I. Stevenson

Stability of polymers under ionising radiation: The many faces of radiation interactions with polymers

Ionising radiations have dramatic effects on the properties of polymers commonly used in hard radiation environments. From the early radiolysis studies, polymers have been classified into those undergoing mainly chain scissions and those being cross-linked. Real situations are however far more complex due to the semi-crystalline organisation of most polymers implying transition temperatures (glass transition, fusion temperature of the crystalline fraction), oxygen diffusion driven formation of oxidised species and effects of the polymer formulation (anti-oxidants, influence of the fillers). Progress has been made in the understanding of such phenomena and different examples have been considered illustrating the different faces of the polymer evolution under ionising radiation. Particular emphasis has been given to the changes of the final polymer properties in their conditions of use. Two examples of materials of widespread importance in nuclear technology: elastomers and epoxy resins, will be discussed.

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6 naphthalene dicarboxylate). I. Amorphous films

Abstract The dielectric and dynamic mechanical behaviour of amorphous poly(ethylene 2,6 naphthalene dicarboxylate) (PEN) and of thermally annealed samples is reported as a function of the morphology. During the treatment at temperatures close to the melting temperatures the samples become semi-crystalline. differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment. However the degree of crystallinity as well as the melting temperatures increase with the annealing temperature. Both dielectric (DEA) and dynamic mechanical (DMA) analysis display three relaxation processes. In order of decreasing temperature the α-relaxation due to the glass transition, the β ∗ -process assigned to the out of plane movements of the naphthalene rings or aggregates of it and the β-relaxation due to local molecular motions of carbonyl groups. The α-relaxation process shifts to higher temperatures for the semi-crystalline samples compared to the amorphous one. On the contrary, at a fixed frequency the temperature associated to β ∗ -relaxation is higher for the amorphous sample than for the semi-crystalline ones. The associated apparent activation energies are rather high and depend on the thermal treatment and also surprisingly on the method of measurement. It is concluded that the β ∗ -relaxation is probably due to cooperative molecular motions of the naphthalene groups which aggregate in the amorphous state and that these aggregates are prevented from forming when the degree of crystallinity changes due to the thermal treatment. Finally, the activation energy for the β-process is nearly independent of the thermal treatment and the value agrees with that found for poly(ethylene terephthalate) (PET).

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the a-relaxation due to the glass transition, the b p -process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the b-relaxation due to local fluctuations of the carbonyl groups. The a-relaxation process shifts to higher temperatures for the 250 and 260 8C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 8C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to b p -relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the b-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the b p -a nda-relaxation.

Influence of the molecular modifications on the properties of EPDM elastomers under irradiation

The degradation of the mechanical behaviour of EPDM elastomers used as cable insulation materials has been investigated by mechanical spectroscopy and tensile tests for different formulations: unvulcanised EPDM, vulcanised and stabilised elastomer with an antioxidant. In all cases, γ-irradiation of EPDM under oxygen leads to a reduction of the molecular mobility indicated by the shift of the glass transition relaxation temperature towards higher temperatures. Moreover, the molecular flow occurring above Tg is suppressed after irradiation for the unvulcanised EPDM providing evidence of cross-linking. The competition between cross-linking and chain scissions is shown by the decrease of the storage modulus above the crystallites melting temperature (∼40 °C) at doses larger than 100 kGy. A strong increase of the Young modulus and reduction of the elongation at break of the non-vulcanised EPDM becoming more brittle are shown by stress/strain characterisations performed at 80 °C. At the opposite vulcanised EPDM exhibits higher elongation at break after crystallites melting. This evolution is interpreted by the competition between cross-linking and chain scissions, being hindered by the crystallites at room temperature. The intrinsic irradiation effects can be isolated after crystallite melting. The reduction of the molecular mobility can be explained by a chemi-crystallisation process assisted by chain scissions, leading to a more rigid phase upon irradiation.

Relaxation behaviour of radiochemically aged EPDM elastomers

Abstract EPDM-type elastomers (70% ethylene; 28% propylene; diene monomer: 2% norbornene) were characterised before and after irradiation using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Three formulations were studied: pure elastomer, cross-linked elastomer with dicumyl peroxide and protected with a phenolic-type antioxidant agent. Two irradiation methods were used: γ radiation, under oxygen, at a fixed dose rate of 1 kGy/h, for total integrated doses ranging from 50 to 450 kGy and electron irradiation, under inert atmosphere, at a dose rate of 500 kGy/h, for doses of 100 and 500 kGy. For both types of irradiation, for the three EPDM formulations and for any total integrated dose, DSC and DMA show that a cross-linking phenomenon predominates. Cross-linking increases with the total integrated dose. It is also observed that cross-linking is stronger with the γ irradiation than with the electron irradiation, which confirms the influence of the oxidising atmosphere.

Dielectric relaxation behaviour of poly(ethylene naphthalene 2,6 dicarboxylate) (PEN)

Abstract Dielectric spectroscopy has been used to study poly(ethylene naphthalene 2,6 dicarboxylate) (PEN) samples of different morphologies obtained by thermally treating bi-axially stretched PEN films. Neat and thermally treated samples of PEN films have been characterised by differential scanning calorimetry in order to measure the glass transition and melting temperatures as well as the degrees of crystallinity. Dielectric analysis has allowed the observation of the evolution of molecular relaxation phenomena with morphology changes and has revealed three relaxation processes: β-, β*- and α-relaxation (increasing temperature). The β-relaxation is associated with local motions of ester and the β*-relaxation with partially cooperative motions of naphthalene groups. The latter has been shown to be related to the morphology of the materials under study. The α-relaxation associated to the glass transition of PEN corresponds to cooperative motions induced by conformational rearrangements of the main chain and depends also on the morphology of the PEN films. Dielectric relaxation behaviours were compared using the activation energies calculated from Arrhenius equation formalism for the two sub-glass processes. Vogel–Fulcher–Tammann fits were performed on the α-relaxation. Our important contribution for the bi-axially stretched PEN films study is related to the assignment of the β*-relaxation that can be attributed to naphthalene aggregates.

Gamma irradiation-induced modifications of polymers found in nuclear waste embedding processes Part I: The epoxy/amine resin

Abstract Ion exchange resins (IERs) saturated in cesium and borate ions are well representative of low and medium activity nuclear waste to be embedded in an epoxy resin/amine hardener, such a conditioning procedure being under qualification. In order to test these materials in realistic conditions they are externally irradiated (air and water), in mixed beds saturated in fixed ions (cesium and borate) and water. Irradiation effects are evidenced with the HSF-SIMS technique by the variation of the emission characteristic of both the fixed ions, the chemical structure of the IERs and their interrelationship, both from the analysis of the solid material and of the residual or rinsing water. It appears that the fixed ions can be released in surrounding water as a consequence of radiation-induced resin fragments solubility.

Effect of electron irradiation on the mechanical properties of DGEBA/DDM epoxy resins

Abstract The thermomechanical properties of diglycidyl ether of bisphenol A (DGEBA)/4-4 diaminodiphenylmethane (DDM) resins are analysed as a function of the irradiation dose. Irradiation results in a decrease of the glass transition temperature and of the elastic modulus in the rubbery region, ascribed to a destruction of crosslinks. The Arrhenius diagrams show that the α relaxation becomes faster, that the β process is enhanced but that γ relaxation is less modified by the irradiation. The effects of irradiation on the stress–strain curves suggest a spatial heterogeneity of the crosslink density.