Electrical Conductivity and Linear Rheology of Multi-Walled Carbon Nanotube/Acrylonitrile Butadiene Styrene Polymer Nanocomposites Prepared By Melt Mixing and Solution Casting
Authors: Kobayashi Takafumi , Takeda Shunsuke , Khosla Ajit , Furukawa Hidemitsu , Sugimoto Masataka , Sukumaran Sathish K. 
Published: May 2020

Polymer nanocomposites are potential materials for three dimensional (3D) printing of functional conductive microstructures such as sensors [1] and circuits [2]. For 3D-printing, the materials should possess sufficiently high conductivity while still being able to flow during printing. Typically, obtaining sufficiently high conductivity necessitates the use of conductive fillers in quantities that result in a composite with excessively large viscosity [3]. One route to alleviate this problem is the use of conductive fillers with a high aspect ratio [4], such as carbon nanotubes. In addition, as the physical properties of a composite are expected to depend on the filler dispersion [5], we compared the properties of nanocomposites prepared by melt mixing and solution casting.
Poly(acrylonitorile butadiene styrene) (ABS) was used as the matrix as it is already widely used in 3D printing. The conductive filler was multiwalled carbon nanotubes (MWCNT) at a volume fraction (in %) between 0 – 4.52. For melt mixing, the MWCNT were dispersed in ABS using an internal batch mixer at 200°C for 10 minutes using a roller speed of 50 rpm. For solution casting, ABS dissolved in dichloromethane is added to a suspension of MWCNT/chloroform and sonicated for a further 10 minutes.
We measured the conductivity of the nanocomposites prepared by both methods. Fig. 1 [6] indicates that addition of a small quantity of MWCNT (0.113 vol%) increased the conductivity by several orders of magnitude. Upon further addition of MWCNT, the increase in conductivity becomes more gradual and appears to saturate. The obtained values are comparable to that in the literature [3]. To analyze the origin of the conductivity, the obtained conductivity was fit to a power-law equation to obtain the percolation threshold and the critical exponent. The obtained exponent was close to the theoretical value of 2 [4] which suggests that the conductivity could be due to the formation of a percolating path comprising the filler within the polymer matrix. The values of the threshold and the exponent were similar for the two preparation methods indicating that the preparation method had little effect on the obtained conductivity.
The linear rheology data in Fig. 2 [6] indicates that at low MWCNT additions, the low frequency G’ of the solution cast samples is larger than that of the melt mixed samples, suggesting that the MWCNT are better dispersed in the melt mixed samples – an unexpected result. Upon further addition of MWCNT, the difference in the low frequency G’ gradually disappeared. In Fig. 3, the variation of G’(ω = 0.0215 rad/s) with MWCNT addition was fit to a power-law equation, as done before for the conductivity data. While the difference between the exponents for the samples prepared by the two methods was larger than that seen in the conductivity exponent, both values were close to 2 and similar to the conductivity exponent. However, the rheology threshold was smaller than the conductivity threshold indicating that the percolation-like phenomenon seen in the rheology data could be, at least partly, be attributed to the interaction between the filler and the polymer, i.e., unlike in electrical percolation filler-filler contact is not essential.
In summary, we have achieved an increase in conductivity of several orders of magnitude without significantly altering the linear rheology of the nanocomposite. While the conductivity obtained is still several orders of magnitude smaller than that of metals and hence not sufficient for application such as circuits, the conductivity is sufficient for applications such as electromagnetic shielding.
[1] S. Guo et al., Nanoscale, 7, 6451 (2015).
[2] S. W. Kwok et al., Applied Materals Today, 9, 167 (2017).
[3] D. P. Schmitz et al., Materials Today Communications, 15, 70 (2018)
[4] T. Gkourmpis, in Controlling the Morphology of Polymers, edited by G. R. Mitchell and A. Tojeira, (Springer International Publishing, 2016), pp. 209.
[5] M. H Kim et al., Korea – Australia Rheology Journal, 31, 179 (2019).
[6] S. K. Sukumaran et al., Journal of The Electrochemical Society, 166, B3091 (2019).

Figure 1