Improved Method of Functionalizing Carbon Nanotubes to Form Polymer Nanocomposites

An improved method of functionalizing carbon nanotubes in order to form polymer nanocomposites has been developed by University of Houston Professor of Physics Seamus A. Curran and  Dr. Amanda V. Ellis, a Senior Lecturer in Chemistry/Nanotechnology at the School of Chemistry, Physics and Earth Sciences and Flinders University and the Center for NanoScale Science and Technology in Adelaide South Australia. 

The method provides a practical method for creating transparent antistatic coatings, electromagnetic insulating ("EMI") shields, and reinforced polymeric membranes in fuel cells. The material comprises a smooth nanocomposite with enhanced electrical conductivity up to 33 sm^-1 with 0.90 wt % nanotube loadings and with improved thermal stability as compared with non-covalently linked MWCNT-polystyrene blends.

The manufacturing methods are detailed in U.S. Patent 7,713,508 which is held by Arrowhead Center, Inc. (Las Cruces, NM), the technology transfer and development arm of the University of New Mexico system.

 Curran and Ellis developed carbon nanotube structures that are covalently bonded via chemically reactive groups on the outer walls of the nanotubes and methods for forming the covalently bonded nanotube structures. Their invention also includes materials comprising the functionalized nanotubes covalently bonded to organic based monomers and/or polymers, and methods for their formation.  The manufacturing methods developed by Curran and Ellis allows for facile synthesis of nanotube-dithioester structures, synthesis of nanotube-polystyrene nanocomposites, and  nanotube-styrene nanocomposites.  

Polymer/nanotube nanocomposites are of interest because of their potential structural and electronic applications. However, the chemical interaction between polymers and nanotubes is typically limited to van der Waals forces and weak electrostatic interactions. It is known in the art that functionalization enhances the interaction between nanotubes and other organic matrices. 

FIG. 4 shows a scanning electron micrograph of pristine MWCNT's;

FIG. 5 shows a scanning electron micrograph of functionalized MWCNT's;

FIG. 6 shows (a) a scanning electron micrograph of purified, pristine MWCNT's, (b) a scanning electron micrograph of MWCNT's after a 12 hour acid treatment and thiation with phosphorus pentasulfide, (c) and (d) transmission electron micrographs of functionalized MWCNT's, and (e) the interface regions between a pair of covalently linked MWCNT's.  FIG. 6(e), shows evidence of covalent attachment in which there is a continuation in the graphitic lattice fringes of one nanotube to the next--this is not observed for typical CNT bundles held together via van der Waals forces. 

FIG. 6 shows FESEM images of the purified pristine MWCNT's prepared via the arc-discharge method (a) and MWCNT's after 12 hours of acid treatment and thiolation with phosphorus pentasulfide (b). Also shown are high resolution transmission electron microscopy ("HRTEM") images for the functionalized MWCNT's (.about.20 nm in diameter) (c) and (d) and the expanded region of interface between a pair of covalently linked MWCNT's showing uninterrupted lattice fringes of the outer graphitic walls of the tubes (e). For the HRTEM evaluation, the functionalized MWCNT's were dispersed in toluene and drop cast onto a Cu-grid with a holey carbon support prior to high-resolution transmission electron microscopy. 

The HRTEM micrographs (FIG. 6(c)) illustrate that the reaction products are bundles of aligned and closely bound (fused and tethered) nanotubes. The bundle represented is a group of three tubes overlapped by a fourth tube. FIG. 6(d), illustrates aligned attachment of the ends of two multi-walled nanotubes (.about.20 nm in diameter). Attachment at the ends predominates as acid attack results in a higher concentration of functional groups.

The preferred method for the formation of nanocomposites comprises sonicating dithioester MWCNT's in a solvent, preferably anhydrous toluene, and adding styrene. The mixture is preferably heated, preferably in a silicon oil bath, preferably at approximately 100.degree. C. for approximately 24 hours with continuous stirring. The reaction mixture is preferably cooled, tetrahydrofuran ("THF") is preferably added, and the solution is preferably added to methanol. The resulting precipitate is preferably filtered and volatile materials removed under vacuum to yield the composite material.

Dr. Curran joined the Physics Department at the University of Houston in July 2007 as an Associate Professor specializing in nanocomposites fabrication for applications in nanoelectronics and nanophotonic systems. Previously, he held the position of Assistant Professor at New Mexico State University (NMSU). Prior to joining NMSU he held postdoctoral positions at Rensselaer, the Max Planck Institute in Stuttgart and both postdoctoral and lecturer position at Trinity College.

The fundamental control over composite morphologies and interfacial chemical interactions is critical for dramatically enhancing their performances in electronic applications. Although covalent attachment of polymers to nanotubes has been achieved, high loadings with poor nanotube dispersions dominate composite formation and device fabrication. Ion-conducting polymers, or polymers containing conductive fillers such as carbon black (5-30% by weight loading), are typically employed for electronic applications. However, prior techniques for producing carbon composites suffer from several drawbacks including having high dopant or filler loadings, being brittle, and being opaque as thin films. Carbon nanotubes offer a viable alternative.

However, although single-walled carbon nanotubes (SWCNT's) provide the highest conductivities at low loadings, homogeneous dispersion is still problematic. In the prior art, multi-walled carbon nanotube ("MWCNT") composites have offered a similar electronic potential to SWCNT's, but the loadings have been considerably higher, and the conductivities obtained have been considerably lower.

Nanotubes in polymer composites do not have higher conductivities because of aggregation and tube-to-tube proximity. The result is that charge transport is via a hopping process where the charge carrier motion is determined by scattering from one conductive site to the next. Therefore, poor nanotube dispersion aggravates the poor and reduced carrier transport capability of the composite.

Carbon nanotubes are inert but can be chemically functionalized. Functionalization and derivatization increases interfacial binding in composites thus providing a mechanism for connecting nanotubes together and/or to substrates. Because of the limited scope of direct covalent sidewall functionalization at defect sites, traditional chemical treatments such as wet oxidation in concentrated HNO3/H2SO4 are used to functionalize nanotube surfaces with hydroxyl (--OH), carboxyl (--COOH) and carbonyl (>C.dbd.O) groups. In the art, carboxylic groups have been converted to acid chloride using thionyl chloride with subsequent amide linkage of an aminoalkanethiol to single-walled nanotubes ("SWNT's"). More recently SWNT's have been derivatized by thionyl chloride to produce thiols on the ends of SWNT's.