Self-assembled nano-structured polyelectrolyte composite membranes for pervaporation

Self-assembled nano-structured polyelectrolyte composite membranes for pervaporation

Materials Science and Engineering C 26 (2006) 1 – 8 www.elsevier.com/locate/msec Self-assembled nano-structured polyelectrolyte composite membranes f...

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Materials Science and Engineering C 26 (2006) 1 – 8 www.elsevier.com/locate/msec

Self-assembled nano-structured polyelectrolyte composite membranes for pervaporation Zhaoqi Zhu, Xianshe Feng *, Alexander Penlidis Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received 11 March 2005; accepted 13 March 2005 Available online 16 September 2005

Abstract This work is concerned with nano-structured polyelectrolyte composite membranes for solvent dehydration by pervaporation. The membranes were prepared by the electrostatic layer-by-layer deposition of polyethylenimine and poly(acrylic acid) onto a microporous polyacrylonitrile substrate membrane. Each cycle of alternating deposition of the cationic and anionic polymers formed a bilayer of polyelectrolyte, and multiple deposition cycles were used to achieve the permselectivity of the membrane. The polyacrylonitrile substrate membrane was partially hydrolyzed to improve the initial deposition of polyelectrolytes. It was proposed to use a relatively dilute concentration of the polyelectrolytes in the first few cycles of deposition, followed by depositions with more concentrated polyelectrolyte solutions (but still far below the critical overlapping concentration to ensure well extended conformation of the polyelectrolyte molecules). It was demonstrated that using this technique a good permselectivity could be achieved with less than 10 cycles of deposition, which was much less than the number of cycles used in the literature (e.g. 60 – 90). The membrane showed good separation performance for separation of water from isopropanol; at a feed water concentration of 8 – 10 wt.%, a permeate concentration of over 99 wt.% water was achieved with a permeation flux of about 0.6 kg/m2 h. D 2005 Published by Elsevier B.V. Keywords: Polyelectrolyte; Self-assembly; Pervaporation; Composite membrane

1. Introduction Pervaporation is a promising technology for liquid separation. It could effectively complement and/or potentially replace distillation for the separation of azeotropic and close-boiling point liquid mixtures [1]. Unlike distillation where all components are vaporized under reflux, pervaporation is generally energy efficient because the membrane is usually targeted for preferential permeation of the minor component in the liquid mixture, and as such only a small fraction of the feed needs to be vaporized. Although it has been over 20 years since pervaporation was first used on an industrial scale, the market share of pervaporation technology for liquid separation is still relatively small. High performance pervaporation membranes are needed to make this technology more competitive. * Corresponding author. Fax: +1 519 746 4979. E-mail address: [email protected] (X. Feng). 0928-4931/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.msec.2005.03.008

A good pervaporation membrane must have a high separation factor and high flux. Unfortunately there is usually a trade-off between selectivity and permeability for most conventional polymeric membranes. The development of composite membranes, consisting of a thin dense separating layer and a relatively thick microporous supporting layer, is an effective approach of enhancing the permeation rate while retaining membrane selectivity. In the composite membrane, the separation takes place in the dense separating layer, whereas the microporous substrate provides mechanical support to the separating layer. The separating layer should be thin in order to maximize the permeation flux of the membrane. Solution coating is currently the most common technique for fabricating the separating layer, and the minimum thickness of a solution cast film that can be obtained without having any defect is still limited. Nano-assembly of polyelectrolytes is a simple, versatile and environmental benign technique for making layered

