ISSN:
0022-3832
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Purified natural rubber has been exposed in vacuo to the 4-M.e.v. electron beam from a linear accelerator and attempts have been made to estimate the ratio of fractures of the main chain of carbon atoms to crosslinks, using three different methods. Measurements of the sol fraction (S) as a function of the radiation dose (R) did not conform to a linear relationship between S + S1/2 and R-1 reported by Charlesby and Pinner for a number of other polymers. It appeared that in the present experiments there was little consummated random fracture, and it was found that the sol fraction tended towards zero at radiation doses above about 50 Mrad. The stress in rubbers maintained at constant elongation decreased continuously on irradiation, and it was found that rigorous exclusion of traces of air and impurities was of critical importance. Deficiencies of previous analyses of stress-relaxation data are discussed and an attempt made to provide an internally consistent treatment of the data. In one treatment which made no allowance for the possible contribution of chain entanglements a value for β of 0.18 was obtained. In another treatment an attempt was made to allow for entanglements and a much lower value for β of 0.03 was obtained. This very low value may be due to the inadequacy of the assumption made that fracture inevitably releases entanglements under conditions where a much higher rate of concurrent crosslinking may serve to lock them into the network. Further work is required to establish a reliable basis for the interpretation of stress-relaxation data. In a third method stress-strain measurments of irradiated rubber provided estimates of the number-average molecular weights of both (1) the chain segments between adjacent crosslinks and (2) the primary molecular chains. Comparison with similar data for vulcanizates prepared by thermal decomposition of di-tert-butyl peroxide gave a value for β of 0.11. This value was calculated on the assumption that no fracture occurs on formation of the peroxide vulcanizates and is, therefore, likely to be an underestimate. The nominal G(X) value of 0.9 is increased to 1.05 when an allowance is made for fracture.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1960.1204314104
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