Composites Science and Technology 71 (2011) 1556–1562

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Composites Science and Technology

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High performance polyimide composite films prepared by homogeneityreinforcement of electrospun nanofibers

Dan Chen a, Ruiyu Wang a, Weng Weei Tjiu b, Tianxi Liu a,⇑a Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR Chinab Institute of Materials Research and Engineering, A�STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 January 2011Received in revised form 17 May 2011Accepted 21 June 2011Available online 20 July 2011

Keywords:A. Carbon nanotubesA. NanocompositesA. Polymer–matrix composites (PMCs)B. Mechanical propertiesE. Electrospinning

0266-3538/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compscitech.2011.06.013

⇑ Corresponding author.E-mail address: txliu@fudan.edu.cn (T. Liu).

Highly aligned polyimide (PI) and PI nanocomposite fibers containing carbon nanotubes (CNTs) were pro-duced by electrospinning. Scanning electron microscopy showed the electrospun nanofibers were uni-form and almost free of defects. Transmission electron microscopy indicated that the CNTs were finelydispersed and highly oriented along the CNT/PI nanofiber axis at a relatively low concentration. Theas-prepared well-aligned electrospun nanofibers were then directly used as homogeneity reinforcementto enhance the tensile strength and toughness of PI films. The neat PI nanofiber reinforced PI filmsshowed good transparency, decreased bulk density and significantly improved mechanical properties.Compared with neat PI film prepared by solution casting, the tensile strength and elongation at breakfor the PI film reinforced with 2 wt.% CNT/PI nanofibers were remarkably increased by 138% and 104%,respectively. The significant increases in the overall mechanical properties of the nanofibers reinforcedpolyimide films can be ascribed to good compatibility between the electrospun nanofibers and the matrixas well as high nanofiber orientation in the matrix. Our study demonstrates a good example for fabricat-ing high performance and high toughness polyimide nanocomposites by using this facile homogeneityself-reinforcement method.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In the past two decades, polymer nanocomposites have receivedsignificant attention in both industry and academia for their en-hanced mechanical and other physical properties [1–10]. Fibrousfillers such as carbon nanofibers have been utilized for improvingthe strength and rigidity of polymers due to their high aspect ratioand unique properties [1,5,7,8]. One of the key issues to successfullydevelop nanofiber-filled polymer nanocomposites is fine dispersionand high orientation of nanofibers in polymer matrices. However,on the one hand, it is not easy to achieve homogeneous dispersionof nanofibers in polymer matrices because of their high surfacearea. A poorly dispersed nanomaterial may degrade the mechanicalproperties because of poor adhesion at the fiber–matrix interfaces[10–12]. On the other hand, nanofibers usually cannot be effectivelyaligned in the matrix, which prevents the nanofillers from beingefficiently used as the ideal reinforcing agents [5,8,11–15].

Polyimide (PI) is particularly attractive for their outstandingthermal stability and good mechanical properties and usually usedas high-performance engineering plastics [6,14,16,17]. Polyimidenanocomposites containing fibrous nanoparticles are considered

ll rights reserved.

to be promising for many applications, especially as light-weighthigh-performance materials [13,14,18–20]. Fu and Zheng synthe-sized polyimide/silica tubes nanocomposites with improvedtensile strength and ductility by using c-aminopropyltriethoxysilane as the coupling agent [14]. Li found that the nitric acid oxi-dized carbon fibers can improve the mechanical and tribologicalproperties of the polyimide [19]. However, nearly all the nanofiberreinforced composites need the modification of nanofibers or theuse of coupling agents, which may seriously affect the propertiesof the as-prepared nanocomposites. Besides that, it is difficult toobtain nanocomposites with good nanofibers alignment becausethe nanofibers tend to be randomly distributed in the nanocom-posites. That is why the mechanical properties of the nanofibersreinforced nanocomposites cannot be remarkably improved.

