Preparation of anisotropically proton-conductive materials based on poly(vinylidene fluoride) ion track membranes

Proton exchange membranes (PEMs) for polymer electrolyte fuel cells (PEFCs) have recently been developed by our original techniques of the γ-ray or electron-beam crosslinking and graft polymerization. Our additional main focus is the use of cylindrical damage in polymers produced by bombardment of high-energy ions, that is called a latent track. We are taking the approach using this ion track technology according to two procedures, expecting the preparation of “anisotropically” proton-conductive PEMs. One is ion-track grafting [1], i.e., direct grafting into the activated zone; and the other is the track etching and subsequent chemical modification inside the etched pores. The report in the preceding issue [2] described the former attempt and, therefore, the latter is presented here, including basic characteristics of the resulting PEMs. A commercially available poly(vinylidene fluoride) (PVDF) film with a thickness of 25 μm was bombarded by swift heavy ions from the TIARA cyclotron of JAEA and the UNILAC linear accelerator of GSI. According to the established procedure [3], track etching was then performed in a 9 M KOH aqueous solution at 80 ̊C. It should be noted here that irradiation with high-LET GeV beams and preheating treatment of the irradiated films in air accelerated the track etching [4]. The sulfonic acid groups were introduced into the track-etched cylindrical pores of 100 nm diameter (density 3.0×10 9 cm -2 ) via the γ-rayinduced styrene grafting and subsequent sulfonation. Figure 1 shows the proton conductivity, σ, in the inplane and thickness directions for the resulting PEMs whose ion exchange capacity (IEC) was controlled up to 2.2 mmol g -1 . The proton transport only in the thickness direction, increasing with the IEC, was obtained at < 1.6 mmol g -1 . This anisotropic conductivity strongly indicates the formation of one-dimensional straight proton conductive pathways parallel to the ion-beam incident axis.