effect of functionalized carbon nanotubes on molecular interaction and properties of polyurethane...

Effect of Functionalized Carbon Nanotubes on

Molecular Interaction and Properties of

Polyurethane Composites

Nanda Gopal Sahoo,1 Yong Chae Jung,2 Hye Jin Yoo,2 Jae Whan Cho*1,2

1Artificial Muscle Research Center, Konkuk University, Seoul 143-701, Korea2Department of Textile Engineering, Konkuk University, Seoul 143-701, KoreaE-mail: [email protected]

Received: May 30, 2006; Revised: July 31, 2006; Accepted: August 9, 2006; DOI: 10.1002/macp.200600266

Keywords: composites; crystallization; mechanical properties; multi-walled carbon nanotubes; polyurethane

Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991 by

Iijima,[1] the fabrication of CNT/polymer composites has

received much attention for their many potential applica-

tions.[2–5] Because of the excellent mechanical, electrical,

and magnetic properties as well as nanometer scale diameter

and high aspect ratio,[6,7] CNTs can be used as an ideal

reinforcing agent for high strength polymer composites.

However, since CNTs usually form stabilized bundles due to

van der Walls force, it is extremely difficult to align and

disperse the CNTs in a polymer matrix. The biggest issues in

the preparation of CNT-reinforced composites reside in

efficient dispersion of CNTs into a polymer matrix, the

assessment of the dispersion, and the alignment and control of

the CNTs in the matrix. The solubilization of CNTs via

chemical functionalization is considered as an effective way

to achieve a homogeneous dispersion of CNTs in polymer

matrices.[8–10] The surfactants have also been used to

improve the dispersion and strengthen the interactions

between the CNT and polymer matrix.[11,12] Other methods

for the dispersion of nanotubes in solvents have been tried, for

example, solutioncasting,[13,14]meltmixing,[15,16] and, in situ

polymerization.[17,18]

Polyurethane (PU) was selected as the matrix in this

study because of its superior properties and potential

applications in coatings, adhesives, shape memory poly-

mers, and medical fields. PU is consisting of alternating

hard and soft segments. The hard segment is composed of

alternating diisocyanate and chain-extendermolecules (i.e.,

diol or diamine), whereas the soft segment is formed from a

linear, long-chain diol. Phase separation occurs in PUs

because of the thermodynamic incompatibility of the hard

and soft segments.

Summary: Functionalized MWNTs were incorporated intoPU by solution mixing to improve the mechanical and ther-mal properties of composites. A homogeneous dispersionof MWNTs was successfully achieved in PU matrix asevidenced by scanning electron microscopy. It may beattributed to the hydrogen bonds existing between C Ogroups of hard segments of PU chains and COOH groups ofthe MWNT-COOH. The incorporation of the MWNTseffectively enhanced the crystallization of the PU matrixthrough heterogeneous nucleation, and the nucleation effectwas more evident at 10 wt.-% functionalized MWNTs ascompared to other composite systems. Mechanical proper-ties of the PU-MWNTs composites were assessed asa function of MWNT concentration and dispersion ofMWNT in PU matrix. The most significant improvement inmechanical properties was obtained, e.g., 740% increase in

modulus and 180% increase in tensile strength over purePU with 20% MWNT content. The thermal stability ofcomposites due to thermal gravimetric measurements wassignificantly improved.

A possible interaction of H-bonding existed between PUchain and MWNT-COOH.

Macromol. Chem. Phys. 2006, 207, 1773–1780 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper DOI: 10.1002/macp.200600266 1773

The composites based on CNT have been intensively

studied using different polymers.[19–23] Xia et al.[22] prepared

PU grafted single-walled CNTs (SWNT-g-PU) by in situ

polymerization. The results suggested that SWNT-g-PU

improved the dispersion of SWNT in the matrix and

strengthened the interfacial interaction between PU and

SWNT. Chen et al.[23] demonstrated a simple solvent casting

method for the production of PU nanocomposites with

alignedCNTs.By the addition of a 5wt.-%nanotube into PU,

a 1.9-fold increase in Young’s modulus was achieved. Kuan

et al.[24] synthesized multi-walled carbon nanotube

(MWNT)/waterborne PU nanocomposites. They reported

that the tensile stress and modulus of nanocomposites

increased by 370 and 170.6%, respectively, as compared to

pure PU. Recently, we have proposed a novel concept to

prepare PU-MWNTs composite by the in situ polymerization

of prepolymer in the presence of modified MWNTs. The

MWNT-cross-linked PU showed a higher modulus and

tensile strength compared to the pure PU.[25]

