Electric properties, PVDF

Some electrical properties are shown in Table 3. Values of other parameters have been pubflshed (146). Polymorphism of the PVDF chains and the orientation of the two distinct dipole groups, —CF2— and —CH2—, rather than trapped space charges (147) contribute to the exceptional dielectric properties and the extraordinarily large piezoelectric and pyroelectric activity of the polymer (146,148,149). 

In this book those ferroelectric solids that respond to shock compression in a purely piezoelectric mode such as lithium niobate and PVDF are considered piezoelectrics. As was the case for piezoelectrics, the pioneering work in this area was carried out by Neilson [57A01]. Unlike piezoelectrics, our knowledge of the response of ferroelectric solids to shock compression is in sharp contrast to that of piezoelectric solids. The electrical properties of several piezoelectric crystals are known in quantitative detail within the elastic range and semiquantitatively in the high stress range. The electrical responses of ferroelectrics are poorly characterized under shock compression and it is difficult to determine properties as such. It is not certain that the relative contributions of dominant physical phenomena have been correctly identified, and detailed, quantitative materials descriptions are not available. 

Fluoroplastic FPs have superior heat and chemical resistance, excellent electrical properties, but only moderate strength. Variations include PTFE, FEP, PFA, CTFE, ECTFE, ETFE, and PVDF. Used for bearings, valves, pumps handling concentrated corrosive chemicals, skillet linings, and as a film over textile webs for inflatables such as pneumatic sheds. Excellent human-tissue compatibility allows its use for medical implants. 

Examples of fluoroplastics include polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), ethylene—chlorotrifluoroethylene (ECTFE), ethylene—tetrafluoroethylene (ETFE), poly(vinylidene fluoride) (PVDF), etc (see Fluorine compounds, organic). These polymers have outstanding electrical properties, such as low power loss and dielectric constant, coupled with very good flame resistance and low smoke emission during fire. Therefore, in spite of their relatively high price, they are used extensively in telecommunication wires, especially for production of plenum cables. Plenum areas provide a convenient, economical way to run electrical wires and cables and to interconnect them throughout nonresidential buildings (14). Development of special flame-retardant low smoke compounds, some based on PVC, have provided lower cost competition to the fluoroplastics for indoors application such as plenum cable, Riser Cables, etc. 

The unique dielectric properties and polymorphism of PVDF are the source of its high piezoelectric and pyroelectric activity.75 The relationship between ferroelectric behavior, which includes piezoelectric and pyroelectric phenomena and other electrical properties of the polymorphs of polyvinylidene fluoride, is discussed in Reference 76. 

Attempts have been made to correlate the outstanding electric properties (piezoelectricity, pyroelectricity) of PVDF with parameters obtained from wideline NMR experiments. Samples processed by different electric methods have been included in NMR experiments. Wideline NMR studies of irreversible effects induced by relatively high static electric fields have been reported by several authors. The results of wideline NMR ( H) have been used [53] in a combined investigation with a piezoelectric resonance method to find any effects of electric poling (0.78 MV/cm at 120°C) on structure and/or orientation. However, no indications of structural or orientational changes have been found. 

Table 3.77 gives a number of electrical properties of PVDF measured at room temperature. 

Table 3.77. Typical Electrical Properties of PVDF Film at 23°CP i...

Some electrical properties of ECTFE are given in Table 3.80. The dielectric strength (ASTM D149) is higher than the breakdown strength of PVDF. 

It was discovered that a number of PVDF film properties, including electrical, could be altered by the extrusion and orientation conditions.The relationship between the extrusion and orientation process and the electrical properties of the film are important because of the application of PVDF films in miniaturized capacitors which are used in apparatus such as defibrillators. To compare the properties, extruded film, while in molten state, was brought in contact with chilled rollers at 80°C and cooled rapidly. The oriented film was stretched at 150°C in the longitudinal direction by a stretch ratio of 3.5, that is, its original length was increased by 3.5 times [Eq. (6.6)]. The dielectric constant and dissipation factor were measured as seen in Table 6.8. Orientation increased the dielectric constant and reduced the dielectric loss... 

Table 6.8. Effect of Orientation on Electrical Properties of PVDF Films ...

Polyvinylidene fluoride (PVDF) is a homopolymer of 1,1-difluoroethene with alternating CH2 and CF2 groups along the polymer chain. These groups impart a unique polarity that influences its solubility and electrical properties. The polymer has the characteristic stability of fluoropolymers when exposed to aggressive thermal, chemical, and ultraviolet conditions. 

