Microwave plasma in preparation of new membranes

DESALINATION ELSEVIER

Desalination 163 (2004) 231-238

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Microwave plasma in preparation of new membranes M. Bryjak*, G. Po2niak, I. Gancarz, W. Tylus Wroclaw University of Technology, 50-370 Wrolaw 51, Poland Tel. +48 (71) 320-2383; email: [email protected],pwr.wroc.pl Received 25 July 2003; accepted 29 September 2003

Abstract

The presented paper is a peer-review of plasma action on porous polymer membranes. Considering the plasma medium the discussion is split in two parts: i) the first shows some effects caused by action of non-polymerizing gases, i.e. polymer etching and/or alteration of surface character, and, ii) the second covers action of plasma reagents resulted in the deposition of polymer layers. The authors' attention is focused mainly on alteration of the membranepore structure and surfaces character. Both kinds of plasma can turn porous membrane to new porous species with different surface character and pore size. It is possible also to prepare solid membranes where deposited polymer completely plugs the pores. The presented discussion is illustrated by the results of the authors' own research on preparation of brand-new polymer membranes. Keywords: Polymer membranes; Plasma treatment; Surface modification

1. Introduction

Excellent properties and low price result in a wide use of polymers for membrane manufacturing task. Unfortunately; the number of appropriate polymers, which can be used in such production, is limited. It slows down the development and dissemination of membrane separation technologies in the industry. To overcome this drawback *Corresponding author.

modificationofcommerciaUy available membranes is commonly considered [1,2] with both chemical and physical treatments [3]. Historically; chemical methods were applied earlier. Therein the membranes were put in contact with reagents that introduced functional groups on their surface. The second method; developed in the last two decades, involves physical media-flame, radiation or plasma. Recently; plasma treatment of polymer surfaces has attracted more and more attention. There

Presentedat PERMEA 2003, Membrane Science and Technology Conferenceof l4segrad Countries (CzechRepublic, Hungary, Poland and Slovakia), September 7-11, 2003, TatranslMMatliare, Slovakia.

0011-9164/04/$-- See front matter © 2004 Elsevier B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 4 ) 0 0 1 1 4 - 6

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are two reasons for this phenomenon: i) plasma is considered as an environmentally friendly technique; and ii) it is a very efficient method - - the operation lasts usually no more than several minutes [4]. There is one major drawback, however, of the plasma use - - the properties of the targeted surface alter usually within time of storage. This behavior, called sometimes 'hydrophobic recovery', is believed to be caused by gradual reorientation of the surface chain segments and depletion of polar groups when the surface is exposed to the air or to other nonpolar media. When the treated membrane is stored in water some polar groups are drawn out to the polymer surface and make it hydrophilic. These phenomena are well-documented in literature [5,6]. Applying plasma for the membrane modification purposes one should remember that the highly energetic particles interact with the polymer surface. They may generate several kinds of reactions: breakage of covalent bonds along the chain, crosslinking, grafting, interaction of surface free radicals, alteration of existing functional groups and/or incorporation of chemical groups originated in plasma. The mechanism of these reactions has not been recognized yet. However, three main groups of processes can be distinguished: i) polymer etching (the process called ablation); ii) modification of surface chemistry; and iii) deposition of plasma polymer. The first group comprises a whole bunch of destructive reactions that remove the surface material and, in the case of porous membrane, can enlarge the pore diameter. The second group of plasma processes deals with alteration of surface chemistry by change of functionalities. Finally, the third group includes those processes that allow material to be deposited on the surface in the form of a thin layer. While plasma treatment can be used for modification of all kinds of membranes [7-14] the special attention is paid to the modification of porous membranes. At first, the porous substrate

can form a membrane support when pores are filled by plasma deposited material [15-17]. At the second, plasma treatment is profitable when the surface character of the membrane and its pore diameter control the separation efficiency [ 18-20]. In this ease, the alteration of these both parameters should open new frontiers for membrane applications. There is one more benefit of plasma treatment - - the available membranes are mostly hydrophobic. Plasma activation of their surface allows preparation of new species - - affinity membranes [21], reactive membranes [22] or membranes with brush-like surfaces [23]. The goal of this paper is to show the potential abilities of plasma in preparation of brand-new membranes. For the simplicity of this paper, we have decided to show some effects of microwave plasma action on the surface functionalities and porous structure of the polymers most frequently used in manufacturing of ultrafiltration membranes - - polysulfone and polyacrylonitrile. 2. Methods 2.1. Plasma treatment

