Composites: Part B 62 (2014) 126–136

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Composites: Part B

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Pressure-controlled growth of piezoelectric low-dimensional structuresin ternary fullerene C60/carbon nanotube/poly (vinylidene fluoride)based hybrid composites

http://dx.doi.org/10.1016/j.compositesb.2014.02.0261359-8368/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Key Laboratory of Advanced Technologies of Mate-rials, Ministry of Education, School of Materials Science and Engineering, SouthwestJiaotong University, Chengdu 610031, Sichuan, China. Tel.: +86 28 8760 2714; fax:+86 28 8760 0454.

E-mail address: junluprc@hotmail.com (J. Lu).

Wenjing Huang a, Zhongping Li a, Xin Chen a, Pengfei Tian a, Jun Lu a,c,⇑, Zuowan Zhou a, Rui Huang b,David Hui d, Lupu He a, Chaoliang Zhang e, Xuanlun Wang f

a Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University,Chengdu 610031, Sichuan, Chinab College of Polymer Materials Science and Engineering, Sichuan University, Chengdu 610065, Sichuan, Chinac State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, Chinad Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USAe State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, Chengdu 610041, Chinaf College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China

a r t i c l e i n f o

Article history:Received 2 November 2013Received in revised form 19 February 2014Accepted 21 February 2014Available online 28 February 2014

Keywords:A. Polymer–matrix composites (PMCs)A. Smart materialsB. Electrical propertiesB. Microstructures

a b s t r a c t

Hybrid composites, consisting of polymers and nanostructured carbon allotropes, can exhibit differentproperties than their constituent components. In this work, the synergistic action of fullerene C60 andcarbon nanotube (CNT) on poly (vinylidene fluoride) (PVDF) was evaluated, in terms of the dispersionof the carbonaceous fillers in the polymer matrix during the composite preparation, and the control ofcrystalline morphology of the polymer at high pressure. The synergistic effect of the zero-dimensionaland one-dimensional carbon materials resulted in a well dispersed ternary C60/CNT/PVDF based compos-ite, which was simply fabricated by a physical and mechanical route. Furthermore, the pressure-con-trolled growth of piezoelectric low-dimensional crystalline structures of PVDF, including hollownanowires and extended-chain lamellae, was achieved by the simultaneous introduction of C60 andCNT. Especially, PVDF nanowires with folded- and extended-chain lamellae as their substructures wereobtained respectively by controlling the crystallization conditions of the ternary composite at high pres-sure. Under specific conditions, composite samples, which crystalline structures were totally withextended-chain b- or c-form lamellae, further self-reinforced with extended-chain b-form nanowires,were successfully crystallized. The present study provides a facile and effective approach for the fabrica-tion and the multi-level control of polar crystalline structures of a polymer based hybrid composite, withan overall good dispersion of zero- and one-dimensional nanostructured carbonaceous materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Nanostructured carbon allotropes have been intensively inves-tigated in the past two decades, including single- or multi-walledcarbon nanotubes (CNT), fullerenes, graphene, and their chemicalderivatives [1–6]. Exhibiting different properties than their constit-uent components, various hybrid nanostructured carbon materialshave also been fabricated, and can enable decoupled engineering of

energy conversion and transport functions [3–6]. Especially, thecombinational use of the nanostructured carbonaceous fillers, suchas zero-dimensional fullerenes and one-dimensional carbon nano-tubes, in the production of hybrid polymer composites to enhancetheir properties has attracted considerable industrial attention [7].

The combination of polymers with zero- and one-dimensionalcarbon materials offers an attractive route to combine the meritsof organic and inorganic materials into novel hybrid nanocompos-ites, with synergistic improvement observed in mechanical, elec-trical and thermal properties [8,9]. Fang et al. [10] decoratedmulti-walled carbon nanotubes with fullerene C60 via a three-stepchemical functionalization. Compared with pristine CNTs, C60-decorated CNTs further reduced flammability of polypropylene(PP), due to the free-radical-trapping effect of C60 and the barrier

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effect of the CNT network. Guo et al. [7] reported the design of agrape-cluster-like conductive network in a polypropylene matrix.Oriented multi-walled carbon nanotubes (MWCNTs) served asbranches and provided charge transport over large distances, andgrape-like carbon black (CB) aggregates enriched around theMWCNTs and linked them through the charge transport over smalldistances. The experimental results showed that such grape-clus-ter-like network provided a low percolation threshold for PP/CB/MWCNT composites because of the synergistic effect of CB and ori-ented MWCNTs. Ellis et al. [8,9] evaluated the electrical andmechanical properties of single-walled carbon nanotube (SWCNT)reinforced poly (phenylene sulphide) (PPS) composites. The wrap-ping of SWCNTs in polyetherimide (PEI) and the addition of inor-ganic fullerene-like tungsten disulfide (IF-WS2) nanoparticleswere found to be effective in dispersing the SWCNTs. Significantenhancements were demonstrated in stiffness, strength and tough-ness of the hybrid composites by the addition of both nanofillers,and the electrical conductivity of PPS was improve drastically atlow SWCNT content.

