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  • 1. www.advmat.de www.MaterialsViews.com CORRESPONDENCE Comment on ‘‘The Effect of Stress Transfer Within Double-Walled Carbon Nanotubes Upon Their Ability to Reinforce Composites’’ By Wei Lin and C. P. Wong* In a recent Communication published in Advanced Materials,[1] carbon-fiber moduli and their Raman band shift rates; however, Cui et al. discussed wall-to-wall stress transfer within we do not agree with this being a general relation for evaluating double-walled carbon nanotubes (DWNTs) in a composite the CNT modulus. There are two fundamental and/or logical on the basis of stress-induced Raman band shifts. The Raman problems with this extension. G0 band of the DWNTs was split into two components; only the First, in Ref. [2], it was mentioned that the linear behavior had one corresponding to the outer wall shifted distinctly with been attributed, by Huang and Young,[3] to different micro- strain. This phenomenon was considered a reflection of the structures of the fibers under investigation, where ‘‘microstruc- ineffective stress transfer from the outer wall to the inner wall ture’’ referred specifically to the levels of orientation of the during the DWNT deformation in the composite, which would graphitic grains along a fiber. In the case of the CNTs used in greatly reduce the effective modulus of the DWNTs as Ref. [1], the as-defined microstructure shows little difference reinforcement filler. Coincidently, it was found that the between the SWNTs and the DWNTs, since the graphitic planes Raman band shift rate of the outer wall of the DWNTs was (walls in CNTs) are oriented along the fiber (tube) axis with the smaller than that of single-walled carbon nanotubes (SWNTs) only exception of CNT ends. Thus, even though SWNTs, DWNTs when dispersed in an epoxy matrix. From the G0 -band shift and MWNTs may have different effective moduli, the modulus rates, Cui et al.[1] calculated the effective moduli of the SWNTs difference is not due to the microstructure/orientation (the and the outer wall of the DWNTs to be 762 GPa and 552 GPa, effective modulus of CNTs will be discussed in details later). respectively. This was considered a proof of the reduction in Moreover, it is commonsense that the deformation mechanism effective modulus of the DWNTs, the reason being the and, therefore, performance of a polycrystalline material (carbon ineffective wall-to-wall stress transfer. This work has signifi- fiber in this case), is quite different from its single-crystalline cance in providing an experimental proof on what many in this form (CNT). With these in mind, the ‘‘constant of proportion- research field have long hypothesized regarding the ‘‘poor load ality’’ observed for carbon fibers will not hold for CNT-modulus transfer to the inner walls of a multi-walled carbon nanotube’’ calculations, not to say the ‘‘linearity’’ is not good enough (Fig. 12 (MWNT). However, misinterpretations of some results found in in Ref. [2]). Therefore, the smaller G0 band shift rate of the outer literature regarding the Raman band shift rate and effective wall of the DWNTs compared with that of the SWNTs does not modulus led Cui et al. to the controversial conclusion that the indicate a lower modulus of the DWNTs. Or even if we assume reduced Raman band shift rate indicated a reduced effective that the DWNTs and the SWNTs have the same modulus, their modulus for the DWNTs. Raman responses may not be identical. The theoretical foundation of the study in Ref. [1] is the Second, the essential difference—orders of magnitude conclusion in Ref. [2] (Ref. [4] of Ref. [1]), in which Cooper different in surface area—between carbon fibers and CNTs et al.[2] reported an empirical linear relation (a ‘‘constant of significantly increases the complexity of interpreting Raman band proportionality’’) between the moduli of carbon fibers and their shift rates in CNT/polymer composites. The interface effect or, in Raman band shift rates under tensile strains: the higher the other words, the influence by the interphase, plays a significant shift rate, the higher the effective modulus of a CNT. The role in CNT/polymer composites.