J. Phys. Chem. B 2008, 112, 16539–16545
16539
Multifunctional Membranes for Solvent Resistant Nanofiltration and Pervaporation Applications Based on Segmented Polymer Networks Xianfeng Li,† Malgorzata Basko,†,‡,§ Filip Du Prez,*,‡ and Ivo F. J. Vankelecom*,† Faculty of Bioengineering Sciences, Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, Box 2461, 3001 LeuVen, Belgium, and Polymer Chemistry Research Group, Department of Organic Chemistry, Ghent UniVersity, Krijgslaan 281 (S4bis), B-9000 Ghent, Belgium ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: October 28, 2008
Hydrophilic bis(acrylate)-terminated poly(ethylene oxide) was used as macromolecular cross-linker of different hydrophobic polyacrylates for the synthesis of amphiphilic segmented polymer networks (SPNs). Multifunctional composite membranes with thin SPN toplayers were prepared by in situ polymerization. As the support consisted of hydrolyzed polyacrylonitrile, the high chemical resistance of the composite membrane allowed applications of the SPN-based membranes in solvent-resistant nanofiltration (SRNF) and pervaporation (PV). The membranes show very high retention on Rose Bengal (RB) in different solvents, especially in strong swelling solvents such as tetrahydrofuran (THF) and dimethylformamide (DMF). The membranes were also tested in pervaporation for dehydration of ethanol and isopropanol (IPA). The selectivity of the membranes greatly depends on the composition or the ratio of the hydrophilic and hydrophobic phases of the SPN. Introduction Membranes are powerful tools to separate liquid or gaseous mixtures in an economic, energy efficient, and environmentally friendly way. Nanofiltration (NF) is a pressure-driven membrane process involving pressures between 5 and 20 bar.1-4 Many large-scale applications currently exist in wastewater treatment and drinking water production. A major challenge these days is to broaden the range of NF applications to organic feeds in so-called solvent-resistant nanofiltration (SRNF).5,6 This requires solvent-resistant membranes that preserve their separation characteristics under more aggressive conditions, like at elevated temperatures, or in solvents that might dissolve the membrane polymer or make it swell excessively.5-7 The solvent-stable polymers applied so far hardly possess functional groups. Since some affinity between the membrane polymer and permeating solvent is needed, the application of currently available commercial SRNF membranes is often limited to apolar solvents.8-10 Pervaporation is a membrane process in which a liquid feed is split into a liquid retentate and a vapor-phase permeate through the application of a vacuum or sweep gas at the permeate side.11-13 The transport through a PV membrane is based on solution diffusion, while in SRNF, a transient mechanism between solution diffusion and pore flow is currently believed to take place.14-17 Segmented polymer networks (SPNs) represent a relatively new class of materials which received broad attention for potential applications in pervaporation, controlled drug delivery, catalysis, and as ion-conducting solid-state materials.18-22,25 SPNs are two-component networks of covalently interconnected hydrophilic/hydrophobic phases of cocontinuous morphology. The physicochemical properties of these networks are controlled * To whom correspondence should be addressed. E-mail: ivo.vankelecom@ biw.kuleuven.be. Fax: +32-16-32 19 98. Tel: +32-16-32 15 94(I.F.J.V.); E-mail:
[email protected] (F.D.P.). † Katholieke Universiteit Leuven. ‡ Ghent University. § Current address: Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland.
