Enhanced Piezoelectric Properties of Electrospun Poly(vinylidene

Article pubs.acs.org/JPCC

Enhanced Piezoelectric Properties of Electrospun Poly(vinylidene fluoride)/Multiwalled Carbon Nanotube Composites Due to High β‑Phase Formation in Poly(vinylidene fluoride) 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, Seoul National University, Shillim-9-dong 56-1, Kwanakgu, Seoul, Republic of Korea 151-744 ‡ Hybrid Materials Research Center, Korea Institute of Science and Technology, Hawolgokdong 39-1, Sungbukku, Seoul, Republic of Korea 130-650 § Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yousungku, Taejon, Republic of Korea 305-600 ∥ Department of Polymer Science and Engineering, Inha University, Yonghyun-dong, Namku, Incheon, Republic of Korea 402-751 ABSTRACT: We prepared poly(vinylidene fluoride) (PVDF)/multiwalled carbon nanotube (MWCNT) nanocomposites using the electrospinning process and investigated the effects of varying the MWCNT content, as well as the additional use of drawing and poling on the polymorphic behavior and electroactive (piezoelectric) properties of the membranes obtained. Fourier transform infrared spectroscopy and wide-angle X-ray diffraction revealed that dramatic changes occurred in the β-phase crystal formation with the MWCNT loading. This was attributed to the nucleation effects of the MWCNTs as well as the intense stretching of the PVDF jets in the electrospinning process. The remanent polarization and piezoelectric response increased with the amount of MWCNTs and piezoelectric β-phase crystals. A further mechanical stretching and electric poling process induced not only highly oriented β-phase crystallites, but also very good ferroelectric and piezoelectric performances. In the drawn samples, the interfacial interaction between the functional groups on the MWCNTs and the CF2 dipole of PVDF chains produced a large amount of βphase content. In the poled samples, the incorporation of the MWCNTs made it easy to obtain efficient charge accumulation in the PVDF matrix, resulting in the conversion of the α-phase into the β-phase as well as the enhancement of remanent polarization and mechanical displacement.

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most common and important ones. In the TGTĞ conformation (the α-phase conformation), the dipole is inclined relative to the normal axis, so the average dipole moment for each monomer is very reduced. Furthermore, the unit cell of the α-PVDF lattice consists of two chains in a TGTĞ conformation, whose dipole components normal to the chain axis are antiparallel, thus neutralizing each other.4 As a result, the α-phase can be described as nonpolar, nonpiezoelectric, and nonpyrroelectric. On the other hand, the β-phase, which is in an all-trans (TTTT) conformation, has all of its dipoles aligned in the same direction normal to the chain axis. Its unit cell consists of two all-trans chains packed with their dipoles pointing in the same direction. The molecular dipoles in the βphase are thus entirely aligned in one direction; this crystal form can therefore generate the largest spontaneous polar-

INTRODUCTION Poly(vinylidene fluoride) (PVDF) has been studied extensively because of its unique electroactive properties, including piezo-, pyro-, and ferroelectric properties, as well as its other useful properties, such as its flexibility, light weight, and long-term stability under high electric fields.1,2 PVDF is a semicrystalline polymer with a typical crystallinity of 50%, whose molecular structure consists of the repeated monomer unit (−CH2CF2−)n. It is well-known that PVDF has five distinct crystallite polymorphs.3 The most common polymorph of PVDF is the α-phase, which has a monoclinic unit cell with a TGTĞ (T = trans, G = gauche +, Ğ = gauche −) conformation. The piezoelectric crystallization polymorph is the β-phase, which has an all-trans (TTTT) conformation, with an orthorhombic unit cell. The γ-phase also has an orthorhombic unit cell, with a TTTGTTTĞ chain conformation. The other two (δ and ε) polymorphs are the polar and antipolar analogues of the α and γ forms, respectively.3 The first two conformations (the α-phase and the β-phase) are by far the © XXXX American Chemical Society

