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Research ArticleReceived: 21 July 2008 Revised: 23 October 2008 Accepted: 25 November 2008 Published online in Wiley Interscience: 26 February 2009

(www.interscience.wiley.com) DOI 10.1002/pi.2549

Morphological and physical properties of athermoplastic polyurethane reinforced withfunctionalized graphene sheetDuc Anh Nguyen,a Yu Rok Lee,a Anjanapura V Raghu,a Han Mo Jeong,a∗Cheol Min Shinb and Byung Kyu Kimc

Abstract

BACKGROUND: Functionalized graphene sheet (FGS) was recently introduced as a new nano-sized conductive filler, but littlework has yet examined the possibility of using FGS as a nanofiller in the preparation of polymer nanocomposites. In particular,there are currently no published papers that evaluate polyurethane/FGS nanocomposites. The purpose of this study was toprepare a polyurethane/FGS nanocomposite and examine the morphological and physical properties of the material.

RESULTS: A cast nanocomposite film was prepared from a mixture of thermoplastic polyurethane (TPU) solution and FGSsuspended in methyl ethyl ketone. The FGS dispersed on the nanoscale throughout the TPU matrix and effectively enhancedthe conductivity. A nanocomposite containing 2 parts of FGS per 100 parts of TPU had an electrical conductivity of 10−4 S cm−1,a 107 times increase over that of pristine TPU. The dynamic mechanical properties showed that the FGS efficiently reinforcedthe TPU matrix, particularly in the temperature region above the soft segment melt.

CONCLUSION: Our results show that FGS has a high affinity for TPU, and it could therefore be used effectively in the preparationof TPU/FGS nanocomposites without any further chemical surface treatment. This indicates that FGS is an effective andconvenient new material that could be used for the modification of polyurethane. It could also be used in place of othernano-sized conductive fillers, such as carbon nanotubes.c© 2009 Society of Chemical Industry

Keywords: functionalized graphene sheet; thermoplastic polyurethane; nanocomposite; conductivity; mechanical properties

INTRODUCTIONElectrically conducting composites made of polymers and con-ducting fillers, such as natural graphite, carbon black and metalpowders, have been extensively investigated in the past fewdecades. These materials can be utilized in antistatic coatings,electromagnetic shielding and corrosion-resistant coatings, aswell as in various other applications.1 – 4 However, these compos-ites often contain as much as 15 wt% filler, since a large amountof conventional micrometer-scale conducting filler is required toachieve an electrical conductivity of 10−4 S cm−1. This high levelof filler results in poor mechanical properties, poor processabilityand high density.5

Nanocomposites are composed of polymer matrices withreinforcements less than 100 nm in size. These materials haveattracted considerable attention, because many of the physicalproperties of matrix polymers, such as their electrical, mechanical,barrier, and flame-retarding properties, can be substantiallyenhanced using a very small amount of reinforcement as comparedto conventional composites.6,7 Since the unique properties ofpolymer nanocomposites stem from the maximized interfacialcontact between the polymer matrix and the reinforcement, themost commonly utilized fillers have a high surface-to-volume ratio.For example, layered silicates like montmorillonite are often used

since they are composed of stacks of parallel lamellae with a 1 nmthickness and a high aspect ratio.8 – 11

Graphite has been widely used as a filler in conducting polymercomposites for two main reasons: it is a naturally abundant, cheapmaterial and it has good electrical conductivity in the region of104 S cm−1. Graphite consists of layered graphene, which is a one-atom-thick sheet of hexagonal carbon rings. The typical interlayerspacing between graphene layers is 3.35 Å, and they are weaklybonded to each other by van der Waals forces. Each graphenesheet has a high aspect ratio, high modulus, high surface area andexcellent electrical and thermal conductivity along the grapheneplane. It follows then that if these layers could be separateddown to a nanometer thickness, they could effectively reinforce a

∗ Correspondence to: Han Mo Jeong, Department of Chemistry, University ofUlsan, Ulsan 680-749, Korea. E-mail: hmjeong@mail.ulsan.ac.kr

a Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea

b Research Center, N-Baro Tech Co., 974-1, Goyeon-ri, Ungchon-myon, Ulju-gun,Ulsan 689-871, Korea

c Department of Polymer Science and Engineering, Pusan National University,Busan 609-935, Korea