by the ratio of the two components, their nature, and the molecular weight of the macromolecular cross-linkers. In the case of amphiphilic SPNs, the covalent bonding between the hydrophilic phases and the hydrophobic ones limits the maximal swelling and prevents a swollen network structure from disintegration.22-24 This mechanical stability together with their tunable swelling behavior, hydrophilicity, and nanoseparated morphology offers a unique combination of properties to use them in membrane applications. The similar transport mechanism between PV and SRNF makes it possible to explore one single membrane that can be used for application in both of these membrane processes.11 The unique properties of SPNs, especially their easily tunable swelling properties, have been the starting point for this research to investigate them as a new type of multifunctional membrane for SRNF and a pervaporation application, which is suitable in a wide range of solvents. They are prepared from a twocomponent system, formed by a monomer and a so-called bis macromonomer, a bifunctional cross-linker of polymeric nature. To confirm the uniform distribution of SPN on the membrane support, an in situ polymerization method will be adopted to introduce the SPN toplayer on the porous support. The morphology of the membranes and their separation properties for SRNF and pervaporation will be described in detail. Experimental Section Materials and Methods. 1. Materials. Poly(ethylene glycol) (PEG, MW ) 2000 g mol-1) was dried by azeotropic distillation before use. Methyl acrylate (MA) was purified by distillation in the presence of the radical inhibitor phenothiazine. Isobornyl acrylate (iBorA) was distilled at reduced pressure and stored in the refrigerator. Butyl acrylate (BA) was refluxed over CaH2 in the presence of phenothiazine, before distillation. Bis(4-tertbutylcyclohexyl) peroxydicarbonate (Perkadox) was used as received. N,N-dimethylformamide (DMF, Acros), tetrahydrofuran (THF, AppliChem), 4-(N,N-dimethylamino) pyridine (DMAP, Acros), dicyclohexylocarbodiimide (DCCI, Aldrich), and 2-propanol (IPA, Aldrich) were used as received.
10.1021/jp805117z CCC: $40.75 2008 American Chemical Society Published on Web 12/04/2008
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SCHEME 1: The Preparation of Bis(acrylate) Poly(ethylene oxide)
SCHEME 2: The Preparation of Segmented Polymer Networks
TABLE 1: The Composition of the Casting Solutions for the Preparation of Composite Membranes code PEO20/MA80 PEO50/MA50 PEO80/MA20 PEO20/BA80 PEO50/BA50 PEO80/BA20 PEO20/iBorA80 PEO50/iBorA50 PEO80/iBorA20
weight ratio PEO acrylate toluene Perkadox PEO/acrylate (g) (g) (mL) (g) 20/80 50/50 80/20 20/80 50/50 80/20 20/80 50/50 80/20
0.1 0.2 0.4 0.1 0.2 0.4 0.1 0.2 0.4
0.4 0.2 0.1 0.4 0.2 0.1 0.4 0.2 0.1
0.3 0.9 1.6 0.3 0.9 1.6 0.3 0.9 1.6
0.008 0.007 0.006 0.008 0.007 0.006 0.008 0.007 0.006
2. The Preparation of Bis(acrylate) Poly(Ethylene Oxide) (Bis Macromonomer). The preparation of bis(acrylate) poly(ethylene oxide) started from commercially available PEG (Scheme 1). The polymeric glycol was esterified with acrylic acid (AA) by using the DCCI method. A typical procedure is as follows: 5 mmol of DMAP was introduced in a flask under argon atmosphere. After dissolution in dry toluene, the entire mixture was dried by azeotropic distillation. Everything was then dissolved in 50 mL of dry THF to which 20 mmol of triethylamine (TEA) was added. In parallel, 20 mmol of DDCI and 50 mL of dry THF together with 20 mmol of acylic acid were introduced into another flask and stirred for 1 h. The solution of PEG was then slowly transferred to the solution of AA, which was kept at 25 °C for 48 h. Precipitated dicyclohexylurea was filtered and washed with THF on the filter. The macroinitiator was purified by precipitation in cold ether and dried in vacuum. 3. Preparation of Self-Supporting SPNs. The SPNs were prepared via free radical copolymerization of the bis macromonomer with an acrylate monomer and Perkadox as the radical initiator at 70 °C, which is a slightly adopted procedure used earlier for the synthesis of ion-conducting SPN based on poly(methacrylates).25 A typical procedure is given below (Scheme 2). To a flask containing a bis macromonomer under argon, a certain amount of acrylate monomer (MA, BA, or iBorA) and Perkadox was added and mixed vigorously. For the synthesis of networks with a high content of bis macromonomer, a minimal amount of toluene was added to obtain a less viscous reaction mixture. The viscous solution was degassed for a few seconds before transferring it, by means of a syringe, between two glass plates that were kept at the desired distance by means
TABLE 2: The Soluble Fraction and Cross-Link Density in SPNs Based on PEO (Mn ) 2000) and Three Types of Polyacrylates PEO/acrylate ratio soluble cross-link density (w/w) in the SPN fractiona × 10-2 synthesis mixture (wt %) (mol/m3)b
acrylate methyl acrylate butyl acrylate isobornyl acrylate
20:80 50:50 80:20 20:80 50:50 80:20 20:80 50:50 80:20
2.8 2.8 3.5 8.6 2.7 3.0 0.8 2.0 4.2
2.8 2.5 2.2 c c c c 2.3 2.1
a Obtained by Soxhlet extraction in CH2Cl2. b Determined on the basis of DMTA analysis. c Not determined.
