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polyimide composites – Preparation, morphological and electrical properties
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COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2842–2848 www.elsevier.com/locate/compscitech
Silane grafted MWCNT/polyimide composites – Preparation, morphological and electrical properties Siu-Ming Yuen a, Chen-Chi M. Ma b
a,*
, Chin-Lung Chiang
b
a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan Department of Industrial Safety and Health, HungKuang University, Salu, TaiChung, Taiwan
Received 21 September 2007; accepted 15 October 2007 Available 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 tetracarboxylic dianhydride. Acid modified multiwall carbon nanotubes (MWCNTs) were grafted with 3-isocyanato-propyltriethoxysilane. Silane grafted MWCNTs were then mixed with the polyamic acid and heated to 300 C to form a carbon nanotube/polyimide composite. During the imidization processes, the silanes on the MWCNT surface reacted with each other. TEM microphotographs shows that the silane grafted MWCNTs were connected. The composite material possesses an interpenetrating network in which polyimide molecules were interpenetrated into the MWCNT network. The electrical resistivity of silane grafted MWCNT/polyimide decreased very significantly compared 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 electron microscopy (TEM)
1. Introduction Carbon nanotubes (CNTs) have attracted interest since Iijima identified the structure of singlewalled carbon nanotube (SWCNT) in 1991 [1]. CNTs posses excellent mechanical properties, low density, high surface area, high chemical stability, electrical conductivity, and thermal conductivity [2–7]. The mechanical and electrical properties of the polymeric matrices are improved significantly by the addition of carbon nanotube [8–10]. Modified CNTs can enhance the adhesion between CNTs and polymer matrix. Acid modification is one of the most common methods of CNT modification. CNT can be modified by refluxing with nitric acid or a mixture of nitric acid and sulfuric acid. Carboxyl and hydroxyl functional groups are formed on the CNT surface during acid modification [11]. Acid-modified MWCNT can be *
Corresponding author. Tel.: +886 3 5713058; fax: +886 3 571 5408. E-mail address: [email protected] (C.-C. M. Ma).
0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.10.011
modified with silane coupling agent [12–14]. The silane will react with the hydroxyl groups (–OH) on the surface of MWCNTs. The oxidation of MWCNT may generate carboxylic groups (–COOH) rather than hydroxyl groups. Ma et. al. [12] and Vast et. al. [14] suggested that the acid modified MWCNT can generate more hydroxyl groups by reduction process. Valentini et al. [15] modified SWCNTs using CF4 plasma to obtain fluorinated SWCNT(fSWCNT). The f-MWCNT then reacted with APTES and the amine functional group of APTES was grafted on the f-MWCNT. Polyimide is a high-performance polymer owing to its high thermal stability, low dielectric constant and chemical resistance. Accordingly, it found applications in the composite and microelectronics industries [16]. Various investigations have been performed on CNT/polyimide composites [17]. In our earlier study [17], unmodified, acid modified, and amine modified MWCNT/polyimide composite has been prepared. MWCNT improved themechanical and electrical conductivity of polyimide [17]. However,
S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848
modification of MWCNT can not significantly increase the electrical conductivity without forming more effective electrical pathways. In this investigation, a precursor of polyimide, polyamic acid, was prepared by reacting 4,4 0 -oxydianiline(ODA) with 3,3 0 ,4,4 0 -benzophenone tetracarboxylic dianhydride(BTDA). Multi-walled carbon nanotubes (MWCNT) were modified using mixed acid H2SO4/HNO3. The modified MWCNTs were then reacted with 3-isocyanatopropyltriethoxysilane (IPTES). Silane functional groups were grafted on the acid-modified MWCNT (IPTES-MWCNT). The IPTES-MWCNT was well dispersed in polyamic acid before imidation at 300 C. When the IPTES-MWCNT/ polyamic acid was heated to 300 C, the silane molecules on the MWCNT surface were reacted and connected to other MWCNT. However, polyimide does not connect with MWCNT, but interpenetrates into the connected IPTESMWCNT network. The IPTES-MWCNT network reduces the electrical resistivity, which provides more effective electrical pathways. The IPTES-MWCNT has lower electrical resistivity than that of acid modified MWCNT with the same MWCNT loading. For example, the volume resistivity of 2.44 wt% IPTES-MWCNT-1/polymer was 6.94 · 108 X cm and 2.44 wt% acid modified MWCNT/polyimide was 9.25 · 1015 X cm. Addition of IPTES reduces the percolation threshold for the DC conductivity since the silane on the IPTES-MWCNT reacted with each other and caused the MWCNTs connect each other. The interpenetrates network formed in the polyimide matrix which provides electrical pathways easier than that of acid modified MWCNT.
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H2SO4 and HNO3 was refluxing at 50 C for 24 h. After the reaction, the modified MWCNT was filtrated and washed with pure water to remove the reside acid then dried in 100 C. It was reported previously that a prolonged acid treatment of CNT would significantly damage the CNT structure although this method has been widely used to modify of CNT [17–19]. The acid modified MWCNTs were mixed with 3-isocyanatopropyltriethoxysilane (IPTES) and then reacted with the 1 wt% of catalyst, triethylamine (TEA)(Scheme 1) at 25 C for 48 h. Table 1 summarizes the weight ratios of IPTES to MWCNT. 2.4. Preparation of carbon nanotubes/polyimide nanocomposites IPTES-MWCNTs were added to polyamic acid and heated to 60 C to remove the solvent and then heated to 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) spectra of CNT were recorded between 400 and 4000 cm1 on a Nicolet Avatar 320 FT-IR spectrometer, Nicolet Instrument Corporation, Madison, WI, USA. The sample was washed by THF to remove residual IPTES and milled
2. Experimental 2.1. Materials Multi-walled carbon nanotubes were obtained from the Nanotech Port Company, Shenzhen, China. The diameter of 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 - benzophenone tetracarboxylic dianhydride (BTDA) were obtained from Chris KEV Company, Inc., Terrance Leawood, KS, USA. 3-Isocyanatopropyltriethoxysilane (IPTES) was obtained from Lancaster Synthesis Co., Morecambe, England. Triethylamine was received from TEDIA Company, Fairfield, OH, USA. 2.2. Synthesis of precursors of polyimide (polyamic acid) Scheme 1. IPTES grafted of MWCNT.
