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Graphene Modified Lipophilically byStearic Acid and its Composite With LowDensity PolyethyleneSu Jin Hana, Hyung-Il Leea, Han Mo Jeonga, Byung Kyu Kimb,Anjanapura V. Raghuc & Kakarla Raghava Reddyda Department of Chemistry, Energy Harvest-Storage Research Center,University of Ulsan, Ulsan, Koreab Department of Polymer Science and Engineering, Pusan NationalUniversity, Busan, Koreac Centre for Emerging Technologies, Jain Global Campus,Jakkasandra, Indiad School of Chemical and Biomolecular Engineering, The Universityof Sydney, NSW, AustraliaAccepted author version posted online: 16 Apr 2014.Publishedonline: 17 Jul 2014.

To cite this article: Su Jin Han, Hyung-Il Lee, Han Mo Jeong, Byung Kyu Kim, Anjanapura V. Raghu& Kakarla Raghava Reddy (2014) Graphene Modified Lipophilically by Stearic Acid and its CompositeWith Low Density Polyethylene, Journal of Macromolecular Science, Part B: Physics, 53:7, 1193-1204,DOI: 10.1080/00222348.2013.879804

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Journal of Macromolecular Science R©, Part B: Physics, 53:1193–1204, 2014Copyright © Taylor & Francis Group, LLCISSN: 0022-2348 print / 1525-609X onlineDOI: 10.1080/00222348.2013.879804

Graphene Modified Lipophilically by Stearic Acidand its Composite With Low Density Polyethylene

SU JIN HAN,1 HYUNG-IL LEE,1 HAN MO JEONG,1 BYUNGKYU KIM,2 ANJANAPURA V. RAGHU,3

AND KAKARLA RAGHAVA REDDY4

1Department of Chemistry, Energy Harvest-Storage Research Center, Universityof Ulsan, Ulsan, Korea2Department of Polymer Science and Engineering, Pusan National University,Busan, Korea3Centre for Emerging Technologies, Jain Global Campus, Jakkasandra, India4School of Chemical and Biomolecular Engineering, The University of Sydney,NSW, Australia

Graphene, prepared by the thermal reduction of graphite oxide (GO), was modified withstearic acid to enhance its lipophilicity. A novel method, using the intrinsic epoxy groupson the graphene, was utilized for reaction with stearic acid to minimize the negativeimpact of the normal functionalization method on the π -electronic system of graphene.Gravimetric analysis, thermogravimetric analysis (TGA), Fourier transform infrared(FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that thestearic acid was effectively attached to the graphene. In addition, Raman spectroscopyand electric conductivity of the graphene showed that this novel modification method,utilizing intrinsic defects, did not damage the π -electronic system of the sp2 bondedcarbons. The dispersion of graphene in a low density polyethylene (LDPE) matrix wasenhanced; consequently, the reinforcing effect in tensile testing was improved by thelipophilic modification. The crystallization behavior observed by differential scanningcalorimetry (DSC) showed that the crystallization of LDPE was hindered by dispersedgraphene, more evidently when dispersed uniformly.

Keywords composite, graphene, polyethylene, stearic acid

Introduction

Graphene, a single-atom-thick two-dimensional sheet composed of sp2 bonded carbonatoms arranged in a honeycomb structure, holds great promise for a variety of potentialapplications, such as in microelectronic devices, catalysis, sensors, biomedicines, and com-posite materials, because it not only has an extremely high surface area but also superiorphysical properties.[1,2]

Received 12 September 2013; accepted 26 November 2013.Address correspondence to Han Mo Jeong, Department of Chemistry, University of Ulsan, Ulsan

680-749, Korea. E-mail: hmjeong@mail.ulsan.ac.krColor versions of one or more of the figures in the article can be found online at

www.tandfonline.com/lmsb.

