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Track etching technique in membrane technology
Radiation Measurements 34 (2001) 559–566
www.elsevier.com/locate/radmeas
Invited talk
Track etching technique in membrane technology P. Apel ∗ Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russia Received 28 August 2000; received in revised form 24 January 2001; accepted 19 March 2001
Abstract Track membrane (TM) technology is an example of industrial application of track etching technique. Track-etch membranes o0er distinct advantages over conventional membranes due to their precisely determined structure. Their pore size, shape and density can be varied in a controllable manner so that a membrane with the required transport and retention characteristics can be produced. The use of heavy ion accelerators made it possible to vary LET of track-forming particles, angle distribution of pore channels and pore lengths. So far the track formation and etching process has been studied in much detail for several polymeric materials. Today we understand determining factors and have numerous empirical data enabling us to manufacture any particular product based on polyethylene terephthalate (PET) or polycarbonate (PC) 7lms. Pore shape can be made cylindrical, conical, funnel-like, or cigar-like at will. A number of modi7cation methods has been developed for creating TMs with special properties and functions. Applications of “conventional” track membranes can be categorized into three groups: process 7ltration, cell culture, and laboratory 7ltration. The use in biology stands out among other areas. Nuclear track pores c 2001 Elsevier 7nd diverse applications as model systems and as templates for the synthesis of micro- and nanostructures. Science Ltd. All rights reserved. Keywords: Nuclear tracks; Polymers; Chemical etching; Membranes
1. Introduction The use of nuclear tracks for the production of porous membranes was proposed almost immediately after the discovery of particle track etching in thin sheets of materials (Fleischer et al., 1964). This achievement became an industrial technology quite fast. Since the early 1970s the polycarbonate track membranes were available on the market. Basic information on the properties and possible applications of TMs was presented in a classical monograph (Fleischer et al., 1975). Further progress in this 7eld was associated with new particle sources (accelerators), studies of new polymeric materials, search for new applications and development of numerous methods of modi7cation. The author of the present report has been participating in the R&D work on TMs during the past three decades. The knowledge accumulated during this period refers to a wide ∗ Tel.: +7-9621-63544; fax: +7-9621-65955. E-mail address: [email protected] (P. Apel).
area, from special features of track etching to modeling of biological ion channels. Paying tribute to the pioneering works, we have also selected for this review a number of recent reports from di0erent laboratories, which illustrates a worldwide spread of the technology. 2. Irradiation There are two basic methods of producing latent tracks in the foils to be transformed into porous membranes. The 7rst method is based on the irradiation with fragments from the 7ssion of heavy nuclei such as californium or uranium (Fleischer et al., 1964, 1975). Exposing a uranium target (converter) to a neutron Dux from a nuclear reactor initiates the 7ssion of 235 U. Typical energy losses (linear energy transfer = LET) of the 7ssion fragments are about 10 keV=nm. The 7ssion fragments coming from a thin layer target have an almost isotropic angle distribution. To create an array of latent tracks penetrating the foil, a collimator
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is normally used. The advantages of the 7ssion fragment tracking are (i) good stability of a particle Dux in time; (ii) a non-parallel particle Dux (enables the production of membranes with a high porosity and low percent of overlapping pore channels); (iii) relatively low cost. The limitations of the method are caused by (i) the contamination of the tracked foil with radioactive products (“cooling” of the irradiated material is needed, which usually takes a few months); (ii) a limited range of the 7ssion fragments = limited thickness of the membrane; (iii) limited possibilities of creating various angle distributions of pore channels; (iv) fragments of di0erent masses and energies produce tracks with di0erent etching properties. The second method is based on the use of ion beams from accelerators (Fleischer et al., 1975; Tretyakova et al., 1977; Fischer and Spohr, 1983; Spohr, 1980; Vater, 1988; Flerov et al., 1989; Lueck et al., 1990; Bieth and the SAIF Group, 1991). The intensity of the ion beam should be at least 1011 s−1 to be competitive in the track membrane industry. Modern accelerators provide beams of higher intensities. The energies of accelerated ions are a few MeV per nucleon. The beams can be pulsing or continuous. To irradiate large areas a scanning beam is normally used. The design of irradiation facilities has been described in literature (Flerov et al., 1989; Bieth and the SAIF Group, 1991; Oganessian, 1993; Oganessian et al., 2000). The advantages of the accelerator tracking method are (i) no radioactive contamination of the material when the ion energy is below the Coulomb barrier; (ii) identity of bombarding particles = all tracks show the same etching properties; (iii) higher energy of particles = larger range = thicker foils can be perforated; (iv) better conditions for producing high-density (¿109 cm2 ) track arrays; (v) particles heavier than 7ssion fragments can be used (238 U, for instance); (vi) it is easier to control the impact angle and produce arrays of parallel tracks or create some particular angular distributions for getting rid of merging pores (Flerov et al., 1989). However, the stability of the particle Dux from an accelerator is usually lower. Another disadvantage is a higher cost of irradiation. In the past decade we observed a decrease in the popularity of reactor-based irradiation facilities and an increase in the use of accelerators. Heavy ion accelerators employed for the irradiation of polymeric foils on a commercial scale are a tandem in Brookheaven and cyclotrons in Dubna, Louvain-la-Neuve and Berlin. Research work in the track membrane domain is being carried out using a linear accelerator at GSI, Darmstadt, (Spohr, 1980) and the AVF cyclotron in Takasaki (Hagiwara, 1991). There were also reports from IPN, Orsay, (Guillot and Rondelez, 1981), RIKEN, Saitama, (Nakanishi et al., 1990), ZFK, Rossendorf (Lueck et al., 1990), GANIL, Caen, (Bieth and the SAIF Group, 1991), Io0e institute, S.-Petersburg (Kudoiarov et al., 1998), IPPE, Obninsk, (Romanov et al., 1998) describing the use of accelerators for the production of the track-etch membranes.