TABLE 3: Tg of Homopolymers in SPNs polymer
Tg (°C)
PEO PBA PMA PiBorA
-65 -49 +7 +93
of a rubber spacer with a thickness of 1 mm. The glass plate mold containing the solution was kept in an oven for 20 h at 70 °C. 4. Soluble Fraction Determination. Dried SPNs membranes were extracted with CH2Cl2 in a Soxhlet apparatus for 6 h and then dried at 70 °C under vacuum until constant weight. The soluble fraction was calculated as
% soluble fraction ) (w0 - w0) /w0 × 100% where w0 and w0w′0 represent the weights before and after the extraction process, respectively. The standard deviation on the measurements is about 5%. 5. SPN Swelling. The degree of swelling of the SPNs was determined gravimetrically. Dried SPN films were weighed (Wdry) and then soaked in solvents until the weight remained constant (Wwet) after wiping off the surface with blotting paper. The swelling degree of SPN (S) was defined as (eq 1)
S ) (Wwet - Wdry)/Wdry × 100%
(1)
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Figure 1. The swelling properties of SPNs (numbers 80, 50, and 20 refer to the weight ratio of PEO in the SPNs).
All of the experiments were carried out on three samples, and the average was obtained. The standard deviation on the measurements is about 10%. 6. 1H NMR. 1H NMR spectra were recorded in CDCl3 on a Bruker AM 500. 7. Dynamic Mechanical Thermal Analysis (DMTA). DMTA was conducted with a thermal analysis DMA 2980 apparatus on rectangular films at a heating rate of 10 °C/min with a frequency of 1 Hz. DMTA analysis was used for the determination of the structural heterogenity and cross-link density by applying the method described by Hill.26 8. Preparation of Supported SPNs. A. Preparation of a Hydrolyzed Polyacrylonitrile (PAN-H) Support. A polyacrylonitrile (PAN) support was prepared by the phase inversion technique from solutions containing 15 wt % PAN (Scientific Polymer Product; approximate MW: 150000) in DMSO. The polymer solution was cast on a polypropylene support (FO 2471, Viledon) and then immersed in deionized water. The PAN-H support was obtained by immersing the PAN support in 10 wt % NaOH at 50 °C for 40 min.27 The remaining NaOH was removed by washing with water. B. Preparation of the Polymerization Mixture for Deposition on the Support. To the flask containing bis macromonomer, Mn ) 2000 (PEO), the acrylate monomer (MA, BA, or iBorA) was added under argon at the desired ratio and mixed vigorously (Table 1). For the synthesis of networks with a high content of diacrylate poly(ethylene oxide), toluene was added to obtain a homogeneous reaction mixture with a sufficient viscosity that would still allow deposition on the support. The radical initiator
Figure 2. Tan δ as a function of temperature for SPNs [(a) PEO/ PMA and (b) PEO/PiBorA)] 20/80 (curve I), 50/50 (curve II), and 80/ 20 (curve III).