However, the improvement in mechanical properties of

the reported polymer nanocomposites with higher amounts

of CNTs is usually limited, mainly due to the formation of

CNTs agglomeration and/or poor interfacial interaction

with the matrix. In this paper, we report on successful

preparation of carboxyl-functionalizedMWNT-COOH/PU

composites by solution mixing. The dispersion of nano-

tubes in the polymer matrix has also been examined in the

presence or absence of a surfactant. The results were then

compared with those obtained for composites containing

functionalized MWNTs.

Experimental Part

TheMWNTsused in this studywere purchased from IljinNanoTech, Korea. Their diameter and length were about 10–20 nmand 20 mm, respectively. These nanotubes (purity 95%) wereproduced by chemical vapor deposition method. MWNT-COOH was prepared by oxidation of raw MWNTs withH2SO4/HNO3 (3:1) at 90 8C for 10 min with vigorous stirringas described previously.[26,27]

PU block copolymer was synthesized from its monomers,4,40-methylenebis(phenylisocyanate) (MDI, Junsei Chemical)and poly(e-caprolactone)diol (PCL, Solvay Co., MW¼4 000 g �mol�1) by a two-step process, using 1,4-butanediol(BD, Duksan Chemical) as the chain extender.[21] Amole ratioof 5:1:4 of MDI/PCL/BD was used, indicating 30 wt.-% ofhard-segment content.

For the fabrication of MWNT-PU composite films, thefollowing procedure was used. The necessary weight fractionsof functionalized MWNTs (0.05–2.0 g) were first dispersed in150mlDMF solution in sonication, at room temperature for 1 husing a high power ultrasonic processor. Thereafter, PU (9.95–8.0 g) was added into this solution and stirred for 1 h. Themixtures were then sonicated again for 1 h.

PU composites incorporating raw MWNTs (PRC10,PRCS10) were also prepared by dispersing the raw MWNTs

in dimethylformamide without and with surfactant (sodiumdodecylsulfate) and assisted by sonication for 1 h, respectively.Theweight percent of surfactant to CNTwasmaintained at 1%.After stirring the PU solution for 48 h, all the final compositefilms were cast from the homogeneous solutions on a clean,flat-base Petri dish and drying completely in an oven. Earlier,we have tried many times to prepare composites by adjustingthe experimental time, and found that better dispersion wasobtained after stirring the PU solution for 48 h. Compositefilms of 0.41 mm thickness were obtained from this method.The sample code and weight ratio of MWNT for PUcomposites used in this study are presented in Table 1.

Fourier transform infrared (FT-IR) spectroscopic measure-ments were performed using a Jasco FT-IR 300E with anattenuated total reflectance method. Raman spectroscopy(BRUKER, RFS, 100/s) was used to investigate the structuralchanges of CNTs by acid treatment. A 632.8 nm He-Ne laserwas used as the light source.

X-ray diffraction was studied using 2100 series X-raydiffractometer with CuKa radiation at a scan rate of 58 �min�1.The crystallite size (Phkl) and interplanar distance (dhkl) werecalculated using a half height width of diffraction peaks at (hkl)plane as follows:

Phkl ¼ Kl=b cos yhkl ð1Þ

dhkl ¼ l=2 sin yhkl ð2Þ

where b is a half height width in radian of the crystalline peak,l a wavelength of the X-ray radiation (1.548 A for Cu), andK aScherrer constant taken to be 0.9.[28,29]

DSC measurements were carried out using a TA instrument2010 (DuPont) thermal analyzer in a temperature range of�50to þ250 8C, at a heating rate of 10 K �min�1 in nitrogen. Thefirst cooling and second heating DSC thermograms were usedfor the analysis. The crystallinity of soft segments in PU wasdetermined bymeasuring the heat of crystallization on cooling,and using an enthalpy value of 136 J � g�1 for 100% crystallinePCL.[30] A thermogravimetric analysis was carried out in a TAQ50 systemTGA. The samples were scanned from 0 to 600 8C,at a heating rate of 10 K �min�1 in the presence of nitrogen.CNT dispersion on matrix polymer of nanocomposite was

Table 1. Compounding formulations.