3. Chemical etching Chemical etching is a process of pore formation. During chemical etching the damaged zone of a latent track is removed and transformed into a hollow channel (Fleischer et al., 1964, 1975). It is the pore-size-determining and pore-shape-determining stage of the technology. The simplest description of pore geometry is based on two parameters—the bulk etch rate VB and the track etch rate VT (Fig. 1A). It is applicable for larger (¿1 m in diameter) pores. The conical pore shape is transformed into a cylindrical one at VT VB . The geometry of smaller pores is determined also by the size and structure of the damaged zone around the particle path (Fig. 1B). The submicroscopic kinetics of pore formation can be described using the method proposed by Mazzei et al. (1985). Even more complex kinetics of pore growth takes place in an etchant containing a surfactant (Fig. 1C). The bulk etch rate depends on the material, on the etchant composition and on the temperature. The track etch rate depends on a much greater number of factors. They can be classi7ed into a few categories: sensitivity of the material, irradiation conditions, post-irradiation conditions, and etching conditions. The irradiation conditions include parameters of the bombarding particle (LET characterizes quite well the damaging ability of a particle, Fleischer et al., 1975), atmosphere (vacuum, presence of oxygen) and temperature (O’Sullivan and Thompson, 1980; Apel et al., 1997a). In order to achieve reproducible results in the subsequent track etching process, one has to maintain the mentioned conditions constant. The track etch rate generally increases with increasing LET, however at very high LET values (close to 10 keV=nm and higher) this function tends to saturation or even may show a maximum (Apel et al., 1997b). Part of radiolysis products in heavy ion tracks are chemically active species undergoing post-irradiation reactions, such as oxidation, photo-oxidation, etc. For this reason a storage of the tracked polymers (PC or PET) in air leads to a signi7cant increase in the track etch rate (Benton and Henke, 1969; DeSorbo, 1979; Tretyakova et al., 1980). The acceleration of this process with the help of exposure to ultraviolet light (UV) is used in a technological process of production of polyethyleneterephthalate track membranes (Kuznetsov et al., 1991; Apel, 1995). There are some other methods of track sensitization. Heavy ion tracks in PET can be e0ectively sensitized by the treatment with organic solvents (Lueck et al., 1990). The storage at elevated temperature leads to a fading of tracks in most polymers, however there are some polymers showing the opposite behavior. An example is polypropylene in which a moderate heating enhances the particle tracks due to thermo-oxidation (Apel et al., 1996). In most cases the variations of the etchant composition, component concentrations and temperature give us a chance to vary the track to bulk etch rate ratio in a very wide range.
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Fig. 1. (A) Etched pore geometry to a 7rst approximation. The pore shape (a cone) is determined by the chemical dissolution along the particle track at a rate VT and general attack on the etched surface and on the interior surface of the etched track at a lesser rate VB (Fleischer et al., 1975). (B) Pore shape on a submicroscopic scale. The pore geometry depends on the size of the highly damaged zone (core), dc , and the diameter of the cross-linked halo, dh . The latter is etched out at a rate lower than VB (Mazzei et al., 1986; Apel et al., 1997b). (C) Pore geometry dictated by the presence of a surfactant (Apel et al., 2000). The bottleneck is formed due to the protective layer of the surfactant adsorbed on the surface. The diameter of the neck, ds , depends on the length of the surfactant molecules and their con7guration; the length of the neck, ls , is determined by the hindered di0usion of the surfactant molecules into the etched track.
For PET exposed to Xe ions, a change in the concentration of alkali in aqueous solutions enables us to gradually change the etch rate ratio from units to thousands (Tretyakova et al., 1980). In polycarbonate, the etch rate ratio of ∼105 can be achieved for UV-sensitized heavy ion tracks if etching is performed in aqueous alkali solutions (Guillot and Rondelez, 1981). The same tracks show the etch rate ratio of 2– 4 in etchants composed of alkali and alcohols such as methanol, ethanol or propanol. An addition of alcohol to the etchant makes it possible to control the pore cone angle also in CR-39 (Berndt et al., 1986). The activation energy of the track etching process often di0ers from that of the bulk etching (DeSorbo, 1979; Tretyakova et al., 1980; Lueck, 1982). Thus, changing the temperature during etching can be used to increase or decrease the etch rate ratio. Another key parameter is the size of the damaged zone that dissolves at a di0erent rate compared with that for the bulk material. Systematic measurements have been performed with PC and PET for the estimation of this parameter depending on the LET of bombarding particles (Petersen and Enge, 1995; Apel et al., 1997b, 1998). The heaviest ions produce a larger damaged track core and a larger cross-linked halo. At the same time, the heaviest ions do not necessarily provide the highest etch rate ratio. From the comparison of the response function with the damaged zone diameter vs. LET function the conclusion was made that the ions with moderate masses (Kr or Xe) are favored for tracking the polymer foils.
