PLASMA MODIFICATION OF POLYMER MEMBRANES. Irena GANCARZ, Gryzelda POŹNIAK, Jolanta BRYJAK, Marek BRYJAK, Jerzy KUNICKI*

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1 PLASMA MODIFICATION OF POLYMER MEMBRANES Irena GANCARZ, Gryzelda POŹNIAK, Jolanta BRYJAK, Marek BRYJAK, Jerzy KUNICKI* Department of Chemistry, Wrocław University of Technology, WybrzeŜe Wyspiańskiego Wrocław * Central Laboratory of Batteries and Cells, ul. Forteczna 12, Poznań INTRODUCTION Nowadays, membrane processes are receiving considerable interest in many fields of industry. The number of polymers appropriate for membrane preparations is limited so to fulfill diverse process requirements, modification of known and commonly used membranes is often applied. Plasma is an excellent technique for that for a few reasons. Plasma can activate only the upper layer of the membrane not affecting the bulk properties of the polymer. Additionally, this technique is: versatile when changing plasma gas and process parameters one can obtain the whole bunch of membranes of various properties, from just one membrane very effective process usually lasts only a few minutes, environmentally friendly process produces neither by-products nor wastes. The present paper is a review of our numerous works realized in this field which aim was to improve ultrafiltration membranes properties, to get new nanofiltration and ion-exchange membranes, to introduce the surface functional groups as the anchor sites for covalent immobilization of biomolecules and to find the other application for modified membranes. Two different plasma apparatus (one operating at a frequency 2.45 GHz and other at 75 khz) were used to modify porous and solid hydrophobic polymer membranes including polyethylene (PE), polypropylene (PP), polysulfone (PSU), poly(phenylene oxide) (PPO), polyacrylonitrile (PAN) and poly(etherimide) (PEI). Various plasma processes were applied - treatment with plasma of non-polymerizing gases (air, Ar, CO 2, N 2, NH 3, NH 3 +Ar), plasma polymerization (acrylic acid, allyl alcohol, allylamine, n- butylamine) and plasma-initiated surface grafting (acrylic acid, 4-styrenesulfonic acid). The data of FTIR-ATR, XPS and chemical analysis, used to confirm the chemical structure of the modified surface are given in the corresponding papers cited in the text. 112

2 EXPERIMENTAL Membranes of poly (phenylene oxide) (PPO, M v =27 kda, Aldrich) and polysulfone (PSU, Udel P-1700, M v =33 kda, Amoco) were prepared in laboratory. Dense membranes were cast from DMF or CHCl 3 solution and dried in 120 C. Porous membranes were prepared by the phase inversion method from 15% solution in DMF with 10% of PVP (M w =10 kda) for PSU and a mixture of chloroform and nonyl alcohol (8:2 by weight) for PPO. Coagulation medium was water and methanol, respectively. Porous membranes of poly(etherimide) and poly(acrylonitrile) were gift from Sulzer Chemtech GmbH and polypropylene (Celgard 2500) was the product of Daicel Chemical Industries, Ltd. Two various plasma apparatus were used a microwave plasma generator of 2.45 GHz frequency (Plazmatronika, Poland) and 75 khz plasma generator (Dora, Wroclaw). In the former one, plasma was generated in a quartz tube at the top of the glass reaction chamber. Membranes were attached to a table in the post discharge area at various distances from the lower edge of the plasma. In the latter one sample was attached to the one of the electrodes. The details of parameters of all processes are given in the corresponding papers cited in the text. In plasma induced grafting, the polymer sample after argon plasma treatment was left in air for 10 min, then soaked in a deaerated monomer solution and kept in a constant temperature bath (PSU, PPO) or exposed to wide-range UV irradiation (PP) for predetermined time. PSU and PPO porous membranes were hydrophilized in 1:1 ethyl alcohol-water solution prior to the grafting. After graft polymerization the membranes were extracted in distilled water in order to remove the homopolymer. Contact angles were detected by means of TM 50 System (Technicome SA, France). Data of contact angles of water and diiodomethane were used for calculation of surface tension γ s and its polar γ s p and dispersive γ s d components according to [1]. Sample polarity was defined as ratio of γ s p /γ s. The Amicon 8200 dead-end cell with a membrane surface area of 19.6 cm 2 was used in filtration study. The performance of ultrafiltration (UF) membranes was determined with water, buffers and buffered solutions of bovine serum albumin (BSA, 1g/dcm 3 ). Values of fouling index (FI), flux recovery after cleaning (FR) and reduction of the flow in filtration (RF) were calculated as follows: FI=[1 (J f /J o )]; FR=J c /J o ; RF=[1 (J p /J o )]; where: J o is flow of buffer through the fresh membrane, J p - flow of buffered BSA through the same membrane, J f flow of buffer through the membrane after BSA filtration, and J c flow of buffer after membrane cleaning. In micellar enhanced UF (MEUF), a mixture of 2,4-D-herbicide and the surfactant, hexadecyltrimethylammonium bromide (CTAB), was filtered. The concentration of CTAB (conductivity) and 2,4- D-herbicide (UV absorption, 283 nm) in the permeate was determined. In nanofiltration studies, water solutions of salts (1 mmol/dm 3 ) were filtrated. 113

