Investigation of ferroelectric behavior in electroactive polymer systems.

摘要

Ferroelectric behavior has been investigated in newly synthesized electroactive cyanopolymer systems. These systems are chemical analogs to the well known ferroelectric polymer poly(vinylidene fluoride), PVDF, and its copolymers with trifluoroethylene, P(VDF-TrFE). Various chemical groups have been used to replace the electronegative fluorine and electropositive hydrogen atoms found in PVDF. This substitution maintains the polar nature of the all-trans conformation while increasing the amphiphilic nature of the polymers, enabling the fabrication of ultrathin films using the Langmuir-Blodgett technique. The studied cyanopolymers include poly(methyl vinylidene cyanide), PMVC, and several of its copolymers.;Evidence for ferroelectric behavior has been observed in the 50:50 copolymer of poly(methyl vinylidene cyanide) and vinyl acetate, P(MVDCN-VAC), where, at elevated temperatures, polarization hysteresis, measured using a specialized Chynoweth pyroelectric technique, has been observed. Polarization hysteresis loops, in the case of P(MVDCN-VAC), were measured at elevated temperatures below the melting temperature, Tm, of 170°C. Furthermore, the reversible polarization states have been shown to be quite long lived (+1000 hours), with decay rates consistent with that observed in P(VDF-TrFE) systems. Structural characteristics of the cyanopolymer systems have been probed using Transmission electron microscopy and theta-2theta x-ray diffraction. Single-crystal electron diffraction measurements indicate an orthorhombic unit cell with dimensions a=11.2A. b=6.6A, and c=5.3 A, an expanded analog to the orthorhombic unit cell associated to the ferroelectric beta phase in PVDF.;Fundamental structural and dielectric studies have been performed in order to study the nature of the ferroelectric-paraelectric phase transition in ultrathin (15-100 nm) Langmuir-Blodgett copolymer films of the ferroelectric copolymer P(VDF-TrFE) 70:30.;In-situ theta-2theta XRD was used to monitor changes in inter-layer spacing perpendicular to the film surface, corresponding to the (110) direction, as a function of temperature and applied electric-field. Capacitance versus temperature measurements were used to investigate the behavior of the dielectric constant of the copolymer at increasing applied electric fields. Application of a large electric-field, up to 265 MV/m, raises the ferroelectric phase transition temperature, which results in the conversion of the nonpolar trans-gauche paraelectric phase to the polar all-trans ferroelectric phase in a thermodynamically reversible manner, confirming the first-order nature of the ferroelectric transition in P(VDF-TrFE) 70:30.