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

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dx.doi.org/10.1021/jp4011026 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Materials. The polymer used in this experiment was a semicrystalline PVDF (Atofina, Kynar761) which has a number average molecular weight of 5.2 × 105 Da. N,N-Dimethylacetamide (DMAc) and acetone solvent were purchased from Fisher Scientific. MWCNTs (purity, >95%; average diameter, 10−15 nm) were purchased from Hanwha Nanotech Corp. (Korea). Preparation of Functionalized MWCNTs. Heat treatment at 400 °C was done in an oven for 3 h to remove impurities such as amorphous carbon, catalyst metals, and graphite particles.33,38 MWCNTs were then put in a concentrated H2SO4/HNO3 (1:3, vol %) solution and stirred for 48 h. Ultrasonication was applied for 2 h to remove further impurities and to maximize the number of carboxylic acid groups on the surface of the MWCNTs with little destruction of the tube walls.39,40 Extra acid was removed by filtration through a 0.4 μm PVDF porous membrane. The MWCNT slurry was rinsed with distilled water several times. The MWCNTs were dried in a vacuum oven at 80 °C overnight. The functionalized MWCNTs were dispersed in DMAc solvent by using an ultrasonic bath for 30 min. This MWCNT solution was stable for a month. Fabrication of Electrospun Membranes. Figure 1 shows a schematic illustration of the setup used in this study. The

ization and exhibits strong ferroelectric and piezoelectric properties. These unique β-phase-derived properties of PVDF make it useful in a wide range of applications, including actuators, biosensors, energy-harvesting materials, audio devices, transducers, and nonvolatile memories.3,5−8 However, it is not easy to obtain PVDF consisting of entirely β-phase crystals. In all-trans PVDF, the overlapping of neighboring fluorine atoms occurs, because the diameter of the fluorine atom (0.270 nm) is slightly larger than the space provided by an all-trans carbon chain (0.256 nm).9 To diminish this overlap, CF2 groups are tilted to the right and left, relative to their original conformation. This deflection of CF2 groups converts the all-trans form into TGTĞ (α form) or TTTGTTTĞ (γ form). Hence, the α-phase is more easily formed than the βphase in normal circumstances. Although the crystal lattice energy of the α-phase is slightly higher than that of the β-phase, direct β-phase formation from the melt is prohibited due to the high energy of the all-trans conformations.1 β-Phase formation can be accomplished via a crystal phase transition from the α-phase.10 The most common technique for obtaining polar β-PVDF involves mechanical extension (drawing) and electrical poling.11−13 Mechanical drawing contributes to the transition of the original spherulitic structure into a crystal array, in which the molecules are forced into their most extended conformation (polar β-phase), with all of the dipole moments aligned in the same direction.10,11 The application of an electric field on both sides of the PVDF electrets (poling) also results in the orientation of the crystallite polar axis along the field direction, which promotes a higher spontaneous polarization for the β-phase.12 Conversion of the paraelectric phase to the ferroelectric phase has also been achieved using diverse methods such as crystallization from a polar solution under special conditions,13 crystallization from the melt,14 application of high pressure,13 addition of additive materials to PVDF,15 and formation of PVDF-based copolymers with trifluoroethylene (TrFE) or hexafluoropropylene.16,17 Recent additions to this list of techniques are the use of blending with nanofillers such as inorganic (ceramic, metal, magnetic particles, nanoclay) materials18−25 and electrospinning.26−31 One of the materials recently tried as a nanofiller was carbon nanotubes (CNTs).32−36 The PVDF−CNT composite showed remarkably enhanced ferro-, pyro-, and piezoelectric properties when the carbon nanotubes were well dispersed. Nanocomposites made from PVDF and CNTs have the potential to be smart materials, not only because of the combination of the piezoelectric properties of PVDF and the conducting properties of CNTs, but also because of the higher levels of β-phase formation in the electrospun nanofibers and thus the better piezoelectric properties.32,33 The electrospinning process involves the uniaxial stretching of a viscous polymer solution using a large electric potential;37 this process is expected to transform the α-phase into a highly oriented βphase. In the present study, we examined the formation of the β-phase in PVDF subjected to the electrospinning process; the changes in ferroelectric and piezoelectric properties were compared to those fabricated using a thermal press. 33 Electrospun PVDF membranes containing 0.05−1.0 wt % carbon nanotubes were also produced to determine the effects of multiwalled carbon nanotubes (MWCNTs) on the formation of the induced β-phase crystallites and on the ferroelectric and piezoelectric properties.

