PVDF Properties

For general electrochemical measurements, there are many powerful constraints on the choice of binder. In addition to supplying a strong adhesive bond to the 

To a first approximation, PVDF can be thought of as a bulk combination of two components a high-density crystalline component and a low-density amorphous component. The observed density (p) of a given specimen is then given by the following formula  

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Unique PVDF properties have been observed after polymerization in supercritical carbon dioxide [38,51-53,55] at certain conditions. The polymer synthesized in supercritical carbon dioxide exhibits a bimodal molecular weight distribution (MWD), as illustrated in Fig. 4 [52]. At molar VF2 feed concentrations less than about 1.9 M, the polymer has a unimodal distribution, at the conditions of Fig. 4. As the monomer concentration is increased, the distribution becomes broader and bimodal. Changes in temperature, pressure, and residence time also have an effect on the MWD [51,52]. In Fig. 4, ris the average residence time (the reactor volume divided by the inlet volumetric flow rate). 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

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). 

Because of its excekent combination of properties, processibkity, and relatively low price compared to other fluoropolymers, PVDF has become the largest volume fluoropolymer after PTFE consumption in the United States has grown from zero in 1960 to about 6200 metric tons in 1991 (186). About 49% of the consumed volume is PVDF modified by copolymerization with 5—12-wt % HFP to enhance flexibkity. In 1992, Hst price for homopolymer powders was 15.32/kg, and for pekets 15.42/kg the reported market price was 14.09—14.22/kg (187). In the United States, almost ak PVDF is suppHed by Ausimont USA, Inc., Elf Atochem North America, Inc., and Solvay Polymers, Inc. Ausimont and Elf Atochem are producers Solvay is an importer of the resin. Smak amounts of resin are imported from Germany by Huls America, Inc, and from Japan by Kureha Chemical Industry Co., Ltd. PVDE producers and their trademarks are Hsted in Table 4. 

Property ASTM test PTFE PCTFE PVF - PVDF TFE-HFP TFE- ethylene PFA... 

In order to anticipate problems and to interpret observations under the extreme conditions of shock compression, it is necessary to consider structural and electronic characteristics of PVDF. Although the phenomenological piezoelectric properties of PVDF are similar to those of the piezoelectric crystals, the structure of the materials is far more complex due to its ferroelectric nature and a heterogeneous mixture of crystalline and amorphous phases which are strongly dependent on mechanical and electrical history. 

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. 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

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. 

PVDF has properties similar to PTFE but is easier to fabricate. It has good resistance to inorganic acids and alkalis, and organic solvents. It is limited to a maximum operating temperature of 140°C. 

PVDF is a difluoro derivative, leading to the following formula — (— CH2 — CF2-) - and a fluorine content of 59%. PVDF is semicrystalline with three possible crystal forms, one of which has interesting pyro- and piezoelectric properties. Copolymers are marketed for applications needing flexibility. 

PVDF is appreciated for easier processability good chemical resistance good resistance to heat and low temperature good electrical insulating properties even in hot and wet environments ... 

Figure 4.94 displays two examples of retention of stress at yield and elongation at break (relative to the properties at 20°C) versus temperature for a PVDF. Note the very rapid increase of elongation at break when the temperature rises above 50°C. 

PVDFs are good insulators even in wet environments, with high resistivities, dielectric constants and loss factors. Films with suitable crystallinity have pyro- and piezoelectric properties. 

To achieve the goal of required performance, durability, and cost of plate materials, one approach is improvement of the control of the composition and microstructure of materials, particularly the composite, in the material designing and manufacturing process. For example, in the direction of development of thermoplastics-based composite plate, CEA (Le Ripault Center) and Atofina (Total Group) have jointly worked on an irmovative "microcomposite" material [33]. The small powders of the graphite platelet filler and the PVDF matrix were mixed homogeneously by the dispersion method. The filler and matrix had a certain ratio at the microlevel in the powder according to the optimized properties requirements. The microcomposite powders were thermocompressed into the composite plate. 

The materials used in nonwoven fabrics include a single polyolefin, or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), poly(tetrafluoroethylene) (PTFE), polyvinylidine fluoride (PVdF), and poly(vinyl chloride) (PVC). Nonwoven fabrics have not, however, been able to compete with microporous films in lithium-ion cells. This is most probably because of the inadequate pore structure and difficulty in making thin (<25 /rm) nonwoven fabrics with acceptable physical properties. 

The PVdF—HFP separators used in PLION cells were around 3 mil thick, and had poor mechanical properties. It has been reported that the major source of rate limitation in PLION cells was the separator thickness. The rate capability of these cells can be significantly improved by decreasing the separator thickness to that typically used in liquid electrolyte system. Moreover, in the absence of shutdown function. the separator does not contribute to cell safety in any way. Park et al. reported that the HFP content in separators did not have any significant impact on cell performance. The Bellcore process has proven to be an elegant laboratory process but is difficult to implement in large-scale production. 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

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