4. Some particular polymers for the track membrane production The process of track formation and etching in polyethylene terephthalate is well-studied (Tretyakova et al., 1980;
Lueck, 1982). PET is rather stable in acids, organic solvents, biologically inert, and mechanically strong. The high etch rate ratio is achievable (when using UV sensitization) which makes it possible to produce a wide range of membranes with di0erent pore diameters. The etching procedure is simple and fast. Alkali solutions (sometimes with additives) are used to develop tracks. The membranes are relatively hydrophilic without any additional modi7cation. Polycarbonate is the material that has been used for the track membrane production since the seventies (Fleischer et al., 1975). The production technology is very close to that for PET. Compared to PET, the sensitivity of PC is higher, which makes it possible to produce membranes with a pore diameter as small as ∼0:01 m and omit the UV sensitization stage. Polycarbonate track membranes di0er from PET membranes by a lower resistance to organic solvents and a lower wettability. Polypropylene (PP) was investigated as a raw material for membranes to be used for the 7ltration of some aggressive liquids such as strong alkali solutions or inorganic acids. Chemical etching in strong oxidizers (chromium trioxide) is e0ective in developing the latent tracks in PP (Kravets et al., 1997). The etching procedure is considerably more complex than the etching of PET or PC. Samples of PP track membranes with various pore diameters ranging between 0.1 and 3 m have been fabricated on a laboratory scale. The membranes are hydrophobic. A commercial production of PP membranes has not been launched because of the limited estimated market. Much e0ort was made to develop track membranes from Duorinated polymers. The best results were obtained with polyvinylidene 2uoride (PVDF) that exhibits good chemical, mechanical and particle registration properties. PVDF has also some special properties (piezoelectricity) that attracts the attention of researchers. Samples of PVDF track
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membranes were produced and tested in several laboratories (Vater, 1988; Komaki et al., 1988; Shirkova and Tretyakova, 1997). Similarly to PP, the etching of PVDF with strong oxidizers creates problems. Normally it takes hours to achieve a pore size in the micrometer range. Commercial production of PVDF track membranes was not set up because it cannot compete with the casting PVDF membranes existing on the market. Many other Duorinated polymers have been studied and etchable tracks have been observed in some of them. However, numerous attempts to obtain porous structure in fully Duorinated polymers by the track-etch method were not successful. Polyimides (PI) have an excellent stability at high temperatures, excellent mechanical strength, and extraordinary radiation resistance. Films of polypyrromelithimide di0erent in thickness are available for the past 20 years. Research work on PI as a track-recording medium has been performed in several laboratories (Vater, 1988; Komaki et al., 1989; Vilensky et al., 1993; Samoilova et al., 1993; Trautmann et al., 1996). The possibility of track membrane manufacturing has been shown. A few recipes for track etching in polyimides were reported. NaClO and H2 O2 provide a rather high etch rate ratio enabling one to produce a porous membrane of good quality. However, instability of the etchants in time at high temperatures is a serious problem for the technology. CR-39—a highly sensitive to ionizing particles and highly transparent material—can be used to fabricate track-etch membranes for special purposes (Berndt et al., 1986). Some copolymers of CR-39 have been shown to have even higher sensitivity than CR-39 itself (Ogura et al., 1995). The polymer is brittle which impedes the manufacturing and using thin foils. Attempts have been made to produce track-etch pores in aromatic polysulfones (Apel et al., 1997a) and polyetheretherketones (Daubresse et al., 1997). These highly aromatic materials showed a very low sensitivity to energetic heavy particles and, consequently, a very low etch rate ratio not allowing one to get a homogeneous porous structure. In contrast, the registration properties of polyethylene naphthalate have been found quite good (Komaki and Tsujimura, 1976; Starosta et al., 1999). 5. Structure and transport properties of track membranes Track membranes are known as precise porous 7lms with a very narrow pore size distribution (Fleischer et al., 1964; Fischer and Spohr, 1983; Flerov et al., 1989; Calvo et al., 1995). A unique property of TMs is that the number of pores and the pore size are two almost independent parameters which can be varied in a very wide range. The pore diameter can be from 10 nm to tens of micrometers. The pore density can vary from 1 to 1010 cm−2 . No other type of membranes provides such a possibility. In most cases the pore geometry
is very simple (cylinders, cones or combinations of these) which leads to simple relationships between the membrane structure parameters and transport characteristics such as the gas or water Dow rate. Poiseuille and Knudsen formulae adequately describe the viscous Dow and the molecular Dow through a track membrane (Beck and Schultz, 1972). In the production process the 7nished membrane is normally subjected to a rigorous testing by scanning electron microscopy, gas and water Dow rate methods, bubble point, and some other measurements (Kuznetsov et al., 1991; Apel, 1995). Now we understand the determining factors and have numerous empirical data enabling us to manufacture any particular product based on PET and PC. Pore shape can be made cylindrical, conical, funnel-like, and cigar-like at will. Various asymmetric structures can be also produced (Fischer and Spohr, 1983). The scanning electron micrographs in Fig. 2 illustrate the variety of pore structure in track membranes. The separation properties of TMs depend, 7rst of all, on the structure parameters. In the absence of adsorption, the track membrane acts as a “screen” membrane. The retention of particles is determined by the relationship between the pore diameter and the particle size (Pall et al., 1980; Yasminov et al., 1987). If the surface of the membrane adsorbs the particles, the retention characteristics change drastically. In this case the result depends on the nature of the membrane surface and the nature of the particles, and also depends on pH, presence of surfactants, etc. (Bisio et al., 1980; Keesom et al., 1987). Adsorption of solutes leads to a decrease in the 7ltration rate or to the loss of a substance that is expected to pass through the membrane (Tracey and Davis, 1994; van den Oestelaar et al., 1989). An example is sorption of proteins onto the PET membrane surface that is negatively charged at medium and high pH. To solve such problems, track membranes with various speci7c surface properties have to be manufactured. 6. Modi!cation methods The modi7cation methods that change the surface and bulk properties of track membranes or impart new functions to them are (i) sorption of some small or large molecules on the surface; (ii) surface treatment with plasma; (iii) immobilization of functional substances on the surface by covalent binding using chemical reactions; (iv) graft polymerization of various monomers. Hydrophilization of PET and PC track membranes by adsorption of polyvinyl pyrrolidone is a widely used procedure in industry. Similarly, covering the surface with silicone oil or paraRn can make the membranes hydrophobic. Plasma treatment improves wettability of many polymers due to the formation of new polar groups on the surface. Stability of the modi7ed surface signi7cantly depends on the condition of the plasma treatment (Vilensky et al., 1991; Dmitriev et al., 1998). Electrochemical properties of track-etched
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Fig. 2. A few examples of porous structures produced in thin polymeric 7lms using various methods of irradiation and chemical treatment: (A) cross section of a polycarbonate TM with cylindrical non-parallel pore channels; (B) polypropylene TM with slightly conical (tapered towards the center) parallel pores; (C) polyethylene terephthalate TM with cigar-like pores; (D) polyethylene terephthalate TM with “bow-tie” pores.
