Download - 2013-Enhanced Piezoelectric Properties of Electrospun Poly(Vinylidene Fluoride)Multiwalled Carbon Nanotube Composites Due to High β‑Phase Formation in Poly(Vinylidene Fluoride)

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  • Enhanced Piezoelectric Properties of Electrospun Poly(vinylideneuoride)/Multiwalled Carbon Nanotube Composites Due to HighPhase Formation in Poly(vinylidene uoride)Yongjin Ahn, Jun Young Lim, Soon Man Hong, Jaerock Lee, Jongwook Ha, Hyoung Jin Choi,*,

    and Yongsok Seo*,

    Intellectual Textiles Research Center (ITRC) and RIAM School of Materials Science and Engineering, College of Engineering, SeoulNational University, Shillim-9-dong 56-1, Kwanakgu, Seoul, Republic of Korea 151-744Hybrid Materials Research Center, Korea Institute of Science and Technology, Hawolgokdong 39-1, Sungbukku, Seoul, Republic ofKorea 130-650Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yousungku, Taejon, Republic ofKorea 305-600Department of Polymer Science and Engineering, Inha University, Yonghyun-dong, Namku, Incheon, Republic of Korea 402-751

    ABSTRACT: We prepared poly(vinylidene uoride)(PVDF)/multiwalled carbon nanotube (MWCNT) nano-composites using the electrospinning process and investigatedthe eects of varying the MWCNT content, as well as theadditional use of drawing and poling on the polymorphicbehavior and electroactive (piezoelectric) properties of themembranes obtained. Fourier transform infrared spectroscopyand wide-angle X-ray diraction revealed that dramaticchanges occurred in the -phase crystal formation with theMWCNT loading. This was attributed to the nucleation eectsof the MWCNTs as well as the intense stretching of the PVDFjets in the electrospinning process. The remanent polarization and piezoelectric response increased with the amount ofMWCNTs and piezoelectric -phase crystals. A further mechanical stretching and electric poling process induced not only highlyoriented -phase crystallites, but also very good ferroelectric and piezoelectric performances. In the drawn samples, the interfacialinteraction between the functional groups on the MWCNTs and the CF2 dipole of PVDF chains produced a large amount of -phase content. In the poled samples, the incorporation of the MWCNTs made it easy to obtain ecient charge accumulation inthe PVDF matrix, resulting in the conversion of the -phase into the -phase as well as the enhancement of remanentpolarization and mechanical displacement.

    INTRODUCTIONPoly(vinylidene uoride) (PVDF) has been studied extensivelybecause of its unique electroactive properties, including piezo-,pyro-, and ferroelectric properties, as well as its other usefulproperties, such as its exibility, light weight, and long-termstability under high electric elds.1,2 PVDF is a semicrystallinepolymer with a typical crystallinity of 50%, whose molecularstructure consists of the repeated monomer unit(CH2CF2)n. It is well-known that PVDF has ve distinctcrystallite polymorphs.3 The most common polymorph ofPVDF is the -phase, which has a monoclinic unit cell with aTGTG (T = trans, G = gauche +, G = gauche ) conformation.The piezoelectric crystallization polymorph is the -phase,which has an all-trans (TTTT) conformation, with anorthorhombic unit cell. The -phase also has an orthorhombicunit cell, with a TTTGTTTG chain conformation. The othertwo ( and ) polymorphs are the polar and antipolaranalogues of the and forms, respectively.3 The rst twoconformations (the -phase and the -phase) are by far the

    most common and important ones. In the TGTG con-formation (the -phase conformation), the dipole is inclinedrelative to the normal axis, so the average dipole moment foreach monomer is very reduced. Furthermore, the unit cell ofthe -PVDF lattice consists of two chains in a TGTGconformation, whose dipole components normal to the chainaxis are antiparallel, thus neutralizing each other.4 As a result,the -phase can be described as nonpolar, nonpiezoelectric, andnonpyrroelectric. On the other hand, the -phase, which is inan all-trans (TTTT) conformation, has all of its dipoles alignedin the same direction normal to the chain axis. Its unit cellconsists of two all-trans chains packed with their dipolespointing in the same direction. The molecular dipoles in the -phase are thus entirely aligned in one direction; this crystalform can therefore generate the largest spontaneous polar-

