Multilayered Poly(vinylidene fluoride) Composite Membranes with

Jul 8, 2011 - Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of. Education) ...
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Multilayered Poly(vinylidene fluoride) Composite Membranes with Improved Interfacial Compatibility: Correlating Pervaporation Performance with Free Volume Properties Quanfu An,†,‡ Jung-Tsai Chen,† Manuel De Guzman,† Wei-Song Hung,† Kueir-Rarn Lee,*,† and Juin-Yih Lai† † ‡

R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, China ABSTRACT: A spin-coating process integrated with an ozone-induced graft polymerization technique was applied in this study. The purpose was to improve the poor interfacial compatibility between a selective layer of poly(2-hydroxyethyl methacrylate) (PHEMA) and the surface of a poly(vinylidene fluoride) (PVDF) substrate. The composite membranes thus fabricated were tested for their pervaporation performance in dehydrating an ethyl acetate/water mixture. Furthermore, the composite membranes were characterized by field emission scanning electron microscopy (FE-SEM) for morphological change observation and by Fourier transform infrared spectroscopy equipped with attenuated total reflectance (ATR-FTIR) for surface chemical composition analysis. Effects of grafting density and spin-coating speed on pervaporation performance were examined. The composite membrane pervaporation performance was elucidated by means of free volume and depth profile data obtained with the use of a variable monoenergy slow positron beam (VMSPB). Results indicated that a smaller free volume was correlated with a higher pervaporation performance of a composite membrane consisting of a selective layer of spin-coated PHEMA on a PHEMA-grafted PVDF substrate (S-PHEMA/PHEMA-g-PVDF). The composite membrane depth profile illustrated that an S-PHEMA layer spin-coated at a higher revolutions per minute (rpm) was thinner and denser than that at a lower rpm.

’ INTRODUCTION Ethyl acetate is one of the most practical fatty acid esters as it finds wide-ranging use as a quick-drying solvent. Its industrial production is chiefly by the Fischer esterification reaction of ethyl alcohol and acetic acid: CH3 CH2 OH þ CH3 COOH a CH3 COOCH2 CH3 þ H2 O ethylalcohol

aceticacid

ethylacetate

water

The equilibrium can shift to the right by the removal of water, resulting in higher yield of ethyl acetate. This removal of the byproduct water can be done with a membrane by a pervaporative separation technique.1 Several advantages are provided with such a technique: easy process design, high selectivity, and low energy consumption. Pervaporation offers the possibility of separating azeotropes or liquid mixtures containing components with close boiling points that are difficult to separate by distillation. Pervaporation composite membranes have therefore been used to dehydrate aqueous ethyl acetate mixtures.14 However, the permeation flux obtained was low. To increase the flux without sacrificing the selectivity, composite membranes with a hydrophilic skin layer on the top of a hydrophobic porous support have been applied.59 Since a hydrophilic surface is necessary for waterselective pervaporation composite membranes, this study made r 2011 American Chemical Society

use of a highly hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) polymer as a layer deposited on a substrate by a process of spin-coating. Moreover, the application of PHEMA/ substrate composites for pervaporative separation has not been extensively investigated.1012 Nonetheless, because of the poor compatibility between a hydrophilic skin layer and a hydrophobic sublayer, causing a weak adhesion at the interface, peeling often occurs when drying the coating layer. To overcome this shortcoming, researchers have developed some methods to modify the interfacial adhesion: plasma treatment,1317 interfacial cross-linking,5,18 introduction of an adhesion layer,19,20 grafting polymerization,6,8,2125 and chemical modification.26,27 On the basis of these studies, the conclusion is that a strong adhesion can be achieved either with the presence of a strong physical interaction such as hydrogen bonding or with the formation of chemical bonds between the active layer and the support. It was indicated that a membrane has a greater stability if interfacial layers are mutually bound by covalent bonding. Xu et al.28 modified the surface of microporous polypropylene hollow fiber membranes by graft Received: May 16, 2011 Revised: July 8, 2011 Published: July 08, 2011 11062

