American Institute of Aeronautics and Astronautics

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Carbon Nanotubes and Their Polymer-Based Composites in Space Environment

Department of Aeronautics and Astronautics, Massachusetts Institute of Technology,

and

Department of Physics and Geology, The University of Texas Pan American, Edinburg, TX 78541

The behavior of polymer-single walled carbon nanotube composites in the harsh conditions of the space environment is modeled based on the effects of accelerated proton beams. Radiation-induced modifications in graphite nanoparticles and single walled carbon nanotubes dispersed within high-density polyethylene have been investigated by electron spin resonance spectroscopy. Experimental data revealed that both nanocomposites are sensitive to the incoming radiation; however, single walled carbon nanotubes are less sensitive to proton irradiation (in air) than graphite.

Nomenclature

g = g-factor (equal to 2.0023 for a free uncoupled electronic spin, if relativistic corrections are included)

I = resonance line amplitude

Hr = resonance field

HPP = peak to peak resonance line width

I. Introduction UTURE space exploration will be based on the availability of lightweight materials, with improved physical properties, and multifunctional/smart capabilities. Polymer-based (nano) composites are extremely attractive as materials for future space exploration, due to their reduced weight, wide range of physical properties, and

flexibility in designing polymeric materials for targeted applications. Nanocomposites obtained by dispersing nanoparticles of various functionalities within polymeric matrices are of particular importance due to their huge surface to volume ratio. In such materials, the fraction of polymer chains involved in intermolecular interactions with nanoparticles (interface) is large, controlling the mechanical and thermal features of the composite. This allows us to improve and tailor the physical properties of the polymer by engineering polymer-nanoparticle interfaces that enhance specific physical properties and eventually triggers new ones.

1 MLK Professor, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology Cambridge, MA 02138, and AIAA Associate Fellow 2 Professor, Department of Physics and Geology, The University of Texas Pan American, Edinburg, TX 78541, Non-Member

F

AIAA SPACE 2009 Conference & Exposition14 - 17 September 2009, Pasadena, California

AIAA 2009-6769

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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The outstanding mechanical1,thermal2,3, and electrical properties4-6 of carbon nanotubes and their huge aspect ratio7

incited a sustained research on their physical properties and potential applications8-10. As may be noticed from Fig. 1, single walled carbon nanotubes (SWNTs) have outstanding mechanical properties1, 11, 12

(Young modulus in excess of TPa), excellent thermal conductivity, and exceptional electrical conductivity (for conducting SWNTs). Fig. 1 demonstrates the revolution achieved by replacing micron-sized carbon fibers by carbon nanotubes and reflects the drop of physical properties as the size of carbon structure is increased. In the last two decades, an intense research effort was focused on polymer-based nanocomposites with emphasis on the improvement of the physical properties of polymeric matrices by the dispersion of carbon nanotubes. This research demonstrated a sizable increase of mechanical properties (Young modulus) 10-13, a modest improvement of the temperature stability14, 15, a decrease of the polymer flammability16, and the enhancement of polymer thermal and electrical conductivities10-14. In Fig. 1, the performance of CNTs is compared to current state of the art materials. It shows that as well stiffness and strength increases, improvements in thermal conductivity, and heat transfer open-up new worlds of opportunity in Space Environment.

Commercially available carbon nanotubes are typically a collection of semiconducting and conducting nanotubes10-

16. As most polymers are thermal and electric insulators, the addition of carbon nanotubes enhances the thermal and electrical conductivity adding new features to the polymeric matrix. While the combined enhancement of mechanical strength, thermal conductivity, and electrical conductivity triggered by the dispersion of carbon nanotubes within polymers is beneficial for some applications, there are cases in which such multifunctional features are detrimental (the drop in the electrical resistance in electrical insulators or the increase of the thermal conductivity in thermal insulating materials). Carbon nanotubes have an outstanding thermal stability in vacuum or inert gas atmosphere but they are prone to oxidation processes (in molecular or atomic oxygen contained atmospheres)17. The thermal degradation of raw multiwall carbon nanotubes, in air, is ignited at about 500 oC and the maxim mass loss was noticed at about 590 oC17. The maximum mass loss temperature is increasing in the series raw multiwalled carbon nanotube, diamond, and graphite 17. The stability of carbon nanotubes to ionizing radiation is further decreased if atomic oxygen/zone is interacting with carbon nanotubes. The complex features of certain space environments such as Low Earth Orbit that combines ionizing radiation, electromagnetic radiation, and atomic oxygen may result in a rapid degradation of carbon nanotubes, followed by the failure of the polymer nanocomposite. While there are important technical difficulties associated with the space elevator project, such as the reduced length of carbon and the failure to project the amazing features of single walled carbon nanotubes (SWNTs) at larger scale, recent progress may provide some optimism. The soldering of carbon nanotubes under electron beam has been demonstrated18, 19 and the dispersion of carbon nanotubes within polymeric matrices resulted in an improvement of the mechanical properties of the polymeric matrices. While this improvement was not as large

Figure 1. Comparison of the performance of carbon nanotubes and conventional materials

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as expected, further efforts are concentrated on functionalized carbon nanotubes20. The amazing physical properties of carbon nanotubes reignited the space elevator project; a space exploration endeavor heavily depending on the availability of high strength, lightweight materials capable to sustain the harsh condition of the space environment (LEO, MEO, and GEO).

