Thermal Degradation of Polyethers

The aromatic polyethers are subject to rupture at the ether link at around 300°C [518]. Products from such ruptures are somewhat similar to those obtainable from decompositiOTi of the polysulfones, shown above. 

The thermal degradation of polyoxymethylene was found to be initiated at the chain ends. There appear to be three possible sites for initiation of homolytic bond cleavages [519]  

---

Fluorination of high molecular weight polyethers with elemental fluorine (diluted with helium) at room temperature gave fluorinated polyethers, which were depolymerized by further treatment with flowing F -He at 55-210 "C to produce carbonyl difluoride, amongst the decomposition products [1197c]. COFj is also formed in the oxidative thermal degradation of polyethers derived from hexafluoropropene oxide [1582]. Polymeric C F Oj decomposes, at an unspecified temperature, into carbonyl difluoride [760], The presence of... 

Grassie, N. and Mendoza, G.A.P., "Thermal-Degradation of Polyether-Urethanes. 1. Thermal-Degradation of Poly(Ethylene Glycols) Used in the Preparation of Polyurethanes," Polym. Degrad. Stab. 9,155-165,1984. 

Montaudo, G., Puglisi, C., Scamporrino, E., and Vitalini, D., Thermal Degradation of Aromatic-Aliphatic Polyethers 1. Direct Pyrolysis Mass Spectrometry, Macromolecules, 19, 870, 1986. 

The mechanism is believed to be that a hydrogen atom next to the ether linkage is attacked. This radical reacts with oxygen from the air to form a peroxide radical, which in turn takes another hydrogen atom from the backbone of the chain to form a hydroperoxide. This hydroperoxide breaks down into two more radicals (see Figure 7.12). When polyether polyurethanes are heated in an atmosphere of nitrogen, this thermal degradation does not take place and the material retains its properties. 

Since polyurethanes are frequently used in household objects, their thermal degradation and products generated during burning were studied frequently [3-5]. Among these can be included studies on polyester-urethanes [6], polyether-urethanes [7], phenol-formaldehyde urethane [8], studies on the influence of fire retardants on polyurethane decomposition [9, 10], generation of isocyanates during decomposition [11], and other studies [12-17]. Some reports on thermal decomposition of polyurethanes are summarized in Table 14.1.1. 

Kozlov, G. V. Shustov, G. B. Zaikov, G. E. The structural aspect of the interrelation of the characteristics of thermal and thermooxidative degradation of heterochain polyethers. In book Aging of Polymers, Polymer Blends and Polymer Composites. V. 2. Ed. Zaikov, G. Buchachenko, A. Ivanov, V New York, Nova Science Ihiblishers, Inc. 2002,151-160. 

A serious limitation to the use of organic polymers in general and of adhesives, in particular, is their poor resistance to thermal degradation. Considerable effort has been put into the development of High-temperature adhesives and examples of the materials that have been produced are described in articles on Polybenzimidazoles, Polyether ether ketone, Polyimide adhesives and Polyphenylquinoxalines. Some of the general principles used in the search for enhanced thermal stability are discussed in this article. 

Poly(phenylene oxide) (PPO) is a thermoplastic, linear, noncrystalline polyether commercially produced by the oxidative polymerization of 2,6-dimethylphenol in the presence of a copper-amine catalyst. PPO has become one of the most important engineering plastics widely used for a broad range of applications due to its unique combination of mechanical properties, low moisture absorption, excellent electrical insulation property, dimension stability and inherent flame resistance. This chapter describes the recent development of this polymer, particularly on the production, application, compounding, properties of its alloys and their general process conditions. The polymerization mechanism and thermal degradation pathways are reviewed and new potential applications driven by the increasing environmental concerns in battery industry, gas permeability and proton-conducting membranes are discussed. 

The best-known representative of the class of segmented polyether esters is the combination of polybutylene terephthalate as the hard component with polyether glycol as the soft component. The presence of polyether components causes a sensitivity to thermal-oxidative degradation in this class of materials. In non-stabilized form, these polymers cannot be processed. Their main oxidation product is formic acid. Moreover, re-formed monomers (terephthalic acid) of the polymer can collect on the surface and form a white, hard-to-remove deposit that changes the gloss level and color [512], Oxidative degradation reactions can be inhibited by primary and secondary antioxidants. Acidolysis caused by the formic acid can be controlled by adding acid acceptors that will bind either the precursor of formic acid, formaldehyde, or the acid itself. Acid amides, urethane, or urea are utilized as acid acceptors [86]. 

The wide range of variation in the chemical composition of polyether block amides expresses itself in the varied sensitivity to thermal-oxidative degradation of these polymers. The polyether blocks are very oxidation prone and with increasing number of these blocks, the sensitivity to oxidative attack increases [86]. 

Backus and co-workers [1] investigated the thermal degradation in air of rigid urethane foams by thermogravimetric analysis (TGA), differential thermal analysis (DTA), infrared (IR), and other techniques. Three commercial foams were studied a polyether urethane foam 1, a flame-retardant polyether urethane foam 2, and a chlorinated polyester urethane foam 3 (Table 5.1). 

---