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polymer matrix and create an uninterrupted conducting networkfor electrons within the polymer.12 – 14

The weak interplanar forces allow for a wide range of atoms,molecules and ions to insert themselves into the interplanarspaces of graphite to form graphite intercalation compounds(GICs). Expanded or partially exfoliated graphite can be preparedfrom various GICs by intense, rapid heating with a flame,microwaves, laser irradiation or inductively coupled plasma. Whenthe intercalate is suddenly volatilized, a very large unidirectionalexpansion takes place and the graphite flakes off.15,16 A numberof studies have been conducted on expanded graphite-reinforcedconductive polymer nanocomposites. This is because the graphitenanoplatelets obtained by partial exfoliation, while being cheap,also have a high aspect ratio, and they form a conductive networkat low concentrations.17 – 21

Graphite oxide (GO), prepared from the oxidation of graphite, is acompound with a layered structure. Each layer consists of randomlydistributed unoxidized aromatic regions and six-member aliphaticregions with polar groups attached. These polar groups, such ashydroxyl, epoxide, ether and carboxylate groups, are the resultof oxidation.22 – 25 It has been reported recently that GO can beexfoliated into single sheets if it is sufficiently oxidized. In this case,the inter-graphene spacing associated with the native graphite iscompletely eliminated in the oxidation stage used for exfoliation.In addition, adequate pressure (from CO2 gas evolved by thedecomposition of functional groups) must also be built up at thegallery between the GO sheets during the rapid heating.26,27 Thiscompletely exfoliated GO, in which the inter-graphene spacingassociated with GO and the graphite is completely excluded afterthermal expansion, has an affinity for polar solvents and polymers,as well as good conductivity. These properties stem from thefact that the completely exfoliated GO is composed of singlefunctionalized graphene sheets (FGSs) that still contain the polarfunctional groups that remain after thermal treatment.28,29

Thermoplastic polyurethanes (TPUs) are unique polymeric ma-terials with a wide range of physical and chemical properties.Because a wide range of monomeric materials are now com-mercially available, and because tailor-made properties can beobtained from well-designed combinations of these monomericmaterials, TPUs can be tailored to meet the highly diversifieddemands of modern technologies.30 This versatility allows TPUsto be utilized in various applications where electrical conductivityis critical, and therefore many researchers have studied conduc-tive TPU nanocomposites.31 Most of these nanocomposites havecarbon nanotubes as conducting filler after additional surfacetreatment.32 – 37

In the study reported here, we focused on the preparation ofa new conductive TPU nanocomposite using FGS as a nanofillerwithout further surface treatment. The conductivity and otherphysical properties of the material are discussed.

EXPERIMENTALMaterialsNatural graphite (HC-908) with an average particle size of8 µm was purchased from Hyundai Coma Co. Ltd, Korea.Polycaprolactone (PCL) diol (2000 g mol−1; Solvay SA, Brussels,Belgium) was dried and degassed at 80 ◦C under vacuum for3 h. 4,4′-Methylenebis(cyclohexyl isocyanate) (H12MDI; BASF),1,4-butanediol (BD; BASF), dioctyltin dilaurate (DOT; CNA Co.Ltd, Chungnam, Korea), methyl ethyl ketone (MEK; Aldrich),concentrated H2SO4 (96%; Matsunoen Chemicals Ltd, Osaka,

Japan), fuming HNO3 (Matsunoen Chemicals Ltd) and KClO3

(Samchun Pure Chemical Co. Ltd, Seoul, Korea) were used asreceived.

Preparation of FGSGO was prepared using the Staudenmaier method.26,38 A 500 mLround-bottom flask containing 10 g of graphite powder and270 mL of a concentrated H2SO4/fuming HNO3 mixture (2/1 v/v)was cooled to 0 ◦C and agitated. KClO3 (110 g) was slowly addedto the reaction mixture, taking care that the temperature of themixture remained below 20 ◦C. The reaction mixture was allowedto reach room temperature and stirred for 120 h. Then the mixturewas poured into 10 L of deionized water. The GO was filtered andwashed with distilled water until the pH reached 6.5. The resultingGO was dried at 80 ◦C, and then pulverized and screened with a100 mesh sieve to obtain fine particles. Elemental analysis showedthat the composition of the GO was C10O3.68H2.48.