pores can be modi7ed by covalent binding of charged groups or by adsorption of ionic polyelectrolytes (Froehlich and Woermann, 1986). The immobilization of aminoacids to the PET track membranes based on the reactions of end carboxyl and hydroxyl groups was reported (Marchand-Brynaert et al., 1995; Mougenot et al., 1996). However, the surface density of the immobilized in this way species is rather low. The radiation-induced graft polymerization onto track membranes is a process which has been studied in more detail (Zhitariuk et al., 1989; Zhitariuk, 1993; Tischenko et al., 1991; Shtanko and Zhitariuk, 1995). Styrene (St), methacrylic acid (MAA), N -vinyl pyrrolidone (VP), 2-methyl 5-vinyl pyridine (2M5VP), N -isopropyl acrylamide (NIPAAM) and some other monomers have been grafted onto PET track membranes. Grafting of St increases the chemical resistance and makes the membrane hydrophobic. MAA and VP were grafted onto TMs to increase wettability which is especially important when aqueous solutions are 7ltered through small-pore membranes. 2M5VP was grafted with the aim to make the membrane hydrophilic and change its surface charge from negative to positive. During the past decade the grafting of NIPAAM and other intelligent polymers were extensively studied in the research work carried out at TRCRE (Takasaki) and GSI (Darmstadt) (Yoshida et al., 1993, 1997; Reber et al., 1995).
7. Applications Applications of commercially produced track membranes can be categorized into three groups: (i) process 7ltration; (ii) cell culture; (iii) laboratory 7ltration. The process 7ltration implies the use of membranes mostly in the form of cartridges with a membrane area of at least 1 m2 . Puri7cation of deionized water in microelectronics, 7ltration of beverages, separation and concentration of various suspensions are typical examples. There is a strong competition with other types of membranes available on the market. Casting membranes often provide a higher dirt loading capacity and a higher throughput. For this reason the use of track membranes in this 7eld is still limited (Brock, 1984). In the recent years a series of products were developed for the use in the domain called cell and tissue culture (Stevenson et al., 1988; Sergent-Engelen et al., 1990; Peterson and Gruenhaupt, 1990; Rothman and Orci, 1990). Adapted over the years to a variety of cell types, porous membrane 7lters are now recognized as providing signi7cant advantages for cultivating cells and studying the cellular activities such as transport, absorption and secretion (van Hinsbergh et al., 1990). The use of permeable support systems based on TMs has proven to be a valuable tool in the cell biology (Costar=Nuclepore Catalog, 1992).
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A traditional application of track membranes is the laboratory 7ltration (Nash, 1990). TM is a very good instrument when small particles should be collected onto the membrane surface and analyzed. For such purposes track membranes are delivered in the form of disks and 7ltration kits. The use of track-etch pores (often specially produced for a certain experiment) for solving various scienti7c tasks should be mentioned separately. One-pore (DeBlois and Bean, 1970; Fischer and Spohr, 1983), oligo-pore (Koutsouris et al., 1989) and multi-pore samples can serve as unique models for studying the transport of liquids, gases, particles, solutes, electrolytes (Beck and Schultz, 1972; Meares and Page, 1972; Deen, 1987), and electromagnetic waves (Mitrofanov et al., 1991) through narrow channels. Many experiments of this kind are relevant to biological and medical topics (Pasternak et al., 1995). Track membranes can also serve as templates for making various micro- and nanostructures. Magnetic, conducting and superconducting nanowires possessing special properties have been manufactured in this way (Whitney et al., 1993; Martin, 1994; Piraux et al., 1994; Zhitariuk et al., 1995). Using small-pore TMs as matrices, electrically switchable ion-selective membranes and sensors can be produced (Nishizawa et al., 1995).
8. Conclusion First established on nuclear reactors, nowadays the track-etch method of membrane production is mainly based on the use of heavy-ion accelerators. A few centers in the world manufacture track-etch membranes on a commercial scale. The track formation and etching processes are investigated in detail for polycarbonate and polyethylene terephthalate. The development of TMs on the basis of other polymers is still limited. DiRculties in the etching procedure, poor reproducibility of 7lmy material properties or high cost of the 7nal product impede the manufacture of TMs from new materials. The advantage of TMs over conventional membranes is their precisely determined structure. However, many large-scale applications are “insensitive” to such a brilliant property of TMs. Track membranes occupy a niche of biological, medical, analytical and scienti7c applications. This type of membranes is indispensable for manipulations with small particles of living and any other matter. The track membranes seem to be the best porous material for providing a controllable transport of solutes. Further progress in the TM technology can be connected with the creation of membranes having particular properties for a particular use. Membranes that do not adsorb proteins, membranes with various functional groups on the surface, etc., might be developed and introduced into industry. In the author’s opinion, the unique optical properties of etched tracks deserve more attention than it has been paid to them so far.