Figure 1. Schematic illustration of the electrospinning system.

polymer solution filled the plastic syringe (Hamilton, 10 mL) with a metal needle (0.34 mm in diameter) connected to a high-voltage power supply of 14 kV. The syringe was placed in an automatic pump (KD Scientific, model 220). A grounded stainless steel plate was used for the collection of the electrospun fibrous membrane with a thickness of approximately 100 μm and dimensions of approximately 20 cm × 30 cm. The distance from the needle tip to the collector was set as 15 cm. Electrospinning was done with an ejection rate of 40 μL/min from the syringe. In the electrospinning control chamber, the temperature and humidity were set at 25 °C and 40%, respectively. After spinning, the electrospun membranes were dried under vacuum at room temperature for 24 h. Preparation of Drawn and/or Poled Membranes. The electrospun membranes with a thickness of 100 μm and a width of 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 room temperature. For the poling procedure, conducting aluminum B



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

plates were attached to both sides of the electrospun membranes. An electric field of 500 kV/cm was applied in a silicone oil bath (KF-96, Shin-Etsu Silicone Korea Co., Ltd.) at 120 °C for 20 min. After being poled, the samples were removed from the oil bath and rinsed with hexane and methanol to remove the silicone oil. The samples were dried in a vacuum for 1 day. Morphology Observations. The morphology of the electrospun membranes was observed using a scanning electron microscope (Nova NanoSEM 200, FEI Co.). All specimens were gold-coated by sputter coating prior to scanning electron microscopy (SEM) observations. A transmission electron microscope (Tecnai F20G2, FEI Co.) operated at 200 kV was also used. For transmission electron microscopy (TEM) observation, the fibers were directly spun on the copper grids. Analysis of the Crystalline Structure. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR; Spectrum 65, Perkin-Elmer) was used with an average of 1000 scans in the 500−1600 cm−1 range. The crystal structures were determined by wide-angle X-ray diffraction (WAXD; HTK 1200N high-temperature X-ray diffractometer, PANalytical) using 40 kV and 100 mA. The samples were scanned in the range of 10−40°, with a scan rate of 1 deg/min. The incident and diffraction X-ray beams were in the same plane, vertical to the electrospun fiber mat. Analysis of the Degree of Porosity. The apparent porosity of the membranes (P) was determined as P (%) = (1 − (ρm/ρp)) × 100, where ρm is the membrane density and ρp is the pure polymer density. The density of PVDF was 1.77 g cm−3, and the membrane density was determined using the volume and the weight of the membrane.41 Measurement of Ferroelectric and Piezoelectric Properties. Using a vacuum thermal evaporator (MEP5000, SNTEC), aluminum electrodes were thermally evaporated onto both surfaces of the samples in a 10 × 15 mm rectangular area. The metal deposition was conducted under a pressure of 10−6 Torr until a thickness of 200 nm was reached. Polarization− electric field (P−E) hysteresis loops were obtained using a standard ferroelectric testing system (Radiant Technoloies, RT66A) connected to a high-voltage interface (Trek). A triangular wave with a pulse of 1 ms and a voltage of at least 2 kV was applied to the samples in the oil. The piezoelectric response in the thickness direction, d33, was measured with a PSV-400 scanning vibrometer (Polytec) using a power amplifier with a continuous sine wave at a voltage of 200 V and a resonant frequency of 1 kHz.

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

membranes consisted of fibers, typically with diameters of less than 1 μm, with fully interconnected pores and a high degree of porosity (nearly 80%). No bead or beaded fiber was observed. Several wrinkles were observed on the surfaces of the fibers, as shown in Figure 3b,c. This surface morphology resulted from a mixed solution of DMAc with an equal weight of acetone. A smooth surface without wrinkles was obtained when the amount of DMAc in the mixed solvent was increased (Figure 3d).42 Choi et al. stated that fibers of PVDF formed in a mixed solvent system with high acetone content had pores, wrinkles, and raised areas, whereas those prepared in a cosolvent with increased amounts of DMAc showed a smooth surface.43 Bognitzki et al. also reported that the pore structure in the fibers made using dichloromethane as the solvent, which has a low boiling point and a high vapor pressure, was significantly reduced when dichloromethane was replaced by a solvent with the opposite properties.44 The morphological differences between the two fibers were affected by the volatile solvents during electrospinning. Figure 4 shows TEM images of the nanofibers containing MWCNTs. It is evident that individual MWCNTs are well

