Electrical Conductivity of Multiwalled Carbon Nanotube/Acrylonitrile Butadiene Styrene Polymer Nanocomposites prepared by Melt Mixing: Comparison of Twin Screw Extrusion and Batch Mixing

Abstract

Most polymeric materials are insulators. They can be rendered conductive by the addition of appropriate fillers [1]. To obtain the desired conductivity and to ensure spatial homogeneity of the composite, excellent dispersion of the fillers is considered necessary [1,2]. To prepare the composite, solution casting has been widely used and past work suggests that it can adequately disperse the filler in a polymer matrix [3,4]. However, the method involves long preparation times and large quantities of organic solvents, and is not easily scalable. Melt mixing [4-6] offers an attractive alternative as past work [4,5] suggests that it can yield polymer nanocomposites with conductivities similar to that of solution cast samples. They however utilized laboratory scale batch mixing but industrial scale compounding is often performed using melt extrusion [6]. Therefore, in this work, we have investigated the feasibility of twin-screw melt extrusion for nanocomposite preparation by comparing the electrical conductivity of multiwalled carbon nanotubes (MWCNT) filled acrylonitile-butadiene-styrene (ABS) nanocomposites thus prepared to those obtained using internal batch mixing.First, we measured the electrical conductivity of the polymer nanocomposites prepared by the two methods. As can be seen from Fig. 1, for the samples prepared using the batch mixer, the conductivity exhibited a rapid increase of several orders of magnitude upon addition of a small quantity of MWCNT (less than 1 vol%). Upon further addition of MWCNT, the conductivity increase became less steep. The observed behavior is consistent with results in the literature [1,4] and can be interpreted using percolation theory, which suggests that the sharp increase in the conductivity observed at a certain volume fraction arises because the filler forms a conducting path that percolates across the polymer matrix [1]. Unexpectedly however, for the samples prepared using the twin screw extruder, the results were markedly different in that very little increase in conductivity was observed. If the above interpretation using percolation theory [1] is well founded, this suggests the absence of a percolation path consisting of the MWCNT.Due to van der Waals attraction, MWCNT tend to aggregate into micron scale clusters. During nanocomposite preparation these clusters must be broken down to facilitate the dispersion of the MWCNT in the matrix. As sufficient dispersion is believed to be necessary for the formation of the conducting path [1,2], we performed dynamic viscoelasticity measurements of the samples prepared by both methods to assess the dispersion state of MWCNT. At 0.227 vol% MWCNT, the storage modulus, G’, at low frequencies is lower for the samples prepared using the batch mixer when compared to that prepared using the extruder. Following [1,4], this can be taken to suggest that the dispersion of the MWCNT is poorer in the extruded samples and can be hypothesized to be the reason for the low conductivity. Upon increasing the MWCNT content, the low frequency G’ of the samples prepared by both methods differed little from each other and exhibited a plateau. A low frequency plateau in G’ is typically considered indicative of the presence of a network structure [1,4]. Data in Fig. 2 would suggest that, at the higher volume fractions, both preparation methods results in the formation of a MWCNT network structure. However, the conductivity values for the two methods are vastly different. If high electrical conductivity cannot arise in the absence of a percolating path, that would suggest the absence of a network structure and directly contradicts the implications of the rheology data. Future work will focus on resolving this contradiction using other methods to characterize the dispersion of the MWCNT in the ABS matrix.[1] R. M. Mutiso and K. I. Winey, in Polymer Science: A Comprehensive Reference, edited by K. Matyjaszewski and M. Möller, (Elsevier, Amsterdam, 2012), pp. 327.[2] M. H Kim et al., Korea – Australia Rheology Journal, 31, 179 (2019).[3] M. H. Al-Saleh, H. K. Al-Anid, and Y. A. Hussain, Composites Part A: Applied Science and Manufacturing, 46, 53 (2013).[4] S. K. Sukumaran et al., Journal of The Electrochemical Society, 166, B3091 (2019).[5] S. Dul, A. Pegoretti, and L. Fambri, Nanomaterials, 8, 674 (2018).[6] A. Dorigato, V. Moretti, S. Dul, S. H. Unterberger, and A. Pegoretti, Synthetic Metals, 226, 7 (2017).Fig.1 Variation of σ with ϕ: comparison of nanocomposites prepared using a twin-screw extruder (■) and using an internal batch mixer (●).Fig. 2 Variation of G’ with ω at 200℃ for several ϕ: melt extrusion (solid symbols) and batch mixing (hollow symbols). The data for neat ABS is also shown. Figure 1