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공학석사학위논문
Thin-film composite (TFC) membranes with hydrophilic ethyl
cellulose-graft-poly(ethylene glycol) (EC-g-PEG) substrates for
forward osmosis (FO) application
친수성을 가지는 에틸셀룰로스-폴리에틸렌글라이콜
가지형 고분자의 정 삼투 복합 막 지지 층으로의 응용
2015年 2月
서울대학교 대학원
화학생물공학부
유 윤 아
Thin-film composite (TFC) membranes with hydrophilic ethyl
cellulose-graft-poly(ethylene glycol) (EC-g-PEG) substrates for
forward osmosis (FO) application
친수성을 가지는 에틸셀룰로스-폴리에틸렌글라이콜
가지형 고분자의 정 삼투 복합 막 지지 층으로의 응용
指導敎授 李 鍾 贊
이 論文을 工學碩士 學位論文으로 提出함
2014年 12月
서울大學校 大學院
化學生物工學部
柳 允 兒
柳 允 兒의 工學碩士 學位論文을 認准함
2014年 12月
委 員 長 장 정 식 (印)
副委員長 이 종 찬 (印)
委 員 조 재 영 (印)
Thin-film composite (TFC) membranes with hydrophilic ethyl
cellulose-graft-poly(ethylene glycol) (EC-g-PEG) substrates for
forward osmosis (FO) application
by
Yun Ah Yu
February 2015
Thesis Adviser: Jong-Chan Lee
i
Abstract
Thin-film composite (TFC) membranes with hydrophilic
ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) substrates
for forward osmosis (FO) application
Yun Ah Yu
Polymer Chemistry
Department of Chemical & Biological Engineering
Seoul National University
In this work, ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) was
synthesized by esterification of carboxylic acid functionalized methoxy
polyethylene glycol (MPEG-COOH) with ethyl cellulose (EC) in order to develop
the substrates of thin-film composite (TFC) membranes for forward osmosis (FO)
application. The TFC membrane consists of a selective polyamide active layer
formed by interfacial polymerization on top of EC-g-PEG substrate fabricated by
phase separation. The EC-g-PEG substrates were characterized by water contact
ii
angle, porosity, surface roughness, and morphology. It was found that
hydrophilicity and porosity of the EC-g-PEG substrate were increased as
compared with those of the EC substrate. Using a 2 M NaCl as draw solution and
DI water as feed solution, the TFC membranes of EC-g-PEG (TFC-EP) exhibited
a 15.7 LMH of FO water flux, while TFC membranes of EC exhibited only 6.6
LMH of FO water flux. Enhanced FO performances of TFC-EP membrane are
attributed to increased hydrophilicity and porosity, which play a crucial role in
water flux. Moreover, the increase in water permeability and porosity of TFC-EP
can be attributed to decrease in structural parameter which resulted in decreased
internal concentration polarization (ICP) in EC-g-PEG substrates.
Keywords: Thin-film composite membrane, Interfacial polymerization, Ethyl
cellulose, Hydrophilic substrates, Forward osmosis
Student number: 2013-20981
iii
List of Tables
Table 1. Composition of dope solutions for the fabrication of membrane
substrates.
Table 2. The thickness of membrane substrates before and after drying.
Table 3. The properties of the substrates with respect to contact angle, porosity
and pure water permeability (PWP).
Table 4. XPS elemental composition (in at%) of the surfaces of EC and EP
membrane substrates.
Table 5. The trasnpore properties and structural parameters of TFC-FO
membranes.
iv
List of Schemes
Scheme 1. Synthesis of poly(ethylene oxide-co-ethylene carbonate) (PEOEC).
Scheme 2. Interfacial polymerization to form the polyamide active layer.
v
List of Figures
Figure 1. Forward osmosis experimental setup for TFC-FO membrane
performance testing.
Figure 2. 1H NMR Spectra of MPEG-COOH.
Figure 3. 1H NMR Spectra of (a) EC and (b) EC-g-PEG.
Figure 4. IR spectra of (a) EC and (b) EC-g-PEG.
Figure 5. TGA curves of EC and EC-g-PEG under N2 atmosphere.
Figure 6. SEM and OM images of (a) EC and (b) EP membrane substrates.
Figure 7. Porosity of EC and EP membrane substrates.
Figure 8. Contact angle of EC and EP membrane substrates
Figure 9. XPS of (a) EC and (b) EP substrates
Figure 10. Pure water permeability of EC and EP membrane substrates.
Figure 11. SEM images of (a) TFC-EC and (b) TFC-EP membranes.
Figure 12. XPS of (a) TFC-EC and (b) TFC-EP membranes.
vi
Figure 13. Water flux (a) and Reverse salt flux (b) of TFC-EC and TFC-EP
membrnaes.
Figure 14. Water flux differences between TFC-EC and TFC-EP membranes at
AL-FS mode.
