NANO LETTERS
Reinforcement of Polymers with Carbon Nanotubes: The Role of Nanotube Surface Area
2004 Vol. 4, No. 2 353-356
M. Cadek,† J. N. Coleman,*,† K. P. Ryan,† V. Nicolosi,† G. Bister,‡ A. Fonseca,‡ J. B. Nagy,‡ K. Szostak,§ F. Be´guin,§ and W. J. Blau† Materials Ireland Polymer Research Centre, Department of Physics, Trinity College Dublin, Dublin 2, Ireland, Laboratoire de Re´ sonance Magne´ tique Nucle´ aire, Fac. UniVersite´ N.D. de la Paix, rue de Bruxelles 61, 5000 Namur, Belgium, and Centre de Recherche sur la Matie` re DiVise´ e, 1B, rue de la Fe´ rollerie, 45071 Orle´ ans, France Received November 11, 2003; Revised Manuscript Received December 17, 2003
ABSTRACT Tensile tests were carried out on free-standing composite films of poly(vinyl alcohol) and six different types of carbon nanotubes for different nanotube loading levels. Significant increases in Young’s modulus by up to a factor of 2 were observed in all cases. Theories such as the rule-of-mixtures or the Halpin-Tsai-theory could not explain the relative differences between composites made from different tube types. However, it is possible to show that the reinforcement scales linearly with the total nanotube surface area in the films, indicating that low diameter multiwall nanotubes are the best tube type for reinforcement. In addition, in all cases crystalline coatings around the nanotubes were detected by calorimetry, suggesting comparible polymer−nanotube interfaces. Thus, the reinforcement appears to be critically dependent on the polymer−nanotube interfacial interaction as previously suggested.
Carbon nanotubes have generated a great deal of interest since their discovery1 due to their unique physical properties.2 Much research has focused on nanotubes as fillers in polymer or epoxy matrix composite materials with enhanced mechanical,3-5 electrical,6,7 and thermal8,9 properties. Most significantly, they are considered to be ideal candidates for mechanical reinforcement of polymers.2 CNTs are known to have an extremely high Young’s modulus of up to 1 TPa10,11 and tensile strength approaching 60 GPa.11 However, to access these extraordinary properties, a number of issues must be addressed. Of these issues, identifying the optimal properties of the polymer-nanotube interface is considered the most pressing. Specifically, the polymer-nanotube interfacial stress transfer must be maximized in order to achieve the best possible reinforcement. This topic has been the basis of several studies to date.12-16 Another important issue is the type of nanotubes to use for the highest level of reinforcement. It is generally thought that high quality SWNT13 are the best due to their high modulus and small size. Alternatively, arc produced MWNTs are considered promising due to their high modulus and rigidity. Catalytic MWNTs, however, are generally viewed less favorably as * Corresponding author. E-mail:
[email protected], Tel ++353 1 6083859. † Trinity College Dublin. ‡ Fac. Universite ´ N.D. de la Paix. § Centre de Recherche sur la Matie ` re Divise´e. 10.1021/nl035009o CCC: $27.50 Published on Web 01/07/2004
© 2004 American Chemical Society
their high defect level results in a modulus a factor of 10 lower than the more perfect nanotubes.17 In this paper, we will address these issues by studying the mechanical properties of a range of different types of nanotubes dispersed in one type of polymer matrix. We find that the reinforcement scales with inverse nanotube diameter for MWNT. Relative to MWNTs, SWNTs display poorer reinforcement, probably due to bundle formation. This strongly suggests that low-diameter MWNTs are the optimum material for polymer reinforcement. For this study poly(vinyl alcohol) (PVA) was used as the polymeric matrix as it is known to provide good stress transfer to carbon nanotubes.4 The nanotubes used were purified HipCO-SWNTs supplied by CNI Ltd. (U.S.A.),18 double-walled nanotubes (DWNT) from Nanocyl S. A. (Belgium),19 arc grown MWNT20 produced in our own laboratory (AMWNT), catalytic MWNT from Nanocyl S. A. (NMWNT),21 catalytic MWNT produced in Orle´ans22 (France) (OMWNT), and nanotubes with hydroxyl groups covalently bonded at the tips using ball milling23 under constant hydrogen flow (OHMWNT). In all cases except the AMWNT, the nanotubes were pure when received. For the AMWNT the NT were purified during the composite formation process.4 A range of composite dispersions were prepared for each nanotube type by adding the nanotube material to 30 g/L
