Interfacially polymerized thin-film composite polyamide membranes: Effects of annealing processes on pervaporative dehydration of aqueous alcohol solutions

Separation and Purification Technology 72 (2010) 40–47

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Interfacially polymerized thin-film composite polyamide membranes: Effects of annealing processes on pervaporative dehydration of aqueous alcohol solutions Shu-Hsien Huang a,∗∗ , Wei-Song Hung b , Der-Jang Liaw d , Chia-Hao Lo b , Wei-Chi Chao b , Chien-Chieh Hu c , Chi-Lan Li c , Kueir-Rarn Lee b,∗ , Juin-Yih Lai b a

Department of Chemical and Materials Engineering, National Ilan University, I-Lan 26047, Taiwan R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan Department of Chemical and Material Engineering, Nanya Institute of Technology, Chung-Li 32034, Taiwan d Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan b c

a r t i c l e

i n f o

Article history: Received 18 September 2009 Received in revised form 24 December 2009 Accepted 28 December 2009 Keywords: Polyamide Thin-film composite membrane Interfacial polymerization Pervaporation Positron annihilation spectroscopy

a b s t r a c t High-performance thin-film composite polyamide membranes for pervaporative dehydration processes were prepared by means of the interfacial polymerization of triethylenetetramine (TETA) and trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane support. The effects of annealing processes applied during and after the composite membrane preparation on the pervaporation performance were investigated. Two annealing processes were applied. One was to the composite membrane formed after the interfacial polymerization by subjecting it to different annealing temperatures and the other to the aqueous amine (TETA) solution used during the interfacial polymerization process by varying its temperature. Positron annihilation spectroscopy experiments using a slow positron beam were carried out. One of the positron annihilation techniques, Doppler broadening energy spectroscopy (DBES), was used to study the effect of the annealing processes on the S parameter (corresponding to the free volume in the membrane). The first plateau (S1 = 0.47798 ± 1.86E−4, L1 = 182 ± 84 nm) in the curve obtained for the polyamide layer in the Type-B composite membrane was higher than that (S1 = 0.47659 ± 2.74E−4, L1 = 196 ± 52 nm) in the Type-A composite membrane. In other words, this corresponds to a higher S parameter and a thinner layer for the Type-B membrane than the Type-A membrane. This implies that the free-volume amount in the former membrane is higher than that in the latter. It was found that these two annealing processes greatly improved the pervaporation separation performances of the thin-film composite polyamide membranes prepared in this study in dehydrating aqueous alcohol solutions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Most aqueous alcohol solutions are azeotropic mixtures. Examples are ethanol (EtOH), isopropanol (IPA), and tetrafluoropropanol (TFP). EtOH solutions are being investigated as new energy sources. This investigation is in line with the effort to alleviate problems associated with limited petroleum resources [1]. IPA is a clean energy source, and it is one of the important solvents used in large scale in industries, particularly pharmaceutical industries. IPA is used as a cleaning agent in modern semiconductor and electronic industries, where recycling of waste IPA is essential from an environmental and an economic point of view [2]. TFP is a kind of

∗ Corresponding author. Tel.: +886 3 2654190; fax: +886 3 2654198. ∗∗ Corresponding author. Tel.: +886 3 9357400; fax: +886 3 9357025. E-mail addresses: [email protected] (S.-H. Huang), [email protected] (K.-R. Lee). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.12.026

fluoroalcohol, and it is widely used as solvent for coating dyes employed in manufacturing recording media such as the optical disc of DVD-R or CD-R. TFP has relatively a low level of toxicity to human beings and a low impact to the aerosphere environment. As such, it may have a great potential to replace Freon as a cleaning agent [3]. Aqueous alcohol solutions are concentrated using the distillation process, but purity is restricted by their azeotropes. Further, alcohol dehydration is carried out by means of azeotropic distillation, with the use of either an azeotrope-breaking component (entrainer) or a hybrid system consisting of a multi-stage evaporation and distillation. Such azeotropic distillation consumes large energy and entails cost. As a substitute for those conventional separation processes, the pervaporation separation process which can separate azeotropic mixtures and save energy and cost has been applied in the dehydration of aqueous alcohol mixtures. The key to the pervaporation process success is the fabrication of suitable membranes. Polyamides have been regarded as suitable membrane materials because of their high thermal stability,

