Electrically Conducting and Mechanically Strong GrapheneâPolylactic

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Functional Nanostructured Materials (including low-D carbon)

Electrically Conducting and Mechanically Strong Graphene-Polylactic Acid Composites for 3D Printing Mirae Kim, Jae Hwan Jeong, Jong-Young Lee, Andrea Capasso, Francesco Bonaccorso, Seok-Hyeon Kang, Young Kook Lee, and Gwan-Hyoung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03241 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Applied Materials & Interfaces

Electrically Conducting and Mechanically Strong Graphene-Polylactic Acid Composites for 3D Printing

Mirae Kim1,+, Jae Hwan Jeong1,+, Jong-Young Lee1, Andrea Capasso1, Francesco Bonaccorso2, Seok-Hyeon Kang1, Young-Kook Lee1 and Gwan-Hyoung Lee1* 1

Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea 2

Istituto Italiano di Tecnologia, Graphene Labs, I-16163 Genova, Italy

KEYWORDS: graphene composite, polylactic acid, electrical conductivity, mechanical strength, 3D printing

ABSTRACT

1 ACS Paragon Plus Environment

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The advent of 3D printing has had a disruptive impact in manufacturing and can potentially revolutionize industrial fields. Thermoplastic materials printable into complex structures are widely employed for 3D printing. Polylactic acid (PLA) is among the most promising polymers used for 3D printing, owing to its low cost, bio-degradability, and non-toxicity. However, PLA is electrically insulating and mechanically weak; this limits its use in a variety of 3D-printing applications. This study demonstrates a straightforward and environment-friendly method to fabricate conductive and mechanically reinforced PLA composites by incorporating graphene nanoplatelets (GNPs). To fully utilize the superior electrical and mechanical properties of graphene, liquid-exfoliated GNPs are dispersed in isopropyl alcohol (IPA) without the addition of any surfactant, and combined with PLA dissolved in chloroform. The GNP-PLA composites exhibit improved mechanical properties (improvement in tensile strength by 44% and maximum strain by 57%) even at a low GNP threshold concentration of 2 wt%. The GNP-PLA composites also exhibit an electrical conductivity of over 1 mS/cm at >1.2 wt%. The GNP-PLA composites can be 3Dprinted into various features with electrical conductivity and mechanical flexibility. This work presents a new direction towards advanced 3D-printing technology by providing higher flexibility in designing multifunctional 3D-printed features.

1. INTRODUCTION


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Carbon materials, such as carbon nanotubes and carbon black, have been widely employed as nanofillers for polymer nanocomposites.1-2 Among these materials, graphene, a one-atom thick sp2-bonded carbon material, has attracted considerable attention owing to its high conductivity and mechanical robustness.3-4 Being a semimetal with a linear dispersion of bands at the Dirac point, graphene has an intrinsically ultrahigh carrier mobility (up to 200,000 cm2V-1s-1), even at room temperature.5 Its honeycomb lattice structure, consisting of tightly bonded carbon atoms, leads to a theoretically high elastic modulus of 1 TPa and a superior tensile strength of 130 GPa.4 Polylactic acid (PLA) is a widely employed polymeric material providing a low cost, bio-degradability, and nontoxicity.6-7 PLA can easily be reformed and shaped, owing its thermoplastic properties and low melting point, so that it quickly became a suitable material for fused filament fabrication (FFF), which is a versatile 3D printing technique.6 In the FFF process, a continuous filament, usually a thermoplastic polymer such as PLA, is extruded from a heated and moving print head nozzle, forming 3D shapes through layer-by-layer addition. FFF is one of the most widely adopted 3D printing techniques to date, owing to its simplicity, low cost, and fast printing speed.8 Nevertheless, PLA has some drawbacks as a FFF material, such as mechanical weakness and a lack of electrical conductivity, which limit the range of possible applications.9 In order to overcome these drawbacks, some groups have proposed incorporating graphene into the PLA matrix, thus creating novel 3Dprintable PLA composites.9-10 For the production of a high-quality graphene-polymer composite, graphene nanoplatelets (GNPs) with high purity and long-term stability are



