polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures

polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures

Accepted Manuscript The preparation of polyamide/polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures Hui-...

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Accepted Manuscript The preparation of polyamide/polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures Hui-An Tsai, Teng-Yi Wang, Shu-Hsien Huang, Chien-Chieh Hu, Wei-Song Hung, Kueir-Rarn Lee, Juin-Yih Lai PII: DOI: Reference:

S1383-5866(17)30767-0 http://dx.doi.org/10.1016/j.seppur.2017.06.060 SEPPUR 13838

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

9 March 2017 22 June 2017 22 June 2017

Please cite this article as: H-A. Tsai, T-Y. Wang, S-H. Huang, C-C. Hu, W-S. Hung, K-R. Lee, J-Y. Lai, The preparation of polyamide/polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.06.060

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The preparation of polyamide/polyacrylonitrile thin film composite hollow fiber membranes for dehydration of ethanol mixtures

Hui-An Tsaia,*, Teng-Yi Wanga, Shu-Hsien Huangb, Chien-Chieh Hua, Wei-Song Hunga, Kueir-Rarn Leea, Juin-Yih Laia,c

a

R&D Center for Membrane Technology, Department of Chemical

Engineering, Chung Yuan University, Chungli District, Taoyuan City, 32023, Taiwan b

Department of Chemical and Materials Engineering, National Ilan University, I-Lan 26047, Taiwan

c

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan

Corresponding author: Hui-An Tsai Tel: +886 3 2654190 E-mail address: [email protected]

Abstract In this study, Polyamide (PA)/polyacrylonitrile (PAN) thin-film composite (TFC) hollow fiber membranes were fabricated to investigate the dehydration performances of ethanol mixtures by pervaporation. Amine monomers (ethylenediamine (EDA), 1, 6-hexanediamine (HDA), diethylenetriamine (DETA), and tetraethylenepentamine (TEPA)) with difference number of functional groups or chain length were selected to react with trimesoyl chloride (TMC) to prepare a PA dense thin-film onto the surface of asymmetric PAN hollow fiber membranes by way of interfacial polymerization. Attenuated total reflection infrared spectroscopy (FTIR-ATR), scanning electron microscope (SEM), atomic force microscope (AFM), light transmission, contact angle and positron annihilation spectroscopy (PAS) were used to characterize the physicochemical properties, morphologies and microstructure of the PA thin layer. The pervaporation performances of aqueous ethanol solution showed the permeation flux decreased while water content in permeate increased with increasing number of amine groups. The effects of monomer concentration and reaction time on the pervaporation performances of aqueous ethanol solution through TEPA-TMC/PAN TFC hollow fiber membranes were also investigated. A 342.0±22.3 g/m2h permeation flux and 97.6±0.3 wt.% water content in permeate were obtained for the pervaporation of 90 wt.% aqueous ethanol solution at 25OC through the PA/PAN TFC hollow fiber membrane which was fabricated by immersing PAN hollow fiber membrane into 2 wt.% TEPA aqueous solution for 1 min and then contacting 1 wt.% TMC solution for 0.5 min. Keywords: thin film composite membrane; hollow fiber membrane; pervaporation; interfacial polymerization

1. Introduction Membrane can play the role of a barrier to separate two specimens because of differences in dimension, shape, structure or physicochemical properties [1]. Membrane technology has become a dignified and widely accepted separation technology over the past few decades due to the advantages of no need for additional additives during operation, simplicity of operation, low energy consumption and easy to scale-up. The development of a membrane with excellent separation performances is what we expect. It is well known that asymmetric membrane consisting of a very dense top layer supported by a porous sublayer [1] is a good choice for separation operations. However, for the pervaporation and gas separation processes using the solution-diffusion model as the separation mechanism, it is difficult to obtain an excellent separation performance due to some defects might be formed on the top layer of the asymmetric membrane during membrane formation. Asymmetric hollow fiber membranes, which were spun by wet or dry/wet spinning process, have been widely used in the separation processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and pervaporation due to high packing density (i.e. membrane area per unit module volume) and self-supporting [2-11]. In order to enhance the separation performance of the hollow fiber membrane, there are many ways to modify the asymmetric hollow fiber membrane such as additive addition [4, 10, 12-15], heat treatment [16-19], blending [15, 20, 21], cross-linking [22], grafting [23, 24] and composite hollow fiber membrane preparation [25-36]. A major breakthrough in the development of membrane technology is the evolution of composite membranes, which attach a dense

layer of thin layer to the surface of the porous sublayer. In general, the porous sublayer is prepared by using phase inversion method, and the top dense layer with different polymeric materials as substrate, can be fabricated by plasma polymerization, solution coating, dip coating or interfacial polymerization [1]. Interfacial polymerization is a technique that allows the synthesis of ultrathin functional layers at the interface between two immiscible phases. In 1981, PA TFC membrane was developed via in situ interfacial polycondensation on the surface of porous polysulfone support by Cadotte [37]. Thereafter, the TFC membrane has gradually replaced the asymmetric cellulose acetate membrane used in the reverse osmosis field. In general, the properties of the interfacial polymerized TFC membrane are affected by the reactivity and concentration of the monomers, the number of reactive groups on each monomer, and the immersion or contact time of the monomer solution [29-43]. Pervaporation is a pressure driven separation process for liquid mixtures by means of a membrane. In this process, the membrane acts as a barrier between a liquid phase (feed) and a vapor phase (permeate) due to vacuum or sweeping gas conditions applied. The pervaporation performance can be achieved according to the affinity of the feed mixtures with the membrane material. For classical distillation, separation occurs as a result of differences in volatility of the mixtures. Thus, azeotropic mixtures, heat sensitive mixtures, or close-boiling mixtures can be separated by pervaporation with low energy consumption. Since the development of TFC membranes, these membranes were mostly used for reverse osmosis and nanofiltration, rarely used for pervaporation, especially TFC hollow fiber membranes [29, 31, 40-43]. The diffusivity and reactivity of the

