Model experiment on the protection effect in polymers: radiolysis of liquid squalane in the presence and absence of additives

Abstract

A model experiment to clarify the elementary processes in aliphatic polymers irradiated under vacuum has been carried out. Liquid squalane has been selected as the model of branched polyethylene, ethylene-propylene copolymers or polypropylene. The main gaseous product is H 2. The presence of methyl side groups increases the probability of main chain scission, and side chain scission produces methane as the main hydrocarbon gas product. The prominence of main chain scission and the chemical structure of the parent molecule are believed to be the reasons for formation of less oligomeric products. On the other hand, they promote the combination of the scission fragment with the parent radicals to produce intermediate compounds. In the presence of aromatic additives which have a lower ionization potential and lower excited state energy level than the squalane itself, energy and charge transfer to the added molecules take place and both solvent decomposition and product formation including H 2 evolution are significantly reduced. Due to these transfer processes, the added molecules react with squalane molecules to produce adduct compounds which are clearly observed from the mass analysis. A further inspection of the mass spectra shows that scavenging of hydrogen atoms occurs through hydrogenation of the benzene rings in the adducts. Since the adduct compounds of aromatic additives consume more hydrogen atoms than those of hydroaromatic additives, aromatic compounds reduce H 2 evolution more effectively than do hydroaromatic compounds. Since main chain scission is not affected by any of the additives, it is suggested that this scission process directly originates from the decomposition of the excited cations of squalane. The G-values in the radiolysis of liquid alkanes and base polymers are also compared.