Functionalization of MWCNTs with silver nanoparticles decorated polypyrrole and their application in antistatic and thermal conductive epoxy matrix nanocomposite

Abstract

A facile approach for fabricating antistatic and thermally conductive epoxy matrix nanocomposites has been carried out by mixing terephthalic acid-doped MWCNTs with a silver nanoparticle decorated polypyrrole functional coating (MWCNTs@Ag-PPy@COOH). MWCNTs@Ag-PPy@COOH was synthesized via the following steps. Carboxyl groups were first generated on the surface of MWCNTs (MWCNTs–COOH) after mixed acid treatment. Then, pyrrole was grafted onto MWCNTs–COOH (MWCNTs-Py) by the reaction of carboxyl and amine. Furthermore, pyrrole was polymerized on the surface of MWCNTs (MWCNTs@Ag-PPy) initiated by AgNO₃, and silver nanoparticles were decorated onto PPy at the same time. Lastly, terephthalic acid was doped in the repeat unit of PPy through the coordination of carboxyl and pyrrole. Chemical structures of MWCNTs@Ag-PPy@COOH were characterized by FTIR, EDS, XPS, Raman, TGA, SEM and TEM. Results indicated that Ag decorated PPy was successfully covalently bonded onto the surface of MWCNTs, which enhanced the conductive and heat-conducting capabilities. More importantly, because –COOH groups participated in the curing reaction, the dispersion of MWCNTs@Ag-PPy in epoxy matrix was improved after doping terephthalic acid in the repeat unit of PPy. As a result, antistatic and thermally conductive properties were well acquired. The percolation surface resistivity was decreased to 1.3 × 10⁸ Ω sq⁻¹ at 0.5 wt% concentration and thermal conductivity was increased by more than 170% compared to neat epoxy at 10 wt% concentration of MWCNTs@Ag-PPy@COOH.

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1. Introduction

In some special industries, such as the chemical industry, coal mines and power plants, antistatic materials (the surface resistivity should be in the range of 10⁶ to 10⁹ Ω sq⁻¹ (ref. 1 and 2)) are usually needed. Thermal conductivity materials are also used in semiconductors, micro-heat radiators and aerospace industries. Thus, antistatic and thermal conductivity materials need to be studied and developed. As permanent conductive fillers³ and nice thermal conductors,⁴ carbon nanotubes (CNTs) have been widely researched in the field of antistatic and thermally conductive polymer materials.^5–7 However, because the surface of CNTs is inert without any reactive chemical groups, the CNTs were difficult to disperse and it was difficult to construct electric and thermally conductive paths in the matrix. Thus, the agglomeration of CNTs in the matrix was the greatest challenge and the dispersibility of CNTs was an important subject.⁸

The most effective method to modify CNTs is surface chemical functionalization.^9,10 After an agent with similar polar or chemical structure to the target matrix was grafted onto the CNTs, a compatible interface which could graft to another organism or improve the dispersion of CNTs in nanocomposite would be formed. Thus, an antistatic polymer nanocomposite will then achieve percolation with lower CNTs content. Yu et al.¹¹ synthesized a multi-walled carbon nanotube–polypyrrole (MWCNT–PPy) nano-complex dopant for a polyvinyl chloride (PVC) matrix, and a low percolation threshold in PVC for this nano-complex dopant was found. In our previous study, antistatic polyetherimide (PEI) was effected by introducing poly(sodium acrylate)-bonded multi-walled carbon nanotubes and silver nanoparticles (MWCNTs@PSA@Ag). It was found that the electrostatic discharge channel was well constructed through homogenously dispersed MWCNTs with a polar PSA layer and enhanced by Ag nanoparticles.¹²