In order to obtain the FGS, the dried GO was placed in aquartz tube, and the tube was flushed with argon for 10 min. Thequartz tube was then quickly inserted into a furnace preheatedto 1100 ◦C. The tube was left in the furnace for 5 min whilethe evolution of CO2 split the GO into individual sheets.26 – 29

The apparent specific volume of the FGS was 410 cm3 g−1, andelemental analysis showed that the composition of the FGS wasC10O0.50H0.51.

Preparation of TPU/FGS nanocompositeA 500 mL round-bottom, four-necked separable flask wasequipped with a mechanical stirrer, a nitrogen inlet, a thermome-ter and a condenser with a drying tube. TPUs were prepared viasolution polymerization in MEK under a dry nitrogen atmosphere.That is, PCL diol (0.01277 mol), H12MDI (0.02554 mol) and DOT(0.03 phr based on total solid) were added to the round-bottomreactor, and the reaction was allowed to continue for 1 h at 50 ◦C.BD (0.01277 mol) was then added and the reaction proceeded foranother 3 h at 80 ◦C. As the reaction progressed, the increasingviscosity of the mixture was controlled by adding MEK. Uponcompletion of the reaction, the solid content of the polymer so-lution was about 45%. The number-average and weight-averagemolecular weights of the TPU as measured by gel permeationchromatography relative to polystyrene standards were 24 300and 59 200, respectively. Recipe calculations placed the soft PCLsegment content of the TPU at 70 wt%.

The FGS was immersed in MEK and sonicated for 3 h. Thissonicated mixture, about 1 wt% solid content, was fed into thepolymer solution and agitated at 80 ◦C for 3 h to obtain theTPU/FGS nanocomposite solution.

Nanocomposite films were cast on polypropylene plates at25 ◦C for 24 h, and then at 60 ◦C for 24 h. The sample designationcodes in Table 1 provide information regarding the amount of FGSincluded in the TPU samples. For example, TPUN-5 contains 5 partsof FGS per 100 parts of polymer.

MeasurementsThe molecular weights of the TPU samples were evaluated at43 ◦C by gel permeation chromatography (Waters M510) usingtetrahydrofuran as an eluent.

XRD patterns were obtained with a Rigaku RAD-3C X-raydiffractometer using Cu Kα radiation (λ = 1.54 Å) as the X-raysource. The diffraction angle was scanned from 2◦ at a rate of 1.2◦

min−1.

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Table 1. Thermal properties of TPU/FGS nanocomposites

Soft segment Hard segment

Sample Tg (◦C) Tms (◦C) �Hms (J g−1) Tmh (◦C) �Hmh (J g−1)

TPUN-0 −45.2 41.8 25.8 103.2 1.3

TPUN-0.5 −42.9 41.8 25.3 104.5 1.7

TPUN-1 −45.2 42.2 22.0 101.6 2.0

TPUN-2 −44.5 42.5 21.5 99.7 1.4

TPUN-3 −46.0 40.9 21.1 107.0 1.0

TPUN-4 −45.5 41.0 21.1 105.4 1.5

TPUN-5 −48.4 40.8 6.3 126.7 1.0

TPUN-7 −47.6 41.7 5.2 111.9 2.1

DSC was carried out with a Mettler Toledo DSC 823e instrumentat heating and cooling rates of 10 ◦C min−1 with 7 mg of sample.The samples were loaded at 30 ◦C and cooled to −60 ◦C, and thethermal properties were measured in a subsequent heating scan.

TGA was performed with a Mettler Toledo SDTA 851e instrumentat a heating rate of 10 ◦C min−1 under nitrogen atmosphere with22 mg of sample in a platinum crucible.

