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Composites Science and Technology 68 (2008) 2842–2848

SCIENCE ANDTECHNOLOGY

Silane grafted MWCNT/polyimide composites – Preparation,morphological and electrical properties

Siu-Ming Yuen a, Chen-Chi M. Ma a,*, Chin-Lung Chiang b

a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwanb Department of Industrial Safety and Health, HungKuang University, Salu, TaiChung, Taiwan

Received 21 September 2007; accepted 15 October 2007Available online 24 October 2007

Abstract

Precursor of polyimide, polyamic acid has been prepared by reacting 4,4 0-oxydianiline with 3,3 0,4,4 0-benzophenone tetracarboxylicdianhydride. Acid modified multiwall carbon nanotubes (MWCNTs) were grafted with 3-isocyanato-propyltriethoxysilane. Silanegrafted MWCNTs were then mixed with the polyamic acid and heated to 300 �C to form a carbon nanotube/polyimide composite. Dur-ing the imidization processes, the silanes on the MWCNT surface reacted with each other. TEM microphotographs shows that the silanegrafted MWCNTs were connected. The composite material possesses an interpenetrating network in which polyimide molecules wereinterpenetrated into the MWCNT network. The electrical resistivity of silane grafted MWCNT/polyimide decreased very significantlycompared to those only containing acid treated MWCNTs for the same loading with MWCNTs.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites (PMCs); A. Carbon nanotubes; A. Nano composites; B. Electrical properties; D. Transmission electronmicroscopy (TEM)

1. Introduction

Carbon nanotubes (CNTs) have attracted interest sinceIijima identified the structure of singlewalled carbon nano-tube (SWCNT) in 1991 [1]. CNTs posses excellent mechan-ical properties, low density, high surface area, highchemical stability, electrical conductivity, and thermal con-ductivity [2–7]. The mechanical and electrical properties ofthe polymeric matrices are improved significantly by theaddition of carbon nanotube [8–10].

Modified CNTs can enhance the adhesion betweenCNTs and polymer matrix. Acid modification is one ofthe most common methods of CNT modification. CNTcan be modified by refluxing with nitric acid or a mixtureof nitric acid and sulfuric acid. Carboxyl and hydroxylfunctional groups are formed on the CNT surface duringacid modification [11]. Acid-modified MWCNT can be

0266-3538/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2007.10.011

* Corresponding author. Tel.: +886 3 5713058; fax: +886 3 571 5408.E-mail address: ccma@che.nthu.edu.tw (C.-C. M. Ma).

modified with silane coupling agent [12–14]. The silane willreact with the hydroxyl groups (–OH) on the surface ofMWCNTs. The oxidation of MWCNT may generate car-boxylic groups (–COOH) rather than hydroxyl groups.Ma et. al. [12] and Vast et. al. [14] suggested that the acidmodified MWCNT can generate more hydroxyl groups byreduction process. Valentini et al. [15] modified SWCNTsusing CF4 plasma to obtain fluorinated SWCNT(f-SWCNT). The f-MWCNT then reacted with APTES andthe amine functional group of APTES was grafted on thef-MWCNT.

Polyimide is a high-performance polymer owing to itshigh thermal stability, low dielectric constant and chemicalresistance. Accordingly, it found applications in thecomposite and microelectronics industries [16]. Variousinvestigations have been performed on CNT/polyimidecomposites [17]. In our earlier study [17], unmodified, acidmodified, and amine modified MWCNT/polyimide com-posite has been prepared. MWCNT improved themechan-ical and electrical conductivity of polyimide [17]. However,

Scheme 1. IPTES grafted of MWCNT.

Table 1The ratios of IPTES to acid modified MWCNT for IPTES modifiedMWCNT

ID IPTES:MWCNT (in weight)

IPTES-MWCNT-1 1:1IPTES-MWCNT-2 2:1IPTES-MWCNT-3 3:1

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modification of MWCNT can not significantly increase theelectrical conductivity without forming more effective elec-trical pathways.

