Preparation and properties of carbon nanotube/carbon fiber hybrid reinforcement by a two-step aryl diazonium reaction

Abstract

Raw carbon nanotube (CNT)/carbon fiber (CF) hybrids were achieved through a two-step aryl diazonium reaction in mild, eco-friendly conditions. Raman spectra, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed the grafting of CNT onto the CF surface. The surface topography and surface free energy of the modified CFs were examined by scanning electron microscopy (SEM), transmission electron microscope (TEM) and dynamic contact angle (DCA) tests, respectively. The resulting CNT/CF hybrid formed a strong chemical bond between the fiber and matrix and enhanced the surface wettability of the modified CFs. The interfacial shear strength (IFSS) of the CNT/CF hybrid reinforced composites increased by 104% compared with that of the untreated CFs. Moreover, the mechanical properties of the CNT/CF hybrid showed a slight improvement after modification.

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1. Introduction

Carbon fibers (CFs) have become an ideal reinforcement in the fabrication of advanced composites in the past few decades due to their excellent mechanical, thermal and electrical characteristics. The mechanical properties of these composites depend not only on the inherent characteristics of CF and the matrix, but also on the interphase between them. However, the compatibility of CF with the matrix is weak due to their poor wettability and adhesion, which is caused by the non-polar, stable and smooth graphite surface of CFs. So far many research efforts have been made to modify the CF surface in order to improve the interfacial adhesion of the resulting composite through enhancing the surface polarity,¹ surface energy² and roughness of CFs,^3,4 and so on.

Recently, the hybrid materials by grafting carbon nanotube⁵ (CNT) with remarkable mechanical characteristics onto CF have attracted increasing attention.^6–8 Introduction of CNT into the interphase of CF/epoxy matrix can improve the roughness of the CF surface and therefore improve the adhesion between the CF and the matrix in a way similar to Velcro hooks.^9,10 Various approaches have been developed to bind CNT and CF together chemically.^6,11 Several aliphatic and aromatic amine groups (ethanediamine,¹² 1,3-propodiamine,¹³ 4,4′-diaminodiphenyl ether,¹⁴ poly(amidoamine)¹⁵) as coupling agents were introduced to bridge CNT and carbon fiber. Such functions are able to attach to CF from one side and leave the other amine moiety to bind with epoxy matrix. Nevertheless, CFs have to be vigorously pre-oxidized to afford functionalities that often lead to damage of the CF.^12–15 An alternative approach is diazonium reaction, which can initiate nucleophilic aromatic substitutions and provide a universal method to introduce halogens, CN, OH, NH₂, etc. into an aromatic ring to form a wide range of compounds without damaging the substrates.^16–20 To date, the aryl diazonium process has been widely employed to functionalize carbon nanotubes and graphene,^21,22 while the method is rarely adopted in functionalizing carbon fibers to fabricate carbon fiber/epoxy composites.

In this paper, an integrated CNT/CF hybrid material was prepared to enhance the interfacial adhesion between the CF and the epoxy matrix via two-step diazonium reaction in water. This method is expected to provide a more controllable process for grafting CNTs onto CFs without complicated processing conditions and harmful solvents.

2. Materials and methods

2.1 Materials

Polyacrylonitrile (PAN)-based carbon fibers (3 × 10³ single filaments per tow, average diameter 6.5 μm, density 1.76 g cm⁻³) were obtained from Sino steel Jilin Carbon Co., China. The multi-walled carbon nanotubes (MWCNTs, purity > 97%, diameter 20–40 nm, length < 2 μm) were purchased from Shenzhen Nanotech Port Co., Ltd. Isoamyl nitrite (analytically pure) was supplied by Aladdin Co. p-Phenylenediamine (analytically pure) was purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). All other chemicals (acetone, dimethylformamide (DMF) and ethanol) obtained from Tianjin Bodi Organic Chemicals Co. Ltd. were reagent-grade.

