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Comparing Damping Properties of Singlewalled and Multiwalled Carbon Nanotube Polymer Composites

Jonghwan Suhr*, Linda Schadler**, Pulickel Ajayan** and Nikhil Koratkar***

Rensselaer Polytechnic Institute, 110 8th Street,

Troy, NY 12180-3590, USA

ABSTRACT

In this paper we compare the damping properties of polymer nano-composites filled with singlewalled and multiwalled carbon nanotube fillers. The polymer material chosen for this study is polycarbonate (Lexan 121, General Electric). The nanotube fillers are dispersed in the matrix using a novel solution mixing technique with tetra-hydro-furan as the solvent. Both the multiwalled and the singlewalled nanotube filled samples show higher damping level compared to the pristine (or unfilled) polycarbonate. However, the loss modulus of nano-composite samples with singlewalled nanotubes is significantly greater than with multiwalled tubes. This suggests that since the inner shells of the multiwalled tubes are not in contact with the polymer, they do not contribute to interfacial frictional sliding; this reduces the damping efficiency of the multiwalled tubes relative to the singlewalled tubes.

INTRODUCTION

Carbon nanotubes1-3, were first observed by Sumio Iijima in 1991. Ever since their discovery carbon nanotubes have been extensively researched because of their remarkable mechanical, electrical and electronic properties. A carbon nanotube is a thin graphene sheet rolled into a cylinder with both ends capped. Figure 1 shows a molecular model of a carbon nanotube. These quasi-one-dimensional carbon whiskers are perfectly straight tubules with --------------------------------------------------- * Graduate Research Assistant, Mechanical Eng. Dept. ** Professor, Materials Science Dept. *** Assistant Professor, Mechanical Eng. Dept. Copyright 2005 by J. Suhr, N. Koratkar, L. Schadler, P. Ajayan. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission

diameters of nanometer size, and properties close to that of ideal graphite. Singlewalled nanotubes (SWNTs) with a cylindrical shell (Figure 1a) can be considered as the fundamental structural unit. Such structural units form the building blocks of multiwalled nanotubes (MWNTs), containing multiple co-axial cylinders of increasing diameter about a common axis. Figure 1b-c show High Resolution Transmission Electron Microscopy (HRTEM) images of an MWNT and an SWNT. Many interesting applications4 of carbon nanotube technology has been suggested. These include field emission based flat-panel displays, electromechanical actuators and sensors, nano-composites, hydrogen storage, as probes for scanning probe microscopy, nanotube based field-effect transistors etc. Among these carbon nanotube composites5-12 had gathered a great deal of attention in recent years. The majority of work in the carbon nanotube composites area has focused on stiffness and strength enhancement using nanotube fillers. However it is challenging to use nanotubes for structural reinforcement due to a number of reasons including poor load transfer (leading to interfacial slip), poor alignment of nanotubes and poor dispersion of nanotubes in the polymer matrix. Among these interfacial slip is the most serious impediment to use of nanotubes for structural reinforcement and a large body of research work is focused on techniques to prevent interfacial sliding in nanotube composites. Recently our group13-14 and the Wang group15-16 at Penn. State have attempted to exploit interfacial frictional sliding to inject mechanical damping into carbon nanotube polymer composites. The exposed surface area per unit volume of carbon nanotubes is extremely large (> 109 m-1), providing an opportunity for interfacial frictional sliding to have a significant impact on the material damping behavior. We have

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identified two distinct mechanisms for energy dissipation in such systems: (1) interfacial tube-tube sliding and (2) interfacial tube-polymer sliding. Among these tube-polymer sliding shows great promise to inject damping into macroscopic structural systems. In this paper we focus on the tube-polymer sliding energy dissipation mechanism and compare the performance of singlewalled nanotube vs. multiwalled nanotube fillers. We show that singlewalled nanotube fillers are far more efficient at increasing the damping levels than multiwalled nanotube fillers. FABRICATION OF NANOCOMPOSITES Figure 2 shows a schematic diagram of the protocol that was used for the preparation of the nano-composite samples. In this study, purified nanotubes were purchased commercially. A solution mixing process17-18 with Tetrahydrofuran (THF) as the solvent was used to disperse the nanotubes in the polymer matrix. Note that because THF is a moderate solvent for polycarbonate (PC), it limits the interaction energy between the polymer and the solvent and encourages the physical adsorption of the polymer onto the nanotube surface. The absorbed polymer serves as a surfactant for dispersing the nanotubes in the solution and then in the polycarbonate matrix, and also as a load transfer agent from the bulk polymer to the nanotubes. This results in reasonable good quality dispersion and separation of the nanotubes in the polymer matrix. The nanotubes were first sonicated in THF and polycarbonate was dissolved separately in THF. The nanotube dispersion and PC solution were then mixed in a ratio that resulted in the required nanotube concentration in the polymer, and the mixture was sonicated (750W, 20 KHz) for 15 minutes. To obtain the nanocomposite, the mixture was poured very slowly into methanol (methyl alcohol, anhydrous). The volume ratio between THF and methanol was 1:5. The composite material precipitated immediately (since methanol is an anti-solvent for polycarbonate) and was filtered and dried out under vacuum for 14 hours. A compressive mold (pre-heated to 205º C) was used to prepare the standard tensile (dog-bone shaped) specimens. The weight fraction of nanotubes in the nano-composite was varied in the 0.1 to 5% range. Pure polycarbonate samples (without nanotube fillers) of the same dimensions were also prepared (following protocol of figure 2) to compare the response of the two materials. Figure 3 shows typical Scanning Electron Microscopy (SEM) images of the fracture surface for the nanocomposites with

