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Facilitating Interfacial Slip in Carbon Nanotube Polycarbonate Composites
Jonghwan Suhr* and Nikhil A. Koratkar† Department of Mechanical, Aerospace and Nuclear Engineering
Rensselaer Polytechnic Institute, Troy, NY, 12180-3590
In this paper we examine the effect of pre-strain (or biased strain) and temperature on interfacial slip in singlewalled carbon nanotube polycarbonate composites. The nanotube composite is tested under uni-axial sinusoidal loading to characterize the baseline response (storage, loss modulus) of the material. Next a biased strain (0 to 0.85%) is applied and then the sinusoidal load is superposed on to the pre-strain. The results indicate a significant damping enhancement which suggests that pre-strain facilitates the activation of nanotube polymer interfacial slip. Next we tested the nano-composite under sinusoidal loading with zero pre-strain, while changing the temperature in the 25-90 °C range. The results indicate that temperature also facilitates the activation of interfacial slip leading to a significant increase in energy dissipation.
I. Introduction Applications of damping enhancement range from turbo-machinery and engine/equipment mounts to suspension
and steering systems in automotive engineering. Damping also holds the potential for improving the tracking and pointing characteristics and therefore the accuracy of weapon systems mounted on aircraft and land systems. For these reasons, it is important to design structural components with high levels of inherent mechanical damping.
Viscoelastic materials such as high loss factor polymers [1-2] are rapidly gaining popularity in damping
applications. While viscoelastic damping treatments are shown to be promising, they suffer from several important limitations such as: high weight penalty, compactness issues, poor reliability, low thermal conductivity and poor performance at elevated temperatures. In addition to passive damping, several active damping treatments [3-6] have also been explored. However while these techniques offer enhanced damping they are still limited by the deficiencies present in the basic polymer and incur significant weight and power penalty. Therefore there is a critical need to develop advanced materials for damping applications that can overcome the limitations discussed above.
The high Young’s modulus and tensile strength of carbon nanotubes has generated great interest in the research
community regarding the potential development of super-strong, super-stiff composites with carbon nanotube reinforcement fibers. Several groups [7-10] have shown that the efficiency of load transfer in such systems is critically dependent on the quality of adhesion between the nanotubes and polymer chains. Strengthening nanotube-polymer interfaces (to prevent interfacial slip) is an area of on-going research. However interfacial slip while detrimental to mechanical properties enhancement can have a significant impact on the damping response of the nano-composite. In recent years several groups [11-14] have shown that very significant increase in structural damping is possible in composite structures by the use of carbon nanotube additives. Following up on this work our group has recently [15-16] characterized the effect of pre-strain and temperature on interfacial friction damping in single-walled carbon nanotube polycarbonate composites. In our tests a biased strain or pre-strain (0 to 0.85%) is applied and then the sinusoidal load is superposed on to the pre-strain. The results indicate a significant damping enhancement which suggests that pre-strain facilitates the activation of nanotube polymer interfacial slip. To study the effect of temperature we tested the nano-composite under sinusoidal loading with zero pre-strain, while changing
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the temperature in the 25-90 ° C range. Our results show that temperature also facilitates the activation of interfacial slip leading to a significant increase in energy dissipation.
First, the protocol used to fabricate the nano-composite samples is described. This is followed by details of
uniaxial mode testing of the nano-composites to determine the viscoelastic properties of the material. Finally the test results are presented followed by summary and conclusions.
II. FABRICATION OF NANOCOMPOSITES A solution mixing method (previously reported in our group [12]) was used for the preparation of singlewalled
carbon nanotubes (SWNT) and bisphenol-A-polycarbonate (Lexan 121, General Electric) nanocomposites. In this study, purified SWNT were purchased from CNI in Houston, TX, with an average length of 1 µm and average diameter of 1.4 nm. Prior to dispersion the SWNT were oxidized by sonication in nitric acid. This is necessary to improve the dispersion quality of the nanotubes in the polycarbonate. A solution mixing process with Tetrahydrofuran (THF) as the solvent was used to disperse the SWNT in the polymer matrix. The SWNT were first sonicated in THF and polycarbonate was dissolved separately in THF. The SWNT dispersion and PC solution were then mixed in a ratio that resulted in the required SWNT concentration in the polymer, and the mixture was sonicated (750W, 20 KHz) for 15 minutes. To obtain the SWNT-PC 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 samples have dimensions of ~ 3.2 mm (width), 3.2 mm (thickness) and 63.25 mm in length. The weight fraction of SWNT in the nano-composite was about 1.5%. Pure polycarbonate samples (without nanotube fillers) of the same dimensions were also prepared (following similar protocol of the nano-composites) to compare the response of the two materials. Figure 1 shows a typical Scanning Electron Microscopy (SEM) image of the fracture surface for a SWNT-PC composite sample. As seen in the SEM images, individual SWNTs are very well dispersed in the polymer matrix and are pulling out of the fracture surface. High resolution SEM (not shown here) indicated that the SWNT aggregate into 30-35 nm size bundles. This is expected as dispersion down to the single tube level is not possible with SWNT.
III. NANO-COMPOSITE TESTING Figure 2 shows a schematic for the viscoelastic characterization of SWNT-PC nanocomposites. The samples are
tested under uniaxial cyclic loading using an MTS-858 servo-hydraulic test system. 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 modulus 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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Next we applied a biased strain with a superposed sinusoidal strain to study effect of pre-strain on the storage
and loss modulus response of the SWNT-polycarbonate composites. The pre-strain level was varied in the 0-0.85% range. To study the effect of temperature we used an MTS 651.05E environmental chamber to vary the test temperature in the 25-90 °C range. For each data point, the desired test temperature was maintained for at least 10 minutes prior to the application of the dynamic cyclic loading. Two built-in thermocouples inside the environment chamber were used to monitor the test temperature.
