High-sensitivity ion track-etching of poly(vinylidene fluoride) membranes

Quite recently, ion-track membranes of poly(vinylidene fluoride) (PVDF) have attracted a renewed interest for their applications to next-generation electrochemical devices such as fuel cells [1,2]. In order to produce tracketched pores in PVDF films, several kinds of etching solutions were previously employed. In most cases [3], a highly concentrated aqueous KOH solution with a KMnO4 additive was maintained at a high temperature (85oC). These severe etching conditions provided irreversible chemical damage all over the film including the non-irradiated part, thereby destroying some distinctive properties of PVDF. Therefore, in this study, we attempted to prepare PVDF ion-track membranes of up to a few hundred nm thickness without any oxidant additives in the alkaline etching solution. Instead of the KMnO4 induced activation, ion irradiation with high linear energy transfer (LET) was considered to enhance the etching sensitivity even under milder conditions [4,5]. A 25m thick PVDF film was irradiated at room temperature by swift heavy ions from the TIARA cyclotron of JAEA and the UNILAC linear accelerator of GSI. The fluence ranged from 3 × 10 to 3 × 10 ions/cm. Further irradiation conditions are listed in Table 1. Track etching was then performed in a 9 mol/dm KOH aqueous solution at 80 ̊C. After depositing a gold coating, the surface of the 24-hour etched membranes was imaged by scanning electron microscopy (SEM). In order to measure the track etch rate, VT, the etching was performed in an electrolytic conductivity cell. A bulk etch rate, VB, was estimated to 0.46 nm/h by taking the decrease in film thickness during long-time etching. Using the ratio VT / VB, we obtained the track etching sensitivity, Q, according to Q = (VT / VB) – 1. (1) First of all, as an advantage of the no-oxidant alkaline etching, it must be noted that VB was extremely small, suggesting very mild etching conditions for PVDF, in marked contrast to a few hundred nm/h for etching in the KMnO4 containing solution [3]. In a strong alkaline medium at high temperatures, virgin PVDF undergoes deprotonization and the consecutive elimination of fluorine to afford –CH=CFor –CH=CF2. In the absence of the additives, however, these modifications should be restricted to the outermost film layer because there is basically no wetting at the polymer/solution interface. All results, including the LET evaluated by a TRIM code, as well as the Q value and pore diameter, are shown in Table 1. This table roughly indicates that the highermass ion irradiation led to enhanced etching in both the track-core and -halo; VT and pore diameters measured by SEM increased by about a factor of 3 between Ni and Pb ions. Such a large VT enabled us to obtain very high etching sensitivity for the preparation of cylindrical through-pores. A similar effect on the etching behavior was quite recently observed when performing ion-track etching of poly(p-phenylene terephthalamide) [6]. The change of VT for the different beams is ascribed to a LET effect, because the bombardment with high-LET ions produces more severe molecular damage in each latent track. Interesting is that from Xe to Pb, we found a sharp increase in pore diameter, while the difference in LET between these ions is similar to that between the other nearest-mass ions. Thus, the LET does not seem to be the only significant factor in this case. The so-called “velocity effect” might be manifested for the present etched tracks. High-LET irradiation will be most suitable for preparing, in an effective way, PVDF-based ion-track membranes with a pore diameter of nm to sub-μm.