3 Concentration of salt in permeate was determined (conductivity or atomic absorption). Salt rejection was calculated as: R (%) = (1 C p /C o ) x 100, where C p and C o are salt conc. in permeate and in the initial solution, respectively. Immobilization of chosen enzymes (xylose isomerase and invertase) was conducted on the modified by plasma dense membranes. Activation of functional groups prior to immobilization was done with divinylsulfone (hydroxyl) or glutaraldehyde (amino groups). Concentration of protein and sugars was estimated from UV absorption data. Determination of the electrolytic resistance R (om x cm 2 ) of the membrane was determined by measurement of the potential between two reference electrodes at a fixed current that flowed between platinum electrodes. In all measurements concentrated KOH solution (d=1.28 g/cm 3 ) was used. RESULTS AND DISCUSSION Plasma of non-polymerizable gases Most polymers to be used for preparation of filtration membranes have strongly hydrophobic character. That is one of the reasons why such membranes fouls to the high extend. Plasma hydrophilization seems to be an attractive approach to reduce protein fouling and to improve the cleaning process of the membranes. Plasma treatment almost always causes increase of surface tension and polarity of polymer samples. PAN surface becomes almost twice as polar after air plasma treatment (increase of polarity from 44 to 85%) [2] and PE even 10 times more polar (increase from 6 to 60%) [3] than unmodified polymers. In the same time plasma etching causes increase of membrane pore size. These two phenomena affect the permeate fluxes through membranes. The flux change is sometimes very significant, for example for PEI membrane treated in argon plasma water flux increases 4 to 9 times depending on plasma parameters [4]. The serious drawback of plasma modification is diminishing of gained hydrophilicity with time. The most detailed study of plasma influence on filtration performance was performed on PSU membranes. Plasma of non-polymerizable gases as CO 2 [5], N 2 [6] and NH 3 [7] was applied. In all cases a sharp decrease in water contact angle hence increase of polar component of surface tension, in the first minute of plasma treatment was observed. Prolonged excitation did not cause further changes in surface tension and SEM pictures had shown intensive etching [5]. Filtration process of protein (BSA) using native and modified membranes was characterized by fouling index (FI), reduction of the flux in filtration (RF) and flux recovery after cleaning (FR). It can be seen in Table 1 that CO 2, N 2 and NH 3 plasma modified membranes show better performance than untreated ones. In some cases this performance depends also on ph of protein solution. CO 2 plasma modified PSU works better in basic solution (both BSA and membrane are negatively charged so repulsion takes place), N 2 plasma gives surface of amphotheric character filtration in whole ph range seems to be similar. NH 3 plasma unexpectedly seems to leave slightly acidic surface. In case of PSU, the best performance in BSA filtration 114

4 show the CO 2 plasma modified membranes. Air and argon plasma treated membranes show better filtration performance in acidic solution. In almost all cases of plasma treatment significant improvement of flux recovery after cleaning was noted. It means that plasma hydrophilization helps in protein removal from the fouled surface. Table 1. Surface properties and ultrafiltration performance of chosen membranes Base polymer PSU Plasma medium Surface properties Surface tension, mn/m Characteristic filtration parameters Polarity PH=3 ph=9 % FI FR RF FI FR RF None , CO N NH n-bunh n-bunh 2 +Ar , AllNH AllNH 2 + Ar AAc Plasma polymerization Using vapors of organic compounds as plasma medium makes possible a deposition of layer of a new material on the membrane surface. In this process, the starting monomer is highly degraded, the resulting fragments are scrambled, hence the chemical structure of the deposited film is significantly different from the structure of the monomer. However, in the mild plasma conditions (pulsed, remote plasma, low power) high retention of monomer structure is observed. In that way PSU or PPO membranes bearing amine [8-11], hydroxyl [12] or carboxyl [13] groups were obtained by plasma polymerization of n-butyl amine (n-bunh 2 ) [8,9] and allylamine (AllNH 2 ) [9-11], allyl alcohol (AllOH) [12] and acrylic acid (AAc) [13] respectively. Presence of these groups on the surface can improve filtration performance of membranes and also may play the role of anchoring sites for biomolecules. The final result of plasma polymerization depends strongly on the composition of plasma medium. The presence of neutral gas (argon) stabilizes the plasma but sometimes gives unexpected results. Improvement of UF membranes performance Plasma polymerization of all monomers causes decrease of water permeability through PSU membrane thus only short polymerization time can be 115