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Flake-type graphene can be prepared by a top-down method from graphite becausegraphite consists of a stack of flat graphenes with 3.35 Å interlayer spacing, and graphite isreadily available and cheap. Graphene can be peeled mechanically from graphite; however,this method is not suitable for large-scale production of graphene due to its low productivity.Therefore, graphenes are prepared normally by chemical reduction of graphite oxide (GO)dispersed in a solvent because GO can be easily exfoliated into single- or few-layer GO in asolvent. Graphenes can also be produced effectively in bulk by rapid heating of GO powdersbecause CO2 gas is generated through thermal decomposition of the oxygen-containinggroups of GO. Thereby, the thermally reduced GO sheets are exfoliated simultaneouslyinto individual graphene sheets by the instantaneous gas pressure build up in the gallerybetween the sheets.[3] This method is economical and eco-friendly because it does notuse any solvent. These exfoliated graphene sheets are normally few-layer graphenes withspecific surface areas ranging from 400 to 1500 m2/g according to Brunauer, Emmett,and Teller (BET) measurements using nitrogen adsorption in the dry state.[4,5] However,they have some oxygen-containing functional groups, such as epoxy or hydroxyl groups,remaining even after the thermal reduction.[3,6]

Graphene tends to form irreversible agglomerates or tends to restack to form thelayered structure of graphite during compounding with melted polymers or when solventis evaporated from graphene dispersion, because there is a very large cohesive energy of2 eV/nm2 between grapheme layers.[7] Therefore, effective methods to tailor the surfacestructure of graphene have been developed in order to fully utilize the unique properties ofindividual sheets. Introduction of an appropriate moiety on the basal plane of graphene canhinder the agglomeration of isolated graphene. In addition, a well-wetted graphene particlesurface in a polymer matrix can improve the enthalpic interaction, which stabilizes the finedispersion of graphene in the matrix and enhances the interfacial interactions between thegraphene and the matrix.[8,9]

In order to anchor an appropriate moiety onto the basal plane of graphene, an ap-propriate compound is reacted with C C bonds on the basal plane of graphene, or GO,having many oxygen-containing functional groups, is functionalized with an appropriatemoiety and subsequently reduced to yield functionalized graphene.[8–10] However, thesemethods can engender significant deterioration of the inherent properties of graphene, be-cause the methods damage the π -electronic network or geometric structure of the basalplane.

The damage to the intrinsic novel properties of graphene, such as superior electricalconductivity, by covalent modification could be minimized when the functional groups,which remained in a slight amount even after the reduction of GO, were used as the anchor-ing sites because this method did not induce additional changes to the π -electronic systemof graphene. The epoxy groups can be utilized as active sites for covalent modificationbecause the population of epoxy groups on the graphene is not restricted to the edges,but is distributed evenly on the basal plane surface of the graphene.[11,12] Such chemicalmodification utilizing inherent defects may allow for various promising applications ofgraphene.

The dispersion of graphene in nonpolar media, such as polyethylene, can be im-proved by the modification of the graphene surface with nonpolar materials. Some researchgroups have functionalized graphene with long alkyl groups by the reaction of GO and longalkyl amines and subsequent chemical reduction.[13–15] Other research groups have preparedalkyl-functionalized graphene for improved lipophilicity via the reaction of long alkyl com-pounds with the remnant oxygen-containing groups on the graphene, which was prepared

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Modified Graphene/Polyethylene Composite 1195

O

O

O

OH

H

OH

H

RCOO

O

OH

H

OH

H

OH OHOOCRR C

OOH

tetramethylammoniumbromide

Figure 1. Schematic presentation of graphene modification with stearic acid (R = C17H35).

by the reduction of GO.[16,17] Kim et al. coated exfoliated graphite nanoplatelets physicallywith paraffin to enhance the dispersion of graphene in linear low density polyethylene(LLDPE).[18]

In this study, the graphene prepared by the thermal reduction of GO, was modified by anovel method utilizing the remnant epoxy groups on the graphene for reaction with stearicacid to improve its lipophilicity with minimal damage to the π -electronic network of thegraphene basal plane (Fig. 1). The effect of the modification on the structure and propertiesof graphene and on the dispersion of the graphene in low density polyethylene (LDPE) andthe accompanying property changes of the composite were examined.