2.2 Experimental method

The CFs were firstly extracted by Soxhlet with acetone in order to remove the polymer sizing and contaminants from the surface of CF. Then the CFs were washed with supercritical acetone/water (25 mL acetone and 5 mL deionized water) at 633 K for 20 min, and then were desized in acetone again (denoted as untreated CF).²³

A bundle of untreated CFs were bound on a glass frame (3.3 × 20.0 × 50.0 mm) and added into a 100 mL three-neck flask with 50 mL deionized water and p-phenylenediamine. The glass frame was stuck flatwise in the flask. Then a magnetic stir bar was positioned under the glass frame. After that, the reaction unit was heated to 353 K in a water bath. Then the appropriate amount of isoamyl nitrite was slowly introduced through a syringe. The system was agitated intensely with the magnetic stir bar at 353 K for 12 h. After reaction, the CFs were cooled down to room temperature, washed with DMF and deionized water respectively several times until the solution was colorless in order to remove any p-phenylenediamine from the product, then dried in a dry oven at 393 K for 24 h (denoted as NH₂-CF).

The NH₂-CF tied with the glass frame was immersed in 50 mL deionized water containing 1.5 mg mL⁻¹ of the MWCNTs in a 100 mL three-neck flask. Then the flask was heated to 353 K. The isoamyl nitrite was added into the flask slowly and was stirred vigorously by a magnetic stir bar underneath the glass frame at 353 K for 12 h. Thereafter, the CNT/CF hybrids were washed upon sonication in acetone, ethanol and deionized water sequentially, and then vacuum-dried for testing. The schematic of grafting procedures is illustrated in Fig. 1. For comparison, the hybrids obtained by the same process without adding isoamyl nitrite were marked with CNT@CF.

Fig. 1 Schematic diagram of the grafting of CNT onto CF.

2.3 Characterization methods

Raman spectra were recorded at ambient temperature using an Invia Renishaw 2000 spectrometer equipped with an Ar⁺ laser, which provided a laser beam of 514.5 nm wavelength. The laser was focused with a 50× magnification objective, giving a spot which diameter was 1–2 mm. For each spectrum, the accumulation time was 30 s. The depth of analysis in the material was estimated to be 100 nm. Before analysis, the spectral calibration was done with the 520.5 cm⁻¹ transverse optical phonon–phonon mode peak of a Si wafer reference.

Fourier transform infrared (FT-IR) spectra of samples in KBr pellets were obtained by a spectrometer (Nicolet, Nexus 670, USA). The FT-IR spectra were acquired by scanning the specimens for 64 times in the wave number range of 400–4000 cm⁻¹ with the resolution of 2 cm⁻¹.

X-ray photoelectron spectroscopy (XPS) was carried out to examine the chemical compositions of the carbon fiber surface using a spectrometer (ESCALAB 220i-XL, VG, UK) with monochromatic Al Kα source (1486.6 eV) at a base pressure of 2 × 10⁻⁹ mbar. The XPS was energy referenced to the C1s peak at 284.6 eV. The XPS Peak version 4.1 software was used for data analysis.

Field emission scanning electron microscopy (SEM, Quanta 200FEG, FEI, USA) was used to study the surface morphology and structure of the modified carbon fiber at an accelerating voltage of 20 kV. The samples attached with conductive tapes were coated with a thin gold layer by sputter prior to the SEM observation for capturing clear image.

Transmission electron microscopy (TEM) images were taken with a transmission electron microscope (H-7650, Hitachi, Japan) at an operating voltage of 100 kV. To prepare TEM sample, the carbon fibers were molded in a room-temperature curing epoxy resin. These samples were cross sectioned to a relatively thin section (<100 nm) using a microtome.

