Surfactant-enhanced control of track-etch pore morphology

The influence of surfactants on the process of chemical development of ion tracks in polymers is studied. Based on the experimental data, a mechanism of the surfactant effect on the track-etch pore morphology is proposed. In the beginning of etching the surfactant is adsorbed on the surface and crea tes a layer that is quasi-solid and partially protects the surface from the etching agent. However, some etchant molecules diffuse through the barrier and react with the polymer surface. This results in the formation of a sma ll hole at the entrance to the ion track. After the hole has attained a few nanometers in diameter, the surfactant molecules penetrate into the track and cover its walls. Further diffusion of the surfactant into the growing p ore is hindered. The adsorbed surfactant layer is not permeable for large m olecules. Tn contrast, small alkali molecules and water molecules diffuse i nto the track and provide the etching process enlarging the pore. At this s tage the transport of the surfactant into the pore channel can proceed only due to the lateral diffusion in the adsorbed layer. The volume inside the pore is free of surfactant molecules and grows at a higher rate than the po re entrance. After a more prolonged etching the bottle-like (or "cigar-like ") pore channels are formed. The bottle-like shape of the pore channels dep ends on the etching conditions such as alkali and surfactant concentration, temperature, and type of the surfactant. The use of surfactants enables on e to produce track-etch membranes with improved flow rate characteristics c ompared with those having cylindrical pores with the same nominal pore diam eters. (C) 2001 Elsevier Science B.V. All rights reserved.