SAXS data modelling for the characterisation of ion tracks in polymers.

Here, we present new models to fit small angle X-ray scattering (SAXS) data for the characterization of ion tracks in polymers. Ion tracks in polyethylene terephthalate (PET), polycarbonate (PC), polyimide (PI) and polymethyl methacrylate (PMMA) were created by swift heavy ion irradiation using 197Au and 238U with energies between 185 MeV and 2.0 GeV. Transmission SAXS measurements were performed at the Australian Synchrotron. SAXS data were analysed using two new models that describe the tracks by a cylindrical structure composed of a highly damaged core with a gradual transition to the undamaged material. First, we investigate the 'Soft Cylinder Model', which assumes a smooth function to describe the transition region by a gradual change in density from a core to a matrix. As a simplified and computational less expensive version of the 'Soft Cylinder Model', the 'Core Transition Model' was developed to enable fast fitting. This model assumes a linear increase in density from the core to the matrix. Both models yield superior fits to the experimental SAXS data compared with the often-used simple 'Hard Cylinder Model' assuming a constant density with an abrupt transition.

Ion track etching of polycarbonate membranes monitored by in situ small angle X-ray scattering.

In situ small angle X-ray scattering (SAXS) measurements of ion track etching in polycarbonate foils are used to directly monitor the selective dissolution of ion tracks with high precision, including the early stages of etching. Detailed information about the track etching kinetics and size, shape, and size distribution of an ensemble of nanopores is obtained. Time resolved measurements as a function of temperature and etchant concentration show that the pore radius increases almost linearly with time for all conditions and the etching process can be described by an Arrhenius law. The radial etching shows a power law dependency on the etchant concentration. An increase in the etch rate with increasing temperature or concentration of the etchant reduces the penetration of the etchant into the polymer but does not affect the pore size distribution. The in situ measurements provide an estimate for the track etch rate, which is found to be approximately three orders of magnitude higher than the radial etch rate. The measurement methodology enables new experiments studying membrane fabrication and performance in liquid environments.

Osmotic Effects in Track-Etched Nanopores.

Asymmetrically etched ion-track membranes attract great interest for both fundamental and technical reasons because of a large variety of applications. So far, conductometric measurements during track etching provide only limited information about the complicated asymmetric etching process. In this paper, monitoring of osmotic phenomena is used to elucidate the initial phase of nanopore formation. It is shown that strong alkaline solutions generate a considerable osmotic flow of water through newborn conical pores. The interplay between diffusion and convection in the pore channel results in a substantially nonlinear alkali concentration gradient and a rapid change in the pore geometry after breakthrough. Similar phenomena are observed in experiments with cylindrical track-etched pores of 15-30 nm in radius. A theoretical description of the diffusion-convection processes in the pores is provided.

TiO2, SiO2, and Al2O3 coated nanopores and nanotubes produced by ALD in etched ion-track membranes for transport measurements.

Low-temperature atomic layer deposition (ALD) of TiO2, SiO2, and Al2O3 was applied to modify the surface and to tailor the diameter of nanochannels in etched ion-track polycarbonate membranes. The homogeneity, conformity, and composition of the coating inside the nanochannels are investigated for different channel diameters (18-55 nm) and film thicknesses (5-22 nm). Small angle x-ray scattering before and after ALD demonstrates conformal coating along the full channel length. X-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy provide evidence of nearly stoichiometric composition of the different coatings. By wet-chemical methods, the ALD-deposited film is released from the supporting polymer templates providing 30 µm long self-supporting nanotubes with walls as thin as 5 nm. Electrolytic ion-conductivity measurements provide proof-of-concept that combining ALD coating with ion-track nanotechnology offers promising perspectives for single-pore applications by controlled shrinking of an oversized pore to a preferred smaller diameter and fine-tuning of the chemical and physical nature of the inner channel surface.