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required. GNP dispersions are normally produced from graphene oxide (GO) and reduced graphene oxide (rGO). However, this approach is far from ideal.10 Although GO can easily be dispersed in water and other solvents, it does not possess the required properties in terms of electrical conductivity.1 Therefore, GO is usually processed further to obtain rGO. For this, an additional reduction process is required to recover graphene’s superior electrical and mechanical properties. In general, rGO is obtained from GO via a chemical process in a solution or by annealing at high temperature (800 °C).11 In the former case, the process induces the agglomeration of GNPs thanks to strong attractive dipole forces and adsorbents at defect sites, resulting in their sedimentation within a few days.6 In either case, rGO exhibits a lower electrical conductivity than pristine graphene, owing to a considerably higher defect density.12 To tackle the issues related to the use of rGO, the GNP dispersions can be formed by pristine graphene nanoplatelets produced by the exfoliation of graphite in a liquid. In this case, the choice of solvent is crucial for maximizing the exfoliation yield and preventing re-aggregation and sedimentation.13 N-methyl-pyrrolidone (NMP) has appropriate fluidic properties for the exfoliation of graphite and has been routinely employed to prepare graphene-based dispersions. However, NMP is difficult to remove on account of its high boiling point (202 °C), and it is significantly toxic.13 For these reasons, the use of more environmentally friendly solvents, such as co-mixtures of water and ethanol, have been proposed for the exfoliation.14 Another alternative approach involves the use of stabilizing surfactants, such as hexadecyltrimethylammonium bromide (HTAB).15 However, these surfactants, which are also difficult to remove, deteriorate the


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quality of the graphene-polymer composite, mainly because of residual contaminants. Therefore, in order to retain the superior properties of graphene and produce uniform mixtures of GNPs and PLA, GNPs should be dispersed in a solvent that has no harmful influence on the properties of graphene, without surfactants. Achievement of both high mechanical and electrical properties has been tricky as summarized in Table S1. Here, we demonstrate the fabrication of GNP-PLA composites with electrical conductivity and enhanced mechanical strength, which can be employed as filaments for 3D printing. We prepared GNP-PLA composites with uniformly dispersed GNPs with no contamination and a negligible degradation of graphene. The addition of well-dispersed GNPs to a PLA matrix greatly improved the mechanical properties of the composite, whose tensile strength and maximum strain were increased by 44% and 57%, respectively, and allowed for an electrical conductivity of over 1 mS/cm. The GNP-PLA composite filament prepared by hot extrusion was 3D-printed into designed features using the FFF method, including a flexible and conducting feature. Our work provides a facile method for fabricating a 3D-printable GNP-PLA composite with flexibility and electrical conductivity for novel 3D printing applications.

2. EXPERIMENTAL SECTION 2.1. Preparation of GNP-PLA composite and 3D printing For the GNP solution, GNPs were dispersed in IPA and sonicated for 30 min, followed by centrifugation at 2000 rpm for 45 min to obtain GNPs with uniform thickness of 3–4



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nm. After centrifugation, the top half of the volume was collected after decantation, thus discarding thick graphite and GNP aggregates deposited on the bottom. For the PLA solution, PLA was dissolved in chloroform (1.5 g/ml). Then, the PLA solution was mixed with the exfoliated GNP dispersion, and the mixed dispersion was solidified by drying at 100 °C for 1 h. Compared with other 3D printable composites, because most of the thick GNP aggregates are removed in the centrifuging process no additional filtering process was required at this step.16 For 3D printing of the GNP-PLA composite, the composite should be reformed into a filament, which can be supplied to the 3D printer. GNP-PLA filaments were extruded at 170 °C from the completely dried GNP-PLA composite using a homebuilt extruder shown in Figure S1a. The GNP-PLA composite is slightly melted in the heated cylinder of the extruder and pushed out through a narrow nozzle by a pressing piston. A phenomenon called “solvent popping,” which occurs by the explosion of the trapped solvent in the polymer matrix,17 creates undesirable bubbles and leaves behind small pores in the composite. These pores can be drastically reduced by repeating the extrusion process. The hardened GNP-PLA filament was directly employed for 3D printing (Figure S1b). The GNP-PLA composite filaments were 3D-printed into various features using a commercially available 3D printer, such as the hand-held 3Doodler.