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polymeric multicomposites [2], which could also be used to prepare separation membranes. Because of the high hydrophilicity of polyelectrolytes, the membranes so obtained are expected to be appropriate for solvent dehydration. By alternately depositing a polycation and a polyanion on an initially charged substrate, an ultrathin and chemically welldefined polyelectrolyte film with a layer-by-layer structure can be obtained. Each deposition forms one polyelectrolyte layer, and two consecutive depositions with a polyanion and a polycation will form a paired polycation/polyanion polyelectrolyte layer because of electrostatic interactions. Each single layer can have a uniform thickness of about 0.5 –3 nm [2], and the overall thickness of the membrane formed can be controlled and adjusted by the number of polyelectrolyte depositions. The thickness of a single polyelectrolyte layer is affected by such deposition conditions as concentration and pH of the polyelectrolyte solution, ionic strength, and temperature. During the past few years, self-assembled multilayers of polyelectrolyte have attracted significant attention as a potential membrane for separation applications. Using the polyelectrolyte self-assembly technique, polyelectrolyte membranes for liquid separations by pervaporation have been prepared, and these membranes were found to be highly selective to water permeation [3– 6]. Recently, the structure, properties and various factors influencing the multilayer growth of polyelectrolyte self-assembly systems have been studied extensively [7– 13]. It is relatively easy to form a polyelectrolyte self-assembly when a nonporous substrate is used as the supporting material. However, from the standpoint of membranes for separation applications, microporous substrates are preferred in order to achieve a high permeation flux. Unfortunately, the use of porous substrate poses a technical challenge for the fabrication of defect-free self-assembled polyelectrolyte composite membranes. Stroeve et al. [14] used both nonporous silicone and microporous polypropylene membranes as the substrate for making polyelectrolyte self-assembly membranes from polyallylamine/polystyrenesulfate, and it was shown that the polyion films formed on the nonporous support exhibited certain selectivity for gas permeation, whereas the films formed on the porous support were nonselective even after 40 coatings of the polyions, although no visible pores (i.e. defects) were observed under electron microscopy. The work of Tieke et al. [15 – 21], who have investigated extensively the polyelectrolyte self-assembly membranes for the separation of alcohol/water mixtures by pervaporation, showed that a significantly large number of depositions of anionic/cationic polyelectrolytes are needed in order to achieve a high membrane selectivity. For example, after 120 cyclic dip coatings with cationic poly(allylamine hydrochloride) and various anionic polymers, the selectivities of the self-assembly polyelectrolyte membranes formed on microporous polyacrylonitrile substrate were still rather low for ethanol dehydration by pervaporation in spite of the strong hydrophilicity of the

membrane, and the oxygen/nitrogen permeance ratio (which can be used to determine whether the membranes are defect free) was essentially the same as that exhibited by the microporous substrate itself [15]. In this work attempts were made to substantially reduce the number of deposition cycles required for layer-by-layer building up of polyelectrolyte self-assembly composite membranes for solvent dehydration while still retaining a good permselectivity. To do this, it was proposed to use a relatively dilute concentration of the polyelectrolytes in the first few cycles of deposition, followed by depositions with more concentrated polyelectrolyte solutions (but still far below the critical overlapping concentration to ensure well extended conformation of the polyelectrolyte molecules). In addition to the dip coating technique that has been used extensively in the literature, a single-sided coating process was also studied to improve the membrane permeability. The rationale for selection of the polyelectrolytes and the substrate was discussed. The reduced polyelectrolyte deposition cycles will make the polyelectrolyte self-assembly membranes more acceptable for practical applications from a membrane manufacturing point of view.

2. Experimental 2.1. Materials Microporous polyacrylonitrile (PAN) membrane supported on nonwoven polyester fabric backing, originally designed for ultrafiltration with a nominal molecular weight cut-off of 20,000 and 30,000, was supplied by Sepro Membranes. It was used as the porous substrate for making the composite membranes. Polyethylenimine (PEI) (molecular weight Mw = 750,000) and poly(acrylic acid) (PAA) (molecular weight Mw = 250,000) were used as the polycation and polyanion, respectively. They were all supplied by Sigma-Aldrich Canada in the form of aqueous solutions (50 wt.% for PEI, and 35 wt.% for PAA). The PEI was a branched polymer. Reagent grades of isopropanol and sodium hydroxide were purchased from EM Science. Deionized water was used as the solvent in preparing the polyelectrolyte solutions for layer-by-layer deposition of polyions. 2.2. Preparation of the polyelectrolyte membranes The polyacrylonitrile (PAN) membrane was first hydrolyzed in 1 N aqueous sodium hydroxide solution at different temperatures for different periods of time. The hydrolyzed PAN membrane, which was negatively charged, was thoroughly washed and rinsed with deionized water before being used as a support for the initial deposition of cationic polyelectrolyte. This was followed by alternate depositions with polyanions and polycations.