Furthermore, the synergistic action of the zero- and one-dimen-sional carbonaceous nanofillers has been utilized in polymer forthe conversion and storage of energy [11–13]. Katz et al. [11] stud-ied electrospun sub-micron fibers containing conjugated polymer(poly (3-hexylthiophene), P3HT) with a fullerene derivative, phe-nyl-C61-butyric acid methylester (PCBM) or a mixture of PCBMand SWCNTs. The results provided experimental evidence of elec-tron transfer between PCBM and P3HT components in two-compo-nent (P3HT/PCBM) and three-component (P3HT/PCBM/SWCNT)fibers, and suggested that the presence of the dispersing block-copolymer did not prevent the efficiency of the electron transferat the P3HT–PCBM interface in PCBM–P3HT and SWCNT–PCBM–P3HT fibers. These findings suggested a research perspective to-wards utilization of fibers of functional nanocomposites in fiber-based organic optoelectronic and photovoltaic devices. Honget al. [12] fabricated composites for a photo-active layer in an or-ganic photovoltaic device using homogeneously dispersed CNTsin a polymer:fullerene bulk-heterojunction matrix. The compositesshowed considerable improvements of their optical and electricalproperties due to the effects of the wideband photo-absorptionand high charge carrier mobility of the CNTs. The organic solar cellassembled from these composites showed a remarkable increase ofthe power conversion efficiency compared to its counterpart usinga photo-active layer without CNTs. Pieta et al. [13] devised a (car-bon nanotube)-(fullerene–ferrocene dyad polymer) composite,pyr-SWCNTs/(C60Fc-Pd), and then tested as an active material ofa symmetrical device for electrical energy storage. The compositewas redox conducting at both positive and negative potentialsdue to the Fc/Fc+ and C60�/C60 electrode process of the ferroceneand fullerene moiety of the dyad, respectively.

Piezoelectric low-dimensional structures have exciting applica-tions in electronics, optoelectronics, sensors, and the biological sci-ences [14–19]. For example, nanogenerators that use alignednanowires, based on such piezoelectric materials as zinc oxide,for converting nanoscale mechanical energy into electric energy,have been described recently [14–16]. Particularly, the lightweightand conformable polymeric nanowires with piezoelectricity showcertain advantages in organic-based electronic devices, and couldeventually lead to the realization of all-organic instruments [20–24]. Moreover, the extended-chain crystals of piezoelectric poly-mers, with a combination of three-dimensional crystal orderingand long-chain molecular orientation ordering, are ideal systemsfor studies of low-dimensional physics, in addition to their poten-tial applications as functional components [25].

Poly (vinylidene fluoride) (PVDF) is one of the limited knownpiezoelectric class of polymers, and it promises applicability in di-verse field of technology due to their high piezoelectric activity and

availability as flexible thin films [26–29]. PVDF exhibits a pro-nounced polymorphism, i.e. a, b, c, d and e, transforming betweenseveral crystal forms under certain conditions [30–33]. The suc-cessful development of piezoelectric polymer devices depends onthe effective fabrication of polar crystalline structures, such as band c [20,23,24,26,34,35].

Patterned arrays of isolated c-type domains, embedded in thenon-polar a structure in thin PVDF films, were already fabricatedby Park et al. [20], using micro-imprinting lithography, and acapacitor fabricated with the compressed PVDF thin film showedreasonably high remanent polarization of approximately6 lC cm�2, with a coercive voltage of approximately 11 V [20].Wang et al. [24] showed recently that nanoporous arrays of PVDF,fabricated by a lithography-free, template-assisted preparationmethod, could be used for robust piezoelectric nanogenerators.The as fabricated porous PVDF nanogenerators produced the recti-fied power density of 0.17 mW/cm3 with the piezoelectric poten-tial and the piezoelectric current enhanced to be 5.2 times and6 times those from bulk PVDF film nanogenerators under the samesonic-input.

