Les travaux entrepris dans le cadre de cette thèse portent sur l’étude de la dégradation radiolytique de polymères constitutifs de l’enveloppe isolante de câbles électriques contrôle commande. Pour étudier les mécanismes de dégradation de l’isolant du câble, une approche originale à été suivie via l'utilisation de matériaux modèles, i.e un mélange réticulé non chargé à base d’EVA (Ethylène Vinyle Acétate) et d’EPDM (Ethylène Propylène Diène Monomère) et le composite correspondant chargé avec des ATH (trihydroxyde d’aluminium), ont été soumis à une irradiation gamma. Ces travaux s'inscrivent dans la perspective d'un souhait de prolongation de la période d’exploitation des centrales du parc électronucléaire français par l'exploitant EDF. Une analyse multi-échelle de la dégradation des matériaux modèles et de l’isolant a été effectuée pour comprendre l’évolution des propriétés des matériaux polymères induites par différents types de vieillissement (thermooxydatif et/ou radiooxydatif). Dans l’ensemble de ces conditions, Il a été établi que la dégradation du mélange EVA/EPDM et du composite EVA/EPDM/ATH est gouvernée par un mécanisme de coupures de chaînes, le point faible de la matrice polymère étant la partie vinyle acétate; de plus dans le cas de la radiooxydation, les processus purement radiolytiques jouent un rôle clé. À partir de courbes de corrélation, il a été montré que les propriétés physicochimiques des matériaux polymères, telles que l’évaluation par spectroscopie IR de la quantité de produits d’oxydation formés, la détermination de la fraction de gel, et les mesures d’OIT (Temps d’Induction à l’Oxydation), sont des indicateurs sensibles de vieillissement. L’évolution de ces trois critères étant plus précoce et plus progressive que la perte des propriétés mécaniques de l’isolant, on pourrait donc anticiper une perte brutale des propriétés mécaniques, voire de la fonctionnalité du câble, en faisant un suivi de leur évolution. Plus généralement, ces études soulignent l’importance de travailler à des débits de dose très faibles et à température ambiante pour que le vieillissement accéléré soit représentatif, ce qui a été fait dans le cadre de cette thèse. / This study is focused on the radiooxidative degradation of polymeric insulation of Instrument and Controle (I&C) electric cables. In order to investigate the degradation mechanisms of the insulation, an original approach has been implemented thanks to the use of model composites with ATH (Aluminium TriHydrate) filler and blends (without filler) based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) were submitted to gamma-irradiation. This work has been completed within the goal of extending the lifetime operation of French Nuclear Power Plants (NPPs) by EDF. Multi-scale analysis of the model materials and insulation were performed in order to investigate de modifications and the evolution of the polymer properties induced by different kinds of ageing (thermo and/or radiooxidation). After having highlighted that whatever the ageing conditions are, degradation of polymers is governed by scission process, it is shown that purely radiolytic processes play a key role in the overall degradation scenario. These studies based on correlation curves showed that physico-chemical properties of the polymer, such as the evaluation by InfraRed of the amount of the oxidation products formed, gel fraction and Oxidation Induction Time measurements are relevant indicators of ageing. The evolution of these properties during ageing is even, in the case of the insulation, earlier than the loss of mechanical properties, which may allow to anticipate a sudden loss of mechanical properties and therefore of the functionality of the cable insulation. More generally, these studies points out the needs of using the lowest dose rates possible and room temperature, as done in this work.

This PhD project has expanded the knowledge in the area of profluorescent nitroxides with regard to the synthesis and characterisations of novel profluorescent nitroxide probes as well as physical characterisation of the probe molecules in various polymer/physical environments. The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, was described. This novel nitroxide possesses some of the structural rigidity of the isoindoline class of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the integral aromatic ring imparts fluorescence that is switched on by radical scavenging reactions of the nitroxide, which makes it a sensitive probe for polymer degradation. In addition to the parent TMAO, 5 other azaphenalene derivatives were successfully synthesised. This new class of nitroxide was expected to have interesting redox properties when the structure was investigated by high-level ab initio molecular orbitals theory. This was expected to have implications with biological relevance as the calculated redox potentials for the azaphenalene ring class would make them potent antioxidant compounds. The redox potentials of 25 cyclic nitroxides from four different structural classes (pyrroline, piperidine, isoindoline and azaphenalene) were determined by cyclic voltammetry in acetonitrile. It was shown that potentials related to the one electron processes of the nitroxide were influenced by the type of ring system, ring substituents or groups surrounding the moiety. Favourable comparisons were found between theoretical and experimental potentials for pyrroline, piperidine and isoindoline ring classes. Substitution of these ring classes, were correctly calculated to have a small yet predictable effect on the potentials. The redox potentials of the azaphenalene ring class were underestimated by the calculations in all cases by at least a factor of two. This is believed to be due to another process influencing the redox potentials of the azaphenalene ring class which is not taken into account by the theoretical model. It was also possible to demonstrate the use of both azaphenalene and isoindoline nitroxides as additives for monitoring radical mediated damage that occurs in polypropylene as well as in more commercially relevant polyester resins. Polymer sample doped with nitroxide were exposed to both thermo-and photo-oxidative conditions with all nitroxides showing a protective effect. It was found that isoindoline nitroxides were able to indicate radical formation in polypropylene aged at elevated temperatures via fluorescence build-up. The azaphenalene nitroxide TMAO showed no such build-up of fluorescence. This was believed to be due to the more labile bond between the nitroxide and macromolecule and the protection may occur through a classical Denisov cycle, as is expected for commercially available HAS units. Finally, A new profluorescent dinitroxide, BTMIOA (9,10-bis(1,1,3,3- tetramethylisoindolin-2-yloxyl-5-yl)anthracene), was synthesised and shown to be a powerful probe for detecting changes during the initial stages of thermo-oxidative degradation of polypropylene. This probe, which contains a 9,10-diphenylanthracene core linked to two nitroxides, possesses strongly suppressed fluorescence due to quenching by the two nitroxide groups. This molecule also showed the greatest protective effect on thermo-oxidativly aged polypropylene. Most importantly, BTMIOA was found to be a valuable tool for imaging and mapping free-radical generation in polypropylene using fluorescence microscopy.