(“Perkadox”) was introduced to the flask. The solution was degassed using a water pump for a few seconds before transferring it onto the membrane support. C. Membrane Coating Procedure. Membranes were prepared on the PAN-H support that was taped onto a glass plate. The plate was placed on a platform tilted to form a coating angle of 60°. The polymerization mixture was deposited on the support by means of a Pasteur pipet. Depending on the polymer concentration, the procedure was repeated a few times to obtain a full coverage of the support and to minimize defects. After deposition, the system was closed with a glass cover, leaving a minimal amount of air, and the polymerization was conducted at 70 °C for 6 h. 9. Scanning Electron Microscopy (SEM). SEM (Philips XL FEG30) was carried out to study the cross section and surface structures of the membranes. The cross section was obtained after breaking the membranes in liquid nitrogen. The SEM samples were gold coated before use.
TABLE 4: Filtration Properties of SPNs for Different Dyes
PEO20/MA80 PEO50/MA50 PEO80/MA20 PEO20/BA80 PEO50/BA50 PEO80/BA20 PEO20/iBorA80 PEO50/iBorA50 PEO80/iBorA20 a
thicknessa (µm)
RB Pb (L · m-2 · h-1 · bar-1)
Rc (%)
BTB P (L · m-2 · h-1 · bar-1)
0.90 ( 0.20 1.64 ( 0.10 4.00 ( 0.50 1.29 ( 0.30 9.00 ( 0.50 3.80 ( 0.20 0.15 ( 0.01 0.20 ( 0.01 0.19 ( 0.02
0.027 0.032 0.360 0.027 0.180 0.004 0.032 -
99.0 99.0 99.0 99.0 99.0 99.0 68.0 -
0.216 1.049 1.679 0.322 1.080 0.152 0.036 -
Toplayer thickness. b Permeability. c Solute retention.
R (%) 62.0 70.0 51.0 60.0 16.0 15.0 ∼0.0 28.0
CV P (L · m-2 · h-1 · bar-1) 0.477 1.738 1.800 0.258 0.720 0.190 0.001 0.022 0.004
R (%) 36.0 12.0 5.0 12.0 16.0 8.3 9.0 12.0 4.0
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10. SRNF Experiments. Filtrations were done in a stainless steel dead-end pressure cell with a 15.2 cm2 membrane area. The feed solution was poured into the cell, and the cell was pressurized with nitrogen to 20 × 105 Pa (20 bar). During filtration, the feed solution was stirred at 11.66 Hz (700 rpm). Permeate samples were collected in cooled flasks as a function of time, weighed, and analyzed. The retention (R) was calculated with the permeate concentration Cp and the concentration of the original feed Cf solution according to eq 2
R(%) ) (1 - Cp /Cf) × 100
(2)
The permeation was stopped when the retention reached a constant value. All of the measurements were based on at least three samples, and the average values were used. The permeability was normalized by the thickness (1 µm), which was measured by SEM. The standard deviation on the measurements is about 5%. The properties of the solutes used are shown in Table 1 of the Supporting Information. 11. PerWaporation Experiments. Pervaporation experiments were carried out in a CM-Celfa membrane module at room temperature. The effective surface area of the membranes is 15.26 cm2. The vacuum at the downstream side of the apparatus was kept at 1 kPa using a vacuum pump. After steady state (about 1 h) was reached, the permeate liquid was collected in a liquid nitrogen cold trap, and the composition was determined by gas chromatography (HP 5054). The permeation flux (J) and separation factor (R) were calculated according to eqs 3 and 4
J ) W/At
(3)
Re/w ) (ye /yw)/(xe /xw)
(4)
in which W, A, and t represent the weight of the permeate, the membrane area, and the time; yw, ye, xw, and xe represent water and ethanol or isopropanol (IPA) concentrations in the feed and permeate, respectively. The alcohol concentration used in the feed was 80 wt%. Results and Discussion SPN Synthesis. Commercially available PEG with a molecular weight of 2000 g mol-1 was transformed into R,ωbis(acrylate) PEO via an esterification reaction between acrylic acid and the PEG hydroxyl groups in the presence of triethylamine, DMAP, and DCCI (Scheme 1). The 1H NMR (Supporting Information Figure 1) confirms the structure of the resulting bis macromonomers. SPNs with different contents of PEO and acrylate were obtained by free radical copolymerization of the bis macromonomer in the presence of different amounts of (methyl, butyl, or