Ferroelectric polymers
Encyclopedia
Ferroelectric Polymers
are a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field
.
Ferroelectric polymers, such as polyvinylidene fluoride
(PVDF), are used in acoustic transducers and electromechanical actuators because of their inherent piezoelectric response, and as heat sensors because of their inherent pyroelectric response.
A ferroelectric polymer must contain permanent electrical polarization that can be reversed repeatedly, by an opposing electric field. In the polymer, dipoles can be randomly oriented, but application of an electric field will align the dipoles, leading to ferroelectric behavior. In order for this effect to happen, the material must be below its Curie Temperature. Above the Curie Temperature, the polymer exhibits paraelectric behavior, which does not allow for ferroelectric behavior because the electric fields do not align.
A consequence of ferroelectric behavior leads to piezoelectric behavior, where the polymer will generate an electric field when stress is applied, or change shape upon application of an electric field. This is viewed as shrinking, or changes in conformation of the polymer in an electric field; or by stretching and compressing the polymer, measure generated electric fields. Pyroelectric behavior stems from the change in temperature causing electric behavior of the material. While only ferroelectric behavior is required for a ferroelectric polymer, current ferroelectric polymers exhibit pyroelectric and piezoelectric behavior.
In order to have an electric polarization that can be reversed, ferroelectric polymers are often crystalline, much like other ferroelectric materials. Ferroelectric properties are derived from electrets, which are defined as a dielectric body that polarizes when an electric field and heat is applied. Ferroelectric polymers differ in that the entire body undergoes polarization, and the requirement of heat is not necessary. Although they differ from electrets, they are referred to as electrets often. Ferroelectric polymers fall into a category of ferroelectric materials known as a 'order-disorder' material. This material undergoes a change from randomly oriented dipoles which are paraelectric, to ordered dipoles which become ferroelectric.
After the discovery of PVDF, many other polymers have been sought after that contain ferroelectric, piezoelectric, and pyroelectric properties. Initially different blends and copolymers of PVDF were discovered, such as a polyvinylidene fluoride
with poly(methyl methacrylate).
Other structures were discovered to possess ferroelectric properties, such as polytrifluoroethylene and odd-numbered nylon
.
was first discovered in 1921. This phenomenon began to play a much larger role in electronic applications during the 1950s after the increased use of BaTiO3. This ferroelectric material is part of the corner-sharing oxygen octahedral structure, but ferroelectrics can also be grouped into three other categories. These categories include organic polymers, ceramic polymer composites, and compounds containing hydrogen-bonded radicals. It wasn’t until 1969 that Kawai first observed the piezoelectric effect in a polymer polyvinylidene fluoride
(PVDF). Two years later, the ferroelectric properties of the same polymer were reported. Throughout the 1970s and 1980s, these polymers were applied to data storage and retrieval. Subsequently, there has been tremendous growth during the past decade in exploring the materials science, physics, and technology of poly(vinylidenefluoride) and other fluorinated polymers. Copolymer PVDF with trifluoroethylene and odd-numbered nylons were additional polymers that were discovered to be ferroelectric. This propelled a number of developing applications on piezoelectricity
and pyroelectricity
.
of vinylidene fluoride (VF2), however, the polymerization is not completely regiospecific. The asymmetric structure of VF2 leads to the orientation isomers during the polymerization. The configuration of the monomer in the chain can be either “head to head”, “head to tail” or “tail to tail”.
To get more control on the regiospecific polymer synthesis, copolymerization was proposed. One of these methods is introducing the precursor polymer made from copolymerization of VF2 with either 1-chloro-2,2-difluoroethylene (CVF2) or 1-bromo-2,2-difluoroethylene( BVF2). The chlorinated or brominated monomers are attacked at their CF2 carbon by growing –CH2CF2∙ radical. After reductive dechlorination or debromination with tri-n-butyltin hydride they become a reversed VF2 unit in the final polymer.Therefore, a regioisomer of PVDF is formed.
(PVDF) and poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)], are very attractive for many applications because they exhibit good piezoelectric and pyroelectric responses and low acoustic impedance
, which matches water and human skin. More importantly, they can be tailored to meet various requirements. A common approach for enhancing the dielectric constant
is to disperse a high-dielectric-constant ceramic powder into the polymers. Popular ceramic powders are lead based complexes such as PbTiO3 and Pb(Zr,Ti)O3. This can be disadvantageous because lead can be potentially harmful and at high particulate loading, the polymers lose their flexibility and a low quality composite is obtained. Current advances use a blending procedure to make composites that are based on the simple combination of PVDF and cheap metal powders. Specifically, Ni powders were used to make up the composites. The dielectric constants were enhanced from values there were less than 10 to approximately 400. This large enhancement is explained by the percolation theory
.
