Mechanism of protective action of benzene rings in radiolysis of polystyrene

Abstract

AbstractThe crosslinking energy in the radiolysis of polystyrene is 100‐fold that of polyethylene, whereas the free radical yield in the radiation of linear paraffins is only 5–6 times as much as that from alkylaromatic compounds. Therefore, the high radiation stability of polystyrene cannot be explained by the conventional scheme for the action of the benzene rings. In order to explain the high radiation stability of polystyrene the mechanism of the reactions taking place during radiolysis of the polymer must be examined. A hydrogen atom liberated as the result of rupture of a C–H bond under the action of radiation may react in two ways: (1) abstraction of a hydrogen atom from the aliphatic chain, and (2) addition to the double bond of the benzene ring with the formation of a free radical of the cyclohexadienyl type. Owing to the low rate of diffusion in the solid polymers and the high rate of reaction with atomic hydrogen, the radicals formed in this way should be in the immediate vicinity of the primary radical. Interaction of the latter with the radical formed by removal of a hydrogen atom from the aliphatic chain will lead to the formation of crosslinks. The interaction between the primary radical and the radical of the cyclohexadienyl type may take place in the following two ways. The radicals may recombine with each other to form crosslinks or they may engage in a disproportionation reaction. Disproportionation will lead to restoration of the initial state and hence to the dissipation of energy without crosslink formation. This will result in a drop in efficiency of the crosslinking process, the energy for the formation of one crosslink increasing. In order to obtain experimental verification of the above conceptions as to the mechanism of interaction of the primary and cyclohexadienyl radicals, the radiolysis of deuterated toluene C4H5CD3 was studied at the temperature of liquid nitrogen. The addition to the benzene ring of a deuterium atom formed on rupture of a C–D bond under the action of radiation and the subsequent disproportionation of the cyclohexadienyl and primary radicals should, owing to the isotope effect, lead to transition of deuterium atoms from the aliphatic chain to the ring. The results obtained showed that the rate of these processes should exceed by at least 5–6 fold the rate of the processes leading to crosslinkage. It thus follows that the high radiation stability of polystyrene is to a considerable extent associated with disproportionation of the primary and cyclohexadienyl radicals.