The precursor of polyimide (polyamic acid) was prepared by reacting 4,4 0 -oxydianiline (ODA) with 3,3 0 ,4,4 0 benzophenone tetracarboxylic dianhydride (BTDA) in DMAc. 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 of H2SO4 and 160 g of HNO3. Then the mixture of MWCNT,
Table 1 The ratios of IPTES to acid modified MWCNT for IPTES modified MWCNT ID
IPTES:MWCNT (in weight)
IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
1:1 2:1 3:1
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S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848 -CH-, 3000-2800 cm-1
-COOH, 1720 cm-1
-
-COO , 1610-1550 cm-1
-COOH, 3650-3000 cm-1 4000
3500
3000
2500
2000
1500
Wavenumbers, cm-1
-NH-CH-
Scheme 2. Perparation of silane grafted carbon nanotubes/polyimide composites.
-Si-O-C4000
3000
2000
1000
0
Wavenumbers, cm-1
with KBr and was pressed as a tablet. A minimum of 32 scans was signal-averaged with a resolution of 2 cm1 at the 4000–400 cm1 range.
Fig. 1. FT-IR spectrum of (a) acid modified MWCNT, (b) isocyanatopropyltriethoxysilane(IPTES) modified MWCNT.
2.5.2. Morphological properties Morphological properties were examined using a transmission electron microscope (TEM) (JEOL-2000EX, Japan).
stretching, –OH in the primary alcohol, –CH stretching and –COOH stretching, respectively. The carboxylic groups stretch (COOH) appears at 1720 cm1. Fig. 1b displays the FT-IR spectra of isocyanatopropyltriethoxysilane- modified MWCNT. The wavenumbers 1100 cm1 and 3530–3400 cm1 corresponded to –SiO stretching and –NH stretching, respectively.
2.5.3. CP/MAS solid state 29Si nuclear magnetic resonance (NMR) spectroscopy The high-resolution 29Si solid-state NMR used was a BRUKER DSX 400 MHz NMR. The samples were ground into a fine powder. The 29Si CP/MAS NMR spectra of the composites were used to characterize the degree of condensation of the IPTES-MWCNT/polyimide interpenetrating network with various multi-walled carbon nanotube contents. 2.5.4. Measurements of electrical properties The surface and volume electrical resistivity were measured using an ULTRA Mesohmeter SM-8220, DKK TOA Corporation, Tokyo, Japan. The surface and volume electrical resistivity of the MWCNT/polyimide composites were measured after various amounts of MWCNT were added. The charge time was 30 s, and the current voltage of the measurements was 100 V. An average value was obtained 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 carbon nanotubes, with wavenumbers 1610–1550 cm1, 1075– 1010 cm1, 3650–3000 cm1 and 3000–2800 cm1, corresponding to the absorptions of COO– asymmetrical
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 spectra of IPTES-MWCNT/polyimide composites. These figures indicate that tri-substituted siloxane bonds (T2 shift, d 59.84 ppm, and T3 shift, d 67.002 ppm) and some tetra-substituted siloxane bonds (Q4, Q3 and Q2 shift as 109.13, 101 and 91 ppm) are presented in the IPTESMWCNT/polyimide composites [20]. Table 2 summarizes the percentages of T and Q substitution of the composites. The Q-substituted bond may be formed when the composites were heated to 300 C and the Si–C bonds of the IPTES were broken. At low IPTES-MWCNT content (0.99 wt%), the percentage of substituted Q reaches a minimum at IPTES: MWCNT = 2:1. At high IPTESMWCNT content (6.98 wt%), the percentage of substituted Q is proportional to the ratio of IPTES to MWCNT. The –COOH functional groups on the acid treated MWCNT may cause the breaking of the Si–C bond. The density of the –COOH functional groups of the acid-treated MWCNT was around 2.6 mol/g, as determined by titration with NaOH solution. When the ratio of IPTES to MWCNT was lower (1:1), the –COOH functional groups of the acid-modified MWCNT were in excess.
S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848
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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%).
When the ratio of IPTES to MWCNT was higher (3:1), the quantity IPTES was too large to graft onto the Table 2 Percentages of T- and Q-substitution of IPTES-MWCNT/polyimide composites IPTES:MWCNT (in weight)
MWCNT content, wt%
% of T substituted
% of Q substituted
1:1 1:1 2:1 2:1 3:1 3:1
0.99 6.98 0.99 6.98 0.99 6.98
89.22 84.02 92.08 80.34 84.86 76.00
10.78 15.98 7.92 19.66 15.14 24.00
MWCNT. The Si–C bonds of the ungrafted IPTES were weaker than those of the grafted IPTES. When the ratio of IPTES to MWCNT was 2:1, most of the IPTES were grafted 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 the ratio of IPTES to MWCNT and the percentage of Q substitution is proportional to the ungrafted IPTES content.
