High-Strength Stereolithographic 3D Printed Nanocomposites

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High-Strength Stereolithographic 3D Printed Nanocomposites: Graphene Oxide Metastability Jill Z. Manapat, Joey Dacula Mangadlao, Brylee David Buada Tiu, Grace C. Tritchler, and Rigoberto C Advincula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16174 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

High-Strength Stereolithographic 3D Printed Nanocomposites: Graphene Oxide Metastability Jill Z. Manapat1,2, Joey Dacula Mangadlao3‡, Brylee David Buada Tiu1,4‡, Grace C. Tritchler5, Rigoberto C. Advincula1* 1

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

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Department of Mining, Metallurgical, and Materials Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines 3

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Department of Radiology, Case Western Reserve University, Cleveland, OH 44106, USA

Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

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Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

*[email protected] Keywords:

Graphene

Oxide,

Additive

Manufacturing,

Stereolithography,

Polymer

Nanocomposites, Thermal Post-Curing, Mild Annealing

ACS Paragon Plus Environment

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ABSTRACT The weak thermomechanical properties of commercial 3D printing plastics have limited the technology’s application mainly to rapid prototyping. In this report, we demonstrate a simple approach that takes advantage of the metastable, temperature-dependent structure of graphene oxide (GO) to enhance the mechanical properties of conventional 3D-printed resins produced by stereolithography (SLA). A commercially available SLA resin was reinforced with minimal amounts of GO nanofillers and thermally annealed at 50 °C and 100 °C for 12 hours. Tensile tests revealed increasing strength and modulus at an annealing temperature of 100 °C, with the highest tensile strength increase recorded at 673.6% (for 1wt% GO). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) also showed increasing thermal stability with increasing annealing temperature. The drastic enhancement in mechanical properties, rarely seen to this degree in 3D printed samples reported in literature, is attributed to the metastable structure of GO, polymer-nanofiller crosslinking via acid-catalyzed esterification and removal of intercalated water, thus improving filler-matrix interaction as evidenced by spectroscopy and microscopy analyses.


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INTRODUCTION Additive manufacturing (AM) has democratized the manufacturing industry with the advent of affordable 3D printers.1,2 Through these 3D printing techniques, users can easily fabricate a part from a computer-aided design (CAD) model in a layer-by-layer manner. Various types of 3D printing methods now exist from sophisticated ones used for printing aircraft components3 to more simple techniques for printing food.4 But among all these methods, stereolithography (SLA) remains as one of the most widely used AM technique. Developed by Charles Hull in the 1980's, SLA uses a liquid photopolymer resin, which is cured layer-by-layer using a laser to form a finished part (Scheme 1). SLA-printed products have smooth surface finish5 and high resolution at 20 microns or less.6,7 It is often utilized for rapid prototyping in a wide range of industries spanning from medicine8–10 to aerospace11 to electronics,12,13 among others. Furthermore, SLA resins can be modified to tailor its properties for specific applications such as those that require electrical conductivity, flame retardancy, chemical resistance, and the like.