Dynamic contact angle (DCA) tests were measured with double-liquid method using a dynamic contact angle meter (DCAT21, DataPhysics Instruments, Germany). Here two various immersion liquids were used: deionized water (γ^d = 21.8 mN m⁻¹, γ = 72.8 mN m⁻¹) and diiodomethane (γ^d = 50.8 mN m⁻¹, γ = 50.8 mN m⁻¹, 99% purity, Alfa Aesar, USA). The relevant detailed testing process can be found in literatures.^24,25 The polar component γ^p and dispersive component γ^d of CF can be determined by solving following equation:

γ^T_l(1 + cos θ) = 2(γ^p_lγ^p_f)^1/2 + 2(γ^d_lγ^d_f)^1/2 (1)

γ_f = γ^p_f + γ^d_f (2)

where γ_l^T, γ_l^d and γ_l^p represent the surface tension of immersion liquid, its dispersive and polar component, respectively. θ is the corresponding advancing contact angle. The results were averaged from three repeated measurement.

The interfacial shear strength (IFSS) was adopted to quantify the interfacial properties between CF and resin matrix by an universal material evaluation equipment machine (Tohei Sangyo Co. Ltd., Japan). A carbon fiber monofilament was fixed on a metal holder with adhesive tape. The epoxy resin (E-51) and curing agent (H-256) were mixed in 100 : 32 mass ratio to prepare microdroplets. The microdroplets were cured at 363 K for 2 h, 393 K for 2 h and 423 K for 3 h. The value of IFSS was calculated according to eqn (3),

where F_max is the maximum load recorded, d is the average diameter of fiber, and l is the embedded length of a monofilament in resin droplet. The adopted value of IFSS for each fiber type was averaged from 50 successful measurement data.

Single fiber tensile tests (TS) were performed on an universal testing machine (5500R, Instron, USA) according to ASTM D3379-75. The specimens were fixed with gauge length of 20 mm and tested at cross-head speed of 10 mm min⁻¹. At least 100 samples were analyzed with two-parameter Weibull statistical method.

3. Results and discussion

Raman spectroscopy is adopted to analyze the surface of the untreated CF and the modified CF. It is clearly seen that the two characteristic bands are assigned to the “D-band” (defects-induced Raman band, attributed to an A_1g mode) and “G band” (an ideal graphitic lattice vibration mode with E_2g symmetry), at ∼1350 and ∼1600 cm⁻¹, respectively.^26,27 The ratio of intensities between the D and G bands (I_D/I_G) of the CF in Raman spectroscopy is taken as a measure of defects in CF.^28,29 Additionally, the Raman spectra of CF must be fitted because of the serious overlap between D and G band. A shoulder band, known as A-band (associated with amorphous forms of carbon or interstitial defects), also have been observed at 1500–1550 cm⁻¹.^26,27 The Raman spectra and the best curve fitting are obtained by using Renishaw WiRE 3.3 software.

The wave numbers and relative intensities of the different bands are given in Table 1. Fig. 2a–d show the Raman spectra and the corresponding curve fit spectra for untreated CF, NH₂-CF, CNT@CF and CNT/CF hybrid. Fig. 2e illustrates the intensity ratio I_D/I_G for the different carbon fiber. The I_D/I_G value of NH₂-CF increases from 2.58 to 3.31 compared to the untreated CF. This phenomenon indicates the first aryl diazonium treatment lead to a higher disordered degree, which would come from the breaking of graphite structure of CF surface. This demonstrates that aniline groups are covalently grafted onto the surface of CF. Furthermore, I_D/I_G value of the CF after being combined with CNT decreases slightly, from 3.31 for NH₂-CF to 3.29 for CNT/CF hybrid. This would mean that CNTs with lower I_D/I_G value substitute the NH₂ groups through the second aryl diazonium reaction, which does not affect the ordered degree of CF surface. There is no change in the I_D/I_G value between CNT@CF and NH₂-CF. This indicates that a few physisorbed CNTs can't substantially affect the graphitic order of NH₂-CFs.