PPy has been commonly used in rechargeable batteries,¹³ sensors,¹⁴ antistatic materials,¹⁵ stealth materials¹⁶ and solid electrolytic capacitors¹⁷ due to its excellent environmental stability, good conductivity and redox activity. In addition, the conjugated structure made sure PPy had good thermally conductivity based on electronic heat conduction mechanism.¹⁸ In this study, MWCNTs with a silver nanoparticles decorated polypyrrole functional coating (MWCNTs@Ag-PPy) were designed and utilized as an antistatic agent and heat-conducting filler in EP matrix nanocomposite. As illustrated in Scheme 1, the MWCNTs@Ag-PPy was first generated, then terephthalic acid was doped onto the MWCNTs@Ag-PPy to form covalent bonding at the interface. Thus, an electrostatic discharge channel and heat transfer network was constructed through well-dispersed MWCNTs and enhanced by Ag nanoparticles. The antistatic EP nanocomposite expressed percolation just with 0.5 wt% concentration of MWCNTs@Ag-PPy@COOH and thermal conductivity was increased by more than 170% compared to neat epoxy when the concentration of MWCNTs@Ag-PPy@COOH was 10 wt%.

Scheme 1 Schematic of MWCNTs@Ag-PPy@COOH.

2. Experimental

2.1. Materials

MWCNTs (purity ≥95%, diameter 10–20 nm and length 0.5–500 μm) were provided by Shenzhen Nanotech Port Co., Ltd., China. Concentrated sulfuric acid (98%), concentrated nitric acid (70%), xylene (99.0%), silver nitrate and N-methyl-pyrrolidone (NMP) were purchased from Beijing Chemicals Co. of China. Methylhexahydrophthalic anhydride, pyrrole (purity ≥98.0%, density 0.968–0.971 g mL⁻¹) and N,N-dimethyl benzyl amine (purity ≥98.0%, density 0.896–0.902 g mL⁻¹) were provided by Sinopharm Chemical Reagent Co., Ltd. Sodium dodecyl benzene sulfonate (SDBS) was provided by Tianjin Jinke Fine Chemical Institute. Pyrrole needed to be treated by the method of reduced pressure distillation and stored in the refrigerator. Epoxy rein (E-51) was provided from Changshu Jaffa Chemical Co., Ltd.

2.2. Synthesize acid-oxidized MWCNTs

Concentrated H₂SO₄ (20 mL) was mixed into concentrated HNO₃ (20 mL). Then, the MWCNTs (1 g) were dispersed into the mixed acids prepared above under sonication at room temperature for 30 min refluxing the condensate for 1 h at 140 °C. Subsequently, the sample was collected and washed with deionized water on a nylon membrane with pore size of 0.2 μm and dried in a vacuum.

2.3. Synthesize MWCNTs-Py

In a round-bottomed flask equipped with a magnetic stirring bar, the solution of dimethylbenzene (20 g) and sodium dodecyl benzene sulfonate (SDBS, 2 g) in deionized water (60 mL) was stirred for 30 min at room temperature. This solution was directly introduced into the crushed MWCNTs with ultrasonic vibrating for 30 min. Subsequently, 4 mL pyrrole was added dropwise into the solution with stirring. The mixture was vigorously stirred for 24 h at room temperature. Then, the solution was poured into excess acetone for demulsification and washed twice with each detergent, including deionized water, absolute ethyl alcohol and acetone. Lastly, the collected solid was dried to achieve constant weight under vacuum at 60 °C affording the pyrrole-coated MWCNTs composite (MWCNTs-Py).

2.4. Synthesize MWCNTs@Ag-PPy

Compared with synthesizing MWCNTs-Py, in this step the only difference was that after pyrrole was added, silver nitrate was introduced into the solution with stirring in an ice bath and the drawing speed to add silver nitrate to the solution should be slow. The ice bath and slow drawing speed both make the polymerization reaction proceed mildly.

2.5. Synthesize MWCNTs@Ag-PPy@COOH

In a round-bottomed flask equipped with a magnetic stirring bar, MWCNTs@Ag-PPy (100 mg) was added into the solvent N-methyl-pyrrolidone (60 g) with ultrasonic vibrating for 30 min at room temperature. Subsequently, terephthalic acid (1 g) was introduced and the mixture vigorously stirred at room temperature. After 24 h, MWCNTs@Ag-PPy@COOH was prepared and filtrated by a nylon membrane. Finally, the collected solid was dried under vacuum for 8 h at 80 °C.