singlewalled nanotube fillers. As seen in the SEM images, individual nanotubes are fairly well dispersed in the polymer matrix and are pulling out of the fracture surface. Similar results (not shown here) were also obtained for MWNT dispersion.

TEST PROCEDURE Figure 4 shows a schematic for the viscoelastic characterization of MWNT-PC nanocomposites. The samples are tested under uniaxial cyclic loading using an MTS-858 servo-hydraulic test system. All tests in this study are performed at room temperature. Dynamic strain and stress data are measured using an MTS 632.26E-20 extensometer and the load cell of MTS-858 system. In order to characterize and quantify the damping behavior, the linearized material complex modulus19 was calculated using the measured uniaxial stress (σ) and corresponding strain (ε) response. The linearized stress-strain relation can be expressed as: εσ )" '( jEE += (1) where the in-phase component ( 'E ) determines the storage or elastic modulus (i.e. real part of complex modulus) and the quadrature component ( "E ) determines the loss modulus (i.e. imaginary part of complex modulus). To obtain the storage and loss moduli, we applied sinusoidal (or oscillatory) strain to our sample: ε = ε0 sin(ωt), then we measured the resulting stress response, σ = (σ0 cosδ) sin(ωt) + (σ 0 sinδ) cos(ωt), where σs = σ0 cosδ represents the component of the stress that is in phase with the strain and σc = σ0 sinδ represents the component of the stress that is out of phase with respect to the strain. Note that σ0 is the amplitude of the stress, ω is the angular frequency of the applied strain and δ is a phase angle related to material viscoelasticity. The Fourier transform method was used to obtain the in-phase (σs) and out-of-phase (σc) components of the measured uniaxial stress response in the frequency domain. The elastic and loss moduli were then calculated as follows:

0/' εσsE =

0/" εσcE = (2)

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TEST RESULTS

Figure 5 shows test data for the loss modulus of a nano-composite sample with 1.5 wt. % of singlewalled carbon nanotube fillers plotted as a function of strain amplitude. Results for a pristine polycarbonate sample (without any fillers) is also shown for comparison. The baseline pure polycarbonate sample shows strain-independent behavior over the entire strain range. In contrast for the nano-composite sample shows a dramatic increase in loss modulus as the strain amplitude is increased. This suggests that as the strain amplitude is increased the critical interfacial shear stress for nanotube-polymer interfacial slip is reached and filler-matrix sliding is activated resulting in an increase in loss modulus. This increase in the material damping with increasing strain amplitude is a gradual process (figure 5) because not all the nanotube-polymer interfaces will fail simultaneously. Since the nanotube distribution in the polymer is random (no preferred orientation) those tubes that are better aligned with the loading direction tend to fail first. As the strain level is increased more and more of the interfaces begin to fail resulting in a progressive increase in the damping effect. Note that below about 0.2% strain amplitude, the loss modulus of the nanocomposite sample was similar to the baseline polycarbonate, which confirms that fiber-matrix sliding is not fully activated at low strain amplitudes. Above about 1% strain level, the loss modulus response plateaus (levels off) indicating that fiber-matrix slip has been activated for a majority of nanotubes in the composite. We also varied the test frequency in the 1-10 Hz frequency range (figure 5); the results indicate that frequency has a weak effect on the energy dissipation in the 1-10 Hz frequency range. These tests were then repeated with multiwalled carbon nanotube (MWNT) fillers. The results are summarized in figure 6. Qualitatively speaking the response closely resembled the SWNT-PC system (figure 5) that we tested previously. Below ~ 0.2% strain amplitude (tube-matrix sliding is not activated), and the loss modulus of the nano-composite sample was close to the baseline polycarbonate. Above ~ 1% strain level, the loss modulus response levels off indicating that tube-matrix slip has been activated for a majority of the MWNT in the composite. However quantitatively speaking there is a clear difference in the results. To get a comparable loss modulus the weight fraction of the MWNT sample (figure 6) had to be increased to about 5% relative to about 1.5% for the SWNT sample (figure 5). We checked the