Experimental

Materials

Expandable graphite (ES350 F5, average particle size: 280 μm) purchased from Qing-dao Kropfmuehl Graphite Co., Ltd. (China) was used for the preparation of graphene.Stearic acid (Sigma-Aldrich Co. LLC., USA), tetramethylammonium bromide (Sigma-Aldrich Co. LLC.), LDPE (Hanwha Chemical Co., Ltd., Korea, 963-LDPE, melt index10 g/10 min), tetrahydrofuran (Sigma-Aldrich Co. LLC.), toluene (Sigma-Aldrich Co.LLC.), and methanol (Sigma-Aldrich Co. LLC.) were used as received.

Preparation of Graphene

Graphite oxide (GO) was prepared using the Brodie method, as described in our previouspaper.[19] Elemental analysis showed that the GO composition was C10O3.45H1.58. TheGO was thermally reduced at 1100◦C for 1 min under a N2 atmosphere by decomposingthe oxygen-containing groups of GO and generating CO2 gas, thus splitting the GO intoindividual reduced graphene sheets.[3,19] Elemental analysis demonstrated that the graphenecomposition was C10O0.78H0.38, indicating that some oxygen-containing functional groups,such as epoxy or hydroxyl groups, remained even after thermal reduction.[3] The surfacearea of the graphene, obtained by a BET measurement using nitrogen adsorption in the drystate, was 428 m2/g.

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1196 S. J. Han et al.

Modification of Graphene with Stearic Acid

One gram of graphene was dispersed in 200 g of tetrahydrofuran and sonicated at room tem-perature for 30 min. After adding 100 g of stearic acid and 0.5 g of tetramethylammoniumbromide as a catalyst, the mixture was mixed with agitation for 2 h at room temperature,heated to 85◦C and tetrahydrofuran was removed by evaporation for 1 h with agitation.The temperature was held for 8 h to induce the reaction between the epoxy groups onthe graphene and the carboxylic acid groups of stearic acid (Fig. 1).[20,21] For comparison,graphene was also treated with stearic acid by the same procedures described above ex-cept that it was treated with stearic acid at 80◦C in the absence of tetramethylammoniumbromide catalyst.

In order to separate the graphene after treatment with stearic acid, the graphene/stearicacid mixture was suspended in 30-fold tetrahydrofuran at room temperature with stirring,and the suspended graphene was separated by filtration. The filtered graphene was thor-oughly washed with tetrahydrofuran in a Soxhlet extractor for 4 days to remove physicallyadsorbed stearic acid and dried under vacuum for 1 day at 60◦C before characterizationor composite preparation. Hereafter, unmodified graphene is designated as PG, and thegraphene separated after the treatment with stearic acid at 85◦C in the presence of catalystis designated as CG. The graphene separated after treatment with stearic acid at 80◦C inthe absence of catalyst is designated as MG.

Preparation of Graphene/LDPE Composite

A graphene suspension in toluene (0.004 mg/mL, 60 min sonicated) was mixed with a5.0 wt% LDPE solution in toluene at 110◦C for 3 h. Then, the dispersion was poured into20-fold of methanol to precipitate the graphene/LDPE composite. The composite was driedat 100◦C under vacuum for 1 day. The sample designation code used in this manuscriptgives information about the characters of the composites. For example, CGC10 is thecomposite made of 1.0 part graphene and 100 parts LDPE (1.0 phr of graphene). CG/LPDEMGC15 is MG/LDPE composite containing 1.5 phr of graphene. The PGC20 is PG/LDPEcomposite containing 2.0 phr of graphene. To prepare CGC10, 1.35 parts of CG weremixed with 100 parts LDPE, because, in the 1.35 parts of CG, 1.00 part is graphene andanother 0.35 parts are stearic acid attached to the graphene. The attached amount of stearicacid was estimated from the weight increase in gravimetric analysis, discussed later in thismanuscript (Table 1). Dried composites were compression molded at 120◦C with a pressureof 3 MPa to make the specimens to examine electrical conductivity and tensile properties.

