NANO LETTERS
Tunable Single-Walled Carbon Nanotube Microstructure in the Liquid and Solid States Using Poly(acrylic acid)
2006 Vol. 6, No. 5 911-915
Jaime C. Grunlan,* Lei Liu, and Yeon Seok Kim Department of Mechanical Engineering, Polymer Technology Center, Texas A&M UniVersity, College Station, Texas 77843-3123 Received December 16, 2005; Revised Manuscript Received February 19, 2006
ABSTRACT Poly(acrylic acid) is shown to control the level of SWNT dispersion in aqueous mixtures and the state of dispersion in a solid composite. At low pH, PAA-stabilized suspensions containing 0.1 wt % SWNT have a waterlike viscosity, but this mixture thickens as the pH is raised. This behavior is reversed when pH is again lowered. Changing pH varies the SWNT microstructure between aggregated and well-exfoliated states, as evidenced by electron microscopy and electrical conductivity measurements.
Interest in single-walled carbon nanotubes (SWNTs) remains strong because of their small size (d ≈ 1 nm and l ≈ 1 µm),1 high modulus (E ≈ 1 TPa),2 high intrinsic electrical conductivity (σ > 104 S/cm),3 and high thermal conductivity (k > 200 W/m‚K).4 SWNTs hold significant promise for imparting electrical conductivity, mechanical strength, and thermal conductivity to polymeric materials.5,6 Despite this potential, the ability to stabilize nanotubes in solution remains a significant hurdle to their widespread use.7 This has led to significant research efforts on the use of noncovalent stabilizing agents8-14 and chemical modification of the nanotubes15-21 to impart solubility. Once stabilized, these nanotubes lend themselves to chemical modification, spectroscopic study, and/or incorporation into ink-based systems. Solubilized nanotubes are currently being studied for use in drug and gene delivery applications.22 Solid films of SWNTfilled polymers are being studied for a variety of uses including thermal management,23 humidity sensing,24 and corrosion resistance.25 The present work demonstrates a method for controlling the microstructure of carbon nanotubes in both aqueous solution and dry composite films using poly(acrylic acid) (PAA). With pH as a stimulus, nanotubes suspended in a PAA solution can be switched reversibly between a more bundled state (at low pH) and a more exfoliated state (at high pH), as shown schematically in Figure 1.26 Both states are relatively stable, but viscosity results suggest that nanotube exfoliation improves as pH is increased. Furthermore, some remnant of the suspension microstructures is maintained when the suspensions are dried and can be used to tailor composite properties. Electron * Corresponding author. Tel: (979) 845-3027. Fax: (979) 862-3989. E-mail:
[email protected]. 10.1021/nl052486t CCC: $33.50 Published on Web 03/30/2006
© 2006 American Chemical Society
microscopy and electrical conductivity are used to characterize these microstructural differences. Raw single-walled carbon nanotubes, produced via the HiPco process,27 are extremely hydrophobic and cannot be suspended in water without the use of a stabilizing agent. In the present work, SWNTs were added to a 1 wt % PAA solution using sonication to achieve a stable suspension with an approximate concentration of 1.11 mg/mL.28 The initial pH of this SWNT-PAA suspension was 2.9 and was found to have a viscosity of approximately 2 cP between shear rates of 150 and 700 s-1. As the pH of the suspension was raised, the viscosity increased gradually, but a discontinuous jump is seen between a pH of 5.6 and 7.4 as shown in Figure 2a.29 At low shear rates (