isobornyl) acrylate by thermal initiation. The selection of the acrylates was based on the different length of R groups (methyl < butyl < isobornyl), which will lead to different free volumes of the resulting polymers. For the further membrane application, the different free volume of polymers was believed to possibly influence the separation properties. Self-supporting networks with a thickness of 0.1 mm were thus obtained. Their composition is presented in Table 1. Table 2 shows the soluble fraction of the networks determined after Soxhlet extraction in CH2Cl2. All networks show low extractable fractions, indicating that the cross-linking reaction between the bis macromonomer and the acrylates proceeds with good yields. Thermogravi-
Figure 3. The cross sections of SPN-based membranes at low (top) and high magnification (below).
TABLE 5: Nanofiltration of RB through SPNs in Strongly Swelling Solvents IPA PEO20/MA80 PEO50/MA50 PEO80/MA20 PEO20/BA80 PEO50/BA50 PEO80/BA20 PEO20/iBorA80 PEO50/iBorA50 PEO80/iBorA20
THF
DMF
P (L · m-2 · h-1 · bar-1)
R (%)
P (L · m-2 · h-1 · bar-1)
R (%)
P (L · m-2 · h-1 · bar-1)
R (%)
0.027 0.032 0.360 0.027 0.180 0.004 0.032 -
99.0 99.0 99.0 99.0 99.0 99.0 68.0 -
0.126 0.702 1.000 0.216 1.260 0.007 0.030 -
99.7 99.9 99.9 99.0 99.3 99.5 99.2 -
4.050 24.928 9.432 8.238 2.700 0.140 0.015 0.040 0.018
8.8 17.0 53.0 ∼0.0 96.0 95.0 96.0 99.0 27.0
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Figure 4. The surface structures of SPN-based membranes.
Figure 5. Pervaporation properties of SPN (a and a′: ethanol/water; b and b′: IPA/water).
metrical and elementary analysis on other series of SPNs already showed earlier that the difference between the composition of the SPNs and of the reaction mixture never exceeded 5%.22,23 Swelling. For each solvent in which membrane filtrations were investigated (see below), the swelling of the SPNs was determined (Figure 1). The data confirm the potential of SPNs to be prepared as macromolecular substances containing covalently bonded segments with an overall tunable polarity. By
changing the ratio of hydrophilic (PEO) to hydrophobic (polyacrylate) constituents, swelling in water, for instance, could be controlled over two orders of magnitude. Also, for the other solvents, the difference in swelling of the SPN with different compositions is tunable. Most SPNs show the highest swelling in DMF and the lowest in IPA. The influence of the acrylate nature on swelling is less clear. DMTA and TGA. SPNs consist of hydrophophilic and hydrophobic chains covalently bonded to each other. Neverthe-
16544 J. Phys. Chem. B, Vol. 112, No. 51, 2008 less, phase separation can still occur to some extent as a result of their incompatibility. In general, however, the covalent crosslinking of the polymer chains prevents a macroscopic demixing, resulting in a nanoseparated morphology.22,24,28 It was shown earlier by solid-state NMR that the final phase behavior of SPNs is governed by the compatibility of the individual components and the molecular weight and end-group nature of the bis macromonomer.28 As can be seen in Table 3, vinyl polymers with a wide range of Tg values have been selected for the construction of the SPNs.29 The influence of the acrylate/PEO weight ratio and the nature of the poly(acrylate) on the phase behavior of the SPNs was studied by DMTA. Due to the similarity of the Tg’s of PEO and PBA, the phase behavior of this particular SPN cannot be studied by DMTA. In Figure 2, the tan δ is shown as a function of temperature for different compositions of (a) PMA- and (b) PiBorA-based SPNs. The PEO/PMA SPNs mostly show a broad phase transition over the whole composition range, whose temperature range is determined by the PEO/PMA weight ratio. There is certainly some degree of phase separation, which is evidenced by the fact that clear shoulders can be seen in curves II and III. In curve I, the PEO transition is difficult to observe because the material contains only 20% PEG and its transition is overlapping the PMA transition. The PiBorA networks show more heterogeneous phase morphology, possibly due to the bulkiness