Development of Highly Water Permeable Robust PSQ-RO
Membranes for Water Separation

儈䘿≤ᙗᴹ₏ȿɲȳ䘶⎨䘿≤࠶䴒㟌ȃ䮻Ⲫ 
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September, 2022
Dian Zhang
Hiroshima University澳澳Contents
Page
Chapter 1 General Introduction

1-15

Chapter 2 Development of PSQ-RO membranes for water desalination

16-38

from tris[(ethoxysilyl) alkyl]amines by SG process and IP process

Chapter 3 Preparation of PSQ-RO membranes with hydroxyl

39-54

groups by copolymerization of bis[3-(triethoxysilyl)propyl]amine
and triethoxy(3-glycidyloxypropyl)silane

Chapter 4 Preparation of high water permeable PSQ-RO membranes by

55-78

introducing hydroxyethylurea-based hydrophilic water channels

Chapter 5 Preparation of robust and high RO performance PSQ-RO

79-94

membranes modified by SiO2 nanoparticles

Chapter 6 Conclusions

95-96

List of Publication

97-98

Acknowledgments

99

澳

澳

澳

澳

General Introduction
Water is a vital resource and the Earth's surface is covered by water to the tune of
71%. However, ninety-nine percent of that water are ground water, which is hard toutilize. Of the remaining 1%, surface water is considered to constitute 0.0067% of total
water, which can be used as a water resource [1]. The global demand for freshwater isincreasing and the freshwater usage from 1900 to 2015 was summarized in Figure 1.1(Data from Global International Geosphere-Biosphere Programme (IGB)). As shownin Figure 1.1, global water use has increased approximately eight-fold in the last
hundred years. However, limited water resources are becoming increasingly scarce, themost obvious manifestation of water scarcity is the fact that there are 785 million people
who lack even basic access to clean drinking water. According to the statistics, there
are approximately two billion people whose drinking water is contaminated with feces,which may cause diseases (2019, WHO). In addition, water supply systems are alreadychallenged by climate change, increasing water scarcity, population growth,demographic changes, and urbanization. According to WHO, half of the world's
population will live in lacking water areas by 2025. Except for drinking water, the lackof freshwater for irrigation is also a very serious issue. For example, producing one tonof rice need more than 1000 m3 of water for irrigation. Therefore, developing effective
tools for water purification is very important.
The development of efficient tools for water desalination is essential. Nowadays,
various approaches for seawater desalination have been reported so far such as reverse
osmosis (RO), electrodialysis (ED), multi-effect distillation (MED), etc. And RO
process is the most effective and advanced technology which accounts for mostdesalination processes with 64% [2-4].
澳

澳

1

澳
Figure 1.1. Global freshwater usage per year from 1900 to 2015, data from Global
International Geosphere-Biosphere Programme (IGB)
The membrane separation is a technique for selectively separating molecules ofdifferent particle sizes at the molecular level through semi-permeable membranes, also
known as a separation membrane or filter membrane. According to the pore size fromlarge to small, membranes can be classified into microfiltration membrane (MF),ultrafiltration membrane (UF), nanofiltration membrane (NF), reverse osmosis
membrane (RO). The applicable separation ranges of these membranes are shown in
Figure 1.2. For water desalination applications, RO technology was widely used, inwhich reverse osmosis technology can be used to separate water from salt aqueous
solution or remove atoms or some small organic molecules to yield pure water. In thiswork, the author will focus on the RO membrane separation technology.

Figure 1.2. Applicable separation ranges of MF, UF, NF, RO technologies.