Sample code PU MWNT-COOH

wt.-% wt.-%

PU 100 0PMC0.5 99.5 0.5PMC1 99 1PMC5 95 5PMC10 90 10PRC10a) 90 10PRCS10b) 90 10PMC20 80 20

a) PRC10: Composite prepared from rawMWNTs and PUmatrix.b) PRCS10: Composite prepared from raw MWNTs, SDS(1 wt.-%) and PU matrix.

1774 N. G. Sahoo, Y. C. Jung, H. J. Yoo, J. W. Cho

Macromol. Chem. Phys. 2006, 207, 1773–1780 www.mcp-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

analyzed by the cross-sectional fracture of composites, using ascanning electron microscope (SEM, JSM-6380, JEOL) atroom temperature.

The mechanical properties of the samples were measured atroom temperature using a tensile tester machine (Instron 4468)according to theASTMD638 testmethod. The sample usedwasa Dog-bone type dumb-bell sample. The dimensions of the testspecimens were 60� 2.9� 0.41 mm3. The gauge length andcross-head speed were 20 mm and 20 mm �min�1, respectively.In each case, at least five measurements were taken.

Results and Discussion

The FT-IR and Raman spectra of functionalized MWNTs

and raw MWNTs are shown in Figure 1 and 2. The FT-IR

spectra of raw MWNTs showed peaks with very low

intensity at 3 440, 1 637, and 1 182 cm�1, corresponding to

OH, C O, and C–C–O stretching, respectively. In the case

of our modified MWNTs, these characteristic bands

appeared with significantly higher intensity, according to

the degree of modification. This was attributed to the

increased number of carboxylic acid groups generated at

the surface of the MWNTs after acid treatment in H2SO4/

HNO3. In Raman spectra (Figure 2), the two bands at

around 1 579 and 1 325 cm�1 in the spectrawere assigned to

tangential mode (G-band) and disorder mode (D-band),

respectively.[24] The D-band intensity was increased in

modified MWNTs compared to raw MWNTs. The peak

intensity ratio (ID/IG) at D-band and G-band for modified

MWNTs exceeded those of unmodified MWNTs. This

result indicates a direct evidence of the modification of

MWNTs. This may also be evidenced by the change of

morphology in TEM measurements, as reported in our

previous paper.[21] The uniform surfaces and clear diffrac-

tion pattern of raw MWNTs have been observed, owing to

the perfect lattice structure of carbon–carbon bonds, while

the modified MWNTs have showed some defects in the

carbon–carbon bonding associated with the formation of

carboxylic acid groups on the surface.[31]

The nature of interaction between chemical groups onPU

and MWNT surface was probed by measuring the extent of

the shift in absorption wavelength of key groups in the

polymer and on theMWNTaftermixing of the components.

Figure 3 shows the FT-IR spectra of PU and PU-MWNT-

COOH composites. The FT-IR spectra of composites

prepared by rawMWNTand PU with or without surfactant

were also included. From Figure 3(a), two characteristic

peaks near 1 614 and 1 718 cm�1 due to the stretching

vibration of –C O group in hard segments can be seen. The

former peak was due to hydrogen bonded carbonyl group

formed with hard segment –NH group, whereas the latter

peak was due to free carbonyl group of urethane linkages. It

can also be clearly seen that the peaks for C O groups was

gradually shifted from1 718 cm�1 in pure PU to 1 685 cm�1

in composites with the content of functionalized MWNTs.

The results suggest that the composites are not the simple

mixtures of polymer and MWNTs but imply the existence

of a strong interaction between them. The presence of

the carboxylic groups on the nanotube surface is likely

to give the interfacial interaction between the polymer

matrix and the nanotubes in polymer composites. That is,

C O groups of hard segments of PU chains may form the

hydrogen bonds with COOH groups of the MWNTs.

However, for the PMC10 composite, the C O stretching

peak [Figure 3(d)] was more shifted than composites

PRC10 [Figure 3(b)] and PRCS10 [Figure 3(c)], suggesting

that more interactions occurred with the functionalized

MWNTs. The possible interaction between PU and

MWNT-COOH is shown in Figure 4.