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With respect to the T substitution, the peak height of T2 ofthe composites with low MWCNT content was higher than that with high MWCNT content. When the IPTESMWCNT content was low, the silane did not easily react with the other silane because the molecular distance between IPTES and MWCNT was very large, and the interpenetrating network could not be formed easily. However, when the IPTES-MWCNT content was high, the silane is more easily reacted with the other silane because the molecular distance between the IPTES and MWCNT was reduced, facilitating the formation of an interpenetrating network. 3.3. Morphological characteristics Fig. 3a–f are TEM microphotographs of the IPTESMWCNT/polyimide composites. Fig. 3a and b demonstrates the IPTES-MWCNTs (ITES-MWCNT-1) coated with silicas, which look like ‘‘needles’’. Some of the ‘‘needle-shaped’’ MWCNTs were connected with other MWCNT and some of the MWCNTs were assembled as bundles. The diameters of the IPTES—MWCNTs (ITESMWCNT-1) were in the range 58–125 nm and the lengths of the IPTES-MWCNT-1 bundles were about 1.5 lm to 2.0 lm (as indicated by blocks). Fig. 3c and d presents the TEM microphotographs of IPTES-MWCNT-2/polyimide composites. The SiOx was grafted on the IPTESMWCNT at the ‘‘connecting junction’’. Fig. 3c and d displays some MWCNTs were aggregated as bundles and the length of the IPTES-MWCNT-2 bundles were about
0.6 lm to 2.0 lm (as indicated by block), some of which were connected with each other. Fig. 3c demonstrates that some of the ungrafted SiOx look like ‘‘cotton tips’’ which were dispersed in the polyimide matrix. Fig. 3d indicates that the diameter of the MWCNTs was approximately 30 nm. The junction point between each pair of MWCNTs can be found and indicated by ‘‘arrows’’ as shown in Fig. 3b, d and f. The size of the junction point was around 83 nm.The MWCNT may be connected by –Si–O–Si– functional groups. Fig. 3e and f presents the TEM microphotographs of the IPTES-MWCNT-3/polyimide composites. Fig. 3e shows the IPTES- MWCNT-3 dispersed uniformly in the polymer matrix. The SiOx was aggregated at the junction of connected MWCNT. Fig. 3f reveals that the diameter of the MWCNT was about 30 nm. It indicates that SiOx does not coat on the surface of MWCNTs. The SiOx was partially grafted on the MWCNTs with diameters of approximately 116-208 nm. The lengths of the MWCNT bundle were shorter than that of the IPTES-MWCNT-1 and IPTES-MWCNT-2. The lengths of the IPTES-MWCNT-3 bundles are about 0.5 lm to 1.5 lm. The TEM microphotograph in our earlier study [17] showed the acid modified MWCNT has length about 0.5 to 1.0 lm and the acid-modified MWCNTs were straight and some of them were aggregated in bundles, which were dispersed in the polymer matrix [17]. The supplier reported that the length of the unmodified MWCNT was 0.5–40 lm. During acid modification, in HNO3/H2SO4 mixed acid condition, the mixed acid will
Fig. 3. TEM microphotograph of 6.98 wt% IPTES-MWCNT/Polyimide nanocomposites with IPTES:MWCNT (a) 1:1 (10,000·), (b) 1:1 (50,000·) (c) 2:1 (10,000·), (d) 2:1 (50,000·) (e) 3:1 (10,000·), (f) 3:1 (50,000·). The arrows indicated the junction points.
S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848
first attack the defect of CNTs at the wall and the caps. Then the mixed acid will create their defect and to eat away the CNTs. The CNTs will become shorter with acid modification but more –COOH and –OH functional groups created. When the acid modified MWCNT grafted will silane, the silane modified MWCNT may be connected together in the polyimide matrix. When IPTES:MWCNT was 1:1, all of the silanes were grafted on the acid-modified MWCNT surface. When the composites were heated to 300 C, the silane on the MWCNTs were reacted and some of the Si– C bonds were broken. Some of the IPTES-MWCNT-1 were connected and became ‘‘needles-shaped’’. When IPTES:MWCNT = 2:1, most of the IPTES were grafted to the surface of acid-modified MWCNTs, but some of them did not. The ungrafted silica was aggregated and migrated to the MWCNT and aggregated on the MWCNT surface. When a large quantity of IPTES was employed as in IPTES-MWCNT-3 system, the ungrafted silica migrated easily and aggregated as a larger size.
