Surfactant-enhanced control of track-etch pore morphology

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Abstract

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 creates 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 small 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 pore is hindered. The adsorbed surfactant layer is not permeable for large molecules. In contrast, small alkali molecules and water molecules diffuse into the track and provide the etching process enlarging the pore. At this stage 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 pore entrance. After a more prolonged etching the bottle-like (or “cigar-like”) pore channels are formed. The bottle-like shape of the pore channels depends on the etching conditions such as alkali and surfactant concentration, temperature, and type of the surfactant. The use of surfactants enables one to produce track-etch membranes with improved flow rate characteristics compared with those having cylindrical pores with the same nominal pore diameters.

Introduction

The formation of submicron pores of controlled geometry by the chemical etching of nuclear tracks in insulators has been first described by Price and Walker [1]. The technique of particle track-etching has found diverse use in science and technology [2]. Mechanisms of track evolution during chemical treatment have been the subject of intensive research aimed at the development of nano- and microstructures with pre-determined characteristics [2], [3], [4], [5], [6]. Among other possible track-recording materials, particular attention has been paid to polymers which serve as matrices for the so-called track-etch porous membranes. A number of parameters, such as bombarding particle energy loss, material properties, post-irradiation treatment conditions (storage in air, heating, exposure to ultraviolet light), composition of the etching solution, etc., have been studied with respect to their influence on the porous structure obtained by the track-etch technique. In this study we are focusing on the effect of surfactants on the etching of small pores. The use of surfactants as wetting agents, increasing the track to bulk etch rate ratio, was suggested a long time ago. Numerous experimental works have been performed with the use of surfactant-containing etchants. However, the role of surfactants in the chemical etching of particle tracks is much more complex than is customarily thought.

In the present work we restrict ourselves to a qualitative description of the surfactant effect on the track-etch pore formation. The function of a surfactant in a heterogeneous system always includes diffusion and adsorption of the surfactant molecules. These two processes can be studied by various experimental methods. In this work a conductometric method was used to monitor the pore growth during the chemical treatment [5], [6] and, thus, to estimate the influence of the adsorbed surfactant on the rate of the chemical reaction at the etchant/polymer interface within narrow pores. The final etched structures were examined by the scanning electron microscopy (SEM) to determine the pore channel profile. Combination of the results obtained by these methods made it possible to reveal a special function of a surfactant in the etching of nanopores in a solid.

Section snippets

Experimental

Polyethylene terephthalate (PET) films 5, 10 and 12 μm in thickness were “lavsan” (the USSR product), Hostaphan RE5 (Kalle) and Melinex (ICI), respectively. All three types were biaxially oriented foils with the density of about 1.39 g/cm2. Polycarbonate (PC) film Makrofol KG was obtained from Bayer (Germany), its thickness was 10 μm and the density was 1.2 g/cm2.

The film samples were irradiated with Kr (253 MeV) and Xe (150 MeV) ions at the cyclotrons of the Flerov Laboratory of Nuclear

Conductance measurements

The resistance R of the membrane itself was derived by subtraction of the resistance of electrolyte Re from the resistance of the cell Rc. The effective pore diameter, as a function of the etching time t, was obtained fromdeff(t)=[4l/πkNR(t)]1/2,where k is the specific conductivity of the solution used for etching, N is the number of tracks in the sample, and l is the sample thickness. No correction was made for the additional resistance at the pore channel ends, because in all our experiments

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