Article Cite This: ACS Appl. Energy Mater. 2018, 1, 1726−1733
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Plastic Metal-Free Electric Motor by 3D Printing of GraphenePolyamide Powder Al C. de Leon,† Bradley J. Rodier,† Cyril Bajamundi,‡ Alejandro Espera, Jr.,§ Peiran Wei,† John G. Kwon,† Jaylen Williams,∥ Fisher Ilijasic,⊥ Rigoberto C. Advincula,*,†,§ and Emily Pentzer*,†,§ †
Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States VTT Technical Research Centre of Finland, Koivurannantie 1, P.O. Box 1603, Jyväskyla FI-40400, Finland § Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ∥ Department of Biology, Miami University, 501 E. High Street, Oxford, Ohio 45056, United States ⊥ Shaker Heights High School, 15911 Aldersyde Drive, Shaker Heights, Ohio 44120, United States
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S Supporting Information *
ABSTRACT: 3D printing has revolutionized a number of industries, but complete extension to electronics, robotics, and machines has yet to be realized. Current limitations are due to the absence of reliable and facile methods and materials for accessing conductive 3D printed materials. Traditional approaches to conducting nanocomposites (melt-mixing and solution-mixing) require high energy, are time-consuming, or demand functionalization for compatibilization between filler and matrix. Moreover, these methods usually require a high loading of nanofiller to establish a network of conductive particles (high percolation threshold). As such, access to conductive structures using standard 3D printing techniques and easily accessible starting materials is ideal for realizing next generation conductive polymer composites, with the added benefit of tailorability of size and shape of objects produced. Herein we present a facile method to prepare conductive polymer-based powder by assembling graphene oxide nanosheets on the surface of commercial polymer powder, then reduce the nanosheets to render them electrically conductive, and 3D print by selective laser sintering. Importantly, this simple and scalable method allows for polymer particles covered with carbon nanoparticles to be used to 3D print useful electrically conductive structures without a change to processing parameters compared to the polymer particles themselves. The chemical composition and mechanical and electrical properties of the composite materials were characterized, and we report the first example of a working electrostatic motor composed completely of 3D printed pieces, without any metal parts. KEYWORDS: 3D printing, nanocomposites, laser sintering, conducting powders, electrostatic motors
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mixer, and compatibility of conducting filler and polymer.21−23 Alternatively, in solution blending, conductive filler particles are dispersed into a polymer solution, and then solvent is evaporated or the composite is precipitated by addition of a bad solvent.24−26 While this route typically yields good dispersion of filler, it requires a solvent compatible for both filler and polymer, or chemical modification of one to be compatible with the other. Whether melt or solution blending is used, unoptimized processes can result in aggregated particles and require a high loading of filler to reach percolation and form electrically conductive composites.12,21 3D printing of polymer/filler composites is an attractive route for accessing conductive polymer structures, with the added benefit of creating nearly any shape or geometry desired.27−33 With 3D printing, computer software is used to
INTRODUCTION Polymer composites have received much attention in academia and industry over the past decades as researchers seek to enhance the properties of soft materials through incorporation of particle additives, especially to make conductive materials.1−9 Unfortunately, a common challenge encountered in producing such systems is the difficulty in blending a filler particle, which is typically nonpolar, with a polymer matrix, which is typically polar, as the two materials tend to separate from one another.10−14 Moreover, to ensure a composite is electrically conductive, enough filler must be present such that a network within the polymer matrix is formed (i.e., the percolation threshold is reached). 1,4,10,11,13−18 The most common techniques to produce composites are melt blending and solution blending.4,5,17,19,20 In melt blending, conductive particle fillers such as carbon nanotubes (CNTs) are dispersed in a melted polymer in a batch mixer, or single- or twin-screw extruder, with the efficiency of dispersion depending heavily on viscosity of the polymer melt, residence time in the extruder/ © 2018 American Chemical Society
Received: February 20, 2018 Accepted: March 27, 2018 Published: March 28, 2018 1726
DOI: 10.1021/acsaem.8b00240 ACS Appl. Energy Mater. 2018, 1, 1726−1733
ACS Applied Energy Materials
