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

Recently, 3D printing technology has emerged as a promising approach to print electrical circuits [1], sensors [2] and other electrical devices [3]. For 3D printing electrical devices, in addition to the required conductivity, the material should possess sufficient fluidity. Due to the wide variety of available matrices and fillers, it is believed [4] that polymer nanocomposites with the required characteristics can be developed. However, large quantities of fillers are typically necessary to obtain the required conductivity. Such nanocomposites exhibit poor fluidity and cannot be easily 3D printed. One solution is the use of high aspect ratio fillers, such as carbon nanotubes [4]. In addition, as the physical properties of the nanocomposites are expected to depend on the dispersion of filler in the matrix, we compared the properties of nanocomposites prepared by melt mixing with those prepared by solution casting.
As it is already widely used in 3D printing, Poly(acrylonitorile butadiene styrene) (ABS) was chosen as the matrix polymer. Multiwalled carbon nanotubes (MWCNT) were used as the conductive at volume fractions 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 was 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 [5] indicates that the 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 [6]. To analyze the origin of the conductivity, the obtained conductivity was fit to a power-law equation and the corresponding percolation threshold and the critical exponent were obtained. The obtained critical exponent was close to the theoretical value of 2 [4], which suggests that the rapid increase in the conductivity could be due to the formation of a percolating path comprising the filler. 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 [5] indicated that, at low MWCNT additions, the low frequency storage modulus, G’, of the solution cast samples was larger than that of the melt mixed samples. This suggests that the MWCNT were better dispersed in the melt mixed samples and is contrary to expectation. 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. 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 in this case too. However, the rheology threshold was smaller than the conductivity threshold suggesting that the percolation-like phenomenon seen in the rheology data could be, at least partly, 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 required for applications such as circuits, the obtained conductivity is sufficient for applications such as electromagnetic shielding.
[1] S. W. Kwok et al., Applied Materials Today, 9, 167 (2017).
[2] S. Guo et al., Nanoscale, 7, 6451 (2015).
[3] J F. Christ et al., Materials and Design, 131, 394 (2017).
[4] T. Gkourmpis, in Controlling the Morphology of Polymers, edited by G. R. Mitchell and A. Tojeira, (Springer International Publishing, 2016), pp. 209.
[5] S. K. Sukumaran et al., Journal of The Electrochemical Society, 166, B3091 (2019).
[6] D. P. Schmitz et al., Materials Today Communications, 15, 70 (2018).

Figure 1