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Figure 1. Schematic of composite membrane preparation.

polymerization of acrylic acid. The permeation rate of the membrane used in water/ethanol pervaporative separation is quite high, but the separation factor is very low due to the low packing density of the graft layer. From the literature, we have gathered that a water-selective composite membrane should have a hydrophilic dense layer on the top of a hydrophobic substrate. In addition, there should be an existing chemical bonding at the interface between hydrophilic and hydrophobic layers. In this study, we applied a spin coating of a PHEMA solution on a poly(vinylidene fluoride) (PVDF) substrate to prepare an S-PHEMA/PVDF composite membrane with an ultrathin selective layer. However, the PHEMA layer could not be deposited uniformly on the PVDF membrane support due to the poor compatibility between PHEMA and PVDF. To improve the interfacial compatibility, a 2-hydroxyethyl methacrylate (HEMA) monomer was grafted onto the PVDF substrate by an ozone-induced graft polymerization, resulting in a PHEMA-g-PVDF composite membrane where chemical bonding existed between PVDF and PHEMA. Then, a PHEMA solution was spin-coated on the grafted PVDF, forming a composite membrane represented as S-PHEMA/ PHEMA-g-PVDF (a uniformly distributed spin-coated PHEMA layer on a PHEMA-grafted PVDF substrate). We investigated in this study the pervaporation performance of composite membranes with a selective layer of a hydrophilic polymer. Limited information on the correlation between free volume distribution and pervaporation performance is available.29 Hence, we used a variable monoenergy slow positron beam (VMSPB) to probe the free volume and positron implantation depth profile in an S-PHEMA/PHEMA-g-PVDF composite membrane. The data obtained were used to elucidate the free volume correlation with the pervaporation performance for dehydrating an aqueous ethyl acetate mixture. The effect of different spin-coating speeds on free volume and pervaporation performance was also discussed. This is with regard to spincoating effects reported by Lu et al.30 and Lu and Mi31 who demonstrated that the main chain of a polymer will orient along a substrate surface when the coating polymer and the substrate can wet each other. These studies and other similar investigations of the spin-coating-induced chain orientation,3239 however, did not deal with free volume behavior and pervaporation applications.

’ EXPERIMENTAL SECTION Materials. 2-Hydroxyethyl methacrylate monomer, purchased from Merck, was first purified by removing the polymerization inhibitor hydroquinone monomethyl ether (MEHQ) by means of vacuum distillation at 67 °C and 3.5 mmHg. Poly(2-hydroxyethyl methacrylate) with a molecular weight of 300 000, poly(vinylidene fluoride) pellets (KYNAR 6000HD), and poly(vinylpyrrolidone) (PVP) with a molecular weight of 10 000 were all supplied by Aldrich Co. N,N-Dimethylacetamide (DMAc; HPLC, 99.5%) solvent was obtained from Tedia C., Inc. The nonwoven polyester material used as a membrane support was a product of Ahlstrom Corp. Pristine Membrane Preparation. To prepare a PVDF substrate, a solution of 20 wt % PVDF in DMAc solvent, with 5 wt % PVP as a porogen (pore-generating agent), was cast with a 300 μm Gardner knife on a nonwoven polyester material. The cast film was immediately immersed in succession in a bath of water, the first one for 10 min to precipitate the PVDF membrane layer and the second for 24 h to remove the residual solvent. The resulting membrane substrate was then dried in an oven at 70 °C for 24 h. Ozone-Induced Graft Polymerization. A PVDF membrane was placed in a stainless reactor. A mixture of ozone and oxygen gases was passed through the reactor at a flow rate of 5 L/min. The concentration of ozone was 35 g/m3. This ozone treatment would lead to the generation of hydroperoxide groups on the surface of the PVDF membrane. The hydroperoxide would be broken down by redox decomposition, resulting in the formation of active species necessary for the subsequent graft polymerization to proceed. After the ozone treatment for 20 min, the reactor was purged with nitrogen gas to ensure that the residual, physically sorbed ozone molecule was removed. A degassed HEMA solution with FeCl2 3 4H2O (104 mol) added as a catalyst for the graft polymerization was then introduced into the reactor. The grafting reaction was allowed to proceed in a water bath at 40 °C for 30 min. The active species formed as a result of the ozone treatment would induce the grafting polymerization with the HEMA monomer.40,41 Following the grafting reaction, the membrane was immersed in boiling ethanol for 3 h to remove the PHEMA homopolymer physically adsorbed on the PVDF membrane surface. The PHEMA-g-PVDF membrane was then dried in a vacuum oven for 24 h. By varying the HEMA concentration (530 vol%) used in the ozone-induced graft polymerization, PHEMA-g-PVDF membranes with different grafting density were obtained. The grafting density (GD) was evaluated as follows: GD = (Wf  Wi)/A, where Wf, Wi, and A denote the weight of the grafted membrane, the weight of the pristine 11063