X-ray diffraction patterns of functionalized MWNTs,

PU, and PU-MWNT composites are presented in Figure 5.

The X-ray patterns of the MWNT displayed the presence

of two peaks at 2y¼ 25.808 (3.47 A) and 42.758 (2.12 A)corresponding to the (002) and (100) reflections of the

carbon atoms, respectively, in good agreement with the

Figure 1. FT-IR spectra of raw and functionalized MWNTs.

Figure 2. Raman spectra of raw and functionalized MWNTs.

Effect of Functionalized Carbon Nanotubes on Molecular Interaction and Properties of Polyurethane Composites 1775


literatures values.[14,32] Pure PU showed diffraction peaks

at 2y¼ 21.28 (4.19 A) and 23.48 (3.81 A) due to the

presence of PCL crystals in soft segments. The X-ray

pattern of the composite showed mixed peaks appearing

in the MWNT and pure PU, respectively. The peak

intensities assigned to the MWNTs in the composite,

increased with increasing MWNT content in the compo-

site. It is very interesting that the new diffraction peak

appeared at 2y¼ 19.28 (4.63 A), which has never been

reported, as far as we know. As the MWNT content

increased, its diffraction intensity became gradually

sharp. It is also worthy to note that the crystallite size

(Table 2) corresponding to each peak position at 19.28 and21.28 increased with the increase in MWNT content.

Usually it is been known that the CNTs as reinforcing

agent do not influence the crystal structure of a matrix

polymer. PMC10 composite showed higher diffraction

intensity and larger crystallite size, compared to PRC10

and PRCS10 composites. As a result, for the PU-MWNT-

COOH composites, the PU crystallinity and crystallite

size became higher with increasing MWNT-COOH

concentration. It indicates that MWNTs promote the

crystallization of PU crystals, which was more prominent

in the case of PU-MWNTs composite with 10 wt.-%

functionalized MWNTs as compared to other composite

systems. More detailed work on the origin of these

crystalline features is ongoing.

DSC thermograms of pure PU and PU-MWNTs nano-

composites are presented in Figure 6 and 7. The

crystallization temperature (Tc), melting temperature

(Tm), heat of crystallization (DHc), heat of fusion (DHf),

and percentage of crystallinity obtained from DSC studies

are summarized in Table 3. The percent crystallinity due to

soft segments increased with the addition of MWNT-

COOH up to 10 wt.-%. In addition, the obtained results

showed that the crystallization temperature increased when

bothMWNT-COOHand rawMWNTswere incorporated in

the PU matrix; the effect being more dominant in the

presence of MWNT-COOH. At the same loading of raw

MWNTs (10 wt.-%), the composite in the presence of SDS

surfactant (PRCS10) did not exhibit significant change in

the crystallization behavior of PU compared to the PRC10

composite. These results suggest that the functionalized

MWNTs significantly affected the crystallization of the PU

matrix. The fillers have often a positive effect on the

crystallization of the polymer in the composite sys-

tem.[33,34] That is, they can have a nucleating effect,

resulting in an increase in crystallization temperature. Our

Figure 3. FT-IR spectra of pure PU and PU-MWNT composites:(a) PU, (b) PRC10, (c) PRCS10, (d) PMC10, and (e) PMC20.

Figure 4. A possible interaction of hydrogen bonding existedbetween the PU chain and MWNT-COOH.

Figure 5. X-ray diffractrograms of samples: (a) MWNTs,(b) PMC10, (c) PRC10, (d) PRCS10, (e) PMC5, and (f) PU.

Table 2. Crystallite size of samples corresponding to X-raydiffraction peaks at 2y¼ 19.28 and 21.28.

