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Chemically Functionalized Reduced Graphene Oxide as Additives in Polyethylene Composites for Space Applications.

INTRODUCTION

A major barrier to long-term space exploration is the development of technologies to protect astronauts and their equipment from harmful levels of radiation exposure [1, 2], which can lead to a variety of serious health effects such as cancer [3-5]. Materials rich in hydrogen are known to attenuate a variety of radiation types encountered during space travel, including high Z element (HZE) ions found in galactic cosmic rays (GCRs), protons found in solar particle events (SPEs), and neutrons resulting from the collisions between GCRs and SPEs and matter [6]. One of the most practical materials featuring rich H content is polyethylene, which has approximately 8.9 * [10.sup.22] H atoms/[cm.sup.3], far more than water or even liquid H2 [7]. As such, polyethylene has been considered in space applications and even deployed on the International Space Station as passive shielding for crew sleeping quarters [6-12]. Despite their favorable performance for radiation attenuation, polyethylene and many other olefins generally possess lower mechanical properties, such as tensile strength and impact toughness, than high-performance polymer systems such as Kevlar or poly(ether ether ketone). One slight advantage in using high-density polyethylene (HDPE) for protective applications is its remarkably low-glass transition temperature of -166[degrees]C that is far below the coldest lunar temperatures of -120[degrees]C.

An emerging approach to enhance mechanical and electrical properties of polymers during melt processing is to incorporate 2D graphene structures to form a nanocomposite [11, 13-18]. The efficacy of this approach is perhaps most limited by the miscibility of the graphene structures with matrix polymers, a topic that has become the focus of many studies and reviews [19-25]. Typically, the conjugated sp2 network of the graphene nanoparticle has little thermodynamic incentive to interact with the polymer matrix and instead prefers to aggregate. Ultimately, these aggregates create weak points that decrease the mechanical properties of the composite and can potentially create defects from which catastrophic fractures originate.

Many reports have emerged utilizing reduced graphene oxide (rGO) chemistry as a means for realizing well-dispersed graphene-like nanoparticles in polymer matrices [15, 19, 20, 23, 25, 26]. Before reduction, the various alcohol, carboxylic acid, and epoxide functional groups of GO confer solubility in solvents, such as water and acetonitrile, which provide numerous chemical modification and processing possibilities. These same functionalities disrupt the conjugated planarity of the GO and thus reduce the mechanical and electrical properties generally associated with graphene materials. After reduction of the GO, the resulting rGO loses solubility, however the material regains much of the lost electrical conductivity and strength generally associated with graphene [27]. As such, new chemical modifications and/or processing strategies are required to realize rGO-containing composites or films at the commercial scale.

In this work, we functionalize GO nanoparticles with dodecyl chains via a Williamson-ether synthesis that is followed by a chemical reduction with hydrazine. Characterization of the resulting alkylated reduced graphene oxides (A-rGO) with FTIR spectroscopy, solid-state NMR, elemental analysis, and conductivity testing are performed on the nanoparticles. To examine changes in the dispersion of the A-rGO nanoparticles, they are incorporated into commercially available HDPE via melt compounding. The composites are pressed into thin films and compared to HDPE control samples fabricated with rGO as a control additive. Finally, dynamic mechanical analysis (DMA) and tensile testing are used to understand changes in mechanical properties from the modified rGO additive. Changes in mechanical performance are linked to differences in the dispersion of rGO nanoparticles by cross-sectional scanning electron microscopy (SEM).

RESULTS AND DISCUSSION

Material Synthesis

The synthetic scheme used to obtain rGO and A-rGO is summarized in Fig. 1. The use of the Williamson-ether synthesis was chosen as it forms a robust ether linkage that can withstand the hydrazine reduction process [28]. This contrasts with GO modification strategies that use TsCl or SO[Cl.sub.2] [29] that result in more vulnerable ester linkages and would cleave upon chemical reduction. As such, this synthetic route provides a unique opportunity to obtain a chemically functionalized reduced graphene structure. The visual appearance of rGO and A-rGO are indistinguishable. As a solid, both are a deep black color typical of graphene. The added dodecyl chains of the A-rGO confer increased dispersibility in acetonitrile and n-methyl pyrrolidone (NMP) compared to rGO. The size and functionality of graphene oxides are notoriously difficult to characterize as they generally feature limited solubility and their complex molecular structure results in convoluted signals with many quantitative techniques [30-32]. As such, establishing an extent of functionalization for the A-rGO is limited by similar complications. Nevertheless, numerous characterization techniques were used to confirm the attachment of dodecyl chains in the A-GO and A-rGO samples, as discussed below.