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Excessive polyions on the surface of the self-assembled layers were washed away by water prior to subsequent deposition of oppositely charged polyelectrolyte. The deposition of individual polyelectrolyte layers was accomplished by immersing the base membrane into the polyelectrolyte solution. The basic steps involved in the self-assembling of the layer-by-layer structure consisted of: (i) immersing the support membrane in the solution of a cationic polyelectrolyte for the initial disposition of polycations, (ii) immersing in pure water to wash the excessive polycations, (iii) immersing in the solution of an anionic polyelectrolyte, and (iv) immersing in pure water again to wash excessive polyanions. As such, a pair of electrostatically self-assembled polyelectrolyte layers was formed. These steps were repeated to form multiple polyelectrolyte layers by depositing polycations and polyanions alternatively. The deposition could actually take place on both sides of the membrane, although the initial deposition of polycations would be easier on the negatively charged PAN side of the substrate membrane than on the uncharged polyester side. The deposition of polyelectrolyte on the polyester backing side will block the pores of the support membrane and affect the membrane permeability negatively. In order to have the layer-by-layer deposition only on the PAN side and keep the polyester backing as open as possible to maximize permeability, a single-sided coating approach was also investigated. Basically, the PAN substrate membrane was mounted in the sealing cap assembly of a wide-mouth bottle so that the PAN side of the membrane faced the bottle. Then, the polyelectrolyte solution to be deposited was added into the bottle, and the bottle was sealed with the cap assembly and set upside down. In this way, the polyelectrolyte in the coating solution would contact the surface of the hydrolyzed PAN only. Preferably, the same cap assembly should fit all three bottles containing, respectively, the anionic and cationic polyelectrolyte solutions (for polyelectrolyte deposition) and water (for removal of excessive polyions) to facilitate the deposition operations. This method is referred to as ‘‘single-sided deposition’’, in contrast to the double-sided deposition that is used extensively in the literature. 2.3. Pervaporation The membranes were tested for the separation of water from isopropanol by pervaporation. The experimental apparatus and procedures for pervaporation have been described previously [22]. The feed solution was pumped from a thermostatted feed tank to the permeation cell, and the residue stream was recycled back to the feed tank. The effective area of the membrane in the permeation cell was 13.9 cm2. The feed circulation rate was kept sufficiently high to minimize the boundary layer effect on the membrane surface. Vacuum was applied to the permeate side of the membrane, and the permeate pressure was maintained

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below 5 mm Hg absolute. The permeate vapor was condensed and collected in a Pyrex glass cold trap immersed in liquid nitrogen. The permeation rate was determined gravimetrically by weighing the permeate sample collected over a given period of time, and the compositions of the feed and permeate streams were analyzed using a Hewlett Packard 5890 gas chromatograph. In order to minimize the variation in the feed composition during a pervaporation run, the quantity of permeate removed by the membrane during the run was maintained below 0.1% of the initial feed loaded in the feed tank. This study was concerned with steady state pervaporation. A steady state was considered to have been reached when both the permeation rate and permeate composition became constant. The membrane performance was characterized in terms of permeation flux and permeate composition. The overall permeation flux was calculated from J¼

Q At

ð1Þ

where Q is quantity of permeate collected over a period of time t and A is the effective permeation area of the membrane. The membrane selectivity can also be measured by the separation factor, which is defined as a¼

y=ð1  yÞ x=ð1  xÞ

ð2Þ

where x and y are water concentrations (in mass fraction) in the feed and permeate, respectively. The partial permeation fluxes can be readily determined from the total permeation flux and the permeate concentration. The feed water concentration was in the range of 8 –9 wt.% during the pervaporation experiments.

