Few- and multi-layer graphene on carbon fibers: synthesis and application

Abstract

In the current study, we investigated the influences of chemical vapor deposition parameters on the formation of uniform structures of few- and multi-layer graphene (FLG and MLG) as a coating phase on carbon fiber (CF). To this end, the process conditions of the chemical vapor deposition method, such as catalyst concentration, reaction temperature and time, and also carbon source flow rate, were optimized. The resulting FLG and MLG with high yields led to the modification of the CF surface by improving its properties. By applying scanning electronic microscopy, transmission electron microscopy and Raman spectroscopy, the surface morphology and structural information of the G–CF were analyzed. It was observed that under different conditions the FLG–CF and MLG–CF were obtained with 54%, 58% yields and also 10.21 m² g⁻¹, 8.78 m² g⁻¹ BET surface areas, respectively. Besides that, the FLG–CF and MLG–CF were used as fillers in the polypropylene (PP) composite and the effects of the number of graphene layers on the mechanical and thermal properties of the composite were analyzed. It is noteworthy to mention, composites based on the CF coated with G with only a few layers presented the highest surface area, strength and thermal resistance compared to those based on multi layers.

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Introduction

Carbon fibers (CF) with excellent properties, such as high strength and low weight have been used in industry.^1,2 For a wide range of potential applications, especially in mechanical engineering, CF is primarily preferred for composite material usage due to its outstanding properties like high specific strength and stiffness, performance to weight ratio, high thermal stability, high conductivity, self-lubrication and corrosion resistance.^3–5

Besides that, modifying the surface of the CF is an important issue to improve the efficiency of this material in the industry. So, the modifications of the CF structure, by carbon nanomaterials, have made a big difference in improving the properties of the CF. The utilization of the nanomaterials for the modification of the carbon fiber surface to form thermally stable coatings⁶ and/or for the improvement of the fiber/matrix interfacial adhesion has been recently appraised.^7–9 Graphene/polymer nanocomposites have been introduced as a new class of polymer composites with superior properties that can be applied in many fields of sciences and industries.¹⁰

Graphene (G) is one of the nanomaterials with a two-dimensional sheet of sp², which has been grown on the CF to improve the CF properties.¹¹

Its extended honeycomb network is the basic building block of other important allotropes; it can be stacked to form 3D graphite, rolled to form 1D nanotubes, and wrapped to form 0D fullerenes.¹² Long-range π-conjugation in graphene yields extraordinary thermal, mechanical, and electrical properties, which have long been the interest of many theoretical studies and more recently have become an exciting area for experimentalists.^12,13 Graphene sheets are divided into various types based on the number of layers such as few-layer graphene (4–10 layers) and multi-layer graphene (<10 layers).^14,15 A variation of the number of graphene layers may result in a striking change in their properties.¹⁶ Accordingly, it is very important to explore the production of graphene with a selected number of layers in large quantities for their further fundamental studies and extensive applications.

In order to determine the number of graphene layers, Raman spectroscopy must be used. Raman spectroscopy is a non-invasive technique, which has been widely used to characterize the structural properties of carbon-based nanomaterials.¹⁴ Besides that, this technique has been used to determine the number of graphene layers.^14,15

There are several techniques to synthesize graphene sheets including mechanical exfoliation and cleavage,^17–19 chemical reduction of GO,¹⁷ thermal decomposition on SiC,^20,21 liquid exfoliation^22,23 and chemical vapor deposition (CVD).^24–27 Synthesis of graphene through thermal chemical vapor deposition (CVD) is quite new. CVD growth has been reported as the most popular method for large-scale production of graphene layers.

To the best of our knowledge, modifying the CF by growing different types of graphene (FLG and MLG) with high surface areas and high yields by CVD has not been reported.

Additionally, the resulting FLG–CF and MLG–CF are used as fillers in a polypropylene matrix and analysis of their effects on the polymer composite properties. Therefore, the polymer composites based on graphene have been fabricated with improved properties.^28,29 The properties of the nanocomposite depend on the types of nanomaterials with high surface areas and excellent properties. So, the presence of graphene in the polymer matrix leads to a robust structure in the composites. The presence of G–CF in the polymer matrix has been studied to improve the composite properties.^30–32 Besides that, the number of graphene layers (few or multi) has a different impact on the polymer composite properties,³³ which will be discussed in this paper.