RESULTS AND DISCUSSION

Figure 2 shows optical images of several electrospun PVDF− MWCNT composite nanofibrous membranes. The MWCNTs are uniformly dispersed due to the surface modification and Figure 4. TEM image showing oriented MWCNT in the PVDF nanofibers.

dispersed in the PVDF matrix. Most of the nanotubes embedded in the fiber matrix were well oriented along the fiber axis without entangled nanotube bundles. Others reported similar morphologies in nanofibers containing CNTs.45,46 It should be emphasized that the preparation of separate PVDF and MWCNT solutions and their subsequent mixing is important to prevent the reaggregation of the MWCNTs into

Figure 2. Optical images of the PVDF nanofibrous membranes with different 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× magnification). C



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Figure 5. (a) FTIR spectra and (b) WAXD patterns of (---) the thermally pressed PVDF films and () the electrospun PVDF nanofibrous membranes.

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

elongated along the local axis, reducing the diameter of the jet further. The elongation of the jet fluids made it easier for the polymer chain orientation along the fiber axis to produce more of the polar β-phase. This could be corroborated in the WAXD patterns shown in Figure 5b, where the four characteristic peaks of the α-phase at 2θ = 18.0°, 18.6°, 20.2°, and 26.8° correspond to (100), (020), (110), and (021) reflections, respectively. All the peak intensities assigned to α-phase crystals with a nonpolar TGTĞ conformation were reduced, whereas the diffraction intensity of the characteristic β phase peaks at 2θ = 20.9° (assigned to the (110) reflection) increased significantly.50 The β-phase content of the PVDF electrospun nanofibers was approximately 33%. This indicates that electrospinning is a more efficient process to provide the polar β-phase than the simple addition of MWCNTs to a PVDF melt.33 Although more β-phase was present due to the nanofiber stretching effect, considerable amounts of the nonpolar α-phase remained, indicating that it was not possible to convert all of the α-phase into the β-phase PVDF using solely the electrospinning process. In our previous study, we reported that the addition of MWCNTs into PVDF could induce the α-phase to convert to the β-phase via the accumulated charge induction at the interphase.33 To precisely determine the influence of the MWCNTs on the formation of the β-phase, the FT-IR spectra (see Figure 6a) were normalized on the basis of the absorbance peak at 877 cm−1, which is proportional to the thickness of the sample.51 The peaks at 840 and 1270 cm−1 slightly increased in intensity with the addition of MWCNTs. The β-phase increase with increasing MWCNT content was further confirmed using

large bundles. The good MWCNT alignment was due to the high extension of the electrospun jet. Well-separated MWCNTs were randomly oriented in the electrospinning solution, but they were aligned along the flow direction when a straight line was developed at the Taylor cone. Moreover, due to the elongation of the fluid during jet travel, the nanotubes were further spread out along the direction of motion of the jet. In the same context, Dror et al. presented a theoretical model to explain the alignment behavior of rodlike CNTs in electrospinning,47 and Ge at el. also suggested that jet elongation is a determining factor in orienting the carbon nanotubes.48 FT-IR spectra of the electrospun PVDF nanofibers and the thermally pressed films are presented in Figure 5a. The characteristic absorption peaks of the α-phase appear at 763, 976, 1150, 1211, and 1384 cm−1, and those of the β-phase appear at 842 and 1274 cm−1.49 The α-phase is the most abundant component in the crystal region of normal PVDF.33 The relative intensity of the β-phase absorption peaks gradually increased with electrospinning, while the intensity of the peaks associated with the α-phase drastically decreased. This demonstrates that the elongational effect in the electrospinning process induces the α-phase conversion into the β-phase. The elongation of the jet occurs when the jet is issued from the electrically charged surface of the droplet. The jet is elongated many times its original length, becoming very long and slender. After the jet is established, the influence of Coulomb repulsion between adjacent charges carried within the jet enables the bending motion of the jet to create a coil shape.27−31,37 This Coulomb interaction causes the swirling motion of the jet to be D



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electrostatic force responsible for drawing the jet; the amount of β-phase obtained from the stretching effect of the electrospinning process therefore reaches a plateau value. More β-phase can be obtained in PVDF using the postdrawing and poling process.33 The amount of β-phase varied significantly with drawing and/or poling, as shown in Figure 8. In samples with an applied external electric field and mechanical stress, the intensities of the α-phase FT-IR absorption band and the α-phase WAXD peaks diminished, while the corresponding intensities for the β-phase increased dramatically. Figure 9 shows a schematic illustration of the orientation of the chains and MWCNTs during the processing of the