monomers during interfacial polymerization might affect the morphology and separation performances of the resultant membranes. It was found in the literature that the composite hollow fiber membranes prepared by the interfacial polymerization method mostly use diamine and trimesoyl chloride (TMC) as the monomers and few reports use multi-amine monomer. Therefore, in this study, the amine monomers with different number of amine function groups were selected to react with TMC monomer to form PA thin dense film on the surface of PAN asymmetric hollow fiber membrane to prepare the PA/PAN TFC hollow fiber membrane for pervaporation. In addition, with the same number of reaction points of the monomer, the molecular chain length will affect the packing state of formed polymer chain. Thus, in this study, diamine monomers with different chain length were also selected to react with TMC to investigate the effect of the molecular chain length of diamine monomer on the pervaporation performances of aqueous ethanol solution through prepared PA/PAN TFC hollow fiber membranes.

2. Experimental 2.1 Materials. PAN polymer was employed as the matrix of TFC hollow fiber membranes substrate which was supplied by the Tong-Hwa synthesis fiber Co. Ltd. (Taiwan). The reagent grade N, N-dimethylformamide (DMF) was used as a solvent to dissolve the PAN polymer and was not further purified before use. Water was used as the external coagulant and bore liquid during the spinning of PAN hollow fiber membrane. Reagent grade ethylenediamine (EDA) and 1, 6-hexanediamine (HDA) were purchased

from

Acros

Organics

Co.

Diethylenetriamine

(DETA),

tetraethylenepentamine (TEPA) and trimesoyl chloride (TMC) were products of TCI

Co. Distilled water was used in aqueous amine solution. Reagent grade toluene purchased from ECHO Chemical Co. Ltd. (Taiwan) was used as the organic solvent of TMC. Sodium hydroxide (NaOH), for the hydrolysis of PAN hollow fiber membrane was purchased from SHOWA Chemical Co. Ltd. The chemical structures of these monomers are shown in Fig. 1.

(A)

(B)

(C)

(D)

(E) Fig. 1 Chemical structure of (A) EDA, (B) HDA, (C) DETA, (D) TEPA, and (E) TMC.

2.2 Fabrication of PAN hollow fiber membranes Asymmetric PAN hollow fiber membranes were fabricated via phase inversion technique using wet spinning process as described elsewhere [18, 19, 25]. The detailed spinning parameters and conditions are listed in Table 1. PAN powder was

dissolved in solvent to form an 18 wt.% PAN/DMF dope solution. The PAN dope was extruded through an orifice-in tube spinneret with the dimensions of 0.83 and 0.25 mm for outer diameter and inner diameter, respectively. The bore liquid, DMF/H2O (9/1, w/w), was delivered using an ISCO syringe pump (Model: 500D). The dope solution and bore liquid were extruded through the spinneret die that was immersed in 30OC water bath simultaneously to form a nascent PAN hollow fiber membrane. The as-spun hollow fiber then was rinsed with water to remove the residual solvent for 3 days. Table 1 Spinning parameter of PAN hollow fiber membrane Parameter

Condition

Spinning process

Wet spinning

Dope composition

PAN (18 wt.%)/DMF

Bore liquid composition

DMF/H2O (9/1, w/w)

Bore liquid flow rate

1 ml/min

External coagulant

H2O

Spinning temperature

30OC

Spinneret diameter

Outer/inner diameter (0.83/0.25, mm)

2.3 Hydrolysis of PAN hollow fiber membranes The hydrophilicity of the PAN hollow fiber membrane can be enhanced by the hydrolysis of NaOH solution [44], which facilitates the interfacial polymerization. The PAN hollow fiber membrane was hydrolyzed by introducing wet-spun PAN asymmetric hollow fiber membrane into 2M aqueous NaOH solution at 50OC for 30 min. The NaOH-hydrolyzed PAN hollow fiber membrane then was washed

thoroughly by using distilled water. 2.4 Preparation of PA/PAN TFC hollow fiber membranes The PA active layer of the PA/PAN TFC hollow fiber membrane was prepared by the interfacial polymerization of amine (EDA, HDA, DETA, or TEPA) and TMC onto the outer surface of PAN hollow fiber membrane. The wetted hydrolyzed asymmetric PAN hollow fiber membrane was first immersed in a determined concentration of aqueous amine solution for certain time to absorb the amine monomer on the outer surface of hollow fiber membrane. Then, the excess amount of aqueous amine solution on the surface of the PAN hollow fiber membrane was removed. After that, the aqueous amine solution soaked PAN hollow fiber membrane then was brought in contact with a toluene solution containing TMC for certain time to carry out the interfacial polymerization. Finally, the resultant PA/PAN TFC hollow fiber membrane was washed in methanol and then dried at room temperature. 2.5 Characterization of hollow fiber membrane The properties of prepared PAN hollow fiber membranes were detected with various analytical methods. The structure of PA thin-film layer was studied by FT-IR spectroscopy (Perkin-Elmer, Model SPECTRUM ONE). The morphology of the membrane was observed with SEM (Hitachi Co., Model S-4800) and AFM (Digital Instrument, DI 5000). The water contact angle of the hollow fiber membrane was measured by using a Thermo Cahn microbalance device. The hollow fiber membrane to be tested was placed at the hook on the measuring table. When the hollow fiber membrane in contact with water, from the force balance, it can be obtained that the resultant force is mainly the gravitational term of the hollow fiber itself, the wettability term of the water to the hollow fiber and the buoyancy term. With these

