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Siu-Ming Yuen, Chen-Chi M. Ma, Chin-Lung Chiang, Chih-Chun Teng, "Morphology and Properties of Aminosilane Grafted MWCNT/Polyimide Nanocomposites", Journal of Nanomaterials, vol. 2008, Article ID 786405, 15 pages, 2008. https://doi.org/10.1155/2008/786405
Morphology and Properties of Aminosilane Grafted MWCNT/Polyimide Nanocomposites
This investigation presents a novel method for modifying multiwalled carbon nanotubes (MWCNTs). The morphology, electrical resistivity, and percolation threshold of MWCNT/Polyimide nanocomposites were studied. Acid-modified MWCNTs reacted with (3-aminopropyl)triethoxysilane by ionic bonding, and were then mixed with polyamic acid via imidization. TEM microphotographs reveal that silane-grafted MWCNTs were connected to each other. The electrical resistivity of silane-grafted MWCNT/polyimide decreased substantially below than that of acid-treated MWCNTs when the silane-modified MWCNT content was lower than 2.4 wt%. The percolation threshold of the MWCNT/polyimide composites is 1.0 wt% for silane-modified MWCNT and exceeds 7.0 wt% for acid-modified MWCNT. The acid-modified MWCNT/polyimide composites possess slightly higher glass transition temperatures than that of pure polyimide. The glass transition temperature of the polyimide increased significantly with silane-modified MWCNT content. Tensile properties of the polyimide have been improved with the MWCNTs content.
Polyimide is a high-performance polymer with high thermal stability, favorable dielectric properties, and chemical resistance. It has been found applications in the microelectronics and composites industries . Carbon nanotubes (CNTs) have attracted much research interest in various areas since their structures were identified in 1991 . They exhibit excellent mechanical and electrical properties, low density, high-surface area, and high chemical resistance [3–8]. CNT/polymer composites are interesting materials whose mechanical properties and electrical conductivity can be improved by the addition of CNT [9–13]. CNT/polyimide composites are interesting materials and have been extensively studied [14–17]. In our previous investigation,  unmodified-, acid-modified-, and amine-modified-MWCNT/polyimide nanocomposites were prepared and their morphology and electrical, thermal, and mechanical properties were examined .
CNT can be modified by refluxing with strong acid or a strong oxidizing agent. Carboxyl and hydroxyl functional groups are formed on the surface of CNTs during acid modification . Acid-modified MWCNT can be modified with silane [19–23]. Shanmugharaj et al.  grafted 3-aminopropyltriethoxysilane (APTES) to acid-modified MWCNT and prepared silane-modified MWCNT/natural rubber composites. They suggested that silane can be reacted with the hydroxyl groups (−OH) on the surface of MWCNTs [19–23]. The oxidation of MWCNT may generate carboxylic groups (−COOH) rather than hydroxyl groups. Valentini et al.  modified SWCNTs using CF4 plasma to obtain fluorinated SWCNT (f-SWCNT). The f-MWCNT then reacted with APTES and the amine functional group of APTES was grafted on the f-MWCNT. Our previous study has successfully modified MWCNT with silane [24–26]. The silane-modified MWCNTs/Poly (urea urethane) composites have been prepared. The molecular structure and molecular mobility of the carbon-nanotube/PUU nanocomposites have been investigated . These references indicated that carbon nanotube may be dispersed effectively. Increase the dispersion does not improve the electrical conductivity nor decrease the percolation threshold effectively.
In this study, acid-modified MWCNT was mixed with (3-aminopropyl)triethoxysilane(APTES). Silane functional groups are grafted on the acid-modified MWCNT (APTES-MWCNT) by ionic bonding or amide bonding, and some ungrafted APTES may react with polyamic acid to form a complex . After modification of the MWCNT, silane functional groups were remained. The APTES-MWCNT was dispersed in the polyamic acid which was imidized at 300°C. When the APTES-MWCNT/polyamic acid was heated to 300°C, the silane reacted on the MWCNT surface, and this reaction was examined. This work studies the electrical resistivity, the percolation threshold, and the thermal properties of the MWCNT/polyimide.