Figure 15. Forward osmosis performance of TFC-EC, TFC-EP and HTI-CTA
membranes.
vii
List of Supporting Figures
Figure S1. 3D AFM images of the top surface of (a) EC and (b) EP substrates
viii
List of Contents
Abstract ------------------------------------------------------------------------------------ⅰ
List of Tables-------------------------------------------------------------------------------ⅲ
List of Schemes----------------------------------------------------------------------------ⅳ
List of Figures-----------------------------------------------------------------------------ⅴ
List of Supporting Figures--------------------------------------------------------------Ⅶ
List of Contents----------------------------------------------------------------------------Ⅸ
1. Introduction-----------------------------------------------------------------------1
2. Experimental----------------------------------------------------------------------5
ix
2.1. Materials. -------------------------------------------------------------------------5
2.2. Functionalization of MPEG. --------------------------------------------------6
2.3. Esterification reaction between MPEG-COOH and EC. ---------------7
2.4 Characterization of EC and EC-g-PEG. ------------------------------------7
2.5. Preparation of EC and EC-g-PEG membrane substrates. -------------8
2.6. Interfacial polymerization of TFC-FO membranes. ---------------------9
2.7. Characterization of EC and EP membrane substrates and TFC-FO
membranes. --------------------------------------------------------------------------10
2.8. Forward osmosis tests. --------------------------------------------------------12
3. Results and Discussion
3.1. Characterization of the functionalized MPEG. --------------------------15
3.2. Synthesis of EC-g-PEG polymer. -------------------------------------------18
3.3. Preparation of membrane substrates. -------------------------------------23
3.4. Characteristics of membrane substrates. ---------------------------------25
x
3.5. Characteristics of TFC-FO membranes. ----------------------------------37
3.6. Forward osmosis performance of TFC-FO membranes. --------------41
4. Conclusion------------------------------------------------------------------------49
5. References------------------------------------------------------------------------50
1
1. Introduction
Nowadays, global water scarcity is the main problems faced by humanity. Since more
than 97% of the water in the world is seawater, desalination technologies have received
attention to solve the fresh water crisis, particularly in coastal areas. [1,2] Pressure-driven
reverse osmosis (RO) and thermally-driven multistage flash distillation (MSF) processes
are widely used as desalination membrane processes, while they involve expensive and
energy intensive processes. Forward osmosis (FO) is an emerging membrane technology
of possible desalination applications due to low operation cost. [3-6] In the FO process,
water permeates across the semi-permeable membrane from a lower osmotic pressure
feed solution to a higher osmotic pressure draw solution driven by the osmotic pressure
difference between the two aqueous solutions. However, solutes diffuse simultaneously
across the membrane from the draw solution into the feed solution. This reverse salt flux
must be minimized for the effective operation. [7-9]
Comparing FO to RO processes, FO does not need a high external hydraulic pressure,
thereby lowering investment and energy costs. Reported literatures have demonstrated a
lower fouling propensity and high water recovery. [10-12] FO is generally conducted
2
under low or no applied pressures, thereby water transport across the membrane is based
on the solution-diffusion mechanism. When water permeates across the membrane
substrate, draw solution at the active layer of the membrane substrates are diluted
simultaneously. Diffusion of salts works to compensate the diluted draw solutes. However,
diffusion process is not rapid resulting in the decrease of the effective osmotic pressure
and the water flux. As a result, the differing salt concentrations at the traverse boundaries
of the FO membrane substrates cause an internal concentration polarization (ICP)
problem. [13-17] So alleviating ICP is crucial to the FO membrane.
For the fabrication of polymeric FO membranes, two fabrication techniques have been
selected : (1) asymmetric membranes made by non-solvent induced phase separation [18-
20] , and (2) thin-film composite (TFC) membranes made by interfacial polymerization
onto porous support layers. Comparing TFC membranes to asymmetric membranes, TFC
membranes are more beneficial due to a higher water permeability and greater solute
rejection [16,21-23]. TFC membranes comprise aromatic polyamide as selective layer
and porous substrate, and thereby they can be independently tailored to obtain desired FO
performance. [21] An ideal substrate of TFC FO membrane should have high water
permeability, low solute permeability and porous in order to decrease ICP. Studies
revealed that the physicochemical properties of the substrates are very important in
3
determining the separation performance of the TFC FO membranes. [23,24] In 2008,
McCutcheon reported that the substrate’s physicochemical properties of hydrophilicity
are crucial to enhancing water flux in osmotically driven membrane processes. [25]
Recently, many researchers studied the modification of hydrophobic polymer for the
fabrication of TFC membrane with improvement of FO performance. The titanium oxide
nanoparticles and porous zeolite nanoparticles have been used for improving
hydrophilicity of substrates of thin film composite in order to increase the water
permeability and porosity, resulting in enhancement of water flux. [26-29] Wang et al. [23]
and Widjojo et al. [24] have revealed that blending hydrophilic substrates such as
sulfonated polysulfone and sPES-co-sPPSf (sulfonated polyethersulfone and
polyphenylsulfone copolymer) into polysulfone (PSf) improve the water flxues. This is
due to increase of wettability relatively hydrophobic PSf substrate, thereby mitigating the
ICP effect.