S.-H. Huang et al. / Separation and Purification Technology 72 (2010) 40–47

excellent mechanical strength, and high resistance to organic solvents. However, the disadvantage of dense polyamide membranes when applied to the pervaporation separation process is their low permeation rates. To increase the permeation rate of polyamide membranes without sacrificing the selectivity, the membrane must be transformed from a dense thick structure into a composite structure. The interfacial polymerization technique is based on a polymerization reaction that forms a polymer film at the interface between two immiscible phases (aqueous and organic phases), each of which has a highly reactive monomer dissolved in it. The polymer film formed at the interface usually grows from the aqueous phase toward the organic phase. This concept of polymer film growth at the interface has been proven by Morgan [4]. Interfacial polymerization is an effective technique for the preparation of a composite membrane consisting of an interfacially polymerized selective thin layer on the surface of a porous membrane support. Interfacially polymerized thin-film composite membranes have been investigated usually in researches on reverse osmosis (RO) [5–9] or nanofiltration (NF) [10–14], but they have been examined only in a few reports on pervaporation [15–17]. Most of the interfacially polymerized thin-film composite membranes applied in the pervaporation separation process are prepared at ambient conditions [15–17]. It should be noted that the characteristics of a composite polymeric material are different from those of a free-standing polymeric material. The separation performance of a thin-film composite polymeric membrane can be affected by its characteristics such as the glass transition temperature (Tg) during the annealing process. In this study, thin-film composite polyamide membranes were prepared by the interfacial polymerization of triethylenetetramine (TETA) and trimesoyl chloride (TMC). The effects of the annealing processes applied during and after the composite membrane preparation on the pervaporation separation performance for dehydrating aqueous alcohol solutions were investigated. Two annealing processes were used. The first one was after interfacial polymerization, in which the composite membrane was subjected to different annealing temperatures. It was interesting to investigate the annealing effect on the pervaporation performance of the thin-film composite polyamide membrane composed of two polymeric materials with different Tgs. The second annealing process was during interfacial polymerization, wherein the temperature of the aqueous amine (TETA) solution was varied. In this second process, the high-temperature aqueous amine solution was used to increase the chain mobility of the mPAN having lower Tg. In this way, more amine monomers could easily penetrate into the mPAN membrane. This is favorable to form denser and thinner selective polyamide layer, which is beneficial to improve the pervaporation performance. To estimate the effects of the annealing processes on the variation in the free volume of the composite TETA-TMC/mPAN membrane and correlate it with the pervaporation performance, the composite membrane was tested by conducting positron annihilation spectroscopy (PAS) experiments with the use of a variable monoenergy slow positron beam [17,18].

2. Experimental 2.1. Materials Polyacrylonitrile (PAN) polymer was supplied by Tong-Hua Synthesis Fiber Co. Ltd. (Taiwan), and it was used as the supporting layer for the composite membrane prepared by interfacial polymerization. Reagent-grade N-methyl-2-pyrrolidone (NMP) was the solvent used in preparing the PAN casting solution. The monomers used in the interfacial polymerization process were

