Dielectric study of the relaxor ferroelectric poly„vinylidene fluoride-trifluoroethylene… copolymer system Vivek Bharti* and Q. M. Zhang Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 Received 12 June 2000; revised manuscript received 22 September 2000; published 18 April 2001 The high-energy electron irradiated polyvinylidene fluoride-trifluoroethylene,PVDF-TrFE, copolymer exhibits many features resembling the relaxor ferroelectric behavior. In polymer systems, there are local dipolar motions at the monomer or unit cell scale, which manifest themselves as various relaxation processes. In this paper we investigate the relationship between the relaxor ferroelectric behavior, especially, Vogel- Fulcher V-F behavior and these local dipolar relaxation processes in irradiated PVDF-TrFE 65/35-mol % copolymer. In order to cover the change in polarization dynamics of the copolymer system, the dielectric behavior of copolymer is measured over a broad frequency 0.01 Hz–10 MHz and temperature -40 to 80 °C range. The results indicate that there is an increased coupling among the local dipolar motions with reduced temperature in the crystalline region. On the other hand, the randomness introduced in the irradiation prevents the formation of a polar phase, on both the macroscale and the microscale, in the polymer. The observed relaxor behavior is a consequence of the competition of these two effects. The results further show that the V-F process of the irradiated copolymer system is different from the glass transition, which occurs in the amor- phous phase of the copolymer. DOI: 10.1103/PhysRevB.63.184103 PACS numbers: 77.84.Jd, 77.22.Gm, 77.80.Bh, 77.90.+k I. INTRODUCTION Relaxor ferroelectric material is a special class of ferro- electric materials that exhibits a diffused broad dielectric peak and that the dielectric maximum ( T m ) is found to shift towards higher temperature with frequency and follows the Vogel-Fulcher V-F law. 1–4 Macroscopically, the dielectric constant peak does not correspond to a ferroelectric transi- tion and under zero electric field, the material remains in a macroscopically nonpolar phase even at a temperature far below the dielectric constant peak temperature. 1,2 On the other hand, a macroscopic polar phase can be induced at a temperature below T m where the polarization hysteresis loop can be observed. Over the past several decades, there has been a great deal of effort devoted to elucidate the funda- mental mechanisms and microscopic and mesoscopic pro- cesses responsible for this peculiar phenomenon. 1–6 Recently, we reported that polyvinylidene fluoride- trifluoroethylene PVDF-TrFE copolymers after high- energy electrons irradiation show many features resembling those observed in relaxor ferroelectric systems. 7–9 It is also interesting to note that all the relaxor ferroelectric phenom- enon reported earlier was observed in inorganic systems. Here the relaxor ferroelectric behavior was observed in a polymer system, the system in which polarization response is directly from local dipolar motions. The phase diagram of PVDF-TrFE copolymer system is shown in Fig. 1, where for composition range with VDF content between 50 and 85 mol %, a ferroelectric- paraelectric F-P transition is observed below their melting temperature. 10–14 The molecular conformation of these co- polymers in the ferroelectric phase consists of the all trans planar zigzag chains while above the Curie temperature, this conformation changes to random sequences of trans and gauche bonds, resulting in a nonpolar phase paraelectric phase. 10–13 Presented in Fig. 2 is the dielectric data as a function of temperature for unirradiated PVDF-TrFE 65/ 35-mol % copolymer measured as the sample is heated from -40 to 120 °C. The two dielectric relaxation processes peaks are observed in this temperature range. The peak at 105 °C, which is related to the -relaxation process, appears to be due to the transition in crystalline region from all-trans ferroelectric phase to distorted trans-gauche paraelectric phase. 11,12 Early studies have shown that the -relaxation process is associated with the local dipolar motions in the crystalline region. 11,12,15 Due to semicrystalline morphology of copolymer, the broad dielectric-loss peak at a temperature near -20 °C is from the glass transition in the amorphous region, i.e., from a glassy state to rubbery state and known as -relaxation process. 12,15 However, for the -relaxation pro- cess the real part of the dielectric constant does not show any significant anomality indicating that the molecular motions associated with this process is only weakly polar in nature. FIG. 1. The phase diagram for VDF/TrFe copolymer system. T c denotes the F-P transition and T m is the melting transition. PHYSICAL REVIEW B, VOLUME 63, 184103 0163-1829/2001/6318/1841036/$20.00 ©2001 The American Physical Society 63 184103-1