The morphology of the nanocomposites was examined usingtransmission electron microscopy (TEM; Hitachi H-8100). Inorder to obtain samples for TEM observations, cast filmsof the nanocomposites were cryogenically pulverized. Thenanocomposite powder was then mixed with epoxy resin andcured at 70 ◦C for 24 h in a vacuum. The cured material wasmicrotomed into slices. A layer of carbon was subsequentlydeposited onto each slice, being on 200 mesh copper net. TheTEM acceleration voltage was 200 kV.

The dynamic mechanical properties of cast films 0.5 mm thickwere analyzed using dynamic mechanical analysis (DMA; DMA-Q800, TA Instruments). Testing was performed in bending modeat 1 Hz and a heating rate of 5 ◦C min−1.

Tensile testing was performed using an OTU-2 tensile tester(Oriental TM Co., Korea). Cast films were cut into micro-tensilespecimens 25 mm in length, 5 mm in width and 0.5 mm inthickness. The specimens were elongated at a rate of 200 mmmin−1 at 25 ◦C.

Direct current conductivity at room temperature across a 0.5 mmthick cast film was measured with a picoamperometer (Keithley237). Circular silver electrodes measuring 0.28 cm2 were attachedat both surfaces of the specimen. Silver paste was used to ensuregood contact between the specimen surface and the electrode.

RESULTS AND DISCUSSIONXRD and thermal propertiesFigure 1 shows the wide-angle XRD patterns of the graphite, GOand FGS specimens. The diffractogram of graphite shows a veryintense and narrow peak at 2θ = 26.5◦, which corresponds to theX-ray reflection on the (002) planes of well-ordered graphenes.This indicates that the interlayer spacing, Ic, between the well-ordered graphenes is 3.35 Å. GO has a broad peak at 2θ = 14.1◦.Again, this peak corresponds to the X-ray reflection on the (002)planes, and it indicates that the interlayer spacing had expandedto 6.27 Å by the accommodation of various functional groups onthe graphene.22,23,28,39 However, the FGS has no visible peak for2θ > 2◦. This indicates that the distances between the graphenelayers have been greatly expanded, and the layers are sufficiently

105 15 20 25 30

Graphite

Inte

nsity

2θ (°)

GO

FGS

Figure 1. XRD patterns of graphite, GO and FGS.

TPUN-0

Temperature (°C)

-50 0 50 100 150

TPUN-3

TPUN-5

TPUN-7

Exo

ther

m

Figure 2. DSC thermograms of TPU/FGS nanocomposites.

disordered. That is, the long-range order of the GO was destroyedby sudden thermal expansion.26 – 29

Figure 2 shows the DSC thermograms of the TPU and itsvarious nanocomposites. The quantitative results are summarizedin Table 1. In the DSC thermogram of TPUN-0, one can see theglass transition temperature, Tg, and a sharp melting endothermicpeak of the soft segment, Tms, at −45.2 and 41.8 ◦C, respectively. Inaddition, a small endothermic melting peak of the hard segment(Tmh) is observed at 103.2 ◦C. Table 1 shows that, as the amountof FGS in the nanocomposites is increased, the heat of fusion atTms (�Hms) decreases and drops abruptly at higher contents ofFGS. This suggests that the crystallization of the PCL segment isinhibited by the FGS, more evidently at higher contents of FGS. Thisdecrease in the crystallinity also seems to cause a decrease in Tg

with increasing FGS content. The heat of fusion at Tmh (�Hmh) doesnot vary as much with the FGS content as does �Hms (Table 1),suggesting the possibility that the FGS is dispersed predominantlyin the soft segment domain. However, Tmh of nanocomposites withhigh FGS content is higher than Tmh of pristine TPU, indicating

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10 15 2050 25 30

Inte

nsity

2θ (°)

TPUN-0

TPUN-3

TPUN-5

TPUN-7

35

Figure 3. XRD patterns of TPU/FGS nanocomposites.

Temperature (°C)

300200100 400 500 600

Wei

ght r

esid

ue (

%)

0

20

40

60

80

100

120TPUN-0TPUN-3TPUN-5TPUN-7

Figure 4. TGA thermograms of TPU/FGS nanocomposites.

that the mobility of the hard segment in the melt state is reducedin the presence of FGS.