Table 1 Wavenumbers (W) of D, G and A bands and the calculated values of “I_D/I_G” acquired from CF

Samples	D	G	A	ID/IG
	W (cm^−1)	W (cm^−1)	W (cm^−1)
CF	1349.42	1603.07	1530.34	2.58
NH2-CF	1351.53	1599.25	1527.72	3.31
CNT@CF	1340.59	1584.82	1503.34	3.31
CNT/CF hybrid	1352.20	1600.18	1526.72	3.29

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Fig. 2 Raman spectra and curve fitting for the all samples (a) untreated CF, (b) NH₂-CF, (c) CNT@CF, (d) CNT/CF hybrid and (e) intensity of I_D/I_G for all CF.

FT-IR has been performed on different CF samples. Fig. 3 shows the FT-IR spectra of the CF at each step. Compared with the spectrum of the untreated CF, some new bands in the spectrum of the NH₂-CF are obviously detected, a new absorption peak at 3737 cm⁻¹ which is attributed to the –N–H stretching vibration on the carbon fiber generated by the first aryl diazonium treatment.³⁰ The peaks at 1512 and 1458 cm⁻¹ can be assigned to C=C stretching vibration from substituted aromatic ring and the peak at 879 cm⁻¹ is attributed to in-plane bending vibration from para-oriented aromatic ring, indicating the aniline groups were grafted to the carbon fiber surface through chemical bonds.³⁰

Fig. 3 FT-IR spectra of (a) untreated CF, (b) NH₂-CF, (c) CNT@CF and (d) CNT/CF hybrid.

For the CNT/CF hybrid, there is a new band at 1735 cm⁻¹, which can be assigned to the CNT characteristic adsorption.³¹ Meanwhile, the –N–H peaks at 3737 cm⁻¹ disappear, but the peaks at 1512, 1458 and 879 cm⁻¹ still exist, indicating the CNT has been grafted to the carbon fiber surface through substituting the NH₂ groups. These data demonstrate that CNTs have been introduced onto the surfaces of CFs.

For the CNT@CF, the only difference is that the band at 3737 cm⁻¹ still exists, compared with the CNT/CF. It indicates that the physisorbed CNTs doesn't substitute the amine group.

XPS survey scans are performed to detect the surface elements of the carbon fiber surface. The wide survey spectra and C1s peak deconvolution of each CF are shown in Fig. 4. For the untreated CF (shown in Fig. 4a), there are carbon, oxygen and insignificant amount of nitrogen on carbon fiber surface. The C1s core-level spectrum of the untreated CF can be deconvoluted to four component peaks: Csp² and Csp³ in the fiber structure (peak C1s (1), 284.4 eV), C–C bonding of amorphous carbon (peak C1s (2), 285.2 eV), C–O bond (peak C1s (3), 286.8 eV) and carboxyl groups (peak C1s (4), 288.4 eV).³² As seen from Fig. 4b, we can find that through the first aryl diazonium reaction, the nitrogen content and N/C ratio increase significantly and there is a new binding energy peak 285.2 eV which can be assigned to new generated C–N bond (peak C1s(5)).³³ This indicates that the aniline groups had been grafted onto the CF surface. The aniline groups grafted onto the fiber surface would effectively increase the polarity of the carbon fiber surface and make the fiber more easily further react with epoxy matrix for enhancing the interfacial strength of the composites. For CNT@CF (shown in Fig. 4c), the N/C ratio and the content of C–N bond all slightly drop. This indicates that there is no reaction between aniline group and physisorbed CNT. However, after combined with CNT through the second aryl diazonium treatment, the nitrogen content and N/C ratio decrease slightly, but the content of C–N bond drops sharply. All this phenomenon demonstrates that the amine groups on the carbon fiber surface have been replaced with CNT by the second aryl diazonium reaction, which is consistent with the results of the Raman and FT-IR discussed above.