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convert digital models into three-dimensional physical objects with deposition of polymer layers one on top of the other until the desired structure is formed.34−36 Fused deposition modeling (FDM) is a common 3D printing technique in which polymer filaments are extruded, with conductive structures accessed by using a polymer filament loaded with a conductive particle, such as CNTs. Unfortunately, printing of a conductive filament does not necessarily translate into a conductive 3D printed structure, as extrusion through a circular die shears the polymer composite, aligning 2D CNTs along the direction of the flow and resulting in poor electrical conductivity (i.e., the CNTs cannot form a percolated network). Thus, to access conductive composites using FDM, the loading of conducting filler is substantially higher than expected (15−30 wt % vs 1 wt %).37 An alternative to FDM, selective laser sintering (SLS) utilizes a laser to sinter polymer or metal powder preheated to just below its melting point.38−40 Thus, in SLS conductive objects can be 3D printed using conductive powders prepared either by physically mixing filler and polymer powder, by cryogenically fracturing polymer nanocomposites prepared by melt- or solution-mixing, or by coating the powder with the conductive filler.41 For example, Goodridge et al. dispersed carbon nanofibers in melted polyamide, prepared a sheet of the composite by injection molding, and then cryogenically fractured the sheet into powder with particles ∼50 μm in diameter.42 Using this powder as a feedstock for SLS gave structures with improved mechanical properties compared to structures 3D printed with bare polyamide; however, no electrical conductivity data was reported. In a similar vein, Gaikward et al. utilized a twinscrew extruder to disperse graphene nanosheets into melted polyamide, and showed that the prepared nanocomposites had better mechanical and electrical properties compared to pure polyamide, but did not demonstrate 3D printability.43 Current issues in 3D printing conductive structures by SLS include differences in density and particle size of filler and polymer powder which can result in phase separation, and extensive energy required to prepare the precursor composites, as with melt-mixing. Even given the vast improvements made in 3D printing technology with respect to printing speed, printable material (i.e., composite powders), and applications, a major current limitation in 3D printing is the ability to produce electrically conducting objects rapidly and with low loading of conductive fillers.3,4,37 Once realized, such 3D printed objects will find widespread application in the fields of electronics, machinery, and robotics. Herein, we report the facile fabrication of conductive polymer powder and printing of 3D objects by SLS, with utility of the process demonstrated by the printing of an electrostatic motor. Polyamide (PA) powder is first dispersed into an aqueous solution of exfoliated graphene oxide (GO), and then the solution ionic strength is increased to cause deposition of GO nanosheets onto the PA particles. Subsequent chemical reduction of GO yields rGO-coated PA powder (PA-rGO); the chemical composition of the material is characterized by XPS, Raman, and TGA. PA-rGO is used as the feedstock to 3D print structures whose physical properties were characterized by compression and tensile testing, and whose electrical properties are characterized by 4-point probe conductivity measurements. We demonstrate the usefulness and application of the 3D printed structures by preparing a metal-free electrostatic motor, illustrating its performance under applied voltage similar to metal-containing systems.
Article
EXPERIMENTAL SECTION
Materials. Graphite flakes, potassium permanganate, sulfuric acid, aqueous hydrogen peroxide solution (30 wt %), isopropanol, sodium chloride, and hydrazine were purchased from Sigma-Aldrich. Polyamide powder for SLS was purchased from Sinterit. Instrumentation. Atomic force microscopy was performed on a NX-10 Park System in tapping mode to acquire the topography image. Survey and high-resolution XPS scans were collected using a PHI Versaprobe 5000 X-ray photoelectron spectrometer with Al Kα radiation and were referenced to internal SiO2. Raman spectra were collected using DXR (Thermo Scientific) and an excitation wavelength of 633 nm. Thermogravimetric analysis was done using a TGA Q500 (TA Instruments) with a heating rate of 3 °C min−1. Differential scanning calorimetry was performed on a DSC Q1000 (TA Instruments) at 10 °C min−1. FTIR was performed using a Cary 600 from Agilent Technologies in ATR mode using a diamond/ZnSe crystal. Scanning electron micrograph (SEM) images were taken with FEI Helios Nanolab 650 using Through Lens Detector (TLD)-SE mode 2 with 1 kV 25 pA by slicing the 3D printed object and coating with 5 nm gold film. 1D and 2D X-ray diffraction experiments were performed using a Bruker Discover D8 X-ray diffractometer, with a monochromated X-ray source (used with a Co Kα X-ray tube), configured in point focus mode. Measurements of water contact angle were done using a CAM 200 optical goniometer by KSV Instrument, Ltd. Thermal Con. Compression testing was studied on an MTS ReNew upgrade package system fitted with a 1 kN load cell. There were 10 3D printed rectangular blocks (2.54 cm × 2.54 cm × 5.08 cm) tested for each composition at a constant rate of 10 mm/min, and their values were averaged. Tensile tests was done on an MTS ReNew upgrade package system at a strain rate of 1.3 mm/min. Preparation of Graphene Oxide (GO). Graphite (6 g) was ground using a laboratory blender and was dispersed in sulfuric acid in a beaker equipped with magnetic stirring at ambient temperature and open to air. Potassium permanganate (6 