and highly apolar character of the side group. This is evidenced by the presence of multiple transitions which are due to the glass transition of PEO- and PiBorA-rich domains. The cross-link density of the SPNs, as calculated on the basis of DMTA analysis, is in the range of 2.1-2.8 × 102 mol/m3 (Table 2), with most polymers showing a similar cross-linking density. The thermal stability of the SPNs was determined via TGA (Figure 2 in Supporting Information). All SPNs show only a 10% weight loss at about 350 °C, proving the excellent thermal stability of the membranes. Preparation and Characterization of SPN-Based Composite Membranes. The preparation of composite membranes with a SPN top layer was quite challenging but crucial to obtain practically useful membranes with a thin active layer and thus high fluxes. It was achieved via copolymerization of liquid films containing a high molecular weight PEO cross-linker with a chosen acrylate comonomer using a radical initiator. Mixtures with different compositions (Table 1) were prepared and deposited on PAN-H supports before starting polymerization at 70 °C. This kind of support was chosen because of its excellent chemical and thermal stability,30 thus not limiting the final application range of the composite membrane. For top layers with high acrylate content, the viscosity of the polymerization mixture was low enough to be transferred onto the support. For higher PEO contents, a certain amount of toluene had to be added to obtain a less viscous, castable medium. Morphology of the Supported Membranes. The cross sections of the supported SPN-based membranes are shown in Figure 3. All supports show relatively open structures and fingerlike pores, in contrast to the top layers, which are very dense, as expected. The SEM pictures show the visibly defect-free deposition of the SPNs on the PAN-H support. The top layer thickness varies from one composition to another due to the intrinsic change in viscosity when applying a different PEO/ acrylate ratio or when adding toluene. Film thickness is related to viscosity through Einstein’s law.31 On top of this dependency on the viscosity, a certain intrusion of the polymer solution into
Li et al. the support might also take place and thus decrease the visible toplayer thickness. The thickness of the toplayer is however not regularly dependent on the composition due to the difficult control over the radical copolymerization process. Indeed, membranes derived from different acrylate monomers at comparable weight ratios (and thus comparable viscosities) show different thicknesses of the top layer (see Table 4). Among all membranes, the PEO/PBA membrane shows the thickest top layer, while the membranes derived from iBorA show the thinnest top layer (only about 100 nm). A remarkable difference in membrane surface structure can be observed in Figure 4. The SPN membranes derived from BA show a much rougher surface than the others, irrespective of the PEO/BA weight ratio. Probably, this difference should also be related to the polymerization process for the different monomers. Table 4 shows the performance of the supported SPN-based membranes in SRNF. Three dyes with different molar mass (Bengal rose B (RB), bromothymol blue (BTB), and crystal violet (CV)), all dissolved in IPA, were selected for the filtration test. As expected, all membranes show the highest retention for RB (almost 100%) and the lowest for CV due to the size difference; RB has a much higher molecular weight (1017 Da) than CV (408 Da). The permeability of the membranes with different composition is closely related to the membrane swelling. For example, the PEO/BA membranes show a decreasing permeability with increasing amount of BA, similar to the swelling tendency in IPA. For the PEO/iBorA membranes, the PEO50/iBorA50 having the highest swelling in IPA and also shows the highest permeability. Rather surprising, an important change in solvent permeability can be observed as a function of the solute that is retained. All of the membranes show the highest IPA permeability in the presence of CV and the lowest in the presence