Materials and methods

Materials

In this research, the carbon fiber (Toho Tenax Co. Ltd) was applied as a filler in the polymer matrix. In order to grow graphene, a high purity acetylene (C₂H₂), nitrogen (N₂) and hydrogen (H₂) were used as the carbon source, carrier and promoter gases, respectively. Besides those, a bimetallic catalyst, including copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) and nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), was used. On the other hand, polypropylene pellets (PP 600G) were used as the polymer matrix and were supplied by Petronas Polymers Marketing and Trading Division, Malaysia.

Graphene growth on the CF surface

In order to synthesize the few layers and multi layers of graphene sheets, the CVD parameters had to be altered. Initially, the CFs were added into a bimetallic catalyst solution (copper nitrate trihydrate and nickel nitrate hexahydrate) with different concentrations (50 mM, 100 mM and 150 mM concentrations with a 1/1 ratio) and then agitated by use of ultrasonic agitation for 2 h. To eliminate the nitrate compound, the CFs were dried under airflow at 200 °C for about 2 hours.

The chemical vapor deposition method was employed to grow the FLG and MLG on the CF at atmospheric pressure at different temperatures from 950 °C to 1050 °C. This process was fulfilled by the decomposition of acetylene on the catalyst surface with different flow rates (25, 50 and 100 standard cubic centimeters per minute (sccm)) on a catalyst surface under a flow of H₂/N₂ (50, 100 sccm) in the reactor for different reaction times (10, 30 and 50 min). Finally, the C₂H₂ flow was stopped, the heater was turned off and then the reactor was cooled under the flow of N₂.

To analyze the structural information, morphology and surface area of the G–CF, Raman spectroscopy, the electron microscopes (SEM, TEM), and BET surface area analyzer were applied, respectively.

BET surface area analysis

The Brunauer–Emmett–Teller (BET) technique was employed to analyze the specific surface area of the resulting samples based on the ISO 9277.

Yield (carbon deposition efficiency%)

The process efficiency was determined through the weight of the deposited carbon, during each run. The yield or carbon deposition efficiency (CDE%), corresponding to the percentage of the deposited carbon in comparison with the introduced quantity of the carbon fiber coated with catalytic particles was calculated as below:where PM is the mass of the product and CM is the mass of the initial carbon fiber coated with the catalyst.

Composite fabrication

To fabricate the polymer composite, initially, the PP was melted by use of a mixer at 180 °C with a 55 rpm rotor speed for 5 min and then the fillers (5 wt%) were added and mixed for 15 min.³⁴ After that, the composite was put in a mold and positioned into a Hot Press Machine under 150 kg cm⁻² pressure at 180 °C for about 6 min and then cooled to 60 °C.

Composite characterization

The resulting composites were formed into bone shapes based on the ASTM D638 standard.³⁵ To do a tensile test, an Instron Universal Testing Machine was used to determine the tensile stress and tensile modulus of the composites (PP, CF–PP and G–CF–PP). The tests were performed with a crosshead speed of 5 mm min⁻¹.³⁶

Besides that, a thermogravimetric analysis (TGA) was used to analyse the thermal resistance of the polymer composite.³⁷ The heating program was performed from 25 °C to 800 °C under a flow of nitrogen.

Result and discussion

This research is divided into two main activities: at first, the synthesis and characterization of graphene on the carbon fiber (few layers and multi layers) and second, an investigation of the effects of the graphene layers on the composite properties and their interaction with a polymer matrix in the formulation of nanocomposites.

Optimization of the CVD parameters to grow graphene on CF

The effects of the catalyst concentration, temperature, carbon source flow rate and reaction time on the quality and quantity of the grown G were investigated and elucidated by means of Raman spectroscopy and BET surface area analyzer. These parameters were varied so that the experimental conditions were optimized. Besides that, the morphology and structure of the resulting few and multi layer graphene (FLG and MLG) were analyzed by SEM and TEM.

