Miyuki Hagiwara

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:

Accumulated Charge Profile in Polyethylene during Fast Electron Irradiations

Space charge profile and consequent field strength profile were obtained by solving the continuity and Poissons simultaneous equations. Experimental electron deposition curve and radiation-induced conductivity curve were used for calculation. The calculated results explained well the observed replacement current and electrical breakdown phenomena. Dose rate, dose, and temperature effects on charge accumulation were also investigated.

Molecular motions of non-crystalline poly(aryl ether-ether-ketone) PEEK and influence of elecron beam irradiation

Abstract The dynamic mechanical relaxation of non-crystalline poly(aryl ether-ether-ketone) PEEK and the one irradiated with electron beam were studied. The three distinct γ, β, α′ relaxation maxima were observed in unirradiated PEEK from low to high temperature. It was revealed from the study on the irradiation effects that three different molecular processes are overlapped in γ relaxation peak, i.e., molecular motion of water bound to main chain (peak temperature; at −100°C), local motion of main chain (at −80°C), and local mode of the aligned and/or oriented moiety (at −40°C). The β relaxation connected with the glass transition occurred at 150°C and it shifted to higher temperature by irradiation. The α′ relaxation which can be attributed to rearrangement of molecular chain due to crystallization was observed in unirradiated PEEK ∼ 180°C and its magnitude decreased with the increase in irradiation dose. This effect indicates the formation of structures inhibiting crystallization such as crosslinking and/or short branching during irradiation. A new relaxation, β′, appeared in the temperature range of 40° to 100°C by irradiation and its magnitude increased with dose. This relaxation was attributed to rearrangement of molecular chain from loosened packing around chain ends, which were introduced into the non-crystalline region by chain scission under irradiation, to more rigid molecular packing, From these observations, we proposed that deterioration in mechanical properties of non-crystalline PEEK by high energy electron beam was brought about not only by chain scission but structural changes such as crosslinking and/or branching in the main chain.

Radiation deterioration of several aromatic polymers under oxidative conditions

Abstract Radiation-induced oxidative irradiation effects (with γ-rays under oxygen pressure) or poly(aryl sulphones) (U-PS and PES), poly(aryl ester) (U-Polymer), poly(aryl amide) (A-Film) and poly(aryl ether ether ketone) (PEEK) have been studied based on changes in tensile properties. The deterioration dose estimated from the decrease in the elongation at break was as low as one-fifth to one-tenth of that in high-dose-rate electron-beam irradiation, but the order of radiation resistance of the polymers did not differ from that in electron-beam irradiation, i.e. PEEK > A-Film > U-Polymer > U-PS > PES. The radiation stability of aromatic units under oxidative conditions was estimated from a comparison of the radiation resistance of the polymers themselves and their chemical structures. The following order was obtained: diphenyl ether, diphenyl ketone > aromatic amide ⪢ bisphenol A > diphenyl sulphone. The deterioration mechanism of PEEK under oxidative irradiation was studied by measuring dynamic viscoelastic properties. It was concluded that deterioration in mechanical properties under oxidative irradiation was brought about by chain scission only.

Mechanical relaxation of crystalline poly(aryl-ether-ether-ketone) (PEEK) and influence of electron beam irradiation

Abstract Mechanical relaxation of semicrystalline PEEK and influence of electron beam irradiation on molecular motions were studied by measuring dynamic viscoelastic properties in the temperature range from − 160°C to 350°C. Irradiation caused the glass transition temperature (β relaxation) to shift upwards, and β′ relaxation, related to loosened chain packing, to appear just below the glass transition temperature, and a small shoulder that appeared at − 40°C to 0°C on the γ relaxation peak and disappeared in the first run after irradiation to reappear in the second run. It was concluded that both crosslinking and chain scission take place under irradiation. The shoulder on the γ relaxation was assigned to segmental motion of the chains at the crystalline-non-crystalline interface. The inferior radiation resistance of semicrystalline PEEK with respect to non-crystalline PEEK is ascribed to preferential disintegration of tie-molecules between crystalline and non-crystalline phases.