The wide-angle XRD patterns of TPU and its nanocompositesare shown in Fig. 3. The diffractogram of TPUN-0 has two peaksat 2θ = 21.1◦ and 23.3◦. These peaks can be attributed to thereflections on the (110) plane and the (200) plane of the PCLcrystal, respectively.40 The intensity of these peaks decreases asthe content of FGS is increased, supporting the notion proposedfrom the DSC results that the crystallinity of the PCL phase isinhibited by increasing amounts of FGS. Figure 3 also shows thatthe dispersed FGS does not give rise to any new wide-angle XRDpeak at 2θ > 2◦. This suggests that either (1) the distance betweenthe FGSs is too great to allow for the observation of any signalsin the wide-angle XRD patterns or (2) no long-range order existseven when the FGS has a stacked structure.

The thermal degradation behaviors of TPU and its nanocom-posites were examined via TGA. Typical TGA thermograms areshown in Fig. 4 and quantitative data are summarized in Table 2. Itis evident that the degradation temperature increases at low FGS

Table 2. Degradation properties of TPU/FGS nanocomposites

Weight loss temperature (◦C)

Sample 2% 5% 10% 15% Residue (%)

TPUN-0 273.5 286.8 299.1 307.5 0.3

TPUN-1 277.6 291.4 302.2 309.8 1.5

TPUN-2 275.2 291.9 301.2 309.4 2.7

TPUN-3 277.8 294.6 309.6 319.2 3.6

TPUN-4 268.7 284.1 298.7 307.4 4.2

TPUN-5 271.0 285.8 299.6 308.2 5.3

TPUN-7 258.0 270.7 282.6 297.8 7.3

concentrations as compared to TPUN-0. However, the degrada-tion temperature decreases at higher FGS levels. In polymer/claynanocomposites, the thermal degradation is retarded by the pres-ence of clay, because the thin silicate lamellae with a high aspectratio provides a barrier that hinders the diffusion of the volatile de-composition products and enhances char formation.41,42 Hence,the improved resistance to thermal degradation at low levels ofFGS shown in Table 2 can be similarly explained by the barrier ef-fect of the thin FGS. However, the fact that this improvement is notas striking as compared to that of the clay nanocomposites30,43 – 45

and the fact that a high FGS content adversely affects the thermaldegradation resistance show that the FGS is not as effective, forthermal decomposition resistance enhancement, as a clay whichhas an inorganic nature.

Morphological and physical propertiesFigure 5 shows TEM images of TPUN-4, in which one can see asingle FGS or stacks of FGSs whose thickness is less than severalnanometers. These FGSs are finely dispersed throughout the TPUmatrix with a wrinkled or hair-like structure. This morphologydemonstrates that the compatibility between the FGS and theTPU is good enough to achieve nano-sized fine dispersion of theFGSs.

Figure 6 shows the DMA results for TPU and TPU/FGS nanocom-posites. The storage tensile modulus, E′, of TPUN-0 decreasesslowly upon heating from −150 ◦C because of thermal expansion.The rate of reduction of E′ increases at Tg, and another suddendrop of E′ occurs at Tms. When the TPU is reinforced with FGS,the E′ values in the temperature regions below Tg and above Tms

are improved in comparison to those of TPUN-0. However, theE′ values between Tg and Tms are reduced compared to those ofTPUN-0. Figure 6 also shows that these variations are more evidentat higher levels of FGS. The E′ increase observed in the temperatureregions below Tg and above Tms indicates that the FGS reinforcesthe TPU matrix, and this reinforcement becomes more evident asthe matrix polymer weakens from the crystal melting. The E′ re-duction in the presence of FGS in the temperature region betweenTg and Tms indicates that the decrease in modulus caused by thereduced PCL crystallinity overshadows the reinforcing effect ofthe FGS.

