Tailoring Temperature Invariant Viscoelasticity of Carbon Nanotube

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Tailoring Temperature Invariant Viscoelasticity of Carbon Nanotube Material Ming Xu,†,‡ Don N. Futaba,*,†,‡ Motoo Yumura,†,‡ and Kenji Hata*,†,‡,§ †

Technology Research Association for Single Wall Carbon Nanotubes (TASC), Tsukuba, 305-8565, Japan Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan § Japan Science and Technology Agency (JST), Kawaguchi, 332-0012, Japan ‡

bS Supporting Information ABSTRACT:

Using carbon nanotubes (CNTs) as building blocks, we fabricated a viscoelastic material. In contrast to existing conventional materials where the stiffness (storage modulus) increases when the viscosity (damping ratio) decreases, both of these two aspects could be simultaneously improved for the viscoelastic CNT material. This allows fabricating both strong and highly viscous materials. This unique phenomenon was explained by a zipping and unzipping of carbon nanotubes at contacts as the origin of viscoelasticity. KEYWORDS: Carbon nanotube, viscoelasticity, nonaligned structure, density, mechanical property

V

iscoelasticity is the material property exhibiting both viscous and elastic characteristics when a material is exposed to deformation and exists in many aspects of our daily life, such as human tissue, tires, and seismic isolators. Viscoelasticity is quantified by three important properties: storage modulus, loss modulus, and damping ratio. Storage modulus measures the elastic nature (stiffness) of a material and describes the ability to instantaneously strain (i.e., deform) and recover when stressed and released, respectively. In contrast, the loss modulus measures the viscous nature of the material and expresses the resistance to strain through energy dissipation. Finally, the damping ratio is the ratio of loss modulus to storage modulus and describes the relative level of viscosity to elasticity, where an ideal elastic material is zero and an ideal viscous fluid would be infinite. Tailoring viscoelasticity is of practical importance because the required mechanical properties can greatly differ with application. Varying crystallinity and porosity are two common approaches to tailor viscoelasticity of materials. For example, polyethylene, with a storage modulus of 100 MPa and damping ratio of 0.05, is used for food packaging. Yet, when the crystallinity is increased by thermoplasticizing, polyethylene shows a higher storage modulus (>3 GPa) and a lower damping ratio r 2011 American Chemical Society

(0.0008) and can be used for sewer drainage construction. In such a case, molecular chain immobilization and stiffening result in increased storage modulus (stiffness) and decreased loss modulus. Alternatively, when polyethylene is aerated into foam, the storage modulus can be reduced to ∼20 kPa with the damping ratio increased to ∼0.2, and such foams are used for seat cushions. As exemplified, varying the porosity by controlling the bulk density is a straightforward method to tune the viscoelasticity. Carbon nanotubes (CNTs), due to their exceptional mechanical properties and thermal stability,1,2 could be building blocks for various viscoelastic materials. While an individual CNT is fundamentally elastic, an ensemble of CNTs, or a CNT composite, can be viscoelastic. Although not addressed explicitly, various reported CNT ensembles, such as sponges,3 films,4 blocks,5 and fibers6 are expected to be viscoelastic. For example, the nonlinear stress
strain curves of the CNT sponge is a sign of viscoelasticity.3,7 In addition, it was reported that the compressibility Received: May 15, 2011 Revised: July 6, 2011 Published: July 14, 2011 3279

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Figure 1. Mechanical characterization of the nonaligned CNT materials. (a
c) Storage modulus, loss modulus, and damping ratio of CNT materials plotted as a function of density. (d, e) Comparison of normalized moduli and damping ratio of the CNT materials vs metal sponge. Inset: SEM images of CNT material and photographs showing the similar nonaligned, entangled structure.