    Received: January 31, 2013Revised: April 29, 2013Published: April 29, 2013

    Article

    pubs.acs.org/JPCC

    2013 American Chemical Society 11791 dx.doi.org/10.1021/jp4011026 | J. Phys. Chem. C 2013, 117, 1179111799

  • ization and exhibits strong ferroelectric and piezoelectricproperties. These unique -phase-derived properties of PVDFmake it useful in a wide range of applications, includingactuators, biosensors, energy-harvesting materials, audiodevices, transducers, and nonvolatile memories.3,58 However,it is not easy to obtain PVDF consisting of entirely -phasecrystals. In all-trans PVDF, the overlapping of neighboringuorine atoms occurs, because the diameter of the uorineatom (0.270 nm) is slightly larger than the space provided byan all-trans carbon chain (0.256 nm).9 To diminish this overlap,CF2 groups are tilted to the right and left, relative to theiroriginal conformation. This deection of CF2 groups convertsthe all-trans form into TGTG ( form) or TTTGTTTG (form). Hence, the -phase is more easily formed than the -phase in normal circumstances.Although the crystal lattice energy of the -phase is slightly

    higher than that of the -phase, direct -phase formation fromthe melt is prohibited due to the high energy of the all-transconformations.1 -Phase formation can be accomplished via acrystal phase transition from the -phase.10 The most commontechnique for obtaining polar -PVDF involves mechanicalextension (drawing) and electrical poling.1113 Mechanicaldrawing contributes to the transition of the original spheruliticstructure into a crystal array, in which the molecules are forcedinto their most extended conformation (polar -phase), with allof the dipole moments aligned in the same direction.10,11 Theapplication of an electric eld on both sides of the PVDFelectrets (poling) also results in the orientation of the crystallitepolar axis along the eld direction, which promotes a higherspontaneous polarization for the -phase.12 Conversion of theparaelectric phase to the ferroelectric phase has also beenachieved using diverse methods such as crystallization from apolar solution under special conditions,13 crystallization fromthe melt,14 application of high pressure,13 addition of additivematerials to PVDF,15 and formation of PVDF-based copoly-mers with triuoroethylene (TrFE) or hexauoropropy-lene.16,17 Recent additions to this list of techniques are theuse of blending with nanollers such as inorganic (ceramic,metal, magnetic particles, nanoclay) materials1825 and electro-spinning.2631 One of the materials recently tried as a nanollerwas carbon nanotubes (CNTs).3236 The PVDFCNTcomposite showed remarkably enhanced ferro-, pyro-, andpiezoelectric properties when the carbon nanotubes were welldispersed.Nanocomposites made from PVDF and CNTs have the

    potential to be smart materials, not only because of thecombination of the piezoelectric properties of PVDF and theconducting properties of CNTs, but also because of the higherlevels of -phase formation in the electrospun nanobers andthus the better piezoelectric properties.32,33 The electro-spinning process involves the uniaxial stretching of a viscouspolymer solution using a large electric potential;37 this processis expected to transform the -phase into a highly oriented -phase. In the present study, we examined the formation of the-phase in PVDF subjected to the electrospinning process; thechanges in ferroelectric and piezoelectric properties werecompared to those fabricated using a thermal press.33

    Electrospun PVDF membranes containing 0.051.0 wt %carbon nanotubes were also produced to determine the eectsof multiwalled carbon nanotubes (MWCNTs) on theformation of the induced -phase crystallites and on theferroelectric and piezoelectric properties.

    EXPERIMENTAL SECTIONMaterials. The polymer used in this experiment was a

    semicrystalline PVDF (Atona, Kynar761) which has a numberaverage molecular weight of 5.2 105 Da. N,N-Dimethylace-tamide (DMAc) and acetone solvent were purchased fromFisher Scientic. MWCNTs (purity, >95%; average diameter,1015 nm) were purchased from Hanwha Nanotech Corp.(Korea).Preparation of Functionalized MWCNTs. Heat treat-

