Gamma radiolysis of dilute polyisobutylene solutions: influence of solvent

Abstract

Solutions of polyisobutylene have been irradiated with gamma rays from a Cobalt source. One percent concentrations (w/v) in the solvents methyl cyclohexane, toluene, and carbon tetrachloride were used, and of the solution in each solvent one portion was subjected to a ca. 10 kGy dose and another to a ca. 25 kGy dose. No detectable cross-linking occurred as a result of the radiolyses, but there were drastic reductions of MW in all cases, the effect being greatest for CCl 4 and smallest for toluene as solvent. The radiation damage was also assessed as the number of chain scissions caused, and in terms of the G values for scission. Attempts were made to correlate the latter parameter with properties of the solvents as indicated by their Hildebrand Solubility Parameters and by their C s values (chain transfer constants), and also with the reactivities of radicals resulting from solvent scission. The results indicate that neither the hydrodynamic volume of the polymer molecules in solution, nor the tendency of the solvent molecule to donate an atom to a polymer radical, have a predominant influence on the mechanism. It is concluded that in dilute solution, the direct interaction of the gamma radiatiuon with the polymer is not the important factor in determining the overall radiation damage, but that the predominant mechanism involves primary radiolysis of solvent molecules to form products which then attack the polymer molecules to cause the scissions. On this basis, recommendations can be made about choice of solvents for polymers to be used in a radiative environment.