Poly(vinylidene fluoride)-based ion-track membranes with asymmetrical pores

We have been working on ion-track membranes of poly(vinylidene fluoride) (PVDF) because they have attracted a renewed interest for their applications to fuelcells [1]. There were two main conclusions: (i) The PVDF ion-track membranes can efficiently be prepared under milder etching conditions, i.e., without any oxidant additives in the alkaline etching solution [2]; and (ii) irradiation with ions of higher LET promotes chemical etching in both the track-core and -halo, thereby increasing the etching sensitivity and the resulting pore diameter [3]. In these previous studies, cylindrical pores were always developed because the energy deposition was almost uniform across the film. On the other hand, we found that an LET gradient along the ion path could lead to development of asymmetric pores [4]. Commercially available PVDF films of thickness 25 and 100 m were irradiated at room temperature by 11.1 MeV/n Au ions from the UNILAC. The fluence was fixed to 3 × 10 ions/cm. Subsequent track etching was performed in a 9 mol/dm aqueous KOH solution at 80 ̊C in a teflon-coated vessel without stirring. After depositing a gold coating, the surface of the etched membranes was imaged by scanning electron microscopy (SEM). Figure 1 illustrates the depth profile of the LET value calculated with the TRIM 1998 code. As can be seen, the LET rapidly decreases with beam energy loss and finally reaches zero at around 130 μm. According to the above concept, we irradiated a stack of two layers of PVDF in the configuration presented in Fig. 1. The first layer exposed to the incoming beam was 100 m thick. The Au ions were sufficiently energetic to pass through this film while they deposited their energy rather uniformly along the full path. In contrast, the thickness of the second layer was only 25 m. The ions also passed through this film, however, the LET value was only 5.2 MeV/μm at the exit face compared with 19.6 MeV/μm at the entry face. Figure 2 shows SEM images of the front and back surface of the second layer recorded after etching for 48 h. In agreement with our expectation, the entry face contained pore apertures with a diameter of 232±14 nm while, in contrast, the exit face had 50% smaller pore orifices with a diameter of 117±14 nm. Considering that in this film the LET continuously and very dramatically decreased along the track, it is reasonable to assume the formation of “conical” pores although the pore cross-section inside the membrane was not yet analyzed. Compared with the widely employed asymmetric etching method involving chemical etching of only one side of the film [5], this new approach is based solely on the LET significantly varying in the thickness direction. One reason for our successful result is probably the very high sensitivity of the etching process assigned to the large difference between the track and bulk etch rate [2,3].