Controlling the crystal phase and morphology of the polymer athigh temperatures was also tried by the researchers, with theintroduction of organic or inorganic fillers, especially those withsize in nano scale [36–38]. Cha and Yang [36] investigated the ef-fect of high-temperature spinning and poly (vinyl pyrrolidone)(PVP) additive on PVDF hollow fiber membranes, together withthe corresponding microfiltration performances such as water flux,rejection rate, and elongational strength. By PVDF crystallizationduring high-temperature spinning, porous hollow fiber mem-branes with particulate morphology were prepared, which werefurther modified by the addition of miscible PVP with PVDF. Theresults showed that the rejection rate and strength of the fiberswere increased at the expense of reduced water flux and meanpore size. This indicates that high-temperature spinning and PVPaddition are very effective to control the morphology of PVDF hol-low fiber membranes for microfiltration. Li et al. [37] systemati-cally investigated the cold crystallization temperature effects onthe crystal morphologies and the shape memory properties for aPVDF/acrylic copolymer (ACP) blend. It was found that tiny crystalsof PVDF formed by annealing served as the physical cross-linkpoints and the amorphous regions among them acted as thereversible phase for the blend materials during the mechanicaldeformations. So the PVDF/ACP blends with tiny crystals showednot only high shape fixity but also excellent recovery ratios. Maitiet al. [38] demonstrated process and nanoparticle induced piezo-electric super toughened PVDF nanohybrids, which were preparedby incorporating organically modified nanoclay through meltextrusion and solution route. Compared to pure PVDF withoutany trade-off, the solution processed nanohybrid exhibit 1100%improvement in toughness as well as adequate stiffness. Thiswas attributed to the unique crystallization behavior of PVDF thatcreated an island type of structure on top of the silicate layers (b-phase, planar zigzag chain conformation, and subsequent polar c-phase and a-phase as layered type). Furthermore, the extent of pie-zoelectric b-phase has been enhanced by controlled stretching ofthe nanohybrid at moderately high temperature for better disen-tanglement, and 90% of the piezoelectric phase has been stabilized,leading to a super toughened lightweight piezoelectric material.However, to the best of our knowledge, no such investigationwas performed on ternary C60/CNT/PVDF composites at highpressure.

Achieved by creating the desired crystal morphologies withideal molecular orientation during the processing, solid phaseforming under high pressure is a more effective route to producepolymer products with greatly improved physical and mechanicalproperties [39]. As for PVDF, pressure treatment has been proved

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to be effective in enhancing the yielding of its b-or c-form crystals[40,41]. Also, nanostructured carbonaceous fillers, such as fuller-ene C60 and C70, were found to be promising in promoting the for-mation of new polymeric structures at high pressure [32,33,42,43].In this work, we evaluated for the first time the synergistic actionof C60 and CNT on PVDF, in terms of the dispersion of the carbona-ceous fillers in the polymer matrix during the composite prepara-tion, and the control of crystalline morphology of the polymer athigh pressure. Pressure-controlled growth of piezoelectriclow-dimensional crystalline structures of PVDF, including hollownanowires and extended-chain lamellae, was achieved by thesimultaneous introduction of C60 and CNT. Particularly, compositesamples, which crystalline structures were totally with extended-chain b- or c-form lamellae, further self-reinforced withextended-chain b-form nanowires, were successfully crystallizedunder specific conditions.

Fig. 1. TEM photograph of the as fabricated C60/CNT/PVDF composite sample with1.0 wt.% C60 and 5.0 wt.% CNT loadings, just before the applied high-pressuretreatment.

2. Experimental part

2.1. Materials

C60 powder (>99.9% wt/wt purity) was purchased from PuyangYongxin Fullerene Co., Ltd. Multi-wall carbon nanotubes(MWCNTs) (>95% wt/wt purity) was obtained form Chengdu Or-ganic Chemicals Co., Ltd, Chinese Academy of Science, and usedas received. PVDF powder, commercial-grade Solef 6010, was sup-plied by Solvay Co., Ltd, Shanghai. The corresponding weight-aver-age molecular weight, Mw, was 322,000 g/mol. C60, CNTs and PVDFwere pre-mixed at 23,000 rpm, room temperature for 10 min in acommercial Joyoung JYL-C012 blender. This was followed by meltcompounding at 180 rpm, 190 �C for 15 min using a ZJL-300 torquerheometer.

2.2. Sample preparation

High-pressure experiments for the as-prepared C60/CNT/PVDFcomposites were carried out with a self-made piston-cylinderhigh-pressure apparatus [32]. The following procedure for crystal-lization was used. After loading the samples, the temperature wasincreased to a level (200 �C) to allow them to be melted. Then a lowpressure (150 MPa) was applied, and the temperature was raisedto a predetermined level. After equilibrium was established, thepressure was further raised to the predetermined level. These sam-ples were kept under these conditions for a predetermined time,and then quenched down to ambient condition. This procedure en-sured the minimum degradation of PVDF at elevated temperature,and the polymer would be in a molten state before crystallizationtook place. As for polymers, it is known their melting points in-creased with the increase of the applied pressure [40]. The pressureand temperature applied for crystallization of the ternary compos-ite samples were according to the P–T phase diagram of PVDF byHattori et al. [40].