Artificially intelligent hydrogels responding to external stimuli such as temperature and pH

Abstract Methacryloyl-D-alanine methyl ester (MA-D-AlaOMe) and methacryloyl-L-alanine methyl ester (MA-L-AlaOMe) were synthesized and polymerized by irradiation using γ-rays from a 60Co source, in order to investigate the effect of optical isomerism on the swelling of homopolymer hydrogels which changes in temperature in water. These hydrogels showed a typical thermo-response such as low-temperature-swelling and high-temperature-deswelling, but no difference in swelling between the two hydrogels could be regarded as significant. The stable swelling-deswelling kinetics on repeated use are characterized by very limited temperature ranges (0–40°C), and above this range (for example, when cycled between 10 and 70°C) the reversible thermo-response perfectly disappeared because of gel collapse. This defect could be overcome by copolymerization with other functional monomers and the resulting copolymers exhibited a reversible thermostability superior to those of homopolymers of MA-D-AlaOMe and MA-L-AlaOMe, depending strongly upon the type of copolymer and its composition. The hydrolysis products of these hydrogels gave a reversible-response not only for temperature but also for pH. The swelling-deswelling kinetics of non-hydrolyzed and hydrolyzed hydrogels were characterized with regard to porous structure formed in the hydrogel.

Annealing effects on the mechanical properties of organic composite materials irradiated with γ-rays

Abstract Four kinds of cloth-filled organic composites (filler: glass or carbon fiber; matrix: epoxy or polyimide resin) and a unidirectional alumina fiber/epoxy composite were irradiated with 60Co γ-rays at room temperature, and then annealed in vacuo 2 h at 180°C. These composites were examined with regard to the mechanical properties at room temperature before and after the annealing. Following irradiation the Young (tensile) modulus of these composites remains practically unchanged up to 2000 Mrad irrespective of the annealing. The shear modulus and the ultimate strength also remain unchanged up to this dose before annealing, but after annealing both of them begin to decrease significantly at doses below 2000 Mrad for all the composites except the glass/polyimide composite. This finding indicates that latent radiation damage is activated by the annealing, thus causing a decrease in the load transfer capacity at the fiber/matrix interface and, hence, losses in the shear modulus and the ultimate strength. As to the fracture behavior, on the other hand, the propagation energy increases only slightly with increasing absorbed dose before annealing, but after annealing it increases significantly, apparently accompanying a decrease in the ultimate strength. This result is also attributed to the annealing-activated damage at the fiber/matrix interface.

Radiation vulcanization of natural rubber latex with polyfunctional monomers. II

Abstract Natural rubber latexes were vulcanized by γ-rays with two polyfunctional monomers, neopentylglycol diacrylate (A-NPG) and dimethacrylate (NPG). In comparison with A-NPG, NPG was more soluble in rubber particles, but it was less effective as an accelerating agent for the vulcanization because of the smaller rate of polymerization. On the other hand, the colloidal stability of the latex containing A-NPG was low because it was localized on the surface of rubber particles due to its poor solubility to the particles. The solubility of A-NPG was improved by adding A-NPG with solvents. The maximum tensile strength of the irradiated latex film was 350kg/cm 2 at 3 Mrad.

Mechanical properties of organic composite materials irradiated with 2 MeV electrons

Abstract Four kinds of cloth-filled organic composites (filler: glass or carbon fiber; matrix: epoxy or polyimide resin) were irradiated with 2 MeV electrons at room temperature, and were examined with regard to the mechanical properties. Following irradiation the Youngs (tensile) modulus of these composites remains practically unchanged even after irradiation up to 15000 Mrad. The shear modulus and the ultimate strength, on the other hand, begin to decrease after the absorbed dose reaches about 2000 Mrad for the glass/epoxy composite and about 5000–10000 Mrad for the other composites. This result is ascribed to the decrease in the capacity of load transfer from the matrix to the fiber due to the radiation damage at the interface, and the dose dependence is interpreted and formulated based on the mechanics of composite materials and the target theory used in radiation biology. As to the fracture behavior, the propagation energy increases from the beginning of irradiation. This result is attributed to the radiation-induced decrease in the bonding energy at the interface.

Development of radiation crosslinking process for high voltage power cable

Abstract This work has been done to develop an electron beam crosslinking process for high voltage power cable by solving problems of the foaming and discharge breakdown of thick polyethylene insulation during irradiation. When a polyethylene plaque (d=0.92, MI=1.0) of 4 mm thick corresponding to the wall of 6.6 kV cable was irradiated by 1.5 MeV electrons, the temperature went up to 95°C and foaming was observed at a dose of 20 Mrad. On the other hand, irradiation of 24 Mrad was required for sufficient heat resistance (75% gel formation) of polyethylene. Although an addition of conventional polyfunctional monomers reduced the crosslinking dose (10 – 12 Mrad), discharge breakdown occured at relatively low doses. The present work reveals that both foaming and discharge breakdown are efficiently prevented by the use of dipropargyl succinate (DPS). A 6.6 kV type cable specimen insulated with polyethylene containing 2 phr of DPS was irradiated by 1.5 MeV electrons. While the cable had properties comparable to those of a steam cured one, very few voids were observed in the insulation.