2. Experimental Section
Multiwalled carbon nanotubes were obtained from the Nanotech Port Company, Shenzhen, China. The diameters of the MWCNTs were 40–60 nm; their lengths were 0.5–40 m, and their special surface areas were 40–3000 m2/g. Both 4,-oxydianiline (ODA) and 3,,4,-benzophenone tetracarboxylic dianhydride (BTDA) were obtained from Chris KEV Company, Inc. Terrance Leawood, KS, USA. (3-Aminopropyl)triethoxysilane (APTES) was obtained from Lancaster Synthesis Co., Morecambe, England. N,N-Dimethylacetamide (DMAc) was obtained from Tedia Company Inc., Fairfield, OH, USA.
2.2. Synthesis of Polyamic Acid
The precursor of polyimide (polyamic acid) was prepared by reacting 4,-oxydianiline (ODA) with 3,,4,-benzophenone tetracarboxylic dianhydride (BTDA) in N,N-Dimethylacetamide (DMAc). The mole ratio of ODA to BTDA was . 4,-Oxydianiline (ODA) was dissolved in DMAc and then 3,,4,-benzophenone tetracarboxylic dianhydride (BTDA) was added to the 4,-oxydianiline (ODA) solution with an ice bath. The polyamic acid was imidized at 300°C to yield polyimide (as shown in Scheme 1).
2.3. Modification of MWCNT
Pristine MWCNTs were functionalized by refluxing with a mixture of H2SO4 and HNO3 (weight ratio of H2SO4 to HNO3 is ) at 50°C for 24 hours. After acid treatment, the MWCNTs were washed using deionized water, filtered and dried at 100°C. Then, the modified MWCNT was dispersed in DMAc and then (3-aminopropyl)-triethoxysilane (APTES) added to the mixture and was stirred for 1 hour (as presented in Scheme 2). Table 1 presents the mass ratio of APTES to MWCNT. The APTES-treated MWCNT is denoted APTES-MWCNT.
2.4. Preparation of Carbon Nanotubes/Polyimide Nanocomposites
APTES-MWCNT was added to polyamic acid then put on to a plastic plate and heated to 60°C to remove the solvent (DMAc). The mixture was then heated to 300°C to produce MWCNT/polyimide composites.
2.5. Measurement of Properties
2.5.1. Fourier Transform Infrared Spectroscopy
The Fourier transform infrared spectroscopy (FT-IR) spectra of unmodified and acid-modified MWCNT were recorded between 800 and 4000 cm−1 using a Nicolet Avatar 320 FT-IR spectrometer, from the Nicolet Instrument Corporation, Madison, WI, USA. The sample was dispersed in THF solvent and then coated on a CaF2 plate and dried in a vacuum oven at 120°C before it was tested. The spectra from a minimum of 32 scans were averaged with a signal resolution of 2 cm−1 within the range 400–4000 cm−1.
2.5.2. X-Ray Photoelectron Spectroscopy (XPS)
The XPS spectra of MWCNTs were obtained using a ULVAC-PHI, PHI Quantera with scanning monochromated A1 anode high-resolution X-ray photoelectron spectrometer.
2.5.3. CP/MAS Solid State 29Si Nuclear Magnetic Resonance (NMR) Spectroscopy
High-resolution 29Si solid-state NMR was conducted using a Bruker DSX 400 MHz NMR. The samples were ground into fine powder. The 29Si CP/MAS NMR spectra of the composites were obtained to characterize the degree of condensation of the APTES-MWCNT/polyimide interpenetrating network with various Multiwalled carbon nanotube contents.
2.5.4. Morphological Properties
Morphological properties were studied using an FE-SEM (S-4200) scanning electron microscope from Hitachi, Japan, and a JEOL-2000EX transmission electron microscope (TEM) from Japan.
2.5.5. Measurement of Electrical Properties
Surface and volume electrical resistivity were measured using a ULTRA Mesohmeter SM-8220, from the DKK TOA Corporation, Tokyo, Japan. The Surface and volume electrical resistivities of the MWCNT/polyimide composites were measured after various MWCNT contents were added. The charge time was 30 s, and the current voltage of the measurements was 100 V. An average value was obtained from five to six measurements for each sample.
2.5.6. Glass Transition Temperature (Tg)
Glass transition temperatures (Tgs) were measured using a differential scanning calorimeter (DSC) (TA Instruments DSC Q-10). Test data were obtained from 50°C to 350°C at a heating rate of 10°C/min. A minimum of three specimens of each composition were tested.
