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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: [email protected] versions
of one or more of the figures in the article can be found online
at
www.tandfonline.com/lmsb.
1193
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1194 S. J. Han et al.
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.
References
1. Kim, H.; Abdala, A.A.; Macosko, C.W. Graphene/polymer
nanocomposites. Macromolecules2010, 43, 6515.
2. Li, D.; Mueller, M.B.; Gilje, S.; Kaner, R.B.; Wallace, G.G.
Processable aqueous dispersions ofgraphene nanosheets. Nat.
Nanotechnol. 2008, 3, 101.
3. McAllister, M.J.; Li, J.-L.; Adamson, D.H.; Schniepp, H.C.;
Abdala, A.A.; Liu, J.; Herrera-Alonso, M.; Milius, D.L.; Car, R.;
Prud’homme, R.K. Single sheet functionalized graphene byoxidation
and thermal expansion of graphite. Chem. Mater. 2007, 19, 4396.
4. Kim, H.; Macosko, C.W. Processing-property relationships of
polycarbonate/graphene compos-ites. Polymer 2009, 50, 3797.
5. Steurer, P.; Wissert, R.; Thomann, R.; Mülhaupt, R.
Functionalized graphenes and thermoplasticnanocomposites based upon
expanded graphite oxide. Macromol. Rapid Commun. 2009, 30,316.
6. Radovic, L.R.; Silva-Tapia, A.B.; Vallejos-Burgos, F. Oxygen
migration on the graphene surface.1. Origin of epoxide groups.
Carbon 2011, 49, 4218.
7. Niyogi, S.; Bekyarova, E.; Itkis, M.E.; McWilliams, J.L.;
Hamon, M.A.; Haddon, R.C. Solutionproperties of graphite and
graphene. J. Am. Chem. Soc. 2006, 128, 7720.
8. Fang, M.; Wang, K.; Lu, H.; Yang, Y.; Nutt, S. Single-layer
graphene nanosheets with controlledgrafting of polymer chains. J.
Mater. Chem. 2010, 20, 1982.
9. Yang, H.; Li, F.; Shan, C.; Han, D.; Zhang, Q.; Niu, L.;
Ivaska, A. Covalent functionalizationof chemically converted
graphene sheets via silane and its reinforcement. J. Mater. Chem.
2009,19, 4632.
10. Yu, W.; Xie, H.; Chen, L.; Li, Y. The functionalization and
potential applications of graphene. InIEEE 2010 Symposium on
Photonics and Optoelectronic (SOPO); IEEE, 2010, pp. 1–4.
11. Oh, S.M.; Oh, K.M.; Dao, T.D.; Lee, H.-I.; Jeong, H.M.; Kim,
B.K. The modification of graphenewith alcohols and its use in shape
memory polyurethane composites. Polym. Int. 2013, 62, 54.
12. Hsiao, M.-C.; Liao, S.-H.; Yen, M.-Y.; Liu, P.-I.; Pu,
N.-W.; Wang, C.-A.; Ma, C.-C.M. Prepara-tion of covalently
functionalized graphene using residual oxygen-containing functional
groups.ACS Appl. Mater. Interfaces 2010, 2, 3092.
Dow
nloa
ded
by [
Seou
l Nat
iona
l Uni
vers
ity]
at 1
6:54
06
Aug
ust 2
014
-
1204 S. J. Han et al.
13. Kuila, T.; Bose, S.; Hong, C.E.; Uddin, M.E.; Khanra, P.;
Kim, N.H.; Lee, J.H. Preparationof functionalized graphene/linear
low density polyethylene composites by a solution mixingmethod.
Carbon 2011, 49, 1033.
14. Wang, G.; Shen, X.; Wang, B.; Yao, J.; Park, J. Synthesis
and characterisation of hydrophilic andorganophilic graphene
nanosheets. Carbon 2009, 47, 1359.
15. Compton, O.C.; Dikin, D.A.; Putz, K.W.; Brinson, L.C.;
Nguyen, S.T. Electrically conductive“alkylated” graphene paper via
chemical reduction of amine-functionalized graphene oxide
paper.Adv. Mater. 2010, 22, 892.
