Irradiation of Atactic Polystyrene: Linear Energy Transfer Effects

Abstract

Atactic glassy polystyrene (PS) has been irradiated in anoxic conditions by electron and ion beams. The induced modifications were followed, in situ, by Fourier transform infrared spectroscopy (FTIR). In-film modifications and hydrocarbon gas release were followed. In-situ measurements allowed one to avoid any spurious oxidation of the films after irradiation and also permitted studying in detail the evolution with dose of the FTIR spectra. The data were quantitatively analyzed, and we present a complete analysis of the effects of the Linear Energy Transfer (LET) on the radiation chemical yields of several radiation-induced modifications (alkynes, allenes, alkenes, benzene, and disubstituted benzenes). For a better understanding of the LET effects, the in-film modifications are compared to H2 release data from the literature and to our measurements of hydrocarbon gaseous molecule yields obtained by us. The overall destruction yield becomes very significant at high LET, and the radiation sensitivity of this aromatic polymer merges with typical values of aliphatic polymers: the radiation resistance conferred at low LET to polystyrene by the phenyl side groups is lost at high LET. This loss of radiation resistance equally affects the aromatic and aliphatic moieties. Monosubstituted alkynes are created above a LET threshold, whereas the other radiation-induced modifications are observed in the whole LET range. Several observations indicate that the phenyl ring is broken at high LET. Comparison of the alkyne yield in PS, polyethylene, and polycarbonate as well as the formation of nitrile bonds in poly(vinylpyridine- co-styrene) are consistent with a cleavage of the phenyl ring as the prominent source of alkynes. As the competing damage mechanisms do not have the same LET evolution, the relative importance of a specific modification on the global damage depends on LET. Some (benzene and disubstituted benzenes) dominate at low LET, while others (in-film alkyne and acetylene release) dominate at high LET.

Energy migration effect on the formation mechanism of different unsaturations in ethylene/styrene random copolymers

“…This result is similar to results obtained on our previous work: when LET increases, i.e. when excitation and ionization density increases, the radiation protection given by the benzene ring is lost [25].…”

“…Results obtained when irradiating polystyrene, PS, could give some insight on the chemical modifications presumably induced in the aromatic moiety of PES. The major modifications observed in irradiated PS are trans-vinylenes, vinylidenes, benzene and alkynes [25]. Vinyl creation cannot be ruled out [26] but this group is likely formed at a much lower yield.…”

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“…5). This compares well, for instance, with the quadratic dependence of damage cross sections on stopping power in poly(p-phenylene sulfide) (PPS), which is independent on the chemical nature of the bonds [26]. This non-linearity implies that the dose from which the IBIL of BC-400 starts deteriorating is variable and depends on the stopping power of the particles.…”

Temperature and LET effects on radiation-induced modifications in non-perfect polyethylenes

“…In our previous work focused on the effect of ester bonds position effect in methylene containing polymers, we have observed that in-chain defects are substantially more efficient that defects position on side-chains [6]. The second hypothesis comes from the fact that the radiation protection effects of protective bonds are diminished when LET is increasing [3,4,26]. It might be concluded that even at low LET, vinyl bonds are less effective than in-chain double bonds to protect polyethylene.…”

“…Regardless of the position of the energy sink, it has been shown by Chang and LaVerne [4] and by Ferry et al [26] that increasing LET induces a loss of radiation protection efficiency, which implies a loss of energy and radical migration. As on side-chains, energy transfers are already drastically reduced compared to in-chains migration under low LET ionizing rays, it can probably be supposed that their protection efficiency will be lost at a LET lower than the one needed to lose the protection induced from in-chains defects.…”