Scheme 1. Stereolithography Process


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Despite its advantages, SLA-printed parts fabricated using acrylate-based resins require post-curing and are prone to shrinkage and warping due to the chemistries involved.5 Moreover, the resulting thermo-mechanical properties are still insufficient to produce functional, end-use products. Consequently, a 2014 survey14 involving over 100 industry manufacturers found that only 0.9% of the respondents utilized 3D printing to produce functional products because of uncertainty in quality, especially in terms of strength and durability.14 In order to address this concern, new materials should be developed for existing 3D printers; a well-known method of doing this is through fabricating polymer nanocomposites – “polymers bonded with nanoparticles (fillers) to produce materials with enhanced properties.”15 These materials have at least one of its phases in the nanometer range16 and have properties that are different from its micro- or macro- counterparts.16 They are especially attractive for lightweight applications as they require low solid loadings15 to bring about property enhancements. To be effective, several considerations must be taken into account when selecting appropriate filler materials namely, surface area-to-volume ratio (SA:V)16 and compatibility with the polymer resin.17 A high SA:V enhances the reactivity of the filler,18 i.e. there is more surface available for adhesion with the polymer resin. Consequently, most reinforcements are in the form of rods (e.g. carbon nanotubes) or platelets/sheets (e.g. nanoclays).16 Compatibility with the polymer resin is important to ensure good interaction so as to enhance interlayer adhesion, which would lead to better load transfer from the resin to the filler, hence giving superior strengthening effects. Among the many fillers available today, graphene oxide (GO) has taken much attention due to its excellent antibacterial,19 electrical,20 and mechanical21 properties. GO satisfies both the SA:V and polymer compatibility requirements. Its well-established sheet-like morphology22,23 gives it a high SA:V, while the presence of oxygenated functional groups improves GO’s interaction with


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most polymer matrices as compared to graphene alone.24 Thus, GO is a promising filler for 3D printing bulk GO nanocomposites with complex shapes that are impossible to achieve with traditional methods of fabrication such as casting. Current studies have successfully fabricated SLA-printed GO nanocomposites using commercially available photopolymer resin. One study25 observed an increase in strength with increasing GO loading from 0.2 wt% to 0.5 wt%, but the highest tensile strength recorded was only around 15 MPa, with the strengthening mechanism explained only in the context of mechanics. To the best of our knowledge, there is currently no research reporting lowtemperature (i.e. “mild”) annealing of 3D-printed parts and mechanical properties explained in the context of GO’s well-established metastable structure.26 Such a process exploits the inherent characteristics of GO to yield a stronger part using a simple and scalable procedure. Mild annealing is preferred in this study to avoid polymer degradation and GO reduction, hence preserving good filler-matrix interaction. High temperature annealing is usually performed to reduce GO to graphene, consequently improving electrical properties.27 However, high temperatures render the resulting nanocomposite susceptible to excessive warping and shrinkage.27 This study aims to use the metastable structure of GO to enhance the thermo-mechanical properties of SLA-printed GO nanocomposites via a simple mild annealing process to potentially increase the range of applications of this material. Comparison is also made with traditionally casted products to demonstrate the advantages of AM. Ultimately, this will aid in converting the SLA process from rapid prototyping to rapid manufacturing.


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EXPERIMENTAL 1.

Materials Graphite flakes were purchased from Sigma-Aldrich (St. Louis, MO). Sulfuric acid

(H2SO4, 98%) was purchased from VWR Analytical (Radnor, PA), while hydrogen peroxide aqueous solution (H2O2, 30%) and isopropyl alcohol (IPA) were obtained from Fisher Scientific (Waltham, MA). Potassium permanganate (KMnO4) was purchased from EMD Millipore Corporation (Billerica, MA). Grey resin (FLGPGRO2) was purchased from Formlabs (Somerville, MA) and a non-stick dry film lubricant was purchased from DuPont (Wilmington, DE). 2.

Synthesis of Graphene Oxide (GO) GO was synthesized using a modified Hummers method.28 3 grams of graphite flakes

was combined with 400 mL H2SO4 in a 1000 mL beaker with continuous stirring. One equivalent weight of KMnO4 was then added every 24 hours for four days. After which, ice-water mixture and H2O2 were added to quench the reaction. A bright yellow solution was obtained, which was consequently purified by first centrifuging for 10 minutes at 4400 rpm with Milli-Q water. The precipitate was then washed with IPA until neutral pH was obtained. The purified material was vacuum dried at room temperature and stored in a freezer before nanocomposite preparation. 3.

