High-temperature polymers ladder

BPA/DC was compounded with high-temperature resistant ladder and semi-ladder polymers. A composition, which contained BPA/DC, a polyimide resin, organic Zn salt and benzyldimethylamine in A -methylpyrrolidone solution was described. The binder was destined for the manufacture of copper clad laminates. Tg of the cross-linked material was 265 °C [50]. Another example is a binder obtained from BPA/DC and polyphenylquinoxaline in chloroform [51]. 

Ladder polymers. A type of high-temperature polymer. Double linear chains of the macromolecules are periodically linked together (Fig. 1.1). They are insoluble and infusible, being unsuitable for thermoplastic processing and thus are limited in applications. In the macromolecules of step-ladder polymers shorter units of cross-linked double linear chains (ladder structures) are joined by single bonds (Fig. 1.2). An example of a step-ladder polymer is polyimide. 

Although there seem to be no true ladder polymers in large-scale commercial production, several semi-ladder polymers that have rather rigid structures are employed where high-temperature strength is important. Among these are... 

A variety of polymers contain the element boron.42,44,52 60 One of the simplest consists of chains of boron fluoride, of repeat unit -BF-, and can be prepared by the reaction of elemental boron with boron trifluoride at high temperatures. The polymer is a rubbery elastomer, but it has been little studied because of its hydrolytic instability and tendency to ignite spontaneously in air.42 However, a variety of other structures are formed with a number of metals, for example chains with Fe, ladders with Ta, sheets with Ti, tetragonal structures with U, and cubic structures with Ca and Ar. Boron also forms chains that are analogues of poly(dimethylsiloxane), with repeat unit -BCH-O-42... 

Pyrolysis is simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes chain scission and decreased molecular weight of the bulk polymer. This results in reduced cohesive strength and increased brittleness. Resistance to pyrolysis is predominantly a function of the intrinsic heat resistance of the polymers used in the adhesive formulation. As a result, many of the aromatic and multifunctional epoxy resins that are used as base resins in high-temperature adhesives are rigidly crosslinked or are made of a molecular backbone referred to as a ladder structure, as shown in Fig. 15.4. 

The structure of a ladder polymer comprises two parallel strands with regular crosslinks [Fig. 1.3(e)], as in polybenzimidazopyrrolone (XII), which is made by polycondensation of pyromellitic dianhydride (XIII) and 1,2,4,5-tetraminobenzene (XIV). This polymer is practically as resistant as pyrolitic graphite to high temperatures and high energy radiation. It does not burn or melt when heated but forms carbon char without much weight loss. 

Stable at relatively high temperatures. Polyimides, such as polybenzimidazole, etc., are at least in part ladder polymers and are also stable at relatively high temperatures. 

Most polymers that function properly at ambient temperature quite frequently have limited performance at sustained elevated temperatures. This invariably limits the utility of polymeric materials. The low thermal stability is generally due to decreased crystallinity and/or thermal decomposition. Polymer chemists have, through some ingenious ways, synthesized polymer — such as aromatic polyimides and the so-called ladder polymers — specifically designed for high-temperature applications. However, it has also been possible to modify polymers to improve their thermal stability and hence extend then-range of utility. A few examples of condensation polymers illustrate this point. 

A variety of polymers are used in engineering and medical applications which have relatively little impact on the environment. These are mainly high performance and relatively expensive polymers such as the silicones in rubbers, the specialised polyamides referred to above in gear wheels, polycarbonates (in office equipment), chlorinated and sulfonated rubbers, fluorinated polymers such as poly tetrafluoroethylene) Teflon ) in metal coating and the polyimides which, owing to their ladder structure, are extremely stable in high temperature apphcations. Since these polymers are high-cost durable materials, they rarely appear in the waste stream. 

Polyimides, Common name of polymers characterized by repeated imide groups in the macromolecule (Fig. 1.4). Their structure corresponds to that of the step-ladder polymers. Thermally stable infusible insoluble thermosets. Their intermediate prepolymers can be shaped. Modified polyimides have repeated, mainly aliphatic, ester or amide groups in the main chain besides the imide units, in order to facilitate processing. Examples of applications high-temperature electric insulating materials... 

There are several other classes of polymers being examined for high-temperature adhesive and composite applications, such as phthalo-cyanines, " aromatic nitriles,polytriazines, ladder polymers such as poly-bis(benzimidazophenanthroline) (BBP polymers), " and polyphenylene." Many of these materials are not commercially available and much more work is needed before successful applications will be found. 

Heterocyclic polymers yield materials with outstanding high-temperature performance see Table 7.2 (2). Many of these polymers have ladder or semiladder chemical structures. If one bond in a ladder structure is broken by heat or oxidation, the chain may retain its original molecular weight. If a single carbon atom chain is oxidized, usually the chain is degraded, with concomitant loss of properties. 

Quite early in the history of high-temperature pol miers, the concept of ladder polymers seemed promising and led to efforts to S5mthesize a perfect ladder structure by novel, as well as classical, S5mthetic approaches. 

Another approach to obtain thermally stabile polymer fibers is to form ladder-type structures through high temperature treatment. One good example is the high temperature treatment of polyacrylonitrile fibers between 200 and 260°C in air. This high temperature treatment also is called the stabilization process. As discussed in Chapter 11, the stabilization of polyacrylonitrile fibers can convert the linear polymer chains into ladder stmctures that are non-meltable and flame-resistant. Fignre 17.15 shows one possible ladder stmcture that is formed through the stabihzation process of polyacrylonitrile fibers. 

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