Naohiro Hayakawa

Radiation induced oxidative degradation of polymers—I: Oxidation region in polymer films irradiated in oxygen under pressure

Abstract The γ-ray induced oxidation of polyethylene and ethylene-propylene copolymer films has been studied to obtain technological information on reducing the period in the radiation resistance testing of polymer materials. The polymers were irradiated under pressurized oxygen atmosphere (0.21∼10 atm) at high dose rate (0.5∼1 Mrad/h) in order to accelerate the oxidative degradation. The depth of oxidation region in the film was estimated by the gel fraction measurement. The depth was well agreed with the oxygen penetration region, which was calculated using the observed diffusion coefficient and solubility constant of oxygen in the film and specific rate of the oxygen consumption during irradiation. It was found that the depth of oxidation region was proportional to the square root of [oxygen pressure/does rate] and that the testing period could be reduced by higher dose rate irradiation at higher oxygen pressure.

Electron spin resonance in electron-irradiated 3C-SiC

Electron‐irradiation‐induced defects in epitaxially grown 3C‐SiC crystals have been studied by electron‐spin‐resonance (ESR) measurements. The results indicate the presence of an isotropic ESR center that consists of five lines equally spaced at about 1.5 G and has a g value of 2.0029±0.0001. Isochronal and isothermal annealing of electron‐irradiated 3C‐SiC showed that this center was annealed at three stages (150, 350, and 750 °C) and that the 750 °C stage exhibited first‐order reaction with an activation energy of 2.2±0.3 eV.

Degradation in tensile properties of aromatic polymers by electron beam irradiation

Electron beam irradiation effects of ten kinds of polymers containing various aromatic rings linked by functional groups in the main chain (aromatic polymer) were studied with reference to change in tensile properties. The polymers studied were polyimides ‘Kapton H’, and ‘UPILEX’, polyetherimide ‘ULTEM’, polyamides ‘A-Film’, and ‘APH-50 (nomex type paper)’, poly-ether-ether-ketone ‘PEEK’, polyarylate ‘U-Polymer’, polysulphones ‘Udel-Polysulphone’, and ‘PES’, and modified poly(phenylene oxide) ‘NORYL’. Irradiation was carried out by use of electron beam at a dose rate of 5 × 103 Gy s−1 at room temperature. The elongation at break was the most severely influenced by the irradiation and it decreased with increasing dose. The order of radiation resistivity which was evaluated from the dose required for the elongation to become 50% and 20% of the initial value was as follows: Polyimide>PEEK>polyamide>polyetherimide>polyarylate>polysulphone, poly(phenylene oxide) Based on the above experimental results, the following order was proposed as for the radiation stability of the aromatic repeating units composing the main chain:

Radiation induced oxidative degradation of polymers—II: Effects of radiation on swelling and gel fraction of polymers

Abstract The degradation of polyethylene (PE) and ethylene-propylene copolymer (EPR) by γ-ray irradiation in oxygen under pressure was studied by gel fraction and swelling measurements. Irradiation in oxygen produced mainly chain scission, therefore the molecular weight decreased with dose. The G values of chain scission are 4±0.5 for PE, and 1 ≈2 for EPR. When the crosslinked PE and EPR are irradiated in oxygen, gel fraction decreases and swelling ratio increases with dose. A linear relation between the swelling ratio (Q) and the dose (R) is observed for EPR, that is, log[Q/Q 0 ] = K·Q 3 2 ·R . With the addition of antioxidants, the relation still holds but the factor K is changed, so the efficiency of antioxidants can be determined from the value of K.

Electron beam irradiation effects on mechanical relaxation of aromatic polysulphones

Abstract The effects of electron beam irradiation on the molecular motion of poly(oxy-1,4-phenylenesulphonyl-1,4-phenylene oxy-1,4-phenyleneisopropylidene-1,4-phenylene) (U-PS) and poly(oxy-1,4-phenylenesulphonyl-1,4-phenylene) (PES) were studied by measuring the dynamic viscoelastic properties. The γ relaxation peaks existing at low temperature (−100°C for U-PS, −110°C for PES) shifted to higher temperature and became narrower with increasing dose. A new relaxation (β′ relaxation) appeared upon irradiation in the range from 40°C to just below the glass transition temperature. The magnitude of the β′ relaxation was reduced by heat treatment and this relaxation disappeared in the second run. The β′ relaxation was assigned as a molecular motion during rearrangement induced by chain scission from a loosened chain packing to a more rigid chain packing. The β relaxation peak responsible for the glass transition shifted to a lower temperature with increasing dose. It was concluded from these facts that polysulphones are mainly degraded by chain scission on the sulphone moiety.