Effect of catalyst concentration

In order to investigate the influence of the catalyst concentration on the morphology and structural properties of the resulting graphene compounds, the synthesis were fulfilled at 1000 °C for 30 minutes, in a flow of N₂/H₂ (100/50 sccm) and acetylene (50 sccm) using three Ni/Cu catalytic systems, x = 50, 100, 150 mM. Raman spectra in Fig. 1 revealed the presence of the characteristic lines associated with the graphene layers. Analysis of these images indicates that, for all three situations, the morphology of the reaction products show characteristics of graphene structures.

Fig. 1 Raman spectra of G on the CF at 50 mM, 100 mM and 150 mM.

The catalyst concentration had two effects in this research; first, the effect on the number of graphene layers and second, on the coating phase grown on the CF surface. Based on this, the optimum catalyst concentration with a uniform and completed coating of graphene on the CF with a minimum graphene layer was required.

In the Raman spectra of the graphene, three major bands are significant: the D band, positioned at about 1340–1350 cm⁻¹, the G band at 1580–1590 cm⁻¹, and the 2D band at 2690 cm⁻¹.

The G band is characteristic of all the graphitic sp²-type structures. The D band is related to the number of structural defects within the graphitic layers, and its intensity increases with the number of layers. The 2D band is sensitive to the number of layers, and it is used to determine the number of graphitic layers.³⁸ The observed 2D band Raman peak intensity decreased with increasing the number of graphene layers.³⁹ The ratio of intensity of the D peak and G peak and also the ration of the intensity of the 2D peak and G peak are often used for estimating the degree of graphitization, number and size of the sp² cluster of carbonic structures. By decreasing the I_D/I_G, the formation of the carbon structure was proved with high graphitization. Besides that, the I_2D/I_G ratio of the samples increased when graphene flakes were present in the fewer layers.

Raman analysis indicates that the height of the 2D band decreased in the following order: 50 mM > 100 mM > 150 mM with different I_D/I_G ratios of 0.8, 0.81 and 0.69 and also I_2D/I_G ratios of 0.85, 0.68 and 0.61, respectively. As it can be seen in the Raman spectra, by increasing the catalyst concentration the number of graphene sheets increased.

Additionally, the BET surface area, yield and catalyst activity for all three-produced G–CF have been reported in Table 1. The total surface area of the G–CF decreased with increasing the bimetal catalyst concentration. From this result, it can be concluded that increasing the catalyst concentration leads to an increase in the amount and layers of the graphene sheet. So, there is a challenge between the amount and number of layers of the graphene sheet. A high amount of graphene causes the covering of the CF surface completely and then leads to an increase in the yield. Contrarily, increasing the number of graphene layers leads to a decreasing of the surface area of the coated CF.

Table 1 Effect of catalyst concentration on G production (on 0.5 g CF)

Catalyst content (mM)	BET surface area (m^2 g^−1)	PM–CM (g)	CDE (%)
50	8.79	0.12	24
100	9.74	0.22	44
150	8.84	0.25	50

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According to the BET surface area analysis, it was found that the 100 mM catalyst concentration produced few layers of graphene and high yields of graphene on the CF and was selected as an optimum catalyst concentration for FLG synthesis. Besides that, for growing MLG, the 150 mM catalyst concentration was used.

Effect of reaction temperature

To analyze the effects of the reaction temperature on the graphene structure, other parameters including the catalyst concentration (100 mM), reaction time (30 min), constant flow rates of acetylene (50 sccm), and N₂/H₂, 100/50 sccm were kept constant, while the reaction temperatures were varied at 950 °C, 1000 °C, and 1050 °C.

The Raman spectroscopy results are shown in Fig. 2. By increasing the temperature from 950 °C to 1050 °C, the ratio of I_D/I_G decreased from 0.96 to 0.53 and also I_2D/I_G increased from 0.65 to 0.82, respectively, which reveals the increment of the graphitization (decreased D peak and an increased G peak). Additionally, the increasing of the 2D peak at 1050 °C, states the presence of the few layers of the graphene. Therefore, when the temperature increased, the number of graphene layers decreased as well (Table 2).

