Radiolysis of polymers

Basic Studies on Product Analysis and Ion Beam Pulse Radiolysis of Polymers. 102... 

From the materials viewpoint, it is of interest to examine the thermodynamics of radiolysis of polymers. Since gamma radiolysis data is readily available, polymers can be compared as in Table X. Note that materials having a large heat of polymerization tend to crosslink under radiolysis. The same polymers are thermally resistant to degradation. Degrading polymers have low ceiling temperatures ( 150°C) and low heats of polymerization. 

The migration of energy between phases will also have an effect in the radiolysis of mixed polymeric systems not associated with transuranic waste. For instance, the radiolysis of polymers attached to silica particles or the radiolysis of rubber in steel belted tires will probably be affected by energy deposited in the non-organic phase. Energy migration to the polymeric phase may lead to the need for lower overall doses than initially anticipated for a... 

The yield of H2 in the radiolysis of polymers with y-rays is well known for several types of polymers [2], However, transuranic waste materials are a-particle emitters. The radiation chemistry induced by a-particles can be very different than that due to y-rays because of the difference in energy deposition density [13], The high linear energy transfer (LET, equal to the stopping power) of heavy particles leads to an increase in second order reactions, which may change the yields of some products. 

The radiolysis of polymer solutions has been reviewed by Chapiro [441] and later by Henglein and Schnabel [442, 443]. Two types of reaction are added to the degradation mechanism for solid polymers. One involves the reactions between polymer radicals and solvent or between solvent radicals and polymer. The other involves energy transfer from solvent to polymer or from polymer to solvent. 

Molecular oxygen (O2) has a significant effect on the radiolysis of polymers, because it adds readily to free radicals (see Section 5.3.3), a fact which is of great importance for practical applications. The radiolysis of polymers in the absence of O2 is detailed in the following section. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

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