Radiation induced oxidative degradation of polymers—III: Effect of radiation on mechanical properties

Abstract The changes of mechanical properties of various kinds of polyethylene (PE) and ethylene-propylene copolymer (EPR) with the irradiation in air, in oxygen of 10 atm, and under vacuum were investigated. The decrease in the elongation (Eb) and the tensile strength (Tb) of PE by the irradiation in oxygen is larger than under vacuum. The changes of Eb well reflect the degradation of PE. In case of EPR, the Tb decreases sharply with dose in any environments, and the Eb decreases under vacuum to a larger extent than in oxygen. The modulus at 200% elongation of EPR increases with dose under vacuum, but decreases in oxygen. When the samples were irradiated in air, the changes of the mechanical properties were the intermediate between oxygen and vacuum and dependent on the ratio of oxidation and non-oxidation layers in the film. The antioxidant (Irganox 1010 or DPPD) mixed in polymers was found to retard effectively the polymer degradation by the irradiation in oxygen.

Radiation induced oxidative degradation of polymers—IV. Dose rate effects on chemical and mechanical properties

Abstract The dose rate effects on the radiation induced oxidative degradation of crosslinked polyethylene and ethylene-propylene copolymer was investigated by the tensile property, gel fraction, and dielectric loss tangent. The polymer films crosslinked by chemical agent were irradiated with various dose rates from 5×105 to 5×10 3 rad hr in oxygen under pressure from 5 to 0.2 atm at room temperature. It was found that the degradation at a given dose depends on the dose rate; Deg r = k·I -1 3 , where Deg is degradation, r dose, I dose rate, and k constant. For the polymers containing antioxidant the dose rate effects was not observed, then the degradation was only dependent on the total dose.

Fast neutron irradiation effects—I. Dosimetry

Abstract A dosimetry of mixed field, neutron and γ-ray, has been carried out by the activation method in a fast neutron source reactor “YAYOI,” which has been used for the study of fast neutron irradiation effects on organic materials and microorganisms. Neutron flux and energy distribution were obtained by an unfolding calculation based on the activation analysis. Absorbed dose was determined as a function of the distance from the center of the reactor core in a hole called “Glory Hole” used for irradiation of various materials. Contribution of absorbed dose from γ-rays coexisting with neutron was evaluated by measuring peroxide radicals formed in irradiated polytetrafluoroethylene, PTFE, which is insensitive to the neutron component. After evaluating the radiation field, relative sensitivities of popular dosimeters such as red acrylate, radiachromic, and alanine dosimeter for last neutron irradiation were determined by comparison with 60Co γ-ray irradiation. Characteristics of the mixed field dosimetry and LET effect of recoil protons in samples at different neutron fields are discussed.

Fast neutron irradiation effect. II: Crosslinking of polyethylene ethylene-propylene copolymer, and tetrafluoroethylene propylene copolymer

Abstract Effect of fast neutron irradiation on polyethylene, ethylene-propylene copolymer, and tetrafluoroethylene-propylene copolymer was compared with those of Co-60 γ-ray by the measurements of gel fraction and of swelling ratio after irradiation under vacuum. Difference in the effect between fast neutron and Co-60 γ-ray for the three polymers was not detected by those methods in spite of a large difference of linear energy transfer (LET). The G values of crosslink and chain scission determined from gel fraction data were the same for both fast neutron and Co-60 γ-ray for each polymer within the experimental error. It is thought that energy absorbed in a local region of polymer by fast neutron irradiation generates radicals in a more spread region and the radicals move in the polymer matrix until their recombination to form crosslink.

Electron spin resonance study of the thermal decay of radicals in irradiated polyethylene

Abstract The role of radical pairs in the decay of alkyl radicals and the correlation of the decay of radicals with the type of trapping region in irradiated polyethylene have been investigated by using various types of polyethylene. When the sample irradiated at 77°K is warmed above this temperature, the decay curve of radicals against temperature shows a three-step decay. The first decay region is ascribed to the decay of radical pairs. The second decay region is connected with radicals trapped in the noncrystalline regions. The third decay region is correlated to radicals trapped in the crystalline regions.