Fig. 2 Raman spectra of G on the CF at 950 °C, 1000 °C and 1050 °C.

Table 2 Effect of growth temperature on G production (on 0.5 g CF)

Temperature (°C)	BET surface area (m^2 g^−1)	PM–CM (g)	Carbon deposition efficiency (%)
950	6.32	0.16	32
1000	9.74	0.22	44
1050	10.21	0.27	54

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The total surface area of the G–CF increased with increasing the reaction temperature. From this result, it can be understood that the reaction-temperature increment leads to an increase in the amount of the thin layers of the graphene nanoparticles as well as the surface area of the resulting G–CF. Moreover, the carbon deposition efficiency also increases with the temperature rise.

Effect of time

Reaction time has a main role to activate the catalyst and produce a high quality of graphene with a different number of layers. Hence, to find the impact of the reaction time on the properties of the produced graphene, the reaction time was conducted at 10, 30, and 50 min while keeping other parameters constant (100 mM catalyst concentration, 1050 °C temperature and N₂, H₂ and acetylene flow rates at 100, 50 and 50 sccm, respectively).

The Raman spectra of the graphene layers grown on the surface of the CF showed that the 2D and D peaks of the spectra changed significantly with the growth time (Fig. 3). By increasing the growth time, the D peak increased and 2D peak decreased. So, the ratios of I_D/I_G were about 0.94, 0.57 and 0.78 and the ratio of I_2D/I_G were 0.97, 0.82 and 0.65 for 10, 30 and 50 min, respectively. It means that by increasing the synthesis time, the coating of the graphene on the CF increased and covered the CF surface completely. On the other hand, increasing the time led to an increase in the number of graphene layers.

Fig. 3 Raman spectra of the CF–G at 10 min, 30 min and 50 min.

This is evidenced from the intensity of the 2D peak in the Raman spectra, which reveals that the presence of a few layers of graphene coated on the CF was low as reflected by the height of the D peak in the Raman spectra. On the other hand, by increasing the time to 50 min, the catalyst was no longer active, thus a greater number of graphene layers were formed on the CF surface, which caused the decrease in the 2D peak.

Moreover, the surface area results reported in Table 3 state that the G sheet on the CF at 30 min has the highest surface area, whereas, the reaction time of 10 min is insufficient to grow graphene completely on the CF so the surface area is low. By increasing the time to 30 min, the surface area of the G–CF increased. Contrarily, by increasing the duration to 50 min, the number of graphene layers increased; so, the minimum effect of graphene growth was observed.

Table 3 Effect of growth time on G production (on 0.5 g CF)

Time (min)	BET surface area (m^2 g^−1)	PM–CM (g)	Carbon deposition efficiency (%)
10	8.07	0.10	20
30	10.21	0.27	54
50	9.96	0.30	60

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A similar trend was observed on the yield, it was increased significantly as the reaction time increased from 10 to 30 minutes. For the samples synthesized at 30 and 50 minutes, no major differences are observed in the yield, purity, and thermal decomposition temperatures, thus it was concluded that the reaction time of 30 min was the optimum for the FLG growth and 50 min was suitable for the MLG synthesis.

Effect of acetylene (C₂H₂) flow rate

To study the effect of the carbon source (acetylene) flow rate on the morphology and properties of the graphitic structures, a reaction synthesis was conducted over the Ni/Cu catalyst at different flow rates of C₂H₂: 25, 50 and 100 sccm. The other parameters including temperature (1050 °C), catalyst concentration (100 mM), reaction time (30 min) and flow rates of N₂/H₂ (100/50 sccm) were fixed.