2.5.7. Measurements of Tensile Properties
Tensile strength test was carried out by using an instron machine model 4488 at room temperature. Test procedure followed the ASTM-D882. Dimensions of test specimens were 50 mm × 5 mm × 0.1 mm. The crosshead speed was 5 mm/min.
3. Results and Discussion
3.1. Fourier Transfer Infrared Spectroscopy
Figure 1(a) shows the FTIR spectrum of unmodified canbon nanotubes; the wavenumbers 3000 and 2800 cm−1 corresponded to −CH stretching and 1711 cm−1 corresponded to c=O stretching. The FTIR result (−CH stretching) demonstrates that MWCNTs contain defects, which may be formed during the manufacturing of MWCNT. Figure 1(b) presents the FTIR spectrum of acid-modified carbon nanotubes. The wavenumbers 3000 and 2800 cm−1 corresponded to the stretching of −CH. Carboxylic group stretching (COOH) occurred at 1720 cm−1. Absorption peaks at wavenumbers 1610 cm−1 and 3430 cm−1 correspond to COO− asymmetric stretching and −COO− stretching, respectively. The FTIR spectrum of acid-modified carbon nanotubes confirmed that pristine MWCNT was modified by H2SO4/HNO3 mixed acid generating carboxylic groups. Figure 1(c) presents the FTIR spectrum of the APTES-MWCNT. The wavenumbers at 3000 and 2800 cm−1 corresponded to stretching of −CH. Carboxylic groups stretching (−COO−) appeared at 1400 cm−1. Stretch of amine salt (−NH−) appeared at 3100 cm−1. Bend of the amine salt (−NH3+) appeared at 1610 cm−1 and 1500 cm−1. Absorption peaks at wavenumbers 1110 cm−1 and 956 cm−1 corresponded to −SiO− stretching. The FTIR spectrum of APTES-MWCNT confirmed that MWCNT was modified with APTES.
3.2. X-Ray Photoelectron Spectroscopy (XPS)
Figure 2 presents XPS spectra of the silane-modified MWCNT. Figures 2(a), 2(c), and 2(e) reveal that silane-modified MWCNTs have peaks at about 103eV, 154eV, 285eV, 400eV, and 531eV. The peak appeared at 103eV was Si2p, 154eV was Si2s, 285eV was C1s, 400eV was N1s, and 531eV was O1s, respectively. Figure 2(b) presents the peaks of APTES-MWCNT-1 at 400eV, (N1s) can be separated as 400.0eV, 401.4eV, and 402.6eV. The peak at 400.0eV was amide and peaks at 401.4eV, and 402.6eV were amino ions. The ratio of 400.0eV : 401.4eV : 402.6eV was 1 : 0.26 : 0.06. Figures 2(d) and 2(f) present the peaks of APTES-MWCNT-2 and APTES-MWCNT-3 at 400eV, (N1s) can be separated as 400.0eV and 401.4eV. The ratio of 400.0eV : 401.4eV of APTES-MWCNT-2 and APTES-MWCNT-3 were 1 : 0.08 and 1 : 0.13, respectively. The XPS data confirm the most of the silane bonding with the acid-modified MWCNT with amide functional groups and some of them bonded with amino ion.
3.3. CP/MAS Solid State 29Si Nuclear Magnetic Resonance (NMR) Spectroscopy
Figures 3(a) to 3(f) present the 29Si solid-state NMR spectra of APTES-MWCNT/polyimide composites. They reveal that trisubstituted siloxane bonds (chemical shift of T2, ppm, and T3, ppm)  and some tetrasubstituted siloxane bonds (associated with chemical shifts Q2, Q3 and Q4 of −91, −101 and −109.13 ppm, resp.)  are present in APTES-MWCNT/polyimide composites. Trisubstituted siloxane bonds are defined as a silicon atom possesses four bonds, three of them are bonded with three oxygen atoms and the fourth bond may be bonded with another atom, such as carbon atom (Figure 4(a)). Tetrasubstituted siloxane bonds are the four bonds of the silicon atom bonded to oxygen atoms (Figure 4(b)).