Characterization

Thermogravimetric analysis (TGA) was performed with a Q50 (TA Instruments, USA)at a heating rate of 10◦C/min with 2 mg of sample in a platinum crucible under a N2atmosphere. Fourier transform infrared (FTIR) spectra were recorded using a FTS 2000FTIR (Varian Inc., USA) employing a KBr tablet that was made by compression moldingof KBr powder mixed with a small amount of sample. X-ray photoelectron spectroscopy(XPS) measurements were performed on a Thermo Fisher K-Alpha spectrometer (ThermoFisher Scientific Inc., USA) using Al Kα X-ray radiation.

Raman spectra of graphene paper were recorded with a Raman spectrometer (WITecInstruments Corp., USA, Alpha 300R) equipped with a microscope (50× objective) anda Nd-YAG laser using an excitation wavelength of 532 nm. Graphene papers, about

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Modified Graphene/Polyethylene Composite 1197

Table 1Characteristics of graphenes

Amount of stearic acidattached to the graphene

(wt%)Raman intensity ratio

Gravimetric ConductivitySample analysis TGA IG/ID I2D/ID (S/cm)

PG — — 1.17 0.12 20.4MG 10.0 6.6 1.18 0.12 15.7CG 26.1 32.7 1.17 0.11 14.4

20-μm-thick, were obtained by vacuum-filtering a dilute graphene suspension in DMF(0.08 mg/mL) through 1 μm pore size Whatman filter paper. The resulting samples weredried for one day at 85◦C under vacuum and were peeled from the filter paper.

Direct current conductivity of the graphene papers was measured using a four-pointmethod with a CMT-SR 1000 N (AIT Co. Ltd, Korea). The direct current conductivityacross a 0.5-mm-thick composite film was measured with a picoamperometer (Keithley237, Keithley Instruments Inc., USA) at room temperature using round silver electrodes of0.28 cm2. Electrodes were attached to both surfaces of the specimen, and silver paste wasused to ensure good contact between the specimen and the electrodes.

Graphene/LDPE composite films (approximately 30-μm-thick) were imaged using anEclipse LV100 optical microscope (Nikon Corp., Japan) equipped with an Artcam-300MI-DS digital camera.

Tensile properties were examined with a tensile tester (OTU-2, Oriental TM Co.,Korea). The compression molded composite film was cut into a micro-tensile specimen25 mm in length, 5 mm in width, and 0.3 mm in thickness. The specimens were elongatedat a rate of 100 mm/min.

Differential scanning calorimetry (DSC) was carried out using a TA Instruments Q20at heating and cooling rates of 20◦C/min with 8 mg of sample. After loading at roomtemperature, the sample was heated to 140◦C and then cooled to −20◦C for measurement ofthe crystallization temperature (Tc) and heat of crystallization (�Hc). Melting temperature(Tm) and heat of fusion (�Hm) were measured in the subsequent heating scan.

Results and Discussion

Analysis of Graphenes

The weight increase of graphene was examined after treatment with stearic acid, becausethe weight of graphene increased when the stearic acid reacted with the epoxy group ofgraphene, as shown in Fig. 1. The weight increase of CG was 35.3 parts per 100 partsof pristine graphene (35.3 phr), and that of MG was 11.1 phr. These results showed thatthe amount of stearic acid attached on the graphene was higher when the graphene wastreated with stearic acid in the presence of a catalyst due to enhanced chemical reaction.The weight percentages of stearic acid attached to CG and MG, calculated based on theassumption that all the weight increases were due to chemical or physical attachment of

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1198 S. J. Han et al.

0 200 400 600 800

0

20

40

60

80

100

Wei

ght r

esid

ue (%

)

Temperature (oC)

97.3%90.9%

65.5%

0.0%

Figure 2. TGA thermograms of (—) PG, (- - -) MG, (-•-•-•-) CG, and (•••) stearic acid.

stearic acid (35.3 ÷ 135.3 × 100 = 26.1% for CG and 11.1 ÷ 111.1 × 100 = 10.0% forMG), are presented in Table 1.