rGO and A-rGO Structural Characterization

Fourier-transform infrared spectra (FTIR) of each graphene product are shown in Fig. 2.

The GO starting material shown in black is consistent with other reports investigating GO chemistry [33-35]. Perhaps most apparent are the broad O--H stretching peaks centered at 3,420 [cm.sup.-1] in the unreduced samples. Other major peaks observed in the GO material include a C--H stretch at 2,900 [cm.sup.-1], C=O vibration at 1,730 [cm.sup.-1], C=C stretch at 1,620 [cm.sup.-1], C--O ether stretch at 1,230 [cm.sup.-1], and epoxy asymmetric and symmetric stretches at 865 [cm.sup.-1] and 1,030 [cm.sup.-1], respectively. The alkylation reaction is confirmed by the shifts of the carbonyl peaks and appearance of new ones in the range of 1,230-1,500 [cm.sup.-1]. The peaks shift to higher wavenumbers as a result of the decrease in conjugation, presumably due to the aliphatic dodecyl chains. These shifts are also accompanied by the formation of a wide doublet peak in the alkane stretch signal at 2,800 [cm.sup.-1] that is much more pronounced than unmodified GO.

Upon reduction of GO and A-GO with hydrazine, most of the chemical functionality is removed, as indicated by the relatively flat profile shown in red and magenta. After reduction of the AGO to A-rGO, shown in pink, there remains a small peak at 2,800 [cm.sup.-1] corresponding to the C--H stretch of the alkyl chains. This feature, combined with the presence of the C--O stretch at 1,230 [cm.sup.-1], which is consistent with C--O ether stretching, confirms that the final A-GO was successfully reduced with the alkyl chains intact via an ether linkage. As expected, the decrease of the carbonyl signal at 1,730 [cm.sup.-1] suggests that most of the ester-functionalized dodecyl chains did not survive the reduction process. This is also supported by the significant loss of alkane signal at 2,800 [cm.sup.-1] in comparison to A-GO. The rGO and A-rGO samples each feature a medium peak at approximately 2,100 [cm.sup.-1] that is not readily identifiable nor shared by the spectra of the unreduced analogues.

Elemental analysis was used to measure the atomic ratios of C, H, N, and O and these results are summarized in Table 1. GO features the highest O levels (50.6%) and lowest C (47.1%) of the sample set, resulting in a C:0 ratio of 0.93. After reducing GO with hydrazine, a C:0 ratio of 4.61 was measured, reflecting the significantly lower levels of O (16.5%). The A-GO product features similar C and O levels of GO but also includes significantly more H, presumably due to the dodecyl functionalities. After reduction, the C:O ratio increases from 1.21 to 2.90, which is substantially lower than the unmodified rGO. This difference is due to the ether linkages of the dodecyl functionalities that are not removed during hydrazine reduction. Trace amounts of N are observed in the rGO, A-GO, and A-rGO samples that arise from two different mechanisms. First, in the chemically reduced samples, rGO and A-rGO, hydrazine accounts for a portion of the N, as it has been shown to install pyrazole-like functionalities about the edge of the GO ring [36]. The second N source could be NMP, which was chosen as the solvent for the Williamson-ether synthesis for its superior ability to dissolve GO [27]. Under high-temperature conditions, it has been reported that NMP can ring open and attach to the basal plane of the resulting graphene oxide disks [37]. Although a much lower temperature was used for our synthesis (60[degrees]C compared to 210[degrees]C as reported by Dubin et al. [37]), it is likely that the N present in the A-GO sample originated from a similar mechanism.

