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2009, 113, 4751–4754 Published on Web 03/03/2009
Raman Response of Carbon Nanotube/PVA Fibers under Strain Noa Lachman,† Christe`le Bartholome,‡ Pierre Miaudet,‡ Maryse Maugey,‡ Philippe Poulin,*,‡ and H. Daniel Wagner*,† Dept of Materials & Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel, and CNRS, Centre de Recherche Paul Pascal, 115 AVenue Schweitzer, 33600 Pessac, France ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: February 17, 2009
We study the strain-induced shift of the D* Raman band of single-wall carbon nanotubes in polyvinyl alcohol-nanotube composite fibers. If embedded in structural components, such strain-sensitive fibers may be considered for potential applications as strain or stress sensors. Due to improved interfacial adhesion, stronger shifts of the D* Raman band are observed when carboxylic functional groups are present at the nanotube surface. This indicates that nanotube carboxylation would yield better efficacy for future sensing applications. However, we also observe that the improvements of interfacial adhesion do not lead to substantially better mechanical properties of the fibers. This effect is discussed by considering possible degradation of nanotubes during surface functionalization. ∆WN ) m · ε
1. Introduction Raman spectroscopy can provide unique information about vibrational and electronic properties of a material. It is nondestructive and readily available, and measurements can be made over a wide range of environmental conditions. In the field of fiber composite materials, it has been known for about three decades that the application of a mechanical strain to fibers such as carbon or kevlar results in shifted frequencies of those Raman bands which are directly related to the interatomic force constants.1 Correlating such shifts with the applied strain, through a calibration procedure, leads to the determination of local stress profiles in the embedded fibers. A similar effect is observed in carbon nanotube-based composites. Indeed, a characteristic shift under strain of the D* band (located at 2609 cm-1, also termed G′) has been observed in various cases2-7 which effectively turns these tubes into molecular strain sensors. In fact, carbon nanotubes embedded in polymers may achieve several roles, including as orientation detectors as well as molecular sensors around structural defects and reinforcements.5-7 Indeed, since the Raman intensity of a vibration or phonon in a crystal depends on the relative directions of the crystal axis and the electric wave polarization of the incident and scattered light, it may also be used to determine the orientation of nanotubes in polymer matrices or within nanotube bundles.7-9 An empirical linear relationship exists between the singlewall carbon nanotube (SWNT) D* wavenumber shift and the applied elastic strain.4,7 If the nanotube D* wavenumber difference between zero strain and the applied strain ε is defined as the Raman wavenumber shift ∆WN, then the empirical slope m of the wavenumber-strain relation can be defined from * To whom correspondence should be addressed. Email: poulin@ crpp-bordeaux.cnrs.fr (P.P.);
[email protected] (H.D.W.). † Weizmann Institute of Science. ‡ Centre de Recherche Paul Pascal.
10.1021/jp900355k CCC: $40.75
(1)
In the linear elastic regime of a tensile test, the stress σ and the strain ε in the polymer are proportional to each other, with the Young’s modulus E as the proportionality constant. The matrix stress may therefore be deduced from the Raman measurement:
σ ) E · ε ) E∆WN/m
(2)
Experiments show that m varies when nanotubes are embedded in different polymer matrices7 and that, for a given matrix, m is a temperature dependent parameter. A Raman investigation of nanotube-based composite materials can therefore be of great interest, as it may provide information on the level of stress transfer and adhesion between the nanotubes and the matrix. More importantly here, shifts in the Raman signature of nanotubes embedded in polymers, resulting from local strain or stress, may be exploited as a sensing device in composite structures. In the present work, we investigate composite fibers made of carbon nanotubes and polyvinyl alcohol (PVA). Such fibers are prepared by a coagulation spinning process previously described in the literature.10-14 The resulting fibers contain a fraction of SWNT of about 15 wt %. The sensitivity of the Raman response is probed by investigating the effects of surface carboxylation on the resulting straininduced shifts of the D* Raman band of single-wall carbon nanotubes. Strain-induced shifts are observed for both pristine (nonfunctionalized) and functionalized nanotubes. However, as shown here, the slope of the shift vs strain is substantially greater for carboxylated nanotubes, which reflects a better adhesion between the carboxylated nanotubes and the PVA. However, in spite of such improved adhesion, the mechanical properties of fibers made from functionalized nanotubes are not increased. This observation differs from previous reports where nanotube functionalization yielded improved mechanical properties, using 2009 American Chemical Society
