Molecular weight Fluoropolymers

In addition to the fluoroplastics and fluororubbers already described other fluoropolymers have been marketed. Polymers of hexafluoropropylene oxide are marketed by Du Pont (Krytox). These materials have a low molecular weight (2000-7000) and are either oils or greases. The oils are uses as lubricants, heat transfer fluids and non-flammable oils for diffusion pumps. The greases are also used as lubricants. They have good heat and oil resistance but it is said that explosions may result from contact with the surfaces aluminium or magnesium cuttings. 

Fluoropolymers utilizing high molecular weights and copolymerized and alloyed with polyethylene, should be used in most radiation applications. High-dose-rate E-beam processing may reduce oxidative degradation. When irradiated, PTFE and PFA are... 

This research was an attempt to develop new polymers with the mechanical properties of polyarylene ethers and the dielectric properties of fluoropolymers. After initially testing the viability of the [2n+ 2n] cyclodimerization reaction for preparing high-molecular-weight polymers and testing the dielectric properties of these polymers, two polymers (one thermoplastic and one thermoset) were prepared in larger quantities to evaluate the thermal and mechanical performance of these novel compositions. The high Te thermoset was also quantitatively tested for thermal/oxidative stability. 

Since PTFE was first synthesized more than 50 years ago, fluoropolymers have been produced by radical polymerization and copolymerizaton processes, but without any functional groups, for several reasons. First, the synthesis of functional vinyl compounds suitable for radical polymerization is much more complicated and expensive in comparison with common fluoroolefins. In radical polymerization of one of the simplest possible candidates—perfluorovinyl sulfonic acid (or sulfonyl fluoride—there was not enough reactivity to provide high-molecular-weight polymers or even perfluorinated copolymers with considerable functional comonomer content. Several methods for the synthesis of the other simplest monomer—trifluoroacrylic acid or its esters—were reported,1 but convenient improved synthesis of these compounds as well as radical copolymerization with TFE induced by y-irradiation were not described until 1980.2... 

The primary advantage of direct fluorination for synthesizing fluoropolymers is that many monomers are difficult to polymerize because of the steric repulsion of fluorine, which leads to relatively low-molecular-weight species also, it is difficult or impossible to synthesize many of the corresponding monomers. 

These fluoropolymers also differ in their crystallinity, molecular weight and, possibly, a minor comonomer. Consequently, each has its set of properties. To try to shed light on this jungle . Figure 4.87(a) to (f) displays their relative positions versus some of the essential properties relating to their main applications ... 

It is interesting that the PEMA-PVdF blends are amorphous up to at least 50 wt % PVdF even though the Tg of the latter is 24°C. The crystallization of PVdF observed in the analogous PMMA blend does not occur under the same conditions with PEMA—PVdF. This suggests that there is a specific interaction between the fluoropolymer and the methacrylate polymer which is sufficient to "dissolve PVdF in the PMMA and PEMA, and that this specific interaction is superimposed on the conventional diluent-crystalline polymer interactions. The complexity of the rate processes involved with high molecular weight systems arising from molecular mobility makes it impossible to elucidate the nature of... 

When the relation between D and M is established, we can easily convert G(D) obtained by dynamic LLS into a differential molecular weight distribution, such as fw(M). We have successfully applied the above methods to various kinds of polymeric and colloidal systems, such as for Kevlar [15, 23], fluoropolymers (Tefzel Teflon) [12,30-35,52], epoxy [53-55],polyethylene [56,57], water-soluble polymers [18,50-51,58,59], copolymers [60-62], thermoplastics [63-65] and colloids [66-72]. Three of those applications are illustrated below. 

Perfluoropolyethers can also be synthesized in C02 (Bunyard et al., 1999). Such polymers have utility as high-performance lubricants and heat-transfer fluids. This class of fluoropolymer is soluble in C02. In this study, the perfluoropolyethers were prepared by the photooxidation of HFP in liquid C02 at —40°C (see Scheme 9.4). The composition and molecular weights were comparable to those of perfluoropolyethers synthesized in perfluoro-... 

Indeed, free radical polymerization of fluoroolefins continues to be the only method which will produce high-molecular weight fluoropolymers. High molecular weight homopolymers of TFE, CFC1 = CF2, CH2CF2, and CH2=CHF are prepared by current commercial processes, but homopolymers of hexafluoro-propylene or longer-chain fluoroolefins require extreme conditions and such polymerizations are not practiced commercially. Copolymerization of fluoroolefins has also led to a wide variety of useful fluoropolymers. Further discussion of the subject of fluoroolefin polymerization may be found elsewhere and is beyond the scope of this review [213-215]. 

Although CO2 dissolves many small molecules readily, it is a very poor solvent for most high molecular weight polymers. Currently, only amorphous or low melting fluoropolymers and silicone polymers are known to be very soluble in CO2 (T < 100 °C, P < 400 bar), or C02-philic, while many industrially important polymers are relatively insoluble. In 1992, we reported the successful homogenous free radical polymerization of a C02-philic fluorinated acrylate, 1,1-dihydroperfluorooctyl acrylate (FOA)2. Homogenous polymer synthesis in CO2 is fundamentally limited however, by the extremely low solubility of most polymers at readily accessible conditions. 

Continuous mns were carried out in a stirred cell ultrafiltration module using the fluoropolymer membrane FS61PP with a nominal molecular weight cut-off of 20kDa. The reactor, loaded with an appropriate amount of resting cells, was fed with a buffered substrate solution by a peristaltic pump with the flow rate set at... 

Supercritical carbon dioxide is a very good solvent for small molecules, but a poor solvent for most high molecular weight polymers at mild conditions (T<100 °C, P<350 bar). Amorphous fluoropolymers and silicones are the only polymers known to be soluble in CO2 at mild conditions [6]. This difference in solubilities is an advantage for C02-based polymerizations, as it can be used to reduce the energy requirements necessary to separate and purify a polymer after synthesis. Consider, for example, a batch precipitation polymerization in... 

Dadalas, M.C., Epsch, R., Kloos, E.,and Harvey, L.W. Fluoropolymer dispersions containing no or little low molecular weight fiuorinated surfactant, U.S. Patent 7,279,522 (October 9, 2007) to 3M Innovative Properties Co. 

---