Electrospinning is a low-cost but effective method to continu-ously produce polymer nanofibers [13,21–25]. As the electrospunnanofibers shows extraordinary properties such as excellentmechanical strength, high aspect ratio and surface-to-volume ra-tios, they are ideal candidates for reinforcing polymer materials[13,21–26]. Herein, we fabricate polyimide films by using thehighly aligned homogeneous PI nanofibers as reinforcing fillers.The good compatibility between the electrospun nanofibers andthe matrix can be obtained easily without chemical modificationof the PI nanofibers or adding any coupling agents, and the highlyaligned PI fibers can be efficient for the load transfer from the PI

D. Chen et al. / Composites Science and Technology 71 (2011) 1556–1562 1557

matrix to the PI nanofibers. Thus, the mechanical properties of PIfilms are remarkably improved by incorporating the highly alignedneat PI and CNT/PI nanofibers. Interestingly, the electrospun nanof-ibers can improve not only the tensile strength but also the tensilestrain at break of polyimide films. The as-prepared high-perfor-mance PI composite films maintain good transparency after theincorporation of nanofibers and the density of the reinforced filmsis lower than that of neat PI. The as-prepared PI films may have po-tential applications in microelectronics, optical and light-weightaerospace materials.

2. Experimental section

2.1. Materials

Pyromellitic dianhydride (PMDA), 4,4’-oxydianiline (ODA),N,N-dimethylacetamide (DMAc) were commercially obtained fromChina Medicine Co. Pristine multiwalled carbon nanotubes wereobtained from Chengdu Institute of Organic Chemistry, ChineseAcademy of Sciences, and the outer diameters of the CNTs areabout 5–10 nm. The pristine CNTs were refluxed in concentratedHNO3 for 12 h with a weight ratio of the acid to the CNTs of 50:1and the acid functionalized CNTs can be stably dispersed in DMAcupon sonication.

2.2. Preparation of electrospinning solutions

The precursor of polyimide, poly(amic acid) (PAA) was synthe-sized from PMDA and ODA with an equivalent molar ratio. Thepolycondensation was performed in DMAc at a temperature ofabout 0 �C, and the solid content of the pristine PAA solution was25 wt.%. For the preparation of neat PAA solution used for electros-pinning, the pristine PAA solution was diluted by DMAc; and forCNT/PAA solutions, the pristine PAA solution was diluted by CNT/DMAc solution. Prior to solution mixing, the CNT/DMAc solutionswere sonicated for 2 h to uniformly disperse the carbon nanotubesand the solid content of all the solutions for electrospinning wasfixed at about 16.7 wt.%.

2.3. Electrospinning and imidization of nanofiber membranes

Electrospinning was carried out using a syringe with a spinnerethaving a diameter of 0.5 mm at an applied voltage of 18–25 kV atambient temperature. The feeding rate was about 0.25 mL per hourand the spinneret-collector distance was set to be 20 cm. The PAAnanofibers were collected using a rotating disk collector with adiameter of 0.30 m and a width of 10 mm. During electrospinning,the linear speed of the rotating collector was about 20 m s�1 be-cause lower rotation speed cannot efficiently align the nanofibersand higher speed may leads to the breakage of the nanofibers. Allthe electrospun nanofiber membranes were dried at 60 �C for 4 hto remove the residual solvent and then thermally imidized usingthe following program to complete the imidization process: heat-ing up at a rate of 3 �C min�1 to 100, 200 and 300 �C, followed byan annealing at each temperature stage for 30 min.

2.4. Preparation of nanofibers reinforced PI films

The as-prepared electrospun (neat PI and CNT/PI nanofibers)membranes were soaked or immersed in the dilute solution ofPAA (with a solid content of 5 wt.%) for 120 s, and then experiencedthe same imidization process as mentioned above to obtain thenanofibers reinforced PI composite films. The soaking processshould be taken carefully as small air bubbles in the reinforcedfilms may lead to the deterioration of film properties.

2.5. Characterization

A scanning electron microscope (SEM, Tescan) performed at anacceleration voltage of 20 kV was used to observe the surface mor-phology of the electrospun nanofiber membranes and the fracturedsurfaces of PI films. Transmission electron microscopy (TEM) obser-vations of nanofibers were performed under an acceleration voltageof 200 kV with a Philips CM 300 FEG TEM. TEM specimens were pre-pared by directly collecting the electrospun nanofibers onto coppergrids during the fabrication of membranes. Tensile tests of the filmsamples were carried out using an Instron universal material testingsystem, and the samples were directly mounted to the sampleclamps and stretched at a speed of 5 mm min�1. The loading direc-tion of the tensile tests for the PI films reinforced with nanofibersis along the fiber alignment direction and parallel to the fiber axis.Tensile property values reported here represent an average of the re-sults for tests run on at least five samples. Thermogravimetric anal-ysis (Pyris 1 TGA) was performed under nitrogen flow from 100 to800 �C at a heating rate of 20 �C min�1. Dynamic mechanical analy-sis (DMA, Netzsch) of the nanocomposite films were carried out un-der tensile membrane mode from 50 to 450 �C at a frequency of 1 Hzand heating rate of 3 �C min�1.