decreased 7 orders of magnitude (IPTES-MWCNT-1 and IPTES-MWCNT-2) and 8 orders of magnitude (IPTESMWCNT-3) and volume electrical resistivity decreased 11 orders of magnitude (IPTES-MWCNT-1 and IPTESMWCNT-2) and decreased 12 orders of magnitude (IPTES-MWCNT-1 and IPTES-MWCNT-2) (Fig. 4b). Compared to acid modified MWCNTs, the IPTESMWCNT-3/polyimide had higher surface resistivity and the IPTES-MWCNT-1/polyimide had higher volume resistivity. When IPTES content was low ((IPTES:MWCNT = 1:1 and MWCNT content is lower than 2.44 wt%), the IPTES-MWCNT network did not form easily. IPTESMWCNT may disperse well if IPTES-MWCNT network does 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 decreased the surface resistivity. When IPTES content was high (IPTES:MWCNT = 3:1 but MWCNT content is lower than 2.44 wt%), IPTESMWCNT network can be formed when MWCNT content was low. This may decrease the volume resistivity. However, on the composites surface, the residual IPTES may isolate the MWCNT. Moreover, during the formation of IPTES-MWCNT network, some of MWCNT may be detached from the surface, the MWCNT content on the surface will be reduced. When the IPTES-MWCNT network was formed which will provide more effective electrical pathways and the interpenetration of the network may reduce the percolation threshold of the MWCNT/polyimide composites. Electrical resistivity decreased rapidly as the percolation threshold. Comparing the acid-modified MWCNT/polyimide composites to the IPTES-MWCNT/polyimide composites, it can be found that at high MWCNT content (more than 4.76 wt%) all of the IPTES- MWCNT/polyimide composites show lower electrical resistivity than that of acidmodified MWCNT. The percolation threshold of the acid-modified MWCNT/polyimide composites is higher than 6.98 wt% MWCNT content. Electrical resistivity decreased most rapidly at the percolation threshold [9]. Percolations threshold was determined by plotting the first derivatives of log(volume resistivity) as a function of MWCNT content, where the MWCNT content corre-
10
14
10
12
10
10
10
8
Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
-cm
16
14
Volume resistivity,
Surface resistivity,
/cm2
CNTs posses high aspect ratio and contain p-bonds (C@C bond). Charges may be transferred through the pbond (C@C bond) in the CNTs. Adding a small quantity of CNT to the polymer matrix will reduces its surface and volume resistivity markedly. When MWCNT was modified by IPTES, although the silane grafted on the MWCNT, which may isolate the MWCNT, the silane functional group can connect with each other. Polyimide molecules did not react with the MWCNT but interpenetrated into the connected IPTES-MWCNT network. It increases the pathway for electrical conductivity, hence, reduce the electrical resistivity. Fig. 4a and b illustrates the surface and volume electrical resistivity of the IPTES-MWCNT/polyimide composites. When 6.98 wt% of acid-modified MWCNT was used, the surface electrical resistivity of the composites decreased 6 orders of magnitude (Fig. 4a). The volume resistivity of the composites decreased 9 orders of magnitude (Fig. 4b). In the IPTES-MWCNT system, the surface electrical resistivity of the composites decreased more significantly than that of acid modified MWCNT. When 6.98 wt% of IPTESMWCNT was added, the surface electrical resistivity
10
9
10
8
Absolute derivative of log (volume resistivity)
3.4. Electrical properties
10
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Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
7 6 5 4 3 2 1 0
4
10 0
1
2
3
4
5
MWCNT content, wt%
6
7
0
1
2
3
4
5
MWCNT content, wt%
6
7
0
0.5
0.99
2.44
4.76
6.98
MWCNT content, wt%
Fig. 4. Effect of MWCNT content on of the IPTES-MWCNT/polyimide nanocomposites (a) surface resistivity (b) volume resistivity (c) the first derivatives of volume electrical resistivity.
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sponding to the highest absolute derivative is taken as the percolation threshold [21]. In Fig. 4c, the highest absolute derivative of the volume resistivity at the IPTESMWCNT content was more than 6.98 wt% for acid modified MWCNT/polyimide and 2.44 wt% for IPTESMWCNT-1/polyimide, IPTES-MWCNT-2/polyimide and IPTES- MWCNT-3/polyimide composites, respectively. IPTES-MWCNT/polyimide composites have a lower percolation threshold than that of acid-modified MWCNT/polyimide. 4. Conclusion Silane-grafted acid-modified MWCNT was successfully prepared by treating MWCNT with 3-isocyanatopropyltriethoxysilane (IPTES) and then added to polyamic acid. FT-IR reveals that IPTES was successfully grafted on MWCNTs. The 29Si solid state NMR shows that T-substituted and Q-substituted siloxane bonds are presented in the IPTES-MWCNT/polyimide composites. The Q-substituted form may be associated with the breaking of Si–C bonds. The TEM microphotograph demonstrates that MWCNT networks were formed and the polyimide molecules may interpenetrate into the crosslinked CNT network. The surface and volume electrical resistivity of the MWCNT/polyimide composites decreased more significantly when IPTES-MWCNTs were used. The percolation threshold of the IPTES-MWCNT/polyimide composites was lower than that of the acid-modified MWCNT/polyimide composites MWCNT content. Acknowledgement The authors would like to thank the National Science Council of the Taiwan, Republic of China, for financially
supporting this research under Contract No. NSC 942622-E-007-010-C13. References [1] Iijima S. Nature 1991;354:56–8. [2] Sandler J, Shaffer M, Prasse T, Bauhofer W, Schulte K, Windle AH. Polymer 1999;40:5967–71. [3] Geng H, Rosen R, Zheng B, Shimoda H, Fleming L, Liu JZ. Adv Mater 2002;14:1387–90. [4] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. San Diego USA: Academic Press; 1996. [5] Beguin F, Ehrburger P. Carbon 2002;40:1619. [6] Subramoney S. Novel nanocarbons. Advanced materials 1998;10: 1157–73. [7] Yakobson BI, Smalley RE. Am Scientist 1997;85:324–30. [8] Thostenson ET, Chou TW. Carbon 2006;44(14):3022–9. [9] Hu GJ, Zhao CG, Zhang SM, Yang MS, Wang ZG. Polymer 2006;47:480–8. [10] Lau KT, Lu M, Lam CK, Cheung HY, Sheng FL, Li HL. Compos Sci Technol 2005;65:719–25. [11] Hirsch A. Angewandte Chemie International Edition 2002;41:1853–9. [12] Ma PC, Kim JK, Tang BZ. Carbon 2006;44:3232–8. [13] Velasco-Santos C, Martınez-Hernandez AL, Lozada-Cassou M. Nanotechnology 2002;13:495–8. [14] Vast L, Philippin G, Destree A, Moreau N, Fonseca A, Nagy JB, et al. Nanotechnology 2004;15:781–5. [15] Valentini L, Macan J, Armentano I, Mengoni F, Kenny JM. Carbon 2006;44:2196–201. [16] Satou S, Suzuki H, Makino D. Polyimides for semiconductor applications. In: Wilson W, Stenzenberger HD, Hergenrother PM, editors. Polyimides, New York: Chapman and Hall; p. 227–51. [17] Yuen SM, Ma CCM, Lin YY, Kuan HC. Compos Sci Technol 2007:2564–73. [18] Andrews R, Weisenberger MC. Current Opin Solid State Mater Sci 2004;8:31–7. [19] Sham ML, Kim JK. Carbon 2006;44:768–77. [20] Shea KJ, Loy DA, Webster O. J Am Chem Soc 1992;114:6700–10. [21] Li J, Sham ML, Kim JK, Marom G. Compos Sci Technol 2007;67:296–305.
