disclaimer - seoul national...

저작자표시-비영리-변경금지 2.0 대한민국

이용자는 아래의 조건을 따르는 경우에 한하여 자유롭게

l 이 저작물을 복제, 배포, 전송, 전시, 공연 및 방송할 수 있습니다.

다음과 같은 조건을 따라야 합니다:

l 귀하는, 이 저작물의 재이용이나 배포의 경우, 이 저작물에 적용된 이용허락조건을 명확하게 나타내어야 합니다.

l 저작권자로부터 별도의 허가를 받으면 이러한 조건들은 적용되지 않습니다.

저작권법에 따른 이용자의 권리는 위의 내용에 의하여 영향을 받지 않습니다.

이것은 이용허락규약(Legal Code)을 이해하기 쉽게 요약한 것입니다.

Disclaimer

저작자표시. 귀하는 원저작자를 표시하여야 합니다.

비영리. 귀하는 이 저작물을 영리 목적으로 이용할 수 없습니다.

변경금지. 귀하는 이 저작물을 개작, 변형 또는 가공할 수 없습니다.

http://creativecommons.org/licenses/by-nc-nd/2.0/kr/legalcodehttp://creativecommons.org/licenses/by-nc-nd/2.0/kr/

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




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)




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

5. References

1. Raphael, S. Energy Issues in Desalination Processes. Environmental Science &

Technology 42, 8193-8201 (2008).

2. Shannon, M. A. Science and Technology for Water Purification in the Coming

Decades. Nature 452, 301-310 (2008).

3. Kessler, J. O. and Moody, C. D. Drinking water from sea water by forward

osmosis. Desalination 18, 297-306 (1976).

4. McCutcheon, Jeffrey R. A novel ammonia--carbon dioxide forward (direct)

osmosis desalination process. Desalination 174, 1-11 (2005).

5. McGinnis, Robert L., and Menachem Elimelech. Global Challenges in Energy

and Water Supply: The Promise of Engineered Osmosis. Environmental Science

& Technology 42 8625-8629 (2008).

6. Miller JE and Evans LR. Forward Osmosis: A New Approach to Water

Purification and Desalination. Sandia National Laboratories Report, 1-51 (2006).

7. Chung, Tai-Shung. Forward Osmosis Processes: Yesterday, Today and Tomorrow.

Desalination 287, 78-81 (2012).

8. William A. Philip. Reverse Draw Solute Permeation in Forward Osmosis:

Modeling and Experiments Environmental Science & Technology 44, 5170-5176

(2010).

9. Nathant, H. Solute Coupled Diffusion in Osmotically Driven Membrane

Processes. Environmental Science & Technology 43, 6769-6775 (2010).

10. Davis, J. A. Desalination of sea water by direct osmosis, Desalination 16, 151-

155 (1975).

11. Phuntsho, S. A Novel Low Energy Fertilizer Driven Forward Osmosis

Desalination for Direct Fertigation: Evaluating the Performance of Fertilizer

Draw Solutions. Journal of Membrane Science 375.172-181 (2011).

12. Yangali-Quintanilla, V. Indirect Desalination of Red Sea Water with Forward

51

Osmosis and Low Pressure Reverse Osmosis for Water Reuse. Desalination

280.160-166 (2011).

13. McCutcheon, Jeffrey R. and Elimelech, M. Influence of Concentrative and

Dilutive Internal Concentration Polarization on Flux Behavior in Forward

Osmosis. Journal of Membrane Science 284.237-247 (2006).

14. Gray, Gordon, T., McCutcheon, Jeffrey R. and Elimelech, M. Internal

Concentration Polarization in Forward Osmosis: Role of Membrane Orientation.

Desalination 197.1-8 (2006).

15. Mehta, G.D. Loeb, S., Performance of permasep B-9 and B-10 membranes in

various osmotic regions and at high osmotic pressures, Journal of Membrane

Science 4. 335-349 (1979).

16. Yip, N.Y. , Tiraferri, A., Phillip, W.A., Schiffman, J.D., Elimelech, M. High

performance thin-film composite forward osmosis membrane Environmental

Science & Technology 44 3812-3818 (2010).

17. Chou, Shuren, et al. Characteristics and Potential Applications of a Novel

Forward Osmosis Hollow Fiber Membrane. Desalination 261.365-372 (2010).

18. Zhang, Sui, et al. Well-Constructed Cellulose Acetate Membranes for Forward

Osmosis: Minimized Internal Concentration Polarization with an Ultra-Thin

Selective Layer. Journal of Membrane Science 360.522-535 (2010).

19. Hausman, Richard, et al. Functionalization of Polybenzimidizole Membranes to

Impart Negative Charge and Hydrophilicity. Journal of Membrane Science

363.195-203 (2010).

20. Q.Yang, K.Y.Wang, T.S.Chung, Dual-Layer Hollow Fibers with Enhanced Flux

As Novel Forward Osmosis Membranes for Water Production, Environmental

Science & Technology 43, 2800-2805 (2009).

21. Tiraferri, Alberto, et al. Relating Performance of Thin-Film Composite Forward

Osmosis Membranes to Support Layer Formation and Structure. Journal of

Membrane Science 367. 340-352 (2011).

22. A.P.Rao, S.V.Joshi, J.J.Trivedi, C.V. Devmurari, V.J. Shah, Structure–

52

performance correlation of polyamide thin film composite membranes: effect of

coating conditions on film formation, Journal of Membrane Science 211.13-24

(2003).

