Chain scission, rupture

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

Supramolecular chain scission differs further from covalent chain scission, because supramolecular recombination is typically the predominant fate of a ruptured chain anthropomorphically speaking, the supramolecular moieties are, by their very nature, predisposed to reassociation rather than alternative reaction pathways. This predisposition is not intrinsic to the products of covalent bond mpture, which might lead either to high-energy intermediates with nonspecific reactivity or to species that require catalyst or elevated temperature to recombine. The... 

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. 

Several factors contribute to the high heat stability of these compounds. There are extremely strong bonds between the carbon atoms in the polymer backbone and the attached fluorine atoms [13]. These factors help the polymer resist chain scission. In addition, the high fluorine to hydrogen ratio and saturation of the backbone increase the strength and stability of that polymer backbone [13]. Table 8.9 shows some bond dissociation energies that must be exceeded to rupture the bond. 

Now let me try to get some insists into molecular mechanisms of the mechanic fracture in polymers 73). The l teilin model was originally proposed in ex]danation of the mechano-radical formatirm in the hi y stretched fibre. However, one can apply the Peterlin model to the fracture ptenomena in crystalline polymers, because large deformations proceed always in advance of a mechanical fracture. Thus, the tie molecules are assumed to be only parts vtdiich are broken in the case of destmction of bulk polymers. The fact that no mechano-radical is formed from the polymer having no tie mdecules even after the milling supports the interpretation mentioned ove. However, for amorjdious pd[ymers such as PMMA and PB, formation of the mechano-radkals is not attributed to the ruptures of the tie molecules, becau% neither the crystalline parts nor frie tie molecule exist in an amorphous polymer and no particular part of the polymer, on which the applied stress is concentrated, can be assumed in the amorphous polymer. It was found that the polymer chains are ruptured even in the case of an amorphous polymer, like PMMA, PB, and other elastomers, as mentioned in the Section III. The medianism other than the Peterlin model is needed to explain the bond scissions of polymer chains in the amorphous pdymers. 

In principle, the rupture of a fibril may occur by at least two or three mechanisms, which are not always clearly separable. One is the end point of the drawing (creep) mechanism at constant mass of polymer in the fibril. A condition of local necking down could develop, leading to failure (22). See Fig. 1. This would be more likely to exist with low than with high MW polymers. Alternatively, and particularly with high MW polymers, chain scission might occur. 

In the adiabatic case, with rapid advance of the failure front, the local temperature rise may lead to heating polymer fibrils to a range where their shear and extensional viscosity are so severely reduced that necking-down and rupture will quickly ensue. This means that there is a much greater probability that fibril rupture occurs by plastic necking down rather than chain scission, in a fast crack than in a slow one. 

It can be therefore concluded that the above results of the mechanochemical experiments are directly related to the slowing down of chain mobility upon deterioration of solvent quality. Consequently, these experiments are uniquely useful to study chain dynamics in semiconcentrated polymer solution as a function of the thermodynamic conditions at high shear rates. Hence, these experiments can give information about molecular parameters not being accessible by any other method. For example, one can quote the probability of chain scission along the backbones obtained by the full kinetic analysis of the data (cf. Sect. 3). This distribution of rupture sites is obviously connected with the distribution of strain along the chains, which may be probed by degradation experiments. 

Campbell and Peterlin and Peterlin concluded from e.s.r. measurements on isotropic and highly drawn nylon 6 and 6.6 fibres that no detectable free radicals were formed in the isotropic state, whereas approximately 1 chain in 250 was fractured in a fibre under high axial tension at failure. These fractured chains were later identified with the tie molecules linking adjacent crystallites together in the fibre direction. Quantitative theories have since been developed by Kausch et and more recently by DeVries et alP which attempt to correlate creep, creep-rupture, and stress-relaxation in fibres in terms of the measured main chain scission. 

Oxygen in the surrounding atmosphere is found to increase crack growth, presumably by an oxidative chain scission reaction catalyzed by mechanical rupture. The minimum energy Go is found to be somewhat larger for experiments carried out in vacuo [55,59,60]. When antioxidants are included in the elastomer formulation, then the results in an oxygen-containing atmosphere approach those obtained in vacuo. 

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