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2842–2848 www.elsevier.com/locate/compscitech
Silane grafted MWCNT/polyimide composites – Preparation, morphological and electrical properties Siu-Ming Yuen a, Chen-Chi M. Ma b
a,*
, Chin-Lung Chiang
b
a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu, Taiwan Department of Industrial Safety and Health, HungKuang University, Salu, TaiChung, Taiwan
Received 21 September 2007; accepted 15 October 2007 Available 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 tetracarboxylic dianhydride. Acid modified multiwall carbon nanotubes (MWCNTs) were grafted with 3-isocyanato-propyltriethoxysilane. Silane grafted MWCNTs were then mixed with the polyamic acid and heated to 300 C to form a carbon nanotube/polyimide composite. During the imidization processes, the silanes on the MWCNT surface reacted with each other. TEM microphotographs shows that the silane grafted MWCNTs were connected. The composite material possesses an interpenetrating network in which polyimide molecules were interpenetrated into the MWCNT network. The electrical resistivity of silane grafted MWCNT/polyimide decreased very significantly compared 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 electron microscopy (TEM)
1. Introduction Carbon nanotubes (CNTs) have attracted interest since Iijima identified the structure of singlewalled carbon nanotube (SWCNT) in 1991 [1]. CNTs posses excellent mechanical properties, low density, high surface area, high chemical stability, electrical conductivity, and thermal conductivity [2–7]. The mechanical and electrical properties of the polymeric matrices are improved significantly by the addition of carbon nanotube [8–10]. Modified CNTs can enhance the adhesion between CNTs and polymer matrix. Acid modification is one of the most common methods of CNT modification. CNT can be modified by refluxing with nitric acid or a mixture of nitric acid and sulfuric acid. Carboxyl and hydroxyl functional groups are formed on the CNT surface during acid modification [11]. Acid-modified MWCNT can be *
Corresponding author. Tel.: +886 3 5713058; fax: +886 3 571 5408. E-mail address: [email protected] (C.-C. M. Ma).
0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.10.011
modified with silane coupling agent [12–14]. The silane will react with the hydroxyl groups (–OH) on the surface of MWCNTs. The oxidation of MWCNT may generate carboxylic groups (–COOH) rather than hydroxyl groups. Ma et. al. [12] and Vast et. al. [14] suggested that the acid modified MWCNT can generate more hydroxyl groups by reduction process. Valentini et al. [15] modified SWCNTs using CF4 plasma to obtain fluorinated SWCNT(fSWCNT). The f-MWCNT then reacted with APTES and the amine functional group of APTES was grafted on the f-MWCNT. Polyimide is a high-performance polymer owing to its high thermal stability, low dielectric constant and chemical resistance. Accordingly, it found applications in the composite and microelectronics industries [16]. Various investigations have been performed on CNT/polyimide composites [17]. In our earlier study [17], unmodified, acid modified, and amine modified MWCNT/polyimide composite has been prepared. MWCNT improved themechanical and electrical conductivity of polyimide [17]. However,
S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848
modification of MWCNT can not significantly increase the electrical conductivity without forming more effective electrical pathways. In this investigation, a precursor of polyimide, polyamic acid, was prepared by reacting 4,4 0 -oxydianiline(ODA) with 3,3 0 ,4,4 0 -benzophenone tetracarboxylic dianhydride(BTDA). Multi-walled carbon nanotubes (MWCNT) were modified using mixed acid H2SO4/HNO3. The modified MWCNTs were then reacted with 3-isocyanatopropyltriethoxysilane (IPTES). Silane functional groups were grafted on the acid-modified MWCNT (IPTES-MWCNT). The IPTES-MWCNT was well dispersed in polyamic acid before imidation at 300 C. When the IPTES-MWCNT/ polyamic acid was heated to 300 C, the silane molecules on the MWCNT surface were reacted and connected to other MWCNT. However, polyimide does not connect with MWCNT, but interpenetrates into the connected IPTESMWCNT network. The IPTES-MWCNT network reduces the electrical resistivity, which provides more effective electrical pathways. The IPTES-MWCNT has lower electrical resistivity than that of acid modified MWCNT with the same MWCNT loading. For example, the volume resistivity of 2.44 wt% IPTES-MWCNT-1/polymer was 6.94 · 108 X cm and 2.44 wt% acid modified MWCNT/polyimide was 9.25 · 1015 X cm. Addition of IPTES reduces the percolation threshold for the DC conductivity since the silane on the IPTES-MWCNT reacted with each other and caused the MWCNTs connect each other. The interpenetrates network formed in the polyimide matrix which provides electrical pathways easier than that of acid modified MWCNT.