2.6. Synthesis of EP/MWCNTs nanocomposite

First, 14 mg pristine MWCNTs (the addition should be adjusted by different content of pristine MWCNTs in the epoxy matrix) were dispersed in 5 mL of NMP by ultrasound for about half an hour. In the next step, 3 g of EP was added into the solutions, stirring at 203 °C for 1 h to remove the solvent. Furthermore, 2.4 g methylhexahydrophthalic anhydride and 10 mg N,N-dimethyl benzyl amine were added into the mixture under vigorously stirring for about 20 min. Then, the mixture was poured into the mould made by polypropylene, and cured under 80 °C for 2 h, 100 °C for 2 h, 120 °C for 2 h. Subsequently, EP/pristine MWCNTs (0.25 wt%) nanocomposite was prepared. The preparation of EP/MWCNTs@Ag-PPy and EP/MWCNTs@Ag-PPy@COOH nanocomposites would follow the same preparation process as EP/pristine MWCNTs nanocomposite.

2.7. Characterization

Infrared analysis was carried out with Fourier transform infrared spectrometry (FTIR, Nexus 670) using MWCNTs samples as pellets with KBr. Raman spectroscopy (JY, HR800, wavelength 532 nm) was used to determine MWCNTs structure variation after PPy was coated around the surface of MWCNTs. Energy dispersive spectroscopy (EDS) was undertaken using a Hitachi-S4700 scanning electron microscope. X-ray photoelectron spectroscopy (XPS) was undertaken using an ESCALAB 250 spectrometer using monochromatized Al-Kα X-ray source at a constant analyzer. Differential scanning calorimetric (DSC) measurement was performed to understand the thermal behavior between MWCNTs and epoxy. DSC thermograms were obtained (DSC 8230, Rigaku Denki, Co., Ltd. Japan) in the temperature range from 25 to 350 °C with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. The morphology of the samples was inspected by high resolution transmission electron microscopy (HRTEM, JEOL JEM-3010 electron microscope) and scanning electron microscopy (SEM, Hitachi-S4700). Surface resistivity was measured by a four-point probe method using a four-probe meter (ST2258A, Suzhou Jingge Electronic Co., LTD). Thermal conductivity was measured by a thermal conductivity measuring instrument (TC 3000E, Xi'an Xiatech Electron).

3. Results and discussion

3.1. Functionalization of MWCNTs with pyrrole

Based on the approach of mixed acid treatment published earlier, a proportion of the carboxyl groups on MWCNTs were replaced by a uniform envelope of pyrrole. MWCNTs-Py showed the characteristic stretch vibration peaks of C–H at 883 cm⁻¹ and N–H at 1373 cm⁻¹ of the pyrrole ring (Fig. 1b). The peaks at 1542 cm⁻¹ and 1450 cm⁻¹ were eigenstate Py's characteristic peaks and the two peaks were generated by the pyrrole ring's symmetric and asymmetric stretching vibrations (Fig. 1b).^19,20 The EDS spectra are shown in Fig. 2. Compared with pristine MWCNTs, the curve of MWCNTs-Py showed the N peak at 0.39 keV, along with O peak at 0.53 keV. Furthermore, in the XPS spectrum (Fig. 3b), the bonding energy of N 1s peak at 414.9 eV and O 1s peak at 547.4 eV may be assigned to pyrrole and carboxyl groups, respectively. Thereby, this evidence confirmed that the pyrrole successfully grafted onto the surface of MWCNTs.

Fig. 1 FTIR spectra of pristine (a) MWCNTs, (b) MWCNTs-Py, (c) MWCNTs@Ag-PPy and (d) MWCNTs@Ag-PPy@COOH.

Fig. 2 EDS spectra of pristine (a) MWCNTs, (b) MWCNTs-Py, (c) MWCNTs@Ag-PPy and (d) MWCNTs@Ag-PPy@COOH.

Fig. 3 XPS spectra of pristine (a) MWCNTs, (b) MWCNTs-Py, (c) MWCNTs@Ag-PPy and (d) MWCNTs@Ag-PPy@COOH.

Raman spectra of pristine (a) MWCNTs, (b) MWCNTs-Py, (c) MWCNTs@Ag-PPy and (d) MWCNTs@Ag-PPy@COOH are presented in Fig. 4. In general, Raman spectra of MWCNTs contain two domains: the tangential G-band at 1550–1605 cm⁻¹ and disorder-induced D-band at around 1350 cm⁻¹.²¹ The pristine MWCNTs showed the G-band at 1580 cm⁻¹, while MWCNTs-Py showed the G-band at 1584 cm⁻¹, which can be regarded as an evidence that the surface of MWCNTs was modified by pyrrole.

