聚偏氟乙烯是受歡迎的壓電高分子材料，因為它具有高彈性、生物可相容性以及製造簡便等優勢。這些特性使得聚偏氟乙烯成為優異的能量轉換材料，能夠將機械力與電能交互轉換，應用於型變感測器、機械制動器以及能量擷取器。以上應用依賴聚偏氟乙烯的壓電特性，而適當的電場極化及機械拉伸能夠得到良好的壓電特性。靜電紡絲技術搭配滾筒收集能同時提供機械拉伸以及電場極化以及製備出均勻分布的奈米纖維膜。除此之外，在先前研究中發現奈米填料的添加像是奈米碳管，能夠幫助提升聚偏氟乙烯中壓電性結晶的含量。

Polyvinylidene fluoride (PVDF) is a popular piezoelectric polymer because of its high flexibility, biocompatibility, and simplicity in production. These features make PVDF attractive in energy conversion, between mechanical force and electrical power, applications such as strain sensors, mechanical actuators and energy harvesters. The aforementioned applications rely on the piezoelectric property of PVDF and it is well-known that appropriate mechanical stretching and electrical polarization are essential factors to achieve good piezoelectricity. Electrospinning (ES) processes can provide PVDF fibers mechanical stretching and electrical poling simultaneously to produce ultrafine and well-distributed nanofibers. Furthermore, previous studies discovered that the addition of nanoparticles, such as carbon nanomaterials or metallic nanoparticles, can help improve the β content of the electrospun PVDF nanofibers.
In this study, a rotation drum was used during the electrospinning process to collect aligned PVDF nanofibers. By adjusting the rotating speed and the applied voltage, the electrospun PVDF nanofiber membranes collected by the rotating drum showed a higher β content, better piezoelectric coefficient (d33) and enhanced mechanical property than the randomly oriented nanofiber membranes collected on a fixed copper grid. The optimal β content and the d33 value of the aligned PVDF fiber membranes reached 88% and 27.4 pC/N at a rotating speed of 3000 rpm with an applied electric field of 1200V/cm. Moreover, the aligned PVDF nanofibers added with carbon nanotubes (CNT) further increased the β phase content to 89% and d33 value to 31.3 pC/N.
The piezoelectric response of the as-received PVDF nanofiber membranes were evaluated by three types of mechanical loading: compression, tensile and bending. In the compressive loading range between 200 N to 350 N, the output voltage of the piezoelectric units with aligned electrospun PVDF/CNT increased linearly with the applied loading and presented good stability during the cyclic loading, where its sensitivity was 2.26 mV/N while the piezoelectric units with aligned electrospun PVDF and with randomly oriented electrospun PVDF had sensitivities of 1.93 mV/N and 1.30 mV/N, respectively. In the applied tensile strain range between 4% to 10%, the piezoelectric units with aligned electrospun PVDF/CNT can output linearly with increasing tensile strain, showing sensitivity of 19.22 mV/N while the piezoelectric units with aligned electrospun PVDF and with randomly oriented electrospun PVDF had sensitivities of 18.52 mV/N and 15.32 mV/N, respectively. In the bending angle range between 10° to 180°, all three prepared piezoelectric units generated highest output voltage at bending angle of 100°. When being bent at 100°, the unit with aligned electrospun PVDF/CNT outputted 1.89 V while the piezoelectric units with aligned electrospun PVDF and with randomly oriented electrospun PVDF outputted 1.52 V and 0.75 V, respectively.

[1] J. Kymissis, C. Kendall, J. Paradiso, N. Gershenfeld, Parasitic power harvesting in shoes, ISWC (1998) 132-139.
[2] E. O Torres, G. A Rincón-Mora, Long-lasting, self-sustaining, and energy-harvesting system-in-package (SIP) wireless micro-sensor solution, INCEED (2005) 24-30.
[3] S.P. Beeby, R.N. Torah, M.J. Tudor, P. Glynne-Jones, T.O. Donnell, C.R. Saha, S. Roy, A micro electromagnetic generator for vibration energy harvesting, Journal of Micromechanics and Microengineering 17(7) (2007) 1257-1265.
