Solubility perfluoropolyether

As a consequence of their high solubility, perfluoropolyethers could be used to synthesize highly soluble amphiphilic copolymers. This will be described in the following sections of this chapter. 

Historically the widespread use of liquid and supercritical C02 has been limited by the low solubility of most nonvolatile materials at reasonably low pressures. However, studies carried out over a decade ago established the solubility of oligometric perfluoropolyethers and poly(chlorotrifluoroethylene) in liquid C02,12 and a few years later it was discovered that other highly fluorinated polymers such as fluorinated acrylates were also quite soluble in liquefied C02 at relatively low pressures.13... 

Although supercritical CO2 is an effective solvent for oils, fats, and similar substances, it is a poor one for nonvolatile hydrophilic (water-loving) substances such as proteins or metallic salts. Adding water as such to the supercritical CO2 is of little help, as the solubility of water in it is limited. Johnson and co-workers216 overcame the latter limitation by forming water-in-C02 emulsions with the aid of an added nontoxic perfluoropolyether surfactant that forms reverse micelles around the water microdroplets, in effect combining the special properties of supercritical CO2 with the solvent power of water. These emulsions can dissolve a variety of biomolecules at near-ambient temperatures, without loss of their biological activity. 

The solubility of PDMS in COo /toluene mixtures has been attributed to comparable solubility parameters and the interaction between CO2 (a weak Lewis acid) and the strong electron donor capacity of the siloxane group. The oxygen in perfluoropolyethers also has an electron donor capacity. The solubility parameter... 

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-... 

Panza et al. synthesized a C02-philic amphiphile from the coenzyme nicatinamide adenine dinucleotide (MW 664) and a covalently attached perfluoropolyether (MW 2500) (Figure 7B) (73). The fluorofunctional coenzyme (FNAD) was soluble up to 5 mM in CO2 at room temperature and 1400 psi. The C02-soluble FNAD was able to participate in a cyclic oxidation/reduction reaction catalyzed by the enzyme horse liver alcohol dehydrogenase (HLADH) in CO2 at room temperature and 2600 psi. 

In 1993, perfluoropolyether (PFPE) carboxylates, with average molecular weights between 2,500 and 7,500, were reported to be soluble in liquid CO2 (19). However, these high MW polymers were not effective at stabilizing w/c microemulsions. Later, Johnson et al. formed w/c microemulsions with an ammonium carboxylate PFPE (PFPE-COO NH4 surfactant of only 740 MW (30). Success with these surfactants was attributed to the chemical structure itself. PFPE constitutes an extremely C02-philic tail group, accentuated by the presence of pendant fluoromethyl groups, which tend to increase the volume at the interface on the CO2 side and thus favor curvature around the water. 

The potential of microemulsions for organometaUic-catalyzed hydrogenations in water/scC02 biphasic systems has been assessed using the rhodium-catalyzed hydrogenation of styrene as a common test reaction [Eq. (7)] [31]. The water-soluble Wilkinson complex [RhCl(TPPTS)3] was applied as catalyst precursor together with anionic perfluoropolyether carboxylates, cationic Lodyne A, or nonionic poly-(butene oxide)-b-poly(ethylene oxide) surfactants. The interfacial tension is small in the presence of the supercritical fluid and small amounts of surfactant (0.1-2.0 wt.%) suffice to form stable microemulsions. The droplet diameter of the microemulsions varied between 0.5 and 15 pm and a surface area of up to 10 m was obtained. 

As previously mentioned, another class of fluoropolymers known to be readily soluble in CO2 are perfluoropolyethers (PFPEs) [1]. A unique class of fluoropolymers, PFPE polymers and copolymers have been established as high performance materials, exhibiting low surface energies and low moduli, as well as excellent thermal and chemical stabilities. PFPEs are primarily found in high-performance lubricant applications, e.g., for magnetic data storage media and as heat exchanger fluids. One of the main industrial processes for the production of PFPEs is photooxidation of fluoroolefins [41]. Currently, only TFE and HFP are used commercially in this process. Typically, HFP is photooxidized in bulk because of its very low reactivity, while TFE requires an inert diluent in order to prevent homopolymerization of the olefin. 

