Composites: Part A 56 (2014) 72–79

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Composites: Part A

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Facile preparation and characterization of free-standing stiffcarbon-based composite films with excellent performance

1359-835X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesa.2013.09.013

⇑ Corresponding authors. Tel./fax: +86 21 3420 5665.E-mail addresses: hunantao@sjtu.edu.cn (N. Hu), yfzhang@sjtu.edu.cn (Y.

Zhang).

Liling Zhang, Nantao Hu ⇑, Hao Wei, Xiaolu Huang, Liangming Wei, Jing Zhang, Yafei Zhang ⇑Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nanometer Science & Technology, Shanghai Jiao Tong University,Shanghai 200240, PR China

a r t i c l e i n f o

Article history:Received 6 July 2013Received in revised form 17 September 2013Accepted 20 September 2013Available online 1 October 2013

Keywords:A. Carbon–carbon composites (CCCs)B. Mechanical propertiesB. MicrostructuresB. Physical properties

a b s t r a c t

Novel free-standing stiff all carbon films based on multi-walled carbon nanotube (MWNT)/glassy carbon(GC) with excellent performance were fabricated. MWNTs, as excellent reinforcing materials, were suc-cessfully dispersed in polyimide (PI) matrix by in situ polymerization. The resultant MWNT/PI nanocom-oposite films were used as precursors and underwent carbonization process. As a result, all carbonconstituted MWNT/GC composite films were obtained. Mechanical results showed the maximum 3-pointbending strength and modulus reached 575.5 MPa and 7.7 GPa respectively, improved by 54% and 78%compared to those of neat GC films. This method is simple, and the free-standing composite films canbe prepared in large scales, which hold great potential in many applications.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Free-standing paper-like or foil-like materials play importantroles in our technological society, including protective layers,chemical filters, components of electrical batteries or supercapaci-tors, adhesive layers, electronic or optoelectronic components, andmolecular storage [1,2]. Carbon-based films, including flexiblegraphite foils [3], ‘bucky paper’ [4], graphene or graphene oxide pa-per [2,5,6] and diamond, etc., attract much attention in manyfields, such as reinforcement materials, the storage of energy, sup-ports for important catalytic process and so on [7–9].

Glassy carbon, as another allotrope of carbon, has glass-likeappearance with low cost, high stiffness, superior thermal andchemical stability in extreme environments [10], which makes itpopular in many applications, such as electrode material [11],mold material [12], carbon foams [13] and so on. It is expected thatfree-standing carbon-based films based on glassy carbon willarouse much attention because of their high modulus, highstrength, as well as high stiffness. However, it is very difficult tofabricate free-standing glassy carbon films owing to their brittle-ness characteristics. In order to obtain this type of free-standingstiff carbon films, it is necessary to modify the matrix. Many typesof filler, such as carbon fibers, glass fibers, carbon nanotubes(CNTs), etc., have been utilized to enhance the mechanical

properties of many matrices. Therein, CNTs have become the idealcandidates as filler materials in composites for mechanicalenhancement [14]. As we know, CNTs, discovered by Iijima in1991 [15], have aroused much attention due to their high aspectratios, remarkable mechanical [16,17], electrical properties [18],optical [19] and thermal properties [20]. CNT-based nanocompos-ites have a wide range of applications, including catalysis, superca-pacitors, lithium batteries, biosensors, and enforcement materials[21–27]. The improvement of mechanical, electrical and thermalproperties of many matrices, including polymer, ceramic and me-tal, have been successfully achieved through the addition of CNTs[28–32]. Through introduction of CNTs, the crack interfaces ofmatrices will be bridged by nanotubes, and consequently, betterperformance can be realized [33]. Therefore, it is worthwhile to de-velop CNT-reinforced GC-based free-standing nanocompositefilms. With the aid of CNTs as the reinforcement materials, it is ex-pected that the free-standing composite films would possess excel-lent performance in mechanical, electrical, and thermal properties.

