one-dimensional conductors [57] since they exhibit ballistic transport of electrons (i.e. absence of inelastic scattering) over mesoscopic distances along their wall axis. Electron transport through MWCNTs appears to be more complex since some electron transfer between the different layers occurs, thus redistributing the current across the walls. However, it was suggested that the inter-tube transfer in long incommensurate disorder-free MWCNTs becomes negligibly small [58–60] and that, at most, only a few layers close to the outermost shell of the MWCNT significantly contribute to the electron transport. This electronic confinement may stem from the impossibility for electrons to pass through semi-conductive shells, which statistically constitute two-third of the total shells of a MWCNT. As a result, electronic transport along a MWCNT or a SWCNT can be considered to be similar in a first approximation, and conduction mechanisms in MWCNT and SWCNT networks are dealt with in the present discussion as if they were fully analogous. 3.4.1. Percolation threshold 3.4.1.1. Electrical percolation threshold. As shown in Table 1, the percolation thresholds in both SWCNT- and MWCNTbased nanocomposites of iPP-g-MA and PS roughly differ by one order of magnitude, i.e. 2–5 102 vol.% for iPPg-MA nanocomposites, and 40–50 102 vol.% for PS nanocomposites. Several parameters determine the value of the electrical percolation threshold in (nano)composite systems: the aspect ratio of the CNTs, their length distribution, as well as the occurrence of polymer–CNT interactions, of which the influences can be antagonistic. The role of each of these factors is discussed below. For nanocomposites exhibiting comparable filler orientation, the higher the filler aspect ratio (i.e. the ratio of length over diameter), the lower the percolation threshold [61,62]. In the present system, the length of the SWCNTs at the end of the exfoliation process, hence their length when dispersed in the nanocomposite, is 700 (±105) nm, as measured by a method based on DLS experiments [25]. Knowing that the diameter of HiPCO SWCNTs is of the order of 1 nm, it follows that their aspect ratio is about 700. On the other hand, the length of the MWCNTs was measured to be 400 nm (±60 nm) by DLS. Since the diameter of this CNT sample is comprised between 10 nm and 30 nm (according to High Resolution Transmission Electron Microscope (HRTEM) images on several tens of CNTs), the range of aspect ratio values of this MWCNT sample is certainly very broad and can be estimated to be lower than 100, most probably even lower than 50. Simple theoretical models using continuum percolation and considering a CNT network as a group of non-interacting sticks of high aspect ratio a, suggest that the percolation threshold value scales as 1/a by considering that the rod number density is roughly the reciprocal of the excluded volume of the rods
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[63]. Consequently, the percolation threshold of SWCNTbased nanocomposites is expected to be at least seven times lower than for the corresponding MWCNT-based system, assuming that the fillers are not interacting with each other. Note that this factor 7 is certainly overestimated for real systems where CNTs – especially SWCNTs displaying lower bending stiffness – are curved [64] and in which, additionally, matrix-particle and particle–particle interactions can influence the dispersion state of the CNTs [65] – both factors notably capable of increasing the percolation threshold. Furthermore, according to the DLS measurements performed on aqueous SDS–CNT dispersions at the end of the sonication process, the SWCNTs appear to be more polydisperse in length than the MWCNTs, an effect that should contribute to lowering the percolation threshold value of the SWCNT-based system even more [66]. Overall, these considerations point at a lower value of percolation threshold for the nanocomposites prepared with the SWCNTs, in line with our experimental data reported in Table 1. Although the influence of aspect ratio of the CNTs on the electrical percolation threshold indisputably is there, we only see relatively small differences in the percolation thresholds for SWCNT- and MWCNT-based nanocomposites. The earlier mentioned viscosity effect and the favorable combination of surface tensions in the case of iPP and the CNTs play a role as well, but the overruling creason for the low percolation threshold of the PP-based composites is without any doubt the intrinsic conductivity of the iPP we have used in this study, allowing a relatively large distance between neighboring CNTs, and still enabling the electrons to travel through the matrix from one tube to another in a highly efficient way. 3.4.1.2. Rheological percolation threshold. A study on the rheological behavior of CNT-filled iPP-g-MA was conducted in order to obtain some insight into the structure build-up in this kind of nanocomposites. Through the relationship that has been evidenced between electrical and geometrical percolation [67,68], the latter information can indeed be directly related to the conductivity properties of the final nanocomposites. Figs. 3 and 4 display ‘modified Cole–Cole plots’, in which the frequency-dependent storage modulus G0 is plotted against the loss modulus G00 . It has been shown that such plots can be used to identify structural differences between neat and particle-filled polymers [67,69]. Below 0.2–0.4 wt.% SWCNT and MWCNT loading, the curves of G0 vs. G00 are very similar to the ones recorded for the unfilled polymer matrix, with a significant change of slope between low and high frequencies. However, above these critical concentrations, the curves become straight over the entire frequency range studied. Additionally, at SWCNT and MWCNT concentrations beyond 0.2–0.4 wt.%, G0 becomes higher than G00 at low frequencies (i.e. a deviation from classical terminal flow behavior). The higher the CNT concentration, the larger the frequency range in which this observation can be made. Put differently, the crossover point at which G0 is larger than G00 shifts to higher frequencies with increasing CNT concentrations. The shift and the change of slope of the curves suggest a liquid-like to
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1E+4 wt% nanotubes 2,0
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1E+3 1,0
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1E+1 0,2 0,1
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1E-1 1E-1
1E+0
1E+1
1E+2
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Fig. 3. Storage modulus G0 plotted as a function of the loss modulus G00 as determined from frequency sweep experiments for MWCNT/iPP-g-MA nanocomposites at 170 °C. The dashed line corresponds to G0 = G00 .
