N. Betz

Swift heavy ion modification of polymers

Abstract We have studied the effects of dense electronic excitations on vinyl linear polymers, by performing swift heavy ion, SHI, irradiations. We used different ion beams, from C to Pb, provided by the GANIL accelerator at energies of a few MeV/u. We mainly studied the physico-chemical modifications induced in polyethylene (PE), but the role of chemical substitutions has also been investigated by studying the swift heavy ion induced damage in polyvinylidene fluoride (PVDF) and polystyrene (PS). The irradiated samples were analysed ex-situ by means of Fourier transform infrared spectroscopy in the transmission mode. Any contact with air was avoided: oxidation does not occur. Swift heavy ion irradiations are characterised by a significant increase of the yield of chain scission associated with unsaturated end groups. Moreover, we have evidences for specific modifications, i.e. modifications which are never observed when using low ionising particles as electrons or gamma-rays; namely the creation of alkyne and allene end groups. These specific modifications are little sensitive to chemical substitutions. For instance, alkyne formation is observed in PE, PVDF and PS. A simultaneous ionisation of the different atoms of a constituting monomer is likely needed for triggering the formation of the specific swift heavy ion induced modifications. On the other hand, if we pay attention to the physico-chemical modifications which are easily induced by low ionising particles, it is observed that increasing the electronic stopping power does not significantly modify the creation yield of, for instance in PE, trans-vinylene or trans-trans diene groups.

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.

Ion track grafting

Abstract Radiation grafting is a well known means to change the physical and chemical properties of a polymer. Ionising particles such as ions induce in their wake through the solid a continuous trail of excitations and ionisations leading to the formation of a latent track. Radiation grafting induced by swift heavy ions (E > 1 MeV/amu) in latent tracks is influenced by many parameters: irradiation parameters (absorbed dose, electronic stopping power), polymer substrate parameters (thickness, processing) and polymerisation parameters (monomer, grafting time). These parameters affect the polydispersity and the length of the grafted chains as well as the structure of the graft copolymer. Their influence is studied on semicrystalline polymers, the poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene. A review of the results obtained in our laboratory is presented. Due to the high electronic stopping power of the particles used (from 2 to 80 MeV cm2 mg−1), the grafting initiated by swift heavy ions is different from the one initiated with γ-rays. Moreover, the values of the grafting yield and the molecular mass distribution vary depending on the kind of ion used.

Structural study of polystyrene grafted in irradiated polyvinylidene fluoride thin films

Abstract A small-angle scattering study of polystyrene grafted in irradiated polyvinylidene fluoride thin films is presented. The effect of grafting is studied depending on the nature of irradiation and on parameters such as the grafting level and the dose. It is shown that the grafting in γ irradiated materials takes place in the amorphous zone while the grafting is located in the latent tracks after swift heavy irradiation. These conclusions are confirmed by preliminary wide-angle X-ray scattering and scanning electron microscopy studies.

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.

Design, Synthesis, and Evaluation of Original Carriers for Targeting Vascular Endothelial Growth Factor Receptor Interactions

PurposeAngiogenesis is a key event in tumor growth and metastasis, chronic inflammatory disease, and cardiovascular disease. It is controlled by positive and negative regulators, which include vascular endothelial growth factor (VEGF) as the most active of these. VEGF/VEGF receptors are important targets not only for therapy but also for imaging. Based on the structural study of VEGF, we developed a novel cyclopeptide (cyclo-VEGI) that exhibits powerful antitumor properties. We herein report the design of novel molecules derived from cyclo-VEGI as potential targeting agents in cancer and other angiogenesis-related diseases.MethodsWe performed selective chemical modification of the most active VEGF-derived cyclopeptide (cyclo-VEGI). Original hydrophilic linkers were synthesized and coupled to cyclo-VEGI. These reactions provide nanocarriers for delivery. The inhibitory effect of the different compounds on VEGF binding was evaluated in competition assays with 125I-VEGF. A fluorescent cyclo-VEGI peptide was synthezised to assess direct binding and internalization of cyclo-VEGI.ResultsChemical modifications of cyclo-VEGI do not diminish the biological activity of cyclo-VEGI as measured in competition assays; in fact, it is even increased. Moreover there is a strong cellular accumulation of the fluorescent-labeled cyclo-VEGI. Conjugates synthesized in this study may be useful leads to design delivery systems for targeting approaches in cancer and other angiogenesis-related diseases.ConclusionThe modified cyclo-VEGIs may have a wide range of applications and represent a useful tool to develop delivery/carrier systems for therapeutic targeting or imaging.

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.