The paper concentrates on the combined effect of ionizing radiation, extreme temperatures, and oxygen/atomic oxygen on polymer carbon nanotube composites. Recent data concerning the ESR data on pristine and irradiated nanocomposites obtained by dispersing single wall carbon nanotubes and graphite nanoparticles within high-density polyethylene are reported.

II. Experimental Methods

Single walled carbon nanotubes and nanometer sized graphite nanoparticles were dispersed within high-density polyethylene as reported elsewhere. The concentration of the filler in all measured samples was 5% (weight). The samples have been irradiated with protons accelerated at various energies ranging from 10 MeV to 40 MeV, at various fluencies ranging from 108 protons/cm2 to 5x1010 protons/cm2. The irradiation has been carried out in air, at room temperature. The samples were stored in air, at room temperature, before electron spin resonance (ESR) measurements. ESR measurements on pristine and irradiated samples have been carried out by using a Bruker Elexsys spectrometer, operating in X-band and equipped with a variable temperature accessory.

III. Experimental Results and Discussions

The ESR spectra of single walled carbon nanotubes dispersed within high-density polyethylene consist of two overlapping signals, both of them is characterized by almost Lorentzian line shapes. The wide signal, located at g-factor values larger than 2.00 has been assigned (see the left side of Fig. 2) to magnetic impurities originating from catalysts residues. The shift of the resonance line position of the wide line from the theoretical value g=2.0023 (including relativistic correction) reflects the competition between the local molecular field acting on electronic spins and the external applied magnetic field. The other line was ascribed to conducting electrons delocalized over the conducting domains of SWNTs. This narrow and weak line (see the right panel of Fig. 2) was assigned to carbon nanotubes. The position of

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Figure 2. ESR spectra of the broad (left side) and narrow (right side) components ofthe ESR spectrum of high-density polyethylene filled with single walled carbonnanotubes. A. Pristine sample. B. Irradiation with protons accelerated at 10 MeVup to a fluence of 3x108 protons/cm2. C. Irradiation with protons accelerated at 10MeV up to a fluence of 3x109 protons/cm2. D. Irradiation with protons acceleratedat 10 MeV up to a fluence of 3x1010 protons/cm2.

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this line is slightly shifted from the theoretical position by spin-orbit interactions; nevertheless, the local magnetic field acting on the uncoupled electronic spin has strong temperature dependence, derived from the temperature dependence of the magnetization. This provides a simple path for the deconvolution of ESR lines.

No ESR line due to free radicals generated by the irradiation of the polymeric matrix was noticed. This behavior was expected as the glass transition temperature of high-density polyethylene is below -60 oC, and consequently the macromolecular chains motions occurring in the amorphous domains are excited at room temperature. This allows for the diffusion and recombination of free radicals. Hence, the changes in the ESR spectra reflect the modifications of carbon nanostructures and of magnetic residues.

In the case of high-density polyethylene filled with graphite the resonance line, at room temperature is highly asymmetric reflecting that the electrons are located in conducting domains, where the microwave field is damped via the skin effect.

The room temperature spectrum of polyethylene-graphite nanocomposites shows the usual perpendicular and perpendicular components, expected for a two-dimensional conducting system.

As may be inferred from Fig. 3, the effect of proton irradiation on polyethylene filled with graphite is unexpectedly strong. Important symmetry changes are triggered by the irradiation; the sample irradiated with 3x1010 protons/cm2 shows a low local symmetry suggesting the disruption of the two dimensional

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Figure 3. The effect of proton irradiation on the ESR spectra of high-density polyethylene-graphite nanocomposites.

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conducting islands. Such changes can be assigned to irreversible local heating effects. The spectral changes shown in Fig. 4 for pristine graphite dispersed within high-density polyethylene can be explained by the fracture of graphite layers under the effect of ionizing radiation. From Fig. 4 it is noticed that the parallel and perpendicular components of the ESR spectrum are moving away as the temperature of the sample is raised from 300 K to 370 K.

IV Conclusions ESR spectroscopy revealed that the composites obtained by the dispersion of carbon nanotubes and graphite nanoparticles within high-density polyethylene are sensitive to the incoming proton beam. In the case of high-density polyethylene-single walled carbon nanotube composites both the broad and the narrow lines are affected by irradiation. However, major and unexpected changes were noticed in the case of high-density polyethylene graphite nanoparticles and assigned to structural modifications ignited by proton’s irradiation.