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triethylenetetramine (TETA) and trimesoyl chloride (TMC). The former monomer, purchased from Merck Co., was used as the aqueous phase monomer and the latter, obtained from TCI Co., was used as the organic phase monomer. The reaction between these two different monomers resulted in the formation of the active polyamide layer. Distilled water was used in preparing aqueous solutions, and reagent-grade toluene was used as the organic solvent. 2.2. Preparation of modified PAN porous membrane supports (mPAN) In preparing the flat asymmetric PAN porous membrane, a solution of 15 wt% PAN in NMP was cast onto a polyester nonwoven substrate using a casting knife with 200 ␮m gap. The cast membrane was precipitated by immersion in a bath of water. The resulting asymmetric PAN porous membrane was washed by soaking it in water overnight and was then stored in a bath of water prior to its use in preparing the modified PAN (mPAN) porous membrane support, The mPAN preparation procedure was described in our previous study [16]. The affinity between the support membrane surface and the aqueous amine solution is affected by the hydrophilicity of the membrane surface. Thus, a support membrane with higher hydrophilicity can make the aqueous amine solution spread on its surface evenly. This is favorable to form more uniform and defectfree polyamide active layer on the support membrane during the interfacial polymerization process. To improve the hydrophilicity of the support membrane surface, mPAN was prepared by immersing the PAN membrane support in a 2 M NaOH solution at 50 ◦ C. The partial –CN groups of PAN can be converted into –COOH or –CONH2 groups as a result of the hydrolysis with NaOH solution [17]. The mPAN membrane was washed in a water bath for several hours, and was then stored in another water bath before its use for interfacial polymerization. The selection of this membrane support is favorable to obtain high flux. The mPAN membrane was applied for the pervaporation separation of a 70 wt% aqueous isopropanol solution at 25 ◦ C. The result showed that the permeation rate and the water concentration in the permeate were 5000–6500 g/m2 h and 45–55 wt%, respectively. Therefore, the mPAN membrane with a high permeation rate was suitable as the support in the composite membrane. 2.3. Preparation of thin-film composite polyamide membranes In the interfacial polymerization process, the mPAN membrane support was immersed in a 0.1 wt% aqueous TETA solution at room temperature for 5 s. A rubber rod was used to remove the excess amount of the aqueous TETA solution from the surface of the mPAN membrane. This mPAN wet with the aqueous TETA solution was then fixed in a special frame-like device. A toluene solution containing 0.05 wt% TMC was poured onto the mPAN membrane and was allowed to get in contact with the membrane surface for 10 s to carry out the process of interfacial polymerization. The thin-film composite polyamide membrane was dried at room temperature and was then washed with methanol. An illustration of the interfacial polymerization process is shown in Fig. 1. In this study, two annealing processes applied during and after the preparation of the thin-film composite polyamide membranes were investigated: (1) during the interfacial polymerization process, the aqueous TETA solution temperature and time of contact with the TMC were varied, (2) after the interfacial polymerization process, the thin-film composite membranes were annealed at different temperatures and treatment times.

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Fig. 1. Illustration of interfacial polymerization process.

2.4. Characterization The chemical structures of the active layers of the thin-film composite membranes were studied by using FTIR-ATR (PerkinElmer Spectrum One) spectroscopy. In our previous study [18], two peaks appeared in the FTIR-ATR spectra at the wave numbers of 1640 and 1540 cm−1 , corresponding to C O (amide I) and N–H (amide II), respectively. It was confirmed that the thin-film composite membrane’s active layer was composed of aromatic polyamide. SEM (HITACHI S-3000N and S-4800) was used to observe the morphologies of the thin-film composite polyamide membranes. Fig. 2 shows the SEM surface images of mPAN and thin-film composite polyamide membranes. As shown in Fig. 2(a), the mPAN membrane exhibited a smooth surface with micropores spread over the surface. It was found that the active polyamide layer (TETA-TMC) (Fig. 2(b) and (c)) formed as a result of the interfacial polymerization process was found to cover the mPAN membrane (Fig. 2(a)) completely. 2.5. Slow positron beam technique A newly built slow positron beam with a variable mono energy at the R&D Center for Membrane Technology in Chung Yuan Uni-

versity in Taiwan was used for this study to define the mean depth of the membrane between 0 and about 10 ␮m (the mean depth was calculated by using an established equation from the positron incident energy from 0 to 30 keV) [18]. This new radioisotope beam uses 50 mCi of 22 Na as the positron source. The positron annihilation spectrometer was installed in this beam for studying the Doppler broadening energy spectroscopy (DBES). The DBES spectra were measured using an HP Ge detector at a counting rate of approximately 2000 cps. The total number of counts for each DBES spectrum was 1.0 million. To estimate the variation in the thickness of the composite TETA-TMC/mPAN membranes, the S parameter data from DBES were fitted using the VEPFIT program.

