The Cross-Linking of Rubber by Pile Radiation

The degree of cross-linking produced in a rubber by high-energy radiation is proportional to the radiation dose. Unit radiation, as defined in the text, links 1.1 per cent of the isoprene units. The distribution of molecular weight prior to cross-linking agrees with a Poisson distribution. Gel formation begins for γ=0.5. From the radiation dose required to initiate gel formation, the initial average molecular weight can be deduced. The increase of gel fraction with radiation dose follows the relationship deduced theoretically in the first part of the article. Measurement of gel fraction gives an alternative method of calculating the initial average molecular weight. Where some cross-linking is present in the rubber prior to cross-linking, this may be evaluated. In accordance with the theory presented in the article, the viscosity of the sol fraction rises initially, then decreases as the radiation dose increases. This provides a third method of measuring molecular weight, or of relating viscosity to molecular weight, which can be deduced from measurement of gel fraction. The swelling of very lightly cross-linked gel has been compared with the Flory-Huggins relationship, which is found to hold down to very lightly cross-linked gels for which the cross-linking index is only 0.2. To obtain this agreement, it is necessary to consider the swelling of the dry gel, rather than the whole specimen, and to ignore the cross-links required to form the gel itself.