g) was then added, and the dispersion was stirred for 24 h, after which another batch of potassium permanganate (6 g) was added. This was repeated until a total of 24 g of potassium permanganate was added. The reaction was quenched 24 h after the last addition of oxidant by pouring the solution into an ice− water mixture, producing a pink/purple solution (excess potassium permanganate). Hydrogen peroxide was then added dropwise until the color of the solution became yellowish brown, representative of graphene oxide. The synthesized graphene oxide was separated from the acidic solution by centrifugation and was washed with isopropanol until neutral pH. Graphene oxide was dried under vacuum at room temperature and kept in the freezer until use. Preparation of PA-rGO. A 500 mL volume of 1 mg/mL GO solution was prepared by dispersing GO in water via vortex mixing and mild sonication. An appropriate amount of PA was then dispersed in the GO solution with magnetic stirring. Sodium chloride (40× of mass of GO) was then added and allowed to dissolve before adding hydrazine (equal mass as GO). The dispersion was then heated to 80 °C for at least 12 h to yield the black powder dispersion, PA-rGO. PArGO was separated from the solution via filtration, washed with water and acetone, and dried under vacuum at 50 °C. PA-rGO was stored in a vacuum desiccator before use. 3D Printing of PA and PA-rGO. 3D models used for compression, tensile, and electrostatic motor were drawn using SketchUp. The 3D printing process was performed using Sintratec Kit SLS 3D printer. PA or PA-rGO was loaded into the powder reservoir. The entire chamber was then preheated for 1 h and 45 min to reach and maintain 140 and 150 °C for the chamber and print surface target temperatures, respectively. Printing started automatically and was set to keep a printing surface temperature of 150 °C and a laser scanning speed of 650 mm/s until the object was completed. The printed object was then unloaded from the printing chamber and manually cleaned with a brush to remove any loose powder. 1727
DOI: 10.1021/acsaem.8b00240 ACS Appl. Energy Mater. 2018, 1, 1726−1733
Article
ACS Applied Energy Materials
Scheme 1. (a) Schematic Representation of the Preparation of Conductive Powder by Assembling GO on the Surface of PA Particles and Reduction with Hydrazine and (b) Optical Image of the PA-rGO Powder with Increasing Loading of rGOa
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The gray color of the 0 vol % sample is due to carbon black present in the PA particles; powders with higher loadings of rGO are not shown since color is consistent above a certain loading.
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RESULTS AND DISCUSSION Conductive powder for SLS was fabricated by assembling GO nanosheets on the surface of PA powder dispersed in water, and then chemically reducing the nanosheets (Scheme 1a). GO was prepared by oxidation of graphite using potassium permanganate in sulfuric acid, as previously reported,44 and was exfoliated into single sheets ∼1 nm thickness, as confirmed by AFM (Figure 1a,b). Exfoliation of the nanosheets is important to evenly coat the PA particles and also ensures a low loading can be obtained.45 GO is highly dispersible in water due to alcohol functionalities on the basal plane and carboxylic acids/ carboxylates along the edges; however, increased ionic strength of the solution renders GO nanosheets less dispersible, and leads to their precipitation, likely due to hydrophobic interactions or charge shielding (Figure S1).46,47 Commercially available PA particles, intended for SLS printing, were dispersed in an aqueous solution of GO, and then sodium chloride granules were added to the solution, causing GO to precipitate onto the PA particles.41 After addition of the salt and once the solution was clear (i.e., all GO was deposited onto the PA particles), hydrazine was added to the solution to chemically reduce the GO nanosheets and instate electrical conductivity (after reduction the C:O ratio of the nanosheets is 4.7:1, as determined by X-ray photoelectron spectroscopy).48,49 Reduction of GO to reduced GO (rGO) led to a color change from brown to black (Figure S2), and increased loading of GO yielded particles of darker color (Scheme 1b). Deposition of GO nanosheets on the PA particles and subsequent reduction did not significantly alter the size of the PA particles, nor lead
to aggregation, as the diameter of as received PA particles was 43 ± 15 μm and the diameter of PA-rGO was 54 ± 10 μm (Figure 1c,d). Moreover, the presence of rGO did not significantly change the appearance of the particle surface (Figure 1e,f). Of note, PA-rGO containing 1 wt % of GO (0.36 vol %, in addition to the carbon black present in commercial PA) will be the focus of the characterization and application in this report unless otherwise specified, as guided by the electrical characterization data (vide infra). The chemical composition of PA and PA-rGO was evaluated by various techniques, with the presence of rGO verified by Xray photoelectron spectroscopy (XPS) and Raman spectroscopy; characterization by FTIR was unfruitful, as the spectra of GO and PA-rGO did not show any differences (Figure S3 and Table S1). The XPS survey scans of PA and PA-rGO show that the major constituent elements are C, O, and N, as expected, with PA-rGO also showing the presence of Na and Cl (Figure S4).50,51 The high-resolution C 1s scans of PA and PA-rGO both show the presence of two oxidation states of carbon, specifically CC/CC at 284.4 eV and CO at 287.5 eV. Compared to GO, the C 1s scan of PA-rGO shows a less significant contribution from CO (Figure 2a and Figure S5a), possibly due to the superficial nature of XPS which leads to a majority of signal due to rGO.52 The high-resolution N 1s scan of PA shows a narrow peak centered at 399.5 eV (Figure 2b and Figure S5c), with the corresponding scan for PA-rGO having a broadened N 1s peak (Figure 2b and Figure S5d), indicating doping by hydrazine. Raman spectroscopy further supports the incorporation of rGO (Figure 2c): PA contains a 1728
