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a polymer containing cyclic imide groups in their main or side chains:
Aromatic linear polyimides with imide rings in the main chain have acquired practical importance because of their valuable physicochemical properties, which remain unchanged over long periods in a wide range of temperatures (from –270° to + 300°C).
Polyimides are solid, heat-resistant, incombustible substances of predominantly amorphous structure, with molecular weight M̅w = 50,000–150,000 and density 1.35–1.48 g/cm3 (20°C). Most polyimides do not dissolve in organic solvents, are inert to the action of oils and remain virtually unchanged under the action of dilute acids but are hydrolyzed by alkalies and superheated steam. Polyimides are resistant to ozone, γ-rays, and fast electrons and neutrons and are very resistant to heat. The most industrially valuable polyimides are the polypyromellitimides:
They do not soften even at the onset of thermal decomposition (500°-520°C) and can withstand a tension of 50 meganewtons per sq m (MN/m2), or 500 kilograms-force per sq cm (kgf/cm2), at 300°C; their tensile strength at 20°C is 180 MN/m2, or 1,800 kgf/cm2, and their temperature for prolonged use is 250° –300°C.
Polyimides are produced mainly by polycondensation of tet-racarboxylic acids and their derivatives, mainly dianhydrides (most often those of pyromellitic acid and 3, 4, 3′, 4′-benzo-phenonetetracarboxylic acid), with diamines (for example, bis[4-aminophenyl] ether and m-phenylenediamine) in one or two stages. Macromolecular soluble polyamic acids are usually produced first. Films and fibers are formed from these materials and then undergo heat treatment. Polyimides are also worked by pressure molding. Polyimides are used to make monolithic items, electrical insulation films, wire and cable insulation, binders for reinforced plastics, adhesives, plastics, foamed plastics, and fibers; they are used in aviation and space technology.
In the USSR, polyimides are used in making PAK-1 varnish, PM film, DFO molding composition, STP-1 glass-fiber-reinforced plastics, SP-1 adhesive, and Arimid fiber; they are used in the USA to produce Kapton H film, Vespel, and M–33.
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IA. S. VYGODSKII