solutions of PVA in water. PVA (Mw ) 30 000-70 000 g/mol) used in this investigation was purchased from SigmaAldrich [product code: 9002-89-5] and used as supplied. In the case of AMWNT a mass fraction of 25% soot relative to the polymer content was added to the polymer solution. This solution was mixed and purified as described previously24,25 and the true nanotube mass fraction measured by thermal gravimetric analysis. For all other nanotube types, the mass fraction added to the polymer solution was 1 wt %. These samples were sonicated for 5 min using a highpower sonic tip followed by a mild sonification for 2 h followed by further high-power sonification for additional 5 min. To fabricate free-standing composite films, 1 mL of each solution was pipetted onto a polished Teflon disk and placed in a 60 °C heated oven to allow evaporation of the solvent. This procedure was repeated four times on each disk in order to obtain films with thicknesses of up to 0.5 mm. The films were peeled off the substrates and cut into strips of ∼10 mm × 4 mm × 0.3 mm to perform mechanical testing. Prior to testing, all specimens underwent an additional drying procedure for 1 h at 60 °C to evaporate any remaining water. The width and thickness of each strip were measured using a low torque digital micrometer. The volume fraction of NT in each film was calculated from the mass fraction using the densities, F ) 1300kg/m3 for PVA, F ) 1500kg/m3 for DWNT and SWNT, and F ) 2150kg/m3 for all MWNT. To measure the average diameter and length for each nanotube type, transmission electron microscopy was performed (Hitachi H-7000). Formvar-coated copper grids (mesh size 300) were dipped into composite solutions and allowed to dry in ambient conditions. The average diameter, D, and length, l, of each type of nanotube were found to be: DWNT, D ) 2.5 nm, l ) 2.2 µm; SWNT (bundles), D ) 9 nm, l > 10 µm; NMWNT D ) 14 nm, l ) 2.1 µm; OHMWNT D ) 15 nm, l ) 1.6 µm; OMWNT D ) 16 nm, l ) 3.8 µm; AMWNT D ) 24 nm, l ) 0.8 µm. Tensile testing was carried out using a Zwick Z100 tensile tester. A 100N load cell and a cross head speed of 0.5 mm/ min were used to obtain the tensile modulus, Y. In all cases, four strips were measured and the mean and standard deviation calculated. Furthermore, morphology and thermal properties of the composites were studied by differential scanning calorimetry (DSC) using a Perkin-Elmer Diamond DSC power compensation instrument. Scanning rate was 40 K/min where approximately 10 mg of each sample was measured and analyzed. Shown in Figure 1 are the normalized tensile moduli for composite films containing all six nanotube types plotted versus the volume fraction of CNT. An increase in tensile modulus of up to a factor of 2 was observed at very low loading levels. In all cases straight lines can be fit. In the case of the DWNT the curve is linear only at low volume fractions. This is probably due to bundle formation for higher volume fractions. Two of the most common theories used to analyze these type of curves are the rule-of-mixtures26 and the HalpinTsai theory.27 Analysis using the rule-of-mixtures gives the 354
Figure 1. Normalized tensile modulus plot versus volume fraction of nanotubes for each composite film.
nanotube tensile modulus while if the NT modulus and diameter are known, analysis using Halpin-Tsai-theory gives the NT length. These parameters have been calculated in all cases and are presented in Table 1 along with the expected or measured values and the slopes, m, of the fit curves shown in Figure 1. None of these calculated values match well to the expected or measured values. This suggests that neither of these theories apply well for these samples. Empirically, it can be seen from Table 1 that the slopes, m, do not scale with the expected modulus or length. However, as the NT diameter increases the slope tends to decrease. To study this in more detail we plot the rate of increase of the normalized modulus with volume fraction (m) versus the NT diameter on a log-log scale in Figure 2. A straight line of slope close to -1 could be fitted to these data, indicating that the rate of increase is inversely proportional to NT diameter. It can be shown as follows that this indicates that the increase in normalized modulus is proportional to the total NT surface area per unit volume of composite film, SA/V. We can define the reinforcement as the increase in modulus on addition of nanotubes relative to the polymer modulus. If the reinforcement is proportional to the total nanotube surface area per volume then we can write YC - YP N ) k (πDL) YP V
(1)
where YC and YP are the Young’s moduli of the composite and polymer, respectively, k is a proportionality constant, N/V is the number of nanotubes per unit volume, and (πDL) is the surface area per nanotube (D and L are the nanotube diameter and length, respectively). By definition, the volume fraction can be written as N D2 Vf ) π L V 4
(2)
Substituting N/V from eq 2 into eq 1 and rearranging gives Vf YC ) 4k + 1 ) mVf + 1 YP D
(3)
Nano Lett., Vol. 4, No. 2, 2004
Table 1. Tube Analysis by Rule-of-Mixtures and the Halpin-Tsai Theorya Rule of mixtures tubes
diameter d (nm)
slope m
DWNT NMWNT OMWNT OHMWNT AMWNT SWNT
2.8 14 16 15 24 9 (bundles)
622 188 120 102 64 56
Halpin-Tsai
coating b (nm)
universal slope k (nm)
Yc (calc)
Yc (pred)
length, µm (calc)
length, µm (measured)
12 16 12 35 10
435 658 480 382 384
5.1 TPa 1.4 TPa 0.9 TPa 0.8 TPa 0.5 TPa 0.4 TPa
∼1TPa