3. Results and discussion

Electrospinning, which utilizes an external electrostatic field togenerate nanofibers, is widely used as an effective method to con-tinuously produce polymer nanofibers with anisotropic physicalproperties. By changing the experimental parameters, such as thepolymer solution properties (e.g., viscosity, surface tension) andelectrospinning conditions (e.g., voltage, spinneret-collector dis-tance), nanofibers with diameters between 200 and 300 nm canbe obtained. As shown in Fig. 1a by SEM, neat PI nanofibers col-lected by a high speed rotating method were highly aligned.Fig. 1b is the SEM image of neat PI nanofibers at a higher magnifi-cation. It can be seen that the diameter of the as-prepared nanofi-bers is uniform, and the surfaces of the electrospun nanofibers aresmooth and almost free of defects such as beads. As we know, thediameter uniformity and absence of beads in the electrospunnanofibers are of vital importance for achieving high strengthand toughness. Polyimide nanofibers containing different CNTloadings were also prepared by electrospinning. By electrospinningof the CNT/PAA solution and subsequent thermal imidization, PInanofibers with various CNT contents were prepared. Fig. 1c andd shows the SEM micrographs under different magnifications forthe electrospun 3.5 wt.% CNT/PI nanofibers. It can be seen thatthe diameter of the nanofibers containing 3.5 wt.% CNTs was notas uniform as that of neat PI nanofibers and the incorporation ofCNTs leads to the widening of the diameter distribution.

During the electrospinning process, some nanofibers were col-lected on copper grids for TEM observations. Fig. 2a–c shows theTEM micrographs of the electrospun PAA nanofibers containing 2,3.5 and 5 wt.% CNTs, respectively. After the acid functionalizationof CNTs, the PAA chains might form a coating layer around eachindividual CNT rope due to hydrogen bonding and keep the nano-tubes from aggregating or bundling together and thus the CNTs canbe well dispersed in the electrospun nanofibers at a low CNT load-ing (Fig. 2a). The finely dispersed CNTs were aligned parallel alongthe axis of the electrospun fibers easily, which is mainly ascribed tothe large electrostatic field and elongation forces applied to the li-quid jet during electrospinning. It can be seen that the surface of2 wt.% CNT/PAA nanofiber was smooth, and no beads or other de-fects were observed in the nanofiber. Further increase of the CNTcontents, especially for the nanofibers with 5 wt.% CNTs, may leadto the agglomeration of CNTs within the nanofibers. As shown in

Fig. 1. SEM micrographs of the electrospun (a and b) neat polyimide nanofibers and (c and d) 3.5 wt.% CNT/PI nanofibers.

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Fig. 2b and c, the CNTs in the PAA fibers tend to form bundles oragglomerates in the nanofibers, thus resulting in the coarsenesson the nanofiber surfaces.

As ideal reproduction and strong interfacial interactions canusually be achieved by using homogeneity self-reinforcementstrategy for making high performance polymer composites, herewe attempted to fabricate the self-reinforced PI films by using theirown electrospun nanofibers. Fig. 3a–c shows the photographs ofsolution cast PI film, the nanofiber reinforced PI film and the elec-trospun PI nanofiber membranes, respectively. Excellent transpar-ency of the yellow PI film prepared by conventional solutioncasting can be observed (Fig. 3a). The PI membrane consisting ofelectrospun PI nanofibers was totally opaque because of light scat-tering (Fig. 3c). It is interesting to find that the nanofiber reinforcedPI film (Fig. 3b) showed considerable transparency because most ofthe voids between the electrospun nanofibers were filled by the PImatrix during the infiltration and subsequent imidization process.The density of the reinforced films was estimated to be about 70%of the cast bulk PI film (1.47 g/cm3), which means that about 30% ofthe reinforced PI film was not filled by the nanofibers. That is whythe transparency of the reinforced film (Fig. 3b) was not as good asthat of the cast PI film (Fig. 3a).