Solid-state [sup.13]C NMR measurements of the graphene oxide before and after functionalization are shown in Fig. 3. Many of the features of unfunctionalized GO are consistent with previously reported measurements in the literature [36, 38, 39]. Peaks at 140 ppm and 60 ppm correspond to [sp.sup.2] carbon-carbon bonds of the planar C network. In the A-GO sample, there is an emergence of a strong peak at 30 ppm corresponding to the alkyl chains that were added during the functionalization reaction. Furthermore, there are peaks corresponding to ester and ether functionalities in the A-GO spectra located at 180 ppm. It bears mentioning that alkyl chains grafted via ester linkages will cleave upon hydrazine reduction and therefore be absent in the final A-rGO product. Spectra of rGO and A-rGO were not possible due to the electrical conductivity resulting from reduction.

Electrical Conductivity Characterization

Electrical conductivity measurements were performed as an assessment of the applicability for this type of modification strategy for electronic and spacesuit applications, where conductivity offers protection against electrostatic charges. Pellets of rGO and A-rGO were pressed using an IR pellet press and their resistivity was measured using the Van der Pauw Method [40] before and after thermal annealing overnight at 120[degrees]C under vacuum. The results of these measurements are summarized in Table 2.

In the as-pressed samples, electrical conductivities of 1.2 S/m and 6.6 * [10.sup.-6] S/m were recorded for rGO and A-rGO, respectively. The significant reduction in conductivity of the A-rGO material is explained by two factors. First, the attachment of dodecyl chains on the basal plane of the GO disc hinders restoration of the [sp.sup.2] network upon hydrazine reduction. The presence of these groups is supported by the increased functionality of the A-rGO observed from FTIR, elemental analysis, and solid-state NMR results discussed previously. Another anticipated effect of the dodecyl chains is that they will prevent interplanar stacking between A-rGO nanoparticles, thereby decreasing the interparticle charge transfer. While this is a drawback regarding electrical conductivity, this effect likely plays a significant role in the enhanced miscibility of the A-rGO in an HDPE matrix.

Another source of insulation could arise from the presence of water or solvent entrapped in the A-rGO materials. To explore this possibility, electrical conductivities were also measured after thermal annealing at 120[degrees]C overnight under vacuum to remove any entrapped water. After thermal annealing, only a marginal enhancement of the conductivity was observed for the rGO sample. Although annealing led to a 10x increase in the conductivity of the A-rGO sample, the resulting conductivity remains low compared to the unmodified rGO counterpart. Therefore, this lack of conductivity is attributed to the abundant functionalization of the A-rGO basal plane.

Dynamic Mechanical Analysis

Dynamic mechanical analysis measurements as a function of temperature were used to examine the impact on composite mechanical properties and obtain a more quantitative assessment of the dispersion of rGO and A-rGO nanoparticles throughout the HDPE matrix. A temperature range of 30-100[degrees]C was chosen to capture the performance of these materials at the upper bound of temperatures experienced on the lunar surface given their potential in spacesuit applications and activities around the Moon (NASA Lunar Gateway). The storage modulus and tan(5) of these measurements are shown in Fig. 4.

In the rGO sample set, none of the samples exhibited higher storage moduli than the neat HDPE sample, and, in general, the storage moduli decreased with rGO loading level. Alternatively, the samples containing the A-rGO additive exhibited a slight enhancement in storage moduli for loading levels of 0.1 and 1.0% compared to the control HDPE sample. In the best performing sample, 0.1% loading of A-rGO, there is a 30% improvement in the measured storage modulus compared to the 0.1% rGO counterpart. This result, in conjunction with the overall enhancement of modulus at all loading levels, indicates that the dodecyl chains are improving the miscibility of the additives within the matrix. Furthermore, the storage moduli for all samples remains sufficient at even the hottest temperatures on the lunar surface that these composites could be used in low-stress applications on the lunar surface.

Thermogravimetric analysis experiments were performed for neat HDPE and composites containing 1.0% rGO and A-rGO to elicit any changes in thermal stability upon inclusion of the nanoparticles. As shown in the Supporting Information Fig. S3, there is no observable difference in the thermal stability between the neat HDPE and the composites containing rGO nanoparticles. Each sample undergoes a single degradation event at [approximately equal to] 440[degrees]C. Only residual rGO and A-rGO (< 1.0%) remain at the end of the temperature ramp to 800[degrees]C.