Cellulose is the most abundant and renewable natural polymer and has been widely used
in membranes. [30] Ethyl cellulose (EC) which is a chemically modified cellulose
derivative has a hydrophobicity and good solubility in organic solvents. For the
purpose of utilizing ethyl cellulose as membrane substrate materials, the modification of
ethyl cellulose is needed due to their hydrophobicity. According to the aforementioned
4
researches for hydrophilic membrane substrates, we believe the chemically incorporation
of PEG moiety into the EC may significantly enhance the performance of the TFC-FO
membranes.
In this study, we prepared ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) as
hydrophilic membrane substrate materials for high performance FO membrane.
Hydrophilic EC-g-PEG (EP) membrane substrates could be prepared using non-solvent
induced phase separation. The hydrophilic nature of the EP membrane substrates was
investigated and compared with that of bare EC membrane substrates by measuring
contact angle and conducting XPS analysis. In comparison to EC and EP membrane
substrates, the EP membrane substrates exhibited higher porosity and pure water
permeability. The TFC-EP membranes are prepared by an interfacial polymerization of
polyamide on top of EP membrane substrates. The discussion mainly focuses on the
effect of incorporation PEG moiety on FO performance of TFC membranes, including
water flux, reverse salt flux, water permeability, salt permeability, and structural
parameter. Based on the understanding of the characteristics of TFC-EP membranes, EC-
g-PEG with hydrophilic nature containing PEG moieties are very feasible candidates for
the membrane substrate materials of the FO applications.
5
2. Experimental
2.1.Materials
Ethyl cellulose (EC, Aldrich), with a degree of ethyl substitution of 2.4, was used as
received. Poly(ethylene glycol) monomethyl ether (MPEG, Mn = 350, Aldrich) and
succinic anhydride (Aldrich, 99%) was used as received. Tetrabutyl ammonium
bromide (TBAB, JUNSEI, 98% ), 4-di(methylamino)pyridine (DMAP, Alfa aesar, 99%),
succinic anhydride (Aldrich, 99%) and N,N’-dicyclohexylcarbodiimid (DCC, Aldrich,
99%) were used as received. Tetrahydrofuran (THF) was distilled by refluxing over
sodium/benzophenone under a nitrogen atmosphere. N-methyl-2-pyrrolidone (NMP,
Merck, 99.5%), polyethylene glycol 300 (PEG, Mw = 300gmol-1, Aldrich) and n-Hexane
(Daejung chemicals, 99%) was used as received. N,N-dimethylacetamide (DMAc,
Daejung chemicals) were used as received. A polyester non-woven fabric (PET, Grade
3249, Ahlstrom) was used as a backing layer for the substrates. m-Phenylenediamine
(MPD, Aldrich, 99%), trimesoyl chloride (TMC, Aldrich, 98%) and sodium chloride
(NaCl, Daejung chemicals, 99%) were used as received. A commercial FO membrane in
6
the FO process was purchased from Hydration Technology Innovations and was made of
cellulose triacetate (HTI-CTA). Deionized (DI) water was obtained from water
purification system (Synergy, Millipore, USA), having a resistivity of 18.3MΩ cm.
2.2.Functionalization of MPEG.
α-Monocarboxy-ω-monomethoxy poly(ethylene glycol), MPEG-COOH was prepared
by reacting MPEG with succinic anhydride in the presence of a catalytic amount of
TBAB. MPEG (MW=350, 10.5g, 30mmol), succinic anhydride (3.03g, 30mmol) and
TBAB (0.168g 0.532mmol) were dissolved in 30ml of THF and stirred for 24 h at 80 oC.
The solvent was evaporated with a rotary evaporator. After filtration, the residue was
dried in vacuo overnight. Yellow liquid was obtained.
1H NMR (300 MHz, CDCl3, δ (ppm), tetramethylsilane (TMS) ref): 4.26 (2H, C(O)O-
CH2-CH2) , 3.64 (4H, O-CH2-CH2), 3.55 (2H, C(O)O-CH2-CH2-O), 3.37 (3H, OCH3),
2.65 (4H,CO-CH2-CH2-CO).
7
2.3. Esterification reaction between MPEG-COOH and EC
EC (2.09g, 5mmol) , MPEG-COOH (2.42g, 5mmol), DCC (1.04g, 5mmol) as a coupling
agent and DMAP (0.154g, 1.25mmol) as a catalyst were dissolved in 100ml of THF and
reacted at room temperature for 72 h. The unreacted MPEG-COOH and DMAP was
removed by water precipataton. The precipatates were redissolved in THF. The reaction
byproduct diclyclohexylcarbodiurea (DCU) was removed by filtration and then
precipatated in n-hexane twice. The obtained ethyl cellulose-graft-poly(ethylene glycol)
(EC-g-PEG) was freeze-dried overnight.