In this work, novel free-standing stiff all carbon films based onMWNTs-reinforced GC membranes with excellent performancewere fabricated. MWNTs, as excellent reinforcing materials, weresuccessfully dispersed in polyimide (PI) matrix by in situ polymer-ization. The resultant MWNT/PI films were used as precursorsthrough the carbonization process, and as a result, MWNT/GC com-posite films were obtained. The resultant free-standing MWNT/GCcomposite films exhibited excellent strength and modulus, lowsquare resistance and other advantages compared with pristinecarbon films.

Fig. 1. The FT-IR spectra of (a) the pristine MWNTs and (b) the acid-treatedMWNTs. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

L. Zhang et al. / Composites: Part A 56 (2014) 72–79 73

2. Experimental section

2.1. Materials

The MWNTs, 10–20 nm in diameter, 10–30 lm in length andwith purity over 95 wt%, were purchased from Chengdu OrganicChemicals Co. Ltd. (China). For the purpose of perfect dispersionof MWNTs in the polymer matrix, we pre-treated the MWNTs un-der sonication with acid treatment for 8 h, followed by filtrationand washing with deionized water and finally vacuum dried inthe oven. As a result, the obtained MWNTs were modified with car-boxylic groups. Pyromellitic dianhydride (PMDA), 4, 40 - diaminodi-phenyl ether (ODA) and N, N-dimethylacetamide (DMAc) werepurchased from Shanghai Chemical Reagent Company (China).

2.2. Preparation of MWNT/polymer composite films by in situpolymerization

The pre-treated MWNTs were firstly dispersed in DMAc undersonication for 30 min. Then ODA was added into the above DMAcsolution with sonication for another 30 min, and equimolaramount of PMDA was added slowly in batches. After further stir-ring for 4 h at room temperature, the sticky and homogenousMWNT/polyamic acid (PAA) solution was obtained as precursorsolution. Afterwards, the as-prepared MWNT/PAA solution wascast onto clean glass plates and dried at 60 �C, 90 �C, 120 �C,150 �C respectively, each for 1 h. The films were subsequentlypeeled off from the glass plates, followed by undergoing heat treat-ment at 350 �C for 1 h in vacuum for total imidization. As a result,the solvent-free MWNT/ PI composite films were obtained.

2.3. Preparation of MWNT/GC composite films

The as-synthesized MWNT/PI composite films were then put inbetween two carbon plates, and underwent direct carbonizationwith a vacuum degree of 10�3 Pa. The heating program of the car-bonization was controlled at 2 �C/min from room temperature to900 �C and held at 900 �C for an hour. After natural cooling, theMWNT/GC free-standing composite films were finally obtained. Aseries of such MWNT/GC composite films were prepared with theconcentrations of MWNTs ranging from 0 wt.% to 20 wt.%.

2.4. Characterization

Transmission electron microscope (TEM) images of the acid-treated MWNTs were obtained by JEM-2100 (JEOL Ltd., Japan) atan acceleration voltage of 200 kV. Static mechanical 3-point bend-ing tests of the composite films were evaluated by dynamic ther-momechanical analysis (DMA) on TA-Q800 (TA Instruments-Waters LLC) under controlled force mode. The test specimens werecut into rectangle shapes with the dimensions of ca. 50 mm � ca.5 mm. The specimens were mounted using 3-point bending clampswith a clamp compliance of 0.278 lm/N. The specific length andwidth were measured using standard calipers (Mitutoyo). The spe-cific thickness was measured by a spiral micrometer gauge. All 3-point bending tests were conducted at room temperature in con-trolled-force mode with a preload of 0.01 N, and force was loadedwith a force ramp rate of 0.05 N/min. Field-emission scanning elec-tron microscopy (FE-SEM, Carl Zeiss Ultra 55) was used to observethe acid-treated MWNTs and the morphologies of MWNT/GC com-posite films and their failure structures after the mechanical tests.Thermal-gravimetric analyses (TGA, PerkinElmer Pyris 1) were car-ried out from 50 �C to 750 �C at a heating rate of 5 �C /min in airand N2. Fourier transform infrared (FT-IR) spectra were recordedon a Bruker (Germany) VERTEX 70 spectrometer (KBr pellets) over

a range of 400–4000 cm�1 with DTGS or MCT as detector. Squareresistance was recorded using RTS-8 4-point probes resistivitymeasurement system. Vickers-hardness was carried out by HXD-1000TMB/LCD micro hardness tester (Shanghai Taiming OpticalInstrument Co., Ltd. in China) with 100 gf test force and 10 s dura-tion time.