Table 2 summarizes the percentages of T and Q substitution of the composites. The tetrasubstituted bond may have been formed when the composites were heated to 300°C and the Si−C bonds of the APTES may have been broken. When the MWCNT content was low (1.0 wt%), the percentage of Q-substitution was proportional to the ratio of APTES to MWCNT. When the ratio of APTES to MWCNT was excessive, most APTES do not graft on the MWCNT. The ungrafted APTES may be responsible for a lower thermal stability, since the Si−C bond of the ungrafted APTES is weak. Hence, the proportion of tetrasubstitution increased with the ratio of APTES to MWCNT. When the MWCNT content was high (7.0 wt%), the percentage of Q-substitution decreased as the ratio of APTES to MWCNT increased. When the MWCNT content and the silane content were both high, the Si network was easily formed. APTES grafted on the SiOx network and increased the thermal stability.
Regarding T substitution (Figure 4), the peak height of T3 of the sample with higher MWCNT content was higher than that with a lower MWCNT content. When the APTES-MWCNT content was low, since the silane in the nanocomposites could not easily react with the other silane because the APTES-MWCNT intermolecular distance was very large, the interpenetrating network could not easily be formed. However, when the APTES-MWCNT content was high, the silane more easily reacted with the other silane, because the ATPES-MWCNT intermolecular distance was shortened.
3.4. Morphology of MWCNT/Polyimide Composites
Figures 5(a)–5(f) present SEM microphotographs of the APTES-MWCNT/polyimide composites. The MWCNT was dispersed in the polymer matrix. Most of the MWCNTs were embedded in the polyimide matrix. Figures 5(a) and 5(b) show SEM microphotographs of the APTES-MWCNT-1/polyimide composites. Most APTES-MWCNT were embedded in the polyimide matrix and only a few were pulled out from the matrix. Figures 5(c) and 5(d) indicate that most of the MWCNTs in APTES-MWCNT-2/polyimide composites were embedded in the polyimide matrix and some of the MWCNTs were pulled out from the matrix. Figures 5(e) and 5(f) reveal that most of the MWCNT in APTES-MWCNT-2/polyimide composites were also embedded. However, it can be seen that more MWCNTs were pulled out than those of the APTES-MWCNT-1/polyimide composites and APTES-MWCNT-2/polyimide composites. The imprint of the MWCNT on the polyimide matrix (indicated by the arrow in Figure 5(f)) was observed.
Figures 6(a) to 6(f) display the TEM microphotographs of the ATPES-MWCNT/polyimide composites. Figures 6(a) and 6(b) present the TEM microphotographs of the APTES-MWCNT-1/polyimide composites. The MWCNTs were coated with Si layers, which looked like “needles” and were dispersed in the polymer matrix. Some of the MWCNTs were connected to each other. Some of the Si was not coated on the MWCNT surface and was aggregated in the shape of cotton balls.
Figures 6(c) and 6(d) show the TEM microphotographs of the APTES-MWCNT-2/polyimide composites. The MWCNTs were also coated with Si layers and dispersed in the polymer matrix. Some of the MWCNTs were also connected to each other. The MWCNTs were assembled in an “H” shape or a “” shape.
Figures 6(e) and 6(f) display TEM microphotographs of the APTES-MWCNT-3/polyimide composites. The Si was aggregated on the MWCNT surface. Figure 6(e) reveals that the MWCNTs were assembled in the shapes of longer needles which differed from those in Figures 6(a), 6(b). The “needle-shaped” MWCNTs in Figures 6(e), 6(f) were longer and thinner than those in Figures 6(a), 6(b). Figure 6(f) shows that some of the MWCNT have been connected. When the ratio of APTES to MWCNT was high, most of the APTES were not grafted on the surface of acid-modified MWCNTs. Some of the APTES reacted with the polyamaric acid to form a complex [1, 28]. The ungrafted APTES bonded with the grafted APTES, and could not easily form an interpenetrating network. The TEM microphotograph shows that concentrations were different even with the same content (7.0 wt%) APTES-MWCNT. APTES-MWCNT-3 show less concentrated than that of APTES-MWCNT-1 and APTES-MWCNT-2 in polyimide matrix with 7.0 wt% MWCNT content. APTES-MWCNT-1 and APTES-MWCNT-2 show similar concentration in polyimide matrix with 7.0 wt% MWCNT content. However, the lengths of the assembled MWCNT were different. Assembled APTES-MWCNT-3 is longer than that of APTES-MWCNT-2 and APTES-MWCNT-1 in the polyimide matrix. APTES-MWCNT-3 was connected with “head-to-tail” and the APTES-MWCNT-3 assembled to long, thin “needle shaped.” The quantity of MWCNT of 7.0 wt% APTES-MWCNT-3 in polyimide matrix was similar to the 7.0 wt% APTES-MWCNT-2 and APTES-MWCNT-1.