    ment at 400 C was done in an oven for 3 h to removeimpurities such as amorphous carbon, catalyst metals, andgraphite particles.33,38 MWCNTs were then put in aconcentrated H2SO4/HNO3 (1:3, vol %) solution and stirredfor 48 h. Ultrasonication was applied for 2 h to remove furtherimpurities and to maximize the number of carboxylic acidgroups on the surface of the MWCNTs with little destructionof the tube walls.39,40 Extra acid was removed by ltrationthrough a 0.4 m PVDF porous membrane. The MWCNTslurry was rinsed with distilled water several times. TheMWCNTs were dried in a vacuum oven at 80 C overnight.The functionalized MWCNTs were dispersed in DMAc solventby using an ultrasonic bath for 30 min. This MWCNT solutionwas stable for a month.Fabrication of Electrospun Membranes. Figure 1 shows

    a schematic illustration of the setup used in this study. The

    polymer solution lled the plastic syringe (Hamilton, 10 mL)with a metal needle (0.34 mm in diameter) connected to ahigh-voltage power supply of 14 kV. The syringe was placed inan automatic pump (KD Scientic, model 220). A groundedstainless steel plate was used for the collection of theelectrospun brous membrane with a thickness of approx-imately 100 m and dimensions of approximately 20 cm 30cm. The distance from the needle tip to the collector was set as15 cm. Electrospinning was done with an ejection rate of 40L/min from the syringe. In the electrospinning controlchamber, the temperature and humidity were set at 25 C and40%, respectively. After spinning, the electrospun membraneswere dried under vacuum at room temperature for 24 h.Preparation of Drawn and/or Poled Membranes. The

    electrospun membranes with a thickness of 100 m and a widthof 50 mm were mounted in the uniaxial stretching device.Drawing was done at a deformation rate of 1 mm/s at 125 C.All of the electrospun samples could be drawn up to 200%elongation without producing any defects. After the drawing,they were removed from the device and rapidly cooled to roomtemperature. For the poling procedure, conducting aluminum

    Figure 1. Schematic illustration of the electrospinning system.

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  • plates were attached to both sides of the electrospunmembranes. An electric eld of 500 kV/cm was applied in asilicone oil bath (KF-96, Shin-Etsu Silicone Korea Co., Ltd.) at120 C for 20 min. After being poled, the samples wereremoved from the oil bath and rinsed with hexane andmethanol to remove the silicone oil. The samples were dried ina vacuum for 1 day.Morphology Observations. The morphology of the

    electrospun membranes was observed using a scanning electronmicroscope (Nova NanoSEM 200, FEI Co.). All specimenswere gold-coated by sputter coating prior to scanning electronmicroscopy (SEM) observations. A transmission electronmicroscope (Tecnai F20G2, FEI Co.) operated at 200 kV wasalso used. For transmission electron microscopy (TEM)observation, the bers were directly spun on the copper grids.Analysis of the Crystalline Structure. Attenuated total

    reection Fourier transform infrared spectroscopy (ATR-FT-IR; Spectrum 65, Perkin-Elmer) was used with an average of1000 scans in the 5001600 cm1 range. The crystal structureswere determined by wide-angle X-ray diraction (WAXD;HTK 1200N high-temperature X-ray diractometer, PANalyt-ical) using 40 kV and 100 mA. The samples were scanned inthe range of 1040, with a scan rate of 1 deg/min. Theincident and diraction X-ray beams were in the same plane,vertical to the electrospun ber mat.Analysis of the Degree of Porosity. The apparent

    porosity of the membranes (P) was determined as P (%) = (1 (m/p)) 100, where m is the membrane density and p isthe pure polymer density. The density of PVDF was 1.77 gcm3, and the membrane density was determined using thevolume and the weight of the membrane.41

    Measurement of Ferroelectric and PiezoelectricProperties. Using a vacuum thermal evaporator (MEP5000,SNTEC), aluminum electrodes were thermally evaporated ontoboth surfaces of the samples in a 10 15 mm rectangular area.The metal deposition was conducted under a pressure of 106

    Torr until a thickness of 200 nm was reached. Polarizationelectric eld (PE) hysteresis loops were obtained using astandard ferroelectric testing system (Radiant Technoloies,RT66A) connected to a high-voltage interface (Trek). Atriangular wave with a pulse of 1 ms and a voltage of at least 2kV was applied to the samples in the oil. The piezoelectricresponse in the thickness direction, d33, was measured with aPSV-400 scanning vibrometer (Polytec) using a poweramplier with a continuous sine wave at a voltage of 200 Vand a resonant frequency of 1 kHz.

