Shape of nanopores in track-etched polycarbonate membranes

Abstract

High aspect-ratio nanopores of nearly cylindrical geometry were fabricated by irradiation of 20 μm thick polycarbonate (PC) foils with Pb ions followed by UV sensitization and etching in 5 M NaOH at 60 °C. Synchrotron-based small-angle X-ray scattering (SAXS) was used to study the morphology and size variation of the nanopores as a function of the etching time and ion fluence. The shape of the nanopores was found to be consistent with cylindrical pores with ends tapering off towards the two polymer surfaces in the last ~1.6 μm. The tapered structure of the nanopores in track-etched PC membranes was first observed more than 40 years ago followed by many other studies suggesting that the shape of nanopores in PC membranes deviates from a perfect cylinder and nanopores narrow towards both membrane surfaces. It was also reported that the transport properties of the nanopore membranes are influenced by the tapered structure. However, quantification of the shape of nanopores has remained elusive due to inherent difficulties in imaging the pores using microscopy techniques. The present manuscript reports on the quantitative measurement of the tapered structure of nanopores using SAXS. Determination of this structure was enabled by obtaining high quality SAXS data and the development of appropriate fitting models. The etch rates for both the radius at the polymer surface and the radius of the pore in bulk were calculated. Both etch rates decrease slightly with increasing fluence. This behavior is ascribed to the overlap of track halos which are characterized by cross-linking of the polymer chains. The halo radius was estimated to be approximately 120 nm. The influence of the observed nanopore shape on the pore transport properties was estimated and found to have a significant influence on the water flow rates compared to cylindrical pores. The results enable a better understanding of track-etched membranes and facilitate improved pore design for many applications.