We have shown how Electrostatic Force Microscopy can be an extremely useful tool to investigate and to modify the electric properties of sample surfaces on a microscopic and even nanoscopic scale and we have presented a phenomenological model to help relating the experimental data to the material properties. Ferroelectric domains can locally be reoriented and their time evolution can be followed, as was shown for PZT. We have also demonstrated how the ferroelectric polymer PVDF-TrFe could be locally modified which can be used to locally vary the optical properties of a LC cell. Finally, we have demonstrated that rubbing polymer substrates can indeed result in electrostatic charging, in particular for PMMA and PI, while no charging is found for PVA. 

The alternating —CH2—and —CF2 groups along the polymer chain provide a unique polarity that influences the polymer s solubility and electric properties. At elevated temperatures PVDF can be dissolved in polar solvents such as organic esters and amines. This selective solubility offers a way to prepare corrosion resistant coatings for chemical process equipment and long-life architectural finishes on building panels. 

Poly(vinylidene fluoride)(PVDF) is one of the most polar polymers among synthetic polymers and shows the most unique phenomena in many fields of pol3nner science. Especially, the electrical properties of PVDF have been the subject of intensive investigations in recent years since it was reported that it could exhibit a large dielectric constant and internal polarization for its B-form films, a very large piezoelectric and pyroelectric effects for polarized monoaxially and biaxially stretched films.[1,2,3]... 

Electrical Properties. The piezoelectricity of the technologically important polymer of vinylidene fluoride (PVDF) has been the subject of modeling for several decades (see Piezoelectric Polymers). An early example of the use of molecular mechanics to aid in the calculation of the mechanical and electrical properties of this polymer is found in the work of Tadokoro and co-workers (390,391). Subsequent investigation of PVDF by Karasawa and Goddard (83) focussed on the prediction of alternative crystal structures with use of the shell model to captiu-e polarization effects. The latter phenomenon was further explored by Carbeck and co-workers (84), who used the shell model to show that the induced moment because of neighboring dipoles in the crystal increases the dipole per repeat imit by about 50% over its value in the isolated molecule. 

One of its earliest applications, in fact, was the primary insulator on computer hookups used in back panels and installed by high speed, automatic wiring machines While satisfactory electrical properties of the polymer were important, automatic wiring equipment also required a wire insulation material of exceptional physical toughness. KYNAR pvDF easily fit this requirement. 

As mentioned before, the alternating CH and CF groups influence PVDF s solubility and electrical properties. It is essentially insoluble in all nonpolar solvents but dissolves in strong polar solvents such as alkyl amides and is readily solvated at elevated temperatures by certain common solvents such as ketones and esters. This selective solubility was the ke> to the development of the now well-known KYNAR 500 finishes and corrosion coatings. The former are solvent based dispersions that arc factory applied and baked on aluminum or galvanized steel, which form wall panels of industrial and monumental buildings. The corrosion coatings protect process equipment from corrosion. 

The newest market for KYNAR resins is perhaps the most exciting and most challenging. This is the KYNAR Piezo Film. By definition, a piezo electric material is one that can change polarization in response to mechanical stress. The CH - CF repeat unit was found to exhibit the strongest piezo electric and pyro electric activity of all known polymers. This property was first reported in 1969 based on experiments using KYNAR PVDF. Since that time the piezo and pyro electric properties of PVDF have been the subject of many publications. 

Electric properties are an essential asset of fluoropolymers, in particular for PVDF. With the recent technological boom of touchpad, this polymer found additional applications in electronic haptic technology. 

Atomistic simulation has also been carried out to calculate electric properties of PVDF Two force fields, MSXX and MSXXS, were specifically built up to address this issue [49]. Dielectric constants and dielectric loss were thus computed for both the crystalline and amorphous phases of PVDF. In agreement with experimental data, the ensuing computational results showed that the amorphous phase exhibits a higher dielectric constant than the crystalline phase. This behavior arises fi om rapid changes in the torsion angles, leading to rapid modulation of the dipole moment perpendicular to the chain axis. It is stimulated by the presence of sohton-like defects that diffuse along the chain, as it was revealed by MD simulation [49]. 

Motion is generated hy the piezo-electric effect in certain crystalline materials (for instance Quartz), ceramics such as PZT - Pb(ZrTi)03, and polymers (Polyvinyli-dene fluoride - PVDF). In addition, thin films with piezo-electric properties such as Aluminum Nitride (AIN)... 

---