    RESULTS AND DISCUSSIONFigure 2 shows optical images of several electrospun PVDFMWCNT composite nanobrous membranes. The MWCNTsare uniformly dispersed due to the surface modication and

    solution blending. SEM images of the electrospun PVDF berbased membranes are shown in Figure 3. The electrospun

    membranes consisted of bers, typically with diameters of lessthan 1 m, with fully interconnected pores and a high degree ofporosity (nearly 80%). No bead or beaded ber was observed.Several wrinkles were observed on the surfaces of the bers, asshown in Figure 3b,c. This surface morphology resulted from amixed solution of DMAc with an equal weight of acetone. Asmooth surface without wrinkles was obtained when theamount of DMAc in the mixed solvent was increased (Figure3d).42 Choi et al. stated that bers of PVDF formed in a mixedsolvent system with high acetone content had pores, wrinkles,and raised areas, whereas those prepared in a cosolvent withincreased amounts of DMAc showed a smooth surface.43

    Bognitzki et al. also reported that the pore structure in thebers made using dichloromethane as the solvent, which has alow boiling point and a high vapor pressure, was signicantlyreduced when dichloromethane was replaced by a solvent withthe opposite properties.44 The morphological dierencesbetween the two bers were aected by the volatile solventsduring electrospinning.Figure 4 shows TEM images of the nanobers containing

    MWCNTs. It is evident that individual MWCNTs are well

    dispersed in the PVDF matrix. Most of the nanotubesembedded in the ber matrix were well oriented along theber axis without entangled nanotube bundles. Others reportedsimilar morphologies in nanobers containing CNTs.45,46 Itshould be emphasized that the preparation of separate PVDFand MWCNT solutions and their subsequent mixing isimportant to prevent the reaggregation of the MWCNTs into

    Figure 2. Optical images of the PVDF nanobrous membranes withdierent amounts of MWCNTs: (a) 0 wt %, (b) 0.05 wt %, (c) 0.1 wt%, (d) 0.2 wt %, (e) 0.5 wt %, (f) 1.0 wt % (400 magnication).

    Figure 3. SEM images of electrospun PVDF brous membranes. (ac) Electrospun from PVDF (15 wt %)/DMAc/acetone (1:1 DMAc/acetone weight ratio) solutions: (a) low-magnication image (3000),(b) middle-magnication image (20000), (c) high-magnicationimage (50000). (d) Electrospun from PVDF solutions with aDMAc/acetone weight ratio of 80:20 (30000).

    Figure 4. TEM image showing oriented MWCNT in the PVDFnanobers.

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  • large bundles. The good MWCNT alignment was due to thehigh extension of the electrospun jet. Well-separatedMWCNTs were randomly oriented in the electrospinningsolution, but they were aligned along the ow direction when astraight line was developed at the Taylor cone. Moreover, dueto the elongation of the uid during jet travel, the nanotubeswere further spread out along the direction of motion of the jet.In the same context, Dror et al. presented a theoretical modelto explain the alignment behavior of rodlike CNTs inelectrospinning,47 and Ge at el. also suggested that jetelongation is a determining factor in orienting the carbonnanotubes.48

    FT-IR spectra of the electrospun PVDF nanobers and thethermally pressed lms are presented in Figure 5a. Thecharacteristic absorption peaks of the -phase appear at 763,976, 1150, 1211, and 1384 cm1, and those of the -phaseappear at 842 and 1274 cm1.49 The -phase is the mostabundant component in the crystal region of normal PVDF.33