澳

2

Reverse osmosis is a leading technology to purified water by using a semipermeable membrane to remove salt ions and/or macromolecular, which works by using
a high feed pressure on the saltwater side of the semi-permeable RO membrane andforce the water across the membrane to yield pure water, as illustrated in Figure 1.3.
Nowadays, Polyamide-based reverse osmosis membranes, which were prepared byinterfacial polymerization with m-phenylene diamine and trimesoyl chloride. RO
membranes based on polyamide is widely used for the industrial applications becauseof their high RO performance of high water permeability (water flux) and high salt
rejection (salt selectivity). By 2000, the amount of desalinated water generated byreverse osmosis process was 65.5 million m3/day, which is 69 % of the volume of total
amount. [5]澳Pressure

Salt
Water

Pure water

Reverseosmosismembrane

Fresh
Water

Water Flow澳
Figure 1.3. A simple diagram of the reverse osmosis process.
Although polyamide-based reverse osmosis membranes exhibit excellent
separation properties, these membranes still present some problems, as illustrated in
Figure 1.4. For example, because chlorine easily attacks the N-H group in the amidegroup [6-8], polyamide-based membranes commonly show poor chlorine resistance.
However, the chlorine-containing solution (such as NaOCl aqueous solution) isconveniently used as an anti-fouling reagent to remove bacteria on the membranes.
Therefore, the service life of polyamide-based RO membranes was limited. In addition,polyamide-based RO membranes generally exhibit low thermal stability, which has

澳

3limited their operation temperature (up to 45 °C) [9]. It has been demonstrated thatmany strategies have been developed to increase the thermal stability and chlorine
resistance of polyamide-based membranes. For example, Ge and coworkers preparedpolyamide-based membranes with high thermal stability and chlorine resistance byusing g-C3N4 nanosheets as nanofiller [10]. Zhang et al. prepared high chlorineresistant polyamide-based RO membranes by modification with natural occurring εpoly-l-lysine [11]. Although many strategies have been reported to enhance the stabilityof polyamide-based membranes, improvement of their stability is still desired. Thus,
the development of RO membranes with high thermal stability and high chlorine
resistance is essential.
Polysilsesquioxanes (PSQs) with the chemical formula of [RSiO1.5], have been
considered as robust materials due to the inorganic siloxane Si-O-Si networks
possessing basic robustness, thus increasing the robustness and stability of PSQ-basedmaterials [12-14]. A simple schematic of hydrolysis/condensation reactions of bridged
trialkoxysilane precursors is shown in Figure 1.5. Organically bridged PSQs can bereadily prepared via hydrolysis/condensation reactions of bridged trialkoxysilanes inthe sol-gel (SG) process, to form a robust three-dimensional Si-O-Si network. Many
applications of PSQ-based material have been reported. For example, Maeda andcoworkers demonstrated that materials prepared by copolymerization of poly(3aminopropyl)silsesquioxane and poly(3-(2-aminoethylaminopropyl)silsesquioxane)
exhibited excellent antifogging ability [15]. Tsuru and coworkers developed PSQ gas
separation membranes that exhibited high selectivity and excellent permeance [16-18].
In addition, other applications such as adsorbents, catalysts, and heat insulators werealso explored [19-23].
Attributable to the robust and porous structure of PSQ-based materials, PSQ-basedmaterials are suitable for RO membranes preparation and the studies of PSQ-RO
membranes have been well examined and reported. Previously, Tsuru and coworkers
first

demonstrated

that

PSQ-RO

membranes

preparedbypolymerizedbis(triethoxysilyl)ethane (BTESE) via SG process exhibited a good salt rejection of澳

4approximately 98 %. Although BTESE-based RO membranes exhibit low water
permeability on the order of 10-13 m3/m2sPa. These membranes exhibit favorable
stability that both treatments of these membranes with water bath at 90 °C for 3 hours,
and 10 hours exposure by using 1000 ppm NaOCl aqueous solution resulted in no
obvious changes in separation property, indicating the high robustness of thesemembranes [24]. In addition, it was reported that the PSQ-RO membranes ofbis[(triethoxysilyl)propyl]amine (BTESPA) exhibited salt rejection of 96 %, and low
water permeability of 1.1 × 10-13 m3/m2sPa [25]. However, those PSQ-RO membranes
exhibited water permeability on the order of 10-13 m3/m2sPa, which is approximately
10-folds lower than the average value of commercially available polyamide RO
membranes (10-12 m3/m2sPa), although salt rejection was close to the polyamide-based
membranes.
澳Figure 1.4. Advantages and disadvantages of polyamide-based RO membranes andPSQ-based RO membranes.