and recoverability of a CNT sponge highly depended on its bulk density (5.8
25.5 mg/cm3) where the low density sponge showed superior compressibility of 90% while the high density sponge showed superior recoverability of 93%. Recently, a CNT ensemble composed of long and entangled CNTs possessing viscoelastic properties (storage modulus, 1 MPa; loss modulus, 0.3 MPa; damping ratio, 0.3), similar to that of silicone rubber, has been reported.8 This result demonstrated that when properly assembled, CNT ensembles could be highly viscoelastic. Moreover, the inherent thermal resistance of CNTs provided temperature invariant viscoelastic properties from
196 to 1000 °C, far exceeding the operational range of conventional viscoelastic materials.8 For CNT composites, CNTs have been used as fillers to tailor the mechanical properties. For example, by increasing the loading level of multiwalled carbon nanotubes (MWNTs) from 2.5 to 25 vol %, the storage modulus of a viscoelastic MWNT/polystyrene composite could be controllably increased from 1.9 to 4.5 GPa.9 A separate study reported a 14-fold increase in the damping ratio compared to that of an epoxy matrix by dispersing 50 vol % CNTs.10 These results demonstrated that the mechanical properties of CNT materials could be tailored by tuning their internal structures. In this paper, we have tailored the viscoelastic properties of a CNT material within the range of polymeric foams to soft rubbers by controlling the density and thus have made a material that is soft and yet operates at high temperatures. In contrast with existing viscoelastic materials, we found that the storage modulus (stiffness) and damping ratio (viscosity) could be simultaneously increased with density, which could provide an approach to make strong yet viscous materials. This unique phenomenon was readily explained by the zipping and unzipping of carbon nanotubes at contacts as the cause of the viscoelasticity. Since the CNT material (density, 3.3
54 mg/cm3) was highly porous where CNTs only occupy 0.27
4.6% of the total volume, we chose density as the key parameter to tailor the viscoelastic

properties. The CNT material was synthesized by exposing the catalyst film to reactive ion etching (RIE), water-assisted chemical vapor deposition (CVD), and compression as described previously.8 Through controlling the RIE exposure time to reduce the density and the compression ratio, the density of the CNT material could be increased from 3.3 to 54 mg/cm3. The viscoelastic properties (storage and loss moduli, damping ratio) for a series of CNT materials with different densities were characterized by torsion-mode dynamic mechanical analysis (DMA) and plotted as a function of density. Up to a density of 36 mg/cm3, the storage, loss moduli, and damping ratio all showed a monotonic increase with density, above which this trend changed. Over this 1 order increase in density (3.3
36 mg/cm3), the storage modulus increased 1 order (0.11
1.05 MPa), while the loss modulus increased 30 times (0.01
0.32 MPa). As a result, the damping ratio increased from ∼0.1 to 0.32. These results clearly demonstrated that density was one of the key parameters to tailor the viscoelastic properties of the CNT material (Figure 1a
c). It is worthwhile to note that the increase of the damping ratio with density is exceptionally different behavior than conventional porous materials as discussed later. Beyond 36 mg/cm3 a different trend emerged, and at 54 mg/cm3, the storage modulus (∼1.15 MPa) plateaued and the loss modulus (0.27 MPa) and the damping ratio (∼0.2) reduced. This was also an unexpected behavior since both moduli were expected to increase with density for porous materials. We have characterized a metal sponge, basically wire wool of entangled steel wires, to demonstrate the behavior of conventional porous viscoelastic materials and measured the viscoelastic properties as a function of density (Figure 1d,e). The metal sponge was chosen for two reasons: the cellular, network structure was similar to the overall structure of our material and the elasticity of coiled metal wire was analogous to that of an individual CNT. When the density of the metal sponge increased 3280

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Figure 3. Plot of the alignment (Herman’s orientation factor, HOF) as a function of bulk density. Inset: SEM images of the alignment for sample spanning the range of control. Scale bars represent 1 μm.

Figure 2. Plot of the density as a function of reactive ion etching (RIE) etching time before CVD growth showing the range of density control. Inset: Extension of the control range by compression.

from 80 to 230 mg/cm3, both the storage and loss moduli increased 1 order. Consequently, the damping ratio remained fairly constant, different from that of the CNT material. In general, the storage modulus and damping ratio are inversely related, where a material with high storage modulus usually exhibits a low damping ratio. This is because the segments of a stiff material (high storage modulus) possess less mobility (low damping ratio).11 As shown, the viscoelastic properties of the CNT material did not follow this rule and both the storage modulus and the damping ratio could be simultaneously increased with density. This interesting trend is of practical use as it might provide a method to make strong yet viscous materials. To find the dependence of the viscoelastic properties on density, it was crucial to make a series of nearly identical CNT materials differing only by the density. Among the four fabrication steps, catalyst preparation, RIE, water-assisted CVD, and compression, the synthesis (water-assisted CVD with ethylene at 750 °C) and catalyst preparation (sputtered Al2O3 (30 nm)/Fe (2 nm thin film) were fixed as to not influence the wall number and diameter of the grown CNTs.12 Water-assisted CVD16 was essential to synthesize a CNT material with heights exceeding 5 mm, and RIE was crucial to synthesize a nonaligned CNT material with low density. Therefore, density control was implemented by tuning the RIE and compression processes. First, we found that the density of the CNT material could be tailored 1 order (10 to 1 mg/cm3) by adjusting the exposure time (5
32 min) of the catalyst films to the RIE process. Importantly, transmission electron microscopy (TEM) of the CNT materials grown from catalysts treated by RIE at the shortest (5 min) and longest (32 min) exposure times showed that both CNT materials were composed from mainly double-walled CNTs with similar average diameters (5.5 nm). This confirmed that RIE