5 applied for UF membranes. Performance of membranes in protein solutions at various ph gives us information on their surface character. Good filtration properties (Tab.1) are observed for AAc modified PSU in basic solution (acidic surface) [13] and in the whole range of ph in case of All-NH 2 (with and without Ar) plasma treated PSU (amphotheric surface) [9]. N-butyl amine plasma makes the surface more hydrophobic and modified membranes with worsened filtration properties [8,9]. Addition of argon to n-butyl amine vapor improves membrane performance only in acidic solution (basic surface) (Tab. 1). Membranes for micellar enhanced UF Sulfonated porous PPO membranes covered with allylamine plasma polymer retained their ultrafiltration structure. We examined the possibility of their application in the process of micellar enhanced ultrafiltration. It is a technology that employs surfactant micelles to increase the ability of UF membranes in separation of small molecules [14]. Beyond the critical micelle concentration, the surfactant forms large aggregates (micelles of size above 10 nm) in which the organic or inorganic contaminants are solubilized. After that they can be separated by means of suitable UF membrane. This technology seems to be very promising in removal of herbicides from drinking water. The results obtained in the process of removing 2,4-D herbicide from water using modified PPO membrane are shown in Table 2. The performance of modified membrane is better than native one the former rejected much more herbicide than PPO did. The probable reason for such behavior is repulsion of positively charged groups of surfactant and amine groups on the polymer surface. Table 2. MEUF properties of PPO membranes Membrane Water flux dm 3 /m 2 h 2,4-D herbicide rejection,% C s /C h * PPO SPPO-ppAllNH *C s /C h molar conc. ratio of CTAB to herbicide Bipolar composite nanofiltration membranes In this study sulfonated PSU (bearing negative charge) of sulfonation degree equal to 0.25 was applied as a support layer. The upper layer of bipolar membrane was prepared using plasma polymerization process of n-butylamine or allylamine, what gave the skin of weakly basic character. Such membranes were examined with solutions of single electrolytes containing cations and anions of various valences as well as the mixture of salts [15]. They showed higher rejection of magnesium sulfate 116

6 than sodium chloride. Differences of rejection of sodium chloride and magnesium chloride as well as Na 2 SO 4 and MgSO 4 are smaller than for monopolar membranes (Table 3). No essential difference between MgSO 4 rejection from single salt solution and mixture was observed. As in the case of monopolar membranes, NaCl rejection was much lower in the mixture of salts solution. Bipolar membranes with allylamine plasma polymer show better separation properties than these with n- butylamine layer. Such bipolar membranes should be effective in water softening process as they show high salt solution fluxes and high rejection of divalent ions. Table 3. Properties of nanofiltration membranes Kind of membrane J w Single salts retention, % NaCl+MgSO 4 dm 3 /m 2 h NaCl MgCl 2 Na SO MgSO MgSO NaCl monopolar* BuNH bipolar AllNH *the best from investigated monopolar nanofiltration membranes PSU+PVP/SPSU [12] Enzyme immobilization PSU and PPO membranes with plasma polymers of various compounds (allyl alcohol, n-butylamine, allylamine) deposited on their surfaces were used for covalent immobilization of two technologically important enzymes xylose isomerase [10,12] and invertase [11]. Presence of argon in plasma medium stabilized the plasma but deposited layer seemed to have less functional groups. The enzyme activity was usually higher (n-butyl amine plasma is an exception) in that case. The enzyme activity varied strongly with plasma parameters as pulses, power, treatment time or sample-to-plasma distance. The last parameter seemed to have the strongest influence on enzyme immobilization. The values of enzyme activity obtained for modified PSU were in the range of 10.3 to 35.2 µmol/min and for plasma treated PPO from 10.8 to 44.9 µg/min. There is no simple correlation between the polarity of modified samples or concentration of active groups on the surface and enzyme activity [11]. Plasma-induced surface grafting Exposure of the polymer to inert gas plasmas (Ar, He, Ne) results mainly in free radical formation on the surface. These radicals can directly initiate polymerization of monomers (vapor phase grafting). In contact with air they can produce on the surface peroxides and hydroperoxides, which can be a source of succesive radicals in solution grafting. 117