澳

5Figure 1.5. The hydrolysis/condensation reactions of bridged trialkoxysilanes.
Although organically bridged PSQ-RO membranes demonstrated favorable thermal
stability and chlorine resistance, these membranes have remained insufficient forpractical industrial use due to their poor water permeability. For improving the water
permeability of PSQ-RO membranes, several strategies have been proposed in ourprevious works. It has been demonstrated that the introduction of rigid and polar
bridged groups PSQ-RO membranes increases water permeability [14, 25, 26]. In
contrast to BTESE-based membranes, bis(triethoxysilyl)ethene (BTESE2)- andbis(triethoxysilyl)ethyne (BTESE3)-based PSQ-RO membranes via the SG process
exhibited higher water permeability, although salt rejection slightly decreased [27]. In
addition, the other strategies to improve the water permeability of PSQ-RO membranes
by the synthesis of polar and rigid bridged trialkoxysilane precursors has also been
proposed in the past work (Figure 1.6 (a)). For example, Yamamoto et al synthesized
1,4-bis(triethoxysilylmethyl)1,2,3-triazole (BTESMAz) to introduce rigid and polar
bridging units, and the BTESMAz-based membranes prepared by the SG process of
this compound exhibited water permeability of 3.7 × 10-13 m3/m2sPa and salt rejectionof 95% [26], of which water permeability was approximately 3-fold higher than that of
BTESE-based membranes. It was also demonstrated that RO membranes based on(2Z,4Z)‐1,6‐diacetoxy‐3,4‐bis(triethoxysilyl)hexa‐2,4‐diene (BTES-EAc) with a polar澳6acetoxymethyl group, exhibited water permeability of 3.5 × 10-13 m3/m2sPa and salt
rejection of 96% [28]. In addition, PSQ-RO membranes prepared from BTES-ED
exhibited water permeability of 1.7 × 10-13 m3/m2sPa and salt rejection of 98.5 % [29].
BTES-MOU- and BTES-POU-based RO membranes exhibited water permeability of
2.6 × 10-13 m3/m2sPa and 7.8 × 10-14 m3/m2sPa, and salt rejection of 90 % and 85 %,respectively [26].
Improvements in RO performance can also be achieved by copolymerization(Figure 1.6 (b)). For example, compared with BTESE-homo-based membranes, RO
membranes

preparedbycopolymerizing

precursors

BTESE

and

hydroxymethyl(triethoxy)silane (HMTES) showed approximately three times
improvement in water permeability [30]. The improvement in water permeability canbe attributed to the introduction of hydroxyl groups, thereby increasing thehydrophilicity. Moreover, the copolymerization of BTESE2 with HMTES produced
membranes with approximately seven times higher water permeability than those basedon BTESE2 [31]. In addition, it has been reported that copolymer-based PSQ-RO
membrane prepared from the copolymerization of BTESE and BTESPA, exhibitedwater permeability of 1.3 × 10-12 m3/m2sPa and salt rejection of 92 % [32], and thecopolymerROmembranewhich

prepared

from

BTESE

and

octakis(triethoxysilylethyl)-substituted POSS exhibited high water permeability of 1.6
× 10-12 m3/m2sPa but low salt rejection of 86 %. However, the RO performance,
especially the permeability, is still insufficient for practical applications. In this thesis,
the author describes the preparation of new PSQ-based RO membranes for further
improving the permeability with keeping the salt rejection at the level of practicalapplications, by applying new strategies including the design and synthesis of new
precursors, the introduction of hydroxyl groups by in-situ ring opening reaction of
epoxide and hydrophilic water channels, and the composite formation with SiO2
nanoparticles.