forces, the water contact angle can be calculated. The contact angle of the aqueous ethanol solution of PA/PAN TFC flat membrane was estimated by Automatic Interfacial Tensiometer (FACE Mode 1 PD-VP) at room temperature. The aqueous ethanol solution was dropped on the membrane surface of 10 different positions. The average of the 10 measurements of the sample was taken as its contact angle. The interfacial polymerization reactivity between the aqueous amine solution and the TMC solution was observed by using light transmission experiments as described elsewhere [45]. In this process, a transparent container containing an aqueous amine solution was placed on the detector and the light source was mounted above the container vertically. TMC solution then was poured into the container slowly along its inside wall. When the TMC solution was contacted with the aqueous amine solution, the light transmission value was started to be recorded by the detector. The light transmittance was calculated by the following equation:

light transmittance

where

and

(1)

were the light transmission value of the container just containing

aqueous amine solution and after pouring TMC solution into the container at each time interval, respectively. When the aqueous amine solution was contacted with the TMC solution, a free-standing PA thin film then was formed at the interface between the solutions by interfacial polymerization. With the increase of time, the interfacial polymerized PA layer continues to grow, which affects the light transmittance and thus can be used to describe the reactivity of the interfacial polymerization. Positron annihilation spectroscopy (PAS) test was carried out to investigate the variation of the free volume in the PA/PAN TFC hollow fiber membranes at the room

temperature. In the experiment, the slow positron beam as a function of positron incident energy (0-30 keV) under a vacuum was used to measure Doppler-broadened energy spectrum (DBES) with a standard γ-ray spectrometer equipped with a Ge detector. By using the detector, DBES was based on the measurement of the width of the annihilation γ photon line, centered at 511 keV. The Doppler-broadened linewidth can be described by using the simple line-shape parameters (S parameters). The S parameter was defined by the ratio of the central part of the annihilation spectrum and the total spectrum reflected positron annihilation with low-momentum valence electrons [46, 47]. 2.6 Pervaporation measurement The hollow fiber membrane module for pervaporation test was plotted in 5-min rapid solidified epoxy resin binder. An aqueous ethanol feed solution was pumped into the shell side of the hollow fiber membrane module, and the permeate came out of the lumen side of the hollow fibers by using a vacuum pump maintained the downstream pressure at 3-5 mmHg. The permeation flux was determined by:

(2)

where P, W, A, and t represented the permeation flux (g/m2 h), weight of permeate (g), effective hollow fiber area based on the outside diameter of the fiber (m2), and operation time (h), respectively. The permeation flux was determined by dividing the measured weight of the permeate by the sampling time. The compositions of the feed aqueous ethanol solution and permeate were measured by gas chromatography (GC, China Chromatography 8700).

3. Results and discussion 3.1 Type of amine monomers effect 3.1.1 Morphology In order to investigate the effect of amine monomer species on the characteristics of PA/PAN TFC hollow fiber membrane prepared by interfacial polymerization, The NaOH-hydrolyzed asymmetric PAN hollow fiber membrane was immersed in a 2 wt.% aqueous amine solution for 1 min and then contacted with 1 wt.% TMC/toluene solution for 0.5 min to fabricate the PA/PAN TFC hollow fiber membrane. Fig. 2 reveals the FTIR spectra of the resultant PA/PAN TFC hollow fiber membranes. As shown in Fig. 2, it can be found that there were two absorption bands around 1618 cm-1 and 1545 cm-1. These two absorption bands are related to the stretching vibration band of the C=O (amide I) group and N–H (amide II) group, respectively. This means that the chemical composition of the TFC layer of prepared PA/PAN TFC hollow fiber membrane was a polyamide polymer containing –NHCO- group.

(A) (B)

(C) T(%) (D)

CN OH

C=O

COONH

Fig. 2 FT-IR spectra of PA/PAN TFC hollow fiber membranes. (A) TEPA-TMC, (B) DETA-TMC, (C) EDA-TMC, (D) HDA-TMC.

The morphology of PAN hollow fiber membranes before and after interfacial polymerization were also observed by SEM and shown in Fig. 3. It can be seen that the PA dense thin layer was coated on the surface of the asymmetric PAN hollow fiber membranes for all of the PA/PAN TFC hollow fiber membranes. Moreover, the granular structure with different shape was also found on the surface of PA dense layer which were fabricated by using different amine monomers. During the interfacial polymerization process on the surface of PAN hollow fiber membrane, the surface pores of PAN hollow fiber membrane served as the reservoirs of amine monomers, which diffuse toward the organic phase to react with TMC and form the nascent PA layer. Different amine monomers with different reactivity let the diffusion or reaction behavior was different, resulting in the formation of PA thin layer with different types of morphology. Due to the vigorous and heterogeneous eruption of amine monomers from the unevenly distributed support pores, the PA layers with different shape of granular structure were formed. These granular structures let the prepared PA/PAN hollow fiber membranes with different surface roughness as shown in Fig. 4.

