A. Le Moël

Electron trapping in amorphous SiO2 studied by charge buildup under electron bombardment

Electron bombardment of thick pure SiO2 induces the buildup of a negative charge which can be observed through a ‘‘mirror’’ effect in a conventional Auger scanning microscope. A mechanism for the creation of this charge is proposed in terms of trapping of an electron in defects due to the irradiating beam. The influence of temperature is studied on amorphous and monocrystalline SiO2. The temperature dependence of the existence of high negative charge shows around 270 °C an anomalous effect which depends on the irradiation time. The role of electronic excitation to produce defects in silica is discussed.

Physico-chemical modifications induced in polymers by swift heavy ions☆

Abstract We have studied the effects of dense electronic excitations on the physico-chemical modifications of polymers by performing heavy ion irradiations in the energy range of some MeV amu −1 . The effects induced by swift heavy ion irradiations were compared to those induced by MeV electron irradiations performed in similar experimental conditions in polyethylene and poly(vinylidene difluoride) films. The irradiated samples were analysed ex situ by means of Fourier transform infrared spectroscopy in the transmission mode. High electronic stopping power, (d E /d x ) e , irradiations are characterised by a significant increase of the yield of chain scission. Vinyl terminal unsaturations are recorded with radiochemical yields increasing markedly as (d E /d x ) e increases. Moreover, characteristic bands of alkyne terminal groups were observed only above a threshold of (d E /d x ) e . The overall set of results is analysed taking into account the radial dose deposition predicted by numerical calculations and the defect creation measured after electron irradiations. This allows us to elucidate the kind of chemical modifications influenced by the local dose rate; i.e. for which a specific effect due to a dense electronic excitation appears.

Degradation of segmented poly(etherurethane) Tecoflex® induced by electron beam irradiation: Characterization and evaluation

Abstract We have studied the influence of electron beam irradiation on a polyurethane Tecoflex ® (TFX) used in medical applications; this study has been performed in order to evaluate the capability of such materials to be sterilized by electrons in industrial conditions. With this aim, thin films have been prepared and have been irradiated under a dose-rate of 5 MGy h −1 , with absorbed doses varying from 25 to 1000 kGy under O 2 . Analytical techniques used were size exclusion chromatography (SEC) and Fourier transform infrared spectroscopy (FTIR). Evolved gas analysis has been performed using thermogravimetric analysis (TGA) coupled with FTIR spectroscopy (TG–FTIR). TFX films analyzed by SEC showed simultaneous scission and cross-linking that were both increasing with the irradiation dose. Various modifications of FTIR spectra were induced, with appearance of oxidation groups, identified as mainly formates, esters and carboxylic acids. Scission of chains were localized in soft (SS) and hard (HS) segments by decrease of both urethane and aliphatic ether absorbance. Finally, TG–FTIR analysis confirmed previous results: TG analysis of non-irradiated films showed a two-steps profile that was globally shifted to lower temperatures after irradiation. The coupling with FTIR allowed identification of degradation molecules: (i) oxidized SS fragments, (ii) long SS slightly oxidized and (iii) HS accompanied by SS.

Functionalisation of PAA radiation grafted PVDF

The covalent bonding of amino-terminated molecules was performed onto acrylic acid radiation induced grafting poly(vinylidene fluoride) (PVDF). The polymer was irradiated with different ionizing radiation: swift heavy ions or electrons. The polymerization of acrylic acid was then performed to confer to PVDF carboxyl groups suitable for condensation with the amino groups of the other molecule. Acrylic acid swelling of PVDF films was investigated as a function of temperature and monomer concentration in order to anticipate the best grafting conditions. Grafted and functionalized films were characterized using infrared spectroscopy (transmission and ATR), and weighing measurements. The PVDF-g-PAA films exhibit different structures depending on the monomer concentration. Immobilization of an amino-terminated molecule and a peptide onto PVDF was achieved using water soluble carbodiimide.

Modifications of polyvinylidene fluoride (PVDF) under high energy heavy ion, X-ray and electron irradiation studied by X-ray photoelectron spectroscopy*

Polyvinylidene fluoride (PVDF) samples have been irradiated with energetic heavy ions. Their structural and electronic modifications were studied by X-ray photoelectron spectroscopy (XPS) using a monochromatised X-ray source. A non-monochromatised X-ray source was used to irradiate the sample and to study the modifications induced by both X-rays and electrons. In addition a foil of polymer which was subjected to energetic heavy ions irradiation in the GANIL accelerator was also studied and compared with the samples irradiated with electron and X-ray. Changes in the shape, intensity and energy position of the C 1s line enable us to suggest a two step mechanism for the polymer evolution after irradiation: i) creation of allenes produced by dehydrofluorination of the polymer backbone; ii) disappearance of the double bonds by addition of species (atoms or free readicals) generated under the beam.

