Akihiro Oshima

Study on poly-electrolyte membrane of crosslinked PTFE by radiation-grafting

Abstract Polymer electrolyte fuel cell membrane based on crosslinked polytetrafluoroethylene (PTFE) [RX-PTFE] has been processed by radiation-grafting with reactive styrene monomers by γ-rays under atmospheric circumstances, and the characteristic properties of the obtained membranes have been studied. The grafting yields of styrene monomer onto RX-PTFE, which have various crosslinking densities, were in the range of 5–100%. At the reaction period of 24 h, the grafting yields for RX-PTFE with low crosslinking density, which was reacted at 60 °C, achieved 94%. As a tendency, the lower grafting temperature gives higher grafting ratio of styrene onto RX-PTFE. Moreover, the yields of subsequent sulfonation for all samples were close to 100%. Mechanical properties were decreased with increasing grafting yields; especially the membrane with higher grafting yields was brittle. Ion exchange capacity of sulfonated RX-PTFE reached 1.1 meq/g while maintaining the mechanical properties.

Micromachining of crosslinked PTFE by direct photo-etching using synchrotron radiation

Micromachining of crosslinked PTFE (polytetrafluoroethylene) using synchrotron radiation direct photo-etching method has been demonstrated. High aspect-ratio microfabrication was carried out. The etching rate of crosslinked PTFE was higher than that of non-crosslinked PTFE. Through the etching rate measurements of various samples, it was found that synchrotron radiation etching rate of crosslinked PTFE only depends on the degree of crosslinking, neither molecular weight nor crystallinity. The effect of molecular motion on etching process was discussed from temperature dependence data on etching rate. Furthermore, the surface region of synchrotron radiation irradiated sample was investigated by Fourier transform infrared spectroscopy and the experimental result showed that the modification induced by synchrotron radiation proceeded before desorption.

Fabrication of a poly-electrolyte membrane based on cross-linked PTFE thin film by EB irradiation grafting

Polymer electrolyte fuel cell (PEFC) membranes based on cross-linked polytetrafluoroethylene (RX-PTFE) very thin film have been fabricated by radiation grafting with reactive styrene monomers using electron beam irradiation under nitrogen atmosphere at room temperature. The characteristic properties of obtained materials have been measured by differential scanning calorimetry, thermogravimetric analysis, FT-IR spectroscopy, etc. The glass transition temperatures and thermal decomposition temperatures of grafted materials strongly depend on the yields of styrene grafting onto base materials. The higher yields of grafting give lower thermal stability. The ion-exchange capacity of sulfonated materials is 3.0 mEquiv/g. The other properties of obtained membranes, such as chemical structure and rates of hydrogen gas transmission, are discussed.

Surface modification of polytetrafluoroethylene by synchrotron radiation

Abstract Surface modification of polytetrafluoroethylene (PTFE) films by the irradiation using synchrotron radiation (SR) below the melting temperature was studied. The changes in chemical structures, surface compositions and thermal properties of the SR-irradiated PTFE were examined by solid-state 19F NMR, X-ray photoelectron spectroscopy (XPS) and differential scanning calorimetry (DSC), respectively. In order to investigate the degree of modification as a function of depth from the irradiated surface, some PTFE thin films were stacked in layers under vacuum and then they were irradiated with SR. It was found that crosslinking reaction was induced by SR-irradiation in the solid-state within 50 μm from the exposed surface and chain scission was dominant at deeper layers.

Radiation-induced phenomena in ethylene-co-tetrafluoroethylene polymer: Temperature and LET effects

Abstract Irradiation temperature and linear energy transfer (LET) dependency on radiation-induced reactions of ethylene-co-tetrafluoroethylene polymer (ETFE) were investigated precisely by using low and high LET beams, and in a wide range of irradiation temperatures from 77 to 573 K including its melting temperature, respectively. At various temperatures irradiation by low LET beam such as γ-rays or electron beams, significant changes were observed in the photo-absorption spectra in the wavelength region between 200 and 500 nm. The general tendency is that the absorption band shifts to longer wavelengths with higher irradiation temperatures. The enhancement of the photo-absorption at 200–500 nm is due to the formation of conjugated double bonds in ETFE by irradiation. By high LET beam irradiation at room temperature such as ion beams, the photo-absorption spectra was different from those of low LET beams, i.e. the new absorption bands around 250–450 nm was appeared. It could be suggested that the high LET beams induced the production of intermediate species in a localized area such as track structure. As a result, reaction kinetics are different from low LET beams.

Positive–negative dual-tone sensitivities of ZEP resist

A chlorinated resist named ZEP changes from positive- to negative-tone with a high dose of electron beam irradiation. The sensitivities to a 100-kV electron beam as a dual positive- and negative-tone resist are evaluated with various developers. Although the positive-tone sensitivity varies with different developers owing to different solvation strengths, the threshold dose for the positive–negative (P–N) inversion is independent of the developer. According to an analysis using X-ray photoelectron spectroscopy and NMR spectroscopy, the amount of chlorine atoms and terminal double bonds generated by chain scission determines the scission/crosslinking probabilities, i.e., the threshold of the P–N inversion.

Evaluation of vacancies in positive-tone non-chemically and chemically amplified euv / eb resists ∼ relationship between free-volume and LER ∼

EUV, X-ray and EB mainly induce ionization in resist materials and its energy deposition process is different from ArF exposure. Linear energy transfer (LET) effect for resist sensitivity is very important issue from the viewpoint of radiation induced chemical reactions for high-volume nanofabrication. Therefore the knowledgebase of radiation chemistry is required for understanding the resist performances for EUV lithography. In order to acquire the knowledge of resist materials for EUV / EB lithography from a viewpoint of the free-volume, the positron annihilation lifetime spectroscopy was carried out using positron probe microanalyzer (PPMA) installed at AIST. The size of free-volume can be evaluated from the lifetime of ortho-Positronium (o-Ps). The lifetime and intensity of o-Ps in EB-exposed positive-tone non-chemically amplified (non-CA, ZEP) and CA (UV-III) EUV / EB resists were observed. Moreover, to evaluate the relationship between line edge roughness (LER) and free-volume, EB lithography was carried out, and then sensitivities (E0 and Esize) and LER were measured. For both non-CA and CA resist materials, the changes of free-volume due to evaporation of outgas, polarity change or chain scission would hardly influence on their LER and resolution.

High-aspect-ratio micromachining of fluoropolymers using focused ion beam

Poly(tetrafluoroethylene) (PTFE) microstructure with high aspect ratio (> 200) and without solid debris along the edge was fabricated with high etch rate by using FIB. Gasification of PTFE by FIB is responsible for the high aspect ratio, the high etch rate, and the no solid debris. Roughness of etched surface of the PTFE increases with fluence, although edge of the etched area has good profiles. The etch mechanism seems to be complicated.