Fig. 4 Raman spectra of pristine (a) MWCNTs, (b) MWCNTs-Py, (c) MWCNTs@Ag-PPy and (d) MWCNTs@Ag-PPy@COOH.

3.2. Functionalization of MWCNTs-Py with silver nanoparticles decorated polypyrrole

In the EDS spectra, it could be found that the curve of MWCNTs@Ag-PPy (Fig. 2c) showed the Ag peak at 3 keV, and the content of nitrogen increased to 14.56%. In XPS spectra, the emergence of Ag_3d5 on the curve of MWCNTs@Ag-PPy (Fig. 3c) confirmed that Ag nanoparticles had, as expected, modified on the surface of MWCNTs. In the Raman spectra, the G-band of MWCNTs@Ag-PPy (Fig. 4c) blue shifted to 1588 cm⁻¹. Moreover, the peaks at 986 cm⁻¹ and 1040 cm⁻¹ on the curve of MWCNTs@Ag-PPy were the Raman peaks of eigenstate polypyrrole. All this indicated that MWCNTs with a silver nanoparticles decorated polypyrrole functional coating were obtained.

The morphology of pristine MWCNTs and MWCNTs@Ag-PPy composite was characterized using TEM. Fig. 5a and b shows the surface morphology of pristine MWCNTs and MWCNTs@Ag-PPy, respectively. It is quite clear that the MWCNTs@Ag-PPy had larger diameter than that of the pristine MWCNTs and there were many Ag nanoparticles distributed around the surface of MWCNTs.

Fig. 5 TEM images of pristine (a) MWCNTs and (b) MWCNTs@Ag-PPy.

TGA curves were used to estimate the weight ratio of the grafted organic material on MWCNTs. Pristine MWCNTs could maintain their weight stable until 800 °C in nitrogen (Fig. 6a). But the weight of acid-oxidized MWCNTs (Fig. 6b) remained at 95.37 wt% at 800 °C. The 4.63 wt% weight loss came from the decomposition of –OH groups, –COOH groups and broken structures during acid treatment. The weight of MWCNTs@Ag-PPy remained at 88.53 wt% at 800 °C (Fig. 6c) and Ag-PPy remained at 55.18 wt% at 800 °C (Fig. 6e). By comparing the three curves (Fig. 6b, c, and e), the weight fraction of Ag-PPy grafted on the acid-oxidized MWCNTs in the composites could be estimated using the equations as follows:

B% = (1 − X) × A% + X × C%

where A%, B%, and C% were the weight loss percent of acid-oxidized MWCNTs, MWCNTs@Ag-PPy, and Ag-PPy at 800 °C, respectively. X denotes the weight fraction of Ag-PPy grafted on the acid-oxidized MWCNTs and the weight fraction of Ag-PPy grafted on the surface of acid-oxidized MWCNTs was about 17.0%.

Fig. 6 TGA analysis of pristine (a) MWCNTs, (b) acid-oxidized MWCNTs, (c) MWCNTs@Ag-PPy, (d) MWCNTs@Ag-PPy@COOH and (e) Ag-PPy in N₂.

3.3. Functionalization of MWCNTs@Ag-PPy with terephthalic acid

After terephthalic acid was doped, the characteristic stretching vibration peak of C=O (1710 cm⁻¹) and C–O (1290 cm⁻¹) in the carboxyl group appeared in the FTIR spectrum and the characteristic stretching vibration peak of benzene ring at 1600 cm⁻¹ appeared (Fig. 1d). In Fig. 2, compared with MWCNTs@Ag-PPy, the content of oxygen in MWCNTs@Ag-PPy@COOH increased from 9.58% to 11.52%. In the Raman spectra (Fig. 4), compared with MWCNTs@Ag-PPy, the G-band of MWCNTs@Ag-PPy@COOH blue shifted from 1588 cm⁻¹ to 1597 cm⁻¹. This evidence confirms that MWCNTs@Ag-PPy@COOH was obtained.