Nanocomposite Fabrication: Resin mixing, SLA Printing, and Casting Nanocomposites with 0.1 wt%, 0.5 wt%, and 1 wt% GO were prepared by first mixing

dried GO in acetone via sonication for 15 minutes. The resulting solution was then mixed magnetically with Formlabs grey resin in a round bottom flask while drying via vacuum


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evaporation with constant stirring. The GO resin was ultrasonicated for 10 minutes using a ColeParmer Ultrasonic Processor with an amplitude of 30%, pulse interval of 2 seconds, and pulse duration of 5 seconds to ensure homogeneity before printing. Specimens were then 3D printed using Formlabs Form 1+ printer equipped with a Class 1, 405nm violet laser with a power of 120mW and laser spot size (FWHM) of 155 microns. Axis resolution (i.e. layer thickness) used for printing was 50 microns. After printing, the printed samples were then manually agitated in an isopropyl alcohol (IPA) bath for two minutes and were left immersed in IPA for another three minutes to remove any unreacted resin. Mild annealing was carried out in a Fisher Scientific Isotemp 280A vacuum oven at 50 °C and 100 °C for 12 hours. Samples were stored in vacuum to avoid moisture absorption and contamination before characterization. Casted samples were fabricated by pouring the GO resin into 3D printed molds fabricated using a Zortrax M200 printer with ABS filament. The molds were sprayed with a non-stick dry film lubricant as a mold release. The part was then UV irradiated for 15 minutes using Honle Bluepoint 4 EcoCure, with a wavelength range of 390-500 nm, a light source-to-specimen distance of 90 mm, and machine intensity of 60%. 4.

Raw Material and SLA-Printed Nanocomposites Characterization Methods

a.

Rheological Characterization Changes in viscosity of the resin with increasing filler loading were measured using

Brookfield DV2T viscometer, at a speed of 1 rpm, multi-point averaging with points taken every 2 seconds and averaged every 10 seconds, an end condition of 3 minutes, and a water bath temperature of 25 °C.


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b.

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Thermomechanical Characterization To determine the effect of GO on the mechanical properties of the nanocomposites,

tensile test was carried out using an MTS Renew Upgrade tensile testing machine with a 1 kN load cell and pneumatic Instron clamps. Tensile test parameters were according to ASTM D638. Three replicates were performed for each setting. Thermal behavior of the nanocomposites was determined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA was carried out using TA Instruments Q500 under nitrogen atmosphere with a ramp rate of 10 °C/min to 900 °C. Similarly, DSC was performed using TA Instruments Q100 under nitrogen atmosphere with a ramp rate of 10 °C/min to a temperature where .5% of the mass has degraded based on the TGA. c.

Structural Characterization Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy was

carried out using an Agilent Cary 600 Series spectrometer. A total of 32 scans was used to get the average spectra. Structural changes in the nanocomposites were further monitored using Jasco NRS-4100 Raman spectrometer with an excitation laser wavelength of 532 nm. An appropriate amount of the vacuum dried GO powder was used directly for FTIR, while a dried GO flake was used for Raman spectroscopy. A portion of the fractured tensile test specimens was used for both FTIR and Raman for nanocomposite characterization. XRD analysis was performed using a Rigaku MiniFlex 600 with Cu K radiation wavelength of 0.154 nm. Samples were prepared by cutting the tensile test specimens to approximately 0.75”x0.5” after mechanical testing. This was then mounted onto the center of a


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glass slide for analysis. Diffractograms were acquired in 1 hour ranges from 10° to 40° with a scan rate of 0.02 degrees/second, accelerating voltage of 40 kV, and current of 15 mA, at ambient temperature. For UV-Vis spectrometry, a 1 mg/mL sample was prepared by ultrasonicating the dried synthesized GO powder in water for 10 minutes using the same procedure discussed above before 3D printing. After which, an appropriate amount was transferred into a glass cuvette with a path length of 1 cm. Absorbance was measured with respect to water as baseline using a StellarNet UV-Vis-NIR system with a scan range of 200-1100 nm. All rheological, thermo-mechanical, and structural characterization data were plotted and analyzed using Origin 7.0. d.