    The relative intensity of the -phase absorption peaks graduallyincreased with electrospinning, while the intensity of the peaksassociated with the -phase drastically decreased. Thisdemonstrates that the elongational eect in the electrospinningprocess induces the -phase conversion into the -phase. Theelongation of the jet occurs when the jet is issued from theelectrically charged surface of the droplet. The jet is elongatedmany times its original length, becoming very long and slender.After the jet is established, the inuence of Coulomb repulsionbetween adjacent charges carried within the jet enables thebending motion of the jet to create a coil shape.2731,37 ThisCoulomb interaction causes the swirling motion of the jet to be

    elongated along the local axis, reducing the diameter of the jetfurther. The elongation of the jet uids made it easier for thepolymer chain orientation along the ber axis to produce moreof the polar -phase. This could be corroborated in the WAXDpatterns shown in Figure 5b, where the four characteristic peaksof the -phase at 2 = 18.0, 18.6, 20.2, and 26.8 correspondto (100), (020), (110), and (021) reections, respectively. Allthe peak intensities assigned to -phase crystals with a nonpolarTGTG conformation were reduced, whereas the diractionintensity of the characteristic phase peaks at 2 = 20.9(assigned to the (110) reection) increased signicantly.50 The-phase content of the PVDF electrospun nanobers wasapproximately 33%. This indicates that electrospinning is amore ecient process to provide the polar -phase than thesimple addition of MWCNTs to a PVDF melt.33 Althoughmore -phase was present due to the nanober stretchingeect, considerable amounts of the nonpolar -phase remained,indicating that it was not possible to convert all of the -phaseinto the -phase PVDF using solely the electrospinningprocess.In our previous study, we reported that the addition of

    MWCNTs into PVDF could induce the -phase to convert tothe -phase via the accumulated charge induction at theinterphase.33 To precisely determine the inuence of theMWCNTs on the formation of the -phase, the FT-IR spectra(see Figure 6a) were normalized on the basis of the absorbancepeak at 877 cm1, which is proportional to the thickness of thesample.51 The peaks at 840 and 1270 cm1 slightly increased inintensity with the addition of MWCNTs. The -phase increasewith increasing MWCNT content was further conrmed using

    Figure 5. (a) FTIR spectra and (b) WAXD patterns of (---) the thermally pressed PVDF lms and () the electrospun PVDF nanobrousmembranes.

    Figure 6. (a) FTIR spectra and (b) WAXD patterns of the electrospun PVDF/MWCNT membrane.

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  • WAXD (Figure 6b). The intensities at 2 = 18.6 and 20.2,which are related to the -phase, decreased while the -phasepeak at 20.9 increased signicantly. When functionalizedMWCNTs are dispersed in PVDF solutions, electrostaticinteractions act between the functional groups and the CF2groups of the PVDF chains. MWCNTs act as nuclei to induce ahigher crystallization rate. In our previous study, we coulddirectly observe that the rate of crystallization increased withdecreasing PVDF crystal size, in step with the MWCNTconcentration.33 The interactions between the functionalgroups on the MWCNTs and the CF2 dipole of PVDFproduce locally oriented -phase with signicant help from theincreased crystallization speed that occurs due to the presenceof the MWCNTs.To quantitatively characterize the -phase content as a

    function of the amount of MWCNTs in the compositenanobers, the -phase content was calculated on the basis ofthe WAXD data (Figure 7). Here, a few facts are worthy of

    note. First, just 0.05 wt % added MWCNTs brought about aremarkable change in the crystalline structure. The extendedMWCNTs in the nanober increased the interaction with thePVDF over a wider area, thus inducing more -phase. Second,the amount of the -phase reaches a plateau at a concentrationof 1 wt % MWCNTs.33 It is well-known that the complexviscosity increases when MWCNTs are homogeneouslydispersed and they have a stronger interaction with thepolymer matrix.52 Then the viscoelastic force which hampersthe elongation of the jet is also increased relative to the

    electrostatic force responsible for drawing the jet; the amountof -phase obtained from the stretching eect of theelectrospinning process therefore reaches a plateau value.More -phase can be obtained in PVDF using the postdrawingand poling process.33 The amount of -phase variedsignicantly with drawing and/or poling, as shown in Figure8. In samples with an applied external electric eld andmechanical stress, the intensities of the -phase FT-IRabsorption band and the -phase WAXD peaks diminished,while the corresponding intensities for the -phase increaseddramatically.Figure 9 shows a schematic illustration of the orientation of

    the chains and MWCNTs during the processing of the

    electrospun PVDF/MWCNT composite. For the drawnsamples, the chain orientation and the higher level ofinteraction with the MWCNTs increase the -phase content.Under the mechanical stress applied during drawing, thealignment of polymer chains in the long planar conformation(the -phase) is favored.4,53 Further conversion to the -phasecan be accomplished using the poling process. When theexternal electric eld is applied in the direction perpendicular tothe chain axis of the PVDF bers, the -sequence chains rotateto align their dipole moments in the direction favored by thepoled electric eld.54,55