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Table 1. Pervaporation Performance of Membranes for Dehydrating 98 wt % Aqueous Ethyl Acetate Mixture at 25 °C

a

permeation

water content in

separation

membrane

rate (g/(m2 h))

permeate (wt %)

factor (R)

pristine PVDF S-PHEMA/PVDF a

17700 ( 1240 9300 ( 3070

3.1 ( 0.7 24.1 ( 7.1

1.5 16.5

Spin-coating speed = 2000 rpm.

membrane, and the membrane surface area (100 cm2), respectively. An average value of GD was reported from measurements of at least three different grafted membranes. Preparation of Composite Membrane. Composite membranes were prepared by the spin-coating method. They were composed of a layer of PHEMA on the top of a substrate, which was either a pristine PVDF or a PHEMA-grafted PVDF (PHEMA-g-PVDF). The PHEMA layer was deposited by spin coating 5 wt % of PHEMA dissolved in methanol. Two types of composite membrane were thus produced: a spin-coated PHEMA on a PVDF substrate (S-PHEMA/PVDF) and a spin-coated PHEMA on a PHEMA-g-PVDF substrate (S-PHEMA/ PHEMA-g-PVDF). The PHEMA active layer thickness was varied with different spin-coating speeds. After the spin-coating procedure, the composite membranes were heat-treated at 120 °C for 1 h. The process of the composite membrane preparation is illustrated in Figure 1. Membrane Characterization. The surface chemical composition of composite membranes was measured by Fourier transform infrared spectroscopy combined with attenuated total reflectance (FTIR-ATR, Perkin-Elmer Spectrum One spectroscope). The surface and crosssectional morphologies of the membranes were obtained by field emission scanning electron microscopy (FE-SEM; S-4800). The thickness of the active layer of the composite membranes was evaluated on the basis of the SEM images and the positron annihilation spectroscopy (PAS) data. Positron Annihilation Spectroscopy. A variable monoenergy slow positron beam (VMSPB) was used to define the mean depth from the membrane surface between 0 and about 10 μm; the mean depth was determined by converting positron incident energy of 030 keV using an established equation.29 This new radioisotope beam used 50 mCi of 22 Na as the positron source. A positron annihilation spectrometer, either a Doppler broadening energy spectroscope (DBES) or a positron annihilation lifetime spectroscope (PALS), was connected to the VMSPB. The DBES spectra were measured using an HPGe detector at a counting rate of ∼2000 counts/s. The energy resolution of the solidstate detector was 1.5 at 511 keV (corresponding to a positron 2γ annihilation peak). The total number of counts for a DBES spectrum was 1.0 million. The obtained DBES spectra were characterized by an S parameter, defined as a ratio of the integrated counts between 510.3 and 511.7 keV (S width) to the total counts after the background was properly subtracted. Since the S parameter represents the relative value of the low-momentum part of the positronium annihilation radiation, it is sensitive to a change of the positron and positronium (Ps) states due to microstructural changes.4245 The positron and Ps are localized in a hole or free volume with a finite size, and the observed S parameter is a measure of the momentum broadening according to the uncertainty principle. A larger hole is equivalent to a larger S parameter value and a greater amount of parapositronium (singlet state). The S parameter has been successfully used in determining the free volume depth profile in polymeric systems.4245 The multilayered structure was obtained by VEPFIT program analysis of the S parameter.46 The PALS data were obtained by taking the coincident events between the start signal detected by a multichannel plate (MCP) from the secondary electrons and the stop signal discerned by a BaF2 detector