2.3. Characterization

Transmission electron microscopy (TEM) detections were per-formed with a Tecnai G2 F20 S-TWIN apparatus (FEI, USA), employ-ing a Leica EMUC6/FC6 microtome for the preparation of ultrathinsections through room-temperature microtomy. Differential scan-ning calorimetry (DSC) measurements were conducted at atmo-spheric pressure by using a TA-Q20 instrument (TA Instruments,USA). The weight of sample was around 5 mg. The melting behav-ior of the crystals was investigated through a heating scan with aheating rate of 10 �C/min at N2 atmosphere. Wide-angle X-ray dif-fraction (WAXD) results were obtained at room temperature with a

PANalytical X’pert PRO diffractometer (PANalytical BV, Almelo, theNetherlands). Attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR) data were obtained using a Nicolet5700 spectrometer (Thermo Electron Scientific Instruments Corp.,USA) in the range of 700–4000 cm�1, with 32 scans conducted ata resolution of 4 cm�1. WAXD and ATR-FTIR data were collectedat the fresh and smooth surfaces of the samples, which were ob-tained through fracture at liquid N2 temperature. After the WAXDand ATR-FTIR characterizations, the sample surfaces were etchedby using a method modified from that developed by Vaughan[44], and then coated with gold for scanning electron microscopy(SEM) observations using a HITACHI S-3400 apparatus (Hitachi,Tokyo, Japan).

3. Results and discussion

TEM was undertaken to reveal the state of dispersion of bothnanofillers in the as fabricated hybrid composites. Fig. 1 shows atypical TEM photograph of the C60/CNT/PVDF composite samplewith 1.0 wt.% C60 and 5.0 wt.% CNT loadings, just before the ap-plied high-pressure treatment. The dark and light areas correspondto the introduced nanofillers and PVDF matrix, respectively. As canbe seen, an overall good dispersion of C60 nano-aggregations wasachieved in the polymer matrix. For CNTs, small loosely entangledbundles, without agglomerates, were observed. Both C60 and CNTswere randomly dispersed by shear forces, leading to a homoge-neous mixture of both reinforcements with the polymer. In ourprevious investigations [32,33], the adopted route herein has beenproven to be effective in the preparation of the well-dispersed bin-ary composite of fullerene C60 or C70 with PVDF. For comparison, aCNT/PVDF (5/95, wt/wt) composite was also prepared by the sameprocess. However, large CNT aggregations, with size around severalhundreds of nanometers, were observed in PVDF matrix [45]. Thissuggested that the dispersion state of CNTs in the C60/CNT/PVDFcomposite was actually tuned by the introduction of C60. It wasthe synergistic effect of the zero-dimensional C60 and one-dimen-sional CNTs that resulted in a well dispersed ternary C60/CNT/PVDF based composite, simply fabricated by a physical and

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mechanical approach. Recently, Dang and co-workers incorporatedanother zero-dimensional filler, nanosized BaTiO3 (NBT) particles,into MWNT/PVDF composites, and also realized the uniform dis-persion of MWNT without sacrificing its inherent properties [46].

The C60/CNT/PVDF composite with 1.0 wt.% C60 and 5.0 wt.%CNT loadings was selected for the high pressure experiments, byvarying temperature, pressure and crystallization time. The crys-tallization conditions, as well as the results of DSC, WAXD andATR-FTIR, are shown in Figs. 2–4. Fig. 2a shows the DSC resultsof the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples crys-tallized at 500 MPa and different maximum quenching tempera-ture. The results revealed that in general the meltingtemperatures of the high-pressure crystallized composite samples

Fig. 2. DSC (a), WAXD (b) and ATR-FTIR (c) results of the C60/CNT/PVDF (1/5/94,wt/wt/wt) composite samples crystallized at 500 MPa and different maximumquenching temperature.

Fig. 3. DSC (a), WAXD (b) and ATR-FTIR (c) results of the C60/CNT/PVDF (1/5/94,wt/wt/wt) composite samples crystallized at 500 MPa, 260 �C for different time.

increased with the increase of crystallization temperature. Twomelting points were detected for the sample crystallized at230 �C maximum quenching temperature. When the maximumquenching temperature was increased to 245 �C, apart from thetwo melting peaks in the low temperature range, a high tempera-ture endotherm (188.47 �C) emerged on the DSC curve. With thecrystallization temperature further increased, all the peaks shiftedto higher temperature, and finally they merged into a strong singlewhole peak at 280 �C maximum quenching temperature. The melt-ing point of the sample crystallized at 280 �C reached 204.67 �C,which are around 30 �C higher than that of the low temperaturepeak of the sample crystallized at 230 �C. The DSC data suggestedthat crystal form transforming took place at high pressure, withthe increase of crystallization temperature [40,41]. Also, the higher

Fig. 4. DSC (a), WAXD (b) and ATR-FTIR (c) results of the C60/CNT/PVDF (1/5/94,wt/wt/wt) composite samples crystallized at different pressure, 260 �C for 40 min.

