Peculiarities of the formation of track-etched membranes by the data of atomic force microscopy and X-ray scattering

Abstract

Atomic force microscopy (AFM) is employed to obtain information on the main stages of the preparation of ultrafiltration track-etched membranes based on poly(ethylene terephthalate) (PET). The surface structure of initial commercial samples of PET films and the same films irradiated with various fluxes of accelerated heavy ions and subsequently treated with alkaline that results in the formation of pores owing to track etching is studied. It is shown that the order of the aforementioned stages of the formation of track-etched membranes with various porosities (from 0.02 to 6%) at a typical pore size of about 50 nm leads only to slight changes in surface structural parameters and does not fundamentally affect the polymer structure formed by spherulites with sizes that are comparable with the pore sizes. In this case, nearly the same content of the crystalline phase in the initial film and track-etched membrane are identified by large-angle X-ray scattering. The picture of X-ray scattering by track-etched membranes at small angles fully corresponds to the scattering on cylindrical pores with a diameter of about 50 nm. The analysis of the set of roughness profiles of the surfaces of initial films and track-etched membranes obtained by the AFM technique makes it possible to determine and introduce—in addition to standard parameters of the surface, the mean value of roughness and its standard deviation the correlation length characterizing the mean distance at which the “memory” of the roughness value is lost. It is shown that treatments resulting in the formation of track-etched membranes favor an increase in the values of roughness and practically do not affect the mean correlation length, thus supporting the conclusion of the invariance of the main structural parameters of the PET surface.