Figure 2. SEM images (50k) of (a) pristine PVDF membrane and (b) S-PHEMA/PVDF composite membrane.

Figure 3. SEM images (50k) of PHEMA-g-PVDF composite membranes with different grafting density: (a) 1.05, (b) 2.41, and (c) 3.27 mg/cm2. from the annihilation photons at a counting rate of approximately 200300 counts/s. A PALS spectrum contains 2.0 million counts.47 The obtained PALS data were fitted into three lifetime components using the PATFIT program.48 Analyzed results of positron lifetimes (τ1, τ2, and τ3) and intensities (I1, I2, and I3) from PALS spectra are attributed to the positron and positronium annihilation in polymeric membrane materials. τ1 (the shortest ∼0.125 ns) is from p-Ps annihilation, τ2 (∼0.45 ns) is from the positron annihilation, and τ3 (on the order of 15 ns in polymeric materials) is due to o-Ps pickoff annihilation with electron in molecules.49 τ3 is used to calculate the mean free-volume radius R (angstroms to nanometers) on the basis of an established semiempirical correlation equation50,51 from a sphericalcavity model. Since Ps is known to preferentially localize in defect sites,52 particularly in the free volume before annihilation takes place, parameters from o-Ps annihilation have been successfully used to obtain the electron properties and depth profiles of free volumes in thin film polymers.5356 Pervaporation Measurement. The apparatus for the pervaporation experiment was described in a previous paper.57 Given in that previous paper are details of calculations for permeation rate and separation factor and the determination of water concentration in permeate. In this study, the effective membrane area was 3.5 cm2. Pervaporation experiments were conducted at 25 °C, using a mixture containing 2 wt % water and ethyl acetate as a feed solution.

’ RESULTS AND DISCUSSION Pervaporative Dehydration of Aqueous Ethyl Acetate Mixture. As can be deduced from Table 1, a pristine PVDF

membrane exhibited a very high permeation flux but an extremely low separation factor for dehydrating a 98 wt % aqueous 11064

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Figure 4. ATR-FTIR spectra for pristine PVDF membrane and PHEMA-g-PVDF composite membranes with different grafting density.

ethyl acetate mixture at 25 °C. An asymmetric PVDF membrane, shown in Figure 2a, displayed many small pores on the top skin, to which could be attributed the very high flux but extremely low separation factor. In an effort to improve this poor pervaporation performance, a composite membrane with a hydrophilic skin layer on the top of an asymmetric PVDF support membrane was prepared by a spin-coating method. The resulting S-PHEMA/ PVDF membrane had minimal pores on the surface (Figure 2b), illustrating that the spin-coated PHEMA layer covered most of the pores that were evident on the pristine PVDF (Figure 2a). Although the S-PHEMA/PVDF composite membrane showed a much higher separation factor (R = 16.5) compared to the pristine PVDF membrane (R = 1.5), the pervaporation performance was still low. This may be attributed to the uneven distribution of the PHEMA layer on the PVDF substrate (Figure 2b), probably due to the poor compatibility between these two polymers. To remedy such a problem of poor compatibility, a HEMA monomer was grafted onto a PVDF substrate by an ozone-induced graft polymerization to form a PHEMA-g-PVDF composite membrane (Figure 3ac), in which there would be a chemical bonding between PVDF and PHEMA. From Figure 3ac, we can observe that the number of pores on pristine PVDF (Figure 2a) was diminished as a result of grafting with HEMA solutions of different concentrations and that the grafted membrane denseness increased with the grafting density of the PHEMA-g-PVDF membrane. PVDF and PHEMA-g-PVDF membranes were characterized by ATR-FTIR with respect to their surface chemical composition (Figure 4). Following the grafting process, the membrane was washed with boiling ethanol for 3 h. This washing procedure would remove only the surface-adsorbed PHEMA homopolymer. The ATR-FTIR results validated the presence of the chemically bound PHEMA graft on the PHEMA-g-PVDF composite membrane on the basis of the appearance of its characteristic absorbed peaks: the -OH stretching vibration peak at 3405 cm1, the CdO vibration peak at 1723 cm1, and the respective CO asymmetric and symmetric stretching vibrations in the ester group at 12751185 and 1160 1050 cm1.58 From the foregoing arguments, we can therefore infer that the grafting polymerization was successful. The