Acknowledgments

The work was conducted under the auspices Dr. Martin Luther King, Jr Visiting Professorship program at MIT contract No. 2741627 and The University of Texas Pan American. Thanks are also due to TELAMS and Space Propulsion Laboratory (SPL) at MIT for assisting in many ways to making the research successful.

References 1Min-Feng Yu, Bradley S. Files, Sivaram Arepalli, Rodney S. Ruoff, Tensile Loading of Ropes of Single Wall

Carbon Nanotubes and their Mechanical Properties, Phys. Rev. Lett. Vol. 84, 2000, pp. 5552 - 5555 2Berber S, Kwon Y. K., Tomanek D., Unusually high thermal conductivity of carbon nanotubes, Phys. Rev. Lett.,

vol. 84 no. 20, 2000, pp. 4613-4616 3Dresselhaus M. S., Eklund P.C., Phonons in carbon nanotubes, Advances in Physics, vol. 49, no. 6, 2000, pp. 705-

814 4Ebbesen T. W., Lezec H. J., Hiura H., Bennett J. W., Ghaemi H. F., Thio T., Electrical conductivity of individual

carbon nanotubes, Nature, vol. 382, no. 6586, 1996, pp. 54-56 5Dai H. J., Wong E. W., Lieber C. M., Probing electrical transport in nanomaterials: Conductivity of individual

carbon nanotubes, Science, vol. 272, no. 5261, 1996, pp. 523-526 6Rao A. M., Eklund P. C., Bandow S, Thess A, Smalley R. E., Evidence for charge transfer in doped carbon

nanotube bundles from Raman scattering, Nature, vol. 388, no. 6639, 1997, pp. 257-259 7Zhu L. B., Xu J. W., Xiu Y. H., Sun Y. Y., Hess D. W., Wong C. P., Growth and electrical characterization of high-

aspect-ratio carbon nanotube arrays, Carbon, vol. 44, no. 2, 2006, pp. 253-258 8Ajayan P. M., Zhou O. Z., Carbon Nanotubes, Book Series: Topics in Applied Physics, Vol. 80, 2001, pp. 391-425 9Subramoney S., Novel nanocarbons - Structure, properties, and potential applications, Advanced Materials, vol. 10,

no. 15, 1998, pp. 1157-1213 10Kin-Tak Lau, M. Chipara, Hang-Yin Ling, D. Hui, On the effective elastic moduli of carbon nanotubes for

nanocomposite structures, Composites B, vol. 35, no. 2, 2004, pp. 95-101.

American Institute of Aeronautics and Astronautics

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11Thostenson E. T., Ren Z. F., Chou T. W., Advances in the science and technology of carbon nanotubes and their

composites: a review, Composites Science and Technology, vol. 61, no. 13, 2001, pp. 1899-1912 12Moniruzzaman M., Winey K. I., Polymer nanocomposites containing carbon nanotubes, Macromolecules, vol. 39,

no. 16, 2006, pp. 5194-5205 13Coleman J. N., Khan U., Gun'ko Y. K., Mechanical reinforcement of polymers using carbon nanotubes, Advanced

Materials, vol. 18, no. 6, 2006, pp. 689-706 14Xiong J. W., Zheng Z., Qin X. M., Li M, Li H. Q., Wang X. L., The thermal and mechanical properties of a

polyurethane/multi-walled carbon nanotube composite, Carbon, vol. 44, no. 13, 2006, pp. 2701-2707 15Biercuk M. J., Llaguno M. C., Radosavljevic M., Hyun J. K., Johnson A. T., Fischer J. E., Carbon nanotube

composites for thermal management, Applied Physics Letters, vol. 80, no.15, 2002, pp. 2767-2769 16Lin Y., Zhou B., Fernando K. A. S., Liu P., Allard L. F., Sun Y. P., Polymeric carbon nanocomposites from carbon

nanotubes functionalized with matrix polymer, Macromolecules, vol. 36, no. 19, 2003, pp. 7199-7204 17Bom D., Andrews R., Jacques D., Anthony J., Chen B. L., Meier M. S., Selegue J. P., Thermogravimetric analysis

of the oxidation of multiwalled carbon nanotubes: Evidence for the role of defect sites in carbon nanotube

chemistry, Nanoletters, vol. 2, no. 6, 2002, pp. 615-619 18Kashiwase Y., Ikeda T., Oya T., Ogino T., Manipulation and soldering of carbon nanotubes using atomic force

microscope, Applied Surface Science, vol. 254, no. 23, 2008, pp. 7897-7900 19Misra A., Daraio C., Sharp Carbon-Nanotube Tips and Carbon-Nanotube Soldering Irons, Advanced Materials,

vol. 21, no. 22, 2009, pp. 2305-2311 20Bahr J. L., Tour J. M., Covalent chemistry of single-wall carbon nanotubes, Journal of Materials Chemistry, vol.

12, no. 7, 2002, pp. 1952-1958