Radical fracture surfaces

A probe of the departure of the fracture surface from equilibrium. As the charge and defects/free radicals decay away, FE may well be a measure of the initial concentrations and the rate of surface reactions such as defect recombination. 

The fracture molecular mechanism as well as the mechano-radicals existence were also proved by grafting reactions with unsaturated monomers, at the fracture surfaces. The reactions with the sable radical DPPH, or grafting reactions occur as follow. 

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. 

Talking into consideration the actually observed linear dependence of logarithm of lifetime on applied load for polymer samples, the non-linear correlation of these two quantities for a covalent chemical bond, the small fraction of ruptured chains, and the identical activation energy for sample failure and plastic drawing one concludes that the failure of a sample in creep is primarily a consequence of mutual shear displacement of adjacent microfibrillar elements, i.e., a frictional mode, which leads to gradual formation of microcracks at their ends. As soon as by axial and radical coalescence of such microcracks a critical size crack is formed the sample fails catastrophically with the chajracteristic fibrillar fracture surface. The rupture of polymer chains is the consequence but not the primary cause of microfibrillar displacement and ensuing sample failure. 

This is consistent with the observations of Andrews and Reed who observed hydrogen gas foaming from the rubber after the yield point was reached. They attributed the evolution of hydrogen gas to free radical formation caused by main chain fracture and subsequent reactions [1]. Microscopic examination of the samples after failure occurred revealed differences in the character of the fracture surface. Micrographs of the fracture surfaces taken at lOOX showed that the surfaces corresponding to ductile failure were much smoother than those for brittle fracture. 

Samples of the second type, tensile specimens, are in principle suitable for studying the time-dependent formation of free radicals and their effect on strength and on other macroscopic properties. It can be said a priori that the free radical intensities of tensile specimens — be it before or after rupture — will be very much lower than the intensities found in finely ground polymers with fracture surfaces of several thousand cm /g. Even with the increased sensitivity of presently available ESR spec-... 

Secondly there are direct techniques, notably electron spin resonance spectroscopy (ESR), in which the free radicals produced by the fracture of covalent bonds are directly observed, both in respect of their chemical nature and their number. Much of this review is orncemed with the results of ESR studies and this technique is therefore treated at some length below. One little used technique for the direct assessment of free radicals produced by n chanical means is that of Pazonyi et al and Salloum and Eckert They dropped various polymers in an ethanolic solution of diphenyl picryl hydrazyl, a chemical indicator, and determined the free radical concentration in the cut surfaces by colorimetrie measurements of the colour change. This method is subject to soixm uncertainty on account of possible side reactions. 

Numerous investigations in the last two decades have questioned whether, in the absence of voids, different mechanisms such as radical mechanisms are responsible for the continued initiation and propagation in soUd or viscous substances. As with initiation, recent research has tended to emphasize contrasting viewpoints, and these are reflected in the discussions of Chapter 9. There are macroscopic models in which surfaces, friction, fracture, shear, and voids continue to play a role in the phenomenology of detonation, and the temperatures and pressures of fast decomposition serve to fortify the initial mechanisms. 

It is by no means certain that the mechanism of fracture-induced decomposition reaction will be the same as that described for slow decomposition. For example, when the crack is running at its maximum velocity, chemical bonds are broken on a time scale comparable to that required for a bond to make one vibration. It can be imagined, therefore, that those bonds which are near their maximum amplitude of vibration when the crack approaches wiU be most likely to break, probably leaving the surface in a highly active state and producing free radicals in the gas phase. Such a process was envisaged by Taylor and Weale... 

Scissions of main-chains by the mechanical destruction of the polymers are experimentally proved by the analyses of the observed ESR spectra for the various pdy-mers PE, PTFE, PB, PP and PMMA. A pair formation of the radicals, (mechano-radicals), after the milling is clearly demonstrated and this pair formation is believed to be the direct evidence for tl mechano-radicals formed primarily by the medianical actions. A model for chain rupture in an amorphous pdymer was proposed. Excess electrons produced by the triboelectricity due to the friction, diich is always accompanied with the mechanical fracture, play an important role, with coexistence of oxygen, in the thermal conversion of the mechano-radicals. The characteristic behaviors of the mechano-radicals, the hi er reactivity with oxygen, complete photoconversion of the peroxy radical, indicate that the mechano-radicak are formed and trapped on the fresh surfaces produced by cleavage in the solid polymer. The polymerizations initiated at the low temperatures by the PTFE mechano-radicals were reported and the copol5mierization is experimentally evidenced. 

According to electron spin resonance data, free radicals are produced at chain ends even before a macroscopic break occurs. The free radical concentration depends only on the extension, and not on the tensile stress. Concentrations of lO -lO free radicals/cm are generally observed. Since free radical concentrations of only about 10 free radicals/cm occur on the surface, free radicals must form in the test sample interior, that is, from the breaking of polymer chains. In addition, chemical decomposition products are produced by a ductile break, but not by a brittle fracture. 

It was confirmed that the fracture of the ethylene monomers at 77 K produced no free radicals. The quintet ESR spectrum shown in Fig. 7.24 can be undoubtedly attributed to the propagating radical of polyethylene, —CpHp2 (Ca ) H 2, when both the polymers and the monomers are simultaneously fractured. The quintet is due to hyperfine splitting of two a- and two fi-hydrogen nuclei. No trace of the Pl FE radical was detected in the observed spectrum. Accordingly the polymerization of ethylene, which was proved by ESR, had been initiated not by the ethylene radicals but by the PTFE mechano radicals at as low a temperature as 77 K. This extremely high reactivity of the radicals is rather surprising because both PTFE and ethylene react in the solid state at 77 K. The mechano radicals newly created by the chain scission are surrounded by the monomer molecules because the radicals are trapped in the fresh surface formed by the mechanical destruction. 

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