In Fig. 6, one can see that tan δ peak temperature at ca −50 ◦C,which is Tg measured by DMA, moves to lower temperature as theFGS content is increased, which shows that Tg is decreased due tothe reduced crystallinity of the soft segment, as found for the DSCresults. Figure 6 also shows that the tan δ value at Tg is increasedas the content of FGS is increased. This suggests that viscous flowrather than elastic deformation of the soft segment by external

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Figure 5. TEM micrographs of TPUN-4.

dynamic force is enhanced at Tg in the presence of FGS. This maybe due to the reduced crystallinity of the soft segment.

Table 3 shows that the tensile modulus of the nanocompositesis improved as compared to TPUN-0 when the reduction of�Hms by the FGS was not very apparent (Table 1). However,when the reduction of �Hms is evident at high FGS levels thetensile modulus decreases. These results also show that theFGS can reinforce the TPU matrix. The tensile strength and theelongation at break generally decreased in the presence of FGSwhen compared with those of TPUN-0. This result indicates thatmolecular rearrangement and orientation with respect to thetensile axis during deformation are inhibited in the presence ofthe FGS. The reduced soft segment crystallinity can also contributeto these reductions of tensile strength and elongation at break.

One can see from Table 3 that a drastic 107 times improvementin conductivity is observed by the addition of FGS at an amountof 2 parts of FGS per 100 parts of TPU. This result shows that finelydispersed FGSs with nano-sized thicknesses and high aspect ratios(Fig. 5) can create an effective conductive channel. Many researchgroups have reported the conductivity of polyurethane/carbonnanotube nanocomposites. Cho and co-workers reported aconductivity of about 0.3 S cm−1 with a carbon nanotube loadingof 3 wt%,37 and Vaia and co-workers reported a conductivity of

Temperature (°C)

E’

(MPa

)

0.1

1

10

100

1000

10000TPUN-0TPUN-3TPUN-5TPUN-7

-150 -100 -50 0 50 100 150

TA

N δ

0.01

0.1

1

Figure 6. Dynamic mechanical properties of TPU/FGS nanocomposites.

Table 3. Physical properties of TPU/FGS nanocomposites

Tensile properties

SampleModulus

(MPa)Tensile strength

(MPa)Elongationat break (%)

Conductivity(S cm−1)

TPUN-0 458 ± 23 20.0 ± 0.1 814 ± 64 4.58 × 10−11

TPUN-0.5 559 ± 113 18.8 ± 0.1 779 ± 40 8.37 × 10−11

TPUN-1 557 ± 85 17.8 ± 0.3 693 ± 34 4.62 × 10−10

TPUN-2 639 ± 70 13.4 ± 0.1 686 ± 43 5.41 × 10−4

TPUN-3 657 ± 157 15.5 ± 0.3 693 ± 52 5.88 × 10−4

TPUN-4 636 ± 47 17.0 ± 0.1 517 ± 13 8.54 × 10−4

TPUN-5 351 ± 2 18.8 ± 0.9 347 ± 73 9.16 × 10−4

TPUN-7 357 ± 43 18.9 ± 0.2 245 ± 22 4.92 × 10−4

about 10−2 S cm−1 with a carbon nanotube loading of 2 vol.%.35

Other research groups have reported conductivities lower than10−5 S cm−1 with carbon nanotube loading of 2 wt%.1,33,34 Theseresults suggest that the FGS can be used as an alternativeconductive nanofiller in place of carbon nanotubes.

CONCLUSIONSIt was demonstrated that FGS can be finely dispersed on thenanoscale in a TPU matrix simply by solution mixing. No furtherchemical surface modification of the FGS was required. The FGSeffectively improved the conductivity of the TPU by a factorof about 107 as compared to pristine TPU. A conductivity of10−4 S cm−1 was achieved in the nanocomposite containing only

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2 parts of FGS per 100 parts of TPU. The FGS effectively reinforcedthe TPU matrix in the temperature region above Tms. However,because the dispersed FGS hindered the crystallization of thesoft segment, this reinforcing effect was overshadowed by thesoftening due to the decreased crystallinity in the temperatureregion between Tg and Tms.

ACKNOWLEDGEMENTSThis research was financially supported by the Ministry ofEducation, Science Technology (MEST) and the Korean IndustrialTechnology Foundation (KOTEF) through the Human ResourceTraining Project for Regional Innovation

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