    The overall -phase contents after drawing and poling areshown in Figure 10. The amount of -phase went over 90% inthe drawn PVDFMWCNT composite membranes, while theelectrospun and drawn pure PVDF membrane had ca. 80% -phase content at a constant draw ratio. For the drawn samples,the addition of more than 0.2 wt % MWCNTs did not produceany noticeable change in the -phase content.33 This indicatesthat the drawing process is quite ecient in producing changesin the chain conformation. Although the poled samples show

    Figure 7. Variation of the -phase content as a function of theMWCNT loading.

    Figure 8. (a) FTIR and (b) WAXD spectra of undrawn and unpoled, drawn, poled, and drawn and poled PVDF/MWCNT (0.2 wt %)nanocomposite membranes.

    Figure 9. Schematic showing the proposed mechanism for chainextension produced by electrospinning and mechanical drawing.

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  • high -phase contents too, stretching is more eective thanpoling. It should be noted that the poling eects appeareddierently here compared to those on the solution castingmembranes. In our previous study, the poling eects increasedsteadily with increasing MWCNT amounts: a noticeableincrease in the -phase was observed when the MWCNTamount reached a conducting threshold.33 However, theelectrospinning process used in this study already had thenanobers elongated. A pure PVDF nanobrous membraneshowed a -phase content of approximately 33%. Stretching bythe electrospinning process helped the PVDF chain con-formation to be arranged more easily (Figure 9). The additionof the MWCNTs induced charge accumulation at theboundary, which helped the PVDF chains to become arrangedin the -phase conformation.33 For the solution castingmembranes, depoling was observed with excessive MWCNTaddition that led to the -phase content decrease. Forelectrospun samples, it seems that depoling did not happendue to better dispersion of MWCNTs so that the chargeaccumulation at the interface did not exceed the coerciveeld.33

    The eects of MWCNT addition on the PE (polarization,P, versus applied voltage, E) hysteresis are shown in Figure 11a.The drawn and poled PVDF/MWCNT nanober compositelms exhibited a larger remanent polarization (Pr) than the

    drawn and poled nanobrous pure PVDF lm. The neatnanobrous PVDF lm still exhibited a higher Pr than athermally pressed PVDF lm.33 The good piezoelectricity inthe PVDF lms stemmed from the high remanent polarization,i.e., the high lattice dipole moment after polarization.58 A well-saturated hysteresis curve (the Pr and coercive eld values were35.0 mC/cm2 and 75 MV/m) was observed for the electrospunnanobrous PVDF lm, whereas no ferroelectric hysteresisloop was observed for the thermally pressed pure PVDF thinlms. This is due to the enhancment of the -phase formproduced by the elongation of the jet in the electrospinningprocess: the -phase exhibits no net polarization, since thechains are packed with dipoles in the opposite direction in theunit cell, whereas the lamellae of the -crystals can rotate tomatch the chain direction, producing a high degree of dipoleorientation.5658 Addition of MWCNTs further enhanced theferroelectric properties (Figure 11b). Pr reached 45 mC/cm

    2

    with the addition of 0.2 wt % MWCNTs. The ferroelectricityimprovement in the composite membrane produced by theinclusion of MWCNTs and the dierent drawing and polingprocesses is summarized in Table 1.

    The drawing and poling processes produced electrospunPVDF/MWCNT composite membranes with larger Pr valuesthan pure electrospun PVDF or nonprocessed electrospunPVDF/MWCNT membranes due to the higher degree of -phase transformation. The stretched and poled PVDF/MWCNT composite membranes showed much larger Pr values(maximum of around 60 mC/cm2) than the neat PVDFmembranes (35 mC/cm2). Figure 12a,b shows the piezoelectricactuation graph in the d33 direction, which is the membrane

    Figure 10. Variation of the -phase content as a function of theMWCNT loading (wt %): () undrawn and unpoled membranes,() poled membranes, () drawn membranes, () drawn and poledmembranes.

