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Aromatic Polysul des
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Poly (p-phenylene sul de) [IUPAC: poly(sulfanediyl-1,4-phenylene) (trade name: Ryton) (PPS) is synthesized by the reaction of sodium sul de with p-dichlorobenzene in a polar solvent such as 1-methyl-2-pyrrolidinone (NMP) at about 250 C and 1.1 MPa (160 psi) [Fahey and Geibel, 1996; Hill and Brady, 1988; Lopez and Wilkes, 1989]. The reaction may be more complicated than a simple nucleophilic aromatic substitution [Koch and Heitz, 1983]. PPS undergoes a slow curing process when heated above the melting point in air. Curing involves chain extension, oxidation, and crosslinking, but is poorly understood. There is spectroscopic evidence for crosslinking via sulfur, oxygen, and aromatic bridges between polymer chains [Hawkins, 1976].
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Uncrosslinked PPS is a highly ($60%) crystalline polymer with Tm 285 C and Tg 85 C. It is rated for continuous service at 200 240 C, placing PPS between the polysulfones and the polyketones. It has inherent ame resistance, and its stability toward both organic and aqueous environments is excellent. PPS is comparable to polysulfones and polyketones in resistance to acids and bases but is somewhat less resistant to oxidants. The resistance of PPS to organic solvents is comparable to that of polyamides and polyketones. More than 20 million pounds of PPS are produced annually in the Unites States. Applications of PPS include automotive (components requiring heat and uid resistance, sockets and re ectors for lights), consumer (microwave oven components, hair-dryer grille), industrial (oil eld downhole components, motor insulation, pumps and valves), blends with uorocarbon polymers (release coating for cookware, appliances, molds), and protective coatings (valves, pipe, heat exchangers, electromotive cells). PPS is an alternative to traditional thermosetting plastis in some of these applications.
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Aromatic Polyimides
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Aromatic polyimides are synthesized by the reactions of dianhydrides with diamines, for example, the polymerization of pyromellitic anhydride with p-phenylenediamine to form poly(pyromellitimido-1,4-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 direct production of high-molecular-weight aromatic polyimides in a one-stage polymerization cannot be accomplished because the polyimides are insoluble and infusible. The polymer chains precipitate from the reaction media before high molecular weights are obtained. Polymerization is accomplished by a two-stage process. The rst stage involves an amidation reaction carried out in a polar aprotic solvent such as NMP or N; N-dimethylacetamide (DMAC) to produce a high-molecular-weight poly(amic acid) (XLIV). Processing of the polymer can be accomplished only prior to the second stage, at which point it is still soluble and fusible. It is insoluble and infusible after the second stage of the process. The poly(amic acid) is formed into the desired physical form of the nal polymer product (e.g., lm, ber, laminate, coating) and then the second stage of the process is performed. The poly(amic acid) is cyclodehydrated to the polyimide (XLV) by heating at temperatures above 150 C.
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The poly(amic acid) is kept in solution during the amidation by using mild temperatures, no higher than 70 C and usually lower than 30 C, to keep the extent of cyclization to a minimum. Less polar, more volatile solvents than NMP and DMAC are sometimes used to facilitate solvent evaporation in the second stage reaction. This requires the use of an acidic or basic catalyst since the reaction rate is decreased in a less polar solvent. The poly(amic acid) is prone to hydrolytic cleavage and has relatively poor storage stability unless kept cold and dry during storage. Temperatures as high as 300 C are used for the second-stage solid-state cyclization reacton, although the use of vacuum or dehydrating agents such as acetic anhydride-pyridine allow lower cyclization temperatures. Polyimides (PIs) are mostly amorphous materials with high glass transition temperatures. The resistance of PI to organic solvents is excellent. Polyimides show good oxidation resistance and hydrolytic stability toward acidic environments, comparable to PET and better than nylon 6/6. However, PI undergoes hydrolytic degradation in strongly alkaline environments, comparable to polycarbonate and poorer than PET and nylon 6/6. The high temperature resistance of polyimides is excellent with continuous use temperatures of 300 350 C possible, especially for structures containing only aromatic rings. Wholly aromatic polyimides are generally too stiff for most applications, and their processability is limited by poor solubility and lack of melt ow at accessible temperatures. One of the rst commercial polyimides was the polymer obtained from 4; 40 -diaminodiphenyl ether and pyromellitic dianhydride (trade names: Kapton, Vespel). This and other similar polyimides are available in the form of poly(amic acid) solutions that are used as high-temperature wire enamels and insulating varnishes and for coating ber glass and other fabrics. Polyimide lms nd applications as insulation for electric motors and missile and aircraft wire and cable. There is also some processing of PI by powder technology (lowtemperature forming followed by sintering) and compression molding (requiring higher pressures than normal) to form such automotive and aircraft engine parts as bushings, seals, piston rings, and bearings. The lack of easy processability initially limited the utilization of this high-performance material. Several different modi cations of the polyimide system have successfully produced