Transaction F: NanotechnologyVol. 17, No. 1, pp. 60{65c Sharif University of Technology, June 2010

Invited Paper

Morphology and Mechanical Propertiesof Polyacrylonitrile/Multi-Walled

Carbon Nanotube (PAN/MWNTs)Nanocomposite Electrospun Nano�bers

M. Yousefzadeh1, M. Amani-Tehran1, M. Lati�1;� and S. Ramakrishan2

Abstract. In this research the morphology and mechanical properties of PAN/MWNTs nano�bers arestudied. The electrospinning process is used to produce �ber at micro and nano scale. The dispersion ofMWNTs in polymer solution is the key factor to obtain desirable properties in the �nal product. Thus,the dispersion condition is investigated using SEM images. The morphological study of �ber mats showthat by increasing the amount of MWNTs in the polymer from 0 to 1 wt%, the surface roughness of �bersis increased. Increasing the percentage of MWNTs in �ber improves the tensile stress at maximum loadfor about 114% and modulus for about 40%.

Keywords: Nano�ber; Electrospinning; Mechanical property; Carbon nanotube; Nanocomposite.

INTRODUCTION

The special properties of carbon nanotubes (CNTs)have caused considerable interest in investigating theirphysical and mechanical potential toward the devel-opment of a variety of technological applications. Ithas been theoretically and experimentally con�rmedthat nanotubes possess remarkably high sti�ness andstrength [1]. Carbon nanotubes also have high elec-trical and thermal conductivities [2]. The uniquemechanical and physical properties of carbon nan-otubes combined with their high aspect ratio andlow density have brought about extensive research inpreparing and synthesizing the various nanocompositesof carbon nanotube to improve mechanical, electrical,thermal and other properties of materials at macro andnano scale [1,3,4]. The e�ective superior propertiesof carbon nanotubes transformed to nanocompositesare crucial. It is necessary to create strong inter-

1. Department of Textile Engineering, Textile Excellence & Re-search Centers, Amirkabir University of Technology, Tehran,P.O. Box 159163-4311, Iran.

2. Nanoscience and Nanotechnology Initiative, National Univer-sity of Singapore, 9 Engineering Drive 1, Singapore 117576,Singapore.

*. Corresponding author. E-mail: lati�@aut.ac.ir

Received 5 August 2010; accepted 6 September 2010

facial bonding between carbon nanotubes and poly-mers.

In recent years, researchers have become inter-ested in the composition of polymeric �bers with car-bon nanotubes for a variety of applications. Di�erenttypes of polymers were used to produce nanocompos-ites through di�erent processes. Electrospinning is aunique method in which CNTs can be embedded innano�bers formed as a non-woven web. The perfor-mance of this method relies on how well the CNTsare dispersed within �bers. It was found that in theelectrospinning process, carbon nanotubes could bewell-aligned along the �ber axis.

In a common electrospinning process, a highvoltage is applied to create an electric �eld betweenthe droplet of polymer solution at the tip of a needleand a collector plate. A voltage source is used tocharge the solution while the collector is electricallyearthed. This creates an electrostatic force between theneedle and the collector. As the voltage is increased,the electric �eld is intensi�ed causing a force to bebuilt up on the pendant drop of polymer solution atthe tip of the needle. This force acts in the directionopposing the surface tension of the drop. The increaseof electrostatic force causes the drop to be elongatedand forms a conical shape known as Taylor cone. Whenthe electrostatic force overcomes the surface tension

Morphology and Mechanical Properties of PAN/MWNTs Nano�bers 61

of the drop, the charged continuous jet of solution isejected from the cone. The jet of solution acceleratestowards the collector, whipping and bending wildly.As the solution moves away from the needle towardthe collector, the jet is rapidly thinned and driedas the solvent evaporates. On the surface of thegrounded collector, nano�bers are deposited as a webwith random orientation [5-7].

The incorporation of carbon nanotubes into nano-�bers has demonstrated the enhancement of electricalconductivity of polyurethane/polyaniline/ MWNT andpoly(vinylidene uoride)/SWNT nano�bers [8,9].

The improved thermal stability and Young's mod-ulus in PAN/SWNT nano�bers have also been ob-served [10].