Figure 5. Effect of grafting density on pervaporation performance of PHEMA-g-PVDF composite membrane for dehydrating 98 wt % aqueous ethyl acetate mixture at 25 °C (blue filled triangles, water concentration in permeate; black filled squares, permeation rate).

intensity of PHEMA characteristic absorbed peaks increased with the grafting density of the PHEMA-g-PVDF composite membrane, which implies that the amount of the PHEMA graft on the substrate surface increased as well. However, at grafting density higher than 2.41 mg/cm2, the peaks intensity had no noticeable change anymore. This may be explained by the limitation of the ATR-FTIR implantation depth of about 1 μm. At high grafting density, the graft was thicker than 1 μm. It appears then that the intensity of peaks for the PHEMAgrafted membranes with high grafting density was similar. The effect of grafting density on PHEMA-g-PVDF composite membrane pervaporation performance is illustrated in Figure 5. With increasing grafting density, the permeation rate was found to decrease and the water concentration in permeate was observed to increase. These results may be attributed to a thicker PHEMA graft layer at a higher grafting density, as indicated in the cross-sectional SEM images of PHEMA-g-PVDF composite membranes (Figure 3ac). However, the pervaporation performance depicted in Figure 5 was still low. A possible reason may be the low packing density of the grafted PHEMA formed during the grafting polymerization, leading to an uneven graft layer with some defects, as shown in the surface SEM images in Figure 3ac. This rationale is similar to that of a study by Xu et al.24 11065

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Figure 6. Effect of grafting density on pervaporation performance of S-PHEMA/PHEMA-g-PVDF composite membrane for dehydrating 98 wt % aqueous ethyl acetate mixture at 25 °C (blue filled triangles, water concentration in permeate; black filled squares, permeation rate).

Figure 8. Effect of spin-coating speed on pervaporation performance of S-PHEMA/PHEMA-g-PVDF composite membrane for dehydrating 98 wt % aqueous ethyl acetate mixture at 25 °C (GD = 2.41 mg/cm2; blue filled triangles, water concentration in permeate; black filled squares, permeation rate).

Figure 7. SEM images (50k) of composite membranes: (a) PHEMAg-PVDF (GD = 2.41 mg/cm2); (b) S-PHEMA/PHEMA-g-PVDF (2000 rpm, GD = 2.41 mg/cm2).

Figure 9. Cross-sectional SEM images (50k) of S-PHEMA/PHEMAg-PVDF composite membranes prepared at different spin-coating speeds: (a) 1000, (b) 2000, (c) 4000, and (d) 6000 rpm.

