Quantum yield chain-breaking process

The quantum yield for the chain-breaking process in a polymer molecule is given by... 

The photolysis of poly-a-methylstyrene, both in solution [46] and as solid films [10], has been studied by Fox and coworkers. For solid films, a rapid drop in the molecular weight has been observed. The quantum yield of this process is 1 x 10-3 at 27°C and 2 x 10-2 at 115°C. Simultaneously, monomer is evolved with quantum yields of 7 x 10"3 and 5 x 10 1 at 27 and 115°C, respectively. Such behaviour has been ascribed to random breaking of the main chain followed by depolymerization [10] namely,... 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

If this process is combined with the chain reactions (2) and (3), one should obtain, in conjunction with a process for the disappearance of the free radicals (chain breaking), an adequate representation of the photochemical reaction. In fact, it has been shown that the experimental material, although incomplete in many respects, is in good agreement with such a mechanism. Kornfeld (26) found that the quantum yield for the decomposition is far greater than unity (values up to 50 were recorded) which clearly indicates the operation of a chain mechanism. The interaction of two radicals such as ... 

The following observations can be made with respect to the data contained in this table. The quantum yields for both onium salts are less than unity, suggesting that chain induced decomposition processes are not occurring in acetonitrile. As expected, the quantum yields are independent of the anions since they are both transparent to ultraviolet irradiation and do not participate directly in the initial bond breaking process. The discrepancy between the quantum yields for acid formation and either iodobenzene or diphenyl sulfide reported by Gatechair is particularly notable. 

Hence, for the current photochemical reaction the presence of quite typical feature shall be outlined. The fast is that the concentration increase of N in evacuated films from 0.2 to 2% causes equal, 6-fold decrease of initial quantum yields of N consumption (yN,o), polymeric chain breaks ( no) and structural-mechanical damage (yd) of the films under mechanical stress. The concentration slowing down of the mentioned processes is described by the empirical equation ... 

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