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van den Oestelaar, P.J.M., Mentink, I.M., Brinks, G.J., 1989. Loss of peptides and proteins upon sterile 7ltration due to adsorption to membrane 7lters. Drug Dev. Ind. Pharm. 15, 97–106. van Hinsbergh, V.W.M., Sche0er, M.A., Langeler, E.G., 1990. Macro- and microvascular endothelial cells from human tissues. In: Piper, H.M. (Ed.), Cell Culture Techniques in Heart and Vessel Research. Springer, Berlin, Germany, pp. 178–204. Vater, P., 1988. Production and applications of nuclear track micro7lters. Nucl. Tracks Radiat. Meas. 15, 743–749. Vilensky, A.I., Berezkin, V.V., Mchedlishvili, B.V., 1991. Modi7cation of track membranes in a glow discharge plasma. Colloid J. (USSR) 51, 117–120 (see Engl. Transl). Vilensky, A.I., Oleinikov, V.A., Markov, N.G., Mchedlishvili, B.V., Dontsova, E.P., 1993. Polyimide track membranes for ultra- and micro7ltration. Russ. High Polym. 36(A), 475 – 485 (see Engl. Transl.). Whitney, T.N., Jiang, J.S., Searson, P.C., Chien, C.L., 1993. Fabrication and magnetic properties of arrays of metallic nanowires. Science 261, 1316–1319. Yasminov, A.A., Blum, G.Z., Efremov, A.A., Volodin, V.F., 1987. Micro7ltration of high-purity substances. High Purity Substances (USSR), No. 3, 160 –170 (in Russian). Yoshida, M., Tamada, M., Asano, M., Omichi, H., Kubota, H., Katakai, R., Spohr, R., Vetter, J., 1993. Stimulus-responsive track pores. Radiat. E0. Defects Solids 126, 409–412. Yoshida, M., Asano, M., Omichi, H., Spohr, R., Katakai, R., 1997. Substrate-speci7c functional membranes based on etched ion tracks. Radiat. Meas. 28, 799–810. Zhitariuk, N.I., 1993. PETP track membranes modi7ed by radiation grafting. In: Perelygin, V.P. (Ed.), II International Workshop on Solid State Nuclear Track Detectors and Their Applications, Dubna, 24 –26 March 1992, JINR E3-93-1, pp. 169 –172. Zhitariuk, N.I., Kuznetsov, V.I., Shtanko, N.I., 1989. Radiation-induced modi7cation of nuclear membranes on the basis of poly(ethylene terephthalate). Environ. Prot. Eng. 15, 111–119. Zhitariuk, N.I., LeMoel, A., Mermilliod, N., Trautmann, C., 1995. Polymerization of pyrrole into track membranes. Nucl. Instrum. Methods B105, 204–207.
www.elsevier.com/locate/radmeas
Invited talk
Track etching technique in membrane technology P. Apel ∗ Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russia Received 28 August 2000; received in revised form 24 January 2001; accepted 19 March 2001
Abstract Track membrane (TM) technology is an example of industrial application of track etching technique. Track-etch membranes o0er distinct advantages over conventional membranes due to their precisely determined structure. Their pore size, shape and density can be varied in a controllable manner so that a membrane with the required transport and retention characteristics can be produced. The use of heavy ion accelerators made it possible to vary LET of track-forming particles, angle distribution of pore channels and pore lengths. So far the track formation and etching process has been studied in much detail for several polymeric materials. Today we understand determining factors and have numerous empirical data enabling us to manufacture any particular product based on polyethylene terephthalate (PET) or polycarbonate (PC) 7lms. Pore shape can be made cylindrical, conical, funnel-like, or cigar-like at will. A number of modi7cation methods has been developed for creating TMs with special properties and functions. Applications of “conventional” track membranes can be categorized into three groups: process 7ltration, cell culture, and laboratory 7ltration. The use in biology stands out among other areas. Nuclear track pores c 2001 Elsevier 7nd diverse applications as model systems and as templates for the synthesis of micro- and nanostructures. Science Ltd. All rights reserved. Keywords: Nuclear tracks; Polymers; Chemical etching; Membranes
1. Introduction The use of nuclear tracks for the production of porous membranes was proposed almost immediately after the discovery of particle track etching in thin sheets of materials (Fleischer et al., 1964). This achievement became an industrial technology quite fast. Since the early 1970s the polycarbonate track membranes were available on the market. Basic information on the properties and possible applications of TMs was presented in a classical monograph (Fleischer et al., 1975). Further progress in this 7eld was associated with new particle sources (accelerators), studies of new polymeric materials, search for new applications and development of numerous methods of modi7cation. The author of the present report has been participating in the R&D work on TMs during the past three decades. The knowledge accumulated during this period refers to a wide ∗ Tel.: +7-9621-63544; fax: +7-9621-65955. E-mail address: [email protected] (P. Apel).
area, from special features of track etching to modeling of biological ion channels. Paying tribute to the pioneering works, we have also selected for this review a number of recent reports from di0erent laboratories, which illustrates a worldwide spread of the technology. 2. Irradiation There are two basic methods of producing latent tracks in the foils to be transformed into porous membranes. The 7rst method is based on the irradiation with fragments from the 7ssion of heavy nuclei such as californium or uranium (Fleischer et al., 1964, 1975). Exposing a uranium target (converter) to a neutron Dux from a nuclear reactor initiates the 7ssion of 235 U. Typical energy losses (linear energy transfer = LET) of the 7ssion fragments are about 10 keV=nm. The 7ssion fragments coming from a thin layer target have an almost isotropic angle distribution. To create an array of latent tracks penetrating the foil, a collimator
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 2 8 - 1
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is normally used. The advantages of the 7ssion fragment tracking are (i) good stability of a particle Dux in time; (ii) a non-parallel particle Dux (enables the production of membranes with a high porosity and low percent of overlapping pore channels); (iii) relatively low cost. The limitations of the method are caused by (i) the contamination of the tracked foil with radioactive products (“cooling” of the irradiated material is needed, which usually takes a few months); (ii) a limited range of the 7ssion fragments = limited thickness of the membrane; (iii) limited possibilities of creating various angle distributions of pore channels; (iv) fragments of di0erent masses and energies produce tracks with di0erent etching properties. The second method is based on the use of ion beams from accelerators (Fleischer et al., 1975; Tretyakova et al., 1977; Fischer and Spohr, 1983; Spohr, 1980; Vater, 1988; Flerov et al., 1989; Lueck et al., 1990; Bieth and the SAIF Group, 1991). The intensity of the ion beam should be at least 1011 s−1 to be competitive in the track membrane industry. Modern accelerators provide beams of higher intensities. The energies of accelerated ions are a few MeV per nucleon. The beams can be pulsing or continuous. To irradiate large areas a scanning beam is normally used. The design of irradiation facilities has been described in literature (Flerov et al., 1989; Bieth and the SAIF Group, 1991; Oganessian, 1993; Oganessian et al., 2000). The advantages of the accelerator tracking method are (i) no radioactive contamination of the material when the ion energy is below the Coulomb barrier; (ii) identity of bombarding particles = all tracks show the same etching properties; (iii) higher energy of particles = larger range = thicker foils can be perforated; (iv) better conditions for producing high-density (¿109 cm2 ) track arrays; (v) particles heavier than 7ssion fragments can be used (238 U, for instance); (vi) it is easier to control the impact angle and produce arrays of parallel tracks or create some particular angular distributions for getting rid of merging pores (Flerov et al., 1989). However, the stability of the particle Dux from an accelerator is usually lower. Another disadvantage is a higher cost of irradiation. In the past decade we observed a decrease in the popularity of reactor-based irradiation facilities and an increase in the use of accelerators. Heavy ion accelerators employed for the irradiation of polymeric foils on a commercial scale are a tandem in Brookheaven and cyclotrons in Dubna, Louvain-la-Neuve and Berlin. Research work in the track membrane domain is being carried out using a linear accelerator at GSI, Darmstadt, (Spohr, 1980) and the AVF cyclotron in Takasaki (Hagiwara, 1991). There were also reports from IPN, Orsay, (Guillot and Rondelez, 1981), RIKEN, Saitama, (Nakanishi et al., 1990), ZFK, Rossendorf (Lueck et al., 1990), GANIL, Caen, (Bieth and the SAIF Group, 1991), Io0e institute, S.-Petersburg (Kudoiarov et al., 1998), IPPE, Obninsk, (Romanov et al., 1998) describing the use of accelerators for the production of the track-etch membranes.