1H NMR (300 MHz, CDCl3, δ (ppm), tetramethylsilane (TMS) ref): 1.15 (3H, -CH2CH3),
3.35 (3H, -OCH3), 2.94 – 4.30 (PEG side-chains and ethyl cellulose backbone)
FT-IR ( ν(solid, cm-1)) : 1736 (C=O)
2.4. Characterization of EC and EC-g-PEG.
1H NMR spectra were recorded on an AscendTM 400 spectrometer (300 MHz) using
CDCl3 (Cambridge Isotope Laboratories) as the solvent at room temperature (with a
8
tetramethylsilane (TMS) reference). Fourier-transform infrared (FT-IR) spectra of
polymers were recorded on a Cary 660 FT-IR spectrometer (Agilent Technology) at
ambient temperature using attenuated total reflectance (ATR) equipment (FT-IR/ATR).
Data were collected over 30 scans at 4 cm -1 resolution. Thermal gravimetric analysis
(TGA) was performed in a Q-5000 IR from TA Instruments. The samples were first
heated to 110 oC and maintained at 120 oC for 10 min in order to remove residual water,
and then heated to 600 oC at a heating rate of 10 oC min–1.
2.5.Preparation of EC and EC-g-PEG Membrane Substrates
The EC membrane substrates were fabricated using EC casting solutions containing 20
wt% EC, 64 wt% DMAc and 16 wt% THF. The EC-g-PEG membrane substrates (EP)
were prepared using EC-g-PEG dope solutions containing 10 wt% EP, 75 wt% NMP and
15wt% PEG300 as a pore forming agent. (Table 1) The homogeneous casting solution
was left in sonication at degassing mode during 1hr to remove air bubbles trapped within
the solution. The non-woven PET fabric was attached to a clean glass plate using
laboratory adhesive tape. Afterward, the membrane was cast on the non-woven fabric
9
using a casting knife setting the gate height to 100μm, and then by immersed immediately
into a water coagulant bath to initiate the non-solvent induced phase separation. The
membranes were peeled off from the glass plate, and then soaked in another water bath at
room temperature overnight to remove the residual solvent.
2.6. Interfacial Polymerization of TFC-FO Membranes
The membrane substrate was first immersed in a 2 wt % MPD aqueous solutions for 2
min. After that the excess MPD solution was removed by rolling a rubber roller. The
membrane substrate top layer was contacted with TMC dissolved in hexane with a
concentration of 0.1 wt% for 30 seconds for polymerization to occur. After removing the
TMC solution, the membrane was dried in air for 2 min and it was placed in the 100 °C
oven for 30 sec to induce the further polymerization. And then it was stored in DI water
prior to testing.
10
2.7. Characterization of EC and EP membrane substrates and TFC-FO
membranes
The membrane substrates and TFC-FO membranes were freeze-dried and their
structures were investigated by scanning electron microscopy (SEM) using a field
emission scanning electron microscope (FESEM, JEOL JSM-6700F) with an accelerating
voltage of 10 kV. The membrane cross-sectional images were obtained by fracturing the
membranes in liquid nitrogen. The membranes were coated with platinum using a coater
before analysis. Optical microscopy
(OM) images were obtained with an optical microscope (ECLIPSE E600 POL, NIKON)
equipped with a digital camera (COOLPIX E500, NIKON). . In order to analyze the
surface composition of the membranes, X-ray photoelectron spectroscopy (XPS, PHI-
1600) measurements were performed on an Axis-HIS XPS (Kratos Analytical,UK) using
Mg Ka (1254.0 eV) as the radiation source. Spectra were collected over a range of 0–
1100 eV, followed by high resolution scan of the C 1s, O 1s, and N 1s regions. Contact
angles from air captive bubble in water were carried out Contact Angle Geniometer
(Krüss DSA10 Germany). The contact angles for each sample were measured a
minimum of five times on three independently prepared membranes.
11
The porosities of membrane substrates were measured by carefully eliminating excess
water droplets on the wetted surface by filter paper. The weight of the wet membrane (m1,
g) is first measured and then the wet membrane is freeze dried overnight and weighed
again (m2, g). The overall porosity, ε was calculated by the following equation:
ε =
( − )
( − ) +
Where ρw is the density of water (1.00 g/cm3) and ρp is the density of polymers.
The density of polymers was determined by a balance according to the Archimedean
principle by measuring the weights of a polymer in air (wair) and n-hexane liquid (wliq)
and was obtained by the following equation:
=
−
Where ρp is the density of polymer and ρ0 is the density of n-hexane liquid.
The pure water permeability (PWP, L m-2 h-1 bar-1, abbreviated as LMH/bar) of
membrane substrate was obtained using a dead-end filtration cell (CF042, Sterlitech
Corp., Kent, WA) at 1 bar. The effective membrane areas were 2.16 × 2.16 ×π cm2. PWP
was calculated as follows.