澳

7

澳

Figure 1.6. Structure of precursors for the preparation of (a) homopolymer-based RO
membranes (b) copolymer-based RO membranes in the previous work. 澳澳

8In Chapter 2, the author examined tris[3-(triethoxysilyl)propyl]amine (TTESPA),
tris[3-(diethoxymethylsilyl)propyl]amine(TDEMSPA),

tris[(triethoxysilyl)

methyl]amine (TTESMA) tris[(triethoxysilyl)propenyl]amine (TTESP2A), and tris[3(triethoxysilyl)prop-2-ynyl]amine (TTESP3A) as new precursors for PSQ-RO
membrane preparation by the sol-gel process and interfacial polymerization. It was
found that a dense separation layer was formed by precursors with three reactive
ethoxysilyl units. Of these, TTESPA-homo-based membranes which prepared by theinterfacial polymerization gave the best RO performance of water permeance and salt
rejection of 7.3 × 10-13 m3/m2sPa and 95.6% respectively, which were close to the best
RO performance of BTESE3-based membranes (8.5 × 10-13 m3/m2sPa and 95 %) in ourprevious work.
澳
Na+Cl-

H2O

Organically bridged polysilsesquioxane-based
reverse osmosis membranes

R = tripropylamine/trimethylamine澳㻌Chart 1.1. Schematic diagram of PSQ-RO membrane and precursors for membranes
preparation, described in Chapter 1.

In

Chapter3,

thecopolymerizationof

BTESPA

withtriethoxy(3-glycidyloxypropyl)silane (TEGPS) for membrane preparation is described. It isdemonstrated that epoxy groups in TEGPS would undergo a ring-opening reaction withthe attack by the amine unit of BTESPA and form hydroxyl groups, which will
significantly improve the water permeability. The RO experiments clearly indicated thatthe introduction of epoxy-containing monomer significantly improved RO performance,yielding excellent water permeability and high salt rejection of 1 × 10-12 m3/m2sPa and98%, respectively.
澳澳9Chart 1.2. Schematic diagram of works described in Chapter 2.

澳In Chapter 4, the author copolymerized BTESPA with N-(2-hydroxyethyl)-N′-[3(triethoxysilyl)propyl]urea (HETESPU), the RO results indicated that the water
permeability was extremely improved approximately 19 times comparing to BTESPA
homo based membrane (1.86 × 10-12 m3/m2sPa), with salt rejection nearly unchanged(96%), which is the highest water permeability so far obtained from PSQ membranes
that show higher than 90% salt rejection. The extreme improvement of water
permeability is attribute to the aggregation of hydroxyethylurea units through hydrogenbonding in the PSQ layer to form hydrophilic water channels.
OH

OH

NaCl
H2O

O

PSQ-RO membrane
P

OH

NH

O

O
NH

NH

Polysulfone support membrane

Si
R
O
Si

NH
NH
NH

O
Si
R

O

O
O
Si

Si

O

Si
O
Si OH
O Si
O

澳
Chart 1.3. Schematic diagram of the aggregation of hydroxyethylurea units throughhydrogen bonding, described in Chapter 3.
澳

In Chapter 5, mixed matrix membranes (MMMs) were prepared by the introductionof nanoparticles into the PSQ separation layer. It was demonstrated that SiO2
nanoparticles provides additional surface area and micropores in the PSQ layer by filler
effects, thus increasing the porosity and improving water permeability. It has been also
demonstrated that the rigidity of the PSQ polymer chain was increased by the澳10interaction between SiO2 nanoparticles and polymer, which results in the improvementof thermal stability. The author anticipated that the introduction of the robust and rigid
inorganic SiO2 nanoparticles would stabilize the PSQ chain by immobilization, thereby
increasing the thermal stability of RO membranes.

Chart 1.4. Preparation of SiO2 nanoparticles modified PSQ-RO membranes, describedin Chapter 4.

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澳

15

and

Chapter 2 Development of PSQ-RO membranes for water desalination fromtris[(ethoxysilyl) alkyl]amines by SG process and IP process
Introduction
The sol-gel (SG) process has been the focus of continuous development for thesynthesis of porous materials in the last two decades [1-3]. In the previous research, theSG process has been used for the preparation of PSQ-RO membranes. The SG process
is a multistep process that includes sol solution preparation, coating of the sol particles
on a support membrane substrate, and calcination of the coated substrate to convert thesol film into the gel. These procedures require exactly controlled sol particle size
suitable for membrane preparation and optimization of the sol coating conditions andthe calcination conditions. Recently, the interfacial polymerization process (IP) has
been reported for the preparation of PSQ-RO membranes [4]. Compared with the SG
process, IP is simple and efficient. PSQ-RO membranes prepared by the IP process
exhibit RO performance close to that of membranes prepared by the SG process. For
example, bis[3-(triethoxysilyl)propyl]amine (BTESPA)-based membranes prepared by
the SG process and the IP exhibited the water permeability of 1.1 × 10-13 and 1.4 × 1013