Morphological Characterization Appropriate amounts of dried GO powder were mixed with 10mL Milli-Q water to

prepare 1 mg/mL and 2 mg/mL solutions for AFM and SEM, respectively. The GO-water mixture was ultrasonicated using the same settings discussed for UV-Vis. The solution was casted on a mica substrate for AFM and a glass slide for SEM. PicoScan 2500 (Agilent Technologies) atomic force microscope was used in tapping mode to characterize the surface morphology of as-synthesized GO. The piezo scanner with tapping mode tips (NSG30, golden silicon AFM probes, NT-MDT, Tempe, AZ) was run at 1-1.5 lines/s with a 10 m x 10 m imaging area. AFM images were processed using Gwyddion 2.45 software.


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Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of the GO nanocomposites. Samples were coated with 5 nm-thick gold using Hummer 6.2 Sputter System from Anatech USA (Alexandria, VA). SEM micrographs were taken using a JEOL scanning electron microscope (JEOL-JSM-6510LV) with a working distance of 16 or 17 mm and acceleration voltage of 30 kV.

RESULTS & DISCUSSION 1.

GO Synthesis and Nanocomposite Fabrication via SLA Printing The successful formation of graphene oxide was confirmed using FTIR, Raman, and UV-

Vis absorbance spectroscopy. The FTIR spectra of the synthesized GO in Figure S3A confirms the presence of various oxygenated functional groups. Bonds corresponding to C−OH stretching (~1226 cm-1), O−H deformation vibration (~1416 cm-1), C=C aromatic ring in-plane stretching (~1600 cm-1), and C=O carboxylic acid vibration (~1693 cm-1) were observed, which agree to the major functional groups present in GO reported by other researchers.28–30 Similarly, the signature D and G bands of graphene oxide were detected at 1340 cm-1 and 1594 cm-1, respectively, using Raman spectroscopy (Figure S3B). The D band corresponds to the in-plane breathing vibration of GO’s aromatic carbon rings due to any structural defects from a perfect graphite architecture.30 On the other hand, the G band is characteristic of the in-plane stretching vibration of carbon atom pairs.30 Moreover, the UV-Vis spectra (Figure S3C) of as-synthesized GO showed the characteristic peak at ~230 nm, attributed to =∗ transitions of C=C in amorphous carbon systems.31 A shoulder band at 300 nm corresponding to the n–π* transition of C=O was also observed.


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A commercially available acrylate-based matrix was used for nanocomposite preparation. GO-resin interaction may affect the viscosity of the matrix, which is an important consideration to ensure good quality of SLA-printed parts.32,33 Low viscosity resins are usually preferred as these allow better resin flow to replenish each layer during printing34 and also makes handling (e.g. refilling and cleaning the resin tank) more convenient.34,35 In practice, the resin should have a viscosity less than 5 Pa s (5000 cP) to be used for SLA.34 Thus, the viscosity of the resin with varying GO concentration was first measured before proceeding with 3D printing. As expected, increasing GO content resulted in an increase in viscosity, but all values were found to be well below the 5000 cP threshold (Table 1). Table 1. Viscosity of GO Nanocomposite Resins

2.

GO Concentration, wt%

Viscosity, cP

0.1

633.0 ± 30.00

0.5

1116 ± 30.00

1

1562 ± 30.00

SLA-Printed Nanocomposite Characterization FTIR analysis of the resin (Figure 1A) showed the presence of functional groups

corresponding to O−H stretching (~3378 cm-1), C−H stretching (CH3 at ~2944 cm-1; CH2 at ~2852 cm-1), C=O stretching (~1707 cm-1), and C−O−C stretching (~1245 cm-1).36 The presence of oxygenated functional groups in the resin makes it possible to interact with GO by forming hydrogen bonds37 (Scheme S1). This enhanced interaction with most polymers is one of the advantages of GO over graphene by virtue of its surface functional groups.24,38,39 On the other hand, the FTIR spectra of the printed nanocomposites (Figure 1B) with varying GO


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concentrations were found to correspond to that of the resin. It is difficult to confirm the presence of GO through FTIR as many of the peaks overlap with that of the resin, specifically those in the 1245 cm-1 (C−O−C stretching), 1707 cm-1 (C=O stretching), and 3378 cm-1 (O−H stretching regions). Raman spectroscopy was used instead, and the signature D and G bands of GO were observed in the 1346 cm-1 and 1600 cm-1 regions, respectively (Figure 1C).