Fig. 4 shows the typical stress–strain curves of neat cast PI filmand highly aligned nanofiber reinforced PI films. The tensile prop-erties such as tensile strength, tensile modulus and elongation atbreak were averaged and listed in Table 1. The tensile strengthand elongation at break of electrospun nanofiber reinforced filmare much higher than those of the cast film, due to the high orien-tation of the electrospun nanofibers in the film. It is known that

electrospinning is a facile method for obtaining high orientationof polymer chains in the nanofibers. Therefore, the significantimprovement of tensile properties of the neat PI nanofiber rein-forced PI film can be ascribed to twofold orientations: (1) high ori-entation of electrospun PI nanofibers in the PI films (due to high-speed rotation collecting method), and (2) high orientation of PIchains in the electrospun PI nanofibers. As shown in Fig. 3b, thenanofiber reinforced PI film was nearly transparent because ofthe good compatibility between the nanofibers and PI matrix. Bythe incorporation of highly aligned PI nanofibers as reinforcingagents, the tensile strength and elongation at break of neat PI filmswere remarkably improved by about 97% and 46%, respectively.The significant improvement in overall mechanical properties ofPI films can be ascribed to the good compatibility and thus stronginterfacial interaction between PI nanofibers and PI matrix as wellas the dual high-orientation factors mentioned above.

It can be seen from Fig. 4 and Table 1 that, the tensile modulusof neat PI nanofiber reinforced film is slightly higher than those of1 wt.% and 2 wt.% CNT/PI nanofibers reinforced films, because ofbetter compatibility between neat PI fibers and the matrix. Withthe increase of CNT content, the moduli of the reinforced PI filmswere gradually increased and the 5 wt.% CNT/PI membrane pos-sessed the highest tensile modulus. The elongation at break forthe PI films was greatly improved by incorporating 1 wt.% and2 wt.% CNTs into the PI nanofibers. And the optimal tensile proper-ties were achieved for the PI film reinforced with 2 wt.% CNT/PInanofibers. Compared with the solution cast PI films, the tensilestrength and elongation at break for the PI film reinforced with2 wt.% CNT/PI nanofibers were significantly increased by 138%

Fig. 2. TEM micrographs of the electrospun (a) 2 wt.% CNT/PAA; (b) 3.5 wt.% CNT/PAA and (c) 5 wt.% CNT/PAA nanofibers.

Fig. 3. Digital photographs of (a) solution cast film; (b) neat PI nanofiber reinforcedfilm and (c) electrospun neat PI nanofiber membrane.

0 20 40 60 800

40

80

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200

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Stre

ss (

MP

a)

Strain (%)

Solution Cast Neat PI film Neat PI Nanofiber Reinforced PI 1 % CNT/PI Nanofiber Reinforced PI 2 % CNT/PI Nanofiber Reinforced PI 3.5 % CNT/PI Nanofiber Reinforced PI 5 % CNT/PI Nanofiber Reinforced PI

Fig. 4. Typical stress–strain curves of solution cast PI film, neat PI nanofibermembrane reinforced PI film, and PI composite nanofibers (with different CNTloadings) reinforced PI films.

Table 1Summary of mechanical properties of solution cast neat PI film, electrospun neat PIand CNT/PI nanofibers reinforced PI films.

Sample Tensilestrength(MPa)

Tensilemodulus(GPa)

Elongationat break(%)

Solution cast neat PI film 88.4 ± 17.6 1.29 ± 0.15 39.3 ± 9.2Neat PI nanofiber reinforced

film174.1 ± 10.4 1.31 ± 0.12 57.2 ± 9.6

1 wt.% CNT/PI reinforced film 176.5 ± 10.2 1.23 ± 0.13 79.3 ± 8.62 wt.% CNT/PI reinforced film 210.4 ± 14.6 1.26 ± 0.13 80.8 ± 9.43.5 wt.% CNT/PI reinforced

