Fluorinated dianhydrides

It should be noted that this molecule has two ether linkages that give flexibility to the molecular structure. In addition, the bulky -Cl groups in P6FDA are replaced by fluorine atoms, lire 8c of lOFEDA (157.5 ppm from I MS) was almost the same as that of P6FDA (157.6 ppm), so this dianhydride should have higher reactivity than unfluorinated and partially fluorinated dianhydrides. 

In recent years a list of fluorinated dianhydrides (3-53 to 3-59) has been identified by several researchers and is provided in Scheme 3.8. Liu et al. [106] synthesized bis(ethsr dianhydride) monomers, namely, 4,4 -(2-[3 -trifluoromethylphenyl]-... 

The gas permeation stability of an asymmetric polyimide membrane with a thin and defect-free skin layer was investigated by H. Kawakami and co-workers [176,177]. They studied both the effect of molecular weight and the structure of polyimide using a common fluorinated dianhydride, 6FDA (Scheme 3.13). They observed that for 2,2 -fciB(4-aminophenyl) hexafluoropropane (6FDA-6FAP)-based polyimide, despite of different molecular weights, the permeability of N2, O2, and CH4 had almost constant values whereas the CO2 permeability of the asymmetric... 

Separation of aromatic/aliphatic mixtures by pervaporation using the PBO membranes was studied by Ribeiro et al. The PBOs were prepared by the TR of ort/jo-fimctionalized fluorinated polyimide films (Figure 5.60). They used different feed streams, such as toluene/ -heptane or benzene/ -heptane mixtures. All the PBOs were selective toward the aromatic hydrocarbon. The PBO membranes showed higher selectivity (a=6.7, at 80 °C) and 25 times higher permeability (toluene, 220 Barrer, at 80 °C) in comparison to their precursor polyimides (toluene, 8.9 Barrer and a =1.9, at 80 °C). These increases in hydrocarbon permeability for the fluorinated PBO membranes are due to their larger hydrocarbon uptake and lower packing efficiency of the polymer chains, that is, increase in FFV in comparison to their precursor polyimides [91]. In all cases for a given diamine, the replacement of the >C(CF3)2 unit in the dianhydride (i.e., with the replacement of the fluorinated dianhydride with a non-fluoiinated one) resulted in an approximate 10 times reduction in hydrocarbon permeability, and consequently a reduction in total flux, with an increase in selectivity. 

M. R. Coleman and W. J. Koros. Isomeric polyimides based on fluorinated dianhydrides and diamines for gas separation applications. J. Membr. Sci. 50, 1990, 285-297. 

Often the substitution of fluorine atoms for hydrogen atoms in a polymer chain markedly increases the thermal stabiUty of the base polymer this is tme for polyimides. A typical fluorinated polyimide is prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis-(4-amino phenyl)hexafluoropropane according to the following reaction (36) ... 

In addition to the research on fluorinated and cardo polyimides, an important work was devoted to the semiaromatic cycloaliphatic polyimides. Volk-sen points out the potential interest of these materials in electronic industry.64 He reports that the simplest procedure to prepare these materials is to use a cycloaliphatic dianhydride and an aromatic diamine (Fig. 5.9) instead of an aliphatic diamine and an aromatic dianhydride, which leads to formation of gels. 

Table 13.1. Chemical Structures of Dianhydrides from Polyimides Where One or Both Monomer Components Are Fluorinated...

Reactivity Fluorine substitution changes -the reactivity of monomers— often dramatically, if adjacent to reactive sites and other times only moderately or minimally if reactive sites are separated from fluorine by unconjugated bonds. Reactivity is generally reduced for fluorine-contain-ins amines and often increased for fluorine-containina dianhydrides. 

A new and important generation of fluorine-containing polyimides began with the introduction of dianhydrides incorporating the hexafluoroisopropylidene (6F) chemical moiety. Hexafluoroisopropylidenc-containing polymers were... 

Recently, perfluorinated polyimides have been synthesized from novel perfluorinated dianhydrides of both the fluorine and trifluoromethyl types (lOFEDA, P2FDA, P3F, P6F) along with a series of perfluoroaromatic diamines (TFPDA, 4FMPD, 8FODA, 8FSDA). These materials were shown to have superior optical transparency, reasonably high TgS, and reasonably low dielectric constants. 

To generate high-molecular-weight perfluorinated polyimides, it is first necessary to determine how fluorine affects the reactivity of the monomers. In particular, the effect of substituting hydrogen with fluorine on diamine reactivity is important because kinetic studies of the acylation of conventional monomers have revealed that acylation rate constants can differ by a factor of 100 among different dianhydrides, and by a factor of 10 among different diamines. ... 

The glass transition temperatures ofthe polyimides are 195-250 °C their 10% weight loss temperamres (dynamic thermogravimetric analysis, air, AT = 4.5 °C/min) are 390-422 °C. Of particular interest are the dielectric constants of these polyimides. At a relative humidity of 50% these constants are 2.70-2.90 and are comparable with constants of the best fluorinated polyimides [21, 50-55]. The lowest dielectric constant (2.70) was observed for polyimide based on 6F dianhydride, containing the highest amount of fluorine. Thermal treatment of this polymer film at 280-290 °C for 1 hour led to a decrease (2.45) of dielectric constant due to the possible formation of nanofoams [56]. 

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