Static Mechanical Properties

The samples' static mechanical performance varied within the selected actuator displacement range, even for the same group of samples and for the neat HDPE samples. Response varied from stretching with no apparent necking areas, to stretching with a clear necking area, to stretching and fracture for few samples, without a clear correlation with loading and functionalization type. The Supporting Information contains images of these various failure modes as shown in Fig. S1, along with stress versus time plots in Fig. S2 for the 0.1% A-rGO, 0.1% rGO and neat HDPE samples. The variety of deformation modes in the same samples is paralleled by scatter of material properties as illustrated in Fig. 5. Among the samples, the 0.1% A-rGO treatment may be the most promising for the yield strength, with lower scatter (notwithstanding only three samples being tested) and a median that is ~14% higher than the baseline median. This is consistent with the DMA plots shown in the previous section. The scatter of deformation modes in the same samples is paralleled by the scatter of material properties, as shown in the form of boxplots for the 0.1% A-rGO, 0.1% rGO and baseline configurations (Fig. 5ac). The scatters of the stiffness and Poisson's ratio values are considerably higher, making assessment of treatments more difficult. The median yield strength of the tested baseline samples, 20.02 MPa, is compatible with the published data of different types of processed HDPE, which range from 2.69 to 200 MPa for extruded HDPE to 15.2-42.1 MPa for blow molding grade HDPE [41, 42]. The median Young's modulus of the tested baseline samples, 1.49 GPa, is consistent with published data: 0.6201.45 GPa for extruded HDPE, 0.650-2.07 GPa for blow molding grade HDPE [41, 42].

Within the selected displacement range and strain rate, the same for all samples, the variation of trends (clearly defined by a maximum, or lack thereof) parallels the variation of necking features in the samples. A possible explanation for the improved tensile strength of 0.1% A-rGO with respect to 0.1% rGO could be the presence of aggregates in the rGO samples, which are compared using SEM in Fig. 5d-(e). Furthermore, the rGO composites were far less homogeneous, which was apparent by sample color and the appearance of aggregates upon visual inspection. It is envisioned that the aggregates cause stress concentrations and, due to their variability in the samples' volume, impact results scatter. Such aggregates are not as evident in the A-rGO samples, which macroscopically are also significantly more uniform in color than the rGO samples.

CONCLUSIONS

This work demonstrates the utility of our synthetic process to remedy troublesome properties of rGO, such as limited solubility and miscibility, which prevent its use in numerous applications. The Williamson-ether synthesis demonstrated herein provides a facile method to functionalize GO nanoparticles and tailor nanoparticle properties to suit application-specific needs. While the electrical conductivity is decreased by these modifications after reduction, the dodecyl-functionalized rGO nanoparticles still show promise in the enhancement of polymer composite properties. By incorporating these materials into a HDPE matrix, an enhancement of approximately 15% in storage modulus, as well as tensile strength, was observed compared to neat HDPE and outperformed a control containing the unmodified rGO additive by 30%. The fact that neat trends are not observed in the DMA results of the A-rGO sample set indicates that aggregation is still likely occurring, however, to a much lesser extent. We believe that this synthetic route offers promise in applications that do not require electrical conductivity and that might benefit from such chemical modifications, such as filtration membranes or biological scaffolds.

EXPERIMENTAL

The following sections provide details of the sample preparation followed to prepare the polyethylene-based composites. Unless stated otherwise, all materials and solvents were purchased from Sigma-Aldrich.