These ferroelectric materials have also been used as sensors. More specifically, these types of polymers have been used for high pressure and shock compression sensors. It has been discovered that ferroelectric polymers exhibit piezoluminescence upon the application of stress. Piezoluminescence has been looked for in materials that are piezoelectric.
It is useful to distinguish among the several regimes in a typical stress-strain curve for a solid material. The three regimes of the stress-strain curve include elastic, plastic, and fracture. Light emitted in the elastic regime is known piezoluminescence. A general stress-strain curve is shown in Figure 7.
These types of polymers have played a role in biomedical and robotic applications and liquid crystalline polymers. In 1974, R.B. Meyer predicted ferroelectricity in chiral smectic liquid crystals by pure symmetry conditions. Shortly after, Clark and Lagerwall had done work on the fast electrooptic effect in a surface-stabilized ferroelectric liquid crystal (SSFLC) structure. This opened up promising possibility of technical applications of ferroelectric liquid crystals in high-information display devices. Through applied research, it was shown that SSFLC structure has faster switching times and bistability behavior in comparison with commonly used nematic liquid crystal displays. In the same time period, the first side-chain liquid crystalline polymers (SCLCP) were synthesized. These comb-like polymers has mesogenic side chains that are covalently bonded (via flexible spacer units) to the polymer backbone. The most important feature of the SCLCP’s is their glassy state. In other words, these polymers have a “frozen” ordered state along one axis when cooled below their glass transition temperature. This is advantageous for research in the area of nonlinear optical and optical data storage devices. The disadvantage is that these SCLCP’s suffered from their slow switching times due to their high rotational viscosity.
loop, which is related to "memory". One application is integrating ferroelectric polymer Langmuir–Blodgett (LB) films with semiconductor
technology to produce nonvolatile ferroelectric random-access memory(NV-FRAM or NV-FeRAM) and data-storage devices.Recent research with LB films and more conventional solvent formed films shows that the VDF copolymers(consisting of 70% vinylidene fluoride (VDF) and 30% trifluoroethylene (TrFE)) are promising materials for nonvolatile memory applications. The device is built in the form of themetal–ferroelectric–insulator–semiconductor (MFIS) capacitance memory. The results demonstrated that LB films can provide devices with low-voltage operation.
Thin Film Electronics
successfully demonstrated roll-to-roll printed
non-volatile memories
based on ferroelectric polymers in 2009.
and bottom surfaces. Contacts to the two electrodes complete the design.
, a mechanical or acoustic
force applied to one of the surfaces causes a compression of the material. Via the direct piezoelectric effect, a voltage is generated between the electrodes.
Soft transducers in the form of ferroelectric polymer foams have been proved to of great potential.
are a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
.
Ferroelectric polymers, such as polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(PVDF), are used in acoustic transducers and electromechanical actuators because of their inherent piezoelectric response, and as heat sensors because of their inherent pyroelectric response.
Background
First reported in 1971, Ferroelectric Polymers are polymer chains that must exhibit ferroelectric behavior, and possibly, but not required, piezoelectric behavior and pyroelectric behavior.A ferroelectric polymer must contain permanent electrical polarization that can be reversed repeatedly, by an opposing electric field. In the polymer, dipoles can be randomly oriented, but application of an electric field will align the dipoles, leading to ferroelectric behavior. In order for this effect to happen, the material must be below its Curie Temperature. Above the Curie Temperature, the polymer exhibits paraelectric behavior, which does not allow for ferroelectric behavior because the electric fields do not align.
A consequence of ferroelectric behavior leads to piezoelectric behavior, where the polymer will generate an electric field when stress is applied, or change shape upon application of an electric field. This is viewed as shrinking, or changes in conformation of the polymer in an electric field; or by stretching and compressing the polymer, measure generated electric fields. Pyroelectric behavior stems from the change in temperature causing electric behavior of the material. While only ferroelectric behavior is required for a ferroelectric polymer, current ferroelectric polymers exhibit pyroelectric and piezoelectric behavior.
In order to have an electric polarization that can be reversed, ferroelectric polymers are often crystalline, much like other ferroelectric materials. Ferroelectric properties are derived from electrets, which are defined as a dielectric body that polarizes when an electric field and heat is applied. Ferroelectric polymers differ in that the entire body undergoes polarization, and the requirement of heat is not necessary. Although they differ from electrets, they are referred to as electrets often. Ferroelectric polymers fall into a category of ferroelectric materials known as a 'order-disorder' material. This material undergoes a change from randomly oriented dipoles which are paraelectric, to ordered dipoles which become ferroelectric.