The Raman analysis (Fig. 4) for the Ni/Cu at different C₂H₂ flow rates indicates that the ratios of I_D/I_G were 0.82, 0.56 and 0.94 and also the ratios of I_2D/I_G were 0.74, 0.82 and 0.69 for 25, 50 and the 100 sccm flow rate which are related to the presence of fewer layers of graphene in the 50 sccm flow rate and amorphous carbon at 25 and 100 sccm flow rate. This could be a consequence of the increase in the flow rate of C₂H₂—a fact confirmed also by the N₂ adsorption–desorption isothermal. Although the 25 sccm flow rate had the potential to grow a single layer graphene sheet but the sheet was incomplete, so the highest surface area of G–CF was at the 50 sccm flow rate. By increasing the flow rate to 100 sccm, the surface area decreased (Table 4).

Fig. 4 Raman spectra of G–CF by use of 25 sccm, 50 sccm and 100 sccm acetylene flow rates.

Table 4 Effect of acetylene flow rate on graphene production

C2H2 flow rate (sccm)	BET surface area (m^2 g^−1)	PM–CM (g)	Carbon deposition efficiency (%)
25	8.12	0.18	36
50	10.21	0.27	54
100	10.33	0.31	62

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Regarding the aforementioned circumstances, by applying different conditions in the CVD method, few-layer graphene (FLG) and multi-layer graphene (MLG) were obtained. Therefore, by using the 100 mM catalyst condition at 1050 °C under a 50 sccm acetylene flow rate for 30 min, the FLG on the CF was produced and contrarily by usage of the 150 mM catalyst concentration at 1000 °C under the 100 sccm acetylene flow rate for 50 min, the MLG on the CF was synthesized. SEM and TEM images of FLG–CF and MLG–CF are presented in Fig. 5 and 6, respectively.

Fig. 5 (a) SEM and (b and c) TEM images of FLG–CF.

Fig. 6 (a) SEM and (b and c) TEM images of MLG–CF.

Fig. 5 and 6 illustrate the representative SEM and TEM images for the FLG–CF and MLG–CF, respectively. The SEM images demonstrated that the coating of graphene layers on the CF surface and TEM images show the structure and number of graphene layers. It is obvious that the yield and the surface area varied as the layers of graphene increased. The BET surface area and the yield were estimated at about 10.21 m² g⁻¹ and 54% for the FLG on the CF and 8.78 m² g⁻¹ and 58% for the MLG on the CF, respectively. Consequently, by increasing the graphene layers on the CF, the coating of the graphene on the CF was denser which led to an increase in the yield and decrease in the surface area. According to the TEM images, it can be seen that different conditions led to the two types of graphene morphology (FLG and MLG). The few layers are presented in the TEM images of Fig. 5 and multi layer in Fig. 6, as well. Regarding the TEM images, the G sheet in the FLG case is clearer and thinner than in the MLG form.

Composite characterization

The SEM micrographs reveal the morphology and structure of the fractured surfaces of the composites, such as CF–PP, FLG–CF–PP and MLG–CF–PP (see Fig. 7). As it is shown in Fig. 7(a), the neat CF had a very low interfacial interaction with the polymer matrix because of its smooth surface. On the other hand, the presence of graphene on the CF led to an increase in the interfacial interaction with the polymer matrix. Hence, in Fig. 7(b) some interfacial adhesion of the FLG–CF with the PP matrix is observed because of the rough surface of the filler. Hence, increasing the amount of the PP matrix on the CF surface proved the rough surface, which was related to the graphene growth. Besides that, the effect of the graphene number on the interaction of the PP with the CF is proved and shown.

Fig. 7 SEM images of (a) CF–PP, (b) FLG–CF–PP and (c) MLG–CF–PP.

By comparing the FLG–CF–PP, MLG–CF–PP and CF–PP, it was realized that the presence of the G flakes in the polymer matrix was more important than the number of graphene layers as a reinforcing factor; however, the effect of the number of graphene layers also had a significant role as an interlocking operation with the matrix of the composite. Furthermore, to probe the interaction between the fibers and polymer matrix, Raman spectroscopy has been applied. Generally, such an interaction is reflected by a peak shift or peak width change. In the field of composite materials, it has been known for more than two decades that the application of the mechanical strain to fibers (in air, thus without a polymer matrix), such as carbon or Kevlar, results in the shifted frequencies of the Raman peaks (usually 2D peak), which are directly related to the interatomic force constants.⁴⁰ Therefore, the peak shift or peak width change states the interaction between the fibers and the polymer matrix. So, the spectra related to the graphene nanocomposite were sharper than the neat PP matrix because of the resonance and absorbance effects.