130 W. Huang et al. / Composites: Part B 62 (2014) 126–136

melting point for the sample crystallized at 280 �C indicated agreater perfection for the crystals of PVDF, which may be assignedto the formation of extended-chain crystals with b or c form[37,38]. In our previous studies on a C60/PVDF (1/99, wt/wt) com-posite [32], the melting point of the sample crystallized at thesame conditions reached 195.30 �C. The relatively low meltingpoint suggested that the as-formed PVDF crystals in the C60/PVDFcomposite were actually with folded-chain substructures. So thegrowth of the PVDF extended-chain crystals in the C60/CNT/PVDFcomposite should be assigned to the simultaneous introduction ofC60 and CNT. Bai and co-workers [47] recently prepared a MWNT/PVDF composite with a remarkable molecular level interaction atinterfaces through a robust and simple melt-mixing process. Pos-sessing a giant dielectric permittivity, the resultant nanocomposite

still retained a low conductivity level and an excellent thermal sta-bility. This was explained by a reinforced Maxwell–Wagner–Sillars(MWS) effect based on the remarkable molecular level interaction.Compared with the previous investigated C60/PVDF based com-posite, such molecular level interaction at interface, between CNTand PVDF, may play a significant role in the unique crystal formtransforming of the C60/CNT/PVDF composite at high pressure.

Commonly, the identification of the crystalline phases of PVDFis carried out by WAXD or FTIR measurements. Referring to theX-ray diffraction data [40,41,48], the reflections of monoclinic aphase of the polymer, at 2h angles 17.7�, 18.5�, 19.9� and 26.5�,are attributed to the (100), (020), (110) and (021) planes,respectively. The apparent single peak of orthorhombic b phase,at 20.4–21.1�, actually comes from the superposition of the(110) and (200) reflections, and the characteristic reflections ofthe monoclinic c phase are assigned to the (020), (200) and(110) planes, overlapped with 020a, 200b and 110b, respectively[49]. As for the IR bands, those at 764, 796, 976 and 1214 cm�1

are characteristic of the a phase, at 840 and 1280 cm�1 of the bphase, and at 840 and 1234 cm�1 of the c phase, respectively[50,51].

The relevant WAXD and ATR-FTIR spectra of the C60/CNT/PVDFcomposite samples, crystallized at 500 MPa and various maximumquenching temperatures, are shown in Fig. 2b and c, respectively.With the increase of crystallization temperature, the intensity ofthe bands characteristic of a and b phase decreased and increased,respectively, both in WAXD and IR spectra. The c form crystals ofPVDF began to appear when the crystallization temperaturereached 245 �C. Their amount increased at 260 �C, and then de-creased at 280 �C. Particularly, no a phase was detected in the sam-ple crystallized at 280 �C, and its crystal forms are totally a mixtureof b and c phase. We also noted that the intensity of the peaks typ-ical of a phase was still strong, whether in WAXD or IR, for the C60/CNT/PVDF composite sample crystallized at 260 �C, if comparedwith its C60/PVDF counterpart [32].

Fig. 3a shows the DSC data of the C60/CNT/PVDF samples crys-tallized at 500 MPa, 260 �C for different time. With the increase ofcrystallization time during 0–10 min, the melting points of all thepeaks increased correspondingly. When crystallization time wasincreased to 20 min, two endotherms were observed on the DSCcurve. The low and high endothermic regions, at 175.60 and200.78 �C, respectively, should associate with the melting of twodistinct populations of PVDF crystals, i.e. folded- and extended-chain crystals [37,38]. Only single endotherms emerged on theDSC curves of the samples crystallized beyond 40 min, and themelting points remained more or less the same with crystallizationtime further prolonged. The relatively high melting temperaturesstill indicated that PVDF crystals with extended-chain lamellae astheir substructures were formed, for these samples crystallizedfor 40–60 min [40,41]. The calorimetric measurement results forthe samples of this group showed that 20 min was enough to crys-tallize a C60/CNT/PVDF composite with a high melting point, at thesame applied pressure and temperature.

Considering the characteristic bands of WAXD and IR shown inFig. 3b and c, the intensity of the reflection bands of a phase de-creased with the increase of crystallization time, and no such phasewas detected for the sample crystallized at 500 MPa, 260 �C formore than 20 min. The band intensity of b and c phase increasedall the time during 0–40 min, and then decreased when the crystal-lization time was increased to 60 min, which may possibly be dueto the high-temperature degradation of the previously formedPVDF crystals in the sample. Considering the thermal analysis re-sults for the sample crystallized for 60 min, there might be a com-petition between crystallization and high-temperaturedegradation, during the polymer crystallization in the ternaryblend at high pressure.

Fig. 5. Secondary electron images of the etched fracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples, crystallized at: (a) and (b) 500 MPa and 260 �Cmaximum quenching temperature; (c) 500 MPa, 260 �C for 60 min; (d) 600 MPa, 260 �C for 40 min. (b) is the magnified view of the portion highlighted by an elliptical framein (a).

