Polymer chain, random scission

Various studies have been made on the effects of radiation on lactide/glycolide polymers (24,38,58). Gilding and Reed (24) reported the effect of y rays on Dexon sutures. Those results confirmed that deterioration of the sutures occurs but that random chain scission is not the primary mechanism. Number average-molecular weight Mn showed a dramatic decrease at doses above 1.0 Mrad. Thus, unzipping of the polymer chain appeared to be the more dominant process, at least in the case of polyglycolide. 

Reaction 6 representing /1-scission of alkoxyl radicals leading to the reduction of molar mass competes with transfer of a free radical centre to surrounding groups with consequent formation of alcoholic groups (reaction 7), which subsequently loose water and C = C unsaturation appears randomly along the polymer chain. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

Iedema, P.D. and Hoefsloot, H.C.J. (2001) Synthesis of branched polymer architectures from molecular weight and branching distributions for radical polymerisation with long-chain branching, accounting for topology-controlled random scission. Macromol. Theor. Simul., 10, 855. 

Random chain scission—scissions occur at random locations along the polymer chain... 

Although population balance methods are relatively undeveloped for interesting polymers, the author believes they have great potential. Each of the four distinct thermal degradation mechanisms mentioned earlier will be analyzed in detail and examples involving specific polymers will be discussed wherever possible. Furthermore, some relevant combinations of mechanisms (such as combined random scission and end-chain scission) will also be discussed. 

McCoy, B. Polymer thermogravimetric analysis Effects of chain-end and reversible random scission. Chemical Engineering Science 2001 56 1525. 

The types of polymer degradation can be divided into three general categories chain depolymerisation, random scission and substituent reactions. 

Molecular Weight Distribution. A given polymer sample is composed of many polymer chains, which in most cases are not of the same length. This variability can be a result of the synthesis process or of possible random scission and cross-linking that can occur upon processing. For economic reasons, it is not possible to separate the various polymer chains by length prior to use so it is important to characterize this distribution in order to describe the polymer and understand its performance. As with any distribution, no single number is a totally satisfactory descriptor. 

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