3. Results and discussion

3.1. Characterization of the acid-treated MWNTs

The FT-IR spectra of the pristine MWNTs and the acid-treatedMWNTs are shown in Fig. 1. The peaks located at 3440 and1630 cm�1 are corresponded to ACH stretching and C@C stretchingrespectively. As for the acid-treated MWNTs (Fig. 1(b)), a new peakappeared at 1715 cm�1, corresponding to carboxylic groupsstretching (ACOOH), which confirms that the carboxylic groupswere attached on the surfaces of MWNTs after the sonication withacid treatment [34]. Besides, the peaks at 3448 and 1575 cm�1 cor-responded to ACH stretching and ACOOA stretching.

Fig. 2 shows the SEM and TEM morphologies of the pristineMWNTs and the acid-treated MWNTs. It can be seen that pristineMWNTs are long, curled and twisted as ropes in Fig. 2(a). After acidtreatment, the acid-treated MWNTs have been cut into shortlengths and become disentangled as shown in Fig. 2(b). The major-ities of acid-treated MWNTs have a length of several micrometers.TEM images in Fig. 2(c) shows that the wall surfaces become roughafter the acid treatment, with some visible defects. Fig. 2(d) is anenlarged image of the individual acid-treated MWNT, which exhib-its a tubular structure with an outer diameter of �ca. 20 nm. Theystill maintain their large aspect ratios, which is crucial to enhancethe mechanical properties of composite films.

3.2. Thermal properties of the MWNT/PI composite films

Fig. 3 shows the digital photograph of the obtained 1 wt.%MWNT/GC composite film. It can be observed that composite filmis free-standing and flat, with a diameter of 3.5 cm. Other MWNT/GC composite films loaded with different MWNT concentrationslook almost the same from the appearance.

Thermal degradation can be monitored by TGA, and the behav-iors of neat PI films and the MWNT/PI composite films are shownin Fig. 4(a). We can observe that neat PI films begin to decomposeat 550 �C. Meanwhile, the MWNT/PI composite films have excel-lent thermal stability and do not decompose until 610 �C–630 �C.It is clearly known that the addition of MWNTs increases the ther-mal stability of the MWNT/PI composite films. In nitrogen (N2)

Fig. 2. SEM images of the pristine MWNTs (a) and the acid-treated MWNTs (b); TEM images of the acid-treated MWNTs (c, d).

Fig. 3. The digital photograph of the obtained MWNT/GC composite film.

74 L. Zhang et al. / Composites: Part A 56 (2014) 72–79

atmosphere (Fig. 4(b)), the char yields of the MWNT/PI compositefilms at 750 �C are similar with those of the neat PI films, whichsuggested that the addition of MWNTs didn not affect the final charyields of the samples. As we know, the incorporation of nanofillersis generally favorable to improve the thermal stability of polymercomposite systems [35]. Obviously, the MWNT/PI composite filmsexhibit higher thermal stability than that of the polyimide aloneaccording to the thermogravimetric analysis.

Fig. 4. TGA curves of neat PI films, 1 wt.% MWNT/PI film, 2 wt.% MWNT/PI film, 5 wt.% M(a) and N2 (b) atmosphere. (For interpretation of the references to color in this figure le

