Radiation effects scission

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

Much research into radiation effects on polymers is done with samples sealed under vacuum. However, polymer materials may, in practical applications, be subjected to irradiation in air. The effect of irradiation is usually substantially different in air, with increased scission at the expense of crosslinking, and the formation of peroxides and other oxygen-containing structures. Diffusion rates control the access of oxygen to radicals produced by the radiation, and at high dose rates, as in electron beams, and with thick samples, the behaviour may be similar to irradiation in vacuum. Surface changes may be quite different from bulk due to the relative availability of oxygen. 

Radiation effects of ion beams on polymers such as polystyrene have been studied using very quantitative, homogeneous, and energetically accurate irradiation data obtained by time-resolved and product analysis [30]. Recently main chain scission, ablative decomposition, and positive-negative inversion of PMMA induced by various ion beams have been investigated. The dependence of the beam energy and atomic number of incident ion beams on radiation effects has been considered. 

Recently spin-coated PMMA thin films with a thickness of 0.45 pm on silicon wafer were irradiated with various ion beams (H+, He+, N+, Ni3+). Ion beam energy regions are from 300 keV to 4 MeV. Irradiated PMMA films were developed by isopropyl alcohol in these experiments. After the irradiation by ion beams on PMMA in a vacuum, the thickness of the films were measured both before and after development. Various radiation effects on PMMA films such as ablation (sputtering), main chain scission, and positive-negative inversion were observed as shown in Fig. 11. These phenomena are very different from those in 60 Co gamma-ray or electron beam irradiation. Large LET effects are considered to be due to high density excitation by ion beams. 

The radiation effects on ethylene-propylene rubber (EPR) are modestly dependent on the ethylene content [63]. As the ethylene content is increased a shift to a larger yield for crosslinking occurs and the polymer is less prone to scission, the relationship is not linear since similar results for yields are found at 42 and 69% ethylene. 

Heavy ion irradiation of PMMA has also received some attention, particularly by researchers at the Japan Atomic Energy Research Institute. Kudoh and co-workers (269-273) have compared the effects of y radiation and electron beams with those for 30 and 45 MeV protons and heavier ion beams on the tensile and molecular weight properties of PMMA. They have found no difference between the sensitivities of the tensile properties of PMMA to y-rays and electron and proton beams as a function of dose. However, the molecular weight dependence for PMMA showed a clear LET dependence for heavy ions above a critical LET, indicating overlapping between spurs. For low LET radiation the scission probability remains constant, but the scission probability decreases with increasing LET for high LET radiation. The critical LET level for PMMA is a few hundred MeV cm /g. 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

The effect of EB-radiation dose intensity on the surface properties of surface and bulk-modified EPDM rubber have been investigated [382]. Predominant chain scission at higher radiation doses... 

Various studies have been made on the effects of radiation on lactide/glycolide polymers (24,38,58). Gilding and Reed (24) reported the effect of y rays on Dexon sutures. Those results confirmed that deterioration of the sutures occurs but that random chain scission is not the primary mechanism. Number average-molecular weight Mn showed a dramatic decrease at doses above 1.0 Mrad. Thus, unzipping of the polymer chain appeared to be the more dominant process, at least in the case of polyglycolide. 

---