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H2SO4 and HNO3 was refluxing at 50 C for 24 h. After the reaction, the modified MWCNT was filtrated and washed with pure water to remove the reside acid then dried in 100 C. It was reported previously that a prolonged acid treatment of CNT would significantly damage the CNT structure although this method has been widely used to modify of CNT [17–19]. The acid modified MWCNTs were mixed with 3-isocyanatopropyltriethoxysilane (IPTES) and then reacted with the 1 wt% of catalyst, triethylamine (TEA)(Scheme 1) at 25 C for 48 h. Table 1 summarizes the weight ratios of IPTES to MWCNT. 2.4. Preparation of carbon nanotubes/polyimide nanocomposites IPTES-MWCNTs were added to polyamic acid and heated to 60 C to remove the solvent and then heated to 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) spectra of CNT were recorded between 400 and 4000 cm1 on a Nicolet Avatar 320 FT-IR spectrometer, Nicolet Instrument Corporation, Madison, WI, USA. The sample was washed by THF to remove residual IPTES and milled
2. Experimental 2.1. Materials Multi-walled carbon nanotubes were obtained from the Nanotech Port Company, Shenzhen, China. The diameter of 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 - benzophenone tetracarboxylic dianhydride (BTDA) were obtained from Chris KEV Company, Inc., Terrance Leawood, KS, USA. 3-Isocyanatopropyltriethoxysilane (IPTES) was obtained from Lancaster Synthesis Co., Morecambe, England. Triethylamine was received from TEDIA Company, Fairfield, OH, USA. 2.2. Synthesis of precursors of polyimide (polyamic acid) Scheme 1. IPTES grafted of MWCNT.
The precursor of polyimide (polyamic acid) was prepared by reacting 4,4 0 -oxydianiline (ODA) with 3,3 0 ,4,4 0 benzophenone tetracarboxylic dianhydride (BTDA) in DMAc. 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 of H2SO4 and 160 g of HNO3. Then the mixture of MWCNT,
Table 1 The ratios of IPTES to acid modified MWCNT for IPTES modified MWCNT ID
IPTES:MWCNT (in weight)
IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
1:1 2:1 3:1
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S.-M. Yuen et al. / Composites Science and Technology 68 (2008) 2842–2848 -CH-, 3000-2800 cm-1
-COOH, 1720 cm-1
-
-COO , 1610-1550 cm-1
-COOH, 3650-3000 cm-1 4000
3500
3000
2500
2000
1500
Wavenumbers, cm-1
-NH-CH-
Scheme 2. Perparation of silane grafted carbon nanotubes/polyimide composites.
-Si-O-C4000
3000
2000
1000
0
Wavenumbers, cm-1
with KBr and was pressed as a tablet. A minimum of 32 scans was signal-averaged with a resolution of 2 cm1 at the 4000–400 cm1 range.
Fig. 1. FT-IR spectrum of (a) acid modified MWCNT, (b) isocyanatopropyltriethoxysilane(IPTES) modified MWCNT.
2.5.2. Morphological properties Morphological properties were examined using a transmission electron microscope (TEM) (JEOL-2000EX, Japan).
stretching, –OH in the primary alcohol, –CH stretching and –COOH stretching, respectively. The carboxylic groups stretch (COOH) appears at 1720 cm1. Fig. 1b displays the FT-IR spectra of isocyanatopropyltriethoxysilane- modified MWCNT. The wavenumbers 1100 cm1 and 3530–3400 cm1 corresponded to –SiO stretching and –NH stretching, respectively.
2.5.3. CP/MAS solid state 29Si nuclear magnetic resonance (NMR) spectroscopy The high-resolution 29Si solid-state NMR used was a BRUKER DSX 400 MHz NMR. The samples were ground into a fine powder. The 29Si CP/MAS NMR spectra of the composites were used to characterize the degree of condensation of the IPTES-MWCNT/polyimide interpenetrating network with various multi-walled carbon nanotube contents. 2.5.4. Measurements of electrical properties The surface and volume electrical resistivity were measured using an ULTRA Mesohmeter SM-8220, DKK TOA Corporation, Tokyo, Japan. The surface and volume electrical resistivity of the MWCNT/polyimide composites were measured after various amounts of MWCNT were added. The charge time was 30 s, and the current voltage of the measurements was 100 V. An average value was obtained 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 carbon nanotubes, with wavenumbers 1610–1550 cm1, 1075– 1010 cm1, 3650–3000 cm1 and 3000–2800 cm1, corresponding to the absorptions of COO– asymmetrical
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 spectra of IPTES-MWCNT/polyimide composites. These figures indicate that tri-substituted siloxane bonds (T2 shift, d 59.84 ppm, and T3 shift, d 67.002 ppm) and some tetra-substituted siloxane bonds (Q4, Q3 and Q2 shift as 109.13, 101 and 91 ppm) are presented in the IPTESMWCNT/polyimide composites [20]. Table 2 summarizes the percentages of T and Q substitution of the composites. The Q-substituted bond may be formed when the composites were heated to 300 C and the Si–C bonds of the IPTES were broken. At low IPTES-MWCNT content (0.99 wt%), the percentage of substituted Q reaches a minimum at IPTES: MWCNT = 2:1. At high IPTESMWCNT content (6.98 wt%), the percentage of substituted Q is proportional to the ratio of IPTES to MWCNT. The –COOH functional groups on the acid treated MWCNT may cause the breaking of the Si–C bond. The density of the –COOH functional groups of the acid-treated MWCNT was around 2.6 mol/g, as determined by titration with NaOH solution. When the ratio of IPTES to MWCNT was lower (1:1), the –COOH functional groups of the acid-modified MWCNT were in excess.