In this investigation, a precursor of polyimide, polyamicacid, was prepared by reacting 4,4 0-oxydianiline(ODA)with 3,3 0,4,4 0-benzophenone tetracarboxylic dianhy-dride(BTDA). Multi-walled carbon nanotubes (MWCNT)were modified using mixed acid H2SO4/HNO3. The modifiedMWCNTs were then reacted with 3-isocyanatopropyltrieth-oxysilane (IPTES). Silane functional groups were grafted onthe acid-modified MWCNT (IPTES-MWCNT). TheIPTES-MWCNT was well dispersed in polyamic acidbefore imidation at 300 �C. When the IPTES-MWCNT/polyamic acid was heated to 300 �C, the silane moleculeson the MWCNT surface were reacted and connected toother MWCNT. However, polyimide does not connect withMWCNT, but interpenetrates into the connected IPTES-MWCNT network. The IPTES-MWCNT network reducesthe electrical resistivity, which provides more effective elec-trical pathways. The IPTES-MWCNT has lower electricalresistivity than that of acid modified MWCNT with the sameMWCNT loading. For example, the volume resistivity of2.44 wt% IPTES-MWCNT-1/polymer was 6.94 · 108 X cmand 2.44 wt% acid modified MWCNT/polyimide was9.25 · 1015 X cm. Addition of IPTES reduces the percola-tion threshold for the DC conductivity since the silane onthe IPTES-MWCNT reacted with each other and causedthe MWCNTs connect each other. The interpenetrates net-work formed in the polyimide matrix which provides electri-cal pathways easier than that of acid modified MWCNT.

2. Experimental

2.1. Materials

Multi-walled carbon nanotubes were obtained from theNanotech Port Company, Shenzhen, China. The diameterof the MWCNTs was 40–60 nm; the lengths were 0.5–40 lm. Both 4,4 0-oxydianiline (ODA) and 3,3 0,4,4 0- benzo-phenone tetracarboxylic dianhydride (BTDA) wereobtained from Chris KEV Company, Inc., TerranceLeawood, KS, USA. 3-Isocyanatopropyltriethoxysilane(IPTES) was obtained from Lancaster Synthesis Co.,Morecambe, England. Triethylamine was received fromTEDIA Company, Fairfield, OH, USA.

2.2. Synthesis of precursors of polyimide (polyamic acid)

The precursor of polyimide (polyamic acid) was pre-pared by reacting 4,4 0-oxydianiline (ODA) with 3,3 0,4,4 0-benzophenone tetracarboxylic dianhydride (BTDA) inDMAc. The mole ratio of ODA to BTDA was 1:1.

2.3. Modification of MWCNT

One gram of pristine MWCNT was mixed with 240 g ofH2SO4 and 160 g of HNO3. Then the mixture of MWCNT,

H2SO4 and HNO3 was refluxing at 50 �C for 24 h. After thereaction, the modified MWCNT was filtrated and washedwith pure water to remove the reside acid then dried in100 �C. It was reported previously that a prolonged acidtreatment of CNT would significantly damage the CNTstructure although this method has been widely used tomodify of CNT [17–19]. The acid modified MWCNTs weremixed with 3-isocyanatopropyltriethoxysilane (IPTES) andthen reacted with the 1 wt% of catalyst, triethylamine(TEA)(Scheme 1) at 25 �C for 48 h. Table 1 summarizesthe weight ratios of IPTES to MWCNT.

2.4. Preparation of carbon nanotubes/polyimide

nanocomposites

IPTES-MWCNTs were added to polyamic acid andheated to 60 �C to remove the solvent and then heatedto 300 �C to produce MWCNT/polyimide composites(Scheme 2).

2.5. Measurement of properties

2.5.1. Fourier transfer infrared spectroscopy (FT-IR)

Fourier transform infrared spectroscopy (FT-IR) spec-tra of CNT were recorded between 400 and 4000 cm�1

on a Nicolet Avatar 320 FT-IR spectrometer, NicoletInstrument Corporation, Madison, WI, USA. The samplewas washed by THF to remove residual IPTES and milled

Scheme 2. Perparation of silane grafted carbon nanotubes/polyimidecomposites.