film169.2 ± 15.1 1.34 ± 0.15 56.4 ± 10.3

5 wt.% CNT/PI reinforced film 171.5 ± 14.3 1.47 ± 0.12 57.3 ± 11.4

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and 104%, respectively. As we know, polyimide usually possesseslow toughness. Here, high-performance homogeneity reinforcedpolyimide nanocomposites were successfully prepared with hightoughness and enhanced overall mechanical properties by incorpo-ration of 2 wt.% CNT/PI nanofibers as reinforcing agents. For the3.5 wt.% and 5 wt.% CNT/PI nanofibers reinforced PI films, the ten-sile properties were moderately decreased (in comparison withthose of the 2 wt.% CNT/PI nanofibers reinforced one), probably be-cause the CNTs may form bundles or aggregates in the electrospunnanofibers with the increase of CNT contents. As shown in Fig. 2band c, the entangled nanotubes were prone to aggregate close tothe nanofiber surface rather than being dispersed and aligned wellin the nanofibers, thus leading to the coarseness of surfaces and theun-uniformity of the diameters of the electrospun nanofibers.These will greatly deteriorate the quality or mechanical perfor-mance of the electrospun nanofibers, thus partially sacrificing theirreinforcement effect. In addition, for the nanofibers reinforced PI

films, good interfacial adhesion between the nanofibers and thematrix is crucial for achieving excellent mechanical properties.When the CNTs form aggregates close to the nanofiber surface,good interfaces cannot be formed between the nanofibers and PImatrix, thus the concept of homogeneity self-reinforcement willnot play a full role.

Fig. 5. SEM micrographs of fractured surfaces of (a) solution cast PI film; (b) neat PI nanofiber reinforced film; (c) 2 wt.% CNT/PI nanofiber reinforced film; and (d) 3.5 wt.%CNT/PI nanofiber reinforced film.

100 200 300 400 500 600 700 800

60

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Solution Cast Neat PI Film Neat PI Nanofiber Reinforced Film 1% CNT/PI Nanofiber Reinforced Film 2% CNT/PI Nanofiber Reinforced Film 3.5% CNT/PI Nanofiber Reinforced Film 5% CNT/PI Nanofiber Reinforced Film

Wei

ght

(%)

Temperature (oC)

Fig. 6. TGA curves of solution cast PI film, electrospun neat PI and its CNTcomposite nanofibers reinforced films.

1560 D. Chen et al. / Composites Science and Technology 71 (2011) 1556–1562

Fig. 5a and b shows the fractured surfaces of cast PI film andneat PI nanofibers reinforced PI film, respectively. It can be seenthat the fractured surface of the cast PI film is smooth and homo-geneous, while that of the PI film reinforced by the electrospun PInanofibers is very rough. As shown in Fig. 5b, the neat PI nanofiberswere intimately surrounded by the PI matrix, which can be as-cribed to the excellent compatibility and strong interfacial bondingbetween them. The highly aligned PI nanofibers embedded in thematrix tended to be broken (as indicated by red arrows) insteadof being simply pulled out from the matrix. That is why themechanical strength of the nanofibers reinforced films is signifi-cantly improved compared with the cast PI films. A steady increaseor hardening of the stress was observed after the yield point for thenanofiber reinforced film (Fig. 4). However, this was not seen forthe solution cast film. This is probably because at early stage ofthe tensile tests, only part of the aligned nanofibers within thefilms may work and bear the load. During the tensile testing, moreand more nanofibers in the film are extended and suffered the load,thus leading to a steady increase of tensile strength and an en-hanced toughness. As electrospinning can be an efficient methodto realize the orientation of polymer chains along the nanofiberaxis direction, the increased orientation of PI chains in the electro-spun nanofibers may also lead to an increase or improvement ofboth tensile toughness and strength. Moreover, the nanofibersmay serve as connecting bridges to prevent the matrix from frac-turing upon mechanical deformation, thus also enhancing themechanical properties of PI films.

Fig. 5c and d shows the SEM images of fractured surfaces of2 wt.% and 3.5 wt.% CNT/PI nanofiber reinforced PI films, respec-

tively. The CNTs (i.e., the bright dots or lines as indicated by theblue arrows) embedded in the electrospun nanofibers can beclearly observed because of their relatively high electrical conduc-tivity. It can be occasionally seen that the electrospun CNT/PInanofibers are individually protruded out of the fractured surfaces,as indicated by the red arrows in Fig. 5c and d. And, some cavities(as indicated by white arrows) are left behind and clearly observedafter the nanofibers are pulled out from the matrix due to debond-ing; whereas the neat PI nanofibers were closely surrounded by the

0

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3000Solution Cast Neat PI Film Neat PI Nanofiber Reinforced Film 1% CNT/PI Nanofiber Reinforced Film 2% CNT/PI Nanofiber Reinforced Film 3.5% CNT/PI Nanofiber Reinforced Film 5% CNT/PI Nanofiber Reinforced Film

