Conductive 3D microstructures by direct 3D printing of polymer/carbon nanotube nanocomposites via liquid deposition modeling

Abstract

In this work, a new three-dimensional (3D) printing system based on liquid deposition modeling (LDM) is developed for the fabrication of conductive 3D nanocomposite-based microstructures with arbitrary shapes. This technology consists in the additive multilayer deposition of polymeric nanocomposite liquid dispersions based on poly(lactic acid) (PLA) and multi-walled carbon nanotubes (MWCNTs) by means of a home-modified low-cost commercial benchtop 3D printer. Electrical and rheological measurements on the nanocomposite at increasing MWCNT and PLA concentrations are used to find the optimal processing conditions and the printability windows for these systems. In addition, examples of conductive 3D microstructures directly formed upon 3D printing of such PLA/MWCNT-based nanocomposite dispersions are presented. The results of our study open the way to the direct deposition of intrinsically conductive polymer-based 3D microstructures by means of a low-cost LDM 3D printing technique.

Introduction

Three-dimensional (3D) printing is a fabrication technology that consists in the creation of a 3D object starting from a digital model. 3D printing technologies have evolved very rapidly in recent years and have shifted apart from their traditional application field, namely rapid prototyping. Indeed, 3D printing is now being used routinely in a variety of manufacturing sectors ranging from aerospace and automotive to bioengineering [1], [2]. At present, stereolithography, selective laser sintering, selective laser melting and fused deposition modeling (FDM) are among the most widely employed and investigated additive manufacturing methods both in academia and in industrial environments [3]. The various 3D printing technologies differ in terms of cost, maximum spatial resolution and type of materials used. In particular, for the first three methods, 3D features with a very high spatial resolution (in the order of a few μm at most) have been demonstrated but at the expense of relatively high equipment costs and the need of specialized personnel to operate them [3], [4]. On the other hand, FDM has recently become fairly popular especially among non-specialized personnel as it represents a very cost effective approach to produce 3D objects with a relatively good resolution, which can approach 40 μm [5]. However, being a thermally-driven process that requires melting of a thermoplastic filament prior to the additive deposition of the extruded feature, it exhibits some limitations related to the materials to deposit, as only relatively few polymers possess the right thermal and rheological properties to be easily processable with this technology (with poly(lactic acid) – PLA and acrylonitrile–butadiene–styrene – ABS being among the most widely employed) [6]. Recently, the FDM approach was shown to allow a high degree of orientation of short reinforcing fibers in polymer-based composites during filament extrusion, resulting in 3D printed components with unique structural properties that can significantly exceed those of traditional compression molded samples [7]. In addition, the potential of the FDM technology for the fabrication of electronic sensors was recently demonstrated by 3D-printing solid filaments obtained starting from a dispersion of conductive carbon black into a solution of a commercial formulation of poly(caprolactone) (PCL) in dichloromethane (DCM) followed by evaporation of the solvent to form the solid filament to be extruded in a table-top FDM 3D printer [8]. Even though the approach presented in this work clearly allows the possibility to 3D-print objects with embedded sensors and electronic functionalities in a relatively simple fashion, it still requires the additional step of the production of a solid (nano)composite filament to be heated and melted in order to be processed with a standard FDM 3D printer.

Very recently solvent-cast 3D printing has emerged as a versatile and cost-effective strategy to overcome some of the limits imposed by the FDM approach [9]. This relatively new technology consists in the additive deposition of material layers directly from a solution in a volatile solvent. By means of this technology, the production of freeform structures, scaffolds and other self-standing microstructures was recently demonstrated using a computer-controlled robot moving along the x, y and z axes a dispensing apparatus equipped with a 100 μm inner-diameter extruding nozzle, starting from a concentrated solution of PLA in DCM [9]. In addition, by sputtering a metallic layer a few tens of μm thick onto the 3D printed structure, electrically conducting objects could also be obtained. However, no examples of the fabrication of intrinsically conductive 3D microstructures via direct 3D printing of conductive polymer-based nanocomposite materials from liquid dispersion have so far been reported in the literature, notwithstanding their enormous technological potential for application in fields such as microelectronics and biomedical engineering, where this approach would allow the direct fabrication of conductive microstructures with tailored 3D architectures in a low-cost and highly versatile fashion.

In the effort to address this issue, a 3D printing technique is developed in this work for the fabrication of conductive 3D microstructures with arbitrary shapes via the deposition of a new conductive nanocomposite from liquid dispersion by means of a low-cost commercial benchtop 3D printer equipped with a syringe dispenser (see Supplementary data). This method, that will be called liquid deposition modeling (LDM) throughout the text in analogy to the FDM approach introduced earlier, is based on the direct deposition of a homogeneous dispersion of multiwall carbon nanotubes (MWCNTs) in PLA using a high volatility solvent (i.e., DCM) as dispersion medium to ensure fast evaporation during wet filament deposition and rapid formation of rigid 3D microstructures. A thorough electrical characterization of the nanocomposite at increasing MWCNT concentrations is performed to evaluate the percolation threshold to achieve electrical conductivity. In addition, the rheological behavior of the nanocomposite dispersion is experimentally investigated at varying solid (PLA) content and a printability window for this system is identified based on the shear-rate of the material at the extrusion nozzle. Finally, examples of conductive 3D microstructures directly formed upon LDM of such MWCNT-based nanocomposite dispersion are presented. Conductive features as small as 100 μm can be reproducibly obtained with this method, indicating the high reliability of our approach. LDM clearly lends itself to the possibility of incorporating different types of structural and functional (nano)fillers in the 3D printed extruded feature without the need of producing a solid (nano)composite filament, thus presenting clear benefits compared to the more common FDM approach.