The cross-linking and degradation of paraffin chains by high-energy radiation

Abstract

n -Paraffins from C 7 H 16 to C 36 H 74 and polyethylene polymers (Polythene and Winnothene) have been subjected to atomic pile radiation. For the paraffins there is a decrease in the melting-point, until, for a radiation dose R , they no longer melt at temperatures of 160° C or above. At about this same radiation the paraffin is turned into an insoluble gel. The product Rn ρ, where n is the number of carbons per atom, and ρ the density, is approximately constant from heptane (n = 7) to Polythene (n ~ 2000), although an anomaly may occur for Winnothene (n ~ 250). This indicates that the energy required to form a cross-link is approximately independent of chain length. An analysis of published experiments on methane and butane extends this conclusion down to n = 1. The results obtained by earlier workers when paraffinic gases are bombarded with deuterons and α-particles are explained in terms of the cross-linking phenomenon. Solubility measurements give similar values for Rn ρ in the case of Polythene and Winnothene, and show that for every cross-link formed, on the average about 0·35 C—C bonds in the main chain are fractured. Similar values are obtained for methane and butane. The energy absorbed per C—H bond fracture is about 12 eV, and the energy per cross-link is 24 eV. This corresponds to 0·5 % of carbons becoming cross-linked per unit radiation, independent of the physical state (solid, liquid or gaseous) of the irradiated paraffin. The importance of these results, as far as polymerization theory is concerned, is briefly discussed.