(A)

(F)

(B)

(G)

(C)

(H)

(D)

(I)

(E)

(J)

Fig. 3 The SEM images of PA/PAN TFC hollow fiber membranes. (A)(F) Hydrolyzed PAN; (B)(G) EDA-TMC/PAN; (C)(H) HDA-TMC/PAN; (D)(I) DETA-TMC/PAN; (E)(J) TEPA-TMC/PAN. (A)-(E) Outer edge cross-section; (F)-(J) Outer surface.

80

R o u g h n e s s (R m s , n m )

70 60 50 40 30 20 10 0

A

B

C

D

E

P A N h ollow fiber membran es

Fig. 4 The surface roughness of hydrolyzed and PA/PAN TFC hollow fiber membranes.

(A)

Hydrolyzed

PAN,

(B)

EDA-TMC/PAN,

(C)

HDA-TMC/PAN, (D) DETA-TMC/PAN, (E) TEPA-TMC/PAN. 3.1.2 Pervaporation performances The pervaporation performances of 90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes which were prepared with various amine monomers were presented in Table 2. The data in Table 2 depicts that the water content in permeate increased while permeation flux decreased with increasing number of functional group of the amine monomer. These results might be due to the fact that TEPA and DETA are multi-amine monomer when compared to EDA and HDA with two amine groups. Thus, a network of crosslinked structures can be formed more easily at the interfacial polymerization for TEPA and DETA systems, leading to the formation of PA layer with more compact structure. Since the molar volume of ethanol (58.7 cm3/mol) is larger than that of water (18.1 cm3/mol), the ethanol molecules were less likely to pass through the more compact structure than the water molecules, resulting in a higher water content in the permeate side, so that the water

content in permeate increased while permeation flux decreased. Moreover, as the number of amine group increased, the reactivity of amine with TMC increased due to the increment of reactive sites. TEPA is an amine monomer with five amine groups; therefore, the reactivity of TEPA with TMC is better than other amine systems. By increasing the reactivity between amine and TMC, the interfacial polymerized PA layer was more compact, resulting in higher water content in permeate. Table 2 The pervaporation performances and water contact angle of PA/PAN TFC hollow fiber membranes Pervaporation performances* Hollow fiber membranes

Water contact Permeation flux 2

Water content in

angle (O)

( g/m h)

permeate (wt.%)

Hydrolyzed PAN

12673±1244

11.4±0.5

33.4±6.9

EDA-TMC/PAN

768±51

81.5±1.6

60.5±3.1

HDA-TMC/PAN

579±90

67.1±2.2

78.9±5.1

DETA-TMC/PAN

358±10

95.4±1.1

51.2±2.9

TEPA-TMC/PAN

342±22

97.6±0.3

53.4±2.2

*Feed: 90 wt.% aqueous ethanol solution at 25OC. For the diamine system of EDA and HDA, the chain length of EDA is shorter than HDA one. In the interfacial polymerization, HDA is more difficult to diffuse from the aqueous solution through the PA layer to the TMC solution than EDA. Meanwhile, the formed HDA/TMC PA layer was less compact than EDA/TMC PA layer because of the chain length of HDA is longer than EDA one. Thus, the water content in permeate of the HDA/TMC system was lower than EDA/TMC one. As for the permeation flux, the HDA/TMC PA layer was relatively hydrophobic than EDA/TMC layer due to the fact that HDA contains more -CH2- group than EDA,

resulting in a lower water affinity during pervaporation operation. So the permeation flux of HDA/TMC system was lower than EDA/TMC system. Table 2 also summarizes the water contact angle of PA/PAN TFC hollow fiber membrane. The water contact angle of HDA-TMC/PAN TFC hollow fiber membrane was higher than EDA-TMC/PAN one. It can be verified that HDA/TMC layer was relatively hydrophobic than EDA/TMC layer. 3.1.3 Light transmission The PA chain packing density was affected by the reaction rate during the interfacial polymerization. Light transmission experiments were used to observe the light transmittance variation of the free-standing PA film formation during the interfacial polymerization processes between amine monomers and TMC monomer. Fig. 5 shows the time dependence light transmittance curves which were represented the reaction rate of amine monomer with TMC to form PA layer. In general, interfacial polymerization occurs as soon as an aqueous monomer solution is contacted with an organic monomer solution. At this time, as the polymerization reaction time increases, the thickness of the polymerization layer increases, resulting in a decrease in light transmittance. The data in Fig. 5 reveals that the diamine system (EDA and HDA) has a faster decline rate in light transmittance. The reason for this is that these diamine have only two amine groups, the formed PA layer was relatively loose so that the aqueous diamine monomer can continue to diffuse to the TMC solution during the polymerization, resulting in the faster descending rate of light transmittance. Compared with the diamine system, the light transmittance values of multi-amine system was higher. It is shown that the PA layer formed in the multi-amine system was more transparent, indicating that due to the larger sizes of TEPA and DETA, the diffusion could be significantly slower, resulting in a slower formation rate of the PA

layer. The low reactivity of the diamine system showed a lower light transmittance values, representing the PA layer structure formed by these two systems were loose and low cross-linking. These results further confirmed that the polymeric layer formed by the EDA/TMC or HDA/TMC system was less compact than the multi-amine systems, resulting in a higher permeation flux and a lower permeate water content in the pervaporation.