Sample Crystallite size at2y¼ 19.28

Crystallite size at2y¼ 21.28

A A

PU – 133PMC5 165 175PMC10 230 197PRC10 145 170PRCS10 170 182



observations suggest that the incorporation of the MWNT

effectively enhanced the crystallization of the PU matrix

through heterogeneous nucleation, and the nucleation effect

was more evident at 10 wt.-% of functionalized MWNTs in

PU composite, as compared to other composites. These

observations are quite in agreement with the findings by

Mitchell and Krishnamoorti[35] while using functionalized

SWNT as nucleating agent for poly(e-caprolactone). Theyreported that the nanotubes are strong nucleators of poly(e-caprolactone) and dramatically accelerate the crystalliza-

tion of the polymer. We also observed a small decrease in

the crystallinity and crystallization temperature for PU-

MWNT-COOH composite with 20 wt.-%. It may indicate

that an excess of filler in the composite may have a limited

ability as a nucleating agent. The heat of fusion (Table 3)

due to soft segments in the PU was the highest for 10 wt.-%

modified MWNTs-PU composite and the lowest for pure

PU. The heat of fusion is proportional to the amount of

crystallinity in the sample, which was also supported by the

crystallization behavior of composites. The melting

temperature of soft segments in the PU was affected by

filler content and crystallinity, which has a small tendency

to increase with crystallinity and MWNT-COOH.

In order to investigate the thermal stability of the

MWNT-PU composites, TGA measurements were carried

out, and the results are shown in Figure 8. The onset of

decomposition of pure PU occurred at 225 8C. Compared to

the pure PU, the PRC10 and PRCS10 did not show a

significant difference in the onset decomposition temper-

ature. If the criteria for stability are taken as the temperature

at which 10 and 50% weight loss occurred, then it can be

concluded that the decomposition started at almost the same

temperature in the case of PRC10 and PRCS10 composites,

but the decomposition was slow in these cases compared to

Figure 6. DSC thermograms of samples obtained on coolingfrom the melt: (a) PMC10, (b) PRCS10, (c) PRC10, (d) PMC20,(e) PMC1, (f) PMC0.5, and (g) PU.

Figure 7. DSC thermograms of different samples with 10 wt.-%MWNT on cooling: (a) PRC10, (b) PRCS10, and (c) PMC10.

Table 3. Thermal properties of composites used in this study.

Sample DHc Tc Tm DHfa) Crystallinityb)

J � g�1 8C 8C J � g�1 %

PMC0.5 17.6 16.4 47.4 16.5 12.9PMC1 19.2 15.8 47.1 17.1 14.1PMC5 19.9 17.7 48.1 22.5 14.6PMC10 20.3 19.6 47.9 21.4 14.9PRC10 15.3 18.0 46.3 16.2 11.2PRCS10 15.4 19.0 46.9 17.1 11.3PMC20 16.5 13.4 47.0 16.2 11.9PU 15.1 12.1 46.0 16.1 11.1

a) Heat of fusion (DHf) was determined from the second heat run.b) The crystallinity of soft segments in PU was determined bymeasuring the heat of crystallization on cooling DSC thermo-grams.

Figure 8. TGA thermograms of samples: (a) PMC10, (b)PRCS10, (c) PRC10, and (d) PU.



pure PU, which was more evident in the PRCS10

composite. In contrast, PMC10 composite showed more

delayed decomposition compared to pure PU and compo-

sites with the same raw MWNT, loading with the

decomposition temperature increasing by about 50 8C.The temperature of 10% (T10) and 50%decomposition (T50)

also increased markedly in this case. It is important to

point out that the extent of interaction between function-

alized MWNTs and the PU matrix could be responsible for

the higher thermal stability of the PMC10 composite

system.

The representative SEM photographs of the cross-

sectional fracture of composites of the achieved dispersion

for the investigated MWNTs are shown in Figure 9 and 10.

Figure 9 showed the well-dispersed bright dots, and lines

are the ends of the brokenMWNTs.Moreover, theMWNTs

were broken rather than pulled out due to the strong

interfacial bonding between the MWNTs and the polymer

matrix. It is apparent from Figure 9 that there were more

bright dots and lines in the composites with higher MWNT

loading. It was also observed that the MWNT-COOH

displayed a high dispersion ability in the PU matrix. The

homogeneous dispersion in the composites was also

achieved by the addition of a higher amount of MWNT-

COOH (20 wt.-%). The carboxylic groups seemed to

stabilize the MWNT dispersion by stronger interactions

with the PU matrix. This can be attributed to the increased

polarity of the MWNTs by the functional groups and the

possible interaction of the carboxylic groups with –NHCO–

of the PU matrix. Figure 9 indicated a very good matrix

adhesion of the functionalized MWNTs.