2.6. Pervaporation measurement The pervaporation separation of aqueous alcohol solutions (EtOH, IPA, and TFP) using thin-film composite polyamide membranes was conducted. The pervaporation apparatus was described in our previous study [19]. The effective surface area of the membrane in direct contact with the feed solution was 11.64 cm2 . The operating temperature (feed solution temperature) was 25 ◦ C. The concentrations of the feed solution and the permeate were

Fig. 2. SEM surface images of (a) modified PAN (mPAN) membrane support and thin-film composite polyamide membrane, (b) TETA-TMC/mPAN prepared at annealing temperature of 110 ◦ C for 30 min and (c) TETA-TMC/mPAN prepared with immersion of mPAN in aqueous TETA solution at 90 ◦ C for 5 s.

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Fig. 3. Effect of annealing temperature on pervaporation performance of composite TETA-TMC/mPAN membranes for dehydrating 90 wt% aqueous ethanol solutions at 25 ◦ C (annealing time: 60 min).

measured by gas chromatography (GC; China Chromatography 9800). The permeation rate (P) was calculated from the following equation: P=

W A×t

(1)

where W is the weight of the permeate, A is the effective membrane area, and t is the sampling time. 3. Results and discussion 3.1. Effects of annealing temperature and time on pervaporation performance In general, the polymer chains expansibility and mobility increase during the annealing process. The polymer chains rearrange and the polymeric packing density increases gradually with the annealing time. However, the characteristics of a composite polymeric material are different from those of a free-standing polymeric material. During the annealing process, the separation performance of a composite polymeric material can be affected by its characteristics such as the glass transition temperature (Tg). In this study, the composite polymeric material characterized was a thin-film composite polyamide membrane. It would be interesting to investigate the effects of annealing conditions (annealing temperature and time) on the pervaporation performance of the composite TETA-TMC/mPAN membrane. Fig. 3 shows the effect of the annealing temperature on the pervaporation performance of composite TETA-TMC/mPAN membranes for separating a 90 wt% aqueous ethanol solution at 25 ◦ C. It was found that with an increase in the annealing temperature from 25 to 50 ◦ C, the permeation rate increased but the water concentration in the permeate decreased. This might be due to the difference between the glass transition temperatures (Tgs) of the polyamide layer and the mPAN membrane support. The Tg of the mPAN is about 104 ◦ C and that of the polyamide is higher than 200 ◦ C. Compared to the polymer chains of the TETA-TMC polyamide layer, those of the mPAN are easier to expand and mobilize at 50 ◦ C annealing temperature. The expanding and mobilizing polymer chains of the mPAN cause the interfacial polymerization layer to stretch, resulting in an increase in the free volume in the TETATMC polyamide layer. Thus, the permeation rate increased and the water concentration in the permeate decreased.

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Fig. 4. Effect of annealing time on pervaporation performance of composite TETATMC/mPAN membranes for dehydrating 90 wt% aqueous ethanol solutions at 25 ◦ C (annealing temperature: 110 ◦ C).

With increasing annealing temperature from 50 to 130 ◦ C, the permeation rate decreased but the water concentration in the permeate increased. The reason might be that the polymer chains of the active TETA-TMC polyamide layer and the mPAN membrane support expand, mobilize, rearrange, and then tend to densify gradually as the annealing temperature steadily approaches their Tgs, even when it is higher than the Tg of the mPAN. The desirable annealing temperature found was 110 ◦ C. This was used in the following annealing experiments. Fig. 4 shows the effect of the annealing time on the pervaporation performance of composite TETA-TMC/mPAN membranes for separating a 90 wt% aqueous ethanol solutions at 25 ◦ C. It was found that the permeation rate decreased with the annealing time. This result might be due to the densification of the polymer chains as a result of the annealing process. However, the water concentration in the permeate initially increased and then remained almost constant as the annealing time progressed. These results might be due to the increase in the free volume in the composite TETA-TMC/mPAN membranes, resulting from the expanding and mobilizing polymer chains of the active TETA-TMC layer and the mPAN supporting layer induced by the annealing process at annealing times less than 30 min. The densification of the polymer chains at annealing times longer than 30 min resulted in the high water concentration in the permeate. From the results shown in Figs. 3 and 4, the desirable annealing conditions applied for preparing the composite TETA-TMC/mPAN membrane were found to be 110 ◦ C annealing temperature and 30 min annealing time. Compared with the pristine composite membrane (permeation rate = 1151 g/m2 h, water concentration in the permeate = 99.4 wt%), the annealed composite membrane prepared at the desirable annealing conditions (annealing temperature of 110 ◦ C and annealing time of 30 min) exhibited better pervaporation performance (permeation rate = 1475 g/m2 h, water concentration in the permeate = 99.2 wt%) for the dehydration of a 90 wt% aqueous ethanol solution at 25 ◦ C. 3.2. Effects of temperature of and immersion time in aqueous TETA solution on pervaporation performance The above Section 3.1 discussed that the polymer chains expansibility and mobility during the annealing process applied after the interfacial polymerization process resulted in a better pervaporation performance of the composite TETA-TMC/mPAN membrane