L ig h t tra n s m itta n c e (% )

100 90 (D)

80 70

(C) (B)

60

(A)

50 10 0 0

200

400

600

800

1000

T ime (s )

Fig. 5 Light transmission curves of free-standing polyamide membranes. (A) EDA-TMC, (B) HDA-TMC, (C) DETA-TMC, (D) TEPA-TMC.

3.1.4 Surface roughness The light transmittance data in Fig. 5 shows that the number of amine functional groups can affect the reactivity of the amine and TMC at the interfacial polymerization to form a polymeric layer with different density and cross-linking degrees, resulting in the effect of pervaporation performances. In general, when the monomer reaction rate is high, the aqueous phase and the organic phase solution can

form a dense layer at the moment of contact, after that, the adsorbed amine monomer on the membrane surface does not easily diffuse cross the dense polymeric layer to react with the TMC, resulting in the formation of a thin and dense PA layer. Also, a relatively rough surface was usually formed due to the lack of time for packing the polymer chain. Therefore, AFM was used to detect the surface roughness of the resulted PA/PAN TFC hollow fiber membranes. Fig. 4 and Fig. 6 show the AFM images and the surface roughness values obtained from the images, respectively. It can be seen from Fig. 4 that the EDA/TMC system has a lower surface roughness, and the surface roughness of the TEPA/TMC, DETA/TMC and HDA/TMC system was close to and higher than that of EDA/TMC. These results might be due to the fact that the monomer has a faster reaction rate in the TEPA/TMC and DETA/TMC systems, so the thinner and dense PA layer was more likely to form and the intermolecular packing time was shorter, resulting in a rougher surface. For the diamine system, HDA has a longer aliphatic chain length than EDA, it is more difficult for HDA monomers to diffuse into the TMC solution through the formed PA layer during the interfacial polymerization process than EDA, and the lower roughness of PA layer can be formed theoretically. However, as shown in Fig. 4, it was found that the HDA/TMC PA layer had a roughness higher than that of EDA/TMC one, which was significantly inconsistent with the assumption that other factors affecting the surface roughness of PA layer should also be considered. One of the reasons might be related to the chain length of the monomer. Both EDA and HDA are diamine monomers, but HDA has more -CH2- than EDA, so the packing of PA polymer chain was more difficult when HDA reacted with TMC, thus forming a relatively smooth surface in EDA/TMC system.

(A)

(B)

(C)

(D)

Fig. 6 AFM images of the PA/PAN TFC hollow fiber membranes. (A) EDA-TMC, (B) HDA-TMC, (C) DETA-TMC, (D) TEPA-TMC.

3.1.5 PAS In order to identify the packing state of PA polymer chain and confirm the effect of amine monomer on the properties of PA/PAN TFC hollow fiber membrane, the fine-structure of hollow fiber membrane was evaluated based on the Doppler broadening energy spectrum (DBES) and the positron annihilation lifetime (PAL) spectrum. The DBES and PAL were connected to a variable monoenergy slow positron beam (VMSPB). This VMSPB was applied to probe the local mean depth (from the membrane surface to about 10 μm); the mean depth was determined by converting positron incident energies (from 0 to 30 keV) using an established

equation [46]. The S parameter derived from DBES was a qualitative measurement of free-volume. Increasing the relative contribution of low momentum electrons to the annihilation of positrons in open volume defects in polymers can cause an increase in the value of S [47]. The relationship between the positron incident energy (keV) and S parameter for the PA/PAN TFC hollow fiber membranes are exhibited in Fig. 7. In the range of the positron incident energy of less than about 0.5 keV is close to the surface of the PA/PAN TFC hollow fiber membrane, it can be seen from Fig. 7 that the S parameter increases with increasing the positron incident energy rapidly which was due to the back-diffusion of the positron annihilation. When the positron incident energy is from 0.7 to 2.0 keV, it indicates that the positron has penetrated into the membrane, which corresponds to the PA layer of the PA / PAN TFC hollow fiber membrane. In this region, the S parameter exhibits a peak-like region. Then, when the positron incident energy is from 2.0 to 4.0 keV, the S parameter comes to a plateau-like region. This region corresponds to a transition layer from the PA layer to the skin layer of the PAN hollow fiber membrane. As for the positron incident energy from 5.0 to 10 keV, the S parameter increases to a maximum. This region refers to a transition layer from dense skin to porous support layer in the PAN hollow fiber membrane. The S parameter then decreases because of the region remains in the porous support of the PAN hollow fiber membrane.

0.492 0.490

S p a ra m e te r

0.488 0.486 0.484 0.482 0.480 TEPA/TMC DETA/TMC EDA/TMC HDA/TMC

0.478 0.476 0.474 0

5

10

15

20

25

P os itron E n ergy (keV)

Fig. 7 S parameter as a function of positron incident energy for PA/ PAN TFC hollow fiber membranes prepared with amine/TMC. (■) TEPA, () DETA, (▲) EDA, (▼) HDA. The data in Fig. 7 shows that the PA layer of TEPA/TMC system has the smallest S parameter. This indicates that the TEPA/TMC PA layer has the smallest free volume size, so the TEPA/TMC PA layer is the most compact in this study. The interfacial polymerization reaction rate increased with increasing number of amine group, thus, forming a denser PA polymeric layer with higher cross-linking degree, resulting in a lower free volume of the resulted polymeric layer. In the diamine system, the HDA/TMC system has a lower reaction rate than the EDA/TMC system because HDA has a longer chain length than EDA, resulting in a steric hindrance effect. Therefore, the HDA/TMC system has higher S parameter than EDA/TMC system. These results are well consistent with the pervaporation results as shown in Table 2. Therefore, the free volume size follows the order of TEPA/TMC < DETA/TMC < EDA/TMC < HDA/TMC. Furthermore, the data in Fig. 7 also can represent the