2.2. Characterization To characterize the graphene flakes and GNP-PLA composite, Raman spectroscopy (inVia, Renishaw, 532 nm laser) was utilized. For a structural analysis, XPS (K-alpha,


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Thermo Scientific) was employed. X-ray diffraction measurements were conducted using an X-ray diffractometer (SmartLab, Rigaku). The shape and thickness of the graphene flakes were characterized by AFM (NX10, Park systems). The fractured surface of the GNP-PLA composite was observed by SEM (VEGA3, Tescan). The electrical properties of the GNPPLA composite were measured by a parameter analyzer (4200-SCS, Keithley). The 𝐴

resistivity of the composite was calculated using the equation of 𝜌 = 𝑅 𝑙 , where 𝜌, R, A, and l are the resistivity, resistance, cross-section area, and length, respectively. The mechanical characterization was conducted with a tensile stress testing system (TST350, Linkam) at a stretching speed of 50 μm/min. To ensure the reliability of the measurements, each sample was fabricated to a uniform rectangular structure (2 mm × 1 mm × 30 mm) for the tensile stress test. The GNP-PLA solution was dried and cut into small pieces, then they were extruded through a hot cylinder and shaped to specimens for mechanical tests. During hot extrusion, the samples experienced annealing and recrystallization processes.

3. Results and discussion GNP-PLA composites were prepared by mixing liquid-exfoliated GNPs with a PLA solution, followed by a drying and extrusion process, as described in Methods and depicted in Figure 1a. Before fabricating the composite, the GNPs dispersed in IPA were spraycoated onto an SiO2 substrate for characterization (Figure 1b). The atomic force microscopy (AFM) image in the inset of Figure 1b shows a representative graphene nanoplatelet and its thickness profile. From the AFM measurements, it was verified that the lateral size and



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thickness of the graphene nanoplatelets are 4.58 μm and 3.2 nm, respectively, on average (Figure S2). The Raman spectra of the graphene nanoplatelets exhibit strong peaks of D (~1350 cm−1), G (~580 cm−1), and 2D (~2680 cm−1) (Figure S3). The small intensity ratios of 2D and G (I2D/IG) and large intensity ratios of D and G peaks (ID/IG) are a result of the high density of defects, such as edges, wrinkles, and surface impurities.18 It has generally been observed that the D peak increases with the sonication time, because the GNPs are cut into smaller (down to submicron-sized) pieces during the process.19

Figure 1. (a) Schematic process flow of the GNP-PLA composite fabrication, extrusion, and 3D printing: (i) GNP dispersion in IPA and dissolution of PLA in chloroform, (ii) mixing and drying of two dispersions, (iii) extrusion of the GNP-PLA filaments, (iv) 3D printing of GNP-PLA object. (b) Optical micrograph of graphene flakes spray-coated on an SiO2


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substrate; AFM image of a representative graphene flake in the inset, with a height profile to measure the flake thickness. (c) Optical image and magnified SEM image of the crosssection of the GNP-PLA filament.

Figure 2. Analysis of the GNP-PLA composite. (a) Raman spectroscopy shows D, G, and 2D peaks from GNP, and PLA peaks around 2900 cm-1. (b) XPS measurement data. This shows peaks from GNP and PLA only, without the appearance of new peaks or disappearance of existing peaks, indicating that no chemical modification or bonding between GNP and PLA occurs. (c) XRD measurement. Both PLA (16.8°, 19.5°, 22.6° and 27.3°) and GNP (26.5°) peaks exhibit no shifts, indicating no interlayer distance changes. PLA and GNP peaks are indicated as blue circle and red square, respectively. For each analysis, a GNP-PLA composite with a different GNP concentration was used.