Synthesis of Graphene Oxide

Graphene oxide was synthesized using the modified Hummer's method [34] that is summarized as follows: First, 3.0 g of graphite flake (Alfa Aesar) and 18.0 g of potassium permanganate were added to a clean 500 mL round bottom flask with a Teflon-coated stir bar. Next, a mixture of sulfuric acid (360 mL) and phosphonic acid (40 mL) was prepared in a beaker and slowly added to the RB flask while mixing. Upon complete addition, a septum was used to cap the RB flask, and a vent needle was added to prevent any pressure build up. The reaction mixture was heated to 50[degrees]C and allowed to stir overnight ([approximately equal to]18 h). The reaction was halted by first cooling the reaction to room temperature and then pouring the contents over 500 mL of ice containing 3 mL of 30% hydrogen peroxide ([H.sub.2][O.sub.2]). The graphene oxide product was purified by filtration through glass wool, and centrifuged at 4,000 RCF for 2 h, after which the supernatant was decanted and disposed. The collected solids were dissolved in deionized water, filtered, and centrifuged again. This wash, filtration, and centrifugation process was repeated sequentially using 30% HCl, DI water, and ethanol. The final GO product was suspended in ether and recovered using a 0.45 [micro]m PTFE membrane filter. After drying in vacuum overnight, the resulting GO had a deep brown color and was characterized using FTR-IR, solid-state [sup.13]C-NMR, and elemental analysis.

Synthesis of Dodecyl-Graphene Oxide

To a clean and freshly dried three-neck RB flask, 40 mg of sodium hydride were added and sealed using rubber septa in a [N.sub.2] filled glovebox. In a separate flame-dried pear flask, 0.64 g GO were dissolved in 150 mL of iV-methyl-2-pyrrolidone (NMP) and sonicated for 20 min using a Fisher Scientific FS20H ultrasonic cleaner. After complete dissolution, the contents of the pear flask were transferred via cannula into the previously prepared threeneck RB flask under inert atmosphere. The RB flask was heated to 60[degrees]C and allowed to react for 2 h. In another dried pear flask, 6.1 g of dodecyl bromide (large excess) were diluted with 50 mL of NMP and sparged with argon for 15 min. The dodecyl bromide solution then was transferred via cannula into the reaction flask and allowed to reach 60[degrees]C for 72 h under an argon atmosphere. The reaction was quenched by adding 2-propanol drop-wise until no bubbles were observed. This same quenching procedure was repeated using DI water. The product was precipitated using ether, collected using a cellulose filter, and dried under vacuum for 72 h at 60[degrees]C to remove residual NMP.

Reduction of Graphene Oxides

The chemical reduction of GO and dodecyl-GO was performed using the same procedure. First, a stir bar and 100 mg of GO were added to a clean 250 mL RB flask with 80 mL of DI water in open atmosphere. After the mixing was stirred for 1 h to ensure complete dissolution, anhydrous hydrazine (2 mL) was added to the flask. The flask was immediately equipped with a condenser, capped with a rubber septum, and heated in an oil bath to 80[degrees]C. The reaction can be observed almost instantly as the reactor contents begin to precipitate and undergo a color change from a deep brown to black. These conditions were maintained for 18 h to ensure complete reduction of the GO. The rGO product was collected using a nylon membrane filter and washed five times using 100 mL of DI water and 100 mL of methanol during each rinse. The final product was dried under vacuum overnight at 60[degrees]C.

Composite Materials Processing

Composite samples were fabricated with rGO (resulting in control composites) and A-rGO at loading levels ranging from 0.1 to 1.0 wt% in HDPE (Sigma-Aldrich, [M.sub.n] of 80,000 g/mol) using a multistep process as shown in Fig. 6. To prevent the powdered graphene additives from being ejected out of the extruder inlet during processing, a premixing step was performed, in which a measured amount of HDPE was first melted in a beaker at 190[degrees]C, and allowed to completely melt as indicated by its color change from an opaque white solid to a clear amorphous solid. Next, a calculated amount of rGO or A-rGO was added to the molten polymer to achieve the target loading level. The molten composite (Fig. 6a) was then folded over the graphene powder and pressed flat. This "puck" of molten HDPE and graphene was then allowed to cool to room temperature and cut into pieces small enough to fit into the extruder inlet.

