Partially fluorinated thermal stability

Partially fluorinated X-IP has been used for a number of years as an additive in the inert lubricant PFPE film on the surface of a magnetic hard disk to enhance start/stop durability of PFPE lubricants [29,30]. Recently it has been used as a vapor lubricated film on the surface of the disks [31 ]. In order to avoid the PFPE being catalyzed to decomposition by the slider material AI2O3 (refer to Section 3.4), XI -P was also examined as a protective film on the surface of the magnetic heads [25,32]. The results of CSS tests indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and the thickness of X-1P film on the slider surface has an important influence on HDD lubrication properties. 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluorocarbon results. Penetration of fluorine and conversion of the hydrocarbon to fluorocarbon to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to have a number of potentially useful applications ranging from increasing the thermal stability of the surface and increasing the resistance of polymer surfaces to solvents and corrosive chemicals, to improving friction and wear properties of polymer surfaces. It is also possible to fluorinate polymers and polymer surfaces partially to produce a number of unusual surface effects. The fluorination process can be used for the fluorination of natural rubber and other elastomeric surfaces to improve frictional characteristics and increase resistance to chemical attack. 

The chemical nature of the base polymer is an important aspect in membrane development. There has been preference for the thermally stable fluorinated polymers over hydrocarbon polymers. Fluorine-containing polymers, characterized by the presence of carbon-fluorine bonds, are widely used as the base matrices owing to their outstanding chemical and thermal stability, low surface energy, and the ease of modiflcation of various properties by the grafting method. Per fluorinated polymers and partially fluorinated polymers combining hydrocarbon and fluorocarbon structures are excellent candidates as base polymers. For instance, fluorinated FEP has drawn wide attention due to its reasonably good radiation stabiUty [58]. 

Chu et al. [166] prepared sulfonated and partially fluorinated block copolymers of high thermal stability containing 6F-BPA and perfluorobiphenylene units. Block copolymers were synthesized from the hydrophilic (29,000 g/mol) and hydrophobic parts (ll,000g/mol) with decafluorobiphenyl, the structure of which are shown in Scheme 2.37 (0 1 1 ratio for block 0, 1 9 10 raho for block 10, 3 7 10 raho for block 30, and 5 5 10 raho for block-50). The of the copolymers was in the range 15,000-90,000g/mol, with D values 1.3. 4. 

A partially fluorinated polymer, also soluble in polar solvent, is PVDF. It is characterized by high performance, chemical resistance, good thermal stability, and resistance to UV light and oxidant attack (Chin et al., 2006). PVDF membranes have been used primarily in the construction of submerged modules, widely used in bioreactors because of their lower costs with respect to other types of photocatalytic membranes (Sopajaree et al., 1999 Chin, Lim, Chiang, Fane, 2007 Jia et al., 2006, Fu, Ji, Wang, Jin, An, 2006). 

Yamaki et al. [14-16] investigated the thermal stability of fluorinated esters, which are the same fluorinated esters used as additives to improve the cycling performance reported by Nakajima et al. [12,13] prior to our study. In this study, partially fluorinated carboxylic acid esters (Table 20.1) were used as the electrolyte solvent and LiPFe as the salt. LiPFe was dissolved in methyl difluoroacetate (MEA) and ethyl difluoroacetate (EFA) to a salt concentration of 1 M. In the other fluorinated esters, however, LiPFe salt could be dissolved to a salt concentration of less than 0.2 M. Therefore, the solutions of fluorinated esters (P and 2 ) with 0.2 M of LiPFe were used, and the other fluorinated esters were saturated with LiPFe. For comparison, the solutions of corresponding esters with 0.2 M LiPFe were prepared. Similar measurements were performed employing the conventional electrolyte solution used in lithium batteries 1 M LiPFe/EC + DMC (1 1 by vol). 

Therefore, in a comparative study, different nonfluorinated and partially fluorinated sulfonated aromatic polymers were synthesized and characterized in terms of chemical and thermal stabilities. It was found that SO3H groups introduced in the electron-deficient sections of the aromatic polymer main chains were much more stable against splitting-off than SO3H groups pendent to electron-rich sections of the polymer backbones [29]. One of the most stable polymers of this series was a polymer prepared by polycondensation of decafluorobiphenyl and bisphenol (AF), followed by sulfonation with... 

Since PPOCH2Br is a nonfluorinated, electron-rich polymer and FPARCH2Br a partially fluorinated arylene polymer, one could conclude that the partially fluorinated arylene polymer introduces better thermal stabilities into the blend membrane (Fig. 4.15). 

From Fig. 4.16 can be read that both covalently cross-linked blend membranes expose nearly similar thermal stabilities, while the thermal stability of pure partially fluorinated B2 is slightly better than that of the pure fluorine-free B4 only above 450 °C which is however not relevant for the intended intermediate-T fuel cell application of these membranes. Therefore, covalent cross-linking can be regarded as a good method to improve thermal (and chemical, see below) stabilities of PBI-type membranes. 

From the reviewed work, it can be concluded that obviously the use of partially fluorinated aromatic cationomers as ionical cross-linkers leads in most cases to better chemical and thermal stabilities of the blend membranes than if nonfluorinated cationomers would be applied as acidic blend components. Among aU acidic cross-linkers, the sulfonated and partially fluorinated ionomer S9 (Fig. 4.5) leads to the best chemical stability of the referring base-excess PBI blend membranes. 

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