Reactions and Means of Identification

For most cases of stress induced free radical formation this relation has been used for evaluation of time-dependent intensities. 

The number of free radicals necessary for detectable peak heights depends on the line shape and the effective volume of the spectrometer cavity. At the time covered by the referenced investigations (1960-1976) the lower limit of detectability of polymer radicals in the solid state can be given as 10 spins/cm.  

It may be mentioned that the number of free radicals present in a liquid or at the surface of a solid can also be determined by a non-spectroscopic method. This method exploits the color changes associated with the consumption of free radicals from a test solution e.g. of DPPH [39]. It should be emphasized again that great caution must be exercised in comparing absolute values of free radical concentrations obtained from differently shaped samples and/or in different cavities. According to general experience deviations of up to a factor of 2 must be expected. 

The assignment of the specific peaks to specific types of free radicals is not always obvious it sometimes involves highly intriguing techniques and the knowledge of related spectra and of chemical reactions to be expected [64, 67]. The main obstacles to be overcome are the broad line widths of resonance lines from solid specimens and the high rate of many radical reactions. It is evident that a large line width often prevents an effective resolution of nuclear hyperfine structure. The so-called 5 + 4 line spectrum of mechanically destroyed methacrylic polymers [4] is an example of a spectrum which had only been identified after comparison with the 16-line-spectrum of an aqueous solution of the polymerization radical of methacrylic acid. Thus it was learned that the former spectrum is the unresolved form of the latter and has to be assigned to the same radical [40]. 

The radicals I are secondary chain radicals which can only have been produced by the reaction of a hydrogen from a methylene group with an other - a primary — radical. Since the secondary radicals are reaction products they give no direct information on the nature and location of the primary process of mechanical degradation of the above solids. 

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