Since the macroscopic orientation of MWNTsin the polymer matrix of nano�bers during electro-spinning is necessary toward many meaningful tech-nological applications, in the present work, the dis-persion and distribution of the multi-walled carbonnanotubes embedded in electrospun polyacrylonitrile(PAN) nano�bers and the morphology and mechanicalproperties of nano�bers formed as a web are investi-gated.

EXPERIMENTAL

Material

Industrial Polyacrylonitrile (PAN) from Iran Poly-acryle Co. with Mw 100,000 g/mol, and N,N-dimethylformamide (DMF) from Sigma-Aldrich as the solventfor PAN were used. Multi-walled carbon nanotube(MWNT) was purchased from Shenzhen Nanotech-nologies Port Co. The purity of carbon nanotubeswas more than 95% with 5-15 �m lengths and 10-20 nm diameters. Figure 1 is the SEM image of thenanotubes.

Figure 1. SEM image of multi-walled carbon nanotubes.

Polymer Solution Preparation

Di�erent percentages of multi-walled carbon nanotubes(0.01, 0.05, 0.1, 0.3, 0.5, 1 and 2 wt.%) were addedto 5 cc of N,N-dimethylformamide (DMF) that wasused for preparing 15% (w/v) PAN solution, in order todisentangle the nanotubes that typically tend to clingtogether and form lumps which are very di�cult to beprocessed [11]. Multi-walled carbon nanotubes weredispersed in DMF at di�erent concentrations usingprobe sonicatore (SONICS) at room temperature forabout 1 hour. Magnet stirrer (Heidolph) was used tomix the polymer with carbon nanotubes solution untilthe polymer was uniformly dissolved in the solvent.

Electrospinning

Figure 2 shows the experimental set-up used to producenano�ber. The electrospinning process was carriedout by connecting a high voltage power supply fromGamma High Voltage Research (� 30 kV, 1.5 mA).The voltage of 14-20 kV was applied to the nozzlewhose tip was at 14 cm distance from the surface ofthe grounded collector. The nozzle was a B-D 22.5Gneedle with a at tip. A KD scienti�c syringe pumpwas used to provide a constant feed rate of solutionduring electrospinning.

Methods

The nano�ber morphology was analyzed by scanningelectron microscope (SEM: Quanta 200F) and trans-mission electron microscopy (TEM: JEM 3010, JEOL).The sample holders for SEM were coated with gold by

Figure 2. Schematic experimental set-up ofelectrospinning.

62 M. Yousefzadeh et al.

ion sputtering for 90 seconds using an auto �ne coater(JEOL JFC-1600).

The diameter of the nano�bers was measured byImageJ software using the SEM images. The mechani-cal properties of electrospun webs were analyzed usinga universal testing machine (INSTRON 2519) under acrosshead speed of 10 mm/min and gauge length of 2cm at room temperature with 10 N and 10 kN loadcell. The nano�bers thicknesses were measured by thedigital micrometer (Mitutoyo).

RESULTS AND DISCUSSION

Polymer Solution Characterization

PAN/MWNTs solution was prepared using ultrasonicenergy to disperse the nanotubes in the solvent and in-corporate them into nanocomposites. Polyacrylonitrilesolutions containing MWNTs were then electrospunto yield nano�ber MWNTs composite. MWNTs werehomogeneously dispersed and stabled having a darkblack ink-like appearance without precipitation forseveral days (Figure 3).

For improving mechanical properties of polymerand preparing nano�bers with homogeneous surfacemorphology, the condition of dispersion is very criticaland plays a signi�cant role in achieving e�ective prop-erties in nano�bers. It is important that nanotubes canbe dispersed without any breakage and aggregation. Tohave the optimum dispersion, the time and intensityof ultrasonic wave were studied in this e�ort. By

considering the SEM images of dispersed nanotubes,it was found that mixing for about 1 hour with 30%magni�cation was su�cient to achieve a well-dispersedsolution. In Figures 4 and 5, two di�erent weightpercentages of nanotubes at two magni�cations areshown. They reveal that by incrassating of carbonnanotubes in the solution, the dispersion action is notproperly performed.