4000 3500 3000 2500 2000 1500

-COOH, 3650-3000 cm-1

-CH-, 3000-2800 cm-1 -COOH, 1720 cm-1

-COO-, 1610-1550 cm-1

Wavenumbers, cm-1

Wavenumbers, cm-1

4000 3000 2000 1000 0

-CH--Si-O-C-

-NH-

Fig. 1. FT-IR spectrum of (a) acid modified MWCNT, (b) iso-cyanatopropyltriethoxysilane(IPTES) modified MWCNT.

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with KBr and was pressed as a tablet. A minimum of 32scans was signal-averaged with a resolution of 2 cm�1 atthe 4000–400 cm�1 range.

2.5.2. Morphological properties

Morphological properties were examined using atransmission electron microscope (TEM) (JEOL-2000EX,Japan).

2.5.3. CP/MAS solid state 29Si nuclear magnetic resonance

(NMR) spectroscopyThe high-resolution 29Si solid-state NMR used was a

BRUKER DSX 400 MHz NMR. The samples wereground into a fine powder. The 29Si CP/MAS NMR spec-tra of the composites were used to characterize the degreeof condensation of the IPTES-MWCNT/polyimide inter-penetrating network with various multi-walled carbonnanotube contents.

2.5.4. Measurements of electrical properties

The surface and volume electrical resistivity were mea-sured using an ULTRA Mesohmeter SM-8220, DKKTOA Corporation, Tokyo, Japan. The surface and volumeelectrical resistivity of the MWCNT/polyimide compositeswere measured after various amounts of MWCNT wereadded. The charge time was 30 s, and the current voltageof the measurements was 100 V. An average value wasobtained from six measurements for each sample.

3. Results and discussion

3.1. Fourier transform infrared spectroscopy

Fig. 1a presents the FT-IR spectra of acid-modified car-bon nanotubes, with wavenumbers 1610–1550 cm�1, 1075–1010 cm�1, 3650–3000 cm�1 and 3000–2800 cm�1, corre-sponding to the absorptions of COO– asymmetrical

stretching, –OH in the primary alcohol, –CH stretchingand –COOH stretching, respectively. The carboxylicgroups stretch (COOH) appears at 1720 cm�1.

Fig. 1b displays the FT-IR spectra of isocyanatopropyltr-iethoxysilane- modified MWCNT. The wavenumbers1100 cm�1 and 3530–3400 cm�1 corresponded to –SiOstretching and –NH stretching, respectively.

3.2. 29Si solid state nuclear magnetic resonance (NMR) of

structure of cured IPTES-MWCNT/Polyimide composites

Fig. 2a–f summarizes the 29Si solid-state NMR spectraof IPTES-MWCNT/polyimide composites. These figuresindicate that tri-substituted siloxane bonds (T2 shift, d�59.84 ppm, and T3 shift, d �67.002 ppm) and sometetra-substituted siloxane bonds (Q4, Q3 and Q2 shift as�109.13, 101 and 91 ppm) are presented in the IPTES-MWCNT/polyimide composites [20]. Table 2 summarizesthe percentages of T and Q substitution of the composites.The Q-substituted bond may be formed when the compos-ites were heated to 300 �C and the Si–C bonds of theIPTES were broken. At low IPTES-MWCNT content(0.99 wt%), the percentage of substituted Q reaches a min-imum at IPTES: MWCNT = 2:1. At high IPTES-MWCNT content (6.98 wt%), the percentage of substitutedQ is proportional to the ratio of IPTES to MWCNT.

The –COOH functional groups on the acid treatedMWCNT may cause the breaking of the Si–C bond. Thedensity of the –COOH functional groups of the acid-trea-ted MWCNT was around 2.6 mol/g, as determined bytitration with NaOH solution. When the ratio of IPTESto MWCNT was lower (1:1), the –COOH functionalgroups of the acid-modified MWCNT were in excess.

