Polyimide Forms

The utilization of N-silylated diamines as polyamide and polyimide forming monomers has recently been exhaustively reported by Kakimoto et al. [105] and Oishi et al. [106]. But their work is beyond the scope of this... 

Imidisation of PCA based on 4,4 -di-(/7-aminophenoxy)-benzophenone was accompanied by the precipitation of the polymers from the reaction solutions, which is partially due to crystallinity in the polyimides formed. This appears to hinder the solubility of the polymers in amide and phenolic solvents. Polyimides based on l,l-dichloro-2,2-di-(/7-aminophenoxyphenyl)-ethylene are more soluble. The polymer formed from this diamine and the dianhydride of benzophenone-3,3 -4,4 -tetracarboxylic acid is soluble in a TCE/phenol (3 1) mixture. 

Aromatic bis(o-phenylendiamines) prepared from chloral derivatives, in particular 3,3, 4,4 -tetraminobenzophenone, were also used for the synthesis of polyimides possessing good solubilities in amide and phenolic solvents in combination with high thermal and heat resistance. In these respects, they were little different from polyimides formed from other bis(o-phenylendiamines) [29, 30]. 

Polyimides for microelectronics use are of two basic types. The most commonly used commercial materials (for example, from Dupont and Hitachi) are condensation polyimides, formed from imidization of a spin-cast film of soluble polyamic acid precursor to create an intractable solid film. Fully imidized thermoplastic polyimides are also available for use as adhesives (for example, the LARC-TPI material), and when thermally or photo-crosslink able, also as passivants and interlevel insulators, and as matrix resins for fiber-reinforced-composites, such as in circuit boards. Flexible circuits are made from Kapton polyimide film laminated with copper. The diversity of materials is very large readers seeking additional information are referred to the cited review articles [1-3,6] and to the proceedings of the two International Conferences on Polyimides [4,5]. 

The oldest reported microporous membranes are based on carbon and are obtained by controlled pyrolysis of suitable polymeric precursors. Koresh and Soffer were the first to report properties of these membranes in a series of papers starting in 1980 (see refs, in Ref. [78]. Recently Linkov et al. [79] improved this method and arrived at mesoporous asymmetric hollow-fibre carbon membranes which could be transformed to microporous systems by coating the carbon membrane by e.g. vapour deposition polymerisation of polyimide forming precursors. 

The polyimide polymers were formed by an established procedure (17) involving the addition of stoichiometric quantities of benzophenone-tetracarboxylic dianhydride (sublimed) to the diamine dissolved in anhydrous dimethylacetamide. The polyamic acid solution was cast on Teflon, warmed (80° to 90°C) to drive off solvent, then gradually heated to a temperature of 180°C. The polyimide forms rapidly at 120 to 150°C. 

Polyimides form the most important group of thermally stable polymers and are characterized by the presence of the... 

With the polyimides produced in situ, the imide group is simultaneously formed with the polymer. Synthesis and processing difficulties with polyimides formed in situ have led to the development of monomers and prepolymers with preformed imide groups. For example, maleic anhydride converts to what is known as bismaleimides with suitable diamines... 

The norbornene concept was further developed by NASA Lewis Research Center and culminated in the so-called PMR-concept, a novel class Of addition-type polyimides formed by in situ polymerization of monomeric reactants. 5 jhe reaction sequence36-37 involves aromatic diamine, S-norbornene-2,3-dicarboxy1ic acid monoalkyl ester and dialkyl ester of an aromatic tetracarboxylic acid which produces methanol, water, and the following intermediates [6]. 

Sulfonated polyimides (SPI, Fig. 11.11) are potential candidates for proton exchange membranes and direct methanol fuel cells (DMFC) because the polyimide forms a network structure to control methanol permeability, and if sulfonic acid groups are introduced in the polymer chain, then it becomes hydrophilic and facilitates proton conduction. Polyimide is mechanically and thermally stable and chemically resistant [77]. 

Polyimides formed by thermolysis of poly(amic acids) have proven to be the least successful of planarizing polymers because their high glass transition temperatures limit flow and conformability to the device topography. However, this high is a performance advantage. For example, polyimides have been used to planarize device structures where the thermal stability of the polymer allows it to remain on the final device. 

Figure 3. Time dependence of inherent viscosity of the polyimide formed by the microwave-assisted polycondensation of 12PMA (- -) and 12PME (- -) in DMI solvent (1 mL), where the monomer was 1 g.

Figure 4 shows a comparison of the time dependence curve of the inherent viscosity of the polyimide formed by the microwave-assisted polycondensation of 12PMA with that by a conventional solid-state polycondensation at 250°C. It is obvious that the microwave-assisted polyo)ndensation for curve A proceeded much faster than the solid-state polycondensation for curve B. Thus it is concluded that the internal heating by the microwave irradiation was highly effective compared with a conventional external heating, yielding the polyimide with a high inherent visocisty in a very short polymerization time. 

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