Chain scission, stress-induced

The G value for crosslinking may also be estimated from the sol-gel data. In the present study the crosslinking rate of stressed samples was essentially the same as for unstressed samples, but the rate of scission in stressed samples was zero compared with 0.013 in the absence of stress. (If one were to envisage the stress plus radiation-biased creep as being caused by radiation scission followed by stress-induced flow of the scissioned chain segments, one could expect a higher rate of perma-... 

The influence of solvents has been touched on in Sect. 2.3.2. In fact, the influence of environment on crazing and fracture properties of polymers is of major importance in the practical uses of these materials. There are many ways for the environment to induce fracture by means of stress cracking, stress crazing, chain scission, chain crosslinking, etc. Therefore, environmental fracture has been widely studied, specially from the experimental point of view. Reference is a review of environmental cracking of polymers. Most work on environmental crazing has been done in liquid environments - (solvents and non solvents of the material), or high pressure gas environment, near condensation pressure (liquid nitro-... 

Straining the macromolecules result in generation of stresses that may activate some bonds. Mechanically induced chain scission has been explored for grafting polymers and rubbers. 

In general, a variety of mechanisms may contribute to the failure of actual components in service. These may include chemical degradation or oxidation a chemical mechanism that may induce cross-linking and chain-scission. Alternately, other physical processes may alter the state of the polsrmer (eg, surface active agents in the presence of stress may induce crazes due to local diflfiision of the agents near defects). These aspects are not discussed in this article. 

One may assume that in stress-free PE chains such transformations do occur with a rate of at least 1 s at a temperature somewhat below the melting point, i.e. at 400 K. One may then calculate the rate of this process at 300 K from Eq. (3.22) as 0.0018 s . If the chain is strained the activation energy for segment rotation only decreases and the rate of stress relieving transitions increases [19]. With such a time-scale one may consider stress-induced rotational transitions as being easily accomplished before a chain segment reaches elastic energies sufficient for chain scission at room temperature, i.e. more than 25 kcal/mol. 

If a semicrystalline microfibril is subjected to stresses, the resulting deformation will be non-homogeneous at the molecular level. The bulk of the deformation will be born by the amorphous regions. As discussed in Chapter 5 the largest stresses are transferred upon extended chain segments which share the strain imparted onto an amorphous region. The stress induced scission of chains must, therefore, be expected to occur within the amorphous regions. 

Fig. 8.17. Mechano-chemical reaction chain after Zakrevskii and Zhiurkov (17, 20, 551 (a) stress-induced chain scission, (b) formation of chain-end radicals, (c) radical reaction leading to main-chain radicals, (d) scission of radicalized chains, (e) formation of a submicrocrack by repetition of the steps (c) and (d) chain-end radical e.g. -CH2-CH2, x main-chain radical e.g. -CH2-CH-CH2-, stable end groups e.g. -CH2-CH3.

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