3. Chemical etching Chemical etching is a process of pore formation. During chemical etching the damaged zone of a latent track is removed and transformed into a hollow channel (Fleischer et al., 1964, 1975). It is the pore-size-determining and pore-shape-determining stage of the technology. The simplest description of pore geometry is based on two parameters—the bulk etch rate VB and the track etch rate VT (Fig. 1A). It is applicable for larger (¿1 m in diameter) pores. The conical pore shape is transformed into a cylindrical one at VT VB . The geometry of smaller pores is determined also by the size and structure of the damaged zone around the particle path (Fig. 1B). The submicroscopic kinetics of pore formation can be described using the method proposed by Mazzei et al. (1985). Even more complex kinetics of pore growth takes place in an etchant containing a surfactant (Fig. 1C). The bulk etch rate depends on the material, on the etchant composition and on the temperature. The track etch rate depends on a much greater number of factors. They can be classi7ed into a few categories: sensitivity of the material, irradiation conditions, post-irradiation conditions, and etching conditions. The irradiation conditions include parameters of the bombarding particle (LET characterizes quite well the damaging ability of a particle, Fleischer et al., 1975), atmosphere (vacuum, presence of oxygen) and temperature (O’Sullivan and Thompson, 1980; Apel et al., 1997a). In order to achieve reproducible results in the subsequent track etching process, one has to maintain the mentioned conditions constant. The track etch rate generally increases with increasing LET, however at very high LET values (close to 10 keV=nm and higher) this function tends to saturation or even may show a maximum (Apel et al., 1997b). Part of radiolysis products in heavy ion tracks are chemically active species undergoing post-irradiation reactions, such as oxidation, photo-oxidation, etc. For this reason a storage of the tracked polymers (PC or PET) in air leads to a signi7cant increase in the track etch rate (Benton and Henke, 1969; DeSorbo, 1979; Tretyakova et al., 1980). The acceleration of this process with the help of exposure to ultraviolet light (UV) is used in a technological process of production of polyethyleneterephthalate track membranes (Kuznetsov et al., 1991; Apel, 1995). There are some other methods of track sensitization. Heavy ion tracks in PET can be e0ectively sensitized by the treatment with organic solvents (Lueck et al., 1990). The storage at elevated temperature leads to a fading of tracks in most polymers, however there are some polymers showing the opposite behavior. An example is polypropylene in which a moderate heating enhances the particle tracks due to thermo-oxidation (Apel et al., 1996). In most cases the variations of the etchant composition, component concentrations and temperature give us a chance to vary the track to bulk etch rate ratio in a very wide range.
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Fig. 1. (A) Etched pore geometry to a 7rst approximation. The pore shape (a cone) is determined by the chemical dissolution along the particle track at a rate VT and general attack on the etched surface and on the interior surface of the etched track at a lesser rate VB (Fleischer et al., 1975). (B) Pore shape on a submicroscopic scale. The pore geometry depends on the size of the highly damaged zone (core), dc , and the diameter of the cross-linked halo, dh . The latter is etched out at a rate lower than VB (Mazzei et al., 1986; Apel et al., 1997b). (C) Pore geometry dictated by the presence of a surfactant (Apel et al., 2000). The bottleneck is formed due to the protective layer of the surfactant adsorbed on the surface. The diameter of the neck, ds , depends on the length of the surfactant molecules and their con7guration; the length of the neck, ls , is determined by the hindered di0usion of the surfactant molecules into the etched track.
For PET exposed to Xe ions, a change in the concentration of alkali in aqueous solutions enables us to gradually change the etch rate ratio from units to thousands (Tretyakova et al., 1980). In polycarbonate, the etch rate ratio of ∼105 can be achieved for UV-sensitized heavy ion tracks if etching is performed in aqueous alkali solutions (Guillot and Rondelez, 1981). The same tracks show the etch rate ratio of 2– 4 in etchants composed of alkali and alcohols such as methanol, ethanol or propanol. An addition of alcohol to the etchant makes it possible to control the pore cone angle also in CR-39 (Berndt et al., 1986). The activation energy of the track etching process often di0ers from that of the bulk etching (DeSorbo, 1979; Tretyakova et al., 1980; Lueck, 1982). Thus, changing the temperature during etching can be used to increase or decrease the etch rate ratio. Another key parameter is the size of the damaged zone that dissolves at a di0erent rate compared with that for the bulk material. Systematic measurements have been performed with PC and PET for the estimation of this parameter depending on the LET of bombarding particles (Petersen and Enge, 1995; Apel et al., 1997b, 1998). The heaviest ions produce a larger damaged track core and a larger cross-linked halo. At the same time, the heaviest ions do not necessarily provide the highest etch rate ratio. From the comparison of the response function with the damaged zone diameter vs. LET function the conclusion was made that the ions with moderate masses (Kr or Xe) are favored for tracking the polymer foils.