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Fig. 5. Effect of temperature of aqueous TETA solution on pervaporation performance of composite TETA-TMC/mPAN membranes for dehydrating 90 wt% aqueous ethanol solutions at 25 ◦ C (immersion time in aqueous TETA solution: 5 s).

Fig. 6. Effect of immersion time in aqueous TETA solution on pervaporation performance of composite TETA-TMC/mPAN membranes for dehydrating 90 wt% aqueous ethanol solutions at 25 ◦ C (temperature of aqueous TETA solution: 90 ◦ C).

for the dehydration of an aqueous ethanol solution at 25 ◦ C. In line with this, we want to further study the effect of temperature at a certain interfacial polymerization step on the pervaporation separation performance. The activity of the molecules increases as a consequence of the heat treatment. The first step in the interfacial polymerization process is immersing the mPAN membrane support in the aqueous amine solution to absorb the amine monomers in it. If the temperature of the aqueous amine solution increases, it will have an opportunity to increase the activity of the amine monomers and the free volume in the mPAN membrane, resulting in an increase in the amount of the monomers absorbed in the mPAN membrane, which react with the acyl chloride monomers during the interfacial polymerization reaction. This condition is favorable to obtain denser and thinner selective polyamide layer, which is beneficial to improve the pervaporation performance. Therefore, the effects of the temperature of and the immersion time in the aqueous TETA solution on the pervaporation performance of the composite TETATMC/mPAN membrane were investigated. Fig. 5 shows the effect of the temperature of the aqueous TETA solution on the pervaporation performance for dehydrating a 90 wt% aqueous ethanol solution at 25 ◦ C using the resulting composite TETA-TMC/mPAN membrane. Fig. 5 indicates an increase in the permeation rate with an increase in the temperature of the aqueous TETA solution. However, the water concentration in the permeate decreased slightly (<1 wt%) and then remained almost constant. These results might be due to the activity of the TETA monomers increasing as a result of increasing temperature of the aqueous TETA solution. At the same time, the free volume in the mPAN polymer increases. This results in an increase in the amount of the TETA monomers penetrating into the mPAN membrane support, as well as an instantaneous formation of a dense selective TETA-TMC polyamide layer during the interfacial polymerization reaction between TETA and TMC. Consequently, the TETA monomers penetration through the TETA-TMC polyamide layer becomes difficult, so is their reaction with the TMC in the organic solution. A thinner and less mass transfer resistant polyamide layer forms on the mPAN membrane support, which causes an increase in the permeation rate. However, the thinner polyamide layer is easily swollen by the aqueous ethanol solution during the pervaporation process. Hence, how to control the amount of the aqueous amine monomer absorbed in the mPAN membrane support is a very import issue in this study.