To improve the pervaporation performance, a spin-coating process was applied. A solution of PHEMA was spin-coated onto a PHEMA-g-PVDF composite membrane to deposit an ultrathin selective PHEMA layer. The result was an S-PHEMA/PHEMAg-PVDF composite membrane. It was expected then that such a spin-coating procedure would result in a PHEMA layer that could cover the defects on the PHEMA-g-PVDF composite membrane (Figure 3ac). As demonstrated in Figure 6, the pervaporation performance could be improved significantly as a result of conducting the spin-coating process described above. The S-PHEMA/PHEMA-g-PVDF composite membrane showed much higher water selectivity (Figure 6) compared with that of the PHEMA-g-PVDF composite membrane (Figure 5). In the case of the former composite membrane, the water concentration in permeate was higher than 96 wt % for a grafting density greater than 2.41 mg/cm2. At this grafting density then, surface morphologies of PHEMA-grafted composite membranes with and without S-PHEMA layer were compared in Figure 7. Since the surface SEM image shown in Figure 7b turned out to have no defects and became smooth after the PHEMA spin-coating process, it was clearly indicated that the spin-coated layer of PHEMA completely covered the PHEMA-g-PVDF composite membrane defective surface (Figure 7a). As such, a high pervaporation performance was obtained (Figure 6). These data provided evidence that spincoating was an effective technique to prepare a high-performance selective layer on a support membrane surface. The

optimum grafting density for the S-PHEMA/PHEMA-g-PVDF composite membrane was 2.41 mg/cm2. Effect of Spin-Coating Speed on Pervaporation Performance of S-PHEMA/PHEMA-g-PVDF Composite Membranes. Figure 8 depicts the effect of spin-coating speed on pervaporation performance of S-PHEMA/PHEMA-g-PVDF composite membrane for dehydrating a 98 wt % aqueous ethyl acetate mixture at 25 °C. An increase in the permeation rate was indicated with a higher spin-coating speed, while the water concentration in permeate was shown to remain at an almost constant value. These results might be a consequence of the effect of spincoating speed: a much faster speed may lead to a much thinner spin-coated PHEMA layer on a substrate, suggesting that a very fast spin-coating speed may cause coiled PHEMA polymer chains being oriented along the substrate surface to be fully stretched.30,31 On the basis of the cross-sectional SEM images in Figure 9 that indicated a decrease in the thickness of the spincoated PHEMA active layer on a composite membrane at a higher spin-coating speed, it was apparent that such behavior caused a lower diffusion resistance for permeating feed molecules and, hence, resulted in a higher permeation rate compared to a composite membrane prepared at a lower spin-coating speed. Table 2 presents the active layer thickness of S-PHEMA/ PHEMA-g-PVDF composite membranes prepared at different spin-coating speeds. Positron Annihilation Spectroscopy of S-PHEMA/PHEMAg-PVDF Composite Membranes. The pervaporation separation 11066

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Table 2. Thickness of Active Layer of S-PHEMA/PHEMA-gPVDF Composite Membranes as Function of Spin-Coating Speed. (GD = 2.41 mg/cm2) thickness of active layer (nm) spin-coating speed (rpm)

from SEMa

from VEPFIT datab

1000

654 ( 22

671

4000

333 ( 3

482

6000

199 ( 7

202

a

Measured from SEM cross-sectional images. b Evaluated from depth profile in Figure 11. Figure 11. Schematic diagram of four-layer depth structure of S-PHEMA/PHEMA-g-PVDF composite membranes prepared at different spin-coating speeds, obtained by VEPFIT program analysis of S parameter data from DBES.

Figure 10. Plots of S parameter as function of positron incident energy data for PHEMA film (red filled triangles) and S-PHEMA/PHEMA-gPVDF composite membranes prepared at different spin-coating speeds: (black filled squares) 1000, (green filled diamonds) 4000, and (blue filled triangles) 6000 rpm (GD = 2.41 mg/cm2).

performance of S-PHEMA/PHEMA-g-PVDF composite membranes could not be thoroughly explained by surface morphological variations. Therefore, microstructural properties such as free volume in the composite membrane were also discussed. On the basis of free volume information, a depth profile of composite membranes can be drawn. Free volume and depth profile data can be obtained from PAS conducted with a VMSPB. This study used two techniques: DBES and PALS. A positron annihilation parameter S can be determined from DBES and is a measure of the free volume within a material. Increasing the relative contribution of low-momentum electrons to the annihilation of positrons in open volume defects in polymers causes an increase in the value of S.59,60 An analysis of the magnitude and amount of free volume as a function of the composite membrane layer depth can be done with DBES data.