WAXD (Figure 6b). The intensities at 2θ = 18.6° and 20.2°, which are related to the α-phase, decreased while the β-phase peak at 20.9° increased significantly. When functionalized MWCNTs are dispersed in PVDF solutions, electrostatic interactions act between the functional groups and the CF2 groups of the PVDF chains. MWCNTs act as nuclei to induce a higher crystallization rate. In our previous study, we could directly observe that the rate of crystallization increased with decreasing PVDF crystal size, in step with the MWCNT concentration.33 The interactions between the functional groups on the MWCNTs and the CF2 dipole of PVDF produce locally oriented β-phase with significant help from the increased crystallization speed that occurs due to the presence of the MWCNTs. To quantitatively characterize the β-phase content as a function of the amount of MWCNTs in the composite nanofibers, the β-phase content was calculated on the basis of the WAXD data (Figure 7). Here, a few facts are worthy of

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

electrospun PVDF/MWCNT composite. For the drawn samples, the chain orientation and the higher level of interaction with the MWCNTs increase the β-phase content. Under the mechanical stress applied during drawing, the alignment of polymer chains in the long planar conformation (the β-phase) is favored.4,53 Further conversion to the β-phase can be accomplished using the poling process. When the external electric field is applied in the direction perpendicular to the chain axis of the PVDF fibers, the α-sequence chains rotate to align their dipole moments in the direction favored by the poled electric field.54,55 The overall β-phase contents after drawing and poling are shown in Figure 10. The amount of β-phase went over 90% in the drawn PVDF−MWCNT composite membranes, while the electrospun and drawn pure PVDF membrane had ca. 80% βphase content at a constant draw ratio. For the drawn samples, the addition of more than 0.2 wt % MWCNTs did not produce any noticeable change in the β-phase content.33 This indicates that the drawing process is quite efficient in producing changes in the chain conformation. Although the poled samples show

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

note. First, just 0.05 wt % added MWCNTs brought about a remarkable change in the crystalline structure. The extended MWCNTs in the nanofiber increased the interaction with the PVDF over a wider area, thus inducing more β-phase. Second, the amount of the β-phase reaches a plateau at a concentration of 1 wt % MWCNTs.33 It is well-known that the complex viscosity increases when MWCNTs are homogeneously dispersed and they have a stronger interaction with the polymer matrix.52 Then the viscoelastic force which hampers the elongation of the jet is also increased relative to the

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. E



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drawn and poled nanofibrous pure PVDF film. The neat nanofibrous PVDF film still exhibited a higher Pr than a thermally pressed PVDF film.33 The good piezoelectricity in the PVDF films stemmed from the high remanent polarization, i.e., the high lattice dipole moment after polarization.58 A wellsaturated hysteresis curve (the Pr and coercive field values were 35.0 mC/cm2 and 75 MV/m) was observed for the electrospun nanofibrous PVDF film, whereas no ferroelectric hysteresis loop was observed for the thermally pressed pure PVDF thin films. This is due to the enhancment of the β-phase form produced by the elongation of the jet in the electrospinning process: the α-phase exhibits no net polarization, since the chains are packed with dipoles in the opposite direction in the unit cell, whereas the lamellae of the β-crystals can rotate to match the chain direction, producing a high degree of dipole orientation.56−58 Addition of MWCNTs further enhanced the ferroelectric properties (Figure 11b). Pr reached 45 mC/cm2 with the addition of 0.2 wt % MWCNTs. The ferroelectricity improvement in the composite membrane produced by the inclusion of MWCNTs and the different drawing and poling processes is summarized in Table 1.

