PVF2 structure

A specific set of experiments which must be mentioned, being directly associated with the main topic of this paper, is the work of Bergman, et. al. (22) dealing with the second-order nonlinear optical properties of polyvinylidene fluoride (PVF2). Nonvanishing the second-order nonlinear electric dipole susceptibility, is expected in PVF2 since it exhibits other properties requiring noncentrosymmetric microscopic structure. These properties appear... 

Fig. 2. Projection of four crystal structures of PVF2 viewed along the molecular axes. Dipole moments are shown by arrows. Large circles represent fluorine, smaller circles represent carbon. Hydrogen atoms are not shown. Figure and caption from Ref. [55])...

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Figure 11.9 (a) Poly(vinyl fluoride), PVF, [CH2— CHF (i) dipoles present in a tetrahedral unit of PVF and (ii) isotactic structure of a polymer chain of PVF. (b) Poly(vinylidene fluoride), PVF2, [CH2—CF2] (i) dipoles present in a tetrahedral unit of PVF2 and (ii) isotactic structure of a polymer chain of PVF2... 

In addition to the presence of elementary dipoles, it is important for the polymer to crystallise or partly crystallise into noncentrosymmetric structures. The polymer chains can usually pack together in several different ways. For example, poly(vinyli-dene fluoride), PVF2, can crystallise in four forms. The arrangement of the chains in one nonpolar and one polar form is drawn schematically in Figure 11.11. Namrally, the degree of crystallinity of the... 

Figure 11.11 The schematic crystal structure of two forms of poly(vinylidene fluoride), PVF2, viewed down the polymer chains, shown as double triangles the electric dipoles in the chains are drawn as arrows, (a) The dipoles cancel in a centrosymmetric stmcture and the material is a nonpiezoelectric, (h) The dipoles add together in a non-centrosymmetric structure and the material is piezoelectric...

The very high durability of PVF2 comes from the polymer structure ... 

ACRONYMS. TRADE NAMES FVDF, PVF2, Kynar, Solef, Neoflon, Foraflon, KF, Soltex CLASS Vinylidene polymers STRUCTURE -(CHjCFj) -... 

It is now known that a wide array of polymers can be etched using potassium permanganate [273] although some care must be taken to limit the effect of artifacts. The list includes linear and branched PE, PP, PS, poly(4-methylpentene-l), poly (butene-1), PVF2, PEEK, PET and various copolymers such as EPDM terpolymers [273]. More recent work has shown that even liquid crystalline polymers can be etched by a variation of this method. Controls and complementary microscopy are essential to ensure that the experimentalist is not led astray imaging artifacts, hills and valleys or nussing fine structure, lost in the wash baths. 

For both types of polymerization the structure of the resulting polymers was studied by x-ray diffraction analysis, F-NMR, and infrared analysis [534]. In the case of the trialkylborane systems the chain defects were given to be 3.2%, which is higher than the imperfections found for Ziegler-Natta polymers (2.7%). However, remarkable differences in the F-NMR of the two polymers were observed. That of the polymer initiated with boronalkyl/oxygen was similar to the spectrum of conventional PVF2. However, the... 

The various crystalline structures of PVF2 obtained by chemical and radiation-initiated polymerization are described by Gal perin et al. [536,537]. PVF2 samples of different molar masses could be prepared (0.3 to 10 x 10 g/mol, determined from viscosimetry in DMF). Radiation polymerization in the gaseous phase resulted in polymers with the highest molar masses. It was also shown that the conformation of PVF2 chains was independent of the method of initiation but was influenced by the polarity of the medium in which polymerization took place. Radiation polymerization in polar solvents promoted formation of the p phase, while nonpolar solvents (or gaseous polymerization) yielded the a phase [521,537]. 

In the addition to homo-PVF2, a large number of copolymers have also been synthesized which allow to optimize the mechanical properties of fluoropolymers. Most common are copolymers with vinyl fluoride, trifluoroethylene, tetrafluoroethylene, hexafiuoropropy-lene, hexafluoroisobutylene, chlorotrifluoroethylene, and pentafiuoro-propene [521,535, 559-562]. Copolymerization with nonfluorinated monomers is possible [563] in principle but has not yet found commercial use. Fluorocarbon monomers that can help to retain or enhance the desirable thermal, chemical, and mechanical properties of the vinylidene structure are more interesting comonomers. Copolymerization with hexafluoropropylene, pentafluoropropylene, and chlorotrifluoroethylene results in elastomeric copolymers [564]. The polymerization conditions are similar to those of homopoly(vinylidene fluoride) [564]. The copolymers have been well characterized by x-ray analysis [535], DSC measurements [565], and NMR spectroscopy [565,566]. 

The formation of some structure units in head-to-head or tail-to-tail position is an unavoidable phenomenon of the radical polymerization of vinyl monomers. In the ease of poly(vinylidene fluoride) these chain defects are of particular importance because they affect the crystallization and thus the properties of the polymer [521]. In general, the percentage of monomer inversion in PVF2 appears to be a function of the temperature of polymerization The number of H H and T-T units increase from 3.5% at 20 °C to 6.0% at 140 °C. A polymer with a very low content of chain defects has been prepared by Butler et al. [579]. The authors carried out the polymerization in bulk at 0 °C using trichloroacetyl peroxide as the initiator and obtained high-molar-mass PVF2 with only 2.85% of reversed monomer units and low content of branches. 

Copolymerization with other vinylic monomers (i.e., styrene, vinyl acetate) allows even further modification. The ease of structural modification to yield desired blend properties (miscibility) is well-documented in the experimental literature. The common acrylate polymer is PMMA and has been noted to be miscible with various other polymers noted in this chapter (PVF2, styrene copolymers, PVC, PVPh, PEO) and thus will not be discussed in this section. 

Thermal conductivity (K) and thermal diffusivity (/c) measurements versus temperature or blend composition can be employed to reveal structural information. However, it is not as sensitive as other methods and relatively few studies have been reported on blends. Thermal diffusitivies of polymers are generally in the range of 10 cm /s. A relevant review of thermal conductivity of polymer blends has been reported by Tsutsumi [197], where PVME/PS, PVC/PCL, PMMA/PC and PVF2/PMMA blend data were reviewed. The thermal conductivity, K, and thermal diffusivity, K, have analogies with permeability, P, and diffusion coefficient, D, respectively. This analogy is the result of the similarities between Fourier s law and Fick s law ... 

One manifestation of the rearrangement of structural features consequent upon plasma polymerization is the rather interesting surface properties which such films possess. Figure 12 for example shows the critical surface tensions measured for plasma polymers produced from the series of fluorobenzenes. For the polymer from perfluorobenzene of composition C F the critical surface tension of 20 dynes may be compared with that for PVF2 of the same C F stoichiometry of 27 dynes. The critical surface tension increases as the fluorine content decreases. 

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