Fig. 2. 29Si solid-state NMR spectra of cured IPTES-MWCNT/Polyimide composites with various IPTES-MWCNT ratios and contents:IPTES:MWCNT (wt%) (a) 1:1 (0.99 wt%) (b) 1:1 (6.98 wt%) (c) 2:1 (0.99 wt%) (d) 2:1 (6.98 wt%) (e) 3:1 (0.99 wt%) (f) 3:1 (6.98 wt%).

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When the ratio of IPTES to MWCNT was higher (3:1),the quantity IPTES was too large to graft onto the

Table 2Percentages of T- and Q-substitution of IPTES-MWCNT/polyimidecomposites

IPTES:MWCNT(in weight)

MWCNTcontent, wt%

% of Tsubstituted

% of Qsubstituted

1:1 0.99 89.22 10.781:1 6.98 84.02 15.982:1 0.99 92.08 7.922:1 6.98 80.34 19.663:1 0.99 84.86 15.143:1 6.98 76.00 24.00

MWCNT. The Si–C bonds of the ungrafted IPTES wereweaker than those of the grafted IPTES. When the ratioof IPTES to MWCNT was 2:1, most of the IPTES weregrafted onto the MWCNT and the excess number of–COOH functional groups matched the number of IPTES,minimizing the Q substitution ratio.

When the MWCNT content was high (6.98 wt%),the percentage of Q substitution was independent of the–COOH content, since the –COOH content was high.The quantity of ungrafted IPTES was proportional to theratio of IPTES to MWCNT and the percentage of Qsubstitution is proportional to the ungrafted IPTEScontent.

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With respect to the T substitution, the peak height of T2

ofthe composites with low MWCNT content was higherthan that with high MWCNT content. When the IPTES-MWCNT content was low, the silane did not easily reactwith the other silane because the molecular distance betweenIPTES and MWCNT was very large, and the interpenetrat-ing network could not be formed easily. However, when theIPTES-MWCNT content was high, the silane is more easilyreacted with the other silane because the molecular distancebetween the IPTES and MWCNT was reduced, facilitatingthe formation of an interpenetrating network.

3.3. Morphological characteristics

Fig. 3a–f are TEM microphotographs of the IPTES-MWCNT/polyimide composites. Fig. 3a and b demon-strates the IPTES-MWCNTs (ITES-MWCNT-1) coatedwith silicas, which look like ‘‘needles’’. Some of the ‘‘nee-dle-shaped’’ MWCNTs were connected with otherMWCNT and some of the MWCNTs were assembled asbundles. The diameters of the IPTES—MWCNTs (ITES-MWCNT-1) were in the range 58–125 nm and the lengthsof the IPTES-MWCNT-1 bundles were about 1.5 lm to2.0 lm (as indicated by blocks). Fig. 3c and d presentsthe TEM microphotographs of IPTES-MWCNT-2/poly-imide composites. The SiOx was grafted on the IPTES-MWCNT at the ‘‘connecting junction’’. Fig. 3c and ddisplays some MWCNTs were aggregated as bundles andthe length of the IPTES-MWCNT-2 bundles were about

Fig. 3. TEM microphotograph of 6.98 wt% IPTES-MWCNT/Polyimide nanoc(10,000·), (d) 2:1 (50,000·) (e) 3:1 (10,000·), (f) 3:1 (50,000·). The arrows ind

0.6 lm to 2.0 lm (as indicated by block), some of whichwere connected with each other. Fig. 3c demonstrates thatsome of the ungrafted SiOx look like ‘‘cotton tips’’ whichwere dispersed in the polyimide matrix. Fig. 3d indicatesthat the diameter of the MWCNTs was approximately30 nm. The junction point between each pair of MWCNTscan be found and indicated by ‘‘arrows’’ as shown inFig. 3b, d and f. The size of the junction point was around83 nm.The MWCNT may be connected by –Si–O–Si– func-tional groups. Fig. 3e and f presents the TEM microphoto-graphs of the IPTES-MWCNT-3/polyimide composites.Fig. 3e shows the IPTES- MWCNT-3 dispersed uniformlyin the polymer matrix. The SiOx was aggregated at thejunction of connected MWCNT. Fig. 3f reveals that thediameter of the MWCNT was about 30 nm. It indicatesthat SiOx does not coat on the surface of MWCNTs. TheSiOx was partially grafted on the MWCNTs with diametersof approximately 116-208 nm.