To ensure complete mixing of the graphenes in the HDPE matrix, the Thermo Scientific HAAKE MiniLab extruder was used to recirculate approximately 5.5 g of composite material per batch. Each batch of material was recirculated for 2 h at 180[degrees]C using a screw speed of 100 rpm. Typically, a torque of 100 N-cm was observed for these mixing parameters. After mixing, the material was removed from the extruder and placed into a Carver hot press (model 3851-0), where films were pressed at 3 tons and 180[degrees]C for 5 min, while using a Teflon mold to obtain consistent thicknesses on the order of 1 mm. To have comparable thermal histories, neat HDPE samples were fabricated using this same process of compounding for 2 h followed by melt pressing into films. For samples used in DMA and tensile testing, each film was cut into "dog bone" samples using an ASTM D638 [43] die, type V sample geometry.

Immediately upon melt processing, the enhanced miscibility of the A-rGO becomes apparent as shown in Fig. 6e, in which a neat HDPE sample is shown alongside 0.5% loading of rGO, and ArGO, respectively. In this comparison, the speckled appearance of the 0.5% rGO sample contains much more aggregation than the analogous sample with 0.5% A-rGO, which is much darker and more uniformly colored throughout. This type of aggregation was observed in rGO samples at all loading levels, while the A-rGO-containing samples only exhibited minimal signs of aggregates when observed with the naked eye, even at the highest loading level of 1.0%.

Samples Preparation for Mechanical Testing and DMA

Neat HDPE, baseline rGO, and A-rGO composites at different loading levels, were tested following ASTM D638 protocols under quasi-static conditions to obtain mechanical properties (yield strength, Young's modulus and Poisson's ratio) in ambient atmosphere. The size of the samples (0.3-0.45 mm thickness range and ~3.1 mm width) required particular care in the selection and application of strain gauges. Strain gauges with a 350 [OMEGA] grid (biaxial rosette, Omega SGT-3BH/350-XY41) were applied following standard surface cleaning and gauge application procedures. However, standard soldering could not be used because of the risk of melting the samples. Wires (30 AWG) were bonded to the gauges' solder pads with conductive epoxy (MG Chemicals 8331-14G), with masking tape applied to provide stress relief to the wires as shown in Fig. 7. Samples were carefully positioned in a screw-driven axial machine (a 22 kN SATEC, controlled by Instron Blue Hill software), and tested at a strain rate of 0.1 mm/mm/min. The strain acquisition was obtained through Vishay System 7000/StrainSmart hardware/ software. The test was stopped when the actuator displacement reached a set value (4.5 mm), which was past the maximum load point for all samples.

Storage and loss moduli as a function of temperature were determine using dynamic mechanical analysis measurements that were recorded using a TA DMA Q800. Measurements were performed in tension using a constant strain of 0.05%, a preload force of 0.001 N, and a frequency of 1.00 Hz while increasing the temperature from 30 to 100[degrees]C at 3[degrees]C/min.

Samples appropriate for cross-sectional imaging were prepared by immersing dog bone samples into liquid nitrogen until they reached thermal equilibrium. Samples were then snapped and cut down to size appropriate for an SEM holder. Each cross section was adhered to a sample pedestal using carbon tape and then subsequently coated in gold using a sputter coater to prevent surface charging. Images were recorded using a Zeiss UltraoO FE-SEM.

ACKNOWLEDGMENTS

The authors acknowledge funding from the NASA Solar System Exploration Research Virtual Institute, agreement #NNA17BF68A. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). M.O. acknowledges support from the Renewable Bioproducts Institute at Georgia Tech. The authors thank for their help with the mechanical properties characterization UC Davis undergraduate students Shreya Rastogi, Bruno Matsui, Linda Wu, Taner Dubie, Richard Bramble, Stanford University intern Ghufran Alkhamis. Finally, the mechanical testing was enabled by Prof. Patrick Homen, who kindly granted access to the laboratory of Mechanical Engineering at Sacramento State University (CA).

CONFLICT OF INTERESTS

The authors declare no competing financial interests.

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[41.] Overview of materials for High Density Polyethylene (HDPE), Extruded, (n.d.). http://www.matweb.com/search/DataSheet.aspx? MatGUID=482765fad3b443169ec28fb6f9606660 (accessed November 21, 2018).

[42.] Overview of materials for High Density Polyethylene (HDPE), Blow Molding Grade, (n.d.). http://www.matweb.com/search/ DataSheet.aspx?MatGUTD=4fl3f71f5f904fal9874597dd53fd754 (accessed November 21, 2018).