Figure 1. FTIR spectra of (A) SLA resin showing major functional groups and (B) Nanocomposite Control (unheated) samples; (C) Raman spectra of as-synthesized GO and 1wt% GO nanocomposite

The average modulus and tensile strength values for all samples with varying GO compositions and annealing temperatures are listed in Table S1 and presented in Figure 2. For unheated samples and those annealed at 50 °C, the overall strength of the 3D-printed parts is steadily decreasing. On the other hand, a drastic increase in strength was observed for samples treated at 100 °C as the GO content was increased. The same trend was observed for the modulus. To explain these observations, we look at the changes happening in both the resin and GO with increasing temperature.


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Figure 2. Tensile strength as a function of GO loading

2.1 Control and Mild Annealing at 50 °C Percolation threshold is a well-known concept in nanocomposite fabrication. It is used for determining the critical concentration of fillers needed, beyond which no reinforcing effect can be observed. Data from Figure 2 suggests a low percolation for the Control samples and those annealed at 50 °C. The increasing concentration of GO might have led to excessive inter-platelet interaction instead of GO-resin hydrogen bonding, thus decreasing GO-resin interaction and leading to the observed decrease in strength. Moreover, the presence of GO in the resin can lower the efficiency of photopolymerization because the filler can serve as a barrier or hindrance to incoming laser light. The possibility of GO acting as a chain transfer agent inhibiting further growth of polymer chain is an interesting subject worthy of careful investigation in the future. Another possible reason for the observed decrease in strength may be the presence of wrinkles in the graphene sheets as observed in AFM (Figure 3A). These wrinkles can affect the stress distribution on the surface of GO,40 hindering good interlayer adhesion between GO and the resin, consequently causing poor load transfer and leading to lower strength. The presence of the star-like pattern (Figure 3A) suggests weak interaction between GO and the substrate41,42


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where the edges of the GO may become restrained due to Van der Waals forces, but the basal faces free to move and bend around to cause folding or wrinkles.41 Wrinkles may also originate from the carboxyl groups at the edges of GO, whereby edge-edge interaction would cause steric hindrance between two adjacent sheets instead of stacking neatly one on top of the other43 (Scheme S1). There are several ways to remove wrinkles, or at least lessen them as they are said to be inevitable.43 These include increasing temperature44,45 or by enhancing interaction between graphene sheets either by using large GO sheets or inducing inter-sheet chemical crosslinking.43 It should be noted that although current literature suggests that the described wrinkling phenomenon may occur, further investigation should be conducted to provide stronger evidence of wrinkling after thermal annealing of GO nanocomposites, which is beyond the scope of this study.

Figure 3. AFM images of as-synthesized GO showing wrinkled morphology (A); SEM images (scale bar = 50) of fracture surfaces for the unheated (left) samples and those annealed at 100°C (right) for the resin (B and C),


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0.1wt% GO (D and E), 0.5wt% GO (F and G), and 1wt% GO (H and I) nanocomposites; Representative DSC curves showing Tg at various annealing temperatures (J);