The lengths of the MWCNT bundle were shorter thanthat of the IPTES-MWCNT-1 and IPTES-MWCNT-2.The lengths of the IPTES-MWCNT-3 bundles are about0.5 lm to 1.5 lm. The TEM microphotograph in our ear-lier study [17] showed the acid modified MWCNT haslength about 0.5 to 1.0 lm and the acid-modifiedMWCNTs were straight and some of them were aggregatedin bundles, which were dispersed in the polymer matrix[17]. The supplier reported that the length of the unmodi-fied MWCNT was 0.5–40 lm. During acid modification,in HNO3/H2SO4 mixed acid condition, the mixed acid will

omposites with IPTES:MWCNT (a) 1:1 (10,000·), (b) 1:1 (50,000·) (c) 2:1icated the junction points.

S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848 2847

first attack the defect of CNTs at the wall and the caps.Then the mixed acid will create their defect and to eat awaythe CNTs. The CNTs will become shorter with acid mod-ification but more –COOH and –OH functional groups cre-ated. When the acid modified MWCNT grafted will silane,the silane modified MWCNT may be connected together inthe polyimide matrix. When IPTES:MWCNT was 1:1, allof the silanes were grafted on the acid-modified MWCNTsurface. When the composites were heated to 300 �C, thesilane on the MWCNTs were reacted and some of the Si–C bonds were broken. Some of the IPTES-MWCNT-1were connected and became ‘‘needles-shaped’’. WhenIPTES:MWCNT = 2:1, most of the IPTES were graftedto the surface of acid-modified MWCNTs, but some ofthem did not. The ungrafted silica was aggregated andmigrated to the MWCNT and aggregated on the MWCNTsurface. When a large quantity of IPTES was employed asin IPTES-MWCNT-3 system, the ungrafted silica migratedeasily and aggregated as a larger size.

3.4. Electrical properties

CNTs posses high aspect ratio and contain p-bonds(C@C bond). Charges may be transferred through the p-bond (C@C bond) in the CNTs. Adding a small quantityof CNT to the polymer matrix will reduces its surfaceand volume resistivity markedly. When MWCNT wasmodified by IPTES, although the silane grafted on theMWCNT, which may isolate the MWCNT, the silanefunctional group can connect with each other. Polyimidemolecules did not react with the MWCNT but interpene-trated into the connected IPTES-MWCNT network. Itincreases the pathway for electrical conductivity, hence,reduce the electrical resistivity.

Fig. 4a and b illustrates the surface and volume electricalresistivity of the IPTES-MWCNT/polyimide composites.When 6.98 wt% of acid-modified MWCNT was used, thesurface electrical resistivity of the composites decreased 6orders of magnitude (Fig. 4a). The volume resistivity of thecomposites decreased 9 orders of magnitude (Fig. 4b). Inthe IPTES-MWCNT system, the surface electrical resistivityof the composites decreased more significantly than that ofacid modified MWCNT. When 6.98 wt% of IPTES-MWCNT was added, the surface electrical resistivity

104

109

1014

Volu

me

resi

stiv

ity,

-cm

MWCNT 0 2

108

1010

1012

1014

1016

Surf

ace

resi

stiv

ity,

/cm

2

MWCNT content, wt%

Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3

1 3 4 5 6 7 0 21 3

Fig. 4. Effect of MWCNT content on of the IPTES-MWCNT/polyimide nderivatives of volume electrical resistivity.

decreased 7 orders of magnitude (IPTES-MWCNT-1 andIPTES-MWCNT-2) and 8 orders of magnitude (IPTES-MWCNT-3) and volume electrical resistivity decreased 11orders of magnitude (IPTES-MWCNT-1 and IPTES-MWCNT-2) and decreased 12 orders of magnitude(IPTES-MWCNT-1 and IPTES-MWCNT-2) (Fig. 4b).