In the TGA curves, the 15.75% weight loss of MWCNTs@Ag-PPy@COOH came from the decomposition of organic material (Fig. 6d). Compared with the 11.47% weight loss of MWCNTs@Ag-PPy (Fig. 6c), the redundant 4.28 wt% weight loss came from the decomposition of carboxyl groups after doping terephthalic acid.

Fig. 7A shows the differential scanning calorimetry (DSC) curve of EP/MWCNTs@Ag-PPy and EP/MWCNTs@Ag-PPy@COOH mixtures (without curing agent). Compared with EP/MWCNTs@Ag-PPy, it can be observed that MWCNTs@Ag-PPy@COOH mixed with epoxy obtains an obvious exothermic peak at around 165 °C. Through the exothermic reaction, carboxyl groups could react with epoxy groups and the interface structure (Fig. 7B, a) was expected to be generated. After DSC measurement, the mixture was washed three times with acetone, centrifuged three times and dried under vacuum for 8 h at 80 °C to wash out unreacted epoxy resin. After this, the remainder of MWCNTs were tested by FTIR. Compared with MWCNTs@Ag-PPy@COOH (Fig. 1d), the characteristic absorption peaks of COOH at 1710 cm⁻¹ disappeared after reaction with epoxy and the characteristic absorption peaks of C=O and C–O–C in the ester group at 1740 and 1165 cm⁻¹, respectively, could be observed. This indicated that a covalent bond was formed between the reaction of –COOH and the epoxy group (Fig. 7B, b). Moreover, the epoxy group (characteristic absorption peak at 1510 cm⁻¹) at the other side of the epoxy molecule was modified onto MWCNTs (scheme Fig. 7B, a). Such a grafted epoxy group could further participate in the curing reaction to form covalent bonding at the interface.

Fig. 7 (A) DSC curve of (a) EP/MWCNTs@Ag-PPy, (b) EP/MWCNTs@Ag-PPy@COOH mixture, (B, a) the scheme of the reaction of EP and MWCNTs@Ag-PPy@COOH and (B, b) FTIR spectra of EP/MWCNTs@Ag-PPy@COOH.

3.4. Properties of epoxy matrix nanocomposite

The fracture surfaces of pristine epoxy and epoxy nanocomposites were investigated by SEM microphotography in Fig. 8. The fracture surfaces of pristine MWCNTs (a) showed that MWCNTs conglomerated into a huddle and the untreated- MWCNTs@Ag-PPy in epoxy (c) were presented mainly in the form of agglomerates, whereas the MWCNTs@Ag-PPy@COOH (e) dispersed more uniformly, indicating much better dispersion in EP matrix. In addition, more details could be observed from higher magnification images. Most of the pristine MWCNTs (b) and some of MWCNTs@Ag-PPy (d) in the epoxy matrix showed sliding and pulling out at the fracture surfaces, whereas the MWCNTs@Ag-PPy@COOH were found to be broken, embedded and tightly held to the matrix rather than just having been pulled out (f). It was reasonable to deduce that the acquired carboxyl made a contribution to the dispersion of MWCNTs in the matrix and that the matrix backbone could tightly hold MWCNTs by interfacial bonding.

Fig. 8 SEM images of fractured surface of (a and b) EP/pristine MWCNTs, (c and d) EP/MWCNTs@Ag-PPy and (e and f) EP/MWCNTs@Ag-PPy@COOH nanocomposite (MWCNTs content was 1 wt%).

The TEM micrographs of the EP nanocomposite films with pristine MWCNTs, MWCNTs@Ag-PPy and MWCNTs@Ag-PPy@COOH are shown in Fig. 9. It was quite clear that the pristine MWCNTs showed poor dispersibility in the matrix, and MWCNTs@Ag-PPy@COOH appeared more uniformly distributed than MWCNTs@Ag-PPy due to the reaction between acid and epoxy group at the interface.

Fig. 9 TEM images of EP/pristine (a) MWCNTs, (b) EP/MWCNTs@Ag-PPy and (c) EP/MWCNTs@Ag-PPy@COOH nanocomposite (MWCNTs content was 1 wt%).