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composites. However, in absolute values, the rheological percolation is detected at somewhat higher CNT concentration than the electrical percolation, i.e. at about 0.2–0.4 wt.% for both SWCNT and MWCNT composites versus 0.05 wt.% and 0.1 wt.% for SWCNT- and MWCNT-filled iPP-g-MA, respectively. In other words, over an intermediate range of filler concentrations, conductivity enhancement upon addition of fillers is achieved without seriously affecting the rheological behavior of the nanocomposite, which remains comparable to that of the neat polymer matrix. As such, this observation may be of significant interest for thin wall molding and coating applications. It is worth mentioning that the above results are in apparent contradiction with those reported by Du et al. [71] and Kota et al. [72] for MWCNT/poly(methyl methacrylate) (PMMA) and MWCNT/PS nanocomposites, respectively. In the latter, the rheological percolation threshold is significantly lower than the electrical percolation threshold, which is explained by the different CNT–CNT distances required for the two types of percolation threshold. The CNT network can indeed effectively restrain the polymer motion as soon as the inter-tube distance is comparable to the diameter of a random coil of the polymer chains (which is of the order of a couple of tens of nm [73]), whereas the distance between two CNTs needs to be smaller (about 5–7 nm [42,74]) to allow electron hopping or tunneling at the junctions of the filler network. However, the inverse trend is observed in the present case (rheological percolation threshold higher than the electrical one), although the radius of gyration of the polymer is expected to be of the order of 9–10 nm [75,76]. This might possibly be due to the fact that the iPP-g-MA matrix used in the present work is intrinsically conductive, thus promoting electron transfer over inter-CNT distances much larger than in insulating polymer matrices such as PS or PMMA.
0,2
1E+0
1E-1 1E-1
0,0
1E+0
1E+1
1E+2
1E+3
1E+4
Loss Modulus G'' (Pa) Fig. 4. Storage modulus G0 plotted as a function of the loss modulus G00 as determined from frequency sweep experiments for SWCNT/iPP-g-MA nanocomposites at 170 °C. The dashed line corresponds to G0 = G00 .
solid-like transition, which has been reported to correspond to the formation of a CNT network [67,70]. From the above data (Figs. 3 and 4), it appears that a change in the nanocomposite structure occurs at about 0.2–0.4 wt.% for both SWCNT/ and MWCNT/iPP-g-MA nanocomposites. These loadings indeed constitute the rheological percolation threshold, [67] below which the rheological (and processing) behavior of the nanocomposite is very similar to that of the unfilled polymer. Above these filler loadings, the CNTs impede the motion of the polymer. Like other researchers [67,71,72], we found that the rheological and electrical percolation thresholds are of the same order of magnitude for the iPP-g-MA-based nano-
3.4.2. Maximum conductivity The conductivity above the percolation threshold is about one order of magnitude higher for MWCNT-based nanocomposites than for those based on SWCNTs, as shown in Figs. 1 and 2. The resistivity of a nanocomposite consisting of a network of fillers dispersed throughout a polymer matrix is mainly determined by (i) the resistivity of the junctions between conductive particles within the network and (ii) the intrinsic resistivity of the fillers themselves. Much like in an electrical circuit, the overall resistivity of the CNT network is the sum of the resistivities over all CNT–CNT junctions that contribute to electron conduction through the conductive path. Consequently, the fewer the junctions (which can be achieved by using fillers with higher length [77] or by tuning the number of contacts between adjacent particles [78]), the lower the overall resistivity of the network. Accordingly, the higher length of SWCNTs (700 nm vs. 400 nm for the MWCNTs, as determined by DLS) should in principle promote a higher conductivity of the SWCNT-based system as compared to MWCNT-based nanocomposites. Our experimental data, however, show the opposite trend in that the MWCNTbased nanocomposites possess higher conductivities than those based on SWCNTs at equal CNT loading (Table 1). Since this holds for both iPP-g-MA and PS systems, it is
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80
60
(B) 40
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20
0 0
100
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300
400
500
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o