Fig. 6 shows the effect of the immersion time in the aqueous TETA solution at 90 ◦ C on the pervaporation performance for dehydrating a 90 wt% aqueous ethanol solution at 25 ◦ C using the resulting composite TETA-TMC/mPAN membrane. As shown in Fig. 6, both the permeation rate and the water concentration in the permeate are indicated to increase with the immersion time in the aqueous TETA solution from 2 to 5 s. When the immersion time in the aqueous TETA solution was longer than 5 s, the permeation rate decreased while the water concentration in the permeate remained almost constant. The low water concentration in the permeate and the low permeation rate for an immersion time in the aqueous TETA solution of 2 s might be because the immersion time is too short, and hence the amount of the TETA monomers penetrating into the mPAN membrane support is not enough and their distribution not uniform. The selective polyamide layer formed during the interfacial polymerization process has a loose structure (low resistance) because of the small amount of TETA monomer absorbed in the mPAN membrane support. This causes the TETA monomers to easily pass through the polyamide layer and to react with the TMC in the organic solution. These results lead to a loose and thick polyamide layer on the mPAN membrane support, causing the low water concentration in the permeate and low permeation rate. At immersion times in the aqueous TETA solution longer than 5 s, the amount of TETA monomers penetrating into the mPAN membrane increased, resulting in the formation of a denser TETA-TMC polyamide layer. Furthermore, higher amount of TETA monomers penetrating into the mPAN membrane is favorable to obtain a thinner selective TETA-TMC polyamide layer, conducive to give a higher permeation rate. However, the permeation rate decreased when the immersion time in the aqueous TETA solution was longer than 5 s, as shown in Fig. 6. This result might be because the skin layer of the mPAN membrane support tends to densify with increasing time of immersion of the mPAN membrane in the aqueous TETA solution at 90 ◦ C. From the results shown in Figs. 5 and 6, the respective desirable temperature of and immersion time in aqueous TETA solution in preparing the composite TETA-TMC/mPAN membrane were found to be 90 ◦ C and 5 s. Compared with the pristine composite membrane (permeation rate = 1151 g/m2 h), the composite TETA-TMC/mPAN membrane prepared from using an aqueous TETA solution at the temperature of 90 ◦ C and immersing mPAN in it for 5 s greatly improved the permeation rate (permeation

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Table 1 Pervaporative separation of aqueous alcohol solutions at 25 ◦ C using composite TETA-TMC/mPAN membranes. Feed solution

Membrane

Permeation rate (g/m2 h)

Water concentration in permeate (wt%)

90 wt% EtOH/water

Type-A Type-Bb

1151 ± 51 1739 ± 213

99.4 ± 0.1 98.4 ± 0.6

70 wt% IPA/water

Type-Aa Type-Bb

1296 ± 82 1521 ± 24

99.7 ± 0.1 99.7 ± 0.3

70 wt% TFP/water

Type-Aa Type-Bb

1689 ± 70 1903 ± 151

85.5 ± 3.1 97.3 ± 0.4

a

a Type-A: pristine composite TETA-TMC/mPAN membrane (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/25 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C). b Type-B: annealed composite TETA-TMC/mPAN membrane prepared with immersion temperature of 90 ◦ C (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/90 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C).

3.3. Pervaporation performance for separating aqueous alcohol solutions

Fig. 7. Long-term pervaporation test of composite TETA-TMC/mPAN membrane prepared with immersion of mPAN in aqueous TETA solution at 90 ◦ C for 5 s for dehydrating 90 wt% aqueous ethanol solution at 70 ◦ C.

rate = 1739 g/m2 h) for the dehydration of a 90 wt% aqueous ethanol solution at 25 ◦ C. The membrane durability is a critical factor in commercial applications. The long-term membrane operating stability was therefore investigated in this study. Fig. 7 presents the data for the operating time effect on the pervaporation separation of a 90 wt% aqueous ethanol solution at 70 ◦ C using the composite TETA-TMC/mPAN membrane prepared with immersion of mPAN in the aqueous TETA solution at 90 ◦ C for 5 s. During the operation for 145 days at 70 ◦ C, the data indicated that the water concentration in the permeate remained unchanged. The permeation rate decreased first and then remained almost the same. These findings demonstrate that the composite TETA-TMC/mPAN membrane exhibited membrane durability during the pervaporation separation process at a high operating temperature. The membrane durability experiment is being conducted continuously in our laboratory.