In order to acquire the actual MWNT concentrations in the finalMWNT/GC composite films, we measured the weight of eachMWNT/PI and MWNT/GC composite film. Yield ratio of MWNT/GC composite films can be calculated by the ratio of the weightsof MWNT/GC and MWNT/PI composite films. All the correspondingresultant values are around 58%–60%. Most importantly, MWNTconcentrations in MWNT/GC composite films have been deter-mined by the amount of MWNTs in the final MWNT/GC compositefilms according to the following equation:

cðwt:%Þ ¼WMWNT=Wcomposite � 100% ð1Þ

where c refers to the MWNT concentration in the MWNT/GC com-posite film, WMWNT refers to the amount of MWNTs used in theexperiment, and Wcomposite refers to the weight of final MWNT/GCcomposite film. Based on experimental results, the final actualMWNT concentrations are nearly doubled according to the theoret-ical MWNT concentrations in the experiment. This is mainly due tothe thermal degradation of MWNT/PI composite films during car-bonization at 900 �C. The thermal degradation ratio is calculatedto be about 50%, which is in accordance with the yield ratio ofMWNT/GC from MWNT/PI composite films ranging from 58% to66%. All the actual MWNT concentrations in the final MWNT/GCcomposite films were recorded in Table 2 for better comparison.

WNT/PI film, 10 wt.% MWNT/PI film, 20 wt.% MWNT/PI film in the atmosphere of airgend, the reader is referred to the web version of this article.)

Table 2The MWNT concentrations in the MWNT/GC composite films.

MWNT concentration a (wt.%) Neat 1 2 5 10 20

MWNT/PI composite film (mg) 269.1 316.7 317.5 326.7 343.8 367.2MWNT/GC composite film (mg) 156.7 183.3 185.6 201.4 213.0 241.5Yield ratio of MWNT/GC composite film b (%) 58.2 57.9 58.5 61.6 62.0 65.8MWNT concentration in MWNT/GC composite film c (wt.%) 0 2.3 4.6 11.0 21.9 43.5

a MWNT concentration indicates the theoretical MWNTs concentration in the experiment.b Yield ratio of MWNT/GC composite film is calculated by the ratio of the weights of MWNT/GC and MWNT/PI composite films.c MWNT concentration in MWNT/GC composite film is determined by the amount of MWNTs in the final MWNT/GC composite film.

Fig. 5. SEM images of the surface morphologies of (a) neat PI film, (b) 1 wt.% MWNT/PI film, (c) 2 wt.% MWNT/PI film, (d) 5 wt.% MWNT/PI film, (e) 10 wt.% MWNT/PI film, and(f) 20 wt.% MWNT/PI film.

Table 1The 3-point bending properties of the samples.

MWNT concentration (wt.%) Neat 1 2 5 10 20

Bending strength (MPa) 262.9 ± 7.9 288.5 ± 3.2 392.0 ± 7.8 575.5 ± 8.6 320.5 ± 5.1 330.5 ± 1.3Bending modulus (GPa) 1.7 ± 0.03 1.4 ± 0.07 3.4 ± 0.08 7.7 ± 0.23 2.2 ± 0.09 3.2 ± 0.13Strain (%) 0.30 ± 0.02 0.24 ± 0.01 1.90 ± 0.07 0.11 ± 0.01 0.16 ± 0.01 0.20 ± 0.01

L. Zhang et al. / Composites: Part A 56 (2014) 72–79 75

3.3. Morphologies of composite films

FE-SEM is usually used to observe the surface and failure struc-tures of specimens. To make specimens electrically conductive, theMWNT/PI composite films were coated with an ultrathin coating ofgold by vacuum sputtering in advance. Fig. 5 illustrates the SEMmicrographs of the MWNT/PI composite films with differentMWNT concentrations. When the MWNT concentration is lowerthan 5 wt.%, the surfaces of the MWNT/PI composite films are

smooth, only several bright dots, which are attributed to theMWNTs, can be observed. When the concentration of MWNTsreaches 5 wt.%, some MWNTs began to agglomerate in someregions on the surfaces. This phenomenon become more obviouswhen the concentrations of MWNTs reach 10 wt.% and 20 wt.%.Even some thread-like structures can be found. Moreover, it issuggested that MWNTs are well dispersed and embedded in thepolyimide matrix as a result of in situ polymerization. Fig. 6exhibits the surface morphologies of MWNT/GC composite films

Fig. 6. SEM images of the surface morphologies of (a) neat GC film, (b) 1 wt.% MWNT/GC film, (c) 2 wt.% MWNT/GC film, (d) 5 wt.% MWNT/GC film ,(e) 10 wt.% MWNT/GCfilm, and (f) 20 wt.% MWNT/GC film.