3. Results and discussion 3.1. Selection of membrane materials 3.1.1. Substrate Essentially all negatively or positively charged materials can be used as the substrate material to form a polyelectrolyte self-assembly composite membrane. The substrate can be microporous or non-porous, symmetric or asymmetric. Both porous and non-porous substrates have been used in prior work for preparing polyelectrolyte composite membranes. However, in order to minimize the resistance of the substrate to mass transport so as to achieve a high permeation flux, asymmetric microporous substrate membranes are preferred. The skin layer of the substrate should be highly microporous, but the pore sizes should be small enough so that the polyelectrolyte layers to be formed can bridge the pores. Otherwise, if the pore sizes are sufficiently large, the polyelectrolyte cannot cover the pores on the membrane surface; it will rather enter the pores, and consequently the resultant polyelectrolyte self-assembly

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composite membranes would be defective, compromising membrane selectivity. When a non-porous substrate membrane is used, the maximum permeability obtainable with a self-assembly polyelectrolyte composite membrane will be limited as the deposited layers of the polyelectrolyte will contribute additional resistance to mass transport. It has been reported [23] that using a non-porous cellulose acetate membrane treated with oxygen plasma as a substrate, the polyelectrolyte selfassembly composite membranes prepared from polyallylamine and polyacrylic acid exhibited a separation factor of 80 and a permeation flux of 0.1 kg/m2 h for ethanol dehydration by pervaporation at 25 -C when the feed contained about 90 wt.% of ethanol. The membrane permselectivity is not much better than the traditional membranes. Because the polyelectrolyte layer in the composite membrane is meant to be more selective than the substrate for the permeation of components in a mixture, it is expected that when a non-porous substrate is used, both permeability and selectivity of the resultant composite membranes will be compromised due to the significant contribution of the substrate to mass transport resistance, and consequently the highly permselective self-assembly polyelectrolyte membranes will not work to their full potential. It is desirable to minimize the resistance of the substrate membrane, and asymmetric microporous membranes are preferred as the primary function of the substrate is to act as a support for layer-by-layer deposition of polyelectrolytes. Membranes with pore sizes in the ultrafiltration range appear to be a good choice. In this study, microporous asymmetric polyacrylonitrile ultrafiltration membrane supported on nonwoven fabric was chosen as the substrate, which could be hydrolyzed to be negatively charged for better deposition and adhesion of the first polycation layer. Note that the pore size and pore-size distribution of the substrate membrane are critical to the minimum number of polyelectrolyte depositions required to form a defect-free composite membrane. If the pores are too large, it will be difficult for the polyelectrolyte to bridge the pores, and the polyelectrolytes may enter the pores, obstructing the passageways of permeate through the substrate. An increased number of coatings of the polyelectrolyte will be needed to cover the defects, and the permselectivity of the composite membranes so obtained will be reduced. It has been reported that the amount of the electrolyte adsorbed per dipping cycle on a porous support was considerably larger than on a pore-free support [21], indicating that the deposition does not only take place on the membrane surface and a considerable amount of polyelectrolytes is also adsorbed on the internal surface of the pores. A series of polyacrylonitrile ultrafiltration membranes with different molecular weight cut-offs were tested in the preliminary screening, and it was found that the membrane with a molecular weight cut-off of 20,000 yielded the best characteristics in terms of the number of polyelectrolyte depositions required to achieve a certain