Stor

age

mod

ulus

(M

Pa)

Temperature (oC)

(a)

50 100 150 200 250 300 350 400 450

50 100 150 200 250 300 350 400 4500.00

0.05

0.10

0.15

0.20

0.25Solution Cast Neat PI Film Neat PI Nanofiber Reinforced Film 1% CNT/PI Nanofiber Reinforced Film 2% CNT/PI Nanofiber Reinforced Film 3.5% CNT/PI Nanofiber Reinforced Film 5% CNT/PI Nanofiber Reinforced Film

Tan

(δ)

Temperature (oC)

(b)

Fig. 7. DMA curves of solution cast PI film, electrospun neat PI and its CNTcomposite nanofibers reinforced films.

D. Chen et al. / Composites Science and Technology 71 (2011) 1556–1562 1561

matrix (Fig. 5b). This is probably because the interfacial bondingbetween the high CNT loading nanofibers and PI matrix is not asstrong as that of the neat PI nanofibers. It was observed that theagglomerated CNTs tended to be protruded out of the electrospunnanofiber surfaces because of their high surface energies (Fig. 3).Here, with the increase of CNT contents in the electrospun nanof-ibers, the CNTs are inclined to form agglomerates close to thenanofiber surfaces. The nanofibers with CNT agglomerates on theirsurfaces possess relatively poor interfacial interaction with the PImatrix.

Fig. 6 shows the TGA curves under nitrogen atmosphere forsolution cast neat PI and different nanofibers reinforced PI films.It can be seen that all the PI films reinforced by electrospun nanof-ibers possess excellent thermal stability. Although the nanofibersexperienced twice high temperature imidization processes, thethermal stability of the reinforced PI films was no worse than thatof solution cast neat PI film.

Fig. 7a and b depict the DMA results showing storage modulus(G0) and tan(d) versus temperature curves for neat PI and its CNTnanofiber reinforced PI films. The steadily increased modulus canbe attributed to the strengthening effect of high oriented electro-spun nanofibers in the matrix and the combined good dispersionand stiffening effect of CNTs. The glass transition temperaturesfor all the films were higher than 350 �C, indicating excellent ther-mal properties of PI composite films. The solution cast PI film pos-sesses the highest tan(d) value probably because the free volume inneat film is very small, thus more energy is required for chainmobility. The tan(d) value of PI was decreased by addition of elec-trospun nanofibers. The reduction of tan(d) may result from the in-crease of G0 by adding nanofillers, indicating strong bonding

between the nanofibers and PI matrix. Generally, the incorporationof CNTs may lead to the increase of glass transition temperature ofpolymer. The nanofibers containing 2 wt.% CNTs reinforced PI filmpossesses the lowest tan(d) and glass transition temperature, indi-cating that the 2 wt.% CNT/PI fiber reinforced film possesses thehighest toughness, which is consistent with the tensile tests. Thus,this homogeneity reinforcing method is very successful for fabri-cating high mechanical performance polyimide nanocompositeswith low cost.

4. Conclusions

In this study, light-weight and high-performance polyimidenanocomposite films with good transparency were fabricated by ahomogeneity self-reinforcing method. By using electrospinningand the subsequent high-speed rotation collecting methods, highlyaligned PI nanofibers with different CNT loadings were prepared.TEM micrographs indicated that the functionalized CNTs werehomogeneously dispersed and well oriented in the nanofibers. Itwas found that the as-prepared aligned electrospun nanofibers areideal and efficient self-reinforcing agents to prepare high-perfor-mance PI nanocomposite films because of good compatibility be-tween the electrospun nanofibers and the matrix. Compared withsolution cast neat PI film, the tensile strength and elongation atbreak for the PI composite film reinforced with 2 wt.% CNT/PI nanof-ibers were increased by 138% and 104%, respectively. Such a homo-geneity self-reinforcement approach to fabricate high-performancePI/CNT nanofiber films is a good attempt toward utilizing CNTs inpolymer matrices to achieve ultrastrong, tough, and light-weightnanocomposites that can be used in defense or aerospace areas.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (20774019; 50873027), the Outstanding DoctoralResearch Foundation of Key Disciplines in Fudan University, andthe ‘‘Shu Guang’’ project (09SG02) supported by Shanghai Munici-pal Education Commission and Shanghai Education DevelopmentFoundation.

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