A laboratory microwave plasma device, assembled with 2.45 GHz frequency generator (Plazmatronika, Poland), a glass reaction chamber and a vacuum line, was used throughout this study. Plasma was generated in a quartz tube at the top of the reaction chamber. Pieces of membranes, each of ca. 25 cm 2 area, were fixed at various distances from the edge of plasma. In all experimental runs the reactor chamber was initially evacuated to 5 x 10-3 mbar. 2.2. Contact angle measurements

Contact angle evaluations were carried out at room temperature by means of TM 50 System (Technicome S.A., France) equipped with Panasonic GL 350 digital camera connected to a PC equipped with appropriate software. The contact

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M. Brijaket aL /Desalination 163 (2004) 231-238

angle was calculated from the droplet screenimage by outlining its shape and reading droplet radius and chord. About 20 droplets of selected liquid, each ca. 3 mL volume, were usedto evaluate the surface character. Two liquids serve as probes: distilled water and diiodomethane. Surface tension, 7, as well as its polar, T, and dispersive, 7t, components were calculated according to harmonic averaging

7 yzPyp ?z (I + c o s ® ) -- 41 ' 7 - - p

LYL +Ys

yzdys ~

(1)

Yz,

where subscript L and S denote solid and liquid; respectively and 0 is the contact angle,

2.3. XPS spectroscopy XPS spectra were analyzed on SPES ESCA system equipped with PHOIBOS 100 analyzer and SPECLAB software. Mg anode, 10 kV and 200 W, served as the X-ray source. The takeoff angle was adjusted to 90 ° with respect to the sample surface. Wide scan was conducted with 30 eV while narrow scan with 5 eV passage energy.

2.4. Determination of pore size distribution Pore size distribution was calculated according to our procedure described earlier [24]. The method is based on determination of permeate and retentate concentration of 8-10 dextranous standard solutions. The method allows determining two parameters of Gauss function - - standard deviation and average pore diameter. 3. Results

3.1. Plasma of non-deposited gases The usage of the microwave plasma and oxidative gases results in degradation of polymers. This fact is commonly referred in literature [1,25, 26]. Plasma of such gases as air, oxygen or carbon

dioxide is known to etch polymer surfaces to a great extent [27,28]. Nitrogen; ammonia or argon plasmas show similar behavior [29-32]. When above plasmas are used to porous material one may achieve a pore diameter enlargement. As a matter of fact such phenomenonis usually observed. When ordinary ultrafiltration polysulfone membrane is exposed to CO z plasma; its pores become larger and larger with the extent of plasma action [27]. The degradation process can be observed by the changes in pore diameter distribution calculated for modified membrane (Table 1). At a first glance one notes that CO 2 plasma etches the polysulfone membrane - - the pore diameter increases with the course of treatment. However, the comparison of the distribution width makes this conclusion not so obvious. Pore distribution becomes narrower for a short time of modification while the prolonged exposure to plasma widens it again. It may be a result of initial deposition of some parts of degraded polysulfone that plug the smallest pores. In the next stages of plasma action, the deposit is also etched and the pores become unplugged. The best way to verify this statement is to determine the water flux for membranes treated within various time-periods. This comparison is shown in Fig. 1. When one notes that the surface tension does not change significantly with time of membrane treatment (for details see [27]), there is one reason for such behavior - - initial deposition of etched material inside the pore mouth and subsequent Table 1 Average pore diameter, , standard deviation, o, and surface tension,T, of polysulfonemembrane modified in COs plasma (data from [271) Plasma treatment , time, min nm 0 3.5 2 18.7 10 25.5

o 1.56 1.22 2.62

7, mN/m 46.6 61.3 61.2

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M. Brijak et al. / Desalination 163 (2004) 231-238 140

120

& lOO

E

8o

x

~

4o

0 0

2

4

6

Treatment time

8

10

12

[rain]

Fig. 1. Water flux through polysulfone membrane treated by CO2plasma. Table 2 Properties of polyaeryloni~-ile membrane modified by air plasma [24] Treatment time, s

Pore diameter, nm

Surface tension, mN/m

Surface polarity, %

0 12 30 60 120 180

3.4 5.4 10.4 27.7 37.7 22.0

52.2 66.8 64.1 70.0 62.6 61.6

22.0 41.3 40.9 61.0 45.0 40.8

degradation of pore walls in the later stages of treatment. The same conclusion may be drawn from the data of air-plasma modification conducted on a polyacrylonitrile membrane [24]. Table 2 shows this situation. Also in this case, the pore diameter increases with the course of modification. However, for the long exposure time it becomes smaller. Almost at the same moment, the values of surface tension and its polarity component turn down. This phenomenon, observed also for polysulfone asymmetric membrane, does not appear when one modifies symmetric structure [28]. It is supposed that plasma used in those studies can penetrate

