Aromatic polymers Atomic oxygen

Highly branched polymers such as polypropylene and polymers with other links, e.g. poly(oxymethylene), are most readily attacked by atomic oxygen. Perfluorinated polymers, rubber vulcanized with sulfur, and highly aromatic polymers are the most resistant. 

The rapid reaction between atomic oxygen and polymer films is discussed. This typical interface reaction is considerably influenced by the structure of the polymer. All polymers immediately react with atomic oxygen there is no evidence of even short induction periods or autocatalysis. Most readily attacked are highly branched polymers such as polypropylene and polymers with ether links for example, polyformaldehyde. Perfluorinated polymers, rubbers vulcanized with sulphur, and highly aromatic polymers are most resistant (Fig. 22). Oxidation of polymers by atomic oxygen occurs only at or near the surface of the polymer. For this reason the elucidation of the reaction kinetics and mechanism is very difficult. The conventional physico-chemical methods, UV and IR spectroscopy, are in this case inadequate. 

Given that the absorption phenomenon at EUV is atomic, almost every element is opaque at EUV (see Eig. 14.22) except thin films of polymers with high carbon content (as in aromatic polymers based on PHOST) and silicon (see Fig. 14.23). Polymers containing high amounts of oxygen and fluorine have very high absorptivity at EUV (see Fig. 14.23). 

Polyaryloxysilane n. A family of polymers, resistant to high temperatures, made up of silicon atoms, oxygen atoms, and thermally stable aromatic rings, part organic and part inorganic in nature, hke the sihcones. 

Poly(aryl sulfone) n. An aromatic polymer consisting of benzene rings finked by both sulfone (-SO2-) groups and ether oxygen atoms. Poly(aryl sulfones) can be synthesized by solution polymerization in a polar solvent. Several commercial products of this type have been developed as reasonably high temperature resistant engineering plastics. 

A number of methods are available for the preparation of aromatic polymers. The majority of these methods involve the formation of a chemical bond between a carbon atom and a heteroatom. Typical of these are, for example, aromatic polycarbonates the latter are prepared by the reaction of a dihydric phenol with phosgene or derivative thereof.The polymerization proceeds via formation of a carbon-oxygen bond. A similar situation is encountered with the class of polyarylates — the polyesters from dihydric phenols and aromatic diacids. " ... 

Atomic oxygen reacts rapidly with several polymers [655, 803, 905-907, 1294, 1362, 1387, 1776, 1777, 1839, 1958]. Figure 7.1 shows the effect of atomic oxygen on the weight loss of different polymers. Highly branched polymers such as polypropylene and polymers with ether links, e.g. poly-(oxymethylene), are most readily attacked by atomic oxygen. Perffuorinated polymers, rubber vulcanized with sulphur and highly aromatic polymers are... 

Crystallisable polymers have also been prepared from diphenylol compounds containing sulphur or oxygen atoms or both between the aromatic rings. Of these the polycarbonates from di-(4-hydroxyphenyl)ether and from di-(4-hydroxy-phenyl)sulphide crystallise sufficiently to form opaque products. Both materials are insoluble in the usual solvents. The diphenyl sulphide polymer also has excellent resistance to hydrolysing agents and very low water absorption. Schnell" quotes a water absorption of only 0.09% for a sample at 90% relative humidity and 250°C. Both the sulphide and ether polymers have melting ranges of about 220-240°C. The di-(4-hydroxyphenyl)sulphoxide and the di-(4-hydroxy-phenyl)sulphone yield hydrolysable polymers but whereas the polymer from the former is soluble in common solvents the latter is insoluble. 

Aromatic cyclic chains are more stable than aliphatic catenated carbon chains at elevated temperatures. Thus linear phenolic and melamine polymers are more stable at elevated temperatures than polyethylene, and the corresponding cross-linked polymers are even more stable. In spite of the presence of an oxygen or a sulfur atom in the backbones of polyphenylene oxide (PPO), polyphenylene sulfide (PPS), and polyphenylene sulfone, these polymers are... 

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