The common surfactants discussed repeatedly here and elsewhere are in general unsuitable for water-supercritical carbon dioxide emulsions of all types (including microemulsions). Selections are made, to start with, on the basis of the water/ carbon dioxide solubility behavior. A surfactant used in some of the initial studies is ammonium carboxylate perfluoropolyether [24,25]. Subsequently, a variety of triblock co-polymers, e.g. poly(propylene oxide-b-ethylene oxide-b-propylene... 

The solubility of PDMS in CO2 /toluene mixtures has been attributed to comparable solubility parameters and the interaction between CO2 (a weak Lewis acid) and the strong electron donor capacity of the siloxane group. The oxygen in perfluoropolyethers also has an electron donor capacity. The solubility parameter of CO2 at the critical point is 5.5-6.0 (cal/cc), which makes it comparable with pentane, but it can be raised as high as 9-9.5 (cal/cc) by increasing the pressure when solvent power is more akin to that of benzene or chloroform. Fluorinated oils have the lowest solubility parameter of any known liquid at 4.5-5.0 (cal/cc) These figures indicate that CO2 should exhibit miscibility with fluorinated oils. Solubility in CO2 may rise upon replacement of-CH2 with -CF2 or CF (CF3)0.3- ... 

The experimental and theoretical studies mentioned in the introduction demonstrate the feasibility of steric stabilization of latexes and emulsions in supercritical CO2. For example polymeric stabilizers based upon highly C02-soluble PFOAs or perfluoropolyethers (PFPEs) can impart a repulsive force between particles in... 

The choice of surfactant depends on the composition of the continuous phase. Both surfactant mixtures and fluid mixtures can be used. For example, while neither of the two surfactant perfluoropolyether phosphate ether (PFPE-PO4) and AOT is soluble in SCCO2, their mixtures with appropriate ratios can form stable microemulsions with water cores in SCCO2 [16]. 

Peifluoropolyethers Similarly to fluorinated olefins, the introduction of fluorine atoms onto the poly(ethylene oxide) (PEO) backbone has been explored through the family of perfluoropolyethers (sometimes coined perfluoroalkylethers or perfluoropolyalkylethers). Functionalized poly(hexafluoropropylene oxides) are representative perfluoropolyethers that actually attracted early attention for their solubility in SC-CO2 (see Figure 13.5). These polymers are produced by anionic ringopening polymerization of HFP oxide and commercialized as industrial lubricants by Dupont under the name of Krytox . 

Similarly to perfluoropolyethers, the widely accepted explanation for the high solubility of fluorinated poly(acrylates) in SC-CO2 is provided by their low cohesive energy densities and weak self-interactions. Johnston and coworkers reported surface tension values of 10-15 mN/m, which is actually lower than the values for perfluo-ropolyether and poly(vinyl acetate), the other aforementioned C02-philic references. 

Recently, the use of microemulsions in supercritical CO2 to produce nanoparticles has received considerable attention (240,246-249). Since conventional hydrocarbon surfactants for oil/water systems often exhibit low solubilities in CO2 and are therefore incapable of solubilizing a significant amount of water (250,251), surfactants with fiuorinated tails have been used for the formation of water-in-C02 microemulsions (251-254). Perfluoropolyether (PFPE) is a popular and commercially available fiuorinated surfactant. Wai and coworkers prepared silver nanoparticles via chemical reduction in a microemulsion of water in supercritical CO2 with PFPE (247). A slow flocculation of the nanoparticles was... 

The monomers (5.2 wt.% content relative to CO2) and perfluoropolyether (10 wt.% content relative to the monomers) were charged into a high-pressure cell at 28.0 MPa and 65°C. All the reactants were soluble in the supercritical medium, and the reaction was allowed to proceed for 24 h under stirring. The obtained polymer was washed continuously with fresh COj for 1 h to remove any residual monomers or crosslinker. Monodisperse crosslinked MGs with an average diameter of 3.0 0.7 pm were obtained. 

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