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other polymer matrices.15-21 In PVA matrices, it was shown that surface transcrystallinity12,14,22-24 as well as covalent or noncovalent functionalizations25-27 could also be involved or used to improve mechanical reinforcements. The present observations are discussed by considering other factors such as possible degradations of carboxylated nanotubes28-30 which could reduce their Young’s modulus and their length and thereby the mechanical response of the composite fibers.31-34 Nevertheless, regardless of the exact limitations of reinforcement, the present results already confirm that nanotube-PVA fibers can be used as strain or stress sensors and that their sensitivity can be improved by functionalizing the surface of nanotubes. 2. Materials and Methods 2.1. Nanotubes and Composite Fibers. The CNT composite fibers have been prepared by using a so-called coagulation process.10 This process consists of injecting a carbon nanotube dispersion into the coflowing stream of a coagulating polyvinyl alcohol solution. The present fibers were made in a rotating bath with the injection of the nanotube dispersion tangential to the rotation of the PVA container.10 The diameter of the spinneret is 500 µm. The injection rate of the nanotube dispersion is 50 mL/h. The spinneret is located at 2 cm from the rotation axis of the PVA container. The latter is rotated at 100 rpm to provide a coflowing stream. The nanotubes coagulate when they meet the PVA solution and form a gel fiber. These fibers are extracted from the coagulating bath and dried vertically.TheresultantsystemsconsistofcompositePVA-nanotube fibers with a fraction of carbon nanotubes of about 15 wt %. This fraction is deduced from thermogravimetric analysis (TGA) measurements performed with a Setaram TAG 16 instrument under argon flow at a heating rate of 5 °C/min. The PVA polymer was purchased from Seppic, France. It has a molecular weight of 195 000 g/mol and a hydrolysis ratio of 98%. The coagulating solution is obtained by dissolving 5 wt % of this polymer in distilled water. We used Elicarb singlewall nanotubes purchased from Thomas Swan, U.K. Raw nanotubes are dispersed in water at a weight fraction of 0.3 wt %. They are stabilized with sodium dodecyl sulfate surfactant (SDS) at a weight fraction of 1.2 wt %. The nanotube dispersions are homogenized by a sonication treatment (90 min, 40 W) using a Branson Sonifier 205A equipped with a horn. These nanotube dispersions served for making fibers of nonfunctionalized nanotubes. Carboxylated single-wall carbon nanotubes (COOHSWNTs) are obtained by adding 150 mg of Elicarb nanotubes to 25 mL of an aqueous solution of 3.6 M nitric acid under reflux. This nitric acid treatment oxidizes the tubes and induces the formation of surface carboxyl groups. After 24 h of acid treatment, the suspension is rinsed with distilled water and a solution of 0.1 M NaOH. Then, the oxidized nanotubes are redispersed in an aqueous solution of distilled water and SDS (1.2 wt %). The dispersions are then homogenized in a sonicator bath to obtain aggregate free dispersions of carboxylated nanotubes with a weight fraction of 0.3 wt %. This procedure allowed us to achieve nanotube dispersions with similar chemical composition. Carboxylation is checked by acid-base titration.35 Following the method proposed by Hu et al.,35 we found a percentage of acidic sites of about 2%. This number of sites is sufficient to substantially alter the phase behavior of the nanotubes which become hydrophilic and water soluble. Nevertheless, the dispersions of functionalized nanotubes are homogenized in a sonicator bath to obtain aggregate free dispersions of carboxylated nanotubes with a weight fraction of 0.3 wt %. This procedure allowed us to achieve nanotube
Figure 1. Typical Raman spectra of pristine and carboxylated SWNTs embedded in PVA fiber. The D, G, and D* bands are located around 1320, 1580, and 2609 cm-1, respectively.
Figure 2. Strain-induced shifts of the D* Raman band for carboxylated and pristine single-wall nanotubes.
dispersions of pristine and functionalized nanotubes with similar chemical composition. 2.2. Raman Response and Measurements of Mechanical Properties. The fibers were tested in a Raman spectrometer (Renishaw Ramanscope), and carbon nanotube spectra were recorded using the 632.8 nm line of a helium-neon laser. The spectral resolution was 1 cm-1. The incident laser beam was focused onto the specimen surface through a ×50 objective lens, forming a laser spot approximately 2-5 µm in diameter. The calibration of the fibers (frequency shift against strain, or ∆ω vs ε) was performed by monitoring the D* peak shift under applied strain using two specimens of each type. The laser polarization was parallel to the loading direction. Moreover, the tensile mechanical properties of both fiber types were measured using a Zwick Z2.5/TN1S instrument at a strain rate of 1%/ min. 3. Results and Discussion The Raman spectra of SWNTs embedded within the PVA fiber are presented in Figure 1, and reveal that a strong signal is still visible when nanotubes are embedded, which enables the use of the fibers as sensors. The strain-induced shifts of the D* band are shown in Figure 2. At low strain (ε < ∼0.01), in the elastic regime, the data can be approximately fitted by linear relationships. At larger strain, in the plastic regime, the Raman response becomes insensitive to strain, likely due to weakening
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Figure 3. Stress-strain curves of carbon nanotube-PVA composite fibers. The fibers containing pristine nanotubes exhibit slightly better mechanical properties than the fibers with carboxylated nanotubes.
