Aromatic polyimide

Aromatic polyimides have excellent thermal stability, but rather poor long term photostability. Under both UV and VIS (350-500 nm) irradiation, they undergo a colour change from light yellow-brown to dark purple [120,1105, 1501]. 

The following aromatic polyimides have been studied in detail  

Pyromellitic-dianhydride based polyamide is the most photostable of the above group (4.69). The photolytic decomposition in air of polyimide films based on a dianhydride and a diarylamine with hexafluoroisopropylidene 6F bridging groups 4.71 and 4.72) is extremely rapid, leading to efficient chain cleavage and subsequent photo-oxidative decomposition. On the basis on the photo-oxidation of several model compounds and IR difference spectroscopy, it has been shown that the photodecomposition process occurs via decomposition of the arylimide linkage [996, 998]  

Aromatic polyimides are characterized by high-temperature stability and good mechanical and dielectric properties, which render them reliable materials for a variety of applications in a number of different advanced technologies [1211. Their ion beam modification leading to changes in conductivity, hardness, surface chemical properties, etc., has been studied intensively in the past few years (e.g., [34, 122-134]). 

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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. 

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. 

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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