of RB. To further investigate the possible reason, dye adsorption was carried out on the membranes. All of the membranes indeed show the strongest adsorption of RB (Supporting Information Figure 3). After adsorption, the membranes were put in fresh IPA to release the adsorbed species. For CV and BTB, nearly all dye was released from the membranes, and no color was left on the membranes. On the other hand, for RB, a certain amount of dye was still left in the membranes, indicating the much stronger interactions of the SPNs with RB compared to those with CV or BTB. These adsorption effects thus explain the lower permeability for the RB feed. Possibly, the better interaction between IPA and CV and BTB, resulting in dragging effects, also partly explains their low retention. Table 5 shows the filtration data of the SPN-based membranes with RB as the solute in different organic solvents. All of the membranes show a very high retention for RB in IPA and THF. For DMF, all of the BA- and most iBorA-based membranes show high RB retention. However, the MA-based SPN membranes show very low retentions for RB in DMF, which may be due to the low stability of these membranes in DMF, as observed visually. The solvent permeability greatly differs for the different solvent systems. All of the membranes show the highest permeability for DMF and the lowest for IPA.14,15 These permeability data can be directly related to the swelling properties of the SPNs. The exact transport mechanism of solutes/solvents in SRNF membranes is a rather complicated process, depending on different factors like properties of solutes, solvents, and membranes.16
Solvent Resistant Nanofiltration and Pervaporation Pervaporation. As the supported membranes are dense, defect-free materials, also pervaporation experiments have been carried out in ethanol/water and IPA/water systems (Figure 5). All membranes preferentially permeated water. This can be explained by considering the glassy nature of SPNs at room temperature, which results in a more important diffusion selectivity, thus slowing down the larger molecules. In addition, sorption selectivity favors the same order as most of the membranes show a higher swelling in water than in IPA, especially when the fraction of the hydrophilic part (PEO) exceeds 50%. All of the membranes show higher selectivity for the ethanol/water than for the IPA/water system. This may be related to the different interaction between the solvent and the membranes. The weaker interaction between the IPA and membranes may lead to easier permeation of IPA compared to that of ethanol and thus in a low selectivity.11 Compared to reported membranes with similar structures, SPNs show relatively lower selectivity due to their reduced thickness of the toplayer, which might not block all defects.22 Conclusion SPNs consisting of a bis macromonomer of hydrophilic PEO as the cross-linker and three different types of hydrophobic polyacrylates have been used for the preparation of a dense top layer on porous supports. The hydrophilicity of the networks was controlled by changing the ratio of bis macromonomer to acrylate monomer, while the cross-link density of all materials was constant. All of the membranes based on SPNs, used for NF, showed a good stability even in aprotic solvents, such as THF and DMF. The application of the SPN-based membranes in PV was investigated for the dehydration of alcohols. The membrane selectivity was greatly dependent on the composition of the membranes. Acknowledgment. Dr Xianfeng Li acknowledges K. U. Leuven for a grant as a postdocoral research fellow. This research was done in the framework of an I.A.P.-P.A.I. Grant (IAP 6/27) on Supramolecular Catalysis sponsored by the Belgian Federal Government, of a GOA grant from the Flemish Government, of an FWO project, and of a Polish-Flemish Bilateral project. Supporting Information Available: Solute properties, 1H NMR of the bis macromonomer, TGA of SPNs, and dye adsorption of membranes. This material is available free of charge via the Internet at http://pubs.acs.org.
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