16. Cao, Y.; Feng, J.; Wu, P. Alkyl-functionalized graphene
nanosheets with improved lipophilicity.Carbon 2010, 48, 1683.
17. Tessonnier, J.-P.; Barteau, M.A. Dispersion of
alkyl-chain-functionalized reduced graphene oxidesheets in nonpolar
solvents. Langmuir 2012, 28, 6691.
18. Kim, S.; Seo, J.; Drzal, L.T. Improvement of electric
conductivity of LLDPE based nanocompositeby paraffin coating on
exfoliated graphite nanoplatelets. Compos. Part A: Appl. Sci.
Manuf. 2010,41, 581.
19. Choi, J.T.; Kim, D.H.; Ryu, K.S.; Lee, H.-i.; Jeong, H.M.;
Shin, C.M.; Kim, J.H.; Kim, B.K.Functionalized graphene
sheet/polyurethane nanocomposites: Effect of particle size on
physicalproperties. Macromol. Res. 2011, 19, 809.
20. Blank, W.J.; He, Z.; Picci, M. Catalysis of the
epoxy-carboxyl reaction. J. Coating. Techn. Res.2002, 74, 33.
21. Do, H.S.; Park, J.H.; Kim, H.J. Synthesis and
characteristics of photoactive-hydrogenated rosinepoxy methacrylate
for pressure sensitive adhesives. J. Appl. Polym. Sci. 2009, 111,
1172.
22. Socrates, G. Infrared Characteristic Group Frequencies; John
Wiley & Sons: Chichester 1994,pp. 21, 68, 123.
23. Mawhinney, D.B.; Naumenko, V.; Kuznetsova, A.; Yates, J.T.;
Liu, J.; Smalley, R. Infraredspectral evidence for the etching of
carbon nanotubes: ozone oxidation at 298 K. J. Am. Chem.Soc. 2000,
122, 2383.
24. Quintana, M.; Spyrou, K.; Grzelczak, M.; Browne, W.R.;
Rudolf, P.; Prato, M. Functionalizationof graphene via 1,3-dipolar
cycloaddition. ACS Nano 2010, 4, 3527.
25. Hontoria-Lucas, C.; Lopez-Peinado, A.; López-González,
J.de D.; Rojas-Cervantes, M.; Martin-Aranda, R. Study of
oxygen-containing groups in a series of graphite oxides: physical
andchemical characterization. Carbon 1995, 33, 1585.
26. Krauss, B.; Lohmann, T.; Chae, D.-H.; Haluska, M.; von
Klitzing, K.; Smet, J.H. Laser-induceddisassembly of a graphene
single crystal into a nanocrystalline network. Phys. Rev. B 2009,
79,165428 (9 pages).
27. Rao, C.N.R.; Sood, A.K.; Subrahmanyam, K.S.; Govindaraj, A.;
Graphene: The new two-dimensional nanomaterial. Angew. Chem. Int.
Ed. 2009, 48, 7752.
28. Su, C.-Y.; Xu, Y.; Zhang, W.; Zhao, J.; Tang, X.; Tsai,
C.-H.; Li, L.-J. Electrical and spectroscopiccharacterizations of
ultra-large reduced graphene oxide monolayers. Chem. Mater. 2009,
21,5674.
29. Yoon, J.T.; Jeong, Y.G.; Lee, S.C.; Min, B.G. Influences of
poly(lactic acid)-grafted carbonnanotube on thermal, mechanical,
and electrical properties of poly(lactic acid). Polym. Adv.Technol.
2009, 20, 631.
30. Jiang, X.; Drzal, L.T. Multifunctional high-density
polyethylene nanocomposites produced byincorporation of exfoliated
graphene nanoplatelets 2: Crystallization, thermal and
electricalproperties. Polym. Compos. 2012, 33, 636.
Dow
nloa
ded
by [
Seou
l Nat
iona
l Uni
vers
ity]
at 1
6:54
06
Aug
ust 2
014