Nonfluorinated polyimide

The primary purpose of incorporating fluorine into polyimide structures is to decrease the dielectric constant. Considerable effort has been devoted to the creation of new chemistry and to understanding the limits and principles behind this approach. 12.25-27,29,31-34,51-60,65,78,158 ygjygg gf (jjg dielectric constant in nonfluorinated polyimides generally range from about 3.0 to 4.0. Fluorinated polyimides generally range from about 2.6 to 3.3 (see Table 13.3). 

Fluorinated polyimide (PMDA/TFDB) and nonfluorinated polyimide (PMDA/DMDB) films prepared on a silicone substrate were introduced into an electron beam lithography system and subsequently exposed for square patterns (4x4 mm). The electron beam energy was 25 keV the beam current was 10 nA, and the beam dose was 300-1500 pC/cm. The 4x4 mm square was written by a 0.1-pm-wide electron beam. 

Figure 15.18 shows the relationship between the percentage of refractive index increase at the wavelength of 1.3 pm and the electron beam dose for both fluorinated and nonfluorinated polyimides. When the fluorinated polyimide PMDA/TFDB is irradiated the refractive index increased with increasing dose. On the other hand, the refractive index of nonfluorinated polyimide PMDA/DMDB was almost constant when the dose increased. These results show that there is a strong relationship between fluorine content and the increase... 

Figure 15.18. Relationship between the percentage of refractive index increase at a wavelength of 1.3 pm and the electron beam dose for both fluorinated and nonfluorinated polyimides. 

Figure 15.19 shows the relationship between the percentage of height decrease (A) calculated from surface profile measurements and the electron beam dose for both fluorinated and nonfluorinated polyimides. The surface profile between the irradiated and nonirradiated areas was measured. The film shrinks because of electron beam irradiation, and the maximum shrinkage ( ) reaches up to 0.4% at the film surface. 

In addition to Nafion-based catalyst layers, additional types have been developed, including CLs with different ion exchange capacities (lECs) [57,58] or with other hydrocarbon-type ionomers such as sulfonated poly(ether ether ketone) [58-60], sulfonated polysulfone [61,62], sulfonated polyether ionomers [63], and borosiloxane electrolytes [64], as well as sulfonated polyimide [65]. These nonfluorinated polymer materials have been targeted to reduce cost and/or increase operating temperature. Unfortunately, such CLs still encounter problems with low Pt utilization, flooding, and inferior performance compared wifh convenfional Nafion-based CLs. 

Table 15.2 shows the characteristics of the high-fluorine-content 6FDA/TFDB polyimide and KAPTON (nonfluorinated PMDA/ODA type polyimide, DuPont). The Tg of 6FDA/TFDB (335°C) is a little lower than that of PMDA/ODA because of its flexible -C(CF3)2- groups. However, 6FDA/TFDB has a high decomposition temperature of 569°C. 

The fluorinated copolyimides (including homopolyimides) are all more transparent than the nonfluorinated PMDA/ODA. The color of 10-pm-thick films changes gradually from bright yellow to colorless as the 6FDA/TFDB content increases. All these copolyimide films are also homogeneous compared with polyimide blends of 6FDA/TFDB and PMDA/TFDB. 

The CTE of 6FDA/TFDB is a little higher than that of PMDA/ODA. The former has a high CTE because the main chains contain bent hexafluoroisopro-pylidene units. We discuss the thermal expansion behavior of GFDA/TFDB in the next section, comparing it with the low-thermal-expansion fluorinated polyimide PMDA/TFDB. In addition, the refractive index of GFDA/TFDB, 1.556 at 589.3 nm, is much lower than that of the nonfluorinated PMDA/ODA. This is because of its low electronic polarizability, as well as its low dielectric... 

Nonfluorinated polymers such as sulfonated polyimides, polystyrene sulfonic acid, sulfonated poly(arylene ether sulfones), polyphosphazene, polybenzimidazole, sulfonatedpolysulfone, sulfonated poly(phthala-zinone ether ketone), and sulfonated poly(ether ether ketone) have been developed as a potential electrolyte membrane for DMFC [4,6,16-23]. 

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