separation factor. Because of the small number of polyelectrolyte coatings required, the composite membranes also exhibited optimal permeability. Consequently, the polyacrylonitrile ultrafiltration membrane with a molecular weight cut-off of 20,000 was used as the substrate throughout the present study. Using a gas flow porosimeter, the mean pore diameter of the substrate membrane has been determined to be 9.2 nm, which is the pore size of the dry substrate membrane. The pure water permeability of the substrate membrane was measured to have 160 L/m2 h at 25 -C at a transmembrane pressure difference of 207 kPa. Hydrolysis of polyacrylonitrile can occur under either acidic or basic conditions, but generally the rate of hydrolysis in an acid is much slower than in an alkaline environment. As the hydrolysis is mainly to change the surface properties of the substrate membrane, preferably only the surface of the substrate should be hydrolyzed. It was thus decided to carry out the hydrolysis quickly in a sodium hydroxide solution so as to accomplish the hydrolysis in a short period of time, thereby minimizing the possible hydrolysis of the surface of the pore walls. However, it was found that the sodium hydroxide solution could penetrate the substrate membrane easily, and the polyether backing in the substrate membrane could also be hydrolyzed. The hydrolyzed polyacrylonitrile layer was brittle without the polyester backing. Excessive hydrolysis would affect the mechanical strength and integrity of the substrate membrane. Therefore, the degree of hydrolysis should be controlled properly to retain the mechanical stability of the substrate membrane. A preliminary study was conducted to establish suitable hydrolysis conditions based on the separation performance and the stability of the resulting composite membranes, and it was found that hydrolysis in 1 N sodium hydroxide solution at 75 -C for a period of 20 min was appropriate. When increasing the hydrolysis temperature or prolonging the reaction time, more and more – CN groups on the polyacrylonitrile membrane surface will be converted into carboxyl groups, and the surface contact angle will decrease. After most – CN groups on the polyacrylonitrile membrane surface are converted, further hydrolysis will not yield a higher concentration of the carboxylic groups on the membrane surface, but will reduce the mechanical strength of the membrane. 3.1.2. Polyelectrolytes In the formation of polyelectrolyte self-assembly composite membranes, the charge density, structure (linear or branched), and electrolyte type (weak or strong) of polyelectrolytes all influence the building up of the layerby-layer structure. Prior work showed that high selectivities were achieved from membranes with multi-polyelectrolyte layers consisting of high charge density and weak polyions [24]. Therefore, polyacrylic acid and polyethylenimine were used in this study. These polyelectrolyte materials were available in different molecular weights, and it is thus

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convenient to tailor-make the polyelectrolyte self-assembled composite membranes for optimal separation performance. The specific polyelectrolytes used in preparing the composite membranes will obviously affect the number of polyelectrolyte coatings required to achieve certain selectivity for separation. If the molecular weights of the polyelectrolytes are not appropriate, they can pass or block the pores of the substrate. If the polyelectrolyte coils enter the pores, more layers of polyelectrolyte deposited to seal the pores will be needed, thus leading to a poor permeation flux. This was found to be especially important as the polyacrylonitrile substrate membrane would be more hydrophilic after hydrolysis, and the polyelectrolyte molecules tended to enter the pores more easily because of the increased capillary force. Therefore, the polyelectrolytes with relatively high molecular weights were chosen in the present study, and all the polyelectrolytes used had a much higher molecular weight than the molecular weight cut-off of the porous substrate membrane. 3.2. Deposition of polyelectrolytes: dip coating The substrate membrane was dip coated with the polyelectrolyte solutions, and deposition of polyelectrolyte took place on both sides of the substrate membrane. The goal of initial coatings is mainly to reduce the pore size on the surface of the substrate membrane, and thus relatively dilute polyelectrolyte solutions should be used. In a dilute solution, the polyelectrolyte molecules remain far apart and there is no molecular overlapping and aggregation, hence the polyelectrolyte molecules can be deposited freely. In addition, the polyelectrolyte molecules will have well stretched molecular conformation that helps bridge the pores. In the initial stages of polyelectrolyte deposition, the deposition is likely to occur in isolated spots. When the pore size became sufficiently small after the initial depositions, the subsequent coatings are used to seal the pores until a dense surface layer is formed so as to achieve a good permselectivity. Thus, a more concentrated polyelectrolyte solution will be more appropriate in order to reduce the number of subsequent depositions required to make the membrane free of defects. Polyelectrolytes with a higher concentration have a better bridging capacity to form an integral layer of the polyelectrolytes on the surface, and thus an increase in the polyelectrolyte concentration in the coating solution is expected to speed up the growth of the surface layer, thereby reducing the number of coatings required to form a permselective ‘‘dense’’ layer. Based on the above considerations, we propose to use dilute solutions for initial depositions and more concentrated solutions in subsequent depositions. To verify this idea, a polyelectrolyte concentration of 0.02 monomol/L was used for the first few cycles of deposition, and thereafter the polyelectrolyte concentration was increased to 0.2 monomol/L. The unperturbed mean end-to-end distance of linear polyacrylic acid (Mw 250,000) molecules can be estimated