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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%).
When the ratio of IPTES to MWCNT was higher (3:1), the quantity IPTES was too large to graft onto the Table 2 Percentages of T- and Q-substitution of IPTES-MWCNT/polyimide composites IPTES:MWCNT (in weight)
MWCNT content, wt%
% of T substituted
% of Q substituted
1:1 1:1 2:1 2:1 3:1 3:1
0.99 6.98 0.99 6.98 0.99 6.98
89.22 84.02 92.08 80.34 84.86 76.00
10.78 15.98 7.92 19.66 15.14 24.00
MWCNT. The Si–C bonds of the ungrafted IPTES were weaker than those of the grafted IPTES. When the ratio of IPTES to MWCNT was 2:1, most of the IPTES were grafted 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 the ratio of IPTES to MWCNT and the percentage of Q substitution is proportional to the ungrafted IPTES content.
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With respect to the T substitution, the peak height of T2 ofthe composites with low MWCNT content was higher than that with high MWCNT content. When the IPTESMWCNT content was low, the silane did not easily react with the other silane because the molecular distance between IPTES and MWCNT was very large, and the interpenetrating network could not be formed easily. However, when the IPTES-MWCNT content was high, the silane is more easily reacted with the other silane because the molecular distance between the IPTES and MWCNT was reduced, facilitating the formation of an interpenetrating network. 3.3. Morphological characteristics Fig. 3a–f are TEM microphotographs of the IPTESMWCNT/polyimide composites. Fig. 3a and b demonstrates the IPTES-MWCNTs (ITES-MWCNT-1) coated with silicas, which look like ‘‘needles’’. Some of the ‘‘needle-shaped’’ MWCNTs were connected with other MWCNT and some of the MWCNTs were assembled as bundles. The diameters of the IPTES—MWCNTs (ITESMWCNT-1) were in the range 58–125 nm and the lengths of the IPTES-MWCNT-1 bundles were about 1.5 lm to 2.0 lm (as indicated by blocks). Fig. 3c and d presents the TEM microphotographs of IPTES-MWCNT-2/polyimide composites. The SiOx was grafted on the IPTESMWCNT at the ‘‘connecting junction’’. Fig. 3c and d displays some MWCNTs were aggregated as bundles and the length of the IPTES-MWCNT-2 bundles were about
0.6 lm to 2.0 lm (as indicated by block), some of which were connected with each other. Fig. 3c demonstrates that some of the ungrafted SiOx look like ‘‘cotton tips’’ which were dispersed in the polyimide matrix. Fig. 3d indicates that the diameter of the MWCNTs was approximately 30 nm. The junction point between each pair of MWCNTs can be found and indicated by ‘‘arrows’’ as shown in Fig. 3b, d and f. The size of the junction point was around 83 nm.The MWCNT may be connected by –Si–O–Si– functional groups. Fig. 3e and f presents the TEM microphotographs of the IPTES-MWCNT-3/polyimide composites. Fig. 3e shows the IPTES- MWCNT-3 dispersed uniformly in the polymer matrix. The SiOx was aggregated at the junction of connected MWCNT. Fig. 3f reveals that the diameter of the MWCNT was about 30 nm. It indicates that SiOx does not coat on the surface of MWCNTs. The SiOx was partially grafted on the MWCNTs with diameters of approximately 116-208 nm. The lengths of the MWCNT bundle were shorter than that of the IPTES-MWCNT-1 and IPTES-MWCNT-2. The lengths of the IPTES-MWCNT-3 bundles are about 0.5 lm to 1.5 lm. The TEM microphotograph in our earlier study [17] showed the acid modified MWCNT has length about 0.5 to 1.0 lm and the acid-modified MWCNTs were straight and some of them were aggregated in bundles, which were dispersed in the polymer matrix [17]. The supplier reported that the length of the unmodified MWCNT was 0.5–40 lm. During acid modification, in HNO3/H2SO4 mixed acid condition, the mixed acid will
Fig. 3. TEM microphotograph of 6.98 wt% IPTES-MWCNT/Polyimide nanocomposites with IPTES:MWCNT (a) 1:1 (10,000·), (b) 1:1 (50,000·) (c) 2:1 (10,000·), (d) 2:1 (50,000·) (e) 3:1 (10,000·), (f) 3:1 (50,000·). The arrows indicated the junction points.
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first attack the defect of CNTs at the wall and the caps. Then the mixed acid will create their defect and to eat away the CNTs. The CNTs will become shorter with acid modification but more –COOH and –OH functional groups created. When the acid modified MWCNT grafted will silane, the silane modified MWCNT may be connected together in the polyimide matrix. When IPTES:MWCNT was 1:1, all of the silanes were grafted on the acid-modified MWCNT surface. When the composites were heated to 300 C, the silane on the MWCNTs were reacted and some of the Si– C bonds were broken. Some of the IPTES-MWCNT-1 were connected and became ‘‘needles-shaped’’. When IPTES:MWCNT = 2:1, most of the IPTES were grafted to the surface of acid-modified MWCNTs, but some of them did not. The ungrafted silica was aggregated and migrated to the MWCNT and aggregated on the MWCNT surface. When a large quantity of IPTES was employed as in IPTES-MWCNT-3 system, the ungrafted silica migrated easily and aggregated as a larger size.