The TETA-TMC/mPAN membranes were applied to the pervaporation separation of a 90 wt% aqueous ethanol, 70 wt% aqueous isopropanol, and 70 wt% aqueous tetrafluoropropanol mixtures at 25 ◦ C. The pervaporation data are summarized in Table 1. Compared with the pristine composite TETA-TMC/mPAN membrane (TypeA), the annealed composite TETA-TMC/mPAN membrane (Type-B), which was prepared with the immersion of the mPAN membrane support in the aqueous TETA solution at 90 ◦ C for 5 s followed by the TETA solution reaction with the organic TMC solution, greatly improved the permeation rate for the dehydration of the aqueous alcohol solutions at 25 ◦ C. These results might be due to the increased amount of TETA monomers which penetrated into the mPAN membrane. Such increase is caused by increasing the temperature of the aqueous TETA solution from room temperature to 90 ◦ C. Thin polyamide film is instantaneously formed at the interface between the mPAN membrane surface wet with the TETA solution and the organic TMC solution. The thin film prevents the TETA monomer from further desorbing from the mPAN membrane to pass through the thin polyamide film and react with the TMC monomer. Thus, the formation of the active polyamide layer is inhibited, leading to a reduced thickness of the polyamide film. For a 90 wt% EtOH/water solution, the water concentration in the permeate obtained with the use of the Type-B composite membrane was slightly lower compared with the Type-A composite membrane. This is due to the easy penetration of the ethanol molecules through the swollen composite membrane. For a 70 wt% IPA/water or TFP/water solution, the water concentration in the permeate obtained with the use of the Type-B membrane was higher compared with the Type-A membrane. This is due to the difficulty for the larger molar volume of TFP or IPA to penetrate through the composite TETA-TMC/mPAN membrane. The swelling effect in the Type-B composite membrane caused by the 70 wt% TFP/water solution is higher than that by the 70 wt% IPA/water solution, causing the permeation rate of the former aqueous alcohol solution to be higher than the latter. To investigate on the effect of the annealing processes applied during the composite membrane preparation on the pervapora-

Table 2 Data on S parameters and thicknesses of layers for composite TETA-TMC/mPAN membranes. Membrane

S1

S2

S3

S4

L1 (nm)

L2 (nm)

L3 (nm)

Type-Aa Type-Bb

0.47659 ± 2.74E−4 0.47798 ± 1.86E−4

0.47857 ± 5.19E−4 0.47993 ± 7.58E−4

0.48315 ± 9.42E−4 0.48285 ± 8.66E−4

0.48013 ± 1.17E−3 0.47942 ± 1.51E−3

196 ± 52 182 ± 84

584 ± 128 439 ± 101

3251 ± 2155 2865 ± 1734

a Type-A: pristine composite TETA-TMC/mPAN membrane (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/25 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C). b Type-B: annealed composite TETA-TMC/mPAN membrane prepared with immersion temperature of 90 ◦ C (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/90 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C).

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Fig. 8. S parameter vs. positron incident energy (corresponding to mean depth) data for composite TETA-TMC/mPAN membranes. (䊉) Type-A: pristine composite membrane (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/25 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C); () Type-B: annealed composite membrane prepared with immersion temperature of 90 ◦ C (concentration of/immersion time in/temperature of aqueous TETA solution: 0.1 wt%/5 s/90 ◦ C; concentration of/contact time with/temperature of organic TMC solution: 0.05 wt%/10 s/25 ◦ C).

Fig. 9. Schematic diagram of four-layer depth structure obtained by using VEPFIT program analysis of S parameter data from DBES for TETA-TMC/mPAN composite membrane. I: active polyamide layer, II: embedded polyamide + dense skin layer of mPAN, III: transition layer from dense skin to porous support layer of mPAN, and IV: porous support layer of mPAN.