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to be about 37.8 nm [25], and the corresponding radius of gyration can be calculated to be 15.4 nm. Its critical overlapping concentration in water can thus be calculated to be 0.114 g/g or 1.6 monomol/L, which is well above the concentrations used. This indicates that the poly(acrylic acid) chains are well extended. No simple estimate can be applied to the branched polyethylenimine. However, the diameter of polyethylenimine (Mw 25,000) molecules has been determined to be 9.2 nm at a pH 7.4 [26]. In the present study, the molecular weight of polyethylenimine is much higher (Mw 750,000). The radius of gyration is proportional to the square root of the molecular weight. In addition, the pH has been adjusted to 5 in order to increase the degree of ionization, which also helps retain an expanded conformation. Consequently, the size of the polyethylenimine molecules is expected to be much larger than the pore size of the substrate membrane, which ensures that the pores on the substrate membrane will be bridged, but not filled, by the polyelectrolyte molecules. Table 1 shows the separation performance of the polyelectrolyte composite membrane formed with 2 cycles of deposition using dilute solutions (0.02 monomol/L) and 4 cycles of deposition using more concentrated solutions (0.2 monomol/L). For convenience of discussion, such a deposition procedure is designated as (2 + 4) coatings, where the two numbers represent the numbers of two polyelectrolyte deposition cycles with the dilute and four with the higher polyelectrolyte concentration, respectively. Obviously, a total of 6 layer pairs of polyelectrolytes formed on each side of the membrane. It is clear that the membrane is very permselective; at a feed water content of 7.9 wt.%, a permeate concentration of 97.6 wt.% was achieved at ambient temperature, and the corresponding separation factor was substantially higher than what would be obtained by distillation. As the operating temperature increases, both permeability and selectivity of the membrane tend to increase. This is quite unusual as compared to the conventional non-electrolytic polymeric membranes, for which the temperature often has opposite effects on permeability and selectivity. This is believed to be caused by the strong interactions between polyelectrolytes and water. Similar results can also be observed for polyelectrolyte membranes prepared from poly(allylamine hydrochloride) and poly(styrenesulfonic acid) for ethanol/water separation [16]. Table 1 Pervaporation performance of polyelectrolyte composite membrane formed with (2 + 4) cycles of double-sided coatings (feed water content 7.9 wt.%) Temperature (-C)

Flux (kg/m2 h)

Water in permeate (wt.%)

Separation factor

22 40 50 60

0.19 0.38 0.50 0.64

97.6 98.8 99.6 100a

474 960 2900 V

a

The content of isopropanol was too small to be measured.

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Table 2 Pervaporation performance of polyelectrolyte composite membrane formed with (8 + 0) cycles of double-sided coatings (feed water content 11 wt.%)

Table 4 Pervaporation performance of polyelectrolyte composite membrane formed with (2 + 8) cycles of double-sided coatings (feed water content 9 wt.%)

Temperature (-C)

Flux (kg/m2 h)

Water in permeate (wt.%)

Separation factor

Temperature (-C)

Flux (kg/m2 h)

Water in permeate (wt.%)