W. Huang et al. / Composites: Part B 62 (2014) 126–136 131

Fig. 4 shows the characterization results for another group C60/CNT/PVDF composite samples, crystallized at different pressure,260 �C for 40 min. A single endotherm was observed on the DSCcurve of the sample crystallized at 200 MPa, with shoulders formedon the low and high temperature sides of the main peak, respec-tively. When the pressure was increased to 300 MPa, three meltingpeaks emerged. Although the highest melting point of the samplecrystallized at 300 MPa was more than 20 �C higher than that ofits 200 MPa counterpart, it was still within the range of that forfolded-chain crystals of PVDF [40,41]. Still, only single peaks weredetected for the DSC profiles of the sample crystallized at 400 and500 MPa, respectively. Nevertheless, the relatively high meltingpoints should be assigned to the melting of extended-chain PVDFcrystals. Further increasing pressure resulted in the multiple melt-ing behaviors once again, as well as the decrease of the meltingpoints. The melting points of the sample crystallized at 600 MPasuggested that PVDF crystals with just folded-chain lamellae astheir substructures were formed. The WAXD and IR bands inFig. 4b and c show that no a-phase crystals were formed whenthe applied pressure was above 300 MPa. Also, the bands charac-teristic of PVDF crystals with c form were observed evidently atthe same pressure. The reflection intensities of both b and c phasesincreased with the increase of pressure during 200–500 MPa, andthen decreased at 600 MPa. The results in Fig. 4 suggested thatan appropriate pressure was also necessary to crystallize aC60/CNT/PVDF composite sample with crystals of entire piezoelec-tric extended-chain substructures, on the condition that the

crystallization occurred at an appropriate temperature for anenough long time.

The above DSC, WAXD and ATR-FTIR data were further con-firmed by SEM observations [52–54]. For example, a large amountof c-form crystallites were observed in the C60/CNT/PVDF compos-ite sample crystallized at 500 MPa and 260 �C maximum quench-ing temperature (Fig. 5a and b). The number of the c crystallitesincreased with the increase of crystallization time. When the crys-tallization time was increased to 60 min, non-textured c spheru-lites, with their diameters around several tens of micrometers,were revealed on the etched fracture surface of the composite sam-ple (Fig. 5c). Especially, many nano-structured crystallites corre-sponding to c form were observed in the sample crystallized at600 MPa, 260 �C for 40 min, and no trace of a spherulites was de-tected for the same sample (Fig. 5d).

Although DSC, WAXD and IR results suggested that extended-chain crystals of PVDF, with b or c form, were obtained in certainsamples, direct morphologies should be given with SEM measure-ments. Fig. 6a shows the secondary electron image of the etchedfracture surface of C60/CNT/PVDF (1/5/94, wt/wt/wt) compositesample, crystallized at 500 MPa, 260 �C for 20 min. As signalizedby an elliptical frame, oriented b-form fibrils with extended-chainlamellae as their substructures were formed in the sample, thoughthe thickness of such crystals was still relatively small. With the in-crease of crystallization time, the lamellar thickness of the b formextended-chain crystals increased. Fig. 6b gives out the typical SEMphotograph of the composite sample crystallized for 60 min, with

Fig. 6. Secondary electron images of the etched fracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples, crystallized at 260 �C: (a) 500 MPa for 20 min;(b) 500 MPa for 60 min; (c) 400 MPa for 40 min; (d) 600 MPa for 40 min.

132 W. Huang et al. / Composites: Part B 62 (2014) 126–136

other conditions being the same. The striated appearance, the mostcharacteristic feature for polymer extended-chain crystals, was ex-posed clearly for the as-crystallized b form PVDF crystals. Accord-ing to the previous investigations [25,55], the striations should runparallel to the molecular chains. Moreover, c-form crystals werealso observed in the same sample. Although the parallel striationsof the c-form crystallites were not revealed by the applied fractureprocess, the high melting temperature (201.64 �C) observed by DSCindicated that extended-chain lamellar crystals of PVDF might stillbe the substructures of these crystallites. An appropriate superco-oling may promote the formation of the extended-chain crystalswith b or c form. As shown in Fig. 6c, for the sample crystallizedat 400 MPa, 260 �C for just 40 min, the fracture surface was cov-ered with extended-chain PVDF crystals with large striation thick-ness. However, randomly packed b fibrils were observed whenpressure was increased to 600 MPa (Fig. 6d), and the relativelylow melting point of the sample suggested that they were actuallywith folded-chain substructures.