After the discovery of PVDF, many other polymers have been sought after that contain ferroelectric, piezoelectric, and pyroelectric properties. Initially different blends and copolymers of PVDF were discovered, such as a polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
with poly(methyl methacrylate).
Other structures were discovered to possess ferroelectric properties, such as polytrifluoroethylene and odd-numbered nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
.
History
The concept of ferroelectricityFerroelectricity
Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...
was first discovered in 1921. This phenomenon began to play a much larger role in electronic applications during the 1950s after the increased use of BaTiO3. This ferroelectric material is part of the corner-sharing oxygen octahedral structure, but ferroelectrics can also be grouped into three other categories. These categories include organic polymers, ceramic polymer composites, and compounds containing hydrogen-bonded radicals. It wasn’t until 1969 that Kawai first observed the piezoelectric effect in a polymer polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(PVDF). Two years later, the ferroelectric properties of the same polymer were reported. Throughout the 1970s and 1980s, these polymers were applied to data storage and retrieval. Subsequently, there has been tremendous growth during the past decade in exploring the materials science, physics, and technology of poly(vinylidenefluoride) and other fluorinated polymers. Copolymer PVDF with trifluoroethylene and odd-numbered nylons were additional polymers that were discovered to be ferroelectric. This propelled a number of developing applications on piezoelectricity
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
and pyroelectricity
Pyroelectricity
Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...
.
Synthesis of Polyvinylidene Fluoride(PVDF)
The easiest way of synthesizing PVDF is the radical polymerizationRadical polymerization
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules...
of vinylidene fluoride (VF2), however, the polymerization is not completely regiospecific. The asymmetric structure of VF2 leads to the orientation isomers during the polymerization. The configuration of the monomer in the chain can be either “head to head”, “head to tail” or “tail to tail”.
To get more control on the regiospecific polymer synthesis, copolymerization was proposed. One of these methods is introducing the precursor polymer made from copolymerization of VF2 with either 1-chloro-2,2-difluoroethylene (CVF2) or 1-bromo-2,2-difluoroethylene( BVF2). The chlorinated or brominated monomers are attacked at their CF2 carbon by growing –CH2CF2∙ radical. After reductive dechlorination or debromination with tri-n-butyltin hydride they become a reversed VF2 unit in the final polymer.Therefore, a regioisomer of PVDF is formed.
Study of the structure of PVDF
To minimize the potential energy of the chains arising from internal steric and electrostatic interactions, the rotation about single bonds happens in the chain of PVDF. There are two most favorable torsional bond arrangements: trans ( t ) and gauche± ( g± ). In the case of “ t”, the substituents are at 180o to each other .In the case of “g±” , the substituents are at ± 60o to each other. PVDF molecules contain two hydrogen and two fluorine atoms per repeat unit, so they have a choice of multiple conformations. However, rotational barriers are relatively high, the chains can be stabilized into favorable conformations other than that of lowest energy. The three known conformations of PVDF are all-trans, tg+tg-, and tttg+tttg- . The first two conformations are the most common ones and are sketched out in the figure on right. In the tg+tg- conformation, the inclination of dipoles to the chain axis leads to the polar components of both perpendicular(4.0 x 10−30 C-m per repeat) and parallel to the chain(3.4 x 10−30 C-m per repeat). In the all trans structure, the alignment of all its dipoles are in the same direction normal to the chain axis. In this way, it can be expected that the all trans is the most highly polar conformation in PVDF (7.0 x 10−30 C-m per repeat). These polar conformations are the crucial factors that lead to the ferroelectric properties.Current Research
Ferroelectric polymers and other materials have been incorporated into many applications, but there is still cutting edge research that is currently being done. For example, there is research being conducted on novel ferroelectric polymer composites with high dielectric constants. Ferroelectric polymers, such as polyvinylidene fluoridePolyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(PVDF) and poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)], are very attractive for many applications because they exhibit good piezoelectric and pyroelectric responses and low acoustic impedance
Acoustic impedance
The acoustic impedance at a particular frequency indicates how much sound pressure is generated by a given air vibration at that frequency. The acoustic impedance Z is frequency dependent and is very useful, for example, for describing the behaviour of musical wind instruments...
