PVDF films

In this section, examples of films made from polyvinylidene fluoride (PVDF) are discussed. Although most of the pol5winylidene fluoride film is in the form of coating on metal substrates, stand-alone PVDF films and sheets are produced by extrusion and film blowing.1 ] ] Blends of PVDF and a number of other polymers such as polymethylmethacrylate (PMMA) are miscible. Films made from these blends have excellent piezoelectric properties. 

Extrusion of PVDF films can be done in a standard single-screw extruder. Gradual transition screws 

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 

In another example, unoriented PVDF films were stretched at 150°C in the longitudinal direction at a stretch ratio of 3.5 to 1. A sample of the longitudinally oriented film was stretched in the transverse direction at 60°C at a stretch ratio of 3.4 to 1, thus producing a biaxially oriented film. Dielectric constant and loss factor values were measured for these films (Table 6.10). 

Another approach to reduce the loss factor was to heat treat (set) the oriented film under tension after it had been elongated or shrunk by less than 5%. For example, an oriented PVDF film was heat set at a temperature of 140°C-160°C and elongated 5%. Results of loss factor measurements can be seen in Fig. 6.26. Loss factor, in general, decreased with an increase in the time of treatment to less than 1%. 

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PVDF film is readily commercially available in thicknesses up to 110 pm. If film of this thickness is bonded to a rigid substrate, the peak response is at a frequency of about 3.7 MHz which corresponds to the thickness of the layer being a quarter wavelength. This frequency is rather high for most Lamb wave testing since Fig 1 shows that even on a I mm thick... 

Fig 3. Predicted electro-acoustic response for PVDF film bonded to aluminium and backed with different thicknesses of copper. (Thicknesses shown in pm.)... 

PVDF film by using a buffer solution with denaturation/reduction effects.5 This technique can denature, reduce, and digest the proteins in the tissue section efficiently and remove the salt from the tissue. Thus, the ionization efficiency for biological molecules is increased. 

The above-mentioned method is effective in identifying the molecules of detected ions. However, because PVDF film is not permeable to light, it is difficult to observe tissue sections. To resolve this problem, we developed a method to fix tissue sections on transparent film, and then performed MS on those sections.6 We used a conductive film because we expected the ionization efficiency would increase when the electric charge accumulation on the sample was reduced. The film used for this purpose was a polyethylene terephthalate (PET) film with a thickness of 75-125 pm, having a 5 15-nm-thick layer of evaporated oxidation indium tin (ITO) upon it (ITO film). This film is used in touch-panel displays because of its high transparency and superior conductivity. We used it to perform MS/MS for tissue sections and succeeded in identifying multiple proteins from mass spectra.6 Therefore, the further development of this method will enable the application of the mass-microscopic method to observe tissue by optical microscope and to perform tandem mass spectrometry (MSn) at the observation part, simultaneously, enabling the identification of molecules included the part. 

Apparently, the formation of the microporous structure within the PVdF—HFP copolymer was of critical importance to the success of Bellcore technology, and the ion conductivity was proportional to the uptake of the liquid electrolyte. To achieve the desired porosity of PVdF film, Bellcore researchers prepared the initial polymer blend of PVdF with a plasticizer dibutylphthalate (DBP), which was then extracted by low boiling solvents after film formation. Thus, a pore-memory would be left by the voids that were previously occupied by DBP. However, due to the incomplete dissolution of such high-melting DBP during the extraction process, the pore-memory could never be restored at 100% efficiency. Beside the total volume of pores thus created by the plasticizer. 

A close correlation between the polarities of piezoelectricity and pyroelectricity was found for PVC and poly (vinylidene fluoride) (PVDF) films (Nakamura and Wada, 1971). However, it must be emphasized that the polarity of piezoelectricity is determined not only by the polarity of the charge distribution but also by that of heterogeneous strain. The origin of heterogeneous strain in the elongation of film may derive from heterogeneity in the structure of the film. 

The piezoelectricity of PVDF film increases in order of magnitude when the film is roll-drawn. Crystals of a-form (or type II) in the original... 

In the drawn film in which the c-axis of the crystal is oriented along the draw-axis (z-axis) and a, b-axes are randomly oriented around the z-axis, the polarization P, along the x-axis of the film is not expected with elongation in the plane yz. The observed piezoelectricity of roll-drawn PVDF film, therefore, may be ascribed to the embedded charge as will be evidenced in the following. 

The question arises whether the piezoelectricity in polarized PVDF film of j8-form originates from the polarization charge coupled with heterogeneous strain or the strain-dependence of spontaneous polarization. The latter explanation seems reasonable at present for the following reasons. 