DOI: 10.1021/acsaem.8b00240 ACS Appl. Energy Mater. 2018, 1, 1726−1733
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ACS Applied Energy Materials
structure, but does not lead to electrical conductivity (vide infra). In contrast to PA particles, the Raman spectrum of PArGO shows the signature D and G bands of (r)GO at 1350 and 1580 cm−1, respectively, as well as peaks due to PA and carbon black.53−55 X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were used to understand the nature of the rGO nanosheets in PA-rGO and their impact on the thermal properties. Little-to-no difference was observed in the XRD spectrum of PA particles and PArGO (Figure S6a); for both samples, the (110) and (100) peaks of semicrystalline polyamide dominate the spectra, with 2θ values at 24.8° and 23.1°, respectively.56 As nonexfoliated GO nanosheets show a broad peak at 2θ = ∼10° corresponding to an interlayer distance of 1.26 nm and confirming stacking (Figure S6b), the absence of this peak from PA-rGO and PAGO indicates the nanosheets do not assemble into a graphitelike structure on the surface particle, and any multilayers do not have regular spacing. Characterization of the samples by 2D XRD gives similar results (Figure S6c−f).57 Characterization of PA and PA-rGO by TGA shows the expected decomposition of polyamide starting at ∼400 °C and reaching a maximum at 450 °C; the residual mass of PA-rGO is substantially higher than PA upon heating the sample to 800 °C, indicative of the presence of rGO (Figure 2d).49,58,59 Further characterization of PA and PA-rGO by DSC (Figure S7) reveals that the presence of rGO does not impact the melting temperature (∼175 °C) or crystallization temperature (∼150 °C) of the polymer. The lack of change in the morphology and thermal properties of the powder upon coating with rGO suggest that PA-rGO will be printable by SLS, possibly without any changes to the printing parameters of commercial PA (e.g., bed and sintering temperature).60 After verification of the presence of rGO in PA-rGO, impact on the conductivity of the material was evaluated by pressing the material (PA or PA-rGO containing various amounts of rGO) into pellets. The conductivity of PA was 0 S/m, indicating that the carbon black present in the material does not form a percolated network, and is not suitable to illuminate an LED light (Figure 3b, left). Moreover, PA-GO, the precursor to PA-rGO, has an electrical conductivity of 0 S/m, and is likewise nonconductive. However, after reduction of the GO nanosheets to form PA-rGO, conductive composites are formed. As can be seen in Figure 3a, as the loading of rGO increases, the conductivity of the material increases, following the percolation power law with percolation threshold calculated at 0.05 vol % (Table S2 and Matlab script and statistical analysis in Appendix 1 in Supporting Information).61,62 Such a low percolation threshold is brought about by the assembly of the conductive filler (rGO) on the surface of the PA particles, that allows the nanosheets to form a network throughout the pellet that is sufficient for illuminating an LED light (Figure 3b, right). Two AA batteries in series were used to apply 3 V; application of 1.5 V also leads to illumination of the LED, but not bright enough to be easily captured by a photograph. We note that pellets of rGO have electrical conductivity of 10.5 S/m and can also light up an LED light when connected to a power source (Figure S8). Thus, after establishing that PA-rGO with suitable rGO loading could be used to prepare conductive composites, the ability to 3D print the material by SLS was then explored. The as-purchased PA particles could be readily printed by SLS following a standard protocol, and fortuitously PA-rGO particles could be 3D printed without modification of
Figure 1. Characterization of GO nanosheets by (a) atomic force microscopy and (b) the line profile of exfoliated GO on a mica substrate. Characterization of PA and PA-rGO by (c, d) optical microscopy and (e, f) scanning electron microscopy.
Figure 2. Characterization of PA and PA-rGO: (a) high-resolution C 1s XPS, (b) high-resolution N 1s XPS, (c) Raman spectra, and (d) TGA weight loss profiles. Data for the PA particles are shown in solid black lines, and PA-rGO particles are shown in dashed blue lines. Deconvoluted XPS spectra are shown in Figure S5.
small amount of carbon black confirmed by the presence of sharp peaks at 1295 and 1634 cm−1, as well as signatures of the polymer (peaks at 1364 and 1435 cm−1 from asymmetric and symmetric deformation of CH2).52 Carbon black is a filler added to the commercially available PA at low levels (