tion performance, positron annihilation spectroscopy experiments using a slow positron beam were carried out. One of the positron annihilation techniques, Doppler broadening energy spectroscopy (DBES), was used in this study. This technique, based on measuring the width of the annihilation gamma photon with line center at 511 keV, is a powerful method for observing the chemical composition of and the physical microstructural change (such as free-volume variation) in materials. The S parameter as a function of the positron incident energy for the composite TETA-TMC/mPAN membranes is shown in Fig. 8. The steep increase in the S parameter close to the membrane surface is a typical phenomenon in positronium annihilation. These S parameter variations which relate to variations in the free volume in physical structures reveal a multilayered structure in the membrane, based on the positron annihilation characteristic difference between the layers. During the VEPFIT fitting process, we found good fitting results with a four-layer model which is illustrated in Fig. 9: active polyamide layer, embedded polyamide + dense skin layer of mPAN, transition layer from the dense skin to the porous support layer of mPAN, and porous support layer of mPAN. The fitting results for each layer are tabulated in Table 2. The data show that the first plateau (S1 = 0.47798 ± 1.86E−4) in the curve obtained for the polyamide layer in the Type-B composite membrane was higher than that (S1 = 0.47659 ± 2.74E−4) in the Type-A composite membrane. In other words, the S parameter value for the TypeB membrane was higher than that for the Type-A membrane. This implies that the free-volume amount in the former membrane is greater than that in the latter. Hence, the permeation rate through the Type-B membrane is higher than that through the Type-A membrane. Furthermore, the thickness of the active layer (L1 = 182 ± 84 nm) in the Type-B composite membrane is less than that (L1 = 196 ± 52 nm) in the Type-A composite membrane. As such, the lower mass transfer resistance of the Type-B composite membrane resulted in a higher permeation rate during the pervaporation separation of the aqueous alcohol mixtures. The foregoing observations agree very well with the result given in Table 1. In addition to its capabilities, DBES can report the 3␥ to 2␥ annihilation ratio (R parameter), which is defined as the ratio of the total count from the valley region with an energy width between 364.2 and 496.2 keV (which is from the 3␥ annihilation) to the total

Fig. 10. R parameter (3␥/2␥ annihilation ratio) vs. positron incident energy (corresponding to mean depth). (䊉) Type-A composite membrane and () Type-B composite membrane.

S.-H. Huang et al. / Separation and Purification Technology 72 (2010) 40–47

count from the 511 keV peak region with a width between 504.3 and 517.6 keV (which is from the 2␥ annihilation). The R parameter provides information about the existence of large pores (nm to ␮m), where o-Ps undergoes 3␥ annihilation, whereas the S parameter is based from p-Ps and o-Ps undergoing 2␥ annihilation (pick-off annihilation) in free volumes (Å to nm). The layer analysis based on the S parameter values (Figs. 8 and 9) is further supported by the R data vs. the mean depth plot in Fig. 10. Shown are large R values near the composite membrane surface, corresponding to the o-Ps annihilation in vacuum, and in the inner micrometer depth (positron energy > 5 keV, equivalent to mean depth > 0.45 ␮m), which refers to the region of large pores in the mPAN membrane. In addition, there is a gradual increase in the R parameter in the positron energy range of 2.5–4.0 keV, which is the same energy range where the existence of a large variation in the S parameter in Fig. 8 can be found. This gradual increase in the R parameter with the mean depth indicates a progressive transition from the embedded polyamide + dense skin layer of mPAN to the transition layer from the dense skin to the porous support layer of mPAN. 4. Conclusions Thin-film composite polyamide membranes were successfully prepared by the interfacial polymerization between TETA and TMC at different heating processes. They were subsequently applied in the dehydration of aqueous alcohol solutions. The appropriate heating condition applied to the thin-film composite polyamide membrane resulted in a great improvement in the dehydration of alcohols using the pervaporation separation process. The best conditions to apply in preparing the composite TETA-TMC/mPAN membrane were a 90 ◦ C temperature of and a 5 s immersion time in the aqueous TETA solution. The first plateau (S1 = 0.47798 ± 1.86E−4, L1 = 182 ± 84 nm) in the curve obtained for the polyamide layer in the Type-B composite membrane was higher in comparison to that (S1 = 0.47659 ± 2.74E−4, L1 = 196 ± 52 nm) in the Type-A composite membrane, which means that such corresponds to a higher S parameter and a thinner layer for the Type-B membrane than the Type-A membrane. This implies that the freevolume amount in the former membrane is higher than that in the latter. Compared with the pristine composite TETA-TMC/mPAN membrane (Type-A), the annealed composite membrane (Type-B) exhibited better pervaporation performance for the dehydration of a 90 wt% aqueous ethanol solution at 25 ◦ C. Acknowledgements The authors wish to sincerely thank the project Toward Sustainable Green Technology in Chung Yuan University, Taiwan, under grant CYCU-98-CR-CE, the Ministry of Economic Affairs, the Min-

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