[4,5] It might be true that for ‘‘constant of proportionality’’ of À0.05 cmÀ1(GPa%)À1 in the carbon fibers and their composites, the Raman band shift rate is linear relation was taken for granted in Ref. [2] and, therefore, in more a reflection of the intrinsic modulus of the carbon fibers Ref. [1] as well, to be valid for calculating the effective moduli of than of the fiber/polymer interfacial load transfer;[2,3] however, carbon nanotubes (CNTs) by investigating the strain-induced the opposite seems to hold for CNT/polymer composites.[6] In Raman band shift rate of CNTs in a polymer matrix. We do fact, Cooper et al.[2] found a sharp difference in modulus between acknowledge the possible ‘‘linear’’ relation between the SWNT-A (SWNTs prepared by an arc-discharge process) and SWNT-P (SWNTs prepared by a pulsed-laser vaporization [*] Prof. C. P. Wong, W. Lin process) derived from their G0 band shift rates. Although it School of Materials Science & Engineering and was postulated that SWNT-A may have a lower intrinsic modulus Packaging Research Center than SWNT-P, it might not be true.[7] A poorer dispersion of Georgia Institute of Technology SWNT-A in the composite was proposed as a second possible 771 Ferst Drive NW., Atlanta, GA 30332 (USA) reason, however, without solid evidence. Neither TEM images nor E-mail: cp.wong@mse.gatech.edu comparisons of the mechanical properties of the composites were DOI: 10.1002/adma.200902189 given. Basically, the poor dispersion, bundling, and consequently Adv. Mater. 2009, 21, 1–3 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Final page numbers not assigned
  • 2. www.advmat.de www.MaterialsViews.com CORRESPONDENCE small effective aspect ratio are all related directly to the surface modulus of the outer wall of a CNT’’. This is probably the right status of the SWNT-A, which, in turn, determines interfacial reason why the Raman G0 band corresponding to the inner wall of stress transfer from the polymer matrix to the CNTs upon the DWNTs showed little shift in Ref. [1]. Based on the deformation. In this sense, the difference in the Raman DWNT-synthesis process and the TEM images provided in the responses between SWNT-A and SWNT-P in Ref. [2] was Supporting Information of Ref. [1], we think that the DWNTs probably a reflection of CNT/polymer interface status rather used in Ref. [1] have the same outer diameter, tube helicity, and than of the intrinsic modulus of the CNTs. As such, the observed surface chemistry as the SWNTs used; as such, the effective relatively small G0 band shift rate from the MWNTs may, modulus of the outer wall of the DWNTs should be the same as compared with the SWNT-P, be also explained by a relatively poor that of the SWNTs.[14] Even a weak, if any, wall-to-wall stress CNT/polymer interface, since the MWNTs were prepared by an transfer should have enhanced rather than reduced the effective arc-discharge process as well.[2] The reason why the MWNTs modulus of the outer shell. However, in Ref. [1], 762 GPa and showed a higher G0 shift rate than the SWNT-A was probably a 552 GPa were obtained for the SWNTs and the outer wall of the better dispersion in the composite. Therefore, the conclusion in DWNTs, respectively, on the basis of the aforementioned Ref. [2], which is the theoretical presumption of Ref. [1], is not questionable linear relation. correct. Cui et al.[1] cited the work on ‘‘interlayer forces and ultralow Not only is the theoretical foundation problematic in Ref. [1], sliding friction in multiwalled carbon nanotubes’’ by Kis et al.,[15] the most important concept—effective modulus—seems to have and claimed that ‘‘it is this relatively easy shear between the layers been used confusingly. Cui et al.[1] thought that the poor load that reduces the stress transfer and leads to the large reduction in transfer from the outer wall of a DWNT to the inner wall resulted effective Young’s modulus for MWNTs for reinforcement in in a reduction in the effective modulus of the DWNT. For composites.’’ In Ref. [15], a MWNT was glued onto a platinum modulus measurements of CNTs,[8–10] the results reported were wire and its outer layers were removed by electrical breakdown. mostly ‘‘effective moduli’’ because in those measurements, ‘‘all The as-exposed CNT core was welded to a force sensor during the layers have the same circumferential strain, since they have the pullout experiment. This situation is, by no means, comparable to same axial strain after the uniform stresses loading’’.[11] In the situation of a close-ended DWNT or MWNT in a polymer addition, in Ref. [11] (Ref. [9] of Ref. [1]), Tu et al. pointed out that composite. In fact, a DWNT or MWNT under tensile load in a ‘‘the value of Ym (effective modulus) does not reflect the physics composite is analogous to the case studied by Yu et al.,[16] where a change in the true lattice rigidity but just a choice of the cross large modulus value was found for the outermost wall of a section’’. Moreover, conclusions in Ref. [11] are ‘‘valid only for MWNT. Although the modulus value of the outermost wall in tubes whose radii are not very small’’. Therefore, the as-defined Ref. [16] seems a little bit lower than the measurement result of a ‘‘effective modulus’’ for a CNTunit is treated as conventional filler SWNT,[17] a direct comparison between the results by different in contrast to that for a specific CNT wall. In comparison, in measurement techniques is not wise. Also, note that sample Ref. [1], the inner wall of a DWNT upon extension seems to take defects, measurement errors, and possibly the Russian roll on little stress and little axial strain. In this sense, the as-defined structure[18,19] of a MWNTmay account for such a difference. One ‘‘effective modulus’’ no longer makes any sense. Alternatively, may claim that an open-ended CNT in a polymer matrix looks Cui et al.[1] turned to a different concept—the effective modulus similar to the situation in Ref. [15]. Not actually. Note that the of the outer-wall material. With this concept, they attempted to wall–wall distance in a DWNT or MWNT is only $0.34 nm but the indicate that the capability of the outer wall of taking on the load radius of gyration of a polymer chain is typically at least tens of transferred from the polymer matrix is reduced due to the poor times larger than 0.34 nm. Therefore, for open-ended small- wall-to-wall stress transfer; we do not agree with them. diameter DWNTs, the probability of a chemical attachment of the First, Cui et al.[1] took for granted that upon tensile ends of the multiple walls to the polymer matrix in such a way that deformation, ‘‘the stress transfer from the matrix to nanotubes they will have the chance to slide relative to each other is in composites and then subsequently through the different layers extremely low. occurs principally through shear’’. A question arises as: why Second, Cui et al.[1] cited the modeling results by Zelamea should the load be transferred from the outer wall to the inner et al.[20] to support their statement of the ‘‘significant reduction in wall? Actually, it is apparent that for tensile deformation the the effective Young’s modulus of the reinforcement by MWNTs’’. wall-to-wall stress transfer is not necessary or should rarely In fact, although lots of papers have been published on happen in ideal cases.[12,13] This can be easily understood in the mechanical properties of CNT reinforced polymer composites, following way: Typically, non-functionalized CNTs are clo- a direct comparison among the experimental data is very se-ended, which means that a CNT has a sealed outer shell to challenging due to the different CNT sources, treatments, prevent the inner walls from communicating with the polymer composite preparation processes, and so on.[21] Fortunately, we chains around the outer shell. Thinking about a tensile can still use the data for discussion from the systematic work by deformation of a close-ended DWNT in a polymer composite, Gojny et al.