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The cross-section of the GNP-PLA composite filament was observed by scanning electron microscopy (SEM), as shown in Figure 1c. The cross-sectional image shows that the GNPs are uniformly dispersed in the PLA matrix. The GNP-PLA composites with different GNP concentrations ranging from 1 to 10 wt% were investigated by Raman spectroscopy (Figure 2a). The Raman bands of pure PLA are located at 2900 – 3000 cm-1. The Raman peaks of graphene increased with the GNP concentration. The GNP-PLA composites also exhibited the Raman peaks of PLA and graphene, regardless of the GNP concentration, which indicates that no chemical or structural change occurred in the PLA matrix during the composite production process. X-ray photoelectron spectroscopy (XPS) was also employed to investigate the chemical bonds of the GNP-PLA composite surface (Figure 2b). The relative intensity of the C-C bond and other oxygen-related bonds at the XPS spectra of GNPs in the PLA matrix exhibit a degree of oxidation. The XPS results also demonstrate that the strong sp2 peak (~284.8 eV) became dominant as the GNP concentration increased. The functional groups of C-O (~286 eV) and O-C=O (~288.5 eV) are generally observed in most cases involving liquid exfoliated graphene.20 However, there were no changes in chemical bonds, i.e., no formation of new bonds or elimination of existing bonds, including no formation of graphene oxide. The X-ray diffraction (XRD) patterns of GNP-PLA are illustrated in Figure 2c. The three distinctive peaks of the PLA matrix are located at 16.8°, 19.5°, 22.6° and 27.3°, with a minor strain-induced shift.21-22 A small and broad (002) peak at 26.5°, which is the characteristic peak of few-layer graphene, only appears when the weight percentage of GNPs is over 3 wt%. Its shape and magnitude demonstrate that the


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GNPs are not A-B-A restacking into graphite, even at the highest loading.23 Unlike previously reported PLA-graphite composites with amorphous structures (evidenced by a single broad PLA peak in XRD),24 our GNP-PLA composite has a highly crystalline PLA phase, which is not influenced by the addition of more GNPs. This is crucial, because the mechanical properties of polymer-based composites can degrade owing to crystalline changes and reduced hardness of the polymers.1, 25-26 However, our structural analyses by Raman, XPS, and XRD methods show that the PLA was not chemically or structurally modified, and therefore the embedded GNPs correctly act as a reinforcement filler. Our solution-based fabrication method for the GNP-PLA composite allows for a uniform physical mixing, without any chemical reaction taking place. This is crucial, because both the graphene and PLA can maintain their intrinsic properties and maximize their combined effect towards mechanical reinforcement.



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Figure 3. (a) I-V curves of GNP-PLA composites with different GNP concentrations of 2, 2.5, 3, 4, 5, and 10 wt%. The inset shows a magnified view. (b) Resistivities of GNP-PLA composites with different GNP concentrations. The inset shows the conductivities of the GNP-PLA composites on a log scale.

To investigate the electrical properties of the GNP-PLA composite, the current-voltage (I-V) curves at different GNP concentrations (0 ~ 10 wt%) were obtained, as shown in Figure 3a. The I-V curves exhibit a linear increase, demonstrating that the GNPs at all concentrations introduce electrical paths within the PLA matrix. The GNP-PLA composites with concentrations of less than 2 wt% exhibited no measurable current, almost insulating (not shown in Figure 3a). The resistivity of the composites in Figure 3b drastically drops from a few MΩ∙cm to below 100 kΩ∙cm when the GNP concentration is higher than 2.5 wt%, maintaining a small resistivity range above this concentration threshold. Our GNP-PLA composites show the highest conductivities at the small concentrations of GNPs, along with high mechanical strength at the same time. (Figure S4). Although a statistical

model is commonly employed for percolation, the power law does not contain the geometrical information of nanofillers, such as orientation and shape.27-28 To explain the effect of the GNP filler on the electrical conductivity of the composite, we employed percolation theory by modeling the GNPs as discs with a high aspect ratio, as shown in the inset of Figure 4a.28-29 The log-scale conductivity shows that the percolation threshold of


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GNP-PLA composite is around 1.2 wt%, as shown in the inset of Figure 3b. Assuming that the graphene fillers are homogeneously distributed within the matrix with an average interparticle distance (dip), the percolation threshold volume ratio (VGNP) of the composite with randomly oriented GNPs can be described as 27𝜋𝐷2𝑡

