The effects of ion-beam irradiation of polymers

Abstract

Polymethacrylonitrile (PMCN) and polystyrene (PSt) were irradiated in the absence of O 2 with heavy-particle radiation: 275 MeV 20 10Ne 7+ ions, 6.5 MeV 40 18Ar 2+ ions and 180 MeV 40 18Ar 8+ ions. In the case of PMCN the 100 eV yield of main-chain scission was found to decrease with increasing LET (-d E/d x in MeV μm -1): G(S) = 3.3−3.6 at -d E/d x = 2 × 10 -4 ( 60Co γ-rays), G(S) = 0.5 at -d E/d x = 0.43 (Ne 7+ ions) and G(S) = 0.4 at -d E/d x = 2.5 (Ar 8+ ions). In the case of PSt the 100 eV yield of crosslinks was found to be independent of LET within the limits of measurement error: G(X) = 0.05 at -d E/d x = 2 × 10 -4 ( 60Co γ-rays), G(X) = 0.05 at -d E/d x = 0.37 (Ne 7+ ions), G(X) = 0.04 at -d E/d x = 2.2 (Ar 2+ ions) and G(X) = 0.04 at -d E/d x = 2.3 (Ar 8+ ions). These results are critically discussed with respect to the up to 100-fold higher G(X) values reported by other researchers for irradiation of PSt with high LET radiation. The dependence of G(S) on -d E/d x observed with PMCN, and also with polymethylmethacrylate, is explained in terms of an increasing probability of radical-radical reaction and an increasing probability of ionization and excitation occurring in neighboring repeating units with increasing LET of the radiation. The independence of G(X) found for PSt is explained in terms of crosslinking being a free-radical process, the occurence of which does not become less likely with increasing LET because of the extremely low free-radical yield in this polymer. Therefore, radical-radical reactions do not become more probable with increasing LET in this case. Poly-2-chloroethylmethacrylate (PMCMA) turned out to be a probe material to establish pronounced temperature increases in the polymer sample during irradiation at high LET: PMCMA was crosslinked to an insoluble gel, if not cooled carefully during irradiation. It underwent predominant main-chain scission, however, if sufficiently cooled.