3.5. Electrical Properties
CNTs have a high aspect ratio and contain numerous -bonds (C=C bond). Electrical charges may be transferred through the -bond (C=C bond) of CNTs. Adding a small quantity of CNT to the polymer matrix substantially reduces the surface and volume electrical resistivities of the matrix.
Figures 7(a) and 7(b) plot the surface and volume electrical resistivities of the APTES-MWCNT/polyimide composites. The surface resistivities were summarized in Table 3 and the volume resistivities were summarized in Table 4. When unmodified and acid-modified MWCNT were utilized, the surface electrical resistivity of the composites decreased from 1.28 × 1015 /cm2 (neat polyimide) to 7.59 × 106 /cm2 (a decrease of nine orders of magnitude with 7.0 wt% unmodified MWCNT) and to 2.53 × 109 /cm2 (a decrease of six orders of magnitude with 7.0 wt% acid-modified CNT) (Figure 7(a)). The volume electrical resistivity of the composites decreased from 1.53 × 1017 cm (neat polyimide) to 4.09 × 104 cm (a decrease of 13 orders of magnitude with 7.0 wt% unmodified MWCNT), and 2.66 × 108 cm (a decrease in nine order of magnitude with 7.0 wt% acid-modified CNT), (Figure 7(b)). In the APTES-MWCNT system, the surface electrical resistivity of the composites decreased by five orders of magnitude from 1.28 × 1015 /cm2 (neat polyimide) to 8.64 × 1010 /cm2 (for 7.0 wt% APTES-MWCNT-1/polyimde composites), by four orders of magnitude to 8.58 × 1011 /cm2 (with 7.0 wt% APTES-MWCNT-2/polyimide composites), and by four orders of magnitude to 4.53 × 1011 /cm2 (with 7.0 wt% APTES-MWCNT-3/polyimide composites) (Figure 7(a)). The volume electrical resistivity of the composites decreased by four orders of magnitude from 1.53 × 1017 cm (neat polyimide) to 5.07 × 1011 cm (with 7.0 wt% APTES-MWCNT-1/polyimide composites), by eight orders of magnitude to 1.68 × 109 cm (with 7.0 wt% APTES-MWCNT-2/polyimide composites), and by seven orders of magnitude to 2.90 × 1010 cm (7.0 wt% APTES-MWCNT-3/polyimide composites) (Figure 7(b)).
Acid-modified MWCNTs and APTES-MWCNTs provide less improved surface and volume electricity conductivities than unmodified MWCNTs over MWCNT/polyimide nanocomposites. The MWCNTs may be shortened and the number of defects increased during acid modification. Our previous study revealed that unmodified MWCNTs were aggregated, but acid-modified MWCNT were dispersed in the polyimide matrix . The aggregated MWCNTs may connect to each other and more easily form charge-conducting pathways. The model of the aggregated unmodified MWCNT is similar to that presented in Figure 8(a) and the electrical charge conducting model of the small amount of dispersed acid-modified MWCNT is similar to that shown in Figure 8(b). Figure 7(b) reveals that the APTES-MWCNT/polyimide composites have a lower percolation threshold (1.0 wt% for APTES-MWCNT-1 and APTES-MWCNT-2 and 2.4 wt% for APTES-MWCNT-3) than that of the unmodified MWCNT/polyimide (4.76 wt%) or acid-modified MWCNT/polyimide (more than 6.98 wt%). APTES-MWCNT-3 has a higher percolation threshold than APTES-MWCNT-1 and APTES-MWCNT-2 in the polyimide matrix, perhaps because APTES-MWCNT-3 cannot form an interpenetrating network easier than APTES-MWCNT-1 and APTES-MWCNT-2 can.