[12] In Ref. [12], SWNT/polymer composites do show it is the outer shell that is taking the load from the matrix and somewhat higher moduli than MWNT/polymer composites with deforming until its final fracture or failure at the polymer/ the same filler weight fraction. However, the essential reason may outer-shell interface.[12] The outer shell itself is eligible to take on not be the postulated inferior effective moduli of the MWNTs. the stress and, ideally, there is no wall-to-wall bridging bond. Note that the SWNTs used in Ref. [12] have an average diameter There is no tendency or need for a wall-to-wall load transfer— smaller than 2 nm, while the MWNTs are much larger (15 nm in keeping in mind that we are not using the concept of an ‘‘effective diameter). This means that the SWNTs have around seven times modulus of a CNT’’ here but, instead, the concept of an ‘‘effective more surface area than the MWNTs in the composites at the same 2 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–3 Final page numbers not assigned
  • 3. www.advmat.de www.MaterialsViews.com CORRESPONDENCE volumetric filler loading. Considering such a big difference in [3] Y. Huang, R. J. Young, Carbon 1995, 33, 97. surface area, we have to say that the better modulus enhancement [4] M. Moniruzzaman, K. I. Winey, Macromolecules 2006, 39, 5194. of the composites by the SWNTs compared with MWNTs is [5] A. Eitan, K. Y. Jiang, D. Dukes, R. Andrews, L. S. Schadler, Chem. Mater. significantly below expectation, not to mention the assumption 2003, 15, 3198. that MWNTs may have lower effective modulus than SWNTs.[12] [6] A. Eitan, F. T. Fisher, R. Andrews, L. C. Brinson, L. S. Schadler, Compos. Sci. Technol. 2006, 66, 1162. In summary, we have found that the theoretical presumption [7] K. Enomoto, S. Kitakata, T. Yasuhara, N. Ohtake, T. Kuzumaki, Y. Mitsuda, of Ref. [1] on the ‘‘linear relation’’ between the Raman band Appl. Phys. Lett. 2006, 88, 153115. shift rate and the CNT modulus is not logically sound. [8] M. M. J. Treacy, T. W. Ebbesen, J. M. Gibson, Nature 1996, 381, 678. Misinterpretations of ‘‘effective modulus’’ and ‘‘load transfer’’ [9] E. W. Wong, P. E. Sheehan, C. M. Lieber, Science 1997, 277, 1971. during CNT deformations resulted in their unreasonable [10] O. Lourie, H. D. Wagner, J. Mater. Res. 1998, 13, 2418. conclusion that the reduced wall-to-wall stress transfer led to a [11] Z. C. Tu, Z. Ou-Yang, Phys. Rev. B 2002, 65, 233407. reduced effective modulus of the outer CNT wall. More detailed [12] F. H. Gojny, M. H. G. Wichmann, B. Fiedler, K. Schulte, Compos. Sci. discussion on the influences of the CNT purity, the DWNT Technol. 2005, 65, 2300. diameter, the wall defect after the DWNT formation, and [13] L. S. Schadler, S. C. Giannaris, P. M. Ajayan, Appl. Phys. Lett. 1998, 73, dispersion in a polymer matrix is desired. A proper reinterpreta- 3842. [14] N. Yao, V. Lordi, J. Appl. Phys. 1998, 84, 1939. tion of their experimental results and a more systematic study [15] A. Kis, K. Jensen, S. Aloni, W. Mickelson, A. Zettl, Phys. Rev. Lett. 2006, 97, could indeed have profound effects on better understandings of 025501. Raman responses of CNTs under strains, especially in [16] M. F. Yu, O. Lourie, M. J. Dyer, K. Moloni, T. F. Kelly, R. S. Ruoff, Science composites. 2000, 287, 637. [17] A. Krishnan, E. Dujardin, T. W. Ebbesen, P. N. Yianilos, M. M. J. Treacy, Received: June 30, 2009 Phys. Rev. B 1998, 58, 14013. Published online: [18] Y. Maniwa, R. Fujiwara, H. Kira, H. Tou, H. Kataura, S. Suzuki, Y. Achiba, E. Nishibori, M. Takata, M. Sakata, A. Fujiwara, H. Suematsu, Phys. Rev. B 2001, 64, 241402. [19] D. Reznik, C. H. Olk, D. A. Neumann, J. R. D. Copley, Phys. Rev. B 1995, 52, [1] S. Cui, I. A. Kinloch, R. J. Young, L. Noe, M. Monthioux, Adv. Mater. 2009, 174514. 21, 3591. [20] L. Zalamea, H. Kim, R. B. Pipes, Compos. Sci. Technol. 2007, 67, 3425. [2] C. A. Cooper, R. J. Young, M. Halsall, Composites Part A 2001, 32, 401. [21] J. N. Coleman, U. Khan, W. J. Blau, Y. K. Gun’ko, Carbon 2006, 44, 1624. Adv. Mater. 2009, 21, 1–3 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3 Final page numbers not assigned