4. Some particular polymers for the track membrane production The process of track formation and etching in polyethylene terephthalate is well-studied (Tretyakova et al., 1980;
Lueck, 1982). PET is rather stable in acids, organic solvents, biologically inert, and mechanically strong. The high etch rate ratio is achievable (when using UV sensitization) which makes it possible to produce a wide range of membranes with di0erent pore diameters. The etching procedure is simple and fast. Alkali solutions (sometimes with additives) are used to develop tracks. The membranes are relatively hydrophilic without any additional modi7cation. Polycarbonate is the material that has been used for the track membrane production since the seventies (Fleischer et al., 1975). The production technology is very close to that for PET. Compared to PET, the sensitivity of PC is higher, which makes it possible to produce membranes with a pore diameter as small as ∼0:01 m and omit the UV sensitization stage. Polycarbonate track membranes di0er from PET membranes by a lower resistance to organic solvents and a lower wettability. Polypropylene (PP) was investigated as a raw material for membranes to be used for the 7ltration of some aggressive liquids such as strong alkali solutions or inorganic acids. Chemical etching in strong oxidizers (chromium trioxide) is e0ective in developing the latent tracks in PP (Kravets et al., 1997). The etching procedure is considerably more complex than the etching of PET or PC. Samples of PP track membranes with various pore diameters ranging between 0.1 and 3 m have been fabricated on a laboratory scale. The membranes are hydrophobic. A commercial production of PP membranes has not been launched because of the limited estimated market. Much e0ort was made to develop track membranes from Duorinated polymers. The best results were obtained with polyvinylidene 2uoride (PVDF) that exhibits good chemical, mechanical and particle registration properties. PVDF has also some special properties (piezoelectricity) that attracts the attention of researchers. Samples of PVDF track
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membranes were produced and tested in several laboratories (Vater, 1988; Komaki et al., 1988; Shirkova and Tretyakova, 1997). Similarly to PP, the etching of PVDF with strong oxidizers creates problems. Normally it takes hours to achieve a pore size in the micrometer range. Commercial production of PVDF track membranes was not set up because it cannot compete with the casting PVDF membranes existing on the market. Many other Duorinated polymers have been studied and etchable tracks have been observed in some of them. However, numerous attempts to obtain porous structure in fully Duorinated polymers by the track-etch method were not successful. Polyimides (PI) have an excellent stability at high temperatures, excellent mechanical strength, and extraordinary radiation resistance. Films of polypyrromelithimide di0erent in thickness are available for the past 20 years. Research work on PI as a track-recording medium has been performed in several laboratories (Vater, 1988; Komaki et al., 1989; Vilensky et al., 1993; Samoilova et al., 1993; Trautmann et al., 1996). The possibility of track membrane manufacturing has been shown. A few recipes for track etching in polyimides were reported. NaClO and H2 O2 provide a rather high etch rate ratio enabling one to produce a porous membrane of good quality. However, instability of the etchants in time at high temperatures is a serious problem for the technology. CR-39—a highly sensitive to ionizing particles and highly transparent material—can be used to fabricate track-etch membranes for special purposes (Berndt et al., 1986). Some copolymers of CR-39 have been shown to have even higher sensitivity than CR-39 itself (Ogura et al., 1995). The polymer is brittle which impedes the manufacturing and using thin foils. Attempts have been made to produce track-etch pores in aromatic polysulfones (Apel et al., 1997a) and polyetheretherketones (Daubresse et al., 1997). These highly aromatic materials showed a very low sensitivity to energetic heavy particles and, consequently, a very low etch rate ratio not allowing one to get a homogeneous porous structure. In contrast, the registration properties of polyethylene naphthalate have been found quite good (Komaki and Tsujimura, 1976; Starosta et al., 1999). 5. Structure and transport properties of track membranes Track membranes are known as precise porous 7lms with a very narrow pore size distribution (Fleischer et al., 1964; Fischer and Spohr, 1983; Flerov et al., 1989; Calvo et al., 1995). A unique property of TMs is that the number of pores and the pore size are two almost independent parameters which can be varied in a very wide range. The pore diameter can be from 10 nm to tens of micrometers. The pore density can vary from 1 to 1010 cm−2 . No other type of membranes provides such a possibility. In most cases the pore geometry
is very simple (cylinders, cones or combinations of these) which leads to simple relationships between the membrane structure parameters and transport characteristics such as the gas or water Dow rate. Poiseuille and Knudsen formulae adequately describe the viscous Dow and the molecular Dow through a track membrane (Beck and Schultz, 1972). In the production process the 7nished membrane is normally subjected to a rigorous testing by scanning electron microscopy, gas and water Dow rate methods, bubble point, and some other measurements (Kuznetsov et al., 1991; Apel, 1995). Now we understand the determining factors and have numerous empirical data enabling us to manufacture any particular product based on PET and PC. Pore shape can be made cylindrical, conical, funnel-like, and cigar-like at will. Various asymmetric structures can be also produced (Fischer and Spohr, 1983). The scanning electron micrographs in Fig. 2 illustrate the variety of pore structure in track membranes. The separation properties of TMs depend, 7rst of all, on the structure parameters. In the absence of adsorption, the track membrane acts as a “screen” membrane. The retention of particles is determined by the relationship between the pore diameter and the particle size (Pall et al., 1980; Yasminov et al., 1987). If the surface of the membrane adsorbs the particles, the retention characteristics change drastically. In this case the result depends on the nature of the membrane surface and the nature of the particles, and also depends on pH, presence of surfactants, etc. (Bisio et al., 1980; Keesom et al., 1987). Adsorption of solutes leads to a decrease in the 7ltration rate or to the loss of a substance that is expected to pass through the membrane (Tracey and Davis, 1994; van den Oestelaar et al., 1989). An example is sorption of proteins onto the PET membrane surface that is negatively charged at medium and high pH. To solve such problems, track membranes with various speci7c surface properties have to be manufactured. 6. Modi!cation methods The modi7cation methods that change the surface and bulk properties of track membranes or impart new functions to them are (i) sorption of some small or large molecules on the surface; (ii) surface treatment with plasma; (iii) immobilization of functional substances on the surface by covalent binding using chemical reactions; (iv) graft polymerization of various monomers. Hydrophilization of PET and PC track membranes by adsorption of polyvinyl pyrrolidone is a widely used procedure in industry. Similarly, covering the surface with silicone oil or paraRn can make the membranes hydrophobic. Plasma treatment improves wettability of many polymers due to the formation of new polar groups on the surface. Stability of the modi7ed surface signi7cantly depends on the condition of the plasma treatment (Vilensky et al., 1991; Dmitriev et al., 1998). Electrochemical properties of track-etched
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Fig. 2. A few examples of porous structures produced in thin polymeric 7lms using various methods of irradiation and chemical treatment: (A) cross section of a polycarbonate TM with cylindrical non-parallel pore channels; (B) polypropylene TM with slightly conical (tapered towards the center) parallel pores; (C) polyethylene terephthalate TM with cigar-like pores; (D) polyethylene terephthalate TM with “bow-tie” pores.