, which matches water and human skin. More importantly, they can be tailored to meet various requirements. A common approach for enhancing the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
is to disperse a high-dielectric-constant ceramic powder into the polymers. Popular ceramic powders are lead based complexes such as PbTiO3 and Pb(Zr,Ti)O3. This can be disadvantageous because lead can be potentially harmful and at high particulate loading, the polymers lose their flexibility and a low quality composite is obtained. Current advances use a blending procedure to make composites that are based on the simple combination of PVDF and cheap metal powders. Specifically, Ni powders were used to make up the composites. The dielectric constants were enhanced from values there were less than 10 to approximately 400. This large enhancement is explained by the percolation theory
Percolation theory
In mathematics, percolation theory describes the behavior of connected clusters in a random graph. The applications of percolation theory to materials science and other domains are discussed in the article percolation.-Introduction:...
.
These ferroelectric materials have also been used as sensors. More specifically, these types of polymers have been used for high pressure and shock compression sensors. It has been discovered that ferroelectric polymers exhibit piezoluminescence upon the application of stress. Piezoluminescence has been looked for in materials that are piezoelectric.
It is useful to distinguish among the several regimes in a typical stress-strain curve for a solid material. The three regimes of the stress-strain curve include elastic, plastic, and fracture. Light emitted in the elastic regime is known piezoluminescence. A general stress-strain curve is shown in Figure 7.
These types of polymers have played a role in biomedical and robotic applications and liquid crystalline polymers. In 1974, R.B. Meyer predicted ferroelectricity in chiral smectic liquid crystals by pure symmetry conditions. Shortly after, Clark and Lagerwall had done work on the fast electrooptic effect in a surface-stabilized ferroelectric liquid crystal (SSFLC) structure. This opened up promising possibility of technical applications of ferroelectric liquid crystals in high-information display devices. Through applied research, it was shown that SSFLC structure has faster switching times and bistability behavior in comparison with commonly used nematic liquid crystal displays. In the same time period, the first side-chain liquid crystalline polymers (SCLCP) were synthesized. These comb-like polymers has mesogenic side chains that are covalently bonded (via flexible spacer units) to the polymer backbone. The most important feature of the SCLCP’s is their glassy state. In other words, these polymers have a “frozen” ordered state along one axis when cooled below their glass transition temperature. This is advantageous for research in the area of nonlinear optical and optical data storage devices. The disadvantage is that these SCLCP’s suffered from their slow switching times due to their high rotational viscosity.
Nonvatile Memory
The ferroelectric property exhibits polarization–electric-field-hysteresisHysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
loop, which is related to "memory". One application is integrating ferroelectric polymer Langmuir–Blodgett (LB) films with semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
technology to produce nonvolatile ferroelectric random-access memory(NV-FRAM or NV-FeRAM) and data-storage devices.Recent research with LB films and more conventional solvent formed films shows that the VDF copolymers(consisting of 70% vinylidene fluoride (VDF) and 30% trifluoroethylene (TrFE)) are promising materials for nonvolatile memory applications. The device is built in the form of themetal–ferroelectric–insulator–semiconductor (MFIS) capacitance memory. The results demonstrated that LB films can provide devices with low-voltage operation.
Thin Film Electronics
Thin Film Electronics ASA
Thin Film Electronics ASA is a Norwegian printed electronics company, headquartered in Oslo with its main R&D offices in Linköping, Sweden....
successfully demonstrated roll-to-roll printed
Printed electronics
Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and...
non-volatile memories
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...
based on ferroelectric polymers in 2009.
Transducers
The ferroelectric effect always relates the various force to electric properties, which can be applied in transducers. The flexibility and low cost of polymers facilitates the application of ferroelectic polymers in transducers. The device configuration is simple, it usually consists of a piece of ferroelectric film with an electrode on the topand bottom surfaces. Contacts to the two electrodes complete the design.
Sensors
When the device functions as a sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
, a mechanical or acoustic
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
force applied to one of the surfaces causes a compression of the material. Via the direct piezoelectric effect, a voltage is generated between the electrodes.
Actuators
In actuators, a voltage applied between the electrodes causes a strain on the film through the inverse piezoelectric effect.Soft transducers in the form of ferroelectric polymer foams have been proved to of great potential.
See also
- Polyvinylidene fluoridePolyvinylidene fluoridePolyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
- FerroelectricityFerroelectricityFerroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...
- PiezoelectricityPiezoelectricityPiezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
- PyroelectricityPyroelectricityPyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...
External links
- Strategic Polymer Sciences, Inc. - Electroactive Polymer (EAP) - Films - High Energy Density Capacitors - High Strain Actuators
- Piezotech - EAP Piezoelectric Polymers & Films - PVDF and P(VDF-TrFE) shock gauges and sensors - Relaxor & Electrostrictive polymers P(VDF-TrFE-CFE)