𝑉𝐺𝑁𝑃 = 4(𝐷 + 𝑑

𝑖𝑝)

3

(1)

where D and t are the diameter and thickness of the GNPs, respectively. If dip is less than 10 nm, which is the minimum distance for electron hopping reported for most nanofillers,30 and the diameter of the GNPs is considerably larger than dip, then the above equation can be reduced to 𝑉𝐺𝑁𝑃 =

21.195 𝛼

(2)

where α is the aspect ratio (D/t) of the GNP. Based on this equation, the theoretical percolation threshold volume ratio decreases as the aspect ratio is increased, as plotted in Figure 4a. For our GNP-PLA composite with an average aspect ratio of 1417.956 (lateral size/thickness = 4580 nm/3.23 nm in Figure S2), the theoretical percolation threshold is 1.49 vol% (indicated by a red star in Figure 4), which corresponds to 0.8 wt%, which is smaller than the experimentally determined value of 1.2 wt% in Figure 3b. This deviation is probably due to the irregular shape of the GNPs and their defects.28 In addition, a small amount of agglomerated GNPs resulting from inhomogeneous or imperfect dispersion in liquid that are not fully removed after the centrifugation process might partially affect the



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predicted performance. From this result, it can be concluded that the aspect ratio of GNPs is significantly important for lowering the percolation threshold. GNPs with a larger lateral size and smaller thickness, i.e., a higher aspect ratio, are required to lower the percolation threshold volume ratio, as estimated in Figure 4b and 4c.

Figure 4. (a) Plot of theoretical percolation threshold volume ratio (VGNP) vs. aspect ratio of GNP (𝛼) in the GNP-PLA composite. The schematic of the inset shows the theoretical


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GNP distribution model used for the percolation threshold calculation. A red star indicates the estimated percolation threshold value for the GNP-PLA composite in this work. The variation of the percolation threshold volume ratio is shown as a function of (b) the thickness of GNPs with different diameters and (c) the diameter of GNPs with different thicknesses. The red stars in (b) and (c) also indicate the values in this work.

Figure 5. (a) Strain-stress curves of the GNP-PLA composites with different GNP concentrations. (b) Elastic modulus (Ey), tensile strength (σtensile), and maximum strain at fracture (εmax) of the GNP-PLA composites (from top to bottom).

To investigate the mechanical properties of the GNP-PLA composite, mechanical tensile tests were conducted. As shown in Figure 5a, distinct behaviors were observed for 15 ACS Paragon Plus Environment


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composites with different GNP concentrations. The tensile strength (σtensile) and maximum strain (εmax) were maximized simultaneously at ~2 wt% (Figure 5b). With the addition of GNPs, the tensile strength and maximum strai were increased by 44% and 57%, respectively. This mechanical reinforcement is attributed to the fact that the fillers of GNPs result in hardening of the composite and allow for the transfer of stress to the GNPs.31 However, the strength and maximum strain of the composite decreased at over 2 wt% of GNPs and elasticity decreased at over 4 wt%, probably owing to the reduced distance between the GNPs at a higher loading. At a high GNP concentration, the portion of directly touched connections between GNPs, which are more slippery than GNP-PLA connections, became significant, leading to more favorable crack propagation.32 The composite become markedly brittle beyond 10 wt%, leading to no flexibility. Furthermore, the additional stress

concentration points generated by an increased free volume fraction at a higher filler content, 2 wt% in our case, might also contribute to the lower mechanical strength.25 The maximum strain at fracture (εmax) steadily declined with an increasing GNP concentration after 2 wt%(bottom graph of Figure 5b), because the viscosity of the GNP-PLA composite increases owing to the reduced mobility of polymer chains at a higher GNP concentration.25, 33


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Figure 6. (a) Schematic of the bridging effect of GNPs in a cracked region of the PLA matrix. (a) Optical image of the fractured GNP-PLA composites with difference GNP concentrations after tensile tests. (b) SEM images of the surfaces of the fractured GNP-PLA composites with difference GNP concentrations. (c) Microstructural analysis performed by scanning electron microscopy (SEM)

