2013-Enhanced Piezoelectric Properties of Electrospun Poly(Vinylidene Fluoride)Multiwalled Carbon...
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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
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,