reduced the material density while not influencing the CNT wall number and diameter. Second, we employed mechanical compression to further our range of density control. By compression, the density scaled with the compression ratio (R = h/h0 ) of the initial height, h, to the compressed height, h0 , i.e., F0 = RF, where F and F0 are the initial and final densities, respectively. By mechanical compressing the CNT materials (h = 5.2 and 4.8 mm, F = 6.8 and 8.7 mg/cm3), the density was increased above 20
54 mg/cm3. Care was taken to control the final sample height in the range of 1.3 ( 0.4 mm to exclude any possible geometric effect on viscoelastic properties (Figure 2). The scanning electron microscope (SEM) structural images of the lateral surfaces of the CNT materials appeared very similar (except at the highest density of 54 mg/cm3) and revealed an intertube structure where long, individual CNTs traversed laterally, making interconnections with other CNTs. To quantify the degree of alignment, the Herman’s orientation factor (HOF) (where 0 is random and 1 is aligned) was calculated from the Fourier transform (FFT) of the SEM images. The HOF of the CNT material was found to be independent of density and fell in the range of 0.13
0.19. This corresponded to a very low degree of alignment. As reported, this nonalignment was crucial for the CNT material to endure high levels of strain without fracture.8 These results indicated that we could fabricate a series of nearly identical CNT materials differing only by the density (3.3
54 mg/cm3) (Figure 3). High-resolution TEM studies of the CNT material revealed an unusual intertube structure characterized by numerous contacts among CNTs, denoted as “nodes”. In contrast to bundled CNTs where many CNTs make contact with the same CNTs over long spans, a node is composed from only a two to four CNTs intermittently contacting in parallel for only short spans. These nodes are proposed to be the origin of the viscoelasticity of the CNT material as discussed later. In each TEM image, the nodes were marked in red, from which the node density, N, and node lengths, L, were estimated, and the relative changes in both were plotted as a function of density (Figure 4b). Up to a density of 36 mg/cm3, TEM images showed that the basic intertube structure remained constant while both the node density and node length increased monotonically with density. Over this 1 order increase in density, not only did the node density increase 10-fold but the node length increased 3-fold. A 10-fold increase in CNT density resulted from a 10-fold increase in bulk density, and the nodes within the internal structure were preserved. Beyond 36 mg/cm3 a different trend emerged, and at 54 mg/cm3 TEM (Figure 4a) showed the onset of CNTs bundling. At this point, the CNT material underwent a fundamental change in internal structure as 3281

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Figure 4. Mechanism of viscoelasticity by density control. (a) TEM images of the as-prepared intertube structure at varying densities. (Left) Selected section indicating nodes; the nodes are marked in red. (b) The relative changes of the node density, N, and node lengths, L, plotted as a function of density. (c
e) Comparisons of relative changes as a function of density between structural properties and viscoelastic properties of storage modulus, loss modulus, and damping ratio, respectively.

evidenced by the dramatic increase in the node length and decrease in the node density caused by merging of neighboring nodes into long bundles. The trends of the relative changes of the structural (Figure 4b) and mechanical properties (Figure 1a
c) as a function of density (Figure 4c
e) were compared (up to a density of 36 mg/cm3) to understand the impact of the internal structure on the mechanical properties. First, the storage modulus followed a similar trend as the node density but not the node length. Fundamentally, an increase in node density would increase the storage modulus because a high density of nodes would result in shorter and stiffer struts (segments of CNTs between nodes), and a material made from shorter struts would possess higher stiffness. This is analogous to nodes of cellular materials, such as textiles, felts, skin, and collagenous tissues, where the storage modulus strongly depends on the node density. At 54 mg/cm3, this explanation became invalid since the node density did not increase with density. Second, the trend of the loss modulus differed from that of both the node density and the node length but interestingly followed the trend of the product of the node density and length (LN). This agreement can be explained by the zipper model.8 In this model, CNTs in a node were proposed to reversibly attach and detach through zipping and unzipping. This process would dissipate energy because of the energy consumed to overcome the large van der Waals (vdW) attraction between CNTs when unzipped,13 yet no energy would be required for zipping. Therefore, this process would act as a source of viscosity and thus would contribute to the loss modulus. The loss modulus, based