[4] J.Q. Liu, H.B. Fang, Z.Y. Xu, X.H. Mao, X.C. Shen, D. Chen, H. Liao, B.C. Cai, A MEMS-based piezoelectric power generator array for vibration energy harvesting, Microelectronics Journal 39(5) (2008) 802-806.
[5] C.T. Pan, Z.H. Liu, Y.C. Chen, C.F. Liu, Design and fabrication of flexible piezo-microgenerator by depositing ZnO thin films on PET substrates, Sensors and Actuators A: Physical 159(1) (2010) 96-104.
[6] P. Curie, J. Curie, Development by pressure of polar electricity in hemihedral crystals with inclined face, Bull. Soc. Min. de France 3 (1880) 90-102.
[7] X.Q. Fang, J.X. Liu, V. Gupta, Fundamental formulations and recent achievements in piezoelectric nano-structures: a review, Nanoscale 5(5) (2013) 1716-1726.
[8] A.K. Nandi, L. Mandelkern, The influence of chain structure on the equilibrium melting temperature of poly(vinylidene fluoride), Journal of Polymer Science Part B: Polymer Physics 29(10) (1991) 1287-1297.
[9] L.E. Cross, Ferroelectric materials for electromechanical transducer applications, Materials Chemistry and Physics 43(2) (1996) 108-115.
[10] T. Furukawa, Ferroelectric properties of vinylidene fluoride copolymers, Phase Transitions 18(3-4) (1989) 143-211.
[11] J. Wang, H. Li, J. Liu, Y. Duan, S. Jiang, S. Yan, On the α → β Transition of Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization, Journal of the American Chemical Society 125(6) (2003) 1496-1497.
[12] A. Salimi, A.A. Yousefi, Analysis Method: FTIR studies of β-phase crystal formation in stretched PVDF films, Polymer Testing 22(6) (2003) 699-704.
[13] J. Humphreys, E.L.V. Lewis, I.M. Ward, E.L. Nix, J.C. McGrath, A study of the mechanical anisotropy of high-draw, low-draw, and voided PVDF, Journal of Polymer Science Part B: Polymer Physics 26(1) (1988) 141-158.
[14] N. Levi, R. Czerw, S. Xing, P. Iyer, D.L. Carroll, Properties of Polyvinylidene Difluoride−Carbon Nanotube Blends, Nano Letters 4(7) (2004) 1267-1271.
[15] Y. Ahn, J.Y. Lim, S.M. Hong, J. Lee, J. Ha, H.J. Choi, Y. Seo, Enhanced Piezoelectric Properties of Electrospun Poly(vinylidene fluoride)/Multiwalled Carbon Nanotube Composites Due to High β-Phase Formation in Poly(vinylidene fluoride), The Journal of Physical Chemistry C 117(22) (2013) 11791-11799.
[16] S. Huang, W.A. Yee, W.C. Tjiu, Y. Liu, M. Kotaki, Y.C.F. Boey, J. Ma, T. Liu, X. Lu, Electrospinning of Polyvinylidene Difluoride with Carbon Nanotubes: Synergistic Effects of Extensional Force and Interfacial Interaction on Crystalline Structures, Langmuir 24(23) (2008) 13621-13626.
[17] G. Mago, D.M. Kalyon, F.T. Fisher, Membranes of Polyvinylidene Fluoride and PVDF Nanocomposites with Carbon Nanotubes via Immersion Precipitation, Journal of Nanomaterials (2008) 1-8.
[18] X. Huang, P. Jiang, C. Kim, F. Liu, Y. Yin, Influence of aspect ratio of carbon nanotubes on crystalline phases and dielectric properties of poly(vinylidene fluoride), European Polymer Journal 45(2) (2009) 377-386.
[19] R.K. Layek, S. Samanta, D.P. Chatterjee, A.K. Nandi, Physical and mechanical properties of poly(methyl methacrylate) -functionalized graphene/poly(vinylidine fluoride) nanocomposites: Piezoelectric β polymorph formation, Polymer 51(24) (2010) 5846-5856.
[20] J. Wang, J. Wu, W. Xu, Q. Zhang, Q. Fu, Preparation of poly(vinylidene fluoride) films with excellent electric property, improved dielectric property and dominant polar crystalline forms by adding a quaternary phosphorus salt functionalized graphene, Composites Science and Technology 91 (2014) 1-7.