The positron energy can be converted to give a mean depth from the surface of a composite membrane to a certain point in the bulk region. Thus, information on the variation in free volume in a composite membrane and the multilayered structure of the composite was obtained by this study from PAS experiments with the use of VMSPB. Figure 10 presents DBES data in the form of plots of S vs positron incident energy data for PHEMA film and S-PHEMA/ PHEMA-g-PVDF composite membranes. For both this film and composite membranes, the S parameter near the surface increased rapidly. This is typical in positron annihilation, the initial dramatic increase in S being due to the back-diffusion and scattering of positrons and positronia (4). On the other hand, a five-layer model gave slightly better χ2 values than a four-layer model, but the fitted results were unstable and the resultant errors were larger than the fitter layer thickness and diffusion lengths. Therefore, the VEPFIT process provided fitting results descriptive of a four-layer model (Figure 11). The four layers 11067

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Table 3. Positron Annihilation Result for PHEMA Dense Film and S-PHEMA/PHEMA-g-PVDF Composite Membranes Prepared at Different Spin-Coating Speeds (Positron Energy Fixed at 1.5 keV) membrane

τ3 (ns)

I3 (ns)

R (Å)

PHEMA film

2.619 ( 0.0194

16.24 ( 0.23

3.3664 ( 0.0107

S-PHEMA/PHEMA-

2.601 ( 0.0144

16.73 ( 0.47

3.3529 ( 0.0080

2.034 ( 0.0129

14.31 ( 0.31

2.8864 ( 0.0097

1.922 ( 0.0137

13.01 ( 0.65

2.7828 ( 0.0110

g-PVDF, 1000 rpm S-PHEMA/PHEMA-

Figure 12. Representation of chain behavior of PHEMA graft and S-PHEMA during film formation.

indicated were labeled as follows: (I) spin-coated PHEMA skin layer, (II) dense PHEMA graft layer, (III) transition layer from dense PHEMA graft layer to surface of porous PVDF support layer, and (IV) porous PVDF support layer. The thickness of the spin-coated PHEMA skin layer of S-PHEMA/PHEMA-g-PVDF composite membranes is listed in Table 2. Although the thickness data from the methods of SEM and PAS had some discrepancies, the two sets of data showed decreasing trend with increasing spin-coating speeds. As shown in Figure 10 for values of positron energy from about 1 to 3.5 keV, the S-PHEMA/PHEMA-g-PVDF composite membrane prepared at 6000 rpm spin-coating speed displayed lower values of S (smaller amount of free volume) and a thinner S-PHEMA layer, compared to the composite membranes prepared at 4000 and 1000 rpm. The smaller amount of free volume at a faster spin-coating speed may be ascribed to the higher extent of uncoiling of PHEMA polymer chains oriented along the substrate surface, leading to a denser, thinner, and more orderly structure of the composite membrane selective layer. The ultrathin PHEMA layer that was spin-coated at 6000 rpm (Figure 9d) caused a decrease in the mass-transfer resistance, thus resulting in a higher permeation rate compared with 1000 rpm (Figure 8). We can conclude, therefore, that a PHEMA selective layer deposited at a higher spin-coating speed tends to have a thinner and denser structure, and, hence, results in a higher flux and a higher selectively. On the basis of Figure 11, it could be observed that the S parameter for the PHEMA graft layer was higher than that for the S-PHEMA layer. A higher S value corresponds to a higher free volume. However, since the PHEMA chemical structure should be the same in the grafted and spin-coated layers, its S parameter should be similar. The discrepancy observed could be explained as follows. The graft layer was formed from the PVDF substrate surface by free radical polymerization. As such, the growth of the graft chain length was not uniform (as shown in Figure 3), leading to an uneven stacking or a low packing density and a random orientation. The result would be a looser structure, and hence a larger free volume, resulting in a low selectivity of the PHEMA-g-PVDF composite membranes (Figure 5). This is in contrast to a denser structure of the spin-coated PHEMA layer deposited at different coating speeds. The structure would be denser and more compact with spin-coating because there may be uncoiling of polymer chains oriented along the substrate surface. The representation of the chain behavior of the graft PHEMA and S-PHEMA during the film formation is shown in Figure 12.