    Figure 11. (a) PE hysteresis loop for the electrospun PVDF membrane and PVDF membrane with MWCNTs (0.1 wt %). (b) Dependence of theremanent polarization on the MWCNT content for the electrospun PVDF membranes.

    Table 1. Summary of the Remanent Polarization (mC/m2)of the PVDF/MWCNT Nanobrous Membranes Preparedby Dierent Processing Conditions

    MWCNTamt

    (wt %)

    electrospunnanobrousmembrane

    poledelctrospunnanobrousmembrane

    drawnelctrospunnanobrousmembrane

    poled and drawnelctrospunnanobrousmembrane

    0 35 43.2 53.8 55.50.05 41.9 49.8 56.8 58.40.1 42.8 51.5 55.4 57.70.2 43.9 51.7 56.5 58.50.5 44.9 51.3 56.8 58.51.0 44.8 52.5 57.7 60.8

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  • thickness direction, under the application of alternating sine-wave voltages of 200 V to the electrospun PVDF membranes.Each membrane experienced reciprocal vertical motions underdierent electric elds. The cycling piezoelectric displacementwas achieved with a maximum displacement of 83 nm. Thefrequency and waveforms of the displacement correspondedwell to the applied sine-wave forms.59,60 Addition of MWCNTsto the membranes and the application of external stresses suchas drawing and poling changed the membrane motion (shownin Figure 13): the onset and trends in these changes were inline with the changes in the -phase content.

    CONCLUSIONSPVDF membranes of high -phase content could be obtainedusing the electrospinning process, with the addition of smallamounts of MWCNTs (0.2 wt %). The electrospinning processis similar in some ways to uniaxial mechanical stretching, whichhelps the -phase conversion into the -phase. The addition oflong MWCNTs promoted the conversion of the PVDFmolecules -phase into the -phase by acting as nuclei in thecrystallization process and inducing charge accumulation at theinterface. The change of the physical properties could beattributed to the alterations in the polymer microstructureproduced by drawing and poling and to the presence of thecarbon nanotubes. The cooperative eect of the drawingprocess and MWCNTs led to a high degree of conversion fromthe nonpolar -phase to the polar -phase in the electrospunPVDF/MWCNT composite nanobers. The signicant eectsof the polymer chain orientation originated mainly from theirdetermining role in conning the dipole through chain rotationunder drawing and poling.Dierently from the solution casting membranes, depoling

    was not observed with the excessive MWCNT addition that ledto the -phase content decrease. For electrospun samples, itseems that depoling did not happen because of betterdispersion of MWCNTs so that the charge accumulation atthe interface did not exceed the coercive eld. For the poledsamples, the amount of the -phase increased with increasingamount of MWCNTs due to the ecient charge accumulation.The high conversion of the -phase into the -phase wasimproved remarkably by further drawing of the membranes,which resulted in the rapid enhancement of the ferroelectricand piezoelectric properties of the PVDF/MWCNT mem-branes.

    AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (Y.S.); [email protected] (H.J.C.).

    NotesThe authors declare no competing nancial interest.

    Figure 12. (a, b) Displacement images of the electrospun PVDFmembrane. (c) Electromechanical displacement image of electrospunPVDF membranes.

    Figure 13. Comparison between the actuation performance of the PVDF and PVDF/MWCNT membranes with various MWCNT contents andfurther treatment (drawing or drawing and poling).

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    dx.doi.org/10.1021/jp4011026 | J. Phys. Chem. C 2013, 117, 117911179911797

  • ACKNOWLEDGMENTSThis work was supported by the National Research Foundationof Korea (NRF) (Basic Research Program RIAM 041-2004-I-D00224, ITSTD Program RIAM AC 2509 (Grant 0417-20096070), RIAM NR03-09 (Grant 0417-20090027), and theSRC/ERC program (Grant R11-2005-065)) and by Ministry ofKnowledge & Econonics (Original Material TechnologyProgram RIAM I-AC14-10 (Grant 0417-20100043)). H.J.C.acknowledges partial nancial support from the NRF (GrantNRF-2009-0080253).

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