Fig. 8(a–d) shows the Raman spectra of the PP, CF–PP, FLG–CF–PP and MLG–CF–PP samples.

Fig. 8 Raman spectra of (a) PP, (b) CF–PP, (c) FLG–CF–PP and (d) MLG–CF–PP.

The peaks of graphene in the Raman spectra are obviously observed but the peaks for the PP did not appear because of their low intensity.⁴¹ So, the presence of the graphene led to an increase in the intensity of G and 2D peaks, which was due to the enhancement of the interaction between the polymer and filler. Finally, it was found that the interaction of the FLG–CF was more than the MLG–CF with the polymer matrix.

Mechanical test

All the mechanical properties that will be shown in this section are a comparison between the bulk mechanical properties of the neat-CF composites and the G–CF composites to demonstrate the superiority of the latter. The tensile strength and Young's modulus results and the stress–strain curves of the composites are presented in Fig. 9 and 10, respectively. In the context of this work, the CF treated with different types of graphene sheets (FLG and MLG) was able to produce the composites with a relatively higher tensile modulus and strength compared to the neat CF–PP composites.

Fig. 9 Tensile strength and modulus of different PP composites with 5 wt% filler.

Fig. 10 Tensile stress–strain graphs of PP, CF–PP, FLG–CF–PP and MLG–CF–PP.

In this research, the tensile stress and Young's modulus of pure PP were about 28 MPa and 1400 MPa, respectively.

Comparing the stiffness of the different resulting composite, illustrates a significant improvement in the tensile modulus. By adding the different fillers into the PP matrix, the tensile stress and modulus of the polymer increased. Besides that, by comparing the tensile results of the filler (CF) with the nanographene (FLG and MLG), it can be concluded that the tensile stress and Young's modulus of the CF–PP are the lowest amounts, which are related to the smooth surface. This leads to the defective flow of the polymer matrix around the CF surface that causes the CF to be easily pulled out of the matrix. The stiffness and the rigidity of the FLG–CF–PP composite were higher than the MLG–CF–PP because of the FLG presence that led to the enhancement of the interfacial adhesion between the filler and the polymer⁴² because of more enhancement in the surface area of the CF. On the other hand, the presence of a few layers of G with a high surface area in comparison with the multi layers of G led to not only high stress transfer but also the high rigidity of the resultant composite.

It is also revealed in Table 5 that the modulus of FLG–CF–PP was more than MLG–CF–PP and both nanocomposites had higher modulus than the CF–PP composite. Such a significant difference may be related to the presence of graphene and also a difference in the graphene layers. The FLG–CF as the strongest filler had a high adhesion with the polymer matrix that was proved with the tensile results. Additionally, the presence of the graphene layers led to an improvement in the surface area of the CF with a tough interlocking with the PP matrix. Hence, the operative reinforcement modulus of the FLG–CF–PP was more than CF–PP (about 1155 MPa) and the MLG–CF–PP (about 380 MPa). So, it can be concluded that the effect of the presence of the graphene is more than the number of the graphene layers on the mechanical properties of the polymer composite.

Table 5 Tensile results for different composite

Sample	Tensile stress (MPa)	Increment (%)	Tensile modulus (MPa)	Increment (%)
PP	28	—	1400	—
CF–PP	30.5 ± 0.5	8.9%	1603.7 ± 24.5	14.5%
FLG–CF–PP	51.2 ± 0.8	82.8%	2758.3 ± 49.8	97.0%
MLG–CF–PP	46.3 ± 0.4	65.4%	2375.2 ± 36.5	69.6%

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Thermogravimetric analysis (TGA)

The thermal degradation/resistance of the polymer composite was investigated to evaluate the effects of different nanofillers, such as the CF, FLG–CF and MLG–CF on the polymer thermal properties. In the TGA analysis, when the sample absorbed a certain amount of heat, a single degradation step related to thermal degradation began to occur. At this step, the structure of the polymer composite broke down.