The GNPs in the composite play an important role in preventing crack propagation by bridging the PLA matrix across a crack, as illustrated in Figure 6a. The GNPs that are strongly adhered to the PLA matrix effectively restrict crack propagation within the composite, by spreading the strain to the neighboring matrix.34-35 In this manner, the GNPs at the optimal concentration (2 wt% in this work) improve the mechanical properties of 17 ACS Paragon Plus Environment


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the GNP-PLA composite. Meanwhile, GNPs beyond this threshold concentration leave small pores and loosen the bonding strength between the graphene and PLA, leading to stress concentration and mechanical weakening.36 To verify the fracture behaviors of the composites, fractured samples were investigated, as illustrated in Figure 6b and c. In optical images of Figure 6b, the GNP-PLA composite with a higher GNP concentration exhibits a more irregular and rugged shape, along with a larger amount of exposed GNPs. The rugged fractured surfaces of the GNP-PLA composites are also observed by SEM in Figure 6c. The rougher structures of the fractured cross-sections in the GNP-PLA composites with higher GNP concentrations indicate a toughening of the composite, which makes the composite progressively stronger.

Figure 7. (a) Various objects 3D-printed from the GNP-PLA composite: The word “LOVE,” a 3D “N,” and a spring. (b) An unstretched flexible and reconfigurable GNP-PLA wire with electrical conductivity. To demonstrate the flexibility and conductivity, a small LED was


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connected to this wire during the stretching test. The composite wire can be repeatedly stretched and recovered, maintaining electrical conductivity.

Finally, the GNP-PLA composite was extruded into a filament and 3D-printed into various features (Figure S1 and Figure 7a). The improved mechanical properties allow us to easily reshape the GNP-PLA composites with a high flexibility. To utilize the electrical conductivity and mechanical properties of the GNP-PLA composite, we fabricated a flexible and reconfigurable wire with an “S” shape, as shown in Figure 7b. This feature is sufficiently conducting to turn on a small LED, while maintaining robustness and elasticity (see movie S1). Therefore, the 3D-printable GNP-PLA composite with improved mechanical properties and electrical conductivity can broaden the application areas of 3D printing, and lead to a higher degree of freedom in designing multifunctional 3D features.

4. CONCLUSIONS In this study, a GNP-PLA composite was produced using a straightforward and environment-friendly method. The GNPs were incorporated into the PLA matrix as conducting reinforcement fillers, resulting in a physically mixed composite without any undesirable chemical reactions. The GNP-PLA composite exhibited improved mechanical properties and electrical conductivity, even with a small addition of GNPs. The GNP-PLA composite was 3D-printed into various features, and is also suitable for conventional 3D printers. Our method of fabricating GNP-PLA composites broadens the possibilities and



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applicability of GNP-PLA composites in the 3D printing field and provides a high flexibility in designing multifunctional 3D-printed features, owing to the mechanical strength and electrical conductivity of the resulting composites.

ASSOCIATED CONTENT Supporting Information Picture of filament extruder and 3D printing pen; lateral and thickness size distribution of GNP; Raman spectra of various graphene flakes; video of conductive PLA/GRP spring elongate.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Gwan-Hyoung Lee: https://orcid.org/0000-0002-3028-867X Author Contributions M. K., J. H. J. and G.H.L. designed the research project and supervised the experiment. M. K. and J. H. J. performed device fabrication and M. K., J. H. J., J. Y. L., and S. H. K. performed


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device measurements under supervision of F. B., Y. K. L., and G. H. L. M. K., J. H. J., A. C. and G. H. L. analyzed the data and wrote the paper. +

These authors contributed equally.

ACKNOWLEDGEMENT This work was supported by the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea (NRF-2016K1A3A1A25003573), Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the

Ministry

of

Science,

ICT

&

Future

Planning

(2016M3A7B4910940,

2017R1A5A1014862, SRC program: vdWMRC center). A.C. is supported by the Yonsei University Research Fund (Yonsei Frontier Lab. Young Researcher Supporting Program) of 2018. Mirae Kim and Jae Hwan Jeong contributed equally to this work.

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