To improve the dispersion of nanotubes at highconcentration, the multi-walled carbon nanotubes werere uxed in nitric acid 65% and stirred at 150�C for8 hours to attach functional groups of carboxyl andhydroxyl groups. MWNTs were then cool dried afterrinsing 3 times by distilled water.

Morphology of Electrospun Nano�bers

The morphology of the nano�bers was consideredby SEM. Figure 6 shows the SEM image of PANnano�bers webs at di�erent concentrations of MWNTs.It was observed that �bers without any beads couldbe produced at 15% (w/v) concentration of polymer.Bead areas act as stress concentration points and a�ectthe mechanical properties of mat.

It was necessary to minimize any aggregatedMWNTs on the surface of �bers to have beadless anduniform nano�bers webs. By increasing the amount ofMWNTs in �ber, the surface roughness of �bers waschanged.

MWNTs aligned along the nano�ber axis wereexpected to enhance the mechanical properties of

Figure 3. Di�erent percentage of MWNTs (0.01, 0.05, 0.1, 0.3, 0.5, 1 and 2 wt%) in DMF that was sonicated for 1 hour.(a) Before sonication; (b) after 1 day; and (c) after 15 days.

Morphology and Mechanical Properties of PAN/MWNTs Nano�bers 63

Figure 4. SEM of thin �lm that was prepared with dispersed 0.01 wt% MWNTs in DMF.

Figure 5. SEM of thin �lm that was prepared with dispersed 1 wt% MWNTs in DMF.

Figure 6. SEM image of nano�bers with di�erent weight percentage of MWNTs. (a) PAN 15%; (b) PAN/MWNT 0.01wt%; (c) PAN/MWNT 0.05 wt%; (d) PAN/MWNT 0.1 wt%; (e) PAN/MWNT 0.5 wt%; and (f) PAN/MWNT 1 wt%.

�bers. In Figure 7, the high-magni�ed SEM imagesof nanocomposite �bers and also the TEM image areshown. In SEM images, nanotubes can be mostlyobserved on the surface of �bers. They show how thehigh modulus nanotubes can bend the nano�bers.

Since the carbon nanotubes possess a high elec-tron density compared with the PAN polymer matrix,the nanotubes are displayed as darker structures em-

bedded in the PAN nano�bers. The MWNTs werealigned along the �ber axis of the nano�bers.

Mechanical Characterization

The traditional stress-strain method using stretchingtest was employed to evaluate the mechanical proper-ties of the electrospun nano�ber mats. 10 pieces of the

64 M. Yousefzadeh et al.

Figure 7. SEM images of PAN/MWNT 1 wt% (left); TEM images of PAN/MWNT 0.5 wt% (right).

Table 1. Mechanical properties of nano�bers mat and their average diameter (with their con�dence level value �).

Modulus(MPa)

Tensile Stress atMaximum Load

(MPa)

Tensile Strain atMaximum Load

(%)

Nano�berDiameter

(nm)

PAN//MWNT 0.0% 0:24� 0:06 1:9� 0:3 33� 5 210� 21

PAN/MWNT 0.01% 0:26� 0:04 1:5� 0:2 14� 2 691� 43

PAN/MWNT 0.05% 0:25� 0:02 1:7� 0:2 21� 4 511� 29

PAN/MWNT 0.1% 0:25� 0:04 2:2� 0:4 53� 7 438� 29

PAN/MWNT 0.5% 0:28� 0:02 2:3� 0:3 48� 4 386� 21

PAN/MWNT 1.0% 0:33� 0:02 3:1� 0:4 68� 5 520� 4

nano�ber mats from each group (PAN15%, PAN15%with 0.01, 0.05, 0.1, 0.5 and 1 wt% of MWNTs) werestretched. The average of tested features was reportedas the properties of composite nano�bers.

The results are presented in Table 1. The resultscon�rm the enhancement of mechanical properties byincreasing the MWNTs contents in the nano�bers.More improvement can be achieved above the perco-lation threshold value of MWNTs in �bers.

For a simple composite system without microme-chanical interlocking and chemical bonding betweenthe �ller and the matrix, load transfer from the matrixto the �ller is realized through a weak van der Waalsbonding between the �ller and the matrix [12]. Thus, itcan be similarly stated that the load can be transferredfrom the PAN matrix to the MWNTs �llers by van derWaals bonding.

