Polybenzimidazoles polymer structure

Figure 3 shows some polymer structures of other sulfonic-acid-based materials and Fig. 4 shows the conductivity of propanesulfonated polybenzimidazole, showing the strong decrease in conductivity of PFSA above 80 °C and the much more thermally stable PBI derivative. 

Aromatic polybenzimidazoles were synthesized by H. Vogel and C. S. Marvel in 1951 with anticipation, later justified, that the polymers would have exceptional thermal and oxidative stability. Subsequently, NASA and the Air Force Materials Laboratory (AFML) sponsored considerable work with polybenzimidazoles for aerospace and defense applications as a non-flammable and thermally stable textile fiber and as high temperature matrix resins, adhesives and foams. The route to fiber used solutions of high molecular weight polymer. Structural applications used low temperature melting pre-polymers that were cured (polymerized) in place. Applications of polybenzimidazoles were not implemented in the 60 s and 70 s since the polymers tetraamine precursors were not commercially available. 

Polybenzimidazoles first appeared in US Patent 2,895,948 in 1959. In 1961, Vogel and Marvel opened the field of high temperature polymers [1] with their studies of the thermal stability of aromatic polybenzimidazoles [3]. Subsequently, AFML and NASA funded programs for basic studies and for structural and textile applications of polybenzimidazoles to meet new aircraft and aerospace material needs. Several reviews were published [4-7]. Structural applications of polybenzimidazoles are still in developmental stages which is discussed below. One polybenzimidazole polymer (PBI) which was developed for fiber applications is now commercial. Details of the polymer synthesis and fiber process are described in the next section. 

The melting temperatures and thermal stabilities of several polybenzimidazoles are shown in TABLE I. For all aromatic polybenzimidazoies, melting points are over 600 C. Generally, the presence of other structural elements will reduce melting points and thermal stabilities. For example, TABLE I includes polybenzimidazoles condensed from either a tetraamine ether or a diacid ether. Strained polymer structures also reduce polybenzimidazole stability as evidenced by the low melting point of a polymer formed with a biphenyl-2,2 -dicarboxylate. Not surprisingly, the aliphatic, adipic acid forms a much less stable polybenzimidazole than the aromatic examples above it in the table. 

A considerable number of non-cross-linked aromatic and heterocyclic polymers has been produced. These include polyaromatic ketones, aromatic and heterocyclic polyanhydrides, polythiazoles, polypyrazoles, polytriazoles, poly-quinoxalines, polyketoquinolines, polybenzimidazoles, polyhydantoins, and polyimides. Of these the last two have achieved some technical significance, and have already been considered in Chapters 21 and 18 respectively. The most important polyimides are obtained by reacting pyromellitic dianhydride with an aromatic diamine to give a product of general structure (Figure 29.17). 

Sulfonated poly(arylene ether)s have shown promise for durability in fuel cell systems, while poly-(styrene)- and poly(imide)-based systems serve as model systems for studying structure-relationship properties in PEMs because their questionable oxidative or hydrolytic stability limits their potential application in real fuel cell systems. Sulfonated high performance polymer backbones, such as poly(phe-nylquinoxaline), poly(phthalazinone ether ketone)s, polybenzimidazole, and other aromatic or heteroaromatic systems, have many of the advantages of poly-(imides) and poly(arylene ether sulfone)s and may offer another route to advanced PEMs. These high performance backbones would increase the hydrated Tg of PEMs while not being as hydrolytically sensitive as poly(imides). The synthetic schemes for these more exotic macromolecules are not as well-known, but the interest in novel PEMs will surely spur developments in this area. 

Many heterocyclic polymers have been produced in an attempt to develop high-temperature-resistant polymers for aerospace applications. Among these are the polybenzimidazoles (PBIs), which are prepared from aromatic tetramines and esters of dicarboxylic acids (structure 4.63). In standardized procedures, the reactants are heated to below 300°C forming soluble prepolymer, which is converted to the final insoluble polymer by further heating ... 

A number of thermally stable polymers have been synthesized, but in general the types of structures that impart thermal resistance also result in poor processing characteristics. Attempts to overcome this problem have largely been concentrated on the incorporation of flexible groups into the backbone or the attachment of stable pendent groups. Among the class of polymers claimed to be thermally stable only a few have achieved technological importance, some of which are polyamides, polyimides, polyquinoxalines, poly quinolines, and polybenzimidazoles. Of these, polyimides have been the most widely explored. 

The e. s. r. and conductivity of polymers such as the polyacene/qui-none radical polymers (PAQR polymers), polyacetylenes and polybenzimidazoles have been investigated (59, 60, 61). The term eka conjugated has been coined to decribe their properties which are very similar to those previously described. The e. s. r. signal is without structure, the activation energy which ranges from 0.2—2.0 ev is considerably... 

Poly-2-2 -(w-phenylene)-5,5 -bibenzimidazole, commonly called polybenzimidazole (PBI), was developed under the aegis of the U.S. Air Force Materials Laboratory in cooperation with the then-existing Celanese Corporation. The fiber went into commercial production in the United States in 1983. It is a condensation polymer obtained from the reaction of tetra-aminobiphenyl and diphenylisophthalate in a nitrogen atmosphere at temperatures that may reach 400°C in the final stages.29 The structure of a repeating unit is shown below. 

These plastics, also known as pyrrones, are experimental materials prepared from aromatic dianhydrides and aromatic tetraamines. The polymer syntheses provide soluble prepolymers that are converted to the pyrrone structures by thermal or chemical dehydration. The precursors can be used to cast films or coatings, or can be molded under very high pressures into filled or unfilled forms. The pyrrones combine some of the best properties of the polybenzimidazoles and polyimides. The pyrrone films are exceptionally radiation resistant and retain their strength properties after 10,000 megarads of 1-MeV electrons. 

Fig. 7 Chemical structures of some sulfonated polymers and a polyimide (A) sulfonated polyetheretherketone, PEEK, PSE (B) sulfonated polyphenylenesulfide, PPS (C) sulfonated polysulfone (D) poly(4,4 -biphenol) (4,4 -dichlorodiphenyl sulfone), BPSH-XX (XX is mol% of disulfdonated units) (E) sulfonated polybenzimidazole, PBI (F) polyimide.

Structural engineering materials Use as structural materials, especially under conditions of ablation or in high-temperature environments, has been one of the prime objectives of polybenzimidazole development ever since Marvel s first publication.This holds to a small extent for molded products and to a predominant extent for laminates. Almost all the polymers evaluated and used in these technical applications are of the type 1. 

Additional polymer blends comprising PAEK s offering property combinations of potential utility include PSF [Robeson and Harris, 1986 Harris and Robeson, 1989] structurally different poly(aryl ketones) [Harris and Robeson, 1986], PAr [Robeson and Harris, 1992], poly(amide-imide) PAI [Harris and Gavula, 1992], PPS [Robeson, 1987], and other PI [Harris et al., 1992]. Mixtures of structurally different PAEK s were noted to be isomorphic within specific limits of ether/ketone ratios [Harris and Robeson, 1987]. Blends of polybenzimidazole, PBI and several commercial PI (Ultem 1000 and Matrimid 5218) have been studied in depth at the University of Massachusetts and found to be miscible. FTIR studies [Guerra et al., 1988 Kim et al., 1993], NMR studies [Grobelny et al., 1990], thermal, dielectric, and mechanical... 

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