The TGA curves of the pure PP, CF–PP, FLG–CF–PP and MLG–CF–PP composites are illustrated in Fig. 11(a). The presence of the fillers, such as the CF and G in the PP matrix caused an increase in the composite degradation temperature because of the higher heat absorption capacity. The neat PP decomposed rapidly starting at about 350 °C to 400 °C and was completely degraded at 490 °C with no residual char left. The CF–PP composite started losing weight at 420 °C, which was at a higher temperature than the pure PP.

Fig. 11 (a) TGA and (b) DTG curves of different composites (1: neat PP, 2: CF–PP, 3: MLG–CF–PP and 4: FLG–CF–PP).

At 500 °C, the sample started to degrade and the graphene layers oxidized. Therefore, the temperatures above 500 °C corresponded to the lost mass of the graphene while the decomposition below this temperature was related to the amorphous carbon.⁴³ The nanocomposites (FLG–CF–PP and MLG–CF–PP) began losing their mass and ended at the temperatures around 520 °C to 600 °C and 470° to 590°, respectively. The FLG–CF–PP had more thermal resistance than the MLG–CF–PP and both of them had more stability than the CF–PP. Subsequently, by adding the G–CF to the PP matrix, the thermal stability of the polymer composite was significantly enhanced in comparison to the composites without graphene flakes. Further evidences from the DTG graphs in Fig. 11(b) show the melting temperatures of the composites. Using fillers with high thermal resistance increased the melting point of the composites. The melting point for the PP was around 470 °C and increased to 520 °C for CF–PP, 580 °C for MLG–CF–PP and 600 °C for FLG–CF–PP.

These increases can be attributed to the heterogeneous nucleation effect of the nanofillers, which facilitates the crystallization of the PP chains when the nanocomposite is cooled down from a temperature above its melting point.⁴⁴ It is known that the value of the degree of super cooling, T_max, can be employed in order to describe the crystallization behavior of the polymers. The lower value of T_max for the nanocomposites compared to the PP shows that the induction time to crystallize is lower for the nanocomposites than that of PP.

Conclusions

The most important contribution of this research is related to modifying the surface of carbon fiber (CF) by means of growing graphene in the chemical vapor deposition technique. In addition, the growing of the few-layer and multi-layer G on the CF was also investigated. The main objective of growing G directly on the CF was to modify the CF surface including enhancement of the surface area and strength. Besides that, the modified CF was used as a filler in the PP matrix in order to improve the polymer composite properties. Hence, the influences of the number of layers of graphene were studied on the resulting CF and also polymer composite. By increasing the FLG on the CF, the surface area of the CF increased which made a robust network with high adhesion between the CF and the polymer matrix. This adhesion led to the improvement of the mechanical and thermal properties of the nanocomposites. Raman spectroscopy was applied to detect the number of grown graphene layers on the CF. Besides that, this method has been used to analyze the filler/polymer interaction in the composite.

To investigate the impacts of the different fillers on the mechanical and thermal properties of the resulting polymer composites, the tensile test and TGA/DTG were performed. Hence, the FLG–CF–PP with the highest surface area was selected as a pioneer filler in the polymer matrix to increase the mechanical properties and thermal stability properties of the polymer. The tensile stress and modulus of the FLG–CF–PP, MLG–CF–PP and CF–PP composites have increased significantly to about 98.2% and 114.2%, 65.3% and 69.6%, and 8.9% and 14.5%, respectively, compared to the neat PP. In addition, the thermal stability of the CF–PP, MLG–CF–PP and FLG–CF–PP increased from 50 °C to 125 °C, respectively, in comparison to the neat PP. Therefore, these results reveal the importance of not only the modification of the CF surface as a filler in the polymer composites but also the impact of the number of graphene layers on the polymer composite properties.

Notes and references

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