Separation factor

50 60

0.65 0.81

89.3 96.4

68 217

40 50 60

0.17 0.26 0.33

98.8 >99 >99

835 >1000 >1000

As a comparison, membranes were also prepared using only the dilute polyelectrolyte solutions (0.02 monomol/L). Table 2 shows the representative pervaporation performance of the composite membrane with 8 layer pairs of the polyelectrolyte deposited on both sides of the membrane. In spite of the greater number of coating cycles, this membrane exhibited much lower selectivity than the membrane formed from the (2 + 4) cycles of polyelectrolyte deposition with a stepwise change in the polyelectrolyte concentration, whereas its permeation flux was not significantly higher. The advantage of using the varying concentrations in the polyelectrolyte solution for the layer-by-layer deposition of the polyelectrolyte is clearly demonstrated. It may be mentioned that relatively low concentrations (0.0002 to 0.01 monomol/L) of polyelectrolytes have been used in most of the work reported in the literature, while the pore size of the substrate membrane is rather large, as shown in Table 3, which summarizes the characteristic properties of

polyelectrolyte deposition systems. This may explain why a significantly large number of deposition cycles (e.g. 60 or 90) were often needed in order to achieve a reasonable selectivity. Considering that each deposition of the polyion layer takes about 30 min to reach saturation of polyion adsorption [21], it can be estimated that the time required for membrane fabrication would be too long to be acceptable for commercial applications from a manufacturing point of view, and the number of polyion depositions must be reduced substantially in order to make the manufacturing of the self-assembly polyelectrolyte composite membranes practically viable. Compared with the polyelectrolyte membranes reported in literature, a 10-fold reduction in the cycles of polyelectrolyte coatings has been achieved by using the modified deposition technique while still retaining a very good permselectivity.

Table 3 Summary of polyelectrolyte deposition conditions for polyelectrolyte composite membranes Substrate (pore size)

Polyelectrolytes

Concentration of coating solution (monmol/L)

No. of deposition cycles

Ref.

PAN/PETa (20 – 200 nm)

Polyvinylamine (Mw 100,000) Poly(allylamine hydrochloride) (Mw 9600) Polystyrene sulfonate sodium salt (Mw 70,000) Poly(ethyleneimine) (Mw 70,000) Chitosan (Mw 100,000) P oly(diallyldimethyl ammonium chloride) (Mw 250,000) Polystyrene sulfonate sodium salt (Mw 70,000) Poly(allylamine hydrochloride) (Mw 9600) Diketo-diphenylpyrrolo-[3, 4-c]-pyrrole disulfonic acid 10,22-Docosadiyne-1,22-disulfate disodium salt Polyallylamine (Mw 9750) Polystyrenesulfonate (Mw 70,000) Poly(allylamine hydrochloride) (Mw 9600) Polystyrene sulfonate sodium salt (Mw 70,000) Poly(allylamine hydrochloride) (Mw 9600) Polystyrene sulfonate sodium salt (Mw 70,000) Chitosan (Mw 100,000) Polyvinyl pyridine (Mw 50,000) Poly(ethyleneimine) (Mw 70,000) Dextran sulfate (Mw 5000) Polyvinylamine (Mw 100,000) Polyvinyl sulfonate potassium salt (Mw 350,000) Polyvinylamine (Mw 100,000) Polyvinylsulfate potassium salt (Mw 350,000) Polyvinylsulfonate sodium salt (Mw 7000) Polyacrylic acid (Mw 5000 and 250,000) Polyvinylamine (Mw 100,000) Polyvinylsulfate potassium salt (Mw 350,000)

0.01

60

[18]

0.0002 – 0.01

60

[15]

0.0005 – 0.00125

40 – 100

[14]

0.01

30 – 90

[16]

0.01

60

[17]

0.01

60

[20]

0.01

60

[19]

PAN/PET (20 – 200 nm) Polypropylene (40  120 nm) Polycarbonate (50 nm) Polypropylene (40  400 nm) PAN/PET (20 – 200 nm) PAN/PET (20 – 200 nm)

PAN/PET (20 – 200 nm)

PAN/PET (20 – 200 nm) a

PAN = polyacrylonitrile, PET = polyethylene terephthalate.

