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

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

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

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

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

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.

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

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.

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

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.

Co-valent Functionalization Epoxide terminated molecule and carboxylated nanotubes Schadler, RPI Andrews, UK

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 Composites through Chemical Functionalization. Chemistry of Materials, 2003. 15: p. 4470-4475.

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

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.

20 wt% MWNT/Carbon Fiber

Baughman Group poly(vinyl alcohol) fibers tensile strength of 1.8GPa 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

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

Kumar et al SWNT/Polymer Fibers Fabricated fibers with 1 to 10 wt% NT 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

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.

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

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

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

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