76 L. Zhang et al. / Composites: Part A 56 (2014) 72–79

by carbonization of the precursors. The morphology change ten-dency is similar with that of the precursors discussed above.

The fracture structures of the MWNT/GC composite films after3-point bending tests were investigated by FE-SEM as shown inFig. 7. The roughness of the cross-sections gradually increases asthe concentrations of MWNTs ranging from 0 wt.% to 20 wt.%.Without the addition of MWNTs, the fracture structure of neatGC film is smooth, even without ridges on the fracture surface.By incorporating 1 wt.% MWNTs in the GC matrix, the fracture sur-face become uneven. Furthermore, with the increase of MWNTconcentration, more MWNTs have been pulled out. And it is obvi-ous that MWNTs are embedded inside the matrix, which provesthat most MWNTs are uniformly dispersed in the matrix duringthe preparation process. As the MWNT concentration reaches 20wt.%, lots of GC flakes accompanied with pulled-out MWNTs existon the surface of the cross-sections (Fig. 7(f)). The length of pulled-out MWNTs is only several hundred nanometers long, which indi-cates that MWNTs well distributed in the matrix have a stronginterfacial bonding with the GC matrix, and play important rolesin the enhancement of the composite films.

3.4. Electrical properties of the MWNT/GC composite films

In order to determine the effects of MWNTs on the resistance ofthe final composites, the square resistances of the MWNT/GC com-posite films have also been studied. As shown in Fig. 8, the in-creased MWNT concentration results in a significant decrease inthe square resistance of the MWNT/GC composite films. With the

increase of MWNT concentrations from 0 wt.% to 20 wt.%, thesquare resistance decreased by 66% from 4.82 X/h to 1.66 X/h.The effects of MWNTs on the electrical properties of the finalMWNT/GC nanocomposites are similar with those of MWNT/poly-aniline nanocomposite systems reported before [36]. It is sug-gested that the incorporation of highly conducting MWNTs in theGC matrix decreases the resistance of the samples to a significantdegree. During the preparation process, the in situ polymerizationcan make sure the uniform dispersion of MWNTs in the matrix[37], and consequently the MWNT network throughout the com-posite can be formed, leading to an excellent conducting networkwell distributed in the matrix [38]. As we know, MWNTs have highaspect ratios and p-bonds, so it is easy for electrons to be trans-ferred through the p-bonds in the nanotube network, i.e., the de-crease of the resistance of the MWNT/GC composite films isattributed to the addition of MWNTs by providing plenty of elec-tronic channels. In addition, MWNTs, as allotropes of GC, have car-bon bonds with sp2 hybridization, which are similar with those ofGC. As a result, excellent compatibility between MWNTs and ma-trix can be ensured. Hence, a rapid decrease of the square resis-tance can be achieved after introduction of the networks ofMWNTs compared to neat GC films.

3.5. The hardness of the MWNT/GC composite films

The result of Vickers-hardness of the MWNT/GC compositefilms was exhibited in Fig. 9. When the MWNT concentrationsincreased from 0 wt.% to 5 wt.%, the hardness barely varied. The

Fig. 7. SEM images of the morphologies of fracture sections with the 3-point bending mode for (a) neat GC film, (b) 1 wt.% MWNT/GC film, (c) 2 wt.% MWNT/GC film, (d)5 wt.% MWNT/GC film, (e) 10 wt.% MWNT/GC film, and (f) 20 wt.% MWNT/GC film. Several pulled-out MWNTs have been marked by red arrows. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Square resistance of the neat GC film and the MWNT/GC composite films.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Fig. 9. The hardness of the neat GC film and the MWNT/GC composite films. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

L. Zhang et al. / Composites: Part A 56 (2014) 72–79 77

Vickers-hardness remained at the range of 750–810 HV. This maybe due to the fact that MWNTs with low concentrations in thematrix were not enough to form a network structure which is ableto enhance the bonding between the MWNTs and the GC matrix.The Vickers-hardness increases up to 850 HV and 880 HV uponincorporating 10 wt.% and 20 wt.% MWNTs. The MWNTs play animportant role in the reinforcement of the matrix, under certainMWNT concentration in the matrix (5 wt.% in this work), theMWNTs could form a complete network structure. As a result,the bonding is enhanced, leading to a higher hardness.