To achieve the signi�cant mechanical enhance-ments, the main challenges lie in obtaining a gooddispersion, optimizing the interface between polymerand nanotubes, and having high quality structures insu�cient quantities.

The mechanical data in Table 1 show that thetensile modulus of PAN/MWNTs composites is im-proved, specially for 0.5 and 1 wt% of MWNTs.For tensile stress and strain at maximum load, the

improvement takes place above 0.1 wt% of MWNTs innano�bers. Considering the higher value of nano�beraverage diameters in 0.01 and 0.05 wt% concentrations,it can be concluded that the homogenous dispersion ofMWNTs seems to be poor for these concentrations. Innon-homogenous dispersion of nanotubes in polymermatrix, the nanotube can be considered as a stressconcentration point in nono�ber's structure and itcan result in the weakness of mechanical properties ofnano�bers.

The volume density of �bers in the mat, themixing level of nano�bers, the fusion frequency ofthe nano�bers, the existence of imperfections andbranching in �bers are expected to a�ect the tensileproperties of a mat electrospun under di�erent processconditions even from the same polymer/solvent system.

CONCLUSION

The morphological and mechanical properties ofhomogeneous PAN/MWNTs nanocomposite matsof nano�bers containing di�erent concentrations ofMWNTs were investigated. TEM images showed thatMWNTs were almost aligned along the electrospunnano�bers. By increasing the amount of MWNTs inpolymer, the surface roughness of �bers was increased.

Morphology and Mechanical Properties of PAN/MWNTs Nano�bers 65

The tensile modulus in PAN with 1 wt% MWNTs wasincreased about 40% in comparison with PAN withoutMWNTs. The tensile stress at maximum load was alsoimproved about 114%.

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BIOGRAPHIES

Maryam Yousefzadeh received her B.S. in TextileTechnology Engineering from Amirkabir University ofTechnology in 2002. She also received her M.S. andPh.D. degrees (with the highest honor) in the same �eldand from the same university in 2004 and 2010, respec-tively. She is a member of Iranian National Elite Foun-dation and Iranian Inventors Association. Her active�elds of research are Electrospinning and Nano�bers,Polymeric Nanocomposite and Image Analysis.

Mohammad Amani-Tehran received his B.S. (1987)from the Isfahan University of Technology and hisM.S. (1990) from Sharif University of Technology inChemical Engineering. His Ph.D. (2001) is in ColorTechnology from Amirkabir University of Technology.In 1991, he was employed as a Lecturer by AmirkabirUniversity of Technology and he is currently an Asso-ciate Professor in Textile Department. In these yearshe has supervised more than 40 M.S. and �ve Ph.D.theses in Color Technology, ANN Modeling, GeneticAlgorithm optimization and Image Processing. He hasauthored over 20 journal papers and over 60 conferencepapers due to the results of these activities.

Masoud Lati� is Professor in the Department ofTextile Engineering, Amirkabir University of Technol-ogy, Tehran, Iran. He is also the member of TextileResearch Institute (ATMT) and Center of Excellencein Textile (CENMIT). He received his EngineeringDegree (B.S.) in Textile Technology from AmirkabirUniversity of Technology in 1985 and Ph.D. in 1990from the University of Leeds, Leeds, UK. His researchareas are Nano Fibrous Structure, Smart, Technicaland Conductive Textiles and Applied Image Processingwith a teaching and research experience of 20 years.Sixty postgraduate students have obtained their degreeunder his supervision. He has authored or co-authoredover 120 research papers.

Seeram Ramakrishna has been the Vice-President(Research Strategy) of the National University of Sin-gapore since 2008. He is a Professor of MechanicalEngineering. He was a former Dean of NUS Faculty ofEngineering from 2003 to 2008, and has been a found-ing Co-Director of NUS Nanoscience & NanotechnologyInitiative from 2003 to 2010. He received his Ph.D.in Materials Science & Engineering from the Univer-sity of Cambridge and General Management Trainingfrom the Harvard University. He is an acknowledgedglobal leader for his pioneering work on Science andEngineering of Nano�bers for Regenerative Medicine,Harvesting Solar Energy, and Water Treatment.