When the MWCNT content is high (7.0 wt%), the volume electrical resistivities of APTES-MWCNT/polyimide composites exceed those of unmodified and acid-modified MWCNT/polyimide composites. Figures 7(a) and 7(b) show that when the MWCNT content was lower than 7.0 wt%, APTES-MWCNT/polyimide composites had a lower surface and volume electrical resistivity than those of acid-modified MWCNT/polyimide composites. The electrical charge transfer model of the APTES-MWCNT in polyimide matrix is similar to that presented in Figure 8(c). The TEM microphotograph indicates that some of the APTES-MWCNTs are connected to each other in the polyimide matrix and the charge transfer pathways may be more effective than those of unmodified and acid-modified MWCNT. When the MWCNT content is higher than 7.0 wt%, the charge transfer pathway may be formed (Figure 8(d)) in acid-modified MWCNT polyimide composites. Some of the APTESs were not bonded with the acid-modified MWCNT, which may bond to the polyamic acid and form a complex [1, 28]. Furthermore, as presented in Figure 8(e), although the APTES-MWCNTs connected with each other in the polyimide matrix, the MWCNTs were separated by SiOx. When the unmodified and acid-modified MWCNT contents were very high, the MWCNTs may be interconnected (Figure 8(f)). The results show that when the MWCNT content exceeds 7.0 wt%, APTES-MWCNT/polyimide composites have a higher surface and volume electrical resistivity than those of acid-modified MWCNT/polyimide composites.
3.6. Glass Transition Temperature (Tg)
The glass transition temperatures (Tgs) of the polymer matrix depend on the free volume of the polymer, which is related to the affinity between the filler and the polymer matrix. A polymer with a lower free volume generally has a higher Tg. A polymer matrix with a higher affinity to filler exhibited less polymer molecular motion and reduced the free volume of the polymer molecules [32, 33]. The −COOH functional groups on the acid-modified MWCNT surface can form hydrogen bonds with the imide functional groups of the polyimide, reducing the free volume of the polymer. When the APTES-MWCNT/polyamic acid was heated to 300°C, the silane molecules on the MWCNT surface reacted and connected with each other. The free volume of the polymer is reduced and molecular motion is restricted.
Figure 9 shows the DSC data of the MWCNT/polyimide composites. The glass transition temperatures (Tgs) of the MWCNT/polyimide composites were summarized in Table 5. When 1.0 wt% unmodified MWCNT was added to the matrix, Tg decreased slightly. The Tg of 1.0 wt% unmodified MWCNT/polyimide is 279°C. When the unmodified MWCNT content was 7.0 wt%, Tg decreased to 277°C. The affinity between the unmodified MWCNT and the polyimide was poor, so the free volume of the composites increased and Tg decreased. The glass transition temperature (Tg) of the pristine polyimide was 280.44°C. When acid-modified MWCNTs were utilized, the Tg of the MWCNT/polyimide composites increased slightly to 287.45°C (2.4 wt% acid-modified MWCNT/polyimide composites). When the acidmodified MWCNT content was 7.0 wt%, Tg decreased slightly to 284.09°C. When the APTES-MWCNT content was high, Tg increased significantly. The Tg of the APTES-MWCNT/polyimide composites increased to 295.22°C (7.0 wt% APTES-MWCNT-1/polyimide composites), 294.88°C (7.0 wt% APTES-MWCNT-2/polyimide composites), and to 303.69°C (4.8 wt% APTES-MWCNT-3/polyimide composites). The results show that Tg increased with the ratio of APTES to MWCNT, since silane may restrict the molecular motion of the polyimide matrix.
3.7. Tensile Properties
The mechanical properties of MWCNT/polymer composites depend on the affinity of the MWCNTs to the polymer matrix. Since modified MWCNTs have a greater affinity to the polymer matrix than that of unmodified MWCNT, modified MWCNTs improve the tensile properties of the polyimide significantly. Hydrogen atoms at the −COOH groups of acid-modified MWCNTs may form hydrogen bonds with the C=O bonds of the PI molecules [31, 32].