on the zipper model, would be proportional to an integration of the vdW adhesion energy per unit length over the node length and summed over all nodes. As such, the relative change in the loss modulus would be expected to scale as the product of node density, N, and node length, L, as observed. At 54 mg/cm3, the loss modulus did not scale with NL, but this deviation can be readily explained since the zipper model becomes invalid for bundled CNTs as bundling is an irreversible process (i.e., the necessary reversible process is gone).6 Since the storage and loss moduli scaled with N and NL (both functions of density), respectively, the damping ratio would be expected to scale with the node length, L. In fact, when the node length increased 3-fold, the damping ratio also increased 3-fold. On the basis of this explanation, the storage modulus and the damping ratio of the CNT material could increase in tandem, since both node density and node length increased with density. All of the CNT material samples with different density showed nearly constant viscoelastic properties (storage modulus, loss modulus, and damping ratio) as measured by DMA in ambient N2 over an exceptionally wide temperature range (
140 to 600 °C). It should be noted that
140 and 600 °C were determined by the limitation of the instrument, and it is expected that the temperature range to be far wider as to follow the thermal stability range of CNTs (
273.5 to ∼2000 °C).14 The temperature invariance of the viscoelastic properties is unique to the CNT material and has not been found in conventional viscoelastic materials because the viscoelastic properties of the CNT material originate from vdW interactions that are temperature invariant.15 3282

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Figure 5. Comparison of CNT material on Ashby maps. (a
c) Temperature invariance of the storage modulus, loss modulus, and damping ratio of the CNT materials with varying densities. (d, e) The viscoelastic properties of the CNT materials with different densities plotted on the Ashby maps (storage modulus vs damping ratio and strength vs maximum service temperature) to compare to existing materials.

The viscoelastic properties of the CNT materials with different densities were plotted on the Ashby maps (storage modulus versus damping ratio and strength versus maximum service temperature) to compare with existing materials. The Ashby map of the storage modulus versus damping ratio showed that the viscoelasticity of the tailored CNT materials covered a region from polymeric foams to soft rubbers. For example, the storage modulus and damping ratio (0.11 MPa and 0.1) of a low density CNT material (3.3 mg/cm3) is comparable to those of open cell polyurethane foam (0.05 MPa and 0.2). The storage modulus and damping ratio (1.2 MPa and 0.27) of the high density CNT material (54 mg/cm3) were comparable to those of carbon black natural rubber (1.5 MPa and 0.17). While exhibiting similar viscoelastic properties, the density of the CNT materials is only 1 /100 to 1/30 that of polymers, and this feature would be advantageous for applications where weight is a concern, such as transportation. The Ashby map of the maximum service temperature (the maximum operation temperature) versus strength (the maximum stress the material can bear) shows that materials with high maximum service temperature are always tough and no compliant (soft) material possesses a high service temperature. As demonstrated in Figure 5e, due to the thermal stability and softness, the CNT material does not follow this general trend and forms a distinct and isolated island in the Ashby map. In conclusion, we have fabricated a series of viscoelastic CNT materials with different densities by tuning the exposure time of RIE to the catalyst films and compression ratio. The viscoelastic properties of the CNT materials showed an interesting trend,

different from existing viscoelastic materials, where both the storage modulus and damping ratio concurrently increased with density. This trend was explained by the behavior of the node (short contacts among CNTs). We found that the storage modulus and loss modulus were proportional to the node density and product of the node density and length, respectively. Therefore the damping ratio became proportional to the node length which increased with density. This unique mechanism could serve as an approach to fabricate a highly viscous and stiff (high storage modulus) material.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of methods used. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: K. Hata ([email protected]); D. N. Futaba (d-futaba@ aist.go.jp).

’ ACKNOWLEDGMENT We acknowledge partial funding by TASC. M.X. acknowledges technical support from Dr. Seisuke Ata and Dr. Yasuaki Seki. 3283

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