12
PWP = ∆
=∆
∆ ∆
The water permeability coefficient (A), salt permeability coefficient (B) and structural
parameter (S) of the TFC-FO membranes were calculated based on FO experimental data
using a Excel-based algorithm developed by Tiraferri et al. [31]
2.8. Forward Osmosis Tests
Forward osmosis experiments were carried out in a lab-scale cross-flow FO system, as
depicted in Figure 1. The volume of the feed and draw solutions was 2.0 L at the start of
each experimental run. The FO membrane cell had an effective membrane area of 7.7 ×
2.6 cm2, and deep of 0.3 cm for co-current cross-flows. The temperature of water bath of
both the feed and draw solutions was maintained at 25± 0.5 oC by a thermostat. Variable
speed gear pumps were used to pump solutions. All membranes were tested in AL-FS
mode(the active layer was oriented towards the feed solution) and AL-DS mode (the
active layer was oriented towards the draw solution) . The weight change of the draw
solution reservoir was measured to calculate the water permeate flux.
NaCl solutions of various concentrations were used as draw solutions whereas DI water
13
was used as feed solutions. The water flux (Jw, L/m2h) was calculated from the weight
change of draw solution reservoir and can be calculated from the following equation.
=∆
∆ =∆ ⁄
∆
Where ∆V (L) is the volume of water permeated across the membrane from the feed to
the draw solution over a predetermined time interval ∆t (h) during FO experiments and
Am is the effective membrane surface area (m2). The weight change of the draw solution
reservoir was monitored by a computer connected to a balance (CUX4200H, CAS
Corporation). The reverse draw salt flux from the draw solution to the feed solution was
calculated by measuring the conductivity of the feed using a conductivity meter (ES-51,
HORIBA, JAPAN). The reverse salt flux (Js, g/m2h) was obtained from the following
equation:
=∆( )
∆
Where Ct and Vt are the salt concentration and the feed volume at the end of a
predetermined time interval, respectively.
14
Figure 1. Forward osmosis experimental setup for TFC FO membrane performance
testing
15
3. Result and Discussion
3.1.Characterization of the functionalized MPEG.
α-Monocarboxy-ω-monomethoxy poly(ethylene glycol) (MPEG-COOH) was
synthesized via reaction between poly(ethylene glycol) monomethyl ether (MPEG) and
succinic anhydride using tetrabutyl ammonium bromide (TBAB) as an catalyst (Scheme
1).
Figure 2 showed two new peaks at 2.64 and 4.26 ppm in the 1H NMR spectrum of
MPEG-COOH in comparison with that of MPEG. The peaks “e” (2.64 ppm) and “d”
(4.26 ppm) corresponded to four protons of -COCH2CH2-COOH and two protons of -
COOCH2CH2O, respectively. The integration area ratio between peak “a” at 3.36 ppm to
peak “d” at 4.26 ppm, was equal to 3 to 2. This is clear evidence that all monohydroxyl
end group of the MPEG was substituted by succinic anhydride.
16
Scheme 1. Synthesis of ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG).
17
Figure 2. 1H NMR Spectra of MPEG-COOH.
18
3.2.Synthesis of EC-g-PEG polymer
The remaining OH functionality on the ethyl cellulose (EC) chains can be used to
introduce other functionalities through conventional organic reactions, such as an
esterification. In this study, ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) was
synthesized by coupling reaction of MPEG-COOH to EC at room temperature in the
presence of DCC as coupling agent and DMAP as catalyst (Scheme 1).[32] The same
feed ratio of [COOH] of MPEG-COOH to [OH] of EC at 1:1 was applied. A
representative 1H NMR spectrum of EC-g-PEG was shown in Figure 2(b), which is
compared with the precursor EC as shown in Figure 3(a). In Figure 3(b), the proton
peaks observed at 3.35 ppm was clearly assigned to methyl proton of MPEG in EC-g-
PEG. In addition, this peak indicates the presence of MPEG units in the polymers. The
degree of PEG substitution (DSPEG) can be estimated by the 1H NMR spectra and the
degree of ethyl substitution (DSEt) using the following equations.[33]
DSPEG = (Ic/ Ia) × DSEt
Where Ia and Ic are the integrated intensities of a, c proton peaks in Figure 3 (b),
19
respectively.
DSPEG is 0.23 for the sample in Figure 3(b), which means 0.23 mol PEG chains per mol
repeating unit of cellulose. The incorporation of PEG into the EC was further confirmed
by IR spectroscopy as shown in Figure 4. The carbonyl stretching vibration of ester
linkage was located at 1730–1735 cm-1, which indicated carbonyl groups of EC-g-
PEG.[34,35] The formation of ester by the reaction between the hydroxyl group in EC
and the carboxylic group of MPEG-COOH was successful.
The thermal stability of EC and EC-g-PEG was investigated by the TGA experiments.
(Figure 5) Results showed the EC-g-PEG in this work had better thermal stability than the
EC. The formation of strong intermolecular reaction between EC and grafted PEG might
enhance cohesive energy resulting in higher thermal stability. [36]
20
Figure 3. 1H NMR Spectra of (a) EC and (b) EC-g-PEG.
21
Figure 4. IR spectra of (a) EC and (b) EC-g-PEG.