The CG and MG were analyzed also with TGA as another means to evaluate againthe amount of stearic acid attached chemically or physically to the graphenes. Figure 2shows that the weight loss of graphene itself during heating was 2.7% at 700◦C, which isw1 in Eq. (1). In contrast, stearic acid exhibits drastic weight loss in the temperature range200–300◦C and leaves almost no residue above 300◦C. Therefore, the amount of stearicacid attached to CG or MG (x%) can be estimated with Eq. (1), because the weight lossof the CG or MG at 700◦C in Fig. 2 (w2%) is the sum of the weight loss of stearic acidon the graphenes [the first term of Eq. (1)] and that of graphene itself [the second termof equation (1)] at 700◦C. The x values determined by TGA are also shown in Table 1.These results also indicate that stearic acid attaches to the graphene more effectively in thepresence of catalyst. This attached amount was much higher than those of previous reportsfor the alkylation of graphene,[15,17] where the attached amounts estimated from TGA datawere less than 20%.

x + (100 − x) w1100

= w2 (1)

Figure 3 shows the IR spectra of the various samples. As shown in Fig. 3(d), the IRspectrum of stearic acid had the characteristic IR absorption band peak of the carboxylicacid C O bond at 1704 cm−1. The PG had broad IR absorption bands around 1540 cm−1

and 1210 cm−1 (Fig. 3(a)), which are due to C C bonds and C O bonds, respectively.[22,23]

Whereas the IR spectrum of CG showed an additional broad IR absorption band in the range1690–1770 cm−1, having a peak at 1744 cm−1 (Fig. 3(c)), which can be attributed to thatof an ester C O bond. This demonstrates that the ester bond was created by the reactionof stearic acid and epoxy groups on graphene, as shown in Fig. 1. The IR spectrum of MGalso showed this absorption band; however, the band was relatively weaker and broader andhad a shoulder around 1706 cm−1 (Fig. 3(b)). This suggests that a relatively large amountof stearic acid, having a carboxylic acid group, was attached physically on the graphene.

The XPS data were acquired to examine the surface characteristics of the graphenesbecause XPS is a quantitative spectroscopic technique which analyzes the average surface

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Modified Graphene/Polyethylene Composite 1199

Wavenumber (cm-1)

100012001400160018002000

Tran

smitt

ance

(a)

(b)

(c)

(d)

1744 1704 1540 1210

Figure 3. FTIR spectra of (a) PG, (b) MG, (c) CG, and (d) stearic acid.

chemistry of an approximately 5 nm depth. The C1s core level photoemission spectra ofthe graphenes are shown in Fig. 4. Because the carbon bound to carbon (C C carbon)has a peak around 284 eV, the carbon singly bound to oxygen (C O carbon) has a peak

Figure 4. XPS spectra in the C1s region of (a) PG, (b) MG, and (c) CG.

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1200 S. J. Han et al.

Table 2C1s peak data of graphene from XPS

C C carbon C O carbon C O carbon

Sample Peak (eV) Area (%) Peak (eV) Area (%) Peak (eV) Area (%)

PG 284.6 64.9 286.0 25.8 288.5 9.3MG 284.5 66.7 286.0 27.6 288.5 5.7CG 284.5 67.7 286.0 26.5 288.5 5.8

around 286 eV, the carbon doubly bound to oxygen (C O carbon) has a peak around288 eV,[12,24,25] the asymmetric photoelectron peaks were deconvoluted into these threepeaks, as shown in Fig. 4. The results of this peak deconvolution, including peak positionand percentages of each peak area, are summarized in Table 2. These results show that theamounts of C C carbon at the surface of CG and MG were increased and the total amountsof carbon bound to oxygen were reduced relative to those at the surface of PG, althoughthe O/C atomic ratio of stearic acid (1.11/10) was larger than that of PG (0.78/10). Theseresults suggested that the surfaces of CG and MG were covered by alkyl groups, and thesurface coverage by the alkyl group was more evident for CG than for MG because of thegreater amount of attached stearic acid brushes on the surface.

