Polymer fluorinated polyimide

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) ... 

The dielectric constant of polyimide films in the wet condition (50% RH atmosphere) is higher than in the dry condition. This is attributed to water absorption by the polyimides. Figure 15.5 also shows the water absorption of die polyimides, which is due to the presence of imide groups in the polymer and decreases widi increasing fluorine content because of the hydrophobic effect of fluorine atoms. The water absorption is related to the stability of the dielectric constant. The dielectric constant variability between the dry and wet conditions of highly fluorinated polyimides, P3FDA/TFDB and P6FDA/TFDB, is smaller than... 

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]. 

The second key factor determining permeability in polymers is the sorption coefficient. The data in Figure 2.18 show that sorption coefficients for a particular gas are relatively constant within a single family of related materials. In fact, sorption coefficients of gases in polymers are relatively constant for a wide range of chemically different polymers. Figure 2.25 plots sorption and diffusion coefficients of methane in Tanaka s fluorinated polyimides [23], carboxylated polyvinyl trimethylsiloxane [37] and substituted polyacetylenes [38], all amorphous glassy polymers, and a variety of substituted siloxanes [39], all rubbers. The diffusion... 

K. Tanaka, H. Kita, M. Okano and K. Okamoto, Permeability and Permselectivity of Gases in Fluorinated and Non-fluorinated Polyimides, Polymer 33, 585 (1992). 

Z. Ge, L. Fan, S. Yang, Synthesis and characterization of novel fluorinated polyimides derived from l,l -fcis(4-aminophenyl)-l-(3-trifluoromethylphenyl)-2,2,2-trifluoroethane and aromatic dianhydrides, Eur. Polym. J. 44 (4) (2008)1252-1260. 

Among various strategies that have been used to synthesize polyimides with lower dielectric constants, the most common approach is to incorporate fluorene, in the form of trifluoromethyl groups, into diamine and dianhydride units that minimize polarizability and increase the free volume [46]. It is well-known that fluorene atom has unique characteristics such as high electronegativity and low electric polarity. These properties give fluorinated polymers (e.g., poly[tetrafluoroethylene]) attractive features such as low water uptake, water and oil repellency, low permittivity, low refractive indices, resistance to wear and abrasion, and thermal and chemical stability. Fluorination is also known to enhance solubility and optical transparency and to lower the moisture absorption of polyimides. Therefore, it is expected that fluorinated polyimides will be widely applied in the electro-optical and semiconductor industries. The polymer series studied was essentially limited mainly to 6F dianhydride because it proved to be the only dianhydride with which many of the fluorinated diamines would form polymer films suitable for physical characterization. 

S. Banerjee, M.K. Madhra, A.K. Salunke, D.K. Jaiswal, Synthesis and properties of fluorinated polyimides. 3. Derived from novel l,3- w[3 -trifluoromethyl-4 (4"-amino benzoxy) benzyl] benzene and 4,4-fc 5 [3 -trifluoromethyl-4 (4-amino benzoxy) benzyl] biphenyl, Polymer 44 (3)(2003) 613-622. 

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