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very likely that this tendency is caused by different intrinsic conductivities of the fillers themselves. In general, the intrinsic electrical conductivity of CNTs strongly depends on their synthesis method and on possible post-treatment steps (such as purification [79] and/or sonication [44,80]). TGA analyses were performed on the as-produced SWCNTs and MWCNTs in order to obtain information with regard to their purity, which indirectly influences their intrinsic conductivity, see Fig. 5. According to TGA data, the weight loss in the temperature range from room temperature to 400 °C amounts to ca. 23 wt.% in the case of SWCNTs. This loss is mainly caused by the degradation of amorphous carbon and of defective CNTs [81]. The additional weight loss from ca. 77 wt.% to ca. 8 wt.% observed in the temperature range between 400 °C and 600 °C is due to the total decomposition of the CNTs [82]. Finally, beyond 800 °C, the residual sample weight levels off at ca. 6 wt.% of the initial value. This residue is composed of oxidized species of the iron catalyst employed during CNT synthesis [83,84]. For the MWCNT sample, on the contrary, no significant weight loss occurs below 550 °C, which indicates that this sample contains a negligible amount of carbonaceous impurities. In addition, the residual catalyst content is extremely low, around 0.3 wt.%. These results hence point at a significantly higher quality, as far as impurities are concerned, for the MWCNTs as compared to the SWCNTs used in this study. Furthermore, MWCNT oxidation only starts around 540 °C, i.e. more than 150 °C higher than for the SWCNTs. In general, a higher oxidation temperature corresponds to less defective CNTs with a higher degree of graphitization, which is without any doubt beneficial for electrical conductivity. In the present comparison the above results may therefore suggest an overall higher wall perfection of the MWCNTs as compared to the SWCNTs.
technology. SWCNTs and MWCNTs were dispersed either into an iPP-g-MA matrix, initially artificially brought into a latex form after polymerization, or into a PS matrix initially prepared by conventional radical emulsion polymerization. Conductivity measurements revealed that especially the iPP-g-MA nanocomposites displayed extremely low electrical percolation threshold values of the order of 0.05 wt.% and 0.1 wt.% for SWCNT- and MWCNT-based systems, respectively. The relatively high intrinsic conductivity, the low viscosity and the low surface tension of the iPP-g-MA matrix leading to a better wettability of the CNTs are believed to be at the origin of this phenomenon. Interestingly, although the percolation thresholds of the nanocomposites prepared using MWCNTs were higher, their maximum conductivity at high loadings was about one order of magnitude higher than for the corresponding SWCNT/polymer nanocomposites, i.e. in the range of 70 S/m instead of 7 S/m. This higher conductivity level is very likely due to the superior intrinsic quality of the MWCNTs used in this study. Moreover, unlike reported in earlier papers, it was found that the rheological percolation threshold of the iPP-g-MA-based nanocomposites was higher than the electrical one, most probably because the intrinsically conductive iPP-g-MA matrix allows for larger inter-nanotube distances to enable electron transport. The exceptional electrical properties of these materials, associated with their favorable rheological properties and strong nucleation induced crystallization by the CNTs (as studied in earlier reports [1,2]), make this type of nanocomposites extremely attractive from a technological point of view. As for the latex-based CNT dispersion method, despite some minor adverse effects that may be related to the use of surfactants, it has been convincingly demonstrated that it provides a valuable alternative to conventional nanocomposites elaboration methods, leading to extremely well-dispersed CNT composites. Acknowledgements
4. Conclusions The present paper reported on several series of CNT/ polymer nanocomposites prepared using a latex-based
This research is part of the research program of the Dutch Polymer Institute (DPI), project number #416. The work of H.E.M. was supported by a grant of the Fund for
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Fig. 5. Thermogravimetric analysis traces in air for as-received CNTs: HiPCO SWCNTs (Curve (A)) and Nanocyl thin MWCNTs (Curve (B)).
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Scientific Research Flanders (FWO, Belgium). The authors are grateful to Solvay S.A. (Belgium) and Nanocyl S.A. (Belgium) for providing iPP-g-MA and MWCNTs, respectively. Cécile Zakri, Maryse Maugey and Philippe Poulin (Centre de Recherche Paul Pascal, CNRS, France) are kindly acknowledged for performing the DLS measurements on aqueous CNT dispersions. We also kindly thank Junrong Yu for many valuable discussions (Donghua University, China).
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