Figure 10. Variation of the β-phase content as a function of the MWCNT loading (wt %): (■) undrawn and unpoled membranes, (○) poled membranes, (△) drawn membranes, (◆) drawn and poled membranes.

high β-phase contents too, stretching is more effective than poling. It should be noted that the poling effects appeared differently here compared to those on the solution casting membranes. In our previous study, the poling effects increased steadily with increasing MWCNT amounts: a noticeable increase in the β-phase was observed when the MWCNT amount reached a conducting threshold.33 However, the electrospinning process used in this study already had the nanofibers elongated. A pure PVDF nanofibrous membrane showed a β-phase content of approximately 33%. Stretching by the electrospinning process helped the PVDF chain conformation to be arranged more easily (Figure 9). The addition of the MWCNTs induced charge accumulation at the boundary, which helped the PVDF chains to become arranged in the β-phase conformation.33 For the solution casting membranes, depoling was observed with excessive MWCNT addition that led to the β-phase content decrease. For electrospun samples, it seems that depoling did not happen due to better dispersion of MWCNTs so that the charge accumulation at the interface did not exceed the coercive field.33 The effects of MWCNT addition on the P−E (polarization, P, versus applied voltage, E) hysteresis are shown in Figure 11a. The drawn and poled PVDF/MWCNT nanofiber composite films exhibited a larger remanent polarization (Pr) than the

Table 1. Summary of the Remanent Polarization (mC/m2) of the PVDF/MWCNT Nanofibrous Membranes Prepared by Different Processing Conditions MWCNT amt (wt %)

electrospun nanofibrous membrane

poled elctrospun nanofibrous membrane

drawn elctrospun nanofibrous membrane

poled and drawn elctrospun nanofibrous membrane

0 0.05 0.1 0.2 0.5 1.0

35 41.9 42.8 43.9 44.9 44.8

43.2 49.8 51.5 51.7 51.3 52.5

53.8 56.8 55.4 56.5 56.8 57.7

55.5 58.4 57.7 58.5 58.5 60.8

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

Figure 11. (a) P−E hysteresis loop for the electrospun PVDF membrane and PVDF membrane with MWCNTs (0.1 wt %). (b) Dependence of the remanent polarization on the MWCNT content for the electrospun PVDF membranes. F



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CONCLUSIONS PVDF membranes of high β-phase content could be obtained using the electrospinning process, with the addition of small amounts of MWCNTs (0.2 wt %). The electrospinning process is similar in some ways to uniaxial mechanical stretching, which helps the α-phase conversion into the β-phase. The addition of long MWCNTs promoted the conversion of the PVDF molecules’ α-phase into the β-phase by acting as nuclei in the crystallization process and inducing charge accumulation at the interface. The change of the physical properties could be attributed to the alterations in the polymer microstructure produced by drawing and poling and to the presence of the carbon nanotubes. The cooperative effect of the drawing process and MWCNTs led to a high degree of conversion from the nonpolar α-phase to the polar β-phase in the electrospun PVDF/MWCNT composite nanofibers. The significant effects of the polymer chain orientation originated mainly from their determining role in confining the dipole through chain rotation under drawing and poling. Differently from the solution casting membranes, depoling was not observed with the excessive MWCNT addition that led to the β-phase content decrease. For electrospun samples, it seems that depoling did not happen because of better dispersion of MWCNTs so that the charge accumulation at the interface did not exceed the coercive field. For the poled samples, the amount of the β-phase increased with increasing amount of MWCNTs due to the efficient charge accumulation. The high conversion of the α-phase into the β-phase was improved remarkably by further drawing of the membranes, which resulted in the rapid enhancement of the ferroelectric and piezoelectric properties of the PVDF/MWCNT membranes.

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

thickness direction, under the application of alternating sinewave voltages of ±200 V to the electrospun PVDF membranes. Each membrane experienced reciprocal vertical motions under different electric fields. The cycling piezoelectric displacement was achieved with a maximum displacement of 83 nm. The frequency and waveforms of the displacement corresponded well to the applied sine-wave forms.59,60 Addition of MWCNTs to the membranes and the application of external stresses such as drawing and poling changed the membrane motion (shown in Figure 13): the onset and trends in these changes were in line with the changes in the β-phase content.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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



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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) (Basic Research Program RIAM 041-2004-ID00224, ITSTD Program RIAM AC 2509 (Grant 041720096070), RIAM NR03-09 (Grant 0417-20090027), and the SRC/ERC program (Grant R11-2005-065)) and by Ministry of Knowledge & Econonics (Original Material Technology Program RIAM I-AC14-10 (Grant 0417-20100043)). H.J.C. acknowledges partial financial support from the NRF (Grant NRF-2009-0080253).

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Enhanced Piezoelectric Properties of Electrospun Poly(vinylidene

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