7 PSU and PPO membranes PSU membranes were grafted with acrylic acid both in vapor (VG-PSU) and solution (SG-PSU) [10] to get weakly acidic surface. Depending on plasma and grafting parameters grafting yield reached values from 26 to 261 µg/cm 2 for VG- PSU and 0.3 to 12.4 µg/cm 2 for SG-PSU. Permeability of water for VG-PSU dramatically decreases when degree of grafting approaches 50 g/cm 2. Below that value filtration of protein is significantly improved in basic medium (Tab. 4). For SG-PSU one observes constant decrease of water flux with grafting yield and its value was too small to study protein filtration. Table 4. Properties of grafted polymer membranes Base polymer monomer yield, µg/cm 2 PSU PPO Grafting Surface properties Characteristic filtration parameters γ s mn/m polarity ph=3 ph=9 % FI FR RF FI FR RF None AAc* NaSS None NaSS PP Electrolytic area resistance, om x cm 2 None AAc* AAc ** ** * grafting in vapor phase ** - depending on plasma and grafting parameters Strongly acidic surface could be obtained by surface grafting of sodium salt of sulfonic acid (NaSS). Degree of grafting for PSU depending on plasma and grafting parameters reached values from 4 to 42.3 µg/cm 2. As in the case of acrylic acid, NaSS grafted PSU membranes show excellent transport properties in solution of ph above 7 (Tab. 4) [4]. In acidic solutions however filtration parameters are even worse then for unmodified PSU membrane. In case of PPO, grafting with NaSS worsen filtration performance of the membranes [14]. The grafted polymer does not form a uniform layer on the surface but it appears as agglomerates of various sizes, scattered on the substrate [14]. PP porous membranes Microporous polypropylene is an excellent membrane from point of view of chemicals and temperature resistance. It is however very hydrophobic with surface tension of 29.3 mn/m and polar component only 0.6 mn/m. Durable, not changing with time wettability is the main condition of its wider application. Plasma-induced 118

8 grafting of acrylic acid seemed to be the proper method to reach this aim. The grafting was conducted both in vapor phase and solution but the first method appeared to be not very effective. Grafting in solution allowed introducing to polypropylene up to 1 mg of acrylic acid per 1 cm 2 of the membrane (Table 4) in form of even polymer layer on the membrane surface (Fig.1). a b Fig.1. SEM of surface of PP virgin (a) and AAc grafted membrane (b) Such membranes became hydrophilic (polarity around 20%) and were characterized by very high ionic conductivity (Table 5) what makes it a good candidate for a separator in rechargeable high-power Ni/Cd cells [16]. CONCLUSION Modification of membranes in plasma is a possible in simple and fast way to improve their performances and to widen their application. REFERENCES 1. S. Wu, Polymer Interface and Adhesion, M. Dekker, New York M. Bryjak, I. Gancarz, A. Krajciewicz, J. Pigłowski, Angew. Makromol. Chem., 234(1996)21 3. M. Bryjak, I. Garncarz, Angew. Makromol. Chem., 219(1994) I. Gancarz, unpublished data 5. I. Gancarz, G. Poźniak, M. Bryjak, Eur. Polym. J., 35(1999) I. Gancarz, G. Poźniak, M. Bryjak, Eur. Polym. J., 36(2000) M. Bryjak, I. Gancarz, G. Poźniak, W. Tylus, Eur. Polym. J., 38(2002) G. Poźniak, I. Gancarz, M. Bryjak, W. Tylus, Desalination, 146(2002) I. Gancarz, G. Poźniak, M. Bryjak, W. Tylus, Eur. Polym. J., 38(2002) I. Gancarz, J. Bryjak, G. Poźniak, W. Tylus, Eur Polym. J., 39(2003) I. Gancarz, J. Bryjak, G. Poźniak G., W. Tylus, Eur. Polym. J., 42(2006)

9 12. I. Gancarz, J. Bryjak, M. Bryjak, G. Poźniak, W. Tylus, Eur. Polym. J., 39(2003) I. Gancarz, G. Poźniak, M.Bryjak, A. Frankiewicz, Acta Polym., 50(1999) G. Poźniak, I. Gancarz, W. Tylu, Desalination, 198(2006) G. Poźniak, I. Gancarz, M.Bryjak, Proceedings of the 27 th International Conference SSCHE, Tatranske Matlare, Slovak Republik, 2001, A. Ciszewski, I. Gancarz, J. Kunicki, M. Bryjak, Surf. Coat. Technol., 201(2006)