2.2 Mild Annealing at 100 °C The SEM images of the Control and annealed fracture surfaces of the resin are shown in Figures 3B and C, respectively. Smaller pore sizes were observed at 100 °C; the same was true for the 0.1 wt% sample (Figures 3D and E). Tensile strength increased by 23.84% for the resin alone when heated to 100 °C (Table S1). This can be attributed to enhanced filler-matrix interaction brought about by the decrease in pore size with increasing annealing temperature. The porosity of the 0.5 and 1 wt% samples was more difficult to evaluate from SEM, but the more fibrous and massive fracture surface became more visible, which is indicative of plastic deformation and good crack deflection by the filler40 (Figures 3F-I). Crack deflection is a wellestablished toughening mechanism for nanocomposites wherein the plane of action of an advancing crack is shifted or, as the name suggests, deflected when it encounters a filler. This results in changes in the stress distribution on the crack tip,46 requiring additional energy for crack propagation and producing a rougher fracture surface, which agrees with SEM observations. Based on the DSC results (Figure 3J), the glass transition temperature of the resin lies around 47 °C. Thus, at 100 °C, the material would have softened and “flowed” in such a way as to eliminate or decrease the size of existing pores, which agrees with SEM observations. Furthermore, Figure 3J shows that there is increasing Tg with increasing annealing temperature. One of the factors affecting glass transition temperature is the ease with which polymer chains can move. The more easily they move, the lower the Tg and the more constrained they are, the


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higher the Tg because more thermal energy is needed to induce motion of the polymer chains. Consequently, the increasing Tg for the nanocomposites show that GO was effective in hindering the motion of polymer matrix chains.47

Figure 4. TGA of 1wt% GO nanocomposites at different annealing temperatures (left) with zoomed regions A (middle) and B (right)

Representative TGA curves (Figure 4) were analyzed to explain the increased strength for the samples annealed at 100 °C. It is apparent from Figure 4A that the Control and sample annealed at 50 °C had a lower initial decomposition temperature (Ti) and that the residual weight is higher with increasing annealing temperature (Figures 4A and B). These observations suggest higher thermal stability48 after annealing at 100 °C. Three assumptions will be presented to explain these phenomena: (1) there is lower defect density49 with increasing annealing temperature, (2) there is enhanced crosslinking between GO and the resin at 100°C due to acidcatalyzed esterification, and (3) the sample lost intercalated water in its structure at 100°C, hence no mass loss was observed in this region. The validity of each assumption was verified through spectroscopic techniques.


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2.2.1 Spectroscopic Analyses of Strength Increase Raman spectroscopy was used to verify any changes in defect density with respect to temperature. The number of point defects ( for any excitation wavelength may be calculated using a formula reported elsewhere.30 Using the integrated intensity values obtained from Raman spectra (Figure 5A), the calculated for the Control and annealed samples were technically the same at 2.40610 defects/nm2 and 2.41210 defects/nm2 (Table S2), respectively. This suggests that the lower Ti experienced by the Control is not due to it having more defects compared to the 100 °C sample.

Figure 5. (A) Raman spectra of as-synthesized GO and 1wt% GO nanocomposites and (B-D) FTIR spectra of GO nanocomposites before and after annealing


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We now verify the second assumption of chemical crosslinking due to acid-catalyzed esterification. The oxygenated functional groups present in GO render it with acidic properties,50 allowing GO to act as a catalyst for various chemical reactions,51 including esterification. It follows from the endothermic nature of esterification that a higher temperature would enhance the extent of the reaction,52 which could explain why the strengthening effect of crosslinking was only observed at 100°C. To prove this, Figure 5B-D show the FTIR spectra of GO nanocomposites before and after annealing. It is apparent that the C=O peak (~1700 cm-1) for all settings decreased after annealing at 100 °C. Consequently, the carboxylic O-H stretch at ~3300 cm-1 especially at 1wt% and 0.5 wt% also markedly decreased at the same temperature. Taken together, the depletion of COOH moieties evidence the occurrence of chemical crosslinking via esterification (Scheme 2).

Scheme 2. Proposed covalent and non-covalent interaction of 3D printed GO nanocomposites.