Fig. 4 XPS survey spectra (inset) and curve fitting for C1s peak of (a) untreated CF, (b) NH₂-CF, (c) CNT@CF, (d) CNT/CF hybrid and (e) ratios of N1s/C1s and C–N peak area for all CF.

Scanning electron microscopy is used to observe the surface morphology of CFs before and after grafting CNT. Fig. 5a shows surface morphology of the untreated CFs: there is a relatively neat and smooth surface, and a few deep grooves distribute along the axial direction of the CF. As shown in Fig. 5b, the roughness of fiber surface has obvious change after the first aryl diazonium reaction. Some particles distribute uniformly on the CF surface in different directions. In Fig. 5c a few CNTs are dispersed on the surface of CFs and some CNTs entangle with each other. However, after grafting CNT through the second aryl diazonium reaction, the CF surface morphology changes more obviously, as shown in Fig. 5d. We can find that a lot of CNTs are stuck uniformly to the CF surface along the outer edges with different angles due to the individual nanotube with high specific surface area. No large aggregates of the raw CNT appeared. The TEM images of CNT/CF hybrid (Fig. 5e and f) show the roots of CNTs with the strong adhesion on the surface of CFs, which further affirms the MWCNTs are firmly grafted onto the CF surface through the second aryl diazonium reaction. Combined with the results discussed above, the CNT/CF hierarchical structure can be achieved. The CNT would be like a bridge to connect CF and matrix, inserting to the composite interface region could significantly enhance the interfacial adhesion between the fiber and the matrix.

Fig. 5 SEM images of (a) untreated CF, (b) NH₂-CF, (c) CNT@CF, (d) CNT/CF hybrid and (e and f) TEM image of CNT/CF hybrid.

It is well known that the surface functionality and topography of CF can affect fiber surface energy as well as its components.^34,35 High fiber surface energy can increase a better wettability between the CF and the matrix to promote the comprehensive performance of the CF reinforced resin composite. The advancing contact angle (θ), the total surface free energy (γ), its dispersion component (γ^d) and polar component (γ^p) of untreated and modified CF are shown in Fig. 6. We can find that after the first aryl diazonium reaction the contact angle of the NH₂-CF decreases from the 78.6 to 52.0 for water and from 51.2 to 34.9 for diiodomethane. The improved polar component could be supplied by the introduced aniline groups in the context of previous studies. The increased dispersion component could be attributed to the increased fiber surface roughness. For the CNT@CF, a little change of the surface free energy occurred due to the slight variation in surface roughness caused by the physisorbed CNTs, without creating additional polar groups. When CNT are grafted onto CF via the second aryl diazonium reaction, the surface free energy shows a further increase. However, the polar component of surface free energy decreases because the aniline groups are replaced by non-polar CNT. Meanwhile, the introduced CNT gives rise to the more visible changes on CF surface as shown in Fig. 5d, which dominates the dispersion component of surface free energy. In summary, the CNT/CF hybrid have improved interfacial wettability by massive surface area and some functional groups, which are beneficial to the subsequent formation of covalent bonding, van der Waals interaction and physical entanglement, and could greatly enhance the interfacial properties of the composites.

Fig. 6 Contact angles and surface free energy of untreated CF, NH₂-CF, CNT@CF and CNT/CF hybrid.

The single fiber tensile strength test was performed to analyze the influence of grafting on the tensile strength of the fiber. The Weibull distribution was always used to assess the tensile strength.³⁶ The lower the Weibull shape parameters are, the more defects the fibers have.³⁷ As shown in Fig. 7 and Table 2, compared with the untreated CF (3.91 GPa), a comparable fiber tensile strength was observed for the NH₂-CF (3.87 GPa), suggesting that the chemical grafting of aniline groups on the CF surface has little effect on the fiber tensile strength. And the tensile strength of CNT@CF (3.86 GPa) also is not affected by the physisorbed CNTs. We also find that the tensile strength (4.25 GPa) of the CNT/CF hybrid shows a slight increase through the CNT grafting treatments, about 8.69%. This suggests that the introduction of CNT onto the surface of carbon fiber helps to “fill” the grooves or edges on the fiber surface without experiencing any oxidation process and therefore reduces stress concentrations, resulting in improved tensile strength of carbon fiber.