Oshiki and Fukada (1971) measured e and k of roll-drawn and polarized PVDF film over a wide temperature range (Fig. 28). Peterlin and Elwell (1967) observed e for a roll-drawn PVDF film. The ratio e(e — 1)/k is almost constant over temperatures from — 50° C to 75° C with the value of —1.7 x 103 cgsesu. If we assume the film to be completely crystalline and completely oriented, the value of PJN of the film would be obtained from Eq. (53) as PJN = 1.7 x 103 cgsesu. The positive sign of... 

Pr is reasonable because the x-axis of the film is taken as the direction of the poling field. On the other hand, the spontaneous polarization of the P-form crystal of PVDF is estimated from the crystal structure as Ps= 1.3 x 10 5 coul/cm2 = 4x 104 cgsesu. Considering the incompleteness in crystallinity ami orientation, the estimated value of PJN is reasonable compared with PF However, it would still be premature to conclude that the piezoelectricity of drawn and polarized PVDF film is due to the strain dependence of spontaneous polarization. 

The bending piezoelectricity in drawn and polarized polymer films was studied in detail by Kawai (1) (1970). Kitayama and Nakayama (1971) reported a very high piezoelectricity in composite films of polymer (PVDF, nylon 11, PVC) and powdered ceramics (barium titanate, PZT) after poling. In the case of PVDF and nylon, the piezoelectric constant increase by a factor of 102 when the ceramics make up 50% of the volume. The pyroelectricity and optical nonlinearity of polarized PVDF films have been studied by Bergmann, McFee, and Crane (1971). 

Fukada and Sakurai (1971) measured the temperature dependence of df3 and df2 for a drawn and polarized PVDF film (Fig. 30). The relaxational behavior is somewhat masked by the rapid monotonic increase with increasing temperature on account of the decrease in the elastic modulus. Details of the curves, however, seem to indicate that d has a maximum at relaxational temperatures and, in accordance with this, d" has a maximum and a succeeding minimum at these temperatures. 

PVDF film PVDF membranes PVDF monofilament... 

With the flexibility of the blown film extrusion process, one-step production of wide multilayer films is possible. Incorporating a built-in adhesive layer, PVDF films can be laminated directly onto various thermo and thermoset plastics or primed metal substrates. PVDF films can protect the esthetics of UV sensitive substrates while also providing a thermal, chemical, and abrasion-resistant barrier. Recently, white PVDF films have been commercialized that are of full opacity in the UV range of 290 00 Most recently,... 

Figure 3. Isolated nanometric size pores In a polyvlnyllde-nefluorlde (PVDF) membrane obtained by Kr Ion Irradiation at GANIL and subsequent oxidative etching of a 15 pm-thick PVDF film (courtesy ofLSI/CEA-CNRS).

Another example of the use of polarized radiation in imaging studies is the analysis of poly(vinylidene fluoride)(PVDF) films, which have been uniaxially elongated at different temperatures. Depending on the thermal, mechanical and electrical pretreatment, PVDF can exist in different modifications [59]. The crystal structure of the cmmpled 11(a) modification can be converted into the aU-tra s 1(P) form by tensile stress below 140°C (see Figure 9.27a). Figure 9.27b shows the stress-strain diagrams of PVDF films in the 11(a) form which have been elongated to 400 % strain at 100 and 150°C. The observed decrease in stress upon elevation of the... 

Figure 9.27 (a) Conformational changes occurring in the 11(a) l(P) transformation of PVDF (b) Stress-strain diagrams of PVDF films elongated to 400% strain at 100 and 150°C. Reproduced with permission from Ref [58] 2008,... 

There is a plastic polymer called polyvinylidene difluoride (PVDF) that can be used to sense a baby s breath and thus be used to prevent sudden infant death syndrome (SIDS). The secret is that this polymer can be specially processed so that it becomes piezoelectric (produces an electrical current when it is physically deformed) and pyroelectric (develops an electrical potential when its temperature changes). When a PVDF film is placed beside a sleeping baby, it will set off an alarm if the baby stops breathing. The structure of this polymer is shown here ... 

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

Excess heating can destroy the piezoelectric effect in PVDF films. 

PVDF films up to 100-125 pm are transparent to translucent when the molten polymer is quenched in water to minimize crystallization. The transmittance spectra of PVDF in the ultraviolet, visible, and... 

Figure 3.110 Ultraviolet, visible, and near-infrared light transmittance spectra of PVDF film (96.5 pm thickness). ...

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