A-chain scission

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

The kinetics of the decomposition of PPC has been estimated from several studies. An analysis from TGA shows that the activation energy for end-capped PPC at temperatures over approximately 250°C is in the range of 130 kJ/mol, a relatively low value (for a chain scission process) [19]. The same analysis for uncapped PPC is complicated by non-linear behavior. Results consistently indicate that, at lower temperatures, a different decomposition reaction takes place than at higher temperatures. 

Selective bond rupture at entanglement points, or other such sites of stress concentration, could magnify the effect of a chain scission in the presence of an external stress, but it seems unlikely that this is occurring since the sol-gel data actually indicated a (slightly) lower ratio of scissions to crosslinks with an imposed stress. It also is difficult to visualize how the formation of free radicals, scissions, and crosslinks could directly cause the radiation expansion noted under no stress. Therefore, the mechanism of accelerated creep is probably not caused by the formation and reaction of macromolecular free radicals in the polymer specimens. 

The quartet spectrum observed for the amorphous samples at temperatures higher than — 70°C. was thought to be caused by rearrangement of radical XVI, giving the radical structure XIX and a chain scission. 

A second olefin copolymer with a more promising structure is that with isobutylene (2-methylpropene-l). Poly isobutylene itself is a chain-scissioning polymer which has been studied often. It is not much used as a positive resist despite its G(s) of 1.5 to 5 (21) because its Tg is so low, about -60°C. 

Figure 14.6 Destruction of elastically active network chains resulting from a chain scission in the case of tetrafunctional (a) and trifunctional nodes (b). 

In an initially ductile polymer, failure properties (ultimate elongation, fracture toughness, impact resistance) decrease rapidly during a chain-scission aging process, whereas elastic and yield properties are practically unaffected at the embrittlement point. 

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

Figure 5 shows (left) that for a fast hydrophobicity transfer, LMW is essentially. Materials with a decreased amount of LMW also show an increase in receding angles but need a significantly longer time. It can be considered that new LMW can be generated within the silicone polymer bulk as result of a chain scission [7], It was shown that additional LMW does not further accelerate the transfer process. 

A. a-chain scission (for this particular reaction it is estimated that the bond dissociation energy is about 83-94 kcal mol ) ... 

The chain scission also can start truly randomly and not only at the weaker bond. For polymers containing linear backbones, in addition to p-scissions, it is possible to have a-scissions, methyl scissions or even hydrogen scissions. The scission of a C-H bond is thermodynamically unfavorable at low temperatures and is not too common at temperatures where the other scission can take place. The a-scission is more frequent. It refers to the breaking of a o bond to an sp2 carbon. For polystyrene for example, the a-scission leads to the formation of a phenyl radical and a polymeric radical (and it is not a chain scission). 

The value of 2 can be used to find a chain scission force//, = arraZ(,/2 if the crazing stress ocraze is known, using reasoning similar to that used for the transition from chain pullout to crazing considered in the previous section. This fracture force should be characteristic of the fracture of a C-C bond and therefore not only independent of N, but also independent of the detailed molecular structure of the polymer. This appears to be indeed the case and the results obtained for several experimental systems [22,37,39,40,46] all yield a very similar value of/, -2x10 N. 

Fig. 17. Dependence of rates of crosslinking and scission on the composition of methyl methacrylate copolymers irradiated in the form of films and A chain scission, crosslinking [reproduced from Ref. 89].

Zhurkov and his collaborators (77) presented a mechanism, by which of a micro-crack is initiated by a pair of mechano-radicals and is formed in a solid polymer under large stress. They assumed a pair formation of the mechano-radicals in a highly strained polymer, as shown as b in Fig. 11. The radicals primarily produced by a chain scission are end radicals having an unpaired electron at the chain end. Each of these end radicals subtracts a hydrogen from neighbouring polymers to leave two chain radicals, in which an unpaired electron exists in the middle of a chain (c in Fig. 11). By this hydrogen subtraction the borken ends are stabilized, while the two chain radicals convert into the two stable ends and the two end radicals, according to the Zhurkov s mechanism (d in Fig. 11). Then, the two end radicals are reproduced... 

In the case of PS, even subtle properties of the reachon could be revealed, e.g., a Diels-Alder rearrangement product due to thermal inihahon as well as a chain scission product due to the MALDI condihons. °... 

Higher conversions in thiirane polymerizations, however, proceed with a chain scission... 

The degradation seems to be heterogeneous and to be limited to the cathode region. Phenolic and carboxylic acid chain ends are the main products that result from a chain scission reaction of the polymer [88]. 

The chemical stability against chain scission and oxygen attack also plays a role in the application of plastics at still higher temperatures. The statistical probability of chain scission is indeed practically the same for linear chains, cross-linked network polymers, and ladder polymers. But, whereas, a chain scission in linear polymers leads to lower degrees of polymerization because of chain degradation, with consequent diminished mechanical properties, chain scission with, for example, a ladder polymer, still leaves one chain of the ladder intact since it is improbable that both chains of the ladder should break at exactly the same distance from each end (see also Chapter 23). 

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