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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.
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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.
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vii
List of Supporting Figures
Figure S1. 3D AFM images of the top surface of (a) EC and (b) EP
substrates
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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
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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
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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
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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
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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.
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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.
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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.
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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