[21] J. Kim, K.J. Loh, J.P. Lynch, Piezoelectric polymeric thin films tuned by carbon nanotube fillers, SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring (2008) 10.
[22] P.W. Gibson, H.L. Schreuder-Gibson, D. Rivin, Electrospun fiber mats: Transport properties, AIChE Journal 45(1) (1999) 190-195.
[23] Y.M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Experimental characterization of electrospinning: the electrically forced jet and instabilities, Polymer 42(25) (2001) 09955-09967.
[24] R. Thiyagarajan, O. Sahu, Preparation and Characterization of Polymer Nano Fibres Produced from Electrospinning, Journal of Optoelectronics Engineering 2(2) (2014) 24-28.
[25] H.R. Darrell, C. Iksoo, Nanometre diameter fibres of polymer, produced by electrospinning, Nanotechnology 7(3) (1996) 216.
[26] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, Journal of Applied Physics 87(9) (2000) 4531-4547.
[27] A.L. Yarin, S. Koombhongse, D.H. Reneker, Taylor cone and jetting from liquid droplets in electrospinning of nanofibers, Journal of Applied Physics 90(9) (2001) 4836-4846.
[28] H. Fong, I. Chun, D.H. Reneker, Beaded nanofibers formed during electrospinning, Polymer 40(16) (1999) 4585-4592.
[29] A. Baji, Y.-W. Mai, S.-C. Wong, M. Abtahi, P. Chen, Electrospinning of polymer nanofibers: Effects on oriented morphology, structures and tensile properties, Composites Science and Technology 70(5) (2010) 703-718.
[30] C.M. Wu, M.H. Chou, Sound absorption of electrospun polyvinylidene fluoride/graphene membranes, European Polymer Journal 82 (2016) 35-45.
[31] L.Y. Wang, S.Y. Ma, D.Z. Wu, Electrospinning of aligned PVDF nanofibers with piezoelectricity and its application in pressure sensors, 2016.
[32] Y.R. Wang, J.M. Zheng, G.Y. Ren, P.H. Zhang, C. Xu, A flexible piezoelectric force sensor based on PVDF fabrics, Smart Materials and Structures 20(4) (2011) 045009.
[33] M.S. Sorayani Bafqi, R. Bagherzadeh, M. Latifi, Fabrication of composite PVDF-ZnO nanofiber mats by electrospinning for energy scavenging application with enhanced efficiency, Journal of Polymer Research 22(7) (2015) 130.
[34] Z.H. Liu, C.T. Pan, L.W. Lin, J.C. Huang, Z.Y. Ou, Direct-write PVDF nonwoven fiber fabric energy harvesters via the hollow cylindrical near-field electrospinning process, Smart Materials and Structures 23(2) (2014) 025003.
[35] L. Yu, S. Wang, Y. Li, D. Chen, C. Liang, T. Lei, D. Sun, Y. Zhao, L. Wang, Piezoelectric performance of aligned PVDF nanofibers fabricated by electrospinning and mechanical spinning, 2013 13th IEEE International Conference on Nanotechnology (IEEE-NANO 2013), 2013, pp. 962-966.
[36] J.R. Gregorio, M. Cestari, Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride), Journal of Polymer Science Part B: Polymer Physics 32(5) (1994) 859-870.
[37] T. Lei, L. Yu, G. Zheng, L. Wang, D. Wu, D. Sun, Electrospinning-induced preferred dipole orientation in PVDF fibers, Journal of Materials Science 50(12) (2015) 4342-4347.
[38] D. Mandal, S. Yoon, K.J. Kim, Origin of Piezoelectricity in an Electrospun Poly(vinylidene fluoride-trifluoroethylene) Nanofiber Web-Based Nanogenerator and Nano-Pressure Sensor, Macromolecular Rapid Communications 32(11) (2011) 831-837.
[39] 吳昌謀, 郭政霖, 許世緯, 楊明達, 胡宛婷, 熔噴彈性不織布之力量感測器之研究, 2017年功能性材料研討會 (2017).
[40] C.R. Bowen, H.A. Kim, P.M. Weaver, S. Dunn, Piezoelectric and ferroelectric materials and structures for energy harvesting applications, Energy & Environmental Science 7(1) (2014) 25-44.