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.

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 [18].Acid-modified MWCNT can be modified with silane [19][20][21][22][23]. Shanmugharaj et al. [20] 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][20][21][22][23].The oxidation of MWCNT may generate carboxylic groups (−COOH) rather than hydroxyl groups.Valentini et al. [23] modified SWCNTs using CF 4 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][25][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 [26].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 [1].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.

Modification of MWCNT
Pristine MWCNTs were functionalized by refluxing with a mixture of H 2 SO 4 and HNO 3 (weight ratio of H 2 SO 4 to HNO 3 is 3 : 2) 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.

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.

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 CaF 2 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 .

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.

CP/MAS solid state 29 Si nuclear magnetic resonance (NMR) spectroscopy
High-resolution 29 Si solid-state NMR was conducted using a Bruker DSX 400 MHz NMR.The samples were ground into fine powder.The 29 Si 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.

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.

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.

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.

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.

CP/MAS solid state 29 Si nuclear magnetic resonance (NMR) spectroscopy
Figures 3(a) to 3(f) present the 29 Si solid-state NMR spectra of APTES-MWCNT/polyimide composites.They reveal that trisubstituted siloxane bonds (chemical shift of T 2 , δ − 59.84 ppm, and T 3 , δ − 67.002 ppm) [27] and some tetrasubstituted siloxane bonds (associated with chemical shifts Q 2 , Q 3 and Q 4 of −91, −101 and −109.13 ppm, resp.)[27] 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 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 SiO x network was easily formed.APTES grafted on the SiOx network and increased the thermal stability.
Regarding T substitution (Figure 4), the peak height of T 3 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.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 AP-TES-MWCNT-1/polyimide composites.The MWCNTs were coated with SiO x layers, which looked like "needles" and were dispersed in the polymer matrix.Some of the MWCNTs were connected to each other.Some of the SiO x was not coated on the MWCNT surface and was aggregated in the shape of cotton balls.

Morphology of MWCNT/polyimide composites
Figures 6(c) and 6(d) show the TEM microphotographs of the APTES-MWCNT-2/polyimide composites.The MW-CNTs were also coated with SiO x 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 "Ψ" shape.
Figures 6(e) and 6(f) display TEM microphotographs of the APTES-MWCNT-3/polyimide composites.The SiO x 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 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     [27] and (b) tetrasubstituted siloxane bonds (Q shift) [27].
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 MWC-NTs were aggregated, but acid-modified MWCNT were dispersed in the polyimide matrix [17].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  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.

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% acidmodified 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.

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 MWCNTs, 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].

Figure 1 (
Figure1(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.Figure1(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

Figures 5 (
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.Figures5(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.Figures5(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.Figures5(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 Figure5(f)) was observed.Figures6(a) to 6(f) display the TEM microphotographs of the ATPES-MWCNT/polyimide composites.Figures6(a) and 6(b) present the TEM microphotographs of the AP-TES-MWCNT-1/polyimide composites.The MWCNTs were coated with SiO x layers, which looked like "needles" and were dispersed in the polymer matrix.Some of the MWCNTs were connected to each other.Some of the SiO x was not coated on the MWCNT surface and was aggregated in the shape of cotton balls.Figures6(c) and 6(d) show the TEM microphotographs of the APTES-MWCNT-2/polyimide composites.The MW-CNTs were also coated with SiO x 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 "Ψ" shape.Figures6(e) and 6(f) display TEM microphotographs of the APTES-MWCNT-3/polyimide composites.The SiO x was aggregated on the MWCNT surface.Figure6(e) reveals that the MWCNTs were assembled in the shapes of longer needles which differed from those in Figures6(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).Figure6(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 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.Figures5(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.Figures5(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.Figures5(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 Figure5(f)) was observed.Figures6(a) to 6(f) display the TEM microphotographs of the ATPES-MWCNT/polyimide composites.Figures6(a) and 6(b) present the TEM microphotographs of the AP-TES-MWCNT-1/polyimide composites.The MWCNTs were coated with SiO x layers, which looked like "needles" and were dispersed in the polymer matrix.Some of the MWCNTs were connected to each other.Some of the SiO x was not coated on the MWCNT surface and was aggregated in the shape of cotton balls.Figures6(c) and 6(d) show the TEM microphotographs of the APTES-MWCNT-2/polyimide composites.The MW-CNTs were also coated with SiO x 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 "Ψ" shape.Figures6(e) and 6(f) display TEM microphotographs of the APTES-MWCNT-3/polyimide composites.The SiO x was aggregated on the MWCNT surface.Figure6(e) reveals that the MWCNTs were assembled in the shapes of longer needles which differed from those in Figures6(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).Figure6(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

Figure 8 :
Figure 8: Diagram of electrical charge transfer on (a) aggregated conductivity material, (b) small amount of dispersed conductivity material, (c) conductivity material connect each other, (d) large amount of dispersed conductivity material, (e) Junction of connected APTES-MWCNT, (f) Junction of connected unmodified or acid-modified MWCNT over percolation threshold.

Table 2 :
Percentages of T and Q substitution of APTES-MWCNT/polyimide composites.