The dispersion of raw MWNTs was found to be poor

in the PU matrix as shown in Figure 10(a) with 10 wt.-

% MWNTs (PRC10). Furthermore, a few of the agglo-

merates were observed in the PU matrix, reducing the

reinforcing effects of the MWNTs. The dispersion was

somewhat better in the PRCS10 sample with the same

CNTs loading and was most likely due to the presence of

a surfactant. When the PU was added to the nanotubes,

the surfactant molecules could serve as a link between

the nanotubes and polymers, providing hydrophobic

interactions that can enhance the contact at the interface.[36]

The stress-strain profiles for pure PU and PU/MWNTs

composites are illustrated in Figure 11. The tensile

strength and modulus were calculated from the stress–

strain profiles and are shown in Figure 12. A pronounced

yield and post yield drop were observed for pure PU

and composites containing MWNTs up to 1 wt.-%,

while there was no noticeable yield for composites

containing higher amounts of MWNTs. The addition of

functionalized MWNTs into the PU matrix improved

the tensile strength and modulus of the PU matrix.

The tensile strength of PMC10 composite was enhanced

by 108% as compared to pure PU, while an increase of

68% was achieved by incorporating the same amount of

raw MWNTs in the PU matrix. The PRCS10 composite

showed a lower tensile strength and modulus than the

PMC10 composite, but higher than PRC10 composite and

PU because of better dispersion and interaction in this

Figure 9. SEM images of the cross-sectional fracture ofcomposites: (a) PMC1, (b) PMC10, and (c) PMC20.



composite due to the presence of the surfactant. Finally, the

tensile strength and modulus of nanocomposites increased

from 7.6 in pure PU to 21.3 MPa (an increase of 180%) and

50 to 420 MPa (an increase of 740%), respectively, when

the functionalized MWNTs content reached 20 wt.-% in

composites.

The functionalized MWNTs prepared by acid treatment

contain manyC–C bond defects and –COOH groups. These

defects can be seen as problematic to the mechanical

strength of the MWNTs themselves; their incorporation

into the polymeric matrix creates some interaction between

MWNTs and polymer chains, thus being favorable to stress

transfer to MWNTs.[37] Consequently, the hydrophilic

functional groups on the MWNTs were helpful in

improving the interaction with –CONH– groups in PU.

Therefore, the strong interaction between the functional-

ized MWNTs and the PU matrix greatly enhanced the

dispersion as well as the interfacial adhesion, thus

strengthening the overall mechanical performance of the

composite.

Figure 10. SEM images of the cross-sectional fracture ofcomposites: (a) PRC10 and (b) PRCS10.

Figure 11. Stress-strain profiles of PU composites at differentMWNT loading.

Ten

sile

str

eng

th (

MP

a)

0

5

10

15

20

25

PU PMC0.5 PMC1 PMC5 PRC10 PRCS10 PMC10 PMC20

a)M

od

ulu

s(M

Pa)

0

100

200

300

400

500

PMC20PMC10PRC10 PRCS10PMC5PMC1PMC0.5PU

b)

Figure 12. (a) Tensile strength of various samples and(b) modulus of various samples.



Conclusion

Raw MWNTs with or without surfactant and carboxylic

functionalized MWNTs have been incorporated into PU

to demonstrate the effect of the functionalized MWNTs

on the crystalline, thermal, morphological, and mechan-

ical properties of MWNT-reinforced composites. PU

containing functionalized MWNTs showed better

mechanical and thermal properties, as well as increased

crystallinity than other composites with the same raw

MWNTs loading, in presence or absence of surfactant

and pure PU. Comparedwith pure PU, the tensile strength

and modulus of the composite with 20 wt.-% functiona-

lized MWNTs were improved by 180 and 740%,

respectively. SEM observation showed that a homoge-

neous dispersion of functionalized MWNTs throughout

the PU matrix and a strong interfacial adhesion between

functionalized MWNTs and the matrix were achieved in

PU composites, which strengthened the mechanical

properties. So, the functional groups on the MWNTs

surface played an important role in accelerating both the

dispersion of MWNTs and the interfacial adhesion in the

composites compared to raw MWNTs and the use of

surfactant.

Acknowledgements: This work was supported by the KoreaResearch Foundation Grant (KRF-2004-005-B00046) and theSRC/ERC program ofMOST/KOSEF (R11-2005-065).

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