Electrically Conductive Polymer Nanocomposites for 3D Printing
Authors: Kobayashi Takafumi , Khosla Ajit , Sugimoto Masataka , Furukawa Hidemitsu , Sukumaran Sathish K. 
Published: February 2020

Recently, 3D printing has been used to print functional conductive microstructures for sensors [1] and other microelectromechanical applications [2,3]. In addition, micromolding [4] and micropatterning [5], including large scale micropattering on flexible substrates [6] have been accomplished using conductive nanocomposites. These conductive nanocomposites comprised multi-walled carbon nanotubes dispersed in crosslinked poly(dimethylsiloxane). A particular disadvantage of this conductive nanocomposite is the multistep, time-consuming preparation process. In addition, there is little understanding of the relationship between the preparation process and the functional properties of the composite. Therefore, here we focus on this more fundamental issue — the effect of the processing on the properties.
In this study, poly(acrylonitorile butadiene styrene) (ABS) is chosen as the matrix polymer because it is already widely used in 3D printing. For reasons to be elaborated upon later, multi-walled carbon nanotubes (MWCNTs) were chosen as the conductive filler. Several nanocomposites were prepared by dispersing various concentrations of MWCNTs in ABS using two different methods: melt blending and solution casting. In melt blending, shear deformation of the molten material is used to disperse the filler in the matrix. This deformation can orient the nanotubes. In contrast, solution casting is expected to randomly disperse the nanotubes. In addition, the degree of dispersion of the nanocomposite produced by the two methods need not be identical. To understand the effect of the shear deformation, we measured the conductivity of the nanocomposites prepared by the two methods. For both preparation methods, addition of a small quantity of MWCNT (0.25 weight%), increased the conductivity by several orders of magnitude. Upon further addition of MWCNT, the conductivity of the melt-blended samples increased more rapidly than that of the solution cast samples such that beyond approximately 1 weight% of MWCNT, the conductivity of the melt-blended samples was almost two orders of magnitude higher than that of the solution cast samples. We also compared two ABS polymers with different acrylonitirle contents, but the measured conductivity was not significantly different.
In order to understand the difference between the two methods of preparation, we investigated the linear rheology of the nanocomposites. For 0.25 and 0.5 weight% of MWCNT, the storage and the loss moduli of the nanocomposites were essentially similar to that of the matrix polymer. Upon increasing the concentration of MWCNT, the material progressively exhibits solid-like characteristics at low frequencies. These findings suggest that nanocomposites with a low concentration of MWCNTs can be used even in narrow nozzle 3D printers in current use. However, the maximum MWCNT concentration is limited by the more solid-like behaviour seen at progressively higher concentrations. In 3D printing, the molten sample is forced through a nozzle under pressure. This is similar to the pressure-driven flow through a capillary tube. Therefore, to understand the effect of the flow through the nozzle on the conductivity of the material after printing, we extruded the prepared composites through a capillary tube and compared the conductivities of the resulting samples. The conductivities before and after capillary extrusion were not significantly different. This suggests that the process of 3D printing itself does not significantly alter the conductivity characteristics of the material used for printing.
In summary, we have achieved an increase in conductivity of several orders of magnitude without significantly altering the rheology of the nanocomposite. Therefore, we expect that such materials can be used in conventional 3D printers, even in the case of narrow nozzles. While the conductivity obtained is still several orders of magnitude smaller than that of metals and hence not sufficient for applications such as circuits, the conductivity is sufficient for applications including electromagnetic shielding and electrostatic discharge.
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