Polytetrafluoroethylene chain scission

Simultaneously with Charlesby s findings, work along similar lines was carried out in G. E. s Research laboratories in Schenectady (22) and also in Research Institutes in the Soviet Union, although the latter only became known several years later (23). The results of this research demonstrated that in addition to polyethylene, many other polymers could be cross-linked by radiation. These include silicones, rubber, poly (vinyl chloride), polyacrylates and, to a lesser extent, polystyrene. In contrast, polymers such as polymethacrylates, polyisobutylene, polytetrafluoroethylene and cellulose underwent "degradation" by main-chain scission. These early findings were confirmed and extended to other compounds by numerous studies. 

The other chain scissions are accompanied by chlorine-atom transfer to the break site to form two fragments (by disproportionation) of lower molecular mass than the original polymer. Trifluoronitrosomethane copolymers are the least thermally stable. Trifluorochloroethylene copolymers occupy an intermediate position, while polyvinylidene fluoride and polytetrafluoroethylene are the most thermally stable. 

If a polymer is heated to the point where chemical bonds begin to break, radicals will normally be formed, rather than ions. Thus if chain scission takes place to produce free radicals at temperatures above T, rapid radical depolymerization, with production of monomer, is expected. This rapid depropagation is often termed unzipping. Table 1 shows some typical values for Tc for common polymers. They range widely from quite close to room temperature for polyacetal to close to 600°C for polytetrafluoroethylene. In reality, side reactions, which are discussed... 

Fully Fluorinated Polymers. The radiation chemistry of fully fluorinated polymers shows remarkable temperature dependence, with all of the fluorinated thermoplastics undergoing degradation, ie, chain scission, at ambient temperatures, but with an increasing yield of cross-linking reactions at elevated temperatures. Over the past 10 years, this has led to renewed interest in the radiation chemistry and applications of these materials (see Perfluorinated Polymers, Polytetrafluoroethylene). 

Thermal decomposition of a y-irradiated poly( vinyl fluoride) occurred in two main steps firstly, elimination of hydrogen fluoride and secondly, main chain scission to yield unsaturated hydrocarbons." Polytetrafluoroethylene and the copolymer of tetrafluoroethylene and hexafluoropropene were degraded in various atmospheres and the decomposition products analysed." - For an inert atmosphere over twenty different fluorinated products were identified. For air the major products were COF, CF , and COj with minor amounts of fluorocarbons. The gaseous and solid decomposition products have also been analysed from the thermal degradation of poly(carbon monofluorides) containing different proportions of fluorine. Kinetic data were also obtained. 

Most striking in this context is the fact that polytetrafluoroethylene (PTFE), a semicrystalline polymer that undergoes predominantly main-chain scission at room temperature, can be crosslinked when irradiated in the absence of O2 with... 

The influence of the irradiation temperature on the ratio of chain scission to branching reactions in electron beam irradiated polytetrafluoroethylene (PTFE). 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

A novel polyfunctional initiator has been prepared by the reaction of a copolymer of styrene and methyl methacrylate with polytetrafluoroethylene (PTFE) radicals generated photochemically from the monomer and manganese carbonyl. These radicals react with aromatic rings by addition and substitution so that the product copolymer from this reaction carries short PTFE chains with Mn(CO)6 end groups of, for example, structures (12). At 100 °C, scission of the CFj— Mn(CO)s bonds occurs with formation of active radicals, and the copolymer behaves as a polyfunctional macroinitiator. On heating this material with styrene or JV-vinyl-2-pyrrolidone a network structure and a graft copolymer respectively are formed. 

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