The successful development of poly(ethylene terephthalate) fibres such as Dacron and Terylene stimulated extensive research into other polymen containing p-phenylene groups in the main chain. This led to not only the now well-established polycarbonates (see Chapter 20) but also to a wide range of other materials. These include the aromatic polyamides (already considered in previous post …..), the polyphenylene ethers, the polyphenylene sulphides, the polysulphones and a range of linear aromatic polyesters.

    The common feature of the p-phenylene group stiffens the polymer backbone so that the polymers have higher Tgs than similar polymers which lack the aromatic group. As a consequence the aromatic polymers tend to have high heat deformation temperatures, are rigid at room temperature and frequently require high processing temperatures The materials in this chapter are not produced on the scale of the major thermoplastics such as the polyolefins and PVC or even of the polyamides, aliphatic polyethers and polycarbonates. They should therefore be considered as special purpose polymers used when other less expensive materials are unsuitable 

21.2 POLYPHENYLENES 

    Poly-p-phenylene has been prepared in the laboratory by a variety methods’ including the condensation of p-dichlorobenzene using the Wun Fittig reaction. Although the polymer has a good heat resistatnce wit decomposition temperatures of the order of 4000Cthe polymer (Firure 21 is brittle, insoluble and infunible. 

     Several substituted linear polypbenylenes have also been prepared none appear to have the resistance to thermal decomposition shown by the simple poly-p-phenylene.

    In 1968 the Monsanto Company announced the availability of novel soluble low molecular weight ‘polyphenylene’ resins. These may be used to impregnate asbestos or carbon fibre and then cross-linked to produce heat resistant laminates. The basic patent (BP 103711I) indicates that these resins are prepared by heating aromatic sulphonyl halides (e.g. benzene-1,3- disulphonyl dichloride) with aromatic compounds having replaceable nuclear hydrogen (e.g. bisphenoxybenzenes, sexiphenyl and diphenyl ether). Copper halides are effective catalysts. The molecular weight is limited initially by a deficiency in one component. This is added later with further catalyst to cure the polymer. 

     The resultant cross-linked polymer is not always entirely polyphenylene because of the presence of ether oxygen in many of the intermediates. Neither do the polymers have the heat resistance of the ultimate in polyphenylenes, graphite, which has a melting point of 3600°C.

    In 1974 another polyphenylene-type material was introduced. This was designated by the manufacturer, Hercules Inc., as H-resin (not to be confused with H-film, a term that has bcen used by Du Pont to describe a polyimide film). The Hercules materials may be described as thermosetting branched oligophenylenes of schematic structure shown in Figure 21.2. The oligomers are soluble in aromatic and chlorinated hydrocarbons, ketones and cyclic ethers. After blending with a cross-linking system, usually of the Ziegler- Natta catalyst type, the compound is shaped, for example by compression moulding and then cured. Form stability is achieved by heating to 160°C but post-curing to 230-300°C is essential to obtain the best solvent resistance and mechanical properties. 

    It is claimed that the cured materials may be used continuously in air up to 300°C and in oxygen-free environments to 400°C. The materials are of interest as heat and corrosion resistant coatings, for example in geothermal wells, high temperature sodium and lithium batteries and in high temperature polymer and metal processing equipment.