The removal of intercalated water was verified through XRD. From Bragg’s law,53 it is expected that a decrease in interlayer distance due to the removal of intercalated water would result in an increase in the angle of diffraction. This was apparent for the 1 wt% GO nanocomposites (Figure 6) wherein a shift to a higher diffraction angle was observed at a higher annealing temperature.


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Figure 6. XRD spectra of 1wt% GO nanocomposites control and those annealed at 50°C and 100°C

With these results, the combined effect of enhanced crosslinking via acid-catalyzed esterification, removal of intercalated water, and decrease in porosity of the resin caused the 1 wt% GO nanocomposites to have better interaction with adjacent GO sheets and the resin at 100°C, consequently improving thermo-mechanical properties.

3.

SLA-Printed vs. Casted Parts To gauge how the properties of the 3D-printed GO nanocomposite compare with

traditional manufacturing methods, a part was casted using the GO-resin mixture and its tensile strength measured. The 3D-printed Control samples (Figure 7A) were found to have a significantly lower tensile strength compared to the casted parts. Upon mild annealing at 100 °C, however, the tensile strength of the 3D-printed part became comparable with the casted one, attaining an even higher average value.


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Figure 7. (A) Tensile strength comparison of casted and 3D-printed parts; SLA-printed complex-shaped GO nanocomposites: (B) Nested dodecahedron and (C) Diagrid ring

This makes mild annealing a simple, yet promising technique for thermal post-curing of SLA-printed GO nanocomposites to produce materials with good mechanical properties and complex shapes (Figures 7B and C) that are essentially impossible to achieve through casting. The process also has potential applications for parts printed using methods not involving heat, such as paste extrusion. The operating temperature of other techniques, such as fused deposition modeling (FDM) or selective laser sintering (SLS), are well beyond the mild annealing temperatures used in our study, which would defeat the purpose of our low-temperature postcuring process.


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CONCLUSIONS A simple process for strengthening SLA-printed GO nanocomposites was presented, making use of GO’s metastable structure through mild annealing after 3D printing. Modulus and tensile strength decreased for both the Control and samples annealed at 50 °C. In contrast, samples annealed at 100 °C experienced a drastic increase in mechanical properties with the highest % increase recorded at 673.6% for the 1 wt% GO nanocomposite. This enhancement was attributed to three reasons: (1) enhanced crosslinking via acid-catalyzed esterification, (2) removal of intercalated water in the GO membranes, and (3) decrease in pore size of the resin with increasing annealing temperature. These results were confirmed by FTIR, XRD, and SEM, respectively. Furthermore, TGA and DSC showed enhancement of thermal behavior with increasing annealing temperature. The mild annealing method employed may easily be scaled up to potentially expand the range of applications of 3D-printed GO nanocomposites. It was demonstrated that its properties are comparable to traditionally casted parts of the same material annealed at 100 °C. Furthermore, the technique is capable of producing nanocomposites with complicated structures with good thermo-mechanical properties, otherwise unattainable through traditional casting methods. The study may be further extended by annealing at temperatures high enough to induce reduction of GO to graphene, giving the resulting parts better electrical properties—promising for fabricating complex-shaped electrodes among other potential applications.


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Acknowledgements This work is supported by the Department of Science and Technology - Philippine Council for Industry, Energy, and Emerging Technology Research and Development (DOSTPCIEERD) and the National Science Foundation (NSF CMMI NM 1333651 and STC-0423914). We would also like to acknowledge the following for their assistance: Dr. Emily Pentzer (Department of Chemistry, Case Western Reserve University, Cleveland, OH), Ms. Sherry Hemmingsen (JASCO), Dr. David Schiraldi and Ms. Kimberly DeGracia (Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH), and Dr. Anna Cristina Samia and Mr. Eric Abenojar (Department of Chemistry, Case Western Reserve University, Cleveland, OH).

Supporting Information. 3D printing processing conditions (PDF), Mechanical properties of GO nanocomposites (PDF), Number of point defects (PDF).

Author Contributions The manuscript was written through contributions of all authors. ‡These authors contributed equally.


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