Comparative Investigation of the Structure and Properties of Ferroelectric Poly(vinylidene fluoride) and Poly(vinylidene fluoride–trifluoroethylene) Thin Films Crystallized on Substrates Shuting Chen, 1,2 Kui Yao, 2 Francis Eng Hock Tay, 1 Lydia Li Shan Chew 2 1 Department of Mechanical Engineering, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 119260 2 Institute of Materials Research and Engineering, Agency for Science, Technology, and Research, 3 Research Link, Singapore 117602 Received 11 August 2009; accepted 15 November 2009 DOI 10.1002/app.31794 Published online 22 February 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Poly(vinylidene fluoride) (PVDF) and copoly- mers of vinylidene fluoride and trifluoroethylene [P(VDF/ TrFE)s] were deposited on silicon substrates, and their struc- ture and properties were comparatively investigated. Com- pared to P(VDF/TrFE), which is the polymer material currently most dominant for ferroelectric thin-film devices applications, our b-phase PVDF homopolymer thin film dem- onstrated several advantages, such as higher dielectric break- down strength, improved polarization fatigue endurance, and larger ferroelectric polarization at elevated temperatures with improved thermal stability. The reasons for the observed different characteristics between the PVDF and P(VDF/TrFE) thin films were analyzed on the basis of their different structures and morphologies. The results indicate that the low-cost b-phase PVDF homopolymer thin films have great potential as an alternative to P(VDF/TrFE) for ferroelectric and piezoelectric thin-film-device applications. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3331–3337, 2010 Key words: ferroelectricity; thin films INTRODUCTION Strong piezoelectric and ferroelectric effects were first observed in poly(vinylidene fluoride) (PVDF) in the 1970s. In the 1980s, it was found that a synthe- sized copolymer of vinylidene fluoride and trifluoro- ethylene [P(VDF/TrFE)] possessed the piezoelectric and ferroelectric properties superior even to that of PVDF homopolymer. Since then, extensive attention has been paid to both PVDF and its copolymers for various device applications on the basis of their fer- roelectric and piezoelectric effects. 1,2 Structural stud- ies have shown that PVDF-based polymers have complicated semicrystalline polymorphs (a-, b-, c-, and d-crystalline phases). For PVDF homopolymer, the nonferroelectric a phase is most stable thermody- namically. Thus, posttreatments, mainly including mechanical stretching during manufacturing, are required to achieve the most polar b-phase PVDF to obtain the desired properties. 3 On the other hand, P(VDF/TrFE) copolymer with an appropriate vinyli- dene fluoride content can easily form the b-phase structure, as suggested by theoretical calculation in which the all-trans conformation (the b phase) is most thermodynamically favored for P(VDF/TrFE) with the appropriate vinylidene fluoride content. 4 Thus, PVDF and P(VDF/TrFE) bulk films without substrates possess different characteristics for their device applications: P(VDF/TrFE) can possess excel- lent ferroelectric and piezoelectric properties without mechanical stretching; PVDF typically has to go through the mechanical stretching process to induce the b phase, but the material cost is significantly lower in comparison to P(VDF/TrFE). However, as for applications of PVDF-based ferro- electric thin films on substrates, the conventional me- chanical stretching process is not applicable for PVDF homopolymer thin films because of the exis- tence of substrates. Thus, P(VDF/TrFE) thin films have predominantly been used in various applica- tions involving substrates, such as tactile sensors 5 and nonvolatile memories. 6 The significantly lower cost and fewer defects in the molecular structure make PVDF homopolymer thin films attractive for replacing P(VDF/TrFE) in many applications. Recently, we succeeded in achieving dense b-phase PVDF homopolymer thin films on silicon substrates without the mechanical stretching process by intro- ducing hydrogen bonds during the thin-film process- ing, 7,8 and furthermore, these films demonstrated useful ferroelectric and piezoelectric properties. Correspondence to: K. Yao (k-yao@imre.a-star.edu.sg). Journal of Applied Polymer Science, Vol. 116, 3331–3337 (2010) V C 2010 Wiley Periodicals, Inc.