Scission efficiency

It is possible to uniquely determine both the scission efficiency Gis) and the crosslinking efficiency G(x) via Equations 9 and 10 providing both the number average molecular weight (M ) and the weight average molecular weight (Af ) can be measured. 

The relative sensitivity of short-chain alkyl branches of different sizes to elimination on irradiation with formation of the corresponding alkane has been variously reported as being constant or varying (13,14). Figure 9 compares G values for formation of the alkane corresponding to the short-chain branch from samples of these three polymers with branch frequencies from 0.5 to 6 per 1,000 C atoms. There is a notably higher scission efficiency for ethyl branches. 

This technique has been used to investigate the distribution of short-chain branches in LDPE and the relatively higher scission efficiency of ethyl branches would rationalize the yields with 13C NMR measurements of branch frequencies (2). 

Figure 9. Relative scission efficiencies of short-chain branches in the 7 radiolysis of ethylene-a-olefin copolymers. Branch types are indicated.

Cobalt-60 y-Radiolysis Studies. Samples of PIPTBK powder were sealed under vacuum and exposed to 60Co 7-radiation at the National Bureau of Standards in Washington D.C. The samples received 0, 6, 12, and 20 Mrads and the number-average and weight-average molecular weight of each polymer, relative to polystyrene, were determined in THF by GPC. The scission efficiency and the cross-linking efficiency were determined from... 

In Equations 4 and 5, G (s) is the scission efficiency G 0c) is the cross-linking efficiency Mn and Mw are the number-average and weight-average molecular weights after exposure to a dose of D (Mn)0 and (Mw)0 are the initial... 

In an effort to increase the efficiency of scission, particularly in the solid-state where chain repair is most competitive, a number of external additives were auditioned. Since both the Xmax and the absorptivity of polysilane derivatives are functions of molecular weight, splitting the main polymer chain results in a rapid bleaching of the original absorption. This unusual feature can be used as a qualitative, diagnostic test of scission efficiency which greatly facilitates the testing of additives. 

Figure 7.5(a) shows the dependencies of site-selectivity and scission efficiency on gap length (number of unpaired nucleotides). When a 3-base gap is formed from T21 to A 23 in DNA(S3) by use of the oligonucleotide additives bearing monophosphate, this gap-site is selectively hydrolyzed by Ce(iv)/EDTA (lane 3). Furthermore, site-selective hydrolysis of a 2-base gap (lane 2) or a 1-base gap (lane 1) is also successful. The total... 

EDTA at pH 7.0 and 37°C. Lanes 1, one-base gap 2, two-base gap 3, three-base gap 4, five-base gap M, the markers (authentic oligonucleotides having 3 -OH termini). These gaps were formed in DNA S1 -DNA S5) (32P-labelled at the 5 -end) usingthe DNA l -L12-P/P-L12-DNA r combination, (b) Quantitative analysis of scission efficiencies (solid part is for the formation of 3,-OH terminus and the broken one for the formation of 3 -phosphate terminus). 

They assumed that the primary cation radical of PMMA spontaneously and quickly dissociated to form carbocation, which then recombined with the liberated electron to form an excited radical with a ferr-alkyl structure. This excited radical was thought to be the precursor of the scission of the main chain. This reaction model interpreted well their observation that the G value for the scission of the side chain was close to that of the main chain and that the mercaptan added to scavenge electrons suppressed the main-chain scission efficiently without affecting the formation of volatile products from the ester side chain. The above reaction model motivated us to apply the ESE method to the study of radicals in irradiated PMMA. The model now seems inadequate, because it cannot accommodate some recent ESE results as mentioned later. 

Some halogenated compounds increase the scission efficiency. Sulphur does not appreciably alter the scission yield, while nitrobenzene increases it. In the presence of nitrous oxide, 40% of the main-chain scission can be inhibited nitrogen and water are formed [391]. 

Kean ZS, Gossweiler GR, Kouznetsova TB, Hewage GB, Craig SL (2015) A coumarin dimer probe of mechanochemical scission efficiency in the sonochemical activation of chain-centered mechanophore polymers. Chem Commun 51 9157... 

A useful parameter is the scission efficiency, c, defined as the number of molecules of oxygen absorbed per cut in the polymer chain. Some collected data for this parameter are represented in Fig. 9.4. 

In spite of the objections to the various methods of measuring scission events, good 1 1 correlations are found between the scission efficiency values obtained by the various methods in the cases of natural rubber latex, uncross-linked natural rubber, uncross-linked gutta percha and peroxide-cured vulcanizates. On the other hand divergent results are obtained using the stress-relaxation technique with sulphur-cured vulcanizates. 

Fig. 9.4. Effect of temperature and composition on scission efficiency. The vertical lines indicate the range where a spread of results has been reported. (Data collected from various sources.)...

Calculated from G values of isolated hydrocarbons assuming scission efficiency of 0.073 X 10 per 1000 carbon atoms. Because short-chain branches outnumber chain ends by 30 1, no correction for the fragmentation products at chain ends was made. + Determined by infrared analysis. Determined by radiolysis. Reproduced with permission from P.M. Kamath and A. haclow. Journal of Polymer Science-. Polymer Chemistry Edition, 1967, 5, 2023. 1967, Wiley ... 

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