formed thickness of PA thin film layer [48]. The PA layer thickness follows the order of HDA/TMC ≒ TEPA/TMC > EDA/TMC ≒ DETA/TMC. As shown in Fig. 7, when the positron incident energy increases, the S parameter first increases rapidly, then exhibits a peak-like region, followed by a platform-like region. When the positron incident energy continues to increase, the S parameter then increases again. When the positron incident energy increases, the S parameter pattern changes represent the back-diffusion of the positron annihilation, the positron penetrated into the PA active layer, transition layer from the PA layer to the skin layer of the PAN hollow fiber and a transition layer from dense skin to porous support layer in the PAN hollow fiber membrane. It can be seen from Fig. 7 that the order of the required positron incident energy of the transition layer from the PA layer to the skin layer of the PAN hollow fiber is HDA / TMC ≒ TEPA / TMC> EDA / TMC ≒ DETA / TMC. These results indicates that the thickness of the PA layer is in the order of HDA / TMC ≒ TEPA / TMC> EDA / TMC ≒ DETA / TMC. The results are consistent with the results obtained by the SEM images shown in Fig. 3. 3.2 Aqueous amine solution concentration effect Fig. 8 demonstrates the morphology of PA/PAN TFC hollow fiber membranes which were fabricated by immersing PAN hollow fiber membranes into determined concentration of aqueous TEPA solution for 1 min and then contacting with 1 wt.% TMC solution for 0.5 min. The data in Fig. 8 reveals that the thickness of PA active layer increased with increasing the TEPA concentration. These phenomena might be due to that the contact opportunity of TMC with amine group increases with increasing TEPA concentration in aqueous solution [49]. The PA thin film formation has been described to take place in three steps: embryonic film formation which was a

fast process followed by slowdown in polymerization depending on the permeability of the initial film formed; and finally shifting to a diffusion controlled process. In the diffusion controlled step film growth takes place until the monomers diffusing through the film get consumed by other monomers and/or unreacted functional groups of the film. At lower concentration of TEPA in the aqueous solution, very little TEPA was absorbed in the PAN hollow fiber membrane. It was not enough TEPA to form an integrated network structure, a thin/loose PA layers was formed. A higher polymerization rate can be obtained at higher TEPA concentrations during interfacial polymerization. The TEPA molecules were likely to diffuse to the aqueous/organic interface and the growth front of the film. This results in increasing active layer thickness. This was because higher cross-linking density can be obtained at the higher monomer concentration during interfacial polymerization. The driving force for the TEPA monomers diffuse to the organic phase solution increased with increasing TEPA concentration, thereby more TEPA monomers can react with TMC, resulting in the increase of the polymerized layer thickness.

(A)

(B)

(C)

(D)

Fig. 8 Effect of TEPA concentration on the cross-sectional morphology of PA/PAN TFC hollow fiber membranes. TEPA concentration: (A) 0.5 wt.%, (B) 1 wt.%, (C) 2 wt.%, (D) 2.5 wt.%. The effect of TEPA concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes were examined and exhibited the data in Fig. 9. The data show that the permeation flux decreased while water content in permeate increased with increasing TEPA concentration. These results might be due to that the PA layer thickness increased with increasing TEPA concentration, resulting in an increase in permeation resistance. Thus, permeation flux decreased and water content in permeate increased as

1400

100

1200

90

1000

80 70

800

60

600

50

400 200 0 0.0

10 0 0.5

1.0

1.5

2.0

T E P A c on c en tration (w t% )

2.5

3.0

W a te r c o n te n t in p e rm e a te (w t% )

2

P e rm e a tio n flu x (g /m h )

increasing the TEPA concentration in the aqueous solution.

Fig. 9 Effect of TEPA concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes. (() Permeation flux, () Water content in permeate) 3.3 Organic acyl chloride solution effects PAN asymmetric hollow fiber membranes were immersed in 2 wt.% TEPA solution for 1 min and then contacted with 1 wt.% TMC solution for certain time to investigate the effect of TMC contact time on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25°C through prepared PA/PAN TFC hollow fiber membrane. As shown in Fig. 10, it can be found that the permeation flux decreased while water content in permeate increased with increasing the contact time. These might be because that the increase in contact time represented an increase in reaction time of interfacial polymerization, so that a thicker reaction layer could be formed. These phenomena can be verified by the SEM observation as presented in Fig. 11.

100

700

2

P e rm e a tio n flu x (g /m h )

95 600 90

500 400

85

300 200

80

100

10

0

0 0

10

20

30

40

50

C ontac t tim e of T MC s olution (s ec )

60

W a te r c o n te n t in p e rm e a te (w t% )

800

Fig. 10 Effect of contact time of TMC solution on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes. (() Permeation flux, () Water content in permeate)

(A)

(B)

(C) Fig. 11 Effect of contact time of TMC solution on the cross-sectional morphology of PA/PAN TFC hollow fiber membranes. Contact time: (A) 5 s, (B) 15 s, (C) 30 s. Fig. 12 shows the effect of TMC concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution through PA/PAN TFC hollow fiber membranes which were fabricated by contacting TMC solution for 0.5 min. The data in Fig. 12 depicts that the permeation flux decreased while water content in permeate increased with increasing TMC concentration. These phenomena might be due to the fact that as increasing the TMC concentration, the acyl chloride group that can be

reacted with the amine group of TEPA also increased, resulting in the formation of thicker polymer layers. By using 0.01 wt.% TMC to fabricate the PA/PAN TFC hollow fiber membrane, the permeation flux was as high as 12400 g/m2h, however, the water content in permeate was only about 13.2 wt.%. That is, at lower TMC concentration, the formed PA layer was not complete, so the pervaporation performance was significantly poor. A 342.0±22.3 g/m2h permeation flux and 97.6±0.3 wt.% water content in permeate can be obtained for the pervaporation of 90 wt.% aqueous ethanol solution through the PA/PAN TFC hollow fiber membrane.