decreased 7 orders of magnitude (IPTES-MWCNT-1 and IPTES-MWCNT-2) and 8 orders of magnitude (IPTESMWCNT-3) and volume electrical resistivity decreased 11 orders of magnitude (IPTES-MWCNT-1 and IPTESMWCNT-2) and decreased 12 orders of magnitude (IPTES-MWCNT-1 and IPTES-MWCNT-2) (Fig. 4b). Compared to acid modified MWCNTs, the IPTESMWCNT-3/polyimide had higher surface resistivity and the IPTES-MWCNT-1/polyimide had higher volume resistivity. When IPTES content was low ((IPTES:MWCNT = 1:1 and MWCNT content is lower than 2.44 wt%), the IPTES-MWCNT network did not form easily. IPTESMWCNT may disperse well if IPTES-MWCNT network does 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 decreased the surface resistivity. When IPTES content was high (IPTES:MWCNT = 3:1 but MWCNT content is lower than 2.44 wt%), IPTESMWCNT network can be formed when MWCNT content was low. This may decrease the volume resistivity. However, on the composites surface, the residual IPTES may isolate the MWCNT. Moreover, during the formation of IPTES-MWCNT network, some of MWCNT may be detached from the surface, the MWCNT content on the surface will be reduced. When the IPTES-MWCNT network was formed which will provide more effective electrical pathways and the interpenetration of the network may reduce the percolation threshold of the MWCNT/polyimide composites. Electrical resistivity decreased rapidly as the percolation threshold. Comparing the acid-modified MWCNT/polyimide composites to the IPTES-MWCNT/polyimide composites, it can be found that at high MWCNT content (more than 4.76 wt%) all of the IPTES- MWCNT/polyimide composites show lower electrical resistivity than that of acidmodified MWCNT. The percolation threshold of the acid-modified MWCNT/polyimide composites is higher than 6.98 wt% MWCNT content. Electrical resistivity decreased most rapidly at the percolation threshold [9]. Percolations threshold was determined by plotting the first derivatives of log(volume resistivity) as a function of MWCNT content, where the MWCNT content corre-
10
14
10
12
10
10
10
8
Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
-cm
16
14
Volume resistivity,
Surface resistivity,
/cm2
CNTs posses high aspect ratio and contain p-bonds (C@C bond). Charges may be transferred through the pbond (C@C bond) in the CNTs. Adding a small quantity of CNT to the polymer matrix will reduces its surface and volume resistivity markedly. When MWCNT was modified by IPTES, although the silane grafted on the MWCNT, which may isolate the MWCNT, the silane functional group can connect with each other. Polyimide molecules did not react with the MWCNT but interpenetrated into the connected IPTES-MWCNT network. It increases the pathway for electrical conductivity, hence, reduce the electrical resistivity. Fig. 4a and b illustrates the surface and volume electrical resistivity of the IPTES-MWCNT/polyimide composites. When 6.98 wt% of acid-modified MWCNT was used, the surface electrical resistivity of the composites decreased 6 orders of magnitude (Fig. 4a). The volume resistivity of the composites decreased 9 orders of magnitude (Fig. 4b). In the IPTES-MWCNT system, the surface electrical resistivity of the composites decreased more significantly than that of acid modified MWCNT. When 6.98 wt% of IPTESMWCNT was added, the surface electrical resistivity
10
9
10
8
Absolute derivative of log (volume resistivity)
3.4. Electrical properties
10
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Acid modified MWCNT IPTES-MWCNT-1 IPTES-MWCNT-2 IPTES-MWCNT-3
7 6 5 4 3 2 1 0
4
10 0
1
2
3
4
5
MWCNT content, wt%
6
7
0
1
2
3
4
5
MWCNT content, wt%
6
7
0
0.5
0.99
2.44
4.76
6.98
MWCNT content, wt%
Fig. 4. Effect of MWCNT content on of the IPTES-MWCNT/polyimide nanocomposites (a) surface resistivity (b) volume resistivity (c) the first derivatives of volume electrical resistivity.
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sponding to the highest absolute derivative is taken as the percolation threshold [21]. In Fig. 4c, the highest absolute derivative of the volume resistivity at the IPTESMWCNT content was more than 6.98 wt% for acid modified MWCNT/polyimide and 2.44 wt% for IPTESMWCNT-1/polyimide, IPTES-MWCNT-2/polyimide and IPTES- MWCNT-3/polyimide composites, respectively. IPTES-MWCNT/polyimide composites have a lower percolation threshold than that of acid-modified MWCNT/polyimide. 4. Conclusion Silane-grafted acid-modified MWCNT was successfully prepared by treating MWCNT with 3-isocyanatopropyltriethoxysilane (IPTES) and then added to polyamic acid. FT-IR reveals that IPTES was successfully grafted on MWCNTs. The 29Si solid state NMR shows that T-substituted and Q-substituted siloxane bonds are presented in the IPTES-MWCNT/polyimide composites. The Q-substituted form may be associated with the breaking of Si–C bonds. The TEM microphotograph demonstrates that MWCNT networks were formed and the polyimide molecules may interpenetrate into the crosslinked CNT network. The surface and volume electrical resistivity of the MWCNT/polyimide composites decreased more significantly when IPTES-MWCNTs were used. The percolation threshold of the IPTES-MWCNT/polyimide composites was lower than that of the acid-modified MWCNT/polyimide composites MWCNT content. Acknowledgement The authors would like to thank the National Science Council of the Taiwan, Republic of China, for financially
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