Effect of crystal structure on polarization reversal and energy storage of ferroelectric poly(vinylidene fluoride-co-chlorotrifluoroethylene) thin films Ruixuan Han a, b , Jiezhu Jin a , Paisan Khanchaitit a , Jingkang Wang b , Qing Wang a, * a Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA b School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China article info Article history: Received 15 November 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Available online 8 February 2012 Keywords: Ferroelectric polymers Polarization Dielectric properties abstract A series of ferroelectric poly(vinylidene fluoride-co-chlorotrifluoroethylene) films with different crys- tallite sizes have been obtained by varying the film processing conditions. The impact of the crystallite size on the dipole switching and the electric energy density has been systematically studied by the electric displacementeelectric field hysteresis loop measurements. The films with smaller crystallite sizes display larger polarizability, as evidenced by higher maximum polarization and lower dipole switching field in the charging process. Small crystals also facilitate fast dipole depolarization during the discharging process. Consequently, superior released energy densities have been achieved in the films containing small sizes of the ferroelectric crystallite domains. On the other hand, large crystallite sizes are beneficial for the dielectric breakdown strength and the Weibull distribution of the breakdown field of the films. This study sheds new fundamental light on the optimization of the crystal structures of the ferroelectric polymers for high electric energy storage applications. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The development of new materials with high energy density is viewed as a critical enabling step for the realization of compact, low-cost and high-performance energy storage devices for appli- cations in portable electronics, electric power systems and next- generation vehicles [1,2]. Extensive studies have been carried out on polymeric materials for film capacitors, owing to their great processability, high breakdown strength, and self-healing mecha- nism [3,4]. Ferroelectric polymers represented by poly(vinylidene fluoride) (PVDF) and its copolymers are emerging as the promising benchmark candidates for this class of materials [5e7]. The pres- ence of strong polarization from CeF bonds and the spontaneous orientation of dipoles in the crystalline phases give rise to high dielectric responses in PVDF. By modifying PVDF with chlorotri- fluoroethylene (CTFE) or hexafluoropropylene (HFP), the resulting P(VDF-CTFE) and P(VDF-HFP) copolymers exhibit capacitive energy densities of w17 J/cm 3 with millisecond discharge rates [8], which are substantially higher than the current state-of-the-art polymer capacitors, biaxially stretched polypropylene (BOPP) with an energy density of w3 J/cm 3 [9]. The molecular origin of these properties, however, has not been well understood. It is recognized that the electrostatic energy storage capacity of the ferroelectric polymers critically depends on their polarization switching ability under the applied electric field, which in turn is closely related to the crystalline structures and morphologies [10e15]. PVDF with different chain conformations can be packed into various crystal lattices to form a range of crys- talline phases such as the non-polar a phase and polar b, d and g phases depending on the processing conditions [5,6]. The existence of bulky CTFE and HFP in the copolymers stabilizes the trans-gauche (TGTG 0 ) chain conformation and the non-polar a phase, which facilitates the phase transformation under an applied field and avoids early polarization saturation [16]. The P(VDF-CTFE) and P(VDF-HFP) copolymers therefore display narrower polarization hysteresis loops and broadened relaxation process, and conse- quently, higher discharged energy densities than those of normal ferroelectrics [8]. More recently, the effect of the crystal orientation on the stored/discharged energy densities of P(VDF-HFP) has been examined [15]. It is observed that the transverse crystals with c- axes oriented perpendicular to the applied field are desired for larger polarizability and higher electric energy storage as compared to those in the crystal with the c-axes aligned parallel to the field. It is the focus of this work to better understand the dipole re- orientation and switching mechanisms of ferroelectric polymers in response to the applied electric field and elucidate their relation- ship with the electric energy storage properties. A variety of * Corresponding author. E-mail address: wang@matse.psu.edu (Q. Wang). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.02.004 Polymer 53 (2012) 1277e1281