100

2

P e rm e a tio n flu x (g /m h )

12000

80

10000 60

8000 6000

40

4000 20

2000 0

W a te r c o n te n t in p e rm e a te (w t% )

14000

0 0.00

0.25

0.50

0.75

1.00

1.25

1.50

T MC c onc entration (w t% )

Fig. 12 Effect of TMC concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25℃ through PA/PAN TFC hollow fiber membranes. (() Permeation flux, () Water content in permeate)

3.4 Feed composition effect Fig. 13 displays the effect of feed composition on the pervaporation performances of TEPA-TMC/PAN TFC hollow fiber membranes. The data shows the permeation flux decreased while water content in permeate increased slightly as

increasing the ethanol content in feed. These results could be attributed to the affinity between PA thin layer and the feed. Table 3 lists the aqueous ethanol solution contact angle of the TEPA-TMC/PAN TFC flat membranes. It can be found that the contact angle decreased with increasing ethanol concentration. In other words, with higher ethanol concentration in the feed, a higher affinity between polymer chain and feed was obtained. Consequently, the PA thin layer was easily swollen by higher concentration of aqueous ethanol solution. Once the swollen space of PA polymer chain was occupied by ethanol molecular, the adsorbed ethanol molecules were not easily desorbed from the PA layer due to the affinity between polyamide and ethanol. Therefore, the pervaporation performances show the permeation flux decreased while water content in permeate increased slightly as increasing the ethanol concentration in the feed [50].

100

2

P e rm e a tio n flu x (g /m h )

4000 90

3500 3000

80

2500 2000

70

1500 1000

20

500

10

0

W a te r c o n te n t in p e rm e a te (w t% )

4500

0 30

40

50

60

70

80

90

E thanol c onc entration in feed (w t% )

Fig. 13 Effect of ethanol concentration in feed on the pervaporation performances of PA/PAN TFC hollow fiber membranes at 25OC. (() Permeation flux, () Water content in permeate)

Table 3 Effect of ethanol concentration on the aqueous ethanol mixture contact angle for the TEPA-TMC/PAN TFC membrane at 25 OC Ethanol concentration (wt.%) Media

Contact angle (°)

0

30

50

70

90

53.6±1.2

45.3±1.0

38.8±1.0

34.3±0.9

15.7±1.1

3.5 Comparison of separation performance The benchmarking of the pervaporation performance of the prepared PA/PAN TFC hollow fiber membranes with some other reported membranes is given in Table 4. Although the dehydration performance of ethanol was not very good for the prepared PA/PAN TFC hollow fiber membranes in this study, however, the pervaporation performance of this study still has a good separation performance compared to most other membranes. Table 4 Comparison of the pervaporation performances of aqueous ethanol solution Feed ethanol Temperature Flux Separation Membrane PSI conc. (wt.%) (OC) (g/m2 h) factor Pervap® 2201 90 60 100 100 10,000

Ref. 51

Pervap® 2210 TETA-TMC/mPAN flat membranes MPD-TMC/mPAN TFC flat membrane Zeolite-MPD-TMC /mPAN TFN flat membrane

80

80

1671

231

386,000

52

90

25

1739

554

963,400

53

90

25

2425

15

36,375

54

90

25

4500

30

135,000

54

PSf hollow fiber

90

25

173

23.9

4,135

10

85

50

479

162

77,598

11

85

50

659

50

32,950

55

6FDA-ODA-NDA/ PEI dual-layer hollow fiber PAI/PEI dual-layer hollow fiber

PAN/PEG heat treatment hollow fiber CS/mPAN composite hollow fiber CS/CA composite hollow fiber NaA zeolite/ α-Al2 O3 hollow fiber NaA zeolite/4CF α-Al2 O3 hollow fiber SPPO carbon hollow fiber MPD-TMC/PES TFC hollow fiber MPD-TMC/PES TFC hollow fiber, silicone rubber coating MPD-TMC/PVDF TFC hollow fiber EDA-TMC/PAN TFC hollow fiber HDA-TMC/PAN TFC hollow fiber DETA-TMC/PAN TFC hollow fiber TEPA-TMC/PAN TFC hollow fiber

90

25

330

8990

2,966,700

18

90

25

162

400

64,800

25

90

25

234

36.2

8,470

56

90

75

11100

>10000

>1.11E+08

57

90

75

12800

>10000

>1.28E+08

58

90

60

240

360

86,400

59

85

50

8378

22.43

187,920

31

85

50

7501

60.36

452,760

31

85

50

1288

40

51,520

60

90

25

768

39.6

30,413

90

25

579

18.4

10,653

90

25

358

186.7

66,839

90

25

342

366

125,172

This work This work This work This work

Abbreviations: MPD: m-phenylenediamine; TFN; thin film nanocomposite; PSf: polysulfone; 6FDA-ODA-NDA: copoly(4,4-diphenyleneoxide/1,5-napthalene-2,2-bis (3,4-dicarboxyl phenyl) hexa fluoropropane diimide); PEI: polyetherimide; PAI: polyamide-imide; PEG: polyethylene glycol; CS: chitosan; CA: cellulose acetate; 4CF: four-channel hollow fiber; SPPO: sulfonated poly(phenylene oxide); PES: polyethersulfone.