dispersion quality of the MWNT and found that it was in fact superior to the SWNT samples shown previously in figure 3. Inspite of the better dispersion the weight fraction of MWNT had to be significantly raised in order to match the performance of the SWNT fillers. We theorize that the main reason for this is that for the MWNT case only the outermost nanotube cylinders are in contact with the matrix. Therefore the inner cylinders do not contribute to the nanotube-matrix sliding energy dissipation process. Consequently the damping performance of MWNT fillers suffers in comparison to the SWNT fillers. We performed additional tests with SWNT fillers to determine the optimal sonication time associated with nano-composite fabrication. We discovered that sonication time plays an important role in controlling the dispersion quality of the nanotubes and this has a strong impact on the material damping response. Figure 7 shows some of our results which indicates that with improved sonication time the loss modulus of the nano-composite sample is significantly improved. However no significant improvement was seen beyond about 15 minutes sonication time.

SUMMARY AND CONCLUSIONS

Our tests results indicate that MWNT fillers are not as effective as SWNT fillers in augmenting damping in polymer nano-composites. To get comparable loss modulus with both types of fillers, the weight fraction of the MWNT samples had to be 3 to 4 times higher than the corresponding weight fraction for the SWNT polymer nano-composite samples. This effect was observed inspite of the fact that the dispersion quality with MWNT is in fact superior to SWNT. The reason for the enhanced performance of the SWNT fillers can be attributed to the fact that for SWNTs each tube is in contact with the matrix, while for the MWNTs, the majority of tubes are internal to the outermost shell and therefore do not make contact with the polymer matrix. This greatly reduces the effectiveness of the nanotube-polymer frictional sliding mechanism for energy dissipation.

ACKNOWLEDGEMENTS Financial support for this research was provided by the Structures and Dynamics Program of the US Army Research Office, with Dr Gary Anderson serving as the Technical Monitor.

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REFERENCES

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nanotube films for damping applications,” Advanced Materials, 14, 997-1000, (2002).

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investigation of carbon nanotube based polymers for improved structural damping. SPIE International Symposium on Smart Structures and Materials, Damping and Isolation Conference, 5386-18, San Diego, CA, (2004).

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FIGURES

Fig. 1 (a) Molecular model of a singlewalled carbon nanotube; (b) TEM image of a singlewalled carbon nanotube. (c) TEM image of a multiwalled carbon nanotube with multiple coaxial concentric cylinders.

Polycarbonate in THF Carbon nanotubes in THF

Precipitation inMethanol (anti - solvent)

Compressivemolding

Filtration

Mix-up

Polycarbonate in THF Carbon nanotubes in THF

Precipitation inMethanol (anti - solvent)

Compressivemolding

Filtration

Mix-up

Figure 2: Procedure used to disperse carbon nanotubes in the polymer matrix

a b

c

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Figure 3: SEM images of the fracture surface of the samples showing fairly uniform dispersion of singlewalled nanotubes in the polymer matrix

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Figure 4: Schematic of viscoelastic characterization of the nano-composite samples

Figure 5: Loss modulus response of polycarbonate with 1.5 wt. % of SWNT fillers.

0 0.25 0.5 0.75 1 1.25 1.50

10

20

30

40

50

Strain (%)

Loss

Mod

ulus

(MPa

)

SWNT(1.5wt%)-PC @1Hz SWNT(1.5wt%)-PC @5Hz SWNT(1.5wt%)-PC @10HzPolycarbonate @1Hz Polycarbonate @5Hz Polycarbonate @10Hz

ε 0 sin(ωt)

σ sin(δ+ωt)

t0

Uniaxial load

ε 0 sin(ωt)

σ sin(δ+ωt)

t0

Uniaxial load

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Figure 7: Effect of sonication time on damping performance for case of SWNT fillers

0 0.25 0.5 0.75 1 1.25 1.50

10

20

30

40

50

Strain (%)

Loss

Mod

ulus

(MPa

)

Sonication time: 15minSonication time: 2min

Regular Sonication Time (15 min)

Reduced Sonication Time (2 min)

0 0.25 0.5 0.75 1 1.25 1.50

10

20

30

40

50

Strain (%)

Loss

Mod

ulus

(MPa

)

MWNT(5.0wt%)-PC Pure Polycarbonate

Figure 6: Loss modulus response of polycarbonate with 5 wt. % of MWNT fillers.