g-PVDF, 4000 rpm S-PHEMA/PHEMAg-PVDF, 6000 rpm

While DBES can give qualitative analysis, PALS spectra can provide quantitative analysis of free volume size and intensity. τ3 and I3 are related to the free volume size and intensity, respectively. As shown in Table 3, all S-PHEMA/PHEMA-gPVDF composite membranes had lower τ3 and I3 compared to PHEMA film prepared by the casting method. Both τ3 and I3 decreased with increasing spin-coating speeds, indicating smaller and less amount of free volume. This behavior might be attributed to the more orderly polymer structure associated with the chain orientation. Several studies have discussed the shearinduced orientation of polymer chains.6165 Li et al.65 expounded that a strong inertial force induces stretching and shearing field in fluid flow during a spin-coating process, resulting in orientation of flexible polymer coils or aggregates. They argued that at higher inertial force or speed-coating speeds, stronger shearing and stretching field causes polymer chains to orientate. Franke and Birnie66 observed that during spin-coating of composite solutions, fibers orient themselves in the radial direction. The free volume radius range shown in Table 3 is higher than the molecular radius of water (1.2 Å)67 but lower than the molecular radius of ethyl acetate (4.95 Å),68 and therefore suitable for separating water from ethyl acetate. In pervaporation separation processes, smaller free volume size provides higher diffusion selectivity. Because of the small pores in the S-PHEMA/PHEMA-g-PVDF composite membrane prepared at 6000 rpm, it maintained a high selectivity. Furthermore, its ultrathin selective layer of 200 nm provided a higher permeation rate compared to the thicker selective layer (∼650 nm) of composite membrane prepared at 1000 rpm (see Table 2). Hence, we can conclude that a multilayered composite membrane with a skin layer consisting of orientated chains induced by a spin-coating method can give a high permeation rate and maintain a high selectivity. The method of spin-coating is, therefore, advantageous for fabricating a highperformance pervaporation membrane.

’ CONCLUSION In this study, a porous PVDF membrane substrate was surfacegrafted with a HEMA monomer solution by an ozone-induced graft polymerization to fabricate a PHEMA-g-PVDF composite membrane with improved interfacial compatibility between PHEMA and PVDF. On the surface of this PHEMA-grafted composite membrane, spin-coating a PHEMA solution resulted in a deposition of a dense selective layer, which improved the pervaporation performance of the composite membrane significantly. The optimum grafting density was found to be 11068

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Langmuir 2.41 mg/cm2. An S-PHEMA/PHEMA-g-PVDF composite membrane with a denser and a thinner selective layer resulting from inducing a faster spin-coating speed gave a higher permeation rate without sacrificing the selectivity. This high pervaporation performance was correlated with lower values of the S parameter and positron lifetime corresponding to smaller free volume in the composite membrane, the depth profile of which showed a thinner S-PHEMA layer at a spin-coating speed of 6000 rpm compared at 4000 and 1000 rpm. PALS data provided quantitative free-volume size and intensity data, which revealed that an S-PHEMA layer deposited on a PHEMA-grafted substrate was suitable for dehydrating water/ethyl acetate mixtures by pervaporation.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +886 3 2654190. Fax: +886 3 2654198 E-mail: krlee@ cycu.edu.tw.

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