Tsuneo Sasuga

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:

Evaluation of polymer free volume by positron annihilation and gas diffusivity measurements

Abstract In this paper we show that there exists a linear relationship between the size of a cavity where an ortho-positronium (o-Ps) atom annihilates by the ‘pick-off’ mechanism and the total free volume of the molecular liquid or polymer under consideration. This relationship is used to determine the free volume of electron irradiated poly(aryl-ether-ether-ketone) (PEEK) samples and the result is compared with that obtained from gas diffusivity measurements.

Irradiation effects on aromatic polymers: 1. Gas evolution by gamma irradiation

Abstract The effects of gamma irradiation on aromatic polymers, such as polyimides (Kapton, Upilex-R and Upilex-S), poly(aryl ether ether ketone) (PEEK), poly(aryl ether sulphone) (PES), bisphenol A type Udel poly(aryl sulphone) (U-PS) and poly(aryl ester) (U-Polymer), were investigated based on gas evolution. The radiation resistance in terms of gas evolution was in the following order: Upilex-R ⩾ Kapton > PEEK > PES > Upilex-S ⪢ U-PS > U-Polymer. The G values of total gases from these aromatic polymers were 1 100 th to 1 1000 th of the G values from aliphatic polymers. The major component gases were: H 2 and N 2 for polyimides; CO 2 and CO for PEEK; CO 2 , CO and SO 2 for polysulphones; and CO and CO 2 for U-Polymer. The influence of crystallinity on the behaviour and yield of gas evolution was investigated in PEEK. Crystalline PEEK gave lower yield compared to amorphous PEEK. The radiolysis mechanism of gaseous products was discussed based on the structures of the aromatic polymers.

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.

Irradiation effects on aromatic polymers: 2. Gas evolution during electron-beam irradiation

The gas evolution caused by electron-beam (e.b.) irradiation of aromatic polymers, such as polyimides (Kapton, Upilex-R and Upilex-S), poly(aryl ether ether ketone) (PEEK), poly(aryl ether sulphone) (PES), bisphenol A type Udel poly(aryl sulphone) (U-PS) and poly(aryl ester) (U-Polymer), has been quantitatively analysed. The radiation resistance in terms of gas evolution was in the following order: Upilex-R ⋍ Upilex-S >Kapton >PEEK >PES ⪢ U-PS >U-Polymer. The component gases evolved from these aromatic polymers by e.b. irradiation were the same and possessed similar behaviour to those produced by gamma irradiation. But differences were found in the G values of some component gases evolved under e.b. irradiation compared with gamma irradiation due to the temperature rise caused by the high dose rate of e.b. irradiation. The effect of molecular structure and chemical unit linkages in the aromatic polymers on gas evolution was investigated. It was found that the biphenyl imide, tetracarboxylic acid anhydride imide, and aryl ether ketone unit linkages exhibited a protective effect against both e.b. and gamma irradiation. But aryl sulphone, isopropylidene aryl sulphone and aryl ester structures are sensitive to radiation. The polyimides and PEEK showed high radiation resistance, which can be of practical importance.

High-energy-ion-irradiation effects on polymer materials: 3. The sensitivity of cellulose triacetate and poly(methyl methacrylate)

Abstract The changes in sensitivity of a cellulose triacetate (CTA) film dosimeter is reported as a function of linear energy transfer (LET). The change in molecular weight of poly(methyl methacrylate) (PMMA) is also reported. For both materials, little or no LET effect was observed up to a threshold LET, but the sensitivity or radiation yield decreased with increasing LET above this threshold level. The threshold LET level was similar for both polymers, occurring at around a few hundreds of MeV cm2g−1, with this level probably corresponding to the overlapping of spurs along the ions path.

Proton irradiation effects on several organic polymers

Abstract High-energy (8 MeV) proton irradiation effects on the mechanical properties of polymers (about 100 μm thickness) with various constituents and chemical structures were studied by tensile tests and compared with those caused by 2 MeV electrons. In the aliphatic polymers studied (PE, PP, EVA, PVDF, ETFE and nylon 6), there is scarcely any difference in the absorbed dose dependence of the tensile strength and ultimate elongation between proton and electron irradiation. In the aromatic polymers studied (PET, PES, U-PS and U-polymer), however, the decrements in the tensile strength and ultimate elongation against proton dose are less than that in electron irradiation. In this manner, linear energy transfer effects were scarcely observed in the aliphatic polymers but were clearly observed in the aromatic polymers.

Molecular motion of several epoxy resins and influence of electron irradiation

Abstract Electron irradiation effects on the molecular motion of epoxy resins and a composite reinforced by carbon fibre were studied. The materials used were three epoxy resins that were cured by diaminodiphenylmethane: diglycidyl ether of bisphenol A (DGEBA), diglycidyl ether of bisphenol F (DGEBF) and tetraglycidyl diaminodiphenylmethane (TGDDM); and carbon-fibre-reinforced TGDDM/diaminodiphenylsulphone. A mechanical loss peak appeared for all epoxy resins in the temperature range from 50 to 100°C. The results of heat treatments indicated that this relaxation is related to the local motion of chains subjected to an internal stress produced during curing. The internal stress was more marked for the tetrafunctional epoxy (TGDDM) than for the difunctional ones (DGEBA and DGEBF). The main reaction induced by irradiation is chain scission, and the order of radiation resistance of the epoxide monomers was revealed to be TGDDM > DGEBF > DGEBA.