pores can be modi7ed by covalent binding of charged groups or by adsorption of ionic polyelectrolytes (Froehlich and Woermann, 1986). The immobilization of aminoacids to the PET track membranes based on the reactions of end carboxyl and hydroxyl groups was reported (Marchand-Brynaert et al., 1995; Mougenot et al., 1996). However, the surface density of the immobilized in this way species is rather low. The radiation-induced graft polymerization onto track membranes is a process which has been studied in more detail (Zhitariuk et al., 1989; Zhitariuk, 1993; Tischenko et al., 1991; Shtanko and Zhitariuk, 1995). Styrene (St), methacrylic acid (MAA), N -vinyl pyrrolidone (VP), 2-methyl 5-vinyl pyridine (2M5VP), N -isopropyl acrylamide (NIPAAM) and some other monomers have been grafted onto PET track membranes. Grafting of St increases the chemical resistance and makes the membrane hydrophobic. MAA and VP were grafted onto TMs to increase wettability which is especially important when aqueous solutions are 7ltered through small-pore membranes. 2M5VP was grafted with the aim to make the membrane hydrophilic and change its surface charge from negative to positive. During the past decade the grafting of NIPAAM and other intelligent polymers were extensively studied in the research work carried out at TRCRE (Takasaki) and GSI (Darmstadt) (Yoshida et al., 1993, 1997; Reber et al., 1995).
7. Applications Applications of commercially produced track membranes can be categorized into three groups: (i) process 7ltration; (ii) cell culture; (iii) laboratory 7ltration. The process 7ltration implies the use of membranes mostly in the form of cartridges with a membrane area of at least 1 m2 . Puri7cation of deionized water in microelectronics, 7ltration of beverages, separation and concentration of various suspensions are typical examples. There is a strong competition with other types of membranes available on the market. Casting membranes often provide a higher dirt loading capacity and a higher throughput. For this reason the use of track membranes in this 7eld is still limited (Brock, 1984). In the recent years a series of products were developed for the use in the domain called cell and tissue culture (Stevenson et al., 1988; Sergent-Engelen et al., 1990; Peterson and Gruenhaupt, 1990; Rothman and Orci, 1990). Adapted over the years to a variety of cell types, porous membrane 7lters are now recognized as providing signi7cant advantages for cultivating cells and studying the cellular activities such as transport, absorption and secretion (van Hinsbergh et al., 1990). The use of permeable support systems based on TMs has proven to be a valuable tool in the cell biology (Costar=Nuclepore Catalog, 1992).
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A traditional application of track membranes is the laboratory 7ltration (Nash, 1990). TM is a very good instrument when small particles should be collected onto the membrane surface and analyzed. For such purposes track membranes are delivered in the form of disks and 7ltration kits. The use of track-etch pores (often specially produced for a certain experiment) for solving various scienti7c tasks should be mentioned separately. One-pore (DeBlois and Bean, 1970; Fischer and Spohr, 1983), oligo-pore (Koutsouris et al., 1989) and multi-pore samples can serve as unique models for studying the transport of liquids, gases, particles, solutes, electrolytes (Beck and Schultz, 1972; Meares and Page, 1972; Deen, 1987), and electromagnetic waves (Mitrofanov et al., 1991) through narrow channels. Many experiments of this kind are relevant to biological and medical topics (Pasternak et al., 1995). Track membranes can also serve as templates for making various micro- and nanostructures. Magnetic, conducting and superconducting nanowires possessing special properties have been manufactured in this way (Whitney et al., 1993; Martin, 1994; Piraux et al., 1994; Zhitariuk et al., 1995). Using small-pore TMs as matrices, electrically switchable ion-selective membranes and sensors can be produced (Nishizawa et al., 1995).
8. Conclusion First established on nuclear reactors, nowadays the track-etch method of membrane production is mainly based on the use of heavy-ion accelerators. A few centers in the world manufacture track-etch membranes on a commercial scale. The track formation and etching processes are investigated in detail for polycarbonate and polyethylene terephthalate. The development of TMs on the basis of other polymers is still limited. DiRculties in the etching procedure, poor reproducibility of 7lmy material properties or high cost of the 7nal product impede the manufacture of TMs from new materials. The advantage of TMs over conventional membranes is their precisely determined structure. However, many large-scale applications are “insensitive” to such a brilliant property of TMs. Track membranes occupy a niche of biological, medical, analytical and scienti7c applications. This type of membranes is indispensable for manipulations with small particles of living and any other matter. The track membranes seem to be the best porous material for providing a controllable transport of solutes. Further progress in the TM technology can be connected with the creation of membranes having particular properties for a particular use. Membranes that do not adsorb proteins, membranes with various functional groups on the surface, etc., might be developed and introduced into industry. In the author’s opinion, the unique optical properties of etched tracks deserve more attention than it has been paid to them so far.
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