Compared to acid modified MWCNTs, the IPTES-MWCNT-3/polyimide had higher surface resistivity andthe IPTES-MWCNT-1/polyimide had higher volume resis-tivity. When IPTES content was low ((IPTES:MWCNT =1:1 and MWCNT content is lower than 2.44 wt%), theIPTES-MWCNT network did not form easily. IPTES-MWCNT may disperse well if IPTES-MWCNT networkdoes not form and the MWCNT may be isolated, hence,the composite shows higher volume resistivity. However,when the silane on the MWCNT surface absorbed mist,the Si–OH functional groups will be formed, and decreasedthe surface resistivity.

When IPTES content was high (IPTES:MWCNT = 3:1but MWCNT content is lower than 2.44 wt%), IPTES-MWCNT network can be formed when MWCNT contentwas low. This may decrease the volume resistivity. How-ever, on the composites surface, the residual IPTES mayisolate the MWCNT. Moreover, during the formation ofIPTES-MWCNT network, some of MWCNT may bedetached from the surface, the MWCNT content on thesurface will be reduced.

When the IPTES-MWCNT network was formed whichwill provide more effective electrical pathways and the inter-penetration of the network may reduce the percolationthreshold of the MWCNT/polyimide composites. Electricalresistivity decreased rapidly as the percolation threshold.Comparing the acid-modified MWCNT/polyimide com-posites to the IPTES-MWCNT/polyimide composites, itcan be found that at high MWCNT content (more than4.76 wt%) all of the IPTES- MWCNT/polyimide compos-ites show lower electrical resistivity than that of acid-modified MWCNT. The percolation threshold of theacid-modified MWCNT/polyimide composites is higherthan 6.98 wt% MWCNT content. Electrical resistivitydecreased most rapidly at the percolation threshold [9].Percolations threshold was determined by plotting the firstderivatives of log(volume resistivity) as a function ofMWCNT content, where the MWCNT content corre-

content, wt%

Acid modified MWCNTIPTES-MWCNT-1IPTES-MWCNT-2IPTES-MWCNT-3

0 0.5 0.99 2.44 4.76 6.98

012345678

Acid modified MWCNTIPTES-MWCNT-1IPTES-MWCNT-2IPTES-MWCNT-3

Abs

olut

e de

rivat

ive

of

log

(vol

ume

resi

stiv

ity)

MWCNT content, wt%4 5 6 7

anocomposites (a) surface resistivity (b) volume resistivity (c) the first

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sponding to the highest absolute derivative is taken asthe percolation threshold [21]. In Fig. 4c, the highestabsolute derivative of the volume resistivity at the IPTES-MWCNT content was more than 6.98 wt% for acidmodified MWCNT/polyimide and 2.44 wt% for IPTES-MWCNT-1/polyimide, IPTES-MWCNT-2/polyimideand IPTES- MWCNT-3/polyimide composites, respec-tively. IPTES-MWCNT/polyimide composites have alower percolation threshold than that of acid-modifiedMWCNT/polyimide.

4. Conclusion

Silane-grafted acid-modified MWCNT was successfullyprepared by treating MWCNT with 3-isocyanatopropyl-triethoxysilane (IPTES) and then added to polyamic acid.FT-IR reveals that IPTES was successfully grafted onMWCNTs. The 29Si solid state NMR shows that T-substi-tuted and Q-substituted siloxane bonds are presented in theIPTES-MWCNT/polyimide composites. The Q-substitutedform may be associated with the breaking of Si–C bonds.The TEM microphotograph demonstrates that MWCNTnetworks were formed and the polyimide molecules mayinterpenetrate into the crosslinked CNT network. The sur-face and volume electrical resistivity of the MWCNT/poly-imide composites decreased more significantly whenIPTES-MWCNTs were used. The percolation thresholdof the IPTES-MWCNT/polyimide composites was lowerthan that of the acid-modified MWCNT/polyimide com-posites MWCNT content.

Acknowledgement

The authors would like to thank the National ScienceCouncil of the Taiwan, Republic of China, for financially

supporting this research under Contract No. NSC 94-2622-E-007-010-C13.

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