The Raman spectrum of graphene has a characteristic G band around 1580 cm−1 anda second prominent 2D band around 2700 cm−1.[26] If defects are present on graphene,the D band around 1350 cm−1 can be observed. The intensity of the D band is a measureof the amount of disorder in graphene, because the activation of the D band is attributedto the breaking of the translational symmetry of the C C sp2 bond.[27,28] Therefore, anincrease in the number of defects would result in an increase of the D band intensity and aconcomitant decrease in the intensity of the intrinsic G band and 2D band of graphene.[26]

Table 1 shows that both the intensity ratio of the G band and the 2D band relative to Dband, i.e., IG/ID and I2D/ID, changed marginally with the treatment with stearic acid. Thisshows that the modification with stearic acid did not damage the graphitic structure, i.e.,the sp2 C C bond network.

The conductivity of the graphene paper, measured by the four-point method, is shown inTable 1; the conductivities of the graphene papers were reduced slightly by the modificationwith stearic acid. The attached electrically insulative alkyl groups can hinder intimatecontact between the conductive graphenes. Therefore, this, rather than the damage tographitic structure, seems to be a reason for the conductivity reduction. The conductivity ofCG was more than 10-fold higher compared to that (around 1 S/cm) of a previous report,[15]

in spite of the fact that the amount of attached alkyl groups estimated from TGA data wasmore than two-fold higher compared to that of the previous report.[15] This also supportsthe suggestion that the graphene modified with stearic acid has a well-developed graphiticstructure.

Graphene/LDPE Composites

In order to evaluate the dispersion of graphenes in the LDPE matrix, the morphology ofthe graphene/LDPE composite films was observed by optical microscopy. Single-layergraphene is transparent because it absorbs only 2.3% of the light intensity, independent ofthe wavelength in the optical domain. Thus, single-layer graphene cannot be adequately

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Modified Graphene/Polyethylene Composite 1201

Figure 5. Optical microscopy images of (a) PGC05, (b) MGC05, and (c) CGC05.

observed with an optical microscope. However, the transparency decreases when the lightencounters many individual graphene molecules while passing through a material or whengraphenes are stacked into multilayers or agglomerated. Figure 5(a) shows that not onlytranslucent finely dispersed PGs, but also larger black PG particles were observed in PGC05.However, the size and the number of black particles are decreased in Fig. 5(b) (MGC05)and even further in Fig. 5(c) (CGC05). This shows that the fine dispersion of graphene inthe lipophilic LDPE matrix was effectively enhanced by the lipophilization of the graphenesurface with stearic acid.

The tensile properties of the graphene/LDPE composites are presented in Table 3.As shown, the modulus and yield strength were enhanced by the reinforcing effect of thegraphenes, and this effect was intensified by the lipophilization of the graphenes. The finedispersion of the graphene in the LDPE matrix and the enhanced interfacial interaction arethe causes of the enhanced reinforcing effect. This reinforcing effect of CG in CGC10 wasmuch greater than that of the exfoliated graphite nanoplatelets coated with paraffin in the

Table 3Tensile properties of graphene/LDPE composites

Sample Modulus (GPa)Yield stress

(MPa)Tensile strength

(MPa)Elongation at

break (%)

LDPE 2.2 ± 0.2 9.0 ± 0.2 11.3 ± 0.6 534 ± 75PGC10 2.4 ± 0.1 9.7 ± 0.2 9.9 ± 0.4 6 ± 1MGC10 2.5 ± 0.1 10.2 ± 0.4 10.5 ± 0.5 4 ± 1CGC10 2.9 ± 0.2 12.0 ± 0.5 12.5 ± 0.6 6 ± 1

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1202 S. J. Han et al.