Recently, we investigated the high-pressure crystallization of aC60/PVDF nano-composite, and obtained crystalline b-form PVDFnanowires, as well as their arrays, with folded-chain lamellae astheir substructures [32]. Fig. 7a and b show the secondary electronimages of an etched fracture surface of the C60/CNT/PVDF (1/5/94,wt/wt/wt) composite sample, crystallized at 500 MPa and 230 �Cmaximum quenching temperature. As can be seen, the fracturesurface was covered with nanowires of PVDF (Fig. 7a). The nano-wires with polygonal shape, formed by a template-free process,were embedded in the polymer matrix (Fig. 7b). Because the nano-wires were observed only after the etching of the amorphous partsof the fracture surface, they totally belonged to a crystalline entity.

It is known that the PVDF nanowires fabricated by a templatemethod commonly were an amorphous and crystalline mixture[21,22]. PVDF nanowires were also observed in the composite sam-ples crystallized at elevated maximum quenching temperatures,and their morphology was somewhat sensitive to the crystalliza-tion conditions. Fig. 7c and d show the secondary electron imagesof the fracture surfaces of the composite samples crystallized at245 and 260 �C, respectively, with the same applied pressure. Somenanowires fractured with the fracture of the samples at liquid N2

temperature, as shown in the highlighted parts in Fig. 7c and d.The fracture of the nanowires revealed that they were also withhollow structures, just as their counterparts in the high-pressurecrystallized C60/PVDF samples. Although PVDF nanowires wereobserved in the C60/CNT/PVDF samples crystallized during 235–260 �C maximum quenching temperature, they did not form theexpected arrays. Furthermore, the relatively low melting pointsof the composite samples indicated that they were still withfolded-chain substructures.

When the maximum quenching temperature was increased to280 �C, as shown in Fig. 8, the etched fracture surface of the com-posite sample was still covered with nanowires of PVDF. Neverthe-less, the polymer matrix in which they embedded showed parallelstriations, the most common feature of PVDF extended-chain crys-tals with b form. Also, the high melting point (204.67 �C) of theC60/CNT/PVDF composite sample suggested that the as-formednanowires were actually with extended-chain lamellae as theirsubstructures, which was quite different from those crystallizedin the a C60/PVDF sample [32]. Thus, by controlling maximumquenching temperature at high pressure, PVDF nanowires withfolded- and extended-chain lamellae as their substructures were

Fig. 7. Secondary electron images of the etched fracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples, crystallized at 500 MPa and differentmaximum quenching temperature: (a) and (b) 230 �C; (c) 245 �C; (d) 260 �C. (b) is the magnified view of the portion highlighted by an elliptical frame in (a).

Fig. 8. Secondary electron images of the etched fracture surface of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite sample, crystallized at 500 MPa and 280 �C maximumquenching temperature. (b) Is the magnified view of the portion highlighted by an elliptical frame in (a).

W. Huang et al. / Composites: Part B 62 (2014) 126–136 133

obtained respectively in the ternary C60/CNT/PVDF basedcomposite.

The collected WAXD and ATR-FTIR data were reexamined toidentify the crystal form of the as-crystallized PVDF nanowires.For the sample crystallized at 500 MPa and 280 �C maximumquenching temperature, which was totally a mixture of b and cphase (Fig. 2b and c), a large numbers of nanowires were formed(Fig. 8). Thus, the existence of a phase can be excluded from suchnanowires. Other conditions being the same, when the sample wascrystallized at 230 �C maximum quenching temperature, no band

characteristic of c phase was detected (Fig. 2b and c). Nevertheless,PVDF nanowire was still observed in the sample (Fig. 7a and b). Sothe formation of c phase in the nanowires can be further excluded.Conclusively, the crystal form for the obtained PVDF nanowires inthe C60/CNT/PVDF composite samples, with folded- or extended-chain as their substructures, should be assigned to crystalline bphase.

Fig. 9 shows the secondary electron images of the etchedfracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) compos-ite samples, crystallized at 500 MPa, 260 �C for 10 and 20 min,

Fig. 9. Secondary electron images of the etched fracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples, crystallized at 500 MPa, 260 �C for differenttime: (a) 10 min; (b) 20 min. (b) and (d) are the magnified views of the portions highlighted by elliptical frames in (a) and (c), respectively.

Fig. 10. Secondary electron image of the etched fracture surface of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite sample, crystallized at 500 MPa, 260 �C for40 min.

134 W. Huang et al. / Composites: Part B 62 (2014) 126–136

respectively. With the increase of crystallization time, approxi-mately, the number of nanowires was more or less the same(Fig. 9a and c). However, the morphology of the polymer matrix,in which the nanowires embedded, showed more difference

(Fig. 9b and d). For the sample crystallized for 10 min, the matrixbackground corresponded to fibrous and randomly-packed b crys-tals with folded-chain substructures (Fig. 9a and b). When the crys-tallization time was increased to 20 min, parallel striations ofextended-chain crystals with b form were observed, as the matrixbackground of the nanowires (Fig. 9c and d). Moreover, the resultsof thermal analysis suggested that the nanowires in the samplescrystallized for 10 and 20 min should be with folded- andextended-chain lamellae as their substructures, respectively. Thec-axis thickness of the background crystals, i.e. b form lamellarcrystals, increased with the increase of crystallization time. Finally,a composite sample, which crystalline structures were totally withextended-chain b- or c-form lamellae of large thickness, furtherself-reinforced with extended-chain b-form nanowires, were suc-cessfully crystallized, with crystallization time increased to40 min (Fig. 10).

