Preparation of ion-track membranes of poly(p-phenylene terephthalamide)

There have been numerous studies on ion-track membranes of poly(ethylene terephthalate) (PET) and polycarbonate (PC), which are easily prepared by ion beam irradiation and subsequent alkaline etching. More importantly, the ion-track membranes of chemically and thermally stable polyimide (PI) and polyamide (PA), with controlled pore shape and size, may be required for the design of materials that must withstand severe conditions (e.g., alkaline solution and high temperatures). PI-based microporous membranes were obtained by ion-track etching with an oxidative solution [1] and precursor method [2]. However, the ion-track membrane of PA has not yet been reported because the etching sensitivity is inversely related to polymer stability. We report here, for the first time, the investigation of ion-track etching of poly(pphenylene terephthalamide) (PPTA), which is one of the PAs with the highest mechanical strength as well as chemical or thermal stability. 16-μm-thick PPTA films were bombarded by swift heavy ions from the TIARA cyclotron of JAEA and the UNILAC linear accelerator of GSI. The ion fluence was 3 × 10 ions cm. Further irradiation conditions are listed in Table 1. The irradiated PPTA films were etched in a sodium hypochlorite (NaClO) solution at 40°C without stirring, whose pH value was adjusted at pH 9±0.5 by adding a 2 mol dm hydrochloric acid (HCl) solution. The etched sample was washed with a large amount of water and dried at 40°C for 2 hours in vacuum. After depositing a Au coating, the surface and cross-section of the membranes were imaged with a scanning electron microscope (SEM). The conductometric method was applied to determine a track etching rate, Vt [3]. A bulk etching rate, Vb, was estimated to 0.05 μm h by taking the decrease in film thickness during one-hour etching. Using the ratio of the two etching rates, we obtained the track formation sensitivity, Q, according to the following simple equation: Q = (Vt / Vb) – 1. (1) Fig. 1 (a) shows SEM micrographs of the surface and cross-section of the membranes, which were obtained by the irradiation with U ions and a 12-hour etching. This clearly confirms the perfectly cylindrical pores with a 0.3μm diameter. The cylindrical shapes were also observed for the Au ion irradiated films. In contrast, as shown in Fig. 1 (b), the tracks of Xe ions were etched for 18 hours, leading to observable tracks with a surface