Table 4Electrical conductivity and thermal properties of graphene/LDPE composites

Thermal properties

Conductivity Tc Tm �Hc �HmSample (S/cm) (◦C) (◦C) (J/g) (J/g)

LDPE 1.8 × 10−12 92.1 108.1 97.6 95.0PGC05 1.8 × 10−12 91.8 108.2 93.8 93.4PGC10 1.8 × 10−12 92.4 107.6 92.9 92.4PGC15 3.6 × 10−9 91.5 108.0 91.4 91.1PGC20 5.3 × 10−8 91.2 109.0 90.3 90.0MGC05 1.8 × 10−12 92.6 108.2 92.2 91.7MGC10 1.9 × 10−12 92.5 108.0 91.9 90.9MGC15 8.2 × 10−9 92.4 108.0 90.1 89.3MGC20 6.4 × 10−8 93.1 107.6 89.6 89.0CGC05 1.8 × 10−12 91.8 108.7 89.9 89.6CGC10 2.0 × 10−12 91.5 108.9 89.4 89.2CGC15 6.7 × 10−10 92.1 108.0 89.4 89.3CGC20 1.5 × 10−8 92.4 107.6 89.3 88.7

LLDPE composite as reported by Kim et al.[18] In Table 3, the properties measured at highdeformation, the tensile strength and the elongation at break, showed a different story. Inthe tensile test, LDPE itself exhibited a yield point, from which necking was initiated, andexhibited stiffening at high elongation due to the molecular orientation toward the tensileaxis. In contrast, necking was absent in the composites, which exhibited very low valuesof elongation at break. This suggests that the molecular rearrangement during deformationwas strictly inhibited by the dispersed graphenes.

Table 4 shows that the percolation threshold of the electric conductivity of PG/LDPEcomposites was around 1.5 phr. This threshold value is quite large relative to the 0.5 phr ofPG/polyurethane composites studied in an earlier study.[19] This shows that the dispersionof PG in lipophilic LDPE was not as fine as that in polar polyurethane. Normally, theconductivity of a composite is enhanced when the dispersion of a conductive filler isimproved; however, the conductivity change caused by the treatment with stearic acid wasmarginal, as shown in Table 4, although the dispersion was enhanced by the lipophilizationwith stearic acid (Fig. 5). The intimate contact between the graphenes can be hindered byinsulative alkyl chains on the lipophilized graphene.[29] Therefore, the marginal changes inelectrical conductivity shown in Table 4 may be due to the combined contribution of thesetwo factors. However, the conductivity values in Table 4 are generally much higher thanthose reported previously for graphene/LLDPE composites.[18]

In the thermal properties measured by DSC (Table 4), the effects of graphene on Tc andTm were marginal; however, both �Hc and �Hm per gram of LDPE, decreased generallyas the content of graphene in the composites was increased. These decreases of �Hc and�Hm by adding graphenes, compared to those of pristine LDPE, were more evident by MGthan those by PG and were most evident by CG. These results showed that the molecu-lar rearrangement necessary for crystallization was hindered by the dispersed graphenes,and this interference was more evident if the graphenes were more finely dispersed inLDPE.[30]

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Modified Graphene/Polyethylene Composite 1203

Conclusions

Gravimetric analysis, TGA, FTIR, and XPS showed that the intrinsic epoxy groups ongraphene, which was prepared by the thermal reduction of GO, can be utilized effectivelyfor lipophilic modification of graphene through reaction with stearic acid. The Ramanspectrum and electric conductivity demonstrated that decreases in the superior propertiesof graphene, originated from the delocalized π -electronic network of sp2 carbon, can beminimized when the intrinsic defects of the epoxy groups are utilized as an anchoring sitefor modification. Thus, the conductivity of the lipophilically modified graphene, in the formof graphene paper, was higher than any ever reported, although the amount of alkyl groupsattached to the graphene was greater.

Optical microscopy showed that the lipophilic modification enhanced the dispersionof graphene in the LDPE matrix. This enhanced dispersion improved the reinforcing effectof graphene, as shown in the tensile modulus and yield strength. The graphene hinderedthe crystallization of LDPE, and this was more evident when the graphene was morefinely dispersed. This shows that the rearrangement of LDPE chains for crystallization washindered by the interaction between graphene and LDPE molecules.

Funding

This work was supported by the 2013 University of Ulsan Research Fund.

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