Water‐Based Single‐Walled‐Nanotube‐Filled Polymer Composite with an Exceptionally Low Percolation Threshold

Abstract

Polymer composites containing carbon-based filler (e.g., carbon black or carbon nanotubes) are becoming increasingly useful due to their unique combination of metallic conductivity and polymer flexibility. These materials are being investigated for use as electromagnetic interference (EMI) shielding, [1] heat dissipation films, [2] chemical sensors, [3] actuators, [4] photoconductors, [5] and impedance adapters for organic lightemitting diodes (OLEDs).[6] Despite these advantages, carbon black concentrations of more than 25 wt.-% are often required to achieve sufficient electrical conductivity in typical solution or melt-based systems. [7,8] High filler loadings can lead to processing difficulties and loss of polymer-like mechanical behavior. The critical filler concentration needed to achieve true electrical conductivity, known as the percolation threshold, [9] can be reduced in a number of ways. For carbon-black-filled composites, percolation thresholds below 5 wt.-% have been achieved by using a polymer emulsion (or latex) binder, [10] while solution-based systems have exhibited thresholds below 1 wt.-% by replacing carbon black with carbon nanotubes.[6]In the present work, these two concepts have been combined to achieve a percolation threshold below 0.1 wt.-%. Single-walled carbon nanotubes (SWNTs) produced via the high pressure carbon monoxide (HiPco) process [11] are extremely hydrophobic and require the use of a stabilizer to achieve a stable aqueous suspension. Gum Arabic (GA), known to be an effective stabilizing agent for SWNTs in water, [12] was used for this purpose. The quality of nanotube stabilization is evidenced by transmission electron micrographs (TEMs) shown in Figure 1. These are images of dried suspensions that originally contained 2 wt.-% GA and 0.4 wt.-% SWNTs in water. At relatively low magnification (Fig. 1a), most of the SWNTs can be seen as aggregated threads. In the higher magnification image (Fig. 1b) it is clear that these clusters are comprised of 20±30 nanotubes, assuming the individual tubes have an average diameter of 1.2 nm.[11] The dark particles in this image are impurities such as metal catalyst and carbonaceous soot attached to the SWNT bundles. Once stabilized, the SWNTs were combined with a poly(vinyl acetate) (PVAc) emulsion to create an aqueous pre-composite mixture that was dried to create electrically conductive composite films. Figure 2 shows how conductivity increases with SWNT concentration. The solid curve is the fit to the experimental data using the classical percolation power law: [13,14] where r is the composite conductivity in S cm ±1 , r 0 is a scaling factor related to the intrinsic conductivity of the filler, s is the power law exponent (typically 1.6±2.0 for random systems), [15] and V c is the volume fraction of filler at the percolation threshold. The values for r 0 , V c , and s are shown in Figure 2. V c is reported as weight fraction in this case due to the uncertainty of the SWNT density, although it is likely that weight fraction is very similar t...

Nano-Scale Reinforcing and Toughening Thermoplastics: Processing, Structure and Mechanical Properties

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The fine dispersion of functionalized carbon nanotubes in acrylic latex coatings

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Design and fabrication of colloidal polymer nanocomposites

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