the membrane with the depth of several micrometers - - deep enough to affect the skin layer of asymmetric membranes but not so efficient to change the pore radius for a membrane with long symmetric pores. In the case of less aggressive plasmas, like those of N 2or NH 3, etching of the membrane skin layer and slight enlargement of the pore diameter is also observed [29,30]. When plasma etches the polymer surface, it simultaneously modifies it. Some new functional groups are anchored, some others are reconstructed from those existed on the virgin material. The effect gained by non-polymerizing gas plasma is not permanent and disappears significantly within several days after treatment. Reactive groups created by plasma, due to relaxation of surface polymer segments, are sucked into the bulk polymer. This may cause the time-related alteration of membrane performances [6,26]. In most cases the recovery is not completed and the surface preserves the functional character it acquired during plasma treatment. As an example; nitrogen [29] or ammonia [30] plasma modification of a polysulfone membrane may be taken. Let us see what happens with the first of them. The surface of virgin polysulfone is completely hydrophobic. After plasma treatment the surface tension clearly increased with the course of the process (Fig. 2a). When the modified membrane was stored in the air, the favorable environment for hydrophobic recovery [6], its polarity decreased to 1/3 of the post-treatment value but did not disappear. There are always some residual functional groups on the surface of such membranes (Fig. 2b). The presence of surface functional groups can be proved by several analytical methods. FTIRATR, bringing data on chemical composition within 200-800 nm layer, makes the analysis of surface functionalities difficult. XPS spectroscopy allows evaluating the depth gradient of functional group concentrations. Contact angle titration [33,34] brings the information about the most outer layer of the membrane. The use of all these methods together allows correlating membrane

M. Brijak et al. /Desalination 163 (2004) 231-238

6°i

7O

6o

E

235

50I

g 40 30 20

~o Io o~ (a)

,

,

5

10

0 0

15

5

(10)

Treatment time, [min]

10

15

20

25

30

Storage time, [day]

Fig. 2. Surface tension of polysulfone membrane (a) membrane modified in nitrogen plasma; (b) 2 mm modified membrane stored in the air. Circles - - polar component; squares - - dispersive eomponent; triangle - - total surface tension.

performances to the effect of plasma action. Below is a short survey through them. Polysulfone membrane was treated with ammonia alone or its mixture with argon that assured plasma process to run in a stable mode [30]. In the case of lack of argon the plasma extinguishes usually after a short time. The XPS studies show the presence of two forms of nitrogen assigned as N-C/N-H and N-C=O, and many various forms of carbon-bearing functionalities. Qualitative data are shown in Table 3. From the collected data ammonia/argon plasma seems to be more aggressive than ammonia plasma alone.

While XPS spectroscopy can analyze polymer surface with the depth of hundreds angstrom, the surface titration method seems to be one of the most effective analytical methods to monitor any change within the surface layer of one nanometer thick. Simplification of the Fowkes' method [33] allows estimation of surface concentration of acidic and/or basic groups present on the surface after plasma treatment [34]. For a polysulfone membrane modified in carbon dioxide, nitrogen or n-butylamine plasmas, calculations revealed various surface concentrations of acidic, C a and basic, Cbgroups. They were as follows: C= = 0.15 pmol/mZand C b = 0.00 gmol/m2 for CO 2

Table 3 Carbon in various chemical structures. Polysulfone modified in ammonia plasma. Data from [30] Plasma treatment

Peak position, eV

Carbon assignment

Relative fraction, %

None

284.8 286.4 288.4

C-C C--O, C-N C=O, N-C=O, N-C=N, O--C=N

89.8 10.2 0

NH3 plasma

284.8 286.4 288.4

C-C C-O, C-N C=O, N-C=O, N--C=N, O-C=N

76.9 19.4 3.7

NH3/Ar plasma

284.8 286.4 288.4

C-C C-O, C-N C=O, N--C=O, N--C=N, OM2=N

70.4 22.7 6.9

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M. Brijak et al. /Desalination 163 (2004) 231-238

plasma; C = 0.07 ~tmol/mz and C b = 0.34 p.mo1]m2 for N 2 plasma; and C = 0.00 ~mol/m ~-and Cb = 2.50 lamol/m 2 for ButNH 2 plasma. Hence the existence of functional groups on the surface of plasma treated membranes was proved experimentally. Modified membranes have altered their properties - - they became less prone to protein fouling and were easier to clean by a conventional method than their untreated analogues [27,29,30]. 3.2. Plasma polymerizable compounds