22
Figure 5. TGA curves of EC and EC-g-PEG under N2 atmosphere.
23
3.3.Preparation of membrane substrates
The EC substrate was prepared as a control group to investigate the effect of
hydrophilicity on the FO performance. However, the synthesized EC-g-PEG (EP)
material cannot form a free standing asymmetric membrane when the same dope solution
composition of EC substrate is used. Its highly hydrophilic nature induced slow phase
separation rate. Consequently, the dope solution composition of EP substrate is
different with that of EC. When the dope solution concentration of EC substrate was
below 20 wt%, the viscosity of EC substrate was not enough to cast onto the non-woven
fabric. Also, when the dope solution concentration of EP substrate was more than 10 wt%,
EP didn’t fully dissolve in the dope solution. So, the polymer composition in dope
solutions for substrate preparation was summarized in Table 1.
24
Table 1. Compositions of dope solutions for the fabrication of membrane substrates.
Dope Solution
Composition
wt % DMAc THF
EC 20 56 24
wt% NMP PEG300
EP 10 75 15
25
3.4.Characteristics of membrane substrate layers
Figure 6 shows the SEM morphology of membrane substrates. The EC and EP membrane
substrates show a similar top surface which has a smooth structure with no visible pores
observed at a magnification of 10,000. The EC membrane substrate could remove the
PET nonwoven fabric for cross-section of SEM images however the EP membrane
experienced shrinkage during drying step shown in Table 2. So, the EP membrane
substrate was prepared through casting onto the glass without nonwoven fabric. The OM
images in Figure 6 indicated similar thickness of membrane substrates in wet condition.
Porosities, water contact angles, and pure water permeability (PWP) of EC and EP
substrates are tabulated in Table 3. EP substrates have higher porosities than EC
substrates with incorporating PEG side chain. (Figure 7) This result might be due to
increasing the hydrophlicity of EP substrates. [23,24,26-29]
In order to confirm the morphology of the membrane substrates, Atomic force
microscopy (AFM) height images (5×5 µm2) were obtained from tapping mode. The
AFM results are given in Figure S1 in terms of the mean roughness (Ra). The mean
roughness observed increase in surface roughness parameters with incorporating PEG
side chain. It means that an increase in surface roughness might be enhanced the increase
26
in flux through the increase of the area available for the membrane transport.[37,38] As
the surface roughness of membrane substrates increases, the captive bubble contact
angles usually increase. [37] But, the EC substrate has a contact angle of 79.3, while the
EP substrate has relatively low contact angles of 47.9 as shown in Figure 8. These results
clearly indicate that the EP substrates has more hydrophilic surface in water environment
than the EC substrates. The decreased water contact angles are mainly because of the
incorporation of hydrophilic PEG side chain. Also, it overcomes the increment due to the
increase of the roughness. In order to compare the surface composition of EC and EC-g-
PEG substrates, X-ray photoelectron spectroscopy (XPS) was investigated. (Table 4) The
content of oxygen increased after the PEG chains were grafted onto the EC. To further
study the chemical compositions of the membrane surfaces, the O/C ratios were
calculated. Although the theoretical values of the O/C ratio were close in both cases, the
membrane surface of EC-g-PEG had a larger O/C ratio than that of EC. In addition, The
O=C-O peak appeared in the XPS C 1s spectra of EC-g-PEG in Figure 9. It indicated the
presence of PEG moiety of EC-g-PEG. The ratio of carbon in C–O to that in C–C (C–
O/C–C) on the EC-g-PEG membrane substrate is larger than that on the EC membrane
substrate. These XPS results clearly indicate that the EC-g-PEG membrane surface is
more hydrophilic than the EC membrane surface. As a result, the pure water flux of EP
27
substrate was significantly improved from 124 to 425 L/m2hbar with incorporating PEG
side chain.(Figure 10) Clearly, this might be due to improved membrane hydrophilicity
(reduced contact angle value) and increased overall porosity. [23,24,26-29]
28
SEM OM
Top surface Cross section
(a)
(b)
Figure 6. SEM and OM images of (a) EC and (b) EP membrane substrates.
29
Table 2. The thickness of membrane substrates before and after drying
Sample Wet Dried
EC 127.5 ± 1.4 µm 110 ± 1.2 µm
EP 126.3 ± 0.8 µm 80 ± 1.1 µm
30
Table 3. The properties of the substrates with respect to contact angle, overall porosity
and pure water permeability (PWP)
Sample Overall porosity (%) Contact angle (o) PWP (L/m2 h bar)
at 1 bar
EC 69.1 ± 0.1 79.3± 1.6 124 ± 3
EP 81.6±0.0 47.9 ± 0.7 425 ± 5
31
Figure 7. Porosity of EC and EP membrane substrates.
32
a)
(b)
Figure S1. 3D AFM images of the top surface of (a) EC and (b) EC-g-PEG substrates
33
Figure 8. Contact angle of EC and EP membrane substrates.
