Spectroscopic study of chemical modifications induced by swift heavy ions on polymers: the contribution of the CIRIL Platform and the CIMAP Laboratory

Yvette Ngono-Ravache
2015 Journal of Physics, Conference Series  
This paper gathers results obtained on the chemical ageing of polymers, at the CIRIL platform, using Swift heavy ions (SHI) from the GANIL accelerator. Swift heavy ions induce high values of electronic stopping power or LET (Linear Energy Transfer) and deposit their energy in the polymer through electronic processes, in a few nanometer size cylinder centered on the ion path. This results in huge local doses and dose rates. Both defects created in the polymer chain and gas release were
more » ... ease were quantified using spectroscopic methods (FTIR and Residual gas analysis (RGA)). Defects created in polymers submitted to SHI can be separated in two main series: defects common to all ionizing radiations and defects specific to SHI. A common trend of the evolution of these defects, under inert environment, is the following: 1) for the first group of defects, in most of the polyolefins, there is a limited (if inexistent) effect of LET on the radiation chemical yield of creation at low doses. Among defects of this first series, the behavior of vinyl groups is particular, 2) LET effect on SHI specific defects (triple bonds and cumulenes) is tremendous. Triple bonds (alkynes, alkyl or aryl cyanates) are created after a LET threshold value, depending on the polymer chemical structure. The dose effect on macromolecular defects, under inert environment, is also presented. The study of the LET effect on gas release, in various polyolefines, gives an insight on the mechanism of bond cleavage in presence of high ionization and excitation densities. Finally, few results on radiation-induced oxidation are presented. Compared to low-ionizing radiations, oxidation is reduced and unsaturated bonds are created under SHI. CIRIL, 30 years of interdisciplinary research at GANIL polyethylene (PE) for goods wrapping are obtained by low dose e-beam irradiation. In the same way, wooden art craft or antiquity can be rehabilitated through -curing of polyester resins. Besides, some of polymer-based medical disposal are sterilized using either -rays or e-beam at low doses. Unfortunately there are situations were irradiation is detrimental for the polymers; that is the case for insulation sheath of electrical cables in nuclear plants or for polymers composing glove boxes used during nuclear fuel handling and analyses processes. In the latter case, polymers are submitted concomitantly to ,  and  rays. First studies on polymer ageing under ionizing radiation were performed using -rays and e-beam. Ion beam irradiation of polymers was belated. This started with the study of the behavior of photoresists used during semiconductor doping. The great development of ion beam irradiation of polymers is related first to the research of particle detectors for monitoring low fluences, and second for either commercial manufacture of filters or polymer micro-and nano-structuration. Ionizing radiations interact with the polymers through electronic processes, resulting in the creation of excitation and ionization. These initial species lead, after a time scale of around 10 -12 s, to the formation of chemical modifications (called hereafter defects) in the polymer and the subsequent mechanical and optical modifications. Organic synthetic polymers are defined as the repetition of a chemical unit, of small mass, through covalent bonds. They are composed of a backbone and, in some cases, have pendant groups or side chains. Polymers are very sensitive to ionizing radiations, as compared to other materials such as ceramics or steel. Ionizing radiations induce bonds scissions leading to chemical defects. These defects can be created either in the polymer chain in the form of new chemical groups (macromolecular defects) or as small molecules leaving the polymer in the form of gas (gas release). Both macromolecular defects and gas release are function of the polymer chemical structure and the irradiation conditions (projectile nature, environment, dose, dose rate, temperature...). Gamma-rays and e-beams deposit their energy in the material rather homogeneously. On the contrary, swift heavy ions (SHI) deposit their energy through electronic processes occurring close to the ion path: the latent track. Most of the primary excitations and ionizations occur in a track core of a few nanometers. High energy electrons, emitted during ion/polymers interactions, induce ionization/excitations at larger distances from the ion trajectory, in the so-called track halo. This halo is defined by the range of secondary electrons, which increases with the projectile velocity [1]. The huge amount of energy locally deposited by SHI within the track can induce specific damage processes, which involve complex molecular rearrangements and collective atoms motion. The LET induced by SHI in polymers are very high and lead to high ionizing and excitation densities. The comprehension of processes triggered by these high ionization/excitation densities was one of the motivation of the research, on SHI/polymers interactions, performed at CIRIL, using the GANIL beam lines, during the last 30 years. Basic research, aiming to the comprehension of the mechanisms leading to defects formation, as well as more applied research was considered. For the latter, the LET domain of interest is that of actinide-emitted  particles. In terms of LET, actinideemitted  particles are simulated by 11 to 13 MeV/A energy C-Ne ion beams. Both macromolecular defects and gas emission were studied and their evolutions as a function of, the polymer chemical structure, the irradiation environment (vacuum or oxygen), the irradiation temperature (8 K, 298 K), the dose, the dose rate and the LET were followed. Experimental One of the advantages of polymer irradiation studies performed at CIRIL is the use of on-line analyses. The analytical approaches are essentially Residual gas analysis (RGA), Fourier transformed infrared and UV-visible spectroscopies. Specific equipments enable to irradiate and analyze the material without removing it from the set-up. There are two main advantages for this procedure. The first advantage is to probe the same point of the sample, thus avoiding possible heterogeneity in the sample thickness. The second advantage is first to prevent the post-irradiation oxidation prior the analysis (as polymers are known to be sensitive to radiation-induced oxidation), and second to avoid annealing, prior to analysis, after low temperature irradiations. A large range of ions and energies was used for the actual studies. Most of them from the medium energy line of GANIL. CIRIL, 30 years of interdisciplinary research at GANIL
doi:10.1088/1742-6596/629/1/012006 fatcat:gumj5gztzzh2bnvmdbdmcezhlu