In addition to temperature and crystallization time, an appro-priate pressure is crucial to the growth of the hollow nanowiresin the ternary composite. The combination of the three parametersdetermined the size, morphology and substructure of the piezo-electric one-dimensional polymeric materials. Fig. 11a shows thesecondary electron image of the etched fracture surfaces of theC60/CNT/PVDF composite sample, crystallized at 200 MPa, 260 �Cfor 40 min. Very few nanowires were observed on the fracture sur-face. Also, the diameter of the as-formed nanowires was not uni-form. However, with the pressure increased to 300 MPa, thenumber of the nanowires increased, with their diameter generallydecreased (Fig. 11b). Furthermore, size uniformity was achievedfor the nanowires, by the appropriate increase of pressure(Fig. 11c).

Fig. 11. Secondary electron images of the etched fracture surfaces of the C60/CNT/PVDF (1/5/94, wt/wt/wt) composite samples, crystallized at different pressure,260 �C for 40 min: (a) 200 MPa; (b) 300 MPa. (c) is the magnified view of the portionhighlighted by an elliptical frame in (b).

W. Huang et al. / Composites: Part B 62 (2014) 126–136 135

For reference, the samples of pristine PVDF were also crystal-lized at high pressure by just the same procedure [32,45]. Treatedwith the same etching process, no trace of nanowire formation was

observed on the fracture surfaces. SEM only disclosed certain or-dinary crystal structures of PVDF [32,45]. So it is concluded thatthe introduced nanostructured carbonaceous fillers play an impor-tant role in the growth of such polymeric nanowires.

The growth of PVDF nanowires in the C60/CNT/PVDF compos-ites may still be attributed to a C60-induced molecular self-assem-bly at high pressure [32]. Self-assembly driven by physical orchemical interaction has been proven to be effective in construct-ing nanoscale materials with a one-dimensional structure [14–16,56–59]. Also, it is known that the sensitivity of different phasetransitions to pressure is different [25]. Especially, with the intro-duction of C60, pressure may shift the phase equilibrium, and bringa new axis to the self-assembly space. This finally resulted in theformation of polymeric nanowires under high pressure.

However, quite different from their C60/PVDF counterparts[32], no nanowire array was observed in the high-pressure crystal-lized C60/CNT/PVDF composites. This may be due to the simulta-neous introduction of CNTs. The formation of the nanowirearrays in the binary C60/PVDF composites was assigned to anotherself-assembly process, i.e. the self-assembly of the formed one-dimensional nanostructures. In the ternary C60/CNT/PVDF com-posites, the existence of CNTs may hinder the self-assembly ofthe one-dimensional nanowires from the formation of the ex-pected arrays. Nevertheless, the introduction of the CNTs indeedinfluenced the self-assembly behaviors of PVDF, and still playedan important role in the C60-induced molecular self-assembly forthe formation of the polymeric nanowires. It was the synergisticaction of CNTs and C60 that finally resulted in the growth of PVDFnanowires with extended-chain lamellae as their substructures,under specific experimental conditions, of course.

4. Conclusions

In summary, the synergistic action of zero-dimensional C60 andone-dimensional CNTs was evaluated on PVDF. The synergistic ef-fect of C60 and CNTs resulted in an overall good dispersion of bothcarbonaceous fillers in the polymer matrix, during the compositepreparation by an easy physical and mechanical route. Also, pres-sure-controlled growth of piezoelectric low-dimensional crystal-line structures, including hollow nanowires and extended-chainlamellae, was achieved for PVDF, by the simultaneous introductionof C60 and CNT. Particularly, PVDF nanowires with folded- and ex-tended-chain lamellae as their substructures were obtainedrespectively by controlling the high-pressure crystallization condi-tions of the ternary composites. Finally, composite samples, whichcrystalline structures were totally with extended-chain b or c formlamellae, further self-reinforced with extended-chain b-form nano-wires, were crystallized under specific conditions.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (Nos. 50973089 and 51373139), the FundamentalResearch Funds for the Central Universities (Nos. SWJTU11CX056and SWJTU11ZT10), State Key Laboratory of Molecular Engineeringof Polymers (Fudan University) (No. K2012-08), and ChongqingScience and Technology Committee, China (No. CSTC,2010BB4086). The authors extended their gratitude to Dr. YajiangHuang (Sichuan University) for valuable discussions.

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