34
Table 4. XPS elemental composition (in at%) of the surfaces of EC and EC-g-PEG
membrane substrates
Sample C 1s O 1s O/C measured O/C theoritical
EC 73.16 26.84 0.37 0.50
EP 67.53 32.47 0.48 0.55
35
(a)
(b)
Figure 9. XPS of (a) EC and (b) EP substrates
36
Figure 10. Pure water permeability of EC and EP membrane substrates.
37
3.5.Characteristics of TFC-FO Membranes.
The interfacial polymerization reaction between m-phenylenediamine (MPD) and 1,3,5-
trimesoylchloride (TMC) forms a thin aromatic polyamide selective layer onto the EC
and EP membrane substrates. (Scheme 2) Figure 11 displays the SEM images of the top
surface of the TFC-EC and TFC-EP membranes, respectively. The typical “ridge-and-
valley” morphology is observed and it is the typical characteristic of the TFC polyamide
layer.[39]
The XPS spectra in Figure 12 of TFC-EC and TFC-EP membranes show three peaks:
C=C (aromatic), C-N, and C=O, respectively. The existence of amide bond in TFC
polyamide membranes could be confirmed by the XPS spectrum of Figure 12. N1s peak
clearly indicates the existence of nitrogen element in TFC polyamide membranes.
Thereby, these results indicated that a functional selective layer was formed.
38
Scheme 2. Interfacial polymerization to form the polyamide layer.
39
Top surface
(a)
(b)
Figure 11. SEM images of (a) TFC-EC and (b) TFC-EP membranes.
40
N1s C1s
(a)
(b)
Figure 12. XPS of (a) TFC-EC and (b) TFC-EP membranes.
41
3.6. Forward Osmosis Performance of TFC-FO Membranes
Figure 13 presents the water flux and reverse salt flux of membranes obtained using a
cross-flow FO process at AL-FS mode (the active layer was oriented towards the feed
solution) and at AL-DS mode (the active layer was oriented towards the draw solution)
with draw solutions of different NaCl concentrations. As the draw solution concentration
increased, all membranes exhibited the higher water fluxes due to higher osmotic pressure
difference across the membrane. Reverse draw salt leakage from draw solution to feed
solution simultaneously increased because of higher salt concentration gradient through
the active layer of TFC membrane. Comparing between AL-FS mode and AL-DS mode,
the water fluxes and reverse salt fluxes at the AL-DS mode are much higher than those at
the AL-FS mode. For instance, the TFC-EP membrane at the AL-DS mode exhibited
around 28.5 L/m2h water flux and 19.4 g/m2h reverse salt flux in comparison to 15.7
L/m2h and 14.1 g/m2h at the AL-FS mode using 2M NaCl as draw solution. The lower
water flux at the AL-FS mode in comparison to that at the AL-DS mode can be caused by
the severe internal concentration polarization (ICP) within the porous substrate at the AL-
FS mode.[43] All the TFC-EP membranes exhibited much greater water fluxes for all
draw solution concentrations and modes compared to the control TFC-EC membrane.
42
This indicates the enhancement in the structural properties of substrates as incorporation
PEG side chain, alleviating the transport resistance of water molecules to permeate.
Similar FO membranes which have hydrophilic substrate were observed by Han et.al [41]
and Zhou et.al [42] and showed the increase of FO water fluxes. Therefore, the increase
of hydrophilicity in substrate has an important role to form a high water flux TFC-FO
membrane according to others. [23,24,26-29,41,42]
Figure 14 presented the water flux at the AL-FS mode versus concentration of the draw
solution for TFC-EC and TFC-EP membranes. TFC-EC membranes gain water flux two
times as high as that of the TFC-EC membrane at all salt concentrations. Both curve
exhibited a nonlinear dependence because of ICP.[13] The less decrease of water fluxes
of the TFC-EC membrane indicated that increasing hydrophilicity of substrates mitigated
ICP.[23,24,26-29] Transverse water flux across the membrane dilutes draw solution at
the active layer of the membrane in substrates, and diffusion of salts works to compensate
the diluted draw solutes. However, diffusion process is not rapid resulting in the decrease
of the effective osmotic pressure and the water flux. So, alleviating ICP is crucial to the
FO membrane. [40] In order to decrease ICP, reducing the resistance to solute diffusion in
the substrate is crucial. It is determined by membrane support structural parameter S
because salt diffusion coefficient is constant. [40,44]
43
In order to investigate the effect of modified substrate on the FO performance, the water
and solute permeability coefficients (A and B) and S of the membranes were calculated
from the simple FO experiment and the results are shown in Table 5. The water
permeability of the TFC-EP membranes increases compared to the TFC-EC membrane,
while, the salt permeability also slightly rises. The increase of water permeability in the
TFC-EP membrane is mainly due to the increase of membrane hydrophilicity. [23,24,26-
29,41,42] The FO water fluxes and reverse salt fluxes followed a similar trend. Also,
the TFC-EP membranes have smaller S value of 452 µm than that of the TFC-EC
membranes. Generally, the membranes which have the small the S value exhibited the
better FO performance due to mitigating ICP. When the membrane substrates have higher
hydrophilic and porous nature, the ICP effect can be significantly minimized.