Fig. 7 Tensile strength (TS) and interfacial shear strength (IFSS) of the composites reinforced by untreated and modified carbon fiber.

Table 2 Single fiber tensile strength of specimens

Samples	σ0	m	Expectation (GPa)
Untreated CF	4.73	3.21	3.91
NH2-CF	4.64	3.17	3.87
CNT@CF	4.65	3.18	3.86
CNT/CF hybrid	4.79	3.26	4.25

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Interfacial shear property is one of the most important performances for carbon fiber reinforced composites. The results are also showed in Fig. 7. The IFSS values of CF, NH₂-CF, CNT@CF and CNT/CF hybrid reinforced composites are 46.80, 81.13, 80.87 and 95.76 MPa, respectively. Compared with untreated CF, NH₂-CF increases IFSS of the composites by 73%, and CNT/CF hybrid significantly increases IFSS of the composites by 104%. Zhao et al.³ grafted POSS onto carbon fibers, which increase IFSS from 46.8 MPa to 80.6 MPa by 72.2%. Wang et al.³⁸ introduced CNT into carbon fiber, the IFSS increased by 45%. Li et al.⁷ reported that the deposition of the COOH–CNTs gives rise to a significant increase of the IFSS (about 43%) for the carbon fiber composite. Peng et al.¹⁵ found that IFSS increases to 67 MPa for PAMAM-absorbed CF composite. Compared to acid-treated composites, the IFSS has a 56% increase after PAMAM absorption. Generally, the mechanism of the improvement of interfacial shear strength is related to mechanical interlocking, surface wettability and chemical bonding between carbon fiber and matrix. After grafting of the aniline groups, the NH₂ functional groups play an important role in improving the interfacial adhesion between the fiber and the matrix. These polar functional groups could increase the surface free energy of carbon fiber, and make the fiber surface easier to be wetted by epoxy, which makes the degree of the molecular contact maximum. In addition, these amine groups could form a strong chemical bonding at the interface between the fiber and the matrix during curing process as a curing agent. But the IFSS of the CNT@CF drops slightly because there is few additional surface roughness derived from the physisorbed CNTs. However, the further improvement on the interfacial strength of CNT/CF hybrid could be attributed to the enhancement of surface free energy, the mechanical interlocking and the increased fiber surface area caused by CNT on the fiber surface. This interlocking effect of CNT can be further confirmed from the fracture morphologies of the CF composites (Fig. 8). The CNTs grafted on the fiber surface stick into the epoxy matrix and work as Velcro hooks to fix in the interface region. All these phenomena increase the interfacial adhesion between the hybrid and the matrix.

Fig. 8 Debonded interfaces of (a) untreated CF, (b) NH₂-CF, (c) CNT@CF and (d) CNT/CF hybrid.

4. Conclusions

In summary, the raw CNT was grafted onto the CF surface by a two-step diazonium reaction route. Raman, FT-IR, and XPS demonstrated that CNTs were grafted onto the CF surface. The SEM, TEM and DCA results manifested that the roughness and the surface free energy of the CF surface increased obviously through grafting CNTs. The IFSS between CNT/CF hybrid and epoxy resin could enhance from 46.8 MPa to 95.76 MPa increased by 104%, which may be attributed to the enhancement in reactive groups, surface free energy and surface roughness. Moreover, the tensile strength also has shown a slight increase of 8.69%, from 3.91 GPa to 4.25 GPa. Therefore, the two-step aryl diazonium reaction provides a simple and efficient method for fabricating the CNT/CF hybrids in aqueous solution without damaging the substrates.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (no. 21174034) and the Program for Young Teachers Scientific Research in Qiqihar University (2012k-M01).

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