Free radicals molecular fracture

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced... 

As mentioned in the preceding section, the spin adducts in polyethylene and polypropylene fractured under the presence of nitrosobenzene at 77 K are nitroxide radicals at or near chain ends. Since it is reported that the mechano radicals (the free radicals produced by mechanical destruction) are trapped on the fractured surface the application of the spin trapping method is useful for examining the molecular motion of the molecules on the surface. 

Free radical formation was identified v/ith e.s.r. during the fracture process by Zurkov , Peterlin and DeVries . Thus, polymer wear due to the fatigue as well as the abrasive processes can lead to molecular degradation or thermal degradation of polymers. The degradation should predominantly take place on the polymer surface as shown by Bely and Sviridyonok. ... 

Mechanical degradation which leads to formation of free radicals can be achieved in various ways but generally two types of samples are being used in experiment powders of finely ground polymers as discussed above or uniaxially stressed fibers, monofilaments, or strips of films. In samples of the first type all molecular fracture processes are completed before the ESR investigation begins. These samples can primarily be used to obtain information on the nature and absolute number of free radicals present within the degraded polymer. 

The increase of temperature has a fourfold effect on the number of free radicals observed at the point of macroscopic fracture. Firstly as seen previously the bond strength is decreased and thus chain scission at a given molecular stress is facilitated. Secondly the decrease in intermolecular attraction and the increase in molecular mobility lead to more rapid relaxation of molecular stresses. For the same reason — thirdly — the density of stored elastic energy at a given strain level is decreased which will in turn affect crack stability and propagation. Fourthly, the increased reactivity of the free radicals may lead to an increased discrepancy between the concentrations of radicals formed and of radicals observed at the point of failure. 

In Sections I and II of this Chapter primary and secondary free radicals have been treated as microprobes which characterize occurrence and molecular environment of chain breakages. As shown in Chapter 6 the primary mechano-radicals are always chain end radicals which are mostly unstable. At a rate depending on temperature these radicals will transfer the free electrons and thus convert to secondary radicals. This reaction and also subsequent conversion and decay reactions including recombination are relevant with respect to an interpretation of the fracture process in two ways. Firstly these reactions interfere with a determination of the concentration and molecular environment of the original chain scission points. Secondly they change the physical properties of other network chains through the introduction of... 

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