In Fig. 10, the surface resistivity increased steadily with increasing loadings of pristine MWCNTs, MWCNTs@Ag-PPy and MWCNTs@Ag-PPy@COOH. As conductive filler, MWCNTs@Ag-PPy@COOH exhibited more pronounced reduction in surface resistivity. This was because the PPy had good conductivity property and Ag nanoparticles could work as a bridge between MWCNTs to increase the electron transport properties of the polymer matrix.²² In addition, the functional carboxyl group encouraged the dispersion of MWCNTs in the matrix, which was beneficial to build up an electrostatic discharge channel. The percolation surface resistivity was 1.3 × 10⁸ Ω sq⁻¹ at 0.5 wt% MWCNTs@Ag-PPy@COOH concentration and compared with the previous research,^23,24 the EP/MWCNTs@Ag-PPy@COOH nanocomposite could reach an antistatic level at lower filler loading.

Fig. 10 Surface resistivity of EP/pristine MWCNTs, EP/MWCNTs@Ag-PPy and EP/MWCNTs@Ag-PPy@COOH nanocomposite.

As shown in Fig. 11a, it was found that the thermal conductivity increased gradually with increasing content of filler, and the EP/MWCNTs@Ag-PPy@COOH nanocomposite had better thermal conductivity. The carriers of thermal conductivity in solid material are the electron, phonon and photon.²⁵ For the studied material, electron and phonon transportation dominates the thermal transport. For electron transportation, the homodispersed MWCNTs could construct a conductive path and make the material have the higher electrical conductivity (Fig. 10), which would be favorable to get better thermal conductivity. Moreover, phonon transportation in the polymer also needed higher filler content and homodispersed filler.^26,27 It could be concluded that the values of the thermal conductivity (λ) were calculated from the equation as follows:

λ = F(λ_i, Φ_i, G, I)

where λ_i represents component thermal conductivity, Φ_i represents component content, G represents the dispersion of filling and I represents the interface state between filling and matrix.

Fig. 11 Thermal conductivity (a) of EP/pristine MWCNTs, EP/MWCNTs@Ag-PPy and EP/MWCNTs@Ag-PPy@COOH nanocomposite and (b) the TEM imagine of EP/MWCNTs@Ag-PPy@COOH (10 wt%) nanocomposite.

According to theory above, the content of filling makes a great contribution to the thermal conductivity. The thermal conductivity would increase with increasing filling content. Furthermore, the distribution of filling was also significant. The MWCNTs@Ag-PPy@COOH in the epoxy had better dispersibility due to the reaction between carboxyl and epoxy group at the interface and the thermal conductive network was also formed under high content of MWCNTs@Ag-PPy@COOH (Fig. 11b). Thus, the modified nanocomposite had better thermal conductivity than previous literature reports.²⁸ When the content of MWCNTs@Ag-PPy@COOH was 10 wt%, the thermal conductivity was improved to 0.47 W m⁻¹ K⁻¹.

4. Conclusions

In conclusion, a facile and effective strategy was demonstrated to prepare functional MWCNTs@Ag-PPy@COOH. The antistatic behavior and heat-conducting property of EP/pristine MWCNTs, EP/MWCNTs@Ag-PPy and EP/MWCNTs@Ag-PPy@COOH nanocomposites was investigated. First, the percolation surface resistivity of EP-based nanocomposite containing MWCNTs@Ag-PPy@COOH was significantly lower than those containing pristine MWCNTs and MWCNTs@Ag-PPy, which confirmed that the PPy and Ag nanoparticles could strengthen the conductivity of MWCNTs, and acid doping contributed much to encourage the dispersion of MWCNTs in the matrix. The surface resistivity of the percolation threshold was 1.3 × 10⁸ Ω sq⁻¹ when the loading of MWCNTs@Ag-PPy@COOH was at 0.5 wt%. In addition, thermal conductivity showed similar variation trends with the surface resistivity. When the content of MWCNTs@Ag-PPy@COOH was 10 wt%, the thermal conductivity of nanocomposite could increase to 0.47 W m⁻¹ K⁻¹ by more than 170% enhancement compared to neat epoxy. It is expected that our present study would have greatly applied value in high performance antistatic and heat-conducting engineering polymers.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 51203007).

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