Chain scission acceleration

On the other hand, rubbers are exposed to repeated mechanical flexing and deformation in service. Previous studies [55] have shown that flex cracking of rubber materials is accelerated by the presence of oxygen. Environmental flex cracking is caused by oxidative chain scission of the rubber polymer at mechanically induced cracks. 

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. 

Kinetics of Chain Scissions during Accelerated Aging of Poly(ethylene oxide)... 

Similar trends have been observed in the carbon black (N347) filled, TBBS accelerated sulfur-vulcanisation of high-c/s-IIR. In contrast to the NR/CB system, the reversion reactions, i.e., the cis-to-trans isomerisation and the chain scission at 3,4-isoprene units, increase with black content during the overcuring. 

Finally, the effect of ordering processes must be considered in the interpretation of results from accelerated aging tests. The activation energies for the chain-scission reaction and for molecular ordering are likely to be quite different. Furthermore, enhancement of the ordering process through the reduction in DP caused by chain scission is likely to manifest itself as an autocatalytic effect wherein the rate of decay of... 

After inactivation treatment, the catalyst is removed, and phenolic antioxydants and other stabilizers are added. Cross-linking curing is realized on unsaturated pendant groups. Peroxides are avoided because they cause chain scission and therefore systems with sulfur as cross-linker and zinc oxide, 2-mercaptobenzothiazole and tetramethylthiuram monosulfide as accelerators are used. 

Thus, transesterification causes branching as well as chain scission Let us consider the branching statistics for the case of a polycarboxylic acid and polyepoxide with groups of independent reactivity. The use of the statistical method is justified because neither the substitution effect nor the initiated chain growth are operative. If only addition esterification occurs (and indeed this reaction can be selectively accelerated by special catalysts the statistics is analogous to the polyamine-polyepoxide addition and even simpler because the substitution effect is here absent. The pgf s for the number of bonds issuing from polyacid (C) and polyepoxide (E) units read ... 

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. 

Poly(acrylates) and (alkyl acrylates) - The 3-relaxation time of poly(methyl methacrylate) (PMMA) is broadened by UV irradiation, indicating that whilst chain scission is occurring depolymerisation is minimal. The presence of polybutadiene accelerates the photooxidation of PMMA as does poly(vinyl acetate) " when used as blends. 

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