Fluoropolymers behavior

A force field for solid state modeling of fluoropolymers predicted a suitable helical conformation but required further improvement in describing intermole-cular effects. Though victory cannot yet be declared, the derived force fields improve substantially on those previously available. Preliminary molecular dynamics simulations with the interim force field indicate that modeling of PTFE chain behavior can now be done in an all-inclusive manner instead of the piecemeal focus on isolated motions and defects required previously. Further refinement of the force field with a backbone dihedral term capable of reproducing the complex torsional profile of perfluorocarbons has provided a parameterization that promises both qualitative and quantitative modeling of fluoropolymer behavior in the near future. 

More sophisticated material models for fluoropolymer behavior are available that provide dramatic improvements in modeling accuracies. These models will increase in power and capability, and the result will be increasingly reliable predictions of fluoropolymer behavior. 

Over the last decade, selected papers1114 have examined the deposition of fluoropolymers, using RF magnetron sputtering. All of these papers have examined the deposition of PTFE, with some of them2314 also studying the deposition of polyimide (PI) films. This chapter extends these studies and will report on the sputter deposition behavior of PTFE (polytetrafluoroethylene), PVDF (polyvinylidenefluoride), and FEP (fluorinated ethylene propylene copolymer) films. 

Like many other fluoropolymers, Nafion is quite resistant to chemical attack, but the presence of its strong perfluorosulfonic acid groups imparts many of its desirable properties as a proton exchange membrane. Fine dispersions (sometimes incorrectly called solutions) can be generated with alcohol/water treatments. Such dispersions are often critical for the generation of the catalyst electrode structure and the MEAs. Films prepared by simply drying these dispersions are often called recast Nafion, and it is often not realized that its morphology and physical behavior are much different from those of the extruded, more crystalline form. 

J. Kedzierski and S. Beny, Engineering the electrocapillary behavior of electrolyte droplets on thin fluoropolymer films Langmuir 22, 5690-5696... 

The short-term dielectric strength of PFA is 80 kV/mm at a thickness of 0.25 mm, measured by ASTM D149. FEP films give similar results while PTFE has at5q)ical dielectric strength of 47 kV/mm. The dielectric strength of PFA decreases in the presence of corona discharge, a behavior similar to other fluoropolymers. 

Physically based constitutive models that accurately capture the behavior of fluoropolymers. 

Fluoropolymers, as well as other thermoplastics, exhibit a complicated nonlinear response when subjected to loads. The behavior is characterized by initial linear viscoelasticity at small deformations, followed by distributed yielding, viscoplastic flow, and material stiffening at large deformations until ultimate failure occurs. The response is further complicated by a strong dependence on strain rate and temperature, as illustrated in Fig. 11.1. It is clear that higher deformation rates and lower temperatures increase the stiffness of the material. 

There are a number of candidate materials models for predicting the behavior of fluoropolymers. Since the models have varying degrees of complexity, computational expense, and difficulty in determining the material parameters, it is a good idea to use the simplest material model that captures the necessary material characteristics for the application and situation at hand. Unfortunately, it is often difficult to determine, in advance, the required conditions needed by the material model. Hence, it is recommended that a more advanced model be used in order to ensure accuracy and reliability of the predicted data. At a later stage, a less advanced model can be attempted if the computational expense is too great. At that time, the accuracy of the different model predictions can also be tested and validated. 

A number of more advanced and general models attempting to predict the yielding, viscoplastic flow, time-dependence, and large strain behavior of fluoropolymers and other thermoplastics have recently been developed.in this section, we discuss the Dual Network Fluoropolymer (DNF) model. 

Coatings with Thermoplastic Fluoropolymers. Poly(vinylidene fluoride), PVDF, is the only conventional thermoplastic fluoropolymer that is used as a commercial product for weather-resistant paints. This crystalline polymer is composed of -CHjCFj- repeating units it is soluble in highly polar solvents such as dimethyl-formamide or dimethylacetamide. Poly(vinylidene fluoride) is usually blended with 20 30 wt% of an acrylic resin such as poly(methyl methacrylate) to improve melt flow behavior at the baking temperature and substrate adhesion. The blended polymer is dispersed in a latent solvent (e.g., isophorone, propylene carbonate, dimethyl phthalate). The dispersion is applied to a substrate and baked at ca. 300 °C for ca. 40-70 s. The weather resistance of the paints exceeds 20 years [2.16]-[2.18]. 

Fluoropolymers are polytetrafluoroethylene (PTFE) and ethylene tetrafluoroethylene (ETFE). They can assist as redundant insulation and are primarily used as a coating to defend conductor wires from corrosion. The advantages of those materials are their inert and biocompatible behavior and high tensile strength. Otherwise, their stiffness, creep. 

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