Tables 6 and 7 exhibit the tensile properties of the MWCNT/polyimide composites. The tensile strength of neat polyimide is 101.71 MPa. The APTES-MWCNT/polyimide exhibits a maximum tensile strength at the MWCNT content of 1.0 wt%. When 1.0 wt% APTES-MWCNT was employed, the tensile strength increased to 126.49 MPa (increased 24.36%) (APTES-MWCNT-1), to 126.66 MPa (increased 24.53%) (APTES-MWCNT-2), and to 128.03 MPa (increased 25.88%) (APTES-MWCNT-3) (Table 6). However, if the MWCNT content exceeds 1.0 wt%, the tensile strength declines. Table 7 shows the Young's modulus of the APTES-MWCNT/polyimide composites. The Young's modulus of neat polyimide was 2.34 GPa. When 1.0 wt% acid-modified MWCNT was used, the Young's modulus increased to 3.68 GPa (increased 57.73%). When 1.0 wt% APTES-MWCNT was used, the Young's modulus increased to 3.81 GPa (increase 62.82%) (APTES-MWCNT-1), to 4.04 GPa (increase 71.79%) (APTES-MWCNT-2), and to 3.51 GPa (increase 50.00%) (APTES-MWCNT-3) (Table 7). When the APTES-MWCNT (APTES-MWCNT-1 and APTES-MWCNT-2) content exceeds 1.0 wt%, the Young's modulus decreased. When the APTES-MWCNT-1 and APTES-MWCNT-2 contents were 7.0 wt%, Young's modulus decreased to 3.54 GPa (APTES-MWCNT-1) and 3.17 GPa (APTES-MWCNT-2). When the APTES-MWCNT-3 contents were 4.8 wt%, Young's modulus increased to 3.48 GPa.
TEM microphotograph shows that some of the APTES-MWCNTs were connected together in the polyimide matrix. MWCNT network may be formed in the polyimide matrix. When MWCNTs networks were formed in the polyimide matrix, it can be improve the tensile strength more significantly than that of acid-modified MWCNT. Tai et al.  reported that the network MWCNT improved the mechanical properties significantly more than that of dispersed MWCNT. They proposed that the lengths of MWCNTs in network were longer . When APTES-MWCNT content was higher than 2.4 wt%, the composites possess lower tensile strength and Young's modulus decreased slightly. The composites became brittle when APTES-MWCNT content is high.
Aminosilane-modified MWCNT/polyimide nanocomposites have been prepared. 29Si solid state NMR demonstrates that T substituted and Q substituted siloxane bonds are present in APTES-MWCNT/polyimide composites. A TEM microphotograph shows that MWCNTs may connect to each other in the polyimide matrix, MWCNT network, and that polyimide molecules may interpenetrate the crosslinked MWCNT network.
The surface and volume electrical resistivity of the APTES-MWCNT/polyimide composites decreased more significantly than that of acid-modified MWCNT/polyimide composites when the APTES-MWCNTs content was lower than 4.8 wt%, the percolation threshold of the APTES-MWCNT/polyimide composites was lower than that of the unmodified and acid-modified MWCNT/polyimide composites. The surface electrical resistivity of the polyimide decreased from 1.28 × 1015 /cm2 (neat polyimide) to 2.73 × 1013 −cm (2.4 wt% acid modified MWCNT) and 8.07 × 1012 −cm (2.4 wt% APTES-MWCNT-1). The volume electrical resistivity of the polyimide decreased from 1.53 × 1017cm (neat polyimide) to 9.25 × 1016 cm (2.4 wt% acid modified MWCNT) and 9.23 × 1010 cm (2.4 wt% APTES-MWCNT-2). The percolation thresholds of the unmodified MWCNT/polyimde, acid-modified MWCNT/polyimide, APTES- MWCNT-1/polyimide, APTES-MWCNT-2/polyimide, and APTES-MWCNT-3/polyimide were 4.76 wt%, more than 7.0 wt%, 1.0 wt%, 1.0 wt%, and 2.4 wt%, respectively.
The glass transition temperature (Tg) of the APTES-MWCNT/polyimide composites increased more than those of unmodified and acid-modified MWCNT/polyimide composites. The Tg of polyimide increased from 281.1°C (neat polyimide) to 303.7°C (4.8 wt% APTES-MWCNT-3).
Tensile strength of MWCNT/polyimide increase form 101.71 MPa (neat polyimide) to 120.87 MPa (7.0 wt% unmodified MWCNT, increased 18.8%), 132.87 MPa (7.0 wt% acid modified MWCNT, increased 30.6%), 126.49 MPa (1.0 wt% APTES-MWCNT-1, increased 24.4%), 126.66 MPa (1.0 wt% APTES-MWCNT-2 increased, 24.5%), and 128.03 MPa (1.0 wt% APTES-MWCNT-3, increased 25.9%)
The authors would like to acknowledge the National Science Council of Taiwan for the financial support under Contract no. NSC 95-2221-E-007-081.
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