Thin films ferroelectric polymers

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Remarkably some polymers are ferroelectric. Polyvinylene fluoride, as a thin film (approximately 25 /mi thick) is used in Shockwave experiments to measure stress. The pressure range over which this polymer operates is an order of magnitude larger than that of quartz or lithium niobate. 

Ferroelectric substances such as LiNb03 or BaTi03 were found to show anomalous photovoltaic effects (APV) of the order of 103-5 V83). A thin film of a ferroelectric polymer such as PVDF also was found to show APV effects84). Voc of 2.5 x 104 V and short circuit current (Isc) of the order of nA/cm2 were reported. Although the output is very small up to now1, it could be noticed as a specific photoeffect of a thin polymer film. 

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

Polymerization of reactive monomeric liquid crystals is one method for stabilizing the liquid-crystalline thin films. Another approach is to form chemical gels of liquid crystal molecules with low molecular weight by construction of a polymer network. This method has been investigated for the stabilization of ferroelectric liquid crystal displays. Guymon et al. reported that a polymer network produced by photochemical cross-linking accumu-... 

Walba, D. M., Keller, P., Parmar, D. S., Clark, N. A., and Wand, M. D., Design and synthesis of new ferroelectric liquid crystals. 9. An approach to creation of organic polymer thin films with controlled, stable polar orientation of functional groups, J. Am. Chem. Soc., Ill, 8273-8274 (1989). 

Newnham RE, Sundar V, Yimnirun R, Su J, Zhang QM (1997) Electrostriction nonlinear electromechanical coupling in solid dielectrics. J Phys Chem 101 10141-10150 Ploss B, Ploss B, Shin FG, Chan HLW, Choy CL (2000) Pyroelectric or piezoelectric compensated ferroelectric composites. Appl Phys Lett 76 2776-2778 Poddar S, Ducharme S (2013) Measurement of the flexoelectric response in ferroelectric and relaxor polymer thin films. Appl Phys Lett 103 202901... 

K. Thshiro, K. Yamamoto. M. Kobayashi. and O. PhaovAml. A prcliraiiiary study on the structure and ferroelectric phase transition of vacuum-evaporated thin films of viayhdeoe fluoride-tTifluataethyleiK copolymers. Rep. Progr. Polymer Pkys. Jpn. J0 123 (1967). 

The electrical and mechanical properties trf piezoelectric polymers make them a possible alternative to ferroelectric ceramics such as lead zirconate titanate. For several reasons, they are attractive for transducer design. The mechanical flexibility and conformability of thin-film PVDF means that it can be configured into a wide range of transducer products. The low acoustic impedance of PVDF is companrf>le to body tissues, which makes it useful for acoustic imaging applications. Short impulse response and high axial resolution in acoustic imaging systems arc possible with PVDF-featured devices because of the robustness and broadband characteristics of the polymer. 

Historically, nearly all the early investigations into the ferroelectric properties of PVDF were carried out on thin ( 25 jam) films. This was necessary for very good reasons. Most importantly, the material must be oriented to develop an incipient ferroelectric structure. This is most conveniently done by using conventional thin film making equipment, which stretches a preformed polymer strip in either one or two in-plane directions. In addition to this requirement, very high fields (typically 100 kV mm ) must be applied to the oriented polymer during the poling... 

Sato H, Fujikake H, Lino Y, Kawakita M, Kikuchi H (2002) Flexible grayscale ferroelectric liquid crystal device containing polymer walls and networks. Jpn J Appl Phys 41 5302-5306 Sato H, Fujikake H, Kikuchi H, Kurita T (2003) Rollable polymer stabilized ferroelectric liquid crystal device using thin plastic substrates. Opt Rev 10(5) 352-356 Schrader DM, Jean YC (1988) Positron and positronium chemistry. Elsevier, Amsterdam Shinkawa K, Takahashi H, Fume H (2008) Ferroelectric liquid crystal cell with phase separated composite organic film. Ferroelectrics 364 107-112 Simha R, Somcynsky T (1969) On the statistical thermodynamics of spherical and chain molecule fluids. Macromolecules 2 342-350... 

These polymers have a typical ferroelectric behaviour (Figure 13). These copolymere are soluble and c stallise directly in a non centrosymetric B phase. So one can make thin layers of film (in the wder of some pm) by spin coating. For x = 0.75, the Curie temperature is Tc = 125°C... 

Usually, artificial muscle based on electrostrictive, piezoelectric, electrostatic, or ferroelectric materials have been manufactured as a film of the dry polymer, both sides coated with a thin metallic film required to apply the electric field. Electrokinetic artificial muscles [5,6] are constituted by films of polymeric gel (polymer, solvent, and salt) and two electrodes, located as close as possible to the material or coating both on sides, which are required to apply the electric field that drives the electroosmotic process. Any of the actuators described in this paragraph has a triple layer structure metal-electroactive polymer-metal (Figure 16.2). 

The structural unit of the copolymer -(-CH2-CF2)n-(-CF2-CHF-)m- contains n and m corresponding monomer links. The ferroelectric properties are attributed to transverse dipole moments, formed by positive hydrogen and negative fluorine atoms. Below the temperature of the ferroelectric phase transition (about 80-100°C), the main chain of the polymer is in all-trans form and the dipole moments are parallel, at least, within ferroelectric domains separated fi-om each other by domain walls. The ferroelectric switching is due to an electric field induced, collective flip-flop of the dipoles around the backbone of the polymer. Several recent studies were devoted to a local ferroelectric switching of the domains in cast P(VDF-TrFE) films [6-8]. To this effect, a powerful technique, called Electrostatic Force Microscopy (EFM) [9] was used which was developed for studies of domains in thin ferroelectric films, see papers [10, 11] and references therein. 

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