Perfluoroalkylene segments

Table 4.14 Structure (top) and physical properties [41] table below) of semi-fluorinated n-alkanes (60-63) and the homologous dialkyl bicyclohexyl liquid c stals 64 and 65 from which they are structurally derived. The spacefill model of 63 shows the helical conformation of the central perfluoroalkylene segment in contrast with the pentyl side-chains with their typical hydrocarbon zigzag conformation. The differences in charge distribution (red and blue denote negative and positive partial charges, respectively) are visualized by mapping of the electrostatic potential on to the electron density of 63 (B3LYP/6-31C //PM3 level of theory) [44, 50].

The available sources of perfluoroalkylene segments are limited to perfluoroalkylene diiodides and dicarboxylic acids of the type I(CF2CF2) I, where n=l to 5, and HOOC(CF2)nCOOH, where n =... 

The Study of the thermal degradation of these same hybrid silicones [35] was achieved in comparison to the classical polydimethyl and polytrifluo-ropropylmethyl siloxanes, and the authors showed that the introduction of perfluoroalkylene segments - C6H4 - (CF2)x - C6H4 - into the main chain of the polysiloxane increased the thermal stabiUty both under inert and oxidative atmosphere. 

Linear perfluoroalkylene ether dicarboxylic acids, HOOC-(CF2CF2OCF2CF2)nCOOH, are reported to have been prepared by UV-catalyzed coupling reactions (6) and oxidative polymerization reactions (11). Although these structures are considered to represent the preferred chain segments, the synthesis methods apparently have not been developed enough for commercial availability. 

Figure 6 Indicates attempts to build elastomeric properties Into a polymer by Incorporating a fluoroether chain Into a polylmlde system (. Line 1 shows an aryl diamine polymer Intermediate prepared In a lengthy series of synthesis steps from a perfluoroalkylene ether dlcarboxyllc acid. The polymer Is prepared from the reaction of the diamine with perfluoro-hexamethylene-bis(phthalic anhydride). As expected, the polymer had excellent thermal stability, but, again. Its low-temperature properties were deficient. For example, en X + y = 0, the Tg was 130 C when x + y = 1, the Tg was 110 C and when x + y = 3, the Tg was 80 C. With this system, we are unable to achieve requisite (lialn flexibility because of a preponderance of rigid aromatic groups that Inhibit elasticity and cause a raising of the T beyond practical limits for our Intended purpose as a sealant material. The fluoroether segment Is simply too short to yield a useful low-temperature elastomer.

---