The observations described above concern the case when the formation of a polymer deposit is not expected unless the deposition of freed parts of the degraded polymer appears. However, when one uses plasma of organic vapors, the phenomenon of polymer deposition becomes fundamental. In very mild plasma conditions (pulsed, remote plasma with low power) some amount of functional groups present in 'monomers' can be preserved. In that way, polysulfone or polyacrylonitrile membranes bearing amine, hydroxyl, carboxyl, fluorine groups were prepared by plasma polymerization of n-butylamine and allylamine [35, 36], allyl alcohol [37], acrylic acid [31] and perfluorohexane [38]. The obtained membranes can be used as ion-exchange membranes, membranes for distillation, pervaporation or filtration, affinity membranes or supports for an enzyme immobilization. Plasma polymerization can cause partial or total plugging of membrane pores. This phenomenon was observed for a polyacrylonitrile membrane coated with plasma polymerized perfluorohexane [38]. The membrane was completely covered by Teflon-like layerjust after 30 s of treatment. With prolonged exposure, the pores became narrower and finally vanished after 10 min of plasma action. Plasma polymerization of acrylic acid on poly-sulfone membrane caused a five-time reduction of the pore diameter just after 2 min of treatment [31]. The same effect was observed when n-butyl-amine plasma was used. However,

addition of argon to the reaction mixture resulted in an opposite effect - - degradation of polymer matrix [35]. Fig. 3 illustrates both these findings. ButNH2/Ar was supposed that plasma to be more aggressive than ButNH 2 alone. However, the thesis was not supported by XPS data obtained. The comparison of N1, components for both plasma polymers is shown in Table 4. The data collected in Table 4 show more fundamental difference between both plasmas studied. ButNH2 plasma introduced much more oxygen bearing species than ButNH2/Ar plasma and this phenomenon is not fully clear to us. The opposite situation was observed when allylamine replaced butylamine [36]. Pore size distribution

a.

2

4

6

8

InR

Fig. 3. Pore size distribution of virgin (circles),ButNH2 (triangles) and ButNHJAr (squares) plasma treated polysulfonemembranes. Table 4 Relative fraction of nitrogen-bearing components in polysulfone membrane treated with suitable plasma for 1 min [35] Treatment None ButNH2 ButNHJAr

N-C, N-H 0 40.0 56.2

C=N, C?N 0 37.5 28.9

N--C=O, N--C(O)O

0 22.5 14.9

M. Bro'ak et al. /Desalination t63 (2004) 231-238

changed insignificantly after treatment with or without argon (Fig. 4). According to XPS data, the relative concentrations o f N-C(O)O and N-C=O features were much higher as expected when argon was present in the mixture (Table 5). The presence of an unsaturated bond might be the reason for various mechanisms of plasma polymerization in the case of butyl- and allylamine. Some differences between plasma stability were observed in these two cases. AllNH 2plasma e x t i n g u i s h e d m u c h faster that its ButNH z analogue.

237

4. Grafting of monomers to activated surface Plasma activated polymer surfaces may serve as the 'hot' places for molecular grafting [31,3739]. In the case of acrylic monomer two methods of grafting were applied - - immersing of the activated porous support in solution of monomer or keeping it in the monomer vapors [31]. The first one resulted in a fast increase of grafting yield and a significant decrease of the pore diameter. The second showed a slow increase of grafting yield and almost unchangeable character of pore diameter distribution. It was assumed that grating in solution gave long chains ofpoly(acrylic acid) while grafting in the vapor phase formed short polymer brushe s.

5. Concluding remarks

g II.

2

L_

4

6

8

InR

Fig. 4. Pore size distributionfor virgin (circles); AllNH2 (triangles) and AIINH2/Ar(squares) plasma treated membranes. Table 5 Relative fraction of nitrogen-bearing components in polysulfone membrane treated with suitable plasma for I rain [361 Treatment None AllNH2 AllNH2/Ar

N-C, N-H 0 54.6 41.1

C=N, C?N 0 32.1 33.8

N--C=O, N-C(O)O 0 13.2 25.1

Plasma treatment of porous polymer membranes may result in preparation of brand new membranes suitable for various processes. The modification process is fast, it is not so difficult to handling and maintenance, environmentally friendly and efficient. Thus, the use of this technique is expected to be the crucial point in the membrane preparation business. However, special work should be done to launch the plasma treatment of membranes to the industrial scale. When modification of polymer filaments or films is in great progress now, treatment of polymer membranes is still at the beginning of its commercial development. It is the authors' belief that the growing demand for new, reliable membranes will activate the research in this field also.

Acknowledgment The authors appreciate the financial support of the Polish Committee of Science, grant # 7 T09C 088 20.

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