Figure 15 showed the trade-off relationship between water flux (Jw) and inverse reverse
salt flux (Js) in FO process.[45] When the mitigating the ICP effect, the water flux rises
due to the increment of the osmotic pressure in membrane substrates. At the same time,
the salt concentration at the active layer of the membrane in substrates increases and then
the reverse salt flux rises. So, high water flux leads high reverse salt fluxes. The TFC-EP
membrane exhibited higher FO water flux than the TFC-EC membrane, better water/salt
selectivity than the HTI-CTA membrane. The incorporating PEG side chain into EC gives
44
the higher hydrophilic and porous nature to TFC-EP membranes. All of these unique
features of TFC-EP suggest that the EC-g-PEG with hydrophilic nature containing PEG
moieties are very promising candidates for the membrane substrate materials of the FO
applications.
45
(a)
(b)
Figure 13. Water flux (a) and Reverse salt flux (b) of TFC-EC and TFC-EP membranes.
46
Figure 14. Water flux differences between TFC-EC and TFC-EP membranes at AL-FS
mode.
47
Table 5. The transport properties and structural parameters of TFC-FO membranes.
Sample
Water permeability,
A (L/m2 h bar)
Salt permeability,
B (L/m2 h)
Structural parameter,
S (µm)
TFC-EC 0.296 0.646 663
TFC-EP 0.695 0.741 452
48
Figure 15. Forward osmosis performance of TFC-EC, TFC-EP and HTI-CTA membranes.
49
4. Conclusion
We have prepared ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) as
hydrophilic membrane substrate materials for high performance FO membrane. EC-g-
PEG (EP) substrates could be prepared using non-solvent induced phase separation.
Results showed that the hydrophilicity and porosity of the EP substrate was improved
upon chemical incorporation of PEG side chain, i.e. EP substrate showed the lower the
contact angle value (more hydrophilicity) and the greater the porosity value than EC
substrate. The increase of hydrophlicity in the membrane substrates also enhances the
pure water permeability. We have shown that the TFC-EP membranes exhibits higher
water flux than the TFC-EC membranes at FO process, demonstrating the hydrophilic
membrane substrate enhanced the FO performance. It has been demonstrated that the
membrane structure parameter, an indicator of internal concentration polarization (ICP),
can be significantly reduced by using hydrophilic EC-g-PEG materials as the membrane
substrates. We expect that our results will provide insight into the utilization of EC-g-
PEG as hydrophilic membrane substrate material for FO applications.
50
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55
국문 요약
Ethyl cellulose-graft-poly(ethylene glycol) (EC-g-PEG) 가지
형 고분자를 합성 하였고 이를 정 삼투 복합 막의 지지 층으로
응용하였다. PEG 도입하기 전의 EC 지 지층 보다 EC-g-PEG
지지 층이 친수성, 다공성 그리고 수투과도에서 더 높은 수치를
보였다. 정 삼투 복합 막을 제작하여 Water flux를 측정한 결과
PEG 도입 전보다 2배 이상의 증가를 나타내었다. 이는 지지층의
친수성 증가로 인해 다공성과 수투과도 가 증가했기 때문이다.
주요어: 정 삼투, 복합막, 친수성, 에틸셀룰로스-폴리에틸렌글라이콜
1.Introduction 2.Experimental 2.1. Materials 2.2. Functionalization of MPEG 2.3. Esterification reaction between MPEG-COOH and EC 2.4 Characterization of EC and EC-g-PEG 2.5. Preparation of EC and EC-g-PEG membrane substrates 2.6. Interfacial polymerization of TFC-FO membranes 2.7. Characterization of EC and EP membrane substrates and TFC-FO membranes 2.8. Forward osmosis tests
3. Results and3.1. Characterization of the functionalized MPEG 3.2. Synthesis of EC-g-PEG polymer 3.3. Preparation of membrane substrates
3.4. Characteristics of membrane substrates 3.5. Characteristics of TFC-FO membranes 3.6. Forward osmosis performance of TFC-FO membranes.
4. Conclusion 5. References
151.Introduction 12.Experimental 5 2.1. Materials 5 2.2. Functionalization of MPEG 6 2.3. Esterification reaction between MPEG-COOH and EC 7 2.4 Characterization of EC and EC-g-PEG 7 2.5. Preparation of EC and EC-g-PEG membrane substrates 8 2.6. Interfacial polymerization of TFC-FO membranes 9 2.7. Characterization of EC and EP membrane substrates and TFC-FO membranes 10 2.8. Forward osmosis tests 123. Results and Discussion 3.1. Characterization of the functionalized MPEG 15 3.2. Synthesis of EC-g-PEG polymer 18 3.3. Preparation of membrane substrates 233.4. Characteristics of membrane substrates 25 3.5. Characteristics of TFC-FO membranes 37 3.6. Forward osmosis performance of TFC-FO membranes. 414. Conclusion 495. References 50