4. Conclusions In this study, PA/PAN TFC hollow fiber membranes for ethanol dehydration were fabricated successfully via interfacial polymerization. Pervaporation data shows the permeation flux decreased while water content in permeate increased as increasing the number of amine functional group. These results were consistent with the observations of PAS and SEM. The permeation flux and water content in permeate for the 90 wt.% aqueous ethanol solution at 25OC through the PA/PAN TFC hollow fiber membrane forming by immersing asymmetric PAN hollow fiber membrane into 2 wt.% TEPA aqueous solution for 1 min and then contacting 1 wt.% TMC solution for 0.5 min were 342.0±22.3 g/m2h and 97.6±0.3 wt.%, respectively. Acknowledgements The authors wish to sincerely thank the Ministry of Science and Technology (Taiwan) for its financial support. References [1] M.H.V. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publisher, The Netherlands, 1996. [2] Y. Liu, T. Liu, Y. Su, H. Yuan, T. Hayakawa, X.L. Wang, Fabrication of a novel PS4VP/PVDF dual-layer hollow fiber ultrafiltration membrane, J. Membr. Sci., 506 (2016), pp. 1-10 [3] B.K. Thakur, S. De, A novel method for spinning hollow fiber membrane and its application for treatment of turbid water, Sep. Purif. Technol., 93 (2012), pp. 67-74 [4] P.Y. Zhang, Y.L. Wang, Z.L. Xu, H. Yang, Preparation of poly (vinyl butyral) hollow fiber ultrafiltration membrane via wet-spinning method using PVP as additive, Desalination, 278 (2011), pp. 186-193

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Figure captions Fig. 1

Chemical structure of (A) EDA, (B) HDA, (C) DETA, (D) TEPA, and (E) TMC.

Fig. 2

FT-IR spectra of PA/PAN TFC hollow fiber membranes. (A) TEPA-TMC, (B) DETA-TMC, (C) EDA-TMC, (D) HDA-TMC.

Fig. 3

The SEM images of PA/PAN TFC hollow fiber membranes. (A)(F) Hydrolyzed PAN; (B)(G) EDA-TMC/PAN; (C)(H) HDA-TMC/PAN; (D)(I) DETA-TMC/PAN;

(E)(J)

TEPA-TMC/PAN.

(A)-(E)

Outer

edge

cross-section; (F)-(J) Outer surface. Fig. 4

The surface roughness of hydrolyzed and PA/PAN TFC hollow fiber membranes.

(A)

Hydrolyzed

PAN,

(B)

EDA-TMC/PAN,

(C)

HDA-TMC/PAN, (D) DETA-TMC/PAN, (E) TEPA-TMC/PAN. Fig. 5

Light transmission curves of free-standing polyamide membranes. (A) EDA-TMC, (B) HDA-TMC, (C) DETA-TMC, (D) TEPA-TMC.

Fig. 6

AFM images of the PA/PAN TFC hollow fiber membranes. (A) EDA-TMC, (B) HDA-TMC, (C) DETA-TMC, (D) TEPA-TMC.

Fig. 7

S parameter as a function of positron incident energy for PA/ PAN TFC hollow fiber membranes prepared with amine/TMC. (■) TEPA, () DETA, (▲) EDA, (▼) HDA.

Fig. 8

Effect of TEPA concentration on the cross-sectional morphology of PA/PAN TFC hollow fiber membranes. TEPA concentration: (A) 0.5 wt.%, (B) 1 wt.%, (C) 2 wt.%, (D) 2.5 wt.%.

Fig. 9

Effect of TEPA concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes. (() Permeation flux, () Water content in permeate)

Fig. 10 Effect of contact time of TMC solution on the pervaporation performances of

90 wt.% aqueous ethanol solution at 25OC through PA/PAN TFC hollow fiber membranes.( () Permeation flux, () Water content in permeate) Fig. 11 Effect of contact time of TMC solution on the cross-sectional morphology of PA/PAN TFC hollow fiber membranes. Contact time: (A) 5 s, (B) 15 s, (C) 30 s. Fig. 12 Effect of TMC concentration on the pervaporation performances of 90 wt.% aqueous ethanol solution at 25℃ through PA/PAN TFC hollow fiber membranes. (() Permeation flux, () Water content in permeate) Fig. 13 Effect of ethanol concentration in feed on the pervaporation performances of PA/PAN TFC hollow fiber membranes at 25OC. (() Permeation flux, () Water content in permeate)

Table captions Table 1 Spinning parameter of PAN hollow fiber membrane Table 2 The pervaporation performances and water contact angle of PA/PAN TFC hollow fiber membranes Table 3 Effect of ethanol concentration on the aqueous ethanol mixture contact angle for the TEPA-TMC/PAN TFC membrane at 25 OC

Highlights PA/PAN TFC hollow fiber membranes are fabricated via interfacial polymerization. Pervaporation dehydration of ethanol by prepared TFC hollow fiber membrane is studied. The monomer structure can affect the physicochemical properties and separation performance of TFC hollow fiber membrane.