Polyimides, aromatic

Aromatic polyimides are synthesized by the reactions of dianhydrides with diamines, for example, the polymerization of pyromellitic anhydride with p-phenylenediamine to form poly(pyromeUitimido-l,4-phenylene) (XLV) [de Abajo, 1988, 1999 Hergenrother, 1987 Johnston et al., 1987 Maier, 2001]. Solubility considerations sometimes result in using the half acid-half ester of the dianhydride instead of the dianhydride. 

The lack of easy processability initially limited the utilization of this high-performance material. Several different modifications of the polyimide system have successfully produced 

Polyetherimides (PEI) are polyimides containing sufficient ether as well as other flexibi-lizing structural units to impart melt processability by conventional techniques, such as injection molding and extrusion. The commercially available PEI (trade name Ultem) is the polymer synthesized by nucleophilic aromatic substitution between 1,3-bis(4-nitrophthalimido) benzene and the disodium salt of bisphenol A (Eq. 2-209) [Clagett, 1986]. This is the same reaction as that used to synthesize polyethersulfones and polyetherketones (Eq. 2-206) except that nitrite ion is displaced instead of halide. Polymerization is carried out at 80-130°C in a polar solvent (NMP, DMAC). It is also possible to synthesize the same polymer by using the diamine-dianhydride reaction. Everything being equal (cost and availability of pure reactants), the nucleophilic substitution reaction is probably the preferred route due to the more moderate reaction conditions. 

Polyamideimides (PAIs) (trade name Torlon), containing both amide and imide functional groups in the polymer chain, are produced by the reaction of trimellitic anhydride (or a derivative) with various diamines (Eq. 2-210). PAI resins are amorphous polymers 

Bismaleimide (BMI) polymers are produced by reaction of a diamine and a bismaleimide (Eq. 2-211) [de Abajo, 1988, 1999 Mison and Sillion, 1999]. Polymerization is carried out with the bismaleimide in excess to produce maleimide end-capped telechelic oligomers (XLVI). Heating at temperatures of 180°C and higher results in crosslinking via radical chain 

BMI polymers have glass transition temperatures in excess of 260°C and continuous-use temperatures of 200-230°C. BMI polymers lend themselves to processing by the same techniques used for epoxy polymers. They are finding applications in high-performance structural composites and adhesives (e.g., for aircraft, aerospace, and defense applications) used at tem-peratrues beyond the 150-180°C range for the epoxies. Bisnadimide (BNI) polymers are similar materials based on bisnadimides instead of bismaleimides. 

A high-temperature-stable polyimide occurs from the reaction of pyromellitic anhydride with aromatic diamines such as p,p-diaminodi-phenyl ether. Pyromellitic acid is produced either by direct oxidation of 1,2,4,5-tetramethyl benzene (Durol) or from xylylene. The xylylene mixture is chloromethylated. Crystalline l,3-dimethyl-4,6-di(chloromethyl)-benzene (I) is separated from the two liquid isomers (II) and (III) and oxidized to pyromellitic acid with HNO3  

Polycondensation is carried out in two stages. In the first, polyamic acid is formed in solvents such as dimethyl formamide, dimethyl acetamide. 

Bonding occurs predominantly at the para position there is relatively little attack at the meta position. 

In order to avoid cross-linking reactions, the solids content of the solution is restricted to 10-15% and the yield to 50%. The molecular weight of the resulting acid polyamide, which is influenced considerably by the mode of addition of the reaction partners, can attain values of up to M = 55,000 and = 240,000. In the second stage, water is eliminated at 300°C  

It has been proposed that the reaction can be carried out with capped amines because of difficulties in the synthesis given above. Such capped amines include isocyanates, O-alkyl carbamic esters, aldimines, and keti-mines. Capped amines are less basic and more easily purified. They react more slowly with anhydrides, thus permitting stepwise conversions. For example, with isocyanates, no water is eliminated  

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Aromatic polyimides are the first example we shall consider of polymers with a rather high degree of backbone ring character. This polymer is exemplified by the condensation product of pyromellitic dianhydride [Vll] and p-amino-aniline [Vlll] ... 

Synthesis and Properties. In 1972, Du Pont marketed a series of linear aromatic polyimides called NR-150 (105) based on... 

Aromatic polyimides are generally produced by the reaction of aromatic dianhydrides with aromatic diamines to yield a material with the general stmcture... 

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

Tan et al. investigated polymers made from bis-benzocyclobutenes [13-15]. As the benzocyclobutane is analogous to tbe dien, tbe Diels-Alder addition takes place. This reaction is applied to the preparation of polyimides. The advantage of this system is that the resultant polymer is oxidized to form thermally stable aromatic polyimides (Fig. 7). 

Makino, H., Y. Kusuki, H. Yoshida, and A. Nakamura, Process for Preparing Aromatic Polyimide Semipermeable Membranes, U.S. Patent No. 4,378,324, March 1983. 

Aromatic polyimides are well known for their unusual array of favorable physical properties, including excellent thermal stability and excimer-laser processing characteristics. The polyimide structure possesses lower-energy transitions such as n —> n, n —> o, n —> n, and a — n (in order of increasing energy71). However, the w — n and o —> n transitions are forbidden by symmetry rules and related absorptions are significantly weaker than those for... 

Asano, N., Aoki, M., Suzuki, S., Miyatake, K., Uchida, H. and Watanabe, M. 2006. Aliphatic/aromatic polyimide ionomers as a proton conductive membrane for fuel cell applications. Journal of the American Chemical Society 128 1762-1769. 

Geometric effects coupled with diffusion and nucleation usually control the rates of all solids deposition phenomena. Such effects can be put to good use in the production of special products such as cellulose yarn (rayon), by the precipitation of cellulose in filament form as it emerges as sodium cellulose xanthate liquid from the spinnerets into a bath containing sulphuric acid, which extracts the sodium as sodium sulphate, and the carbon disulphide. In a similar manner, the fabrication of aromatic polyimide fibres is performed by dissolving the polymer in concentrated sulphuric acid and forcing the solution through spinnerets into water. 

Thermostability requirement for microelectronic applications basically involves only the thermo exposure during processing. Since the devices are not expected to operate at anywhere near the processing temperature. At 400°C in air, even with very thin films polyimide do not show any sign of degradation within the time (30-60 min) processing take place. We, therefore, conclude that fully aromatic polyimide is thermally sufficient for this application. 

Synthesis of PIQ. Very high heat resistance is required in order for a polymer film to be used as an insulator. This is because several heat treatments over 400 C are necessary in LSI interconnection and assembly processes. An aromatic polyimide (I), a reaction product of aromatic diamine and acid dianhydride, is one of the most heat resistant polymeric materials ... 

Aromatic polyimides have the following repeating unit ... 

The classical synthetic pathway to prepare polyimides consists of a two-step scheme in which the first step involves polymerization of a soluble and thus processable poly(amic acid) intermediate, followed by a second dehydration step of this prepolymer to yield the final polyimide. This preparative pathway is representative of most of the early aromatic polyimide work and remains the most practical and widely utilized method of polyimide preparation to date. As illustrated in Scheme 4, this approach is based on the reaction of a suitable diamine with a dianhydride in a polar, aprotic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), or AT-methylpyrrolidone (NMP), generally at ambient temperature, to yield a poly(amic acid). The poly(amic acid) is then cyclized either thermally or chemically in a subsequent step to produce the desired polyimide. This second step will be discussed in more detail in the imidization characteristics section. More specifically, step 1 in the classical two-step synthesis of polyimides... 

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