Modifications induced in polyvinylidene fluoride by energetic ions

Abstract Modifications induced by energetic ions have been investigated in the regime of electronic stopping power by X-ray photoemission spectroscopy (XPS), infrared absorption spectroscopy (IRAS) and differential scanning calorimetry (DSC). Stacks of thin foils of polyvinylidene fluoride (PVDF) have been exposed to ion beams with energies ranging from 1 to 50 MeV/amu and with different atomic number Z ( 16 O , 84 Kr , 129 Xe ) . XPS measurements provide information about the surface modifications of irradiated PVDF: (1) oxygen ions bombardment leads to the creation of cumulene compounds (CCC)n following hydrogen fluoride desorption, the intensity of which increases with the irradiation dose δE, the fluence φt of the ion and the energy loss ΔE in the polymer; (2) for krypton or xenon irradiation, no changes can be detected on the surface composition. Differential scanning calorimetry (DSC) performed on a pristine sample and on PVDF irradiated with xenon ions shows a decrease of the melting point TFand of the enthalpy ΔHF with the irradiation dose δE. This behaviour is characteristic of a loss of crystallinity due to the internal reorganization of the polymer (creation of polyallenic compounds, chain scission, decrease of the molecular weight). Infrared transmission spectra (IRAS) and attenuated total reflexion spectroscopy (ATR) show that only surface modifications are induced by oxygen ions. For energetic heavy ions (Kr, Xe), the bulk of the polymer is loaded with unsaturated compounds like allenes (2038, 1720, 1610 and 1560 cm−1), and is depleted in fluorine atoms. On the other hand, the PVDF surface studied in ATR mode exibits a much smaller evolution. There results are consistent with XPS data. All these experimental data can be explained by gas formation and desorption, migration in the bulk of reactive species like free radicals and internal recombinations.

Kinetics and characterization of radiation-induced grafting of styrene on fluoropolymers

Abstract Grafting of styrene solution onto poly(ethylene-co-tetrafluoroethylene) (ETFE) was carried out by the pre-irradiation method. ETFE films were irradiated by 1.5 MeV electron beams in air. The influence of grafting temperature (50 to 80°C) has been investigated. It was found that the saturation grafting yield and the initial rate follow an Arrhenius law. The volume grafting yields were measured by FTIR spectroscopy in transmission and by weighing and the ‘surface’ grafting yields by FTIR-ATR. The results showed that grafting reaction is not monomer diffusion controlled in 30 μm film, nevertheless heterogeneities are revealed. By in-situ ESR, the decay of peroxy radicals was recorded under various heating and grafting conditions. These experiments suggest that the peroxy radicals react rapidly with monomer, but do not initiate the grafting process. The propagating radicals were not detectable, which may indicate that polystyrene chains are very long.

A FTIR and SEM study of PS radiation grafted fluoropolymers: influence of the nature of the ionizing radiation on the film structure

This study deals with the structure of polymeric films obtained by radiation grafting of polystyrene (PS) in poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride/hexafluoropropylene) P(VDF/HFP). These fluoropolymers differ by the small HFP amount present in the copolymer, which plasticizes PVDF and favors the chain mobility. Peroxide grafting kinetic results obtained according to different parameters such as the irradiation type (swift heavy ions and γ-rays), the absorbed dose, the grafting time and the substrate nature are presented. The lowest absorbed dose is 10 kGy, which is enough to initiate significant grafting. Fourier Transform IR, transmission and Internal Reflection Spectroscopy are used to calculate the PS grafting yields at different depths. In the case of PVDF only, a PS gradient in the thickness is observed which is higher in the case of a γ-rays initiation than in that of a swift heavy ions one. This might be explained by the fact that the PS diffusion is accelerated in the latent tracks formed after the heavy ions irradiation. Whatever the radiation type, the grafting yield is higher in PVDF than in its copolymer, which is due to the difference of crystallinity. The PS layer formation is observed by Scanning Electron Microscopy. At small grafting yields, the PVDF spherulites are covered with a network of PS that tends to form a continuous layer when increasing the grafting yield; whereas in the case of copolymer, PS seems to penetrate faster in the bulk, so that the formation of the superficial layer is delayed. Our results show that using different kinds of ionizing radiations induces differences in the structure of the grafted films.

Electronic and structural modification of polyvinylidene fluoride under high energy oxygen ion irradiation

Stacks of polyvinylidene fluoride (PVDF) foils were exposed to an oxygen beam of 800 MeV. Electronic and structural modifications induced by the radiations were studied by X-ray photoelectron spectroscopy (XPS) using a monochromatized X-ray Al K α source, and by electron spin resonance (ESR) experiments. It was observed that i) the creation of allenes compounds produced by the dehydrofluorination of PVDF; ii) the presence of primary free radicals and peroxide radicals. The amount of the created defects is related to the electronic ion stopping power d E /d x and to the deposited dose.

A study on radiation grafting of styrene induced by swift heavy-ions in poly(vinylidene fluoride)

Abstract Radiation grafting of polymers is nowadays a rather “classical” way of modifying the physicochemical properties of polymers: γ-rays or electrons are used to induce reactive sites on the polymer chains from which can be initiated the polymerisation of monomers different from the initial irradiated polymer. Taking into account the homogeneous distribution of these sites, the final copolymer is homogeneously grafted. Swift heavy ions are another type of ionising particles. These high energy particles create on their wake through the solid a high density of excitations and ionisations which induce a cylindrical damage zone called the latent track. The radicals formed in the latent track can be used to initiate the grafting. We present a study on the post-irradiation grafting of styrene in poly(vinylidene fluoride) (PVDF) induced by swift heavy O and Xe ions (energies > 1 MeV amu−1). The evolution of the grafting yield and the grafting rate with the absorbed dose or fluence shows differences depending on the type of ion used. Higher yields are obtained when the grafting is induced by swift heavy ions rather than by γ-rays.