3.6. Mechanical properties of the MWNT/GC composite films

The variation curves of the mechanical properties as function ofMWNT concentrations are shown in Fig. 10, which clearly illus-trates the effect of MWNT incorporation on mechanical properties.All the specific data of the 3-point bending strength, modulus andstrain of the MWNT/GC composite films are listed in Table 1. Fromthis table, it can be observed that the MWNT/GC composite filmloaded with 5 wt.% MWNTs exhibits the maximum 3-point bend-ing strength and modulus of 575.5 MPa and 7.7 GPa respectively,

Fig. 10. Mechanical properties of the samples with the 3-point bending mode. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

78 L. Zhang et al. / Composites: Part A 56 (2014) 72–79

improved by 54% and 78% compared to neat GC films. When theMWNT concentration is below and above 5 wt.%, the 3-point bend-ing strength and modulus decreases significantly.

The mechanical reinforcement maybe ascribed to the followingitems: (1) Efficient dispersion of MWNTs in the matrix. The in situpolymerization method can ensure the nanotubes disperse in theprecursor (polymer) efficiently. Therefore, the excellent distribu-tion of MWNTs can be maintained in GC matrix after the carbon-ization process, which is very important to achieve efficient loadtransfer from GC matrix to the MWNT network [29]. (2) The highaspect ratios of MWNTs. The MWNTs we used here have high as-pect ratios, although acid-treatment has been applied. This canbe directly observed in SEM image as shown in Fig. 2. The majori-ties of MNWTs are micrometer long, which is favorable to maxi-mize the load transfer to the nanotubes. And this is also verycrucial for the purpose of optimizing the strength and stiffness ofthe composites [39]. (3) The affinity of the MWNTs to the matrix.In our work, MWNT/PI composite films have been used as precur-sors for the preparation of MWNT/GC composite films. The carbox-ylic groups on the surfaces of MWNTs can react with PAA duringthe imidization process, resulting in covalent bonding betweenthe surfaces of MWNTs and PI matrix [40]. Hence, excellent inter-faces can be formed between MWNTs and PI precursors matrix.During the carbonization process, PI turns into GC gradually asthe increase of the temperature. Since MWNTs and GC are allo-tropes of carbon, the compatibility of the interfaces betweenMWNTs and GC matrix can be easily achieved, i.e., excellent affin-ity of the MWNTs to the matrix can be successfully obtained.Therefore, the mechanical enhancement of carbon-based compos-ite films can be realized through the incorporation of MWNTs bythe in situ polymerization and carbonization process.

4. Conclusion

The free-standing stiff MWNT/GC composite films with variousMWNT concentrations were successfully synthesized through insitu polymerization and carbonization process. The results showedthat MWNT/GC film containing 5 wt.% MWNTs has the maximum3-point bending strength and modulus of 575.5 MPa and 7.7 GParespectively, improved by 54% and 78% compared to neat GC films.TGA determines the better thermal stability of the composite filmswith the addition of MWNTs. In addition, there is no obviouschange of hardness after introduction of MWNTs, and the squareresistance reduces rapidly with the increase of MWNT concentra-tions. This method is very simple, and the resultant free-standingcomposite films with excellent mechanical performance hold greatpotential in a wide range of applications.

Acknowledgments

The authors gratefully acknowledge financial supports by theNational Natural Science Foundation of China (Nos. 51102164,61376003, and 51272155), Medical-Engineering (Science) cross-Research Fund of Shanghai Jiao Tong University (No.YG2012MS37), the Hi-Tech Research and Development Programof China (No. 2011AA050504), and the Foundation for SMC Excel-lent Young Teacher in Shanghai Jiao Tong University. We alsoacknowledge the analysis support from Instrumental AnalysisCenter of Shanghai Jiao Tong University.

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