IV. RESULTS AND DISCUSSION Figure 3 and figure 4 show data for storage and loss moduli of SWNT-PC composite and pure polycarbonate
samples. We applied a static or mean pre-strain to the composite prior to the application of the dynamic cyclic loading. Our test results indicate that significant improvement in damping performance is possible with the application of pre-strain. This is because pre-strain raises the interfacial shear stress for the nanotube inclusions allowing the critical stress for tube-matrix interfacial slip to be reached at a relatively lower strain amplitude. Figures 3-4 indicate that for 0.35% strain amplitude, as the pre-strain is increased from 0 to 0.85%, the storage modulus decreases from 2700 to about 2450 MPa and the loss modulus increases from 37 MPa to about 55 MPa. In contrast, for the pure polycarbonate sample no significant change in the storage/loss modulus response was observed with the application of pre-strain.
Figure 5 and figure 6 show the effect of temperature on the nanotube-polymer sliding energy dissipation mechanism. We observed enhanced damping above room temperature. The reason for this is that at elevated temperatures proximity to the polymer’s glass transition temperature enhances the mobility of the polymer chains making it relatively easier to activate tube-polymer sliding. As the test temperature is raised, interfacial slip is activated at a relatively lower strain amplitude causing a decrease in the storage modulus (figure 5) coupled with a simultaneous increase in the loss modulus response (figure 6).
V. SUMMARY AND CONCULSIONS In this study we investigated the effect of pre-strain and temperature on the damping properties of nano-
composite materials. We show that the damping properties of nanotube composites are significantly enhanced as the pre-strain levels are increased. At elevated pre-strains, interfacial nanotube-matrix slip can be activated at lower dynamic strain levels, thereby maximizing the energy dissipation capability of the system. In typical operational environments, both static as well as dynamic loads are reacted by the structure. As a consequence the nano-composite’s strain response will be comprised of a dynamic (oscillatory) component superposed on a static component (or a pre-strain). In this paper we show that such a static component of the strain is beneficial from a damping perspective.
We also show in this paper that the damping properties of carbon nanotube composites are enhanced as the
operating temperatures are increased. At elevated temperatures, interfacial nanotube-matrix slip can be activated at lower strain levels, leading to enhanced dissipation. These results suggest the strong potential of nano-composites as high temperature damping materials for vibration and acoustic suppression in a variety of dynamic systems.
Acknowledgments 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.
References
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3Baz, A. & Ro, J. “The concept and performance of active constrained layer damping treatments”, J. Sound Vibrat. 28, 18-21 (1994). 4Liao, W. H. & Wang, K. W. “On the analysis of viscoelastic materials for active constrained layer damping treatments”, J. Sound Vibrat. 207, 319-334 (1997). 5Liao, W. H. & Wang, K. W. “Characteristics of enhanced active constrained layer damping treatments with edge elements”, J. Vib. Acoust. 120, 886-893 (1998). 6Liu, Y. & Wang, K. W. “A non-dimensional parametric study of enhanced active constrained layer damping treatments”, J. Sound Vibrat. 223, 611-644 (1999). 7Shadler, L. S., Giannaris, S. C. & Ajayan, P. M. “Load transfer in carbon nanotube epoxy composites”, Appl. Phys. Lett. 73, 26, 3842-3844 (1998). 8Ajayan, P. M., Shadler L. S., Giannaris C. & Rubio, A. “Single-walled carbon nanotube-polymer composites: strength and weakness”, Adv. Mater. 12, 750-753 (2000). 9Wagner, H. D., Lourie, O., Feldman, Y. & Tenne, R. “Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix”, Appl. Phys. Lett. 72, 188-190 (1998). 10Thostenson, E. T. & Chou, T-W. “Aligned multi-walled carbon nanotube-reinforced composites: processing and mechanical characterization”, J. Phys. D: Appl. Phys. 35, L77–L80 (2002). 11Suhr, J., Koratkar, N., Keblinski, P., Ajayan, P. M. “Viscoelasticity in Carbon Nanotube Composites”, Nature Materials 4, 134-137 (2005). 12Koratkar, N., Suhr, J., Joshi, A., Kane, R., Schadler, L., Ajayan, P., Bertolucci, S. “Characterizing energy dissipation in single-walled carbon nanotube polycarbonate composites”, Appl. Phys. Lett. 87, 06312 (2005). 13Zhou, X., Wang, K. W., Bakis “Damping Characteristics of Nanotube Enhanced Composites”, C. E. Composites Science and Technology 64, 2425-2437 (2004). 14Rajoria, H. & Jalili, N. “Passive vibration damping enhancement using carbon nanotube-epoxy reinforced composites”, Composites Science and Technology 65, 2079-2093 (2005). 15Suhr, J. & Koratkar, “Effect of Pre-Strain on Interfacial Friction Damping in Carbon Nanotube Polymer Composites”, N. Journal of Nanoscience and Nanotechnology 6, 483-486 (2006). 16Suhr, J., Zhang, W., Ajayan P., Koratkar “Temperature-Activated Interfacial Friction Damping in Carbon Nanotube Polymer Composites”, Nano Lett. 6, 219-223 (2006).
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Figure 1: Scanning Electron Microscopy (SEM) image of the fracture surface of a typical nano-composite sample showing uniform dispersion of singlewalled nanotubes in the polycarbonate matrix.
Figure 2: Schematic of viscoelastic (uniaxial mode) testing of the nanocomposite samples
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