[43.] ASTM. ASTM D638-14 Standard Test Method for Tensile Properties of Plastics, ASTM D638 (n.d.).

Zach Seibers (iD), (1) Matthew Orr, (2) Graham S. Collier, (1) Adriana Henriquez, (3) Matthew Gabel, (3) Meisha L. Shofner, (2) Valeria La Saponara, (3) John Reynolds (1)

(1) School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics (COPE), Georgia Tech Polymer Network (GTPN), Georgia Institute of Technology,

Atlanta, Georgia, 30332

(2) School of Materials Science and Engineering, and Renewable Byproducts Institute, Georgia Institute of Technology, Atlanta, Georgia, 30332

(3) Department of Mechanical and Aerospace Engineering, University of California, Davis, California, 95616

Additional Supporting Information may be found in the online version of this article.

Correspondence to: Z. Seibers; e-mail: zachseibers@gmail.com Contract grant sponsor: NASA Solar System Exploration Research Virtual Institute; contract grant number: NNA17BF68A. contract grant sponsor: National Science Foundation; contract grant number: ECCS-1542174.

DOI 10.1002/pen.25262

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Synthetic approach used to make the reduced graphene oxide via the modified Hummer's method (top). Process for alkylation and subsequent reduction to form an alkylated-reduced graphene oxide (A-rGO) (bottom).

Caption: FIG. 2. FTIR spectra of the various graphene products. From bottom to top: Graphene oxide (black), reduced graphene oxide (red), alkylated graphene oxide (blue), and reduced alkylated graphene oxide (pink). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Solid-state [sup.13]C NMR spectra of graphene oxide (black) and alkylated graphene oxide (red) before reduction via hydrazine. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Dynamic mechanical analysis thermal sweeps of composites containing rGO (a) and A-rGO (b). The temperatures were chosen based on temperatures experienced on the lunar surface. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Box plots for (a) tensile strength, (b) Poisson's ratio, and (c) Young's moduli of neat HDPE (group 1), 0.1% A-rGO (group 2), and 0.1% rGO (group 3). Cross-sectional scanning electron micrographs for the (d) rGO-HDPE composite exhibits large voids and increased roughness indicative of rGO aggregation compared to the (e) A-rGO HDPE composite that lacks any voids and is much smoother. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Overview of composite fabrication process, (a) Premixing, (b) melt compounding use a recirculating extruder (c and d) melt pressing, and (e) final composite form after cutting using ASTM D638 die. Melt processing was performed at 190[degrees]C and the batch size was nominally 5.5 g, which was limited by the extruder capacity. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Neat HDPE sample being prepared for tensile testing. The cardboard (left) was removed after the sample was aligned in the grips before testing. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Elemental analysis (wt% of graphene oxide and the various
derivatives included in this study. Oxygen was calculated as the
remaining mass.

Materials     C      H     N     O     C/O

GO           47.1   2.3   --    50.6   0.93
rGO          76.0   1.5    6    16.5   4.61
AGO          50.6   3.7   3.9   41.8   1.21
A-rGO        67.4   3.3   6.1   23.2   2.90

TABLE 2. Surface resistivity and electronic conductivity of rGO
and A-rGO pellets before and after annealing under vacuum
at 120[degrees]C overnight.

              Material   t (mm)   [rho] ([OMEGA]-m)

As pressed      rGO       0.15           1.3
               A-rGO      0.14    2.5 * [10.sup.5]

Annealed        rGO       0.15    8.0 * [10.sup.-1]
               A-rGO      0.14    8.0 * [10.sup.4]

              Material   [SIGMA] (S [m.sup.-1])

As pressed      rGO               1.2
               A-rGO       6.6 * [10.sup.-6]

Annealed        rGO               1.8
               A-rGO       2.3 * [10.sup.-5]
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Author:Seibers, Zach; Orr, Matthew; Collier, Graham S.; Henriquez, Adriana; Gabel, Matthew; Shofner, Meisha
Publication:Polymer Engineering and Science
Date:Jan 1, 2020
Words:5936
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