23. Wang, Kai Yu, Tai-Shung Chung, and Gary Amy. Developing Thin-Film-

Composite Forward Osmosis Membranes on the PES/SPsf Substrate through

Interfacial Polymerization. AIChE Journal 58. 770-781 (2012).

24. Widjojo, Natalia, et al. The Role of Sulphonated Polymer and Macrovoid-Free

Structure in the Support Layer for Thin-Film Composite (TFC) Forward Osmosis

(FO) Membranes. Journal of Membrane Science 383.214-223 (2011).

25. McCutcheon, Jeffrey R., and Menachem Elimelech. Influence of Membrane

Support Layer Hydrophobicity on Water Flux in Osmotically Driven Membrane

Processes. Journal of Membrane Science 318.458-466 (2008).

26. Ma, Ning, et al. Nanocomposite Substrates for Controlling Internal Concentration

Polarization in Forward Osmosis Membranes. Journal of Membrane Science

441,54-62 (2013).

27. Emadzadeh, D., et al. Synthesis and Characterization of Thin Film

Nanocomposite Forward Osmosis Membrane with Hydrophilic Nanocomposite

Support to Reduce Internal Concentration Polarization. Journal of Membrane

Science 449, 74-85 (2014).

28. Emadzadeh, D., W. J. Lau, and A. F. Ismail. Synthesis of Thin Film

Nanocomposite Forward Osmosis Membrane with Enhancement in Water Flux

without Sacrificing Salt Rejection. Desalination 330, 90-99 (2013).

29. Emadzadeh, D., et al. A Novel Thin Film Composite Forward Osmosis

Membrane Prepared from Psf–Tio2 Nanocomposite Substrate for Water

Desalination. Chemical Engineering Journal 237, 70-80 (2014).

30. Klemm, D., et al. Cellulose: Fascinating Biopolymer and Sustainable Raw

Material. Angew Chem Int Ed Engl 44.3358-3393 (2005).

31. Tiraferri, Alberto, et al. A Method for the Simultaneous Determination of

Transport and Structural Parameters of Forward Osmosis Membranes. Journal of

53

Membrane Science 444,523-538 (2013).

32. Bernhard Neises and Wolfgang Steglich, Simple Method for the Esterification of

Carboxylic Acids, Angew. Chem. Int. Ed. Engl. 17, 522-524 (1978)

33. Hu, Tianhui, et al. Effect of Flexible Spacer Length on the Mesophase Structures

of Main-Chain/Side-Chain Liquid Crystalline Polymers Based on Ethyl Cellulose.

Polymer Journal 42.752-758 (2010).

34. Li, Yanxiang, et al. Synthesis, Self-Assembly, and Thermosensitive Properties of

Ethyl Cellulose-G-P(Pegma) Amphiphilic Copolymers. Journal of Polymer

Science Part A: Polymer Chemistry 46.6907-6915 (2008).

35. Pronoy, K. C. Investigation by Infrared Absorption of the By-Products of the

Cyanoethylation of Cotton Cellulose. Journal of Polymer Science Part A:

Polymer Chemistry 4, 233-243 (1966)

36. Li, Yanxiang, Ruigang Liu, and Yong Huang. Synthesis and Phase Transition of

Cellulose-Graft-Poly(Ethylene Glycol) Copolymers. Journal of Applied Polymer

Science 110.1797-1803 (2008).

37. Rana, D. Surface Modifications for Antifouling Membranes. Chemical Reviews

110, 2448-2471 (2010).

38. Masahiko H. Effect of skin layer surface structures on the flux behaviour of RO

membranes. Journal of Membrane Science 121,209-215 (1996).

39. Ghosh, A. Sim K., et al. Impacts of Reaction and Curing Conditions on

Polyamide Composite Reverse Osmosis Membrane Properties. Journal of

Membrane Science 311. 34-45 (2008).

40. McCutcheon, Jeffrey R., and Menachem Elimelech. Modeling Water Flux in

Forward Osmosis: Implications for Improved Membrane Design. AIChE Journal

53.1736-1744 (2007).

41. Han, Gang, et al. Thin-Film Composite Forward Osmosis Membranes with Novel

Hydrophilic Supports for Desalination. Journal of Membrane Science 423,543-

555 (2012).

42. Zhou, Zhengzhong, Jim Yang Lee, and Tai-Shung Chung. Thin Film Composite

54

Forward-Osmosis Membranes with Enhanced Internal Osmotic Pressure for

Internal Concentration Polarization Reduction. Chemical Engineering Journal

249, 236-245 (2014).

43. Xie, Ming, William E. Price, and Long D. Nghiem. "Rejection of

Pharmaceutically Active Compounds by Forward Osmosis: Role of Solution pH

and Membrane Orientation." Separation and Purification Technology 93,107-114

(2012).

44. Sidney Loeb, Effect of porous support fabric on osmosis through a Loeb-

Sourirajan type asymmetric membrane. Journal of Membrane Science 129, 243-

249 (1997).

45. Han, Jungim, et al. Preparation and Characterization of Novel Acetylated

Cellulose Ether (Ace) Membranes for Desalination Applications. Journal of

Membrane Science 428, 533-545 (2013).

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

disclaimer - seoul national...

Documents