of the interfacial adhesion as the polymer chains possibly slide at the nanotube interface. As mentioned, the crossover from the elastic to the plastic domain occurs at about 1% strain. This transition in the Raman signature around 1% will be confirmed below by direct mechanical characterizations under tensile load. As seen in Figure 2, the slope in the elastic regime for pristine nanotubes in PVA is -359 cm-1/strain, whereas it is -557 cm-1/strain for carboxylated nanotubes in PVA. In other words, fibers containing carboxylated SWNTs exhibit significantly higher sensitivity to strain than fibers processed in the same way but with pristine nanotubes. Note also that the sensitivity observed here with carboxylated nanotubes is definitely greater than that observed in previous work with a similar PVA but a different nanotube surface treatment: in ref 27, a slope of -370 cm-1/strain was obtained following treatment with hydroxyl groups (with a base D* band shift of -260 cm-1/strain for pristine CNT/PVA), compared to -557 cm-1/strain here (with a base D* band shift of -360 cm-1/strain for pristine CNT/ PVA). Carboxylic groups play a major role in the stress-transfer mechanism, as they improve the transfer of stress from the matrix to the SWNTs, and thus the composite Raman sensitivity to strain. This can be explained by considering that carboxyl groups can form hydrogen bonds with the PVA chains. It is also possible that the carboxyl groups form covalent ester bonds with the hydroxyl groups of the PVA. Both types of bonds are stronger than van der Waals interactions and can explain the better stress transfer observed with carboxylated nanotubes. However, despite the improved stress transfer, the mechanical properties of the fibers are not improved, as seen from the stress-strain curves shown in Figure 3. The fibers exhibit an elastic regime at low strain, up to approximately 1%, in good agreement with the Raman characterizations. The Young’s modulus of pristine nanotube-composite fibers is 22 GPa, whereas it is 19 GPa for carboxylated nanotube-composite fibers. This slightly lower value is somewhat unexpected, as several other authors found that surface functionalization of nanotubes could yield improved mechanical properties of nanocomposites.15-21,25-27 The present observations can be rationalized by considering factors other than interfacial adhesion: (i) Sonication of the tubes may lead to breakage and shortening, even more so for carboxylated than for pristine nanotubes.36-38 (ii) Carboxylation by nitric acid treatments can degrade the wall structure of single-wall carbon nanotubes28-30 and thus reduce their Young’s modulus and tensile strength.
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Figure 4. Effect of applied strain on the integrated peak intensity ratio, ID/IG, for pristine SWNT/PVA and COOH-SWNT/PVA fibers. The intensity ratio calculation is based on the measurement of the areas under the D and G peaks.
Indeed, the integrated peak intensity ratio, ID/IG (based on the area under the D and G peaks, see Figure 1), of pristine SWNT/ PVA is found here to be 0.16, whereas that of COOH-SWNT/ PVA is 0.73. Such a large difference confirms that COOHSWNTs contain significantly more defects than pristine SWNTs.39,40 In other words, carboxylation by nitric acid treatment has significantly degraded the wall structure of singlewall carbon nanotubes. Moreover, referring to Figure 4, upon straining, we find the ID/IG ratio of the COOH-based specimen to progressively decrease down to a constant value, whereas this ratio remains constant for the pristine tube-based specimen. This leads to a smaller but clear difference between the ID/IG ratios at higher strains. This CNT wall structure degradation effect is most likely the main cause of the lack of improvement in the mechanical properties of the fibers. In such conditions, it is possible that, in spite of a better chemical adhesion to the matrix, the weakening or shortening of carboxylated nanotubes produces no improvement in the mechanical properties of the nanocomposites. 4. Conclusions We have shown that carbon nanotubes embedded in composite fibers allow the fibers to be used as strain or stress sensors by measuring the shift of the D* Raman band under mechanical load. Carboxylation of the nanotubes does not improve the mechanical properties of the fibers. However, surface functionalization with carboxylic groups, which provides better tube-polymer adhesion and thus stress transfer, leads to substantial improvements of the fiber as a Raman strain sensor. Future work will consider the inclusion of such fibers in structural composites to probe their sensing ability in practical applications. Acknowledgment. This work was supported by the NOESIS European FP6 project on “Aerospace Nanotube Hybrid Composite Structures with Sensing and Actuating Capabilities”. H.D.W. is the Livio Norzi Professor in Materials Science. References and Notes (1) Mitra, V. K.; Risen, W. M.; Baughman, R. H. J. Chem. Phys. 1977, 66 (6), 2731–2736. (2) Lourie, O.; Wagner, H. D. J. Mater. Res. 1998, 13 (9), 2418–2422. (3) Wood, R.; Frogley, M. D.; Meurs, E. R.; Prins, A. D.; Peijs, A. T.; Dunstan, D. J.; Wagner, H. D. J. Phys. Chem. B 1999, 103, 10388–10392.
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JP900355K
