Carbon Nanotube Polymer Composites: A Review of Recent Developments

1. Carbon Nanotube Polymer Composites: A Review of Recent Developments Rodney Andrews & Matthew Weisenberger University of Kentucky Center for Applied Energy Research

2. Nanotube composite materials are getting stronger, but… …not there yet…

3. Nanotube Composite Materials • Engineering MWNT composite materials • Lighter, stronger, tougher materials • Lighter automobiles with improved safety • Composite armor for aircraft, ships and tanks • Conductive polymers and coatings • Antistatic or EMI shielding coatings • Improved process economics for coatings, paints • Thermally conductive polymers • Waste heat management or heat piping • Multifunctional materials

4. High Strength Fibers • To achieve a high strength nanotube fiber: • High strength nanotubes (> 100 GPa) • Good stress transfer from matrix to nanotube • Or, nanotube to nanotube bonding • High loadings of nanotubes • Alignment of nanotubes (< 5° off-axis) • Perfect fibers • Each defect is a separate failure site

5. Issues at the Interface • Interfacial region, or interaction zone, can have different properties than the bulk polymer: • chain mobility, • entanglement density, • crosslink density • geometrical conformation • Unique reinforcement mechanism • diameter is of the same size scale as the radius of gyration • can lead to different modes of interactions with the polymer. • possible wrapping of polymer chains around carbon

6. MWNT/Matrix Interface • The volume of matrix that can be affected by the nanotube surface is significantly higher than that for traditional composites due to the high specific surface area. • 30nm diameter nanotubes have about 150 times more surface area than 5 µm fibers for the same filler volume fraction Ding, W., et al., Direct observation of polymer sheathing in carbon nanotube-polycarbonate composites. Nano Letters, 2003. 3(11): p. 1593-1597.

7. Interphase Region • Nanotube effecting crystallization of PP • Sandler et al, J MacroMol Science B, B42(3&4), pp 479-488,2003

8. Two Approaches for Surface Modification of MWNTS • Non-covalent attachment of molecules • van der Waals forces: polymer chain wrapping • Alters the MWNT surface to be compatible with the bulk polymer • Advantage: perfect structure of MWNT is unaltered • mechanical properties will not be reduced. • Disadvantage: forces between wrapping molecule / MWNT maybe weak • the efficiency of the load transfer might be low. • Covalent bonding of functional groups to walls and caps • Advantage: May improve the efficiency of load transfer • Specific to a given system – crosslinking possibilities • Disadvantage: might introduce defects on the walls of the MWNT • These defects will lower the strength of the reinforcing component.

9. Polymer Wrapping • Polycarbonate wrapping of MWNT (Ruoff group) Ding, W., et al., Direct observation of polymer sheathing in carbon nanotube-polycarbonatecomposites. Nano Letters, 2003. 3(11): p. 1593-1597.

10. Shi et al - Polymer Wrapping • Activation/etching of MWNT surface • Plasma deposition of 2-7 nm polystyrene • Improved dispersion • Increased tensile strength and modulus • Clearly defined interfacial adhesion layer • Shi, D., et al., Plasma coating of carbon nanofibers for enhanced dispersion and interfacial bonding in polymer composites. Applied Physics Letters, 2003. 83(25): p. 5301-5303.

11. Co-valent Functionalization Epoxide terminated molecule and carboxylated nanotubes Schadler, RPIAndrews, UK

12. Velasco-Santos et. Al. • Functionalization and in situ polymerization of PMMA • COOH and COO- functionalities • in situ polymerization with methyl methacrylate • increase in mechanical properties for both nanotube composites compared to neat polymer • improvements in strength and modulus of the functionalized nanotube composite compared to unfunctionalized nanotubes • The authors conclude that “functionalization, in combination with in situ polymerization , is an excellent method for producing truly synergetic composite materials with carbon nanotubes” • Velasco-Santos, C., et al., Improvement of Thermal and Mechanical Properties of Carbon Nanotube Compositesthrough Chemical Functionalization. Chemistry of Materials, 2003. 15: p. 4470-4475.

13. In Situ Polymerization of PAN • Acrylate-functionalized MWNT which have been carboxilated • Free-radical polymerization of acrylonitrile in which MWNTs are dispersed • Hope to covalentely incorporate MWNTs functionalized with acrylic groups

14. Strong Matrix Fiber Interaction • SEM images of fracture surfaces indicate excellent interaction with PAN matrix, note ‘balling up’ of polymer bound to the MWNT surface. This is a result of elastic recoil of this polymer sheath as the fiber is fractured and these mispMWNTs are pulled out.

15. 20 wt% MWNT/Carbon Fiber

16. Baughman Group • poly(vinyl alcohol) fibers • containing 60 wt.% SWNTs • tensile strength of 1.8GPa • 80GPa modulus for pre-strained fibers • High toughness • energies-to-break of 570 J/g • greater than dragline spider silk and Kevlar • Dalton, A.B., et al., Super-tough carbon-nanotube fibres. NATURE, 2003. 423: p. 703

17. Kearns et al – PP/SWNT Fibers • SWNT were dispersed into polypropylene • via solution processing with dispersion via ultrasonic energy • melt spinning into filaments • 40% increase in tensile strength at 1wt.% SWNT addition, to 1.03 GPa. • At higher loadings (1.5 and 2 wt%), fiber spinning became more difficult • reductions in tensile properties • “NTs may act as crystallite seeds” • changes in fiber morphology, spinning behavior • attributable to polymer crystal structure. • Kearns, J.C. and R.L. Shambaugh, Polypropylene Fibers Reinforced with Carbon Nanotubes. Journal of Applied Polymer Science, 2002. 86: p. 2079-2084

18. Kumar et al • SWNT/Polymer Fibers • PMMA • PP • PAN • Fabricated fibers with 1 to 10 wt% NT • Increases in modulus (100%+) • Increases in toughness • Increase in compressive strength • Decrease in elongation to break • Decreasing tensile strength

19. Kumar – PBO/SWNT Fibers • high purity SWNT (99% purity) • PBO poly(phenylene benzobisoxazole) • 10 wt% SWNT • 20% increase in tensile modulus • 60 % increase in tensile strength (~3.5 GPa) • PBO is already a high strength fiber • 40% increase in elongation to break • Kumar, S., et al., Fibers from polypropylene/nano carbon fiber composites. Polymer, 2002. 43: p. 1701-1703. • Kumar, S., et al., Synthesis, Structure, and Properties of PBO/SWNT Composites. Macromolecules, 2002. 35: p. 9039-9043. • Sreekumar, T.V., et al., Polyacrylonitrile Single-Walled Carbon Nanotube Composite Fibers. Advanced Materials, 2004. 16(1): p. 58-61.

20. Electrospun Fibers • (latest Science article) • Leaders in Field • Frank Ko – Drexel University • ESpin Technologies (TN) • Ko has done extensive work for DoD • Reasonable strengths, but poor transfer fibril to fibril • Not a contiguous graphite structure

21. Conclusions • Nanotubes are > 150 GPa in strength. • Strain-to-break of 10 to 20% • Should allow 100 GPa composites • Challenges still exist • Stress transfer / straining the tubes • Controlling the interface • Eliminating defects at high alignment • Work is progressing among many groups

22. University of Kentucky Center for Applied Energy Research Acknowledgements • Financial Support of the Kentucky Science and Engineering Foundation under grant KSEF-296-RDE-003 for “Ultrahigh Strength Carbon Nanotube Composite Fibers”

23. Questions???