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It should be pointed out that after a good selectivity has been reached, a further increase in the deposition cycles will not continue to improve the membrane selectivity but rather will reduce the permeation rate. Table 4 shows the performance of a composite membrane prepared with (2 + 8) cycles of double-sided coatings for isopropanol dehydration at a feed concentration of 9 wt.%. A comparison of the data in Tables 1 and 4 shows that the flux was almost halved by applying four additional cycles of polyelectrolyte deposition. This is understandable because every layer pair of the deposited polyelectrolyte will contribute to the resistance of the membrane, as the resistances of the polyelectrolyte layer pairs are additive. It may be mentioned that using an asymmetric substrate membrane produced from chemically modified polyamide-6 by the phase inversion technique, a polyelectrolyte selfassembly composite membrane was prepared from various polyelectrolytes, and a good selectivity was obtained with a polyelectrolyte membrane consisting of six layer pairs of poly(acrylic acid) and polyethylenimine [24,27]. However, an examination of the data shows that there existed a significant variation in the properties of the substrate, as reflected by the change in the permeation flux of the substrate membrane (i.e. the composite membrane with zero double layers). Because of the non-uniformity of the substrate membrane, it is not clear how the membrane reproducibility will be affected. Nevertheless, it has been demonstrated that poly(acrylic acid) and polyethylenimine are one of the polyelectrolytes pairs that could be used to achieve a highly permselective membranes for liquid pervaporation. 3.3. Single-sided coating Dip-coating of polyelectrolyte has been used in essentially all studies reported in the literature. While the dip coating process is operationally simple, the membrane permeability is affected by the fact that the self-assembly of the polyelectrolytes also occurs on the backside of the substrate membrane. Because of the large mesh pores in the polyester backing, the polyelectrolyte coating solution can penetrate the mesh pores and block the passageways for the permeate. As a result, the permeation rate will be lowered. To overcome this shortcoming, an attempt is made to prevent the polyelectrolyte in the solution from contacting the polyester backing, while depositing the polyelectrolytes only on the polyacrylonitrile surface of the substrate membrane. In addition, from the standpoint of membrane manufacturing at a large scale on a continuous basis, single-sided coating would be easier to operate than double-sided coating. Table 5 shows the separation performance of a polyelectrolyte membrane formed by (3 + 7) cycles of single-sided depositions. It can be seen that by using the single-sided coating, the permeation flux was higher than with the double-sided dip-coated membranes, but the membrane selectivity was slightly decreased. Nevertheless,

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Table 5 Pervaporation performance of polyelectrolyte composite membrane formed by (3 + 7) cycles of single-sided depositions (feed water content 8.6 wt.%) Temperature (-C)

Flux (kg/m2 h)

Water in permeate (wt.%)

Separation factor

25 40 50 60 70

0.391 0.520 0.648 0.786 0.989

95.7 96.2 96.5 96.6 96.2

205 233 254 261 233

the feasibility of fabricating polyelectrolyte composite membranes using single-sided coating with a relatively small number of coating cycles is demonstrated, and further studies on optimization of the parameters involved in the coating procedure (i.e. polyelectrolyte concentration, drying conditions) are needed to improve the membrane permselectivity.

4. Conclusions Nano-structured polyelectrolyte composite membranes for isopropanol dehydration were prepared by the electrostatic layer-by-layer deposition of polyethylenimine and poly(acrylic acid) onto a microporous polyacrylonitrile substrate membrane. The polyacrylonitrile substrate membrane was partially hydrolyzed to improve the deposition and adhesion of polyelectrolytes in the early stage of polyelectrolyte depositions. It was shown that by using a relatively dilute polyelectrolyte solutions in the first few cycles of deposition and a more concentrated polyelectrolyte solution (but still far below the critical overlapping concentration to retain the well stretched conformation of the polyelectrolyte molecules) in subsequent depositions, a good permselectivity could be achieved with less than 10 cycles of deposition, which was much less than those commonly used in the literature (e.g. 60 – 90). The membrane showed very good separation performance for separation of water from isopropanol; at a feed water concentration of 8 –10 wt.%, a permeate concentration of over 99 wt.% water was achieved with a permeation flux of about 0.6 kg/m2 h.

Acknowledgement Research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is gratefully acknowledged.

References [1] R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, 1991. [2] G. Decher, Science 277 (1997) 1232.

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