m3/m2sPa and NaCl rejection of 96 % and 93.6 %, respectively. However, only

precursors containing a self-catalyzing amino group can be used in IP process. For
example, bis(triethoxysilyl)ethane (BTESE)-based membranes prepared by the IP
process in the presence of an acid catalyst showed only 57.3% NaCl rejection.
To further improve the RO performance of PSQ-RO membranes, the author
designed and synthesized precursors with three reactive ethoxysilyl units, namely,
tris[(triethoxysilyl)propyl]amine (TTESPA), tris[(diethoxymethylsilyl)propyl] amine(TDEMSPA), and tris[(triethoxysilyl)methyl]amine (TTESMA), and prepared RO
membranes on polysulfone support membranes by the SG process and the IP process
to obtain information of structure-performance relationship. It is also expected thatthese precursors with three reactive ethoxysilyl units would form dense RO membranes,
thereby improving RO performance. The RO results exhibited that TTESPA-based
membranes prepared by the SG process and the IP process exhibited favorable RO澳

16performance. In addition, the author prepared tris[(triethoxysilyl)propenyl]amine(TTESP2A) and tris [3-(triethoxysilyl)prop-2-ynyl]amine (TTESP3A) as the
precursors, in which rigid and polar ethenylene and ethynylene linkages were expected
to improve RO performance of the resultant membranes, as observed for the BTESE2and BTESE3-derived membranes [5].
澳

Scheme 2.1. Structures of precursors in this chapter.

澳Results and discussion
Preparation of membranes
As the five precursors TTESPA, TDEMSPA, TTESMA, TTESP2A, and TTESP3A
used for membrane preparation had different reactivities with water in the SG process,the author carefully optimized the molar ratio of water for the formation of sols with
1.6-2.0 nm particle size, which would be suitable to prepare RO membranes on thepolysulfone support membrane (Nitto Denko NTR-7450) [6]. Figure 2.1 presents DLS
particle size distribution for the sols obtained from 2 h treatment of TTESPA with 23
eq of water, TTESMA with 1.5 eq of water, TDEMSPA with 60 eq of water, TTESP2A
with 10 eq of water, and TTESP3A with 10 eq of water, respectively, revealing theformation of sols with ideal particle sizes.

澳

17


Figure 2.1. Particle size distribution of sols determined by DLS measurement.
澳For the PSQ-RO membranes preparation by the SG process, calcination
temperatures should be as high as possible to promote the gelation leading to dense
membranes without decomposing the organic bridges. Decomposition of the organic
bridges may lead to significant membrane shrinkage, thereby resulting in crack andpinhole formation in the membranes and hydrophilicity suppression. To determine thecalcination temperature, the author conducted a thermogravimetric (TG) analysis of
TTESPA, TDEMSPA, TTESMA, TTESP2A, and TTESP3A gels obtained by drying
their sol solutions at 60 °C in air for 2 days. The TG analysis was performed under anitrogen flow at the heating rate of 10 °C/min. Figure 2.2 shows the TG curves of thegels. The first weight loss at temperatures lower than 250 °C is likely to be ascribed tothe desorption of adsorbed water in the gel in nitrogen flow, and water liberation by thecondensation reaction between Si-OH groups to form Si-O-Si linkages. The second
weight loss is probably due to the decomposition of organic bridges in the gels,occurring at temperatures higher than 300 °C. The lesser weight loss at 1,000 °C for
TTESMA than the other membranes would be due to its shorter organic bridges. Basedon these results and the thermal stability of the polysulfone support membrane(NTR7450), the author set the optimum calcination temperature at 150 °C for themembrane preparation.

澳

18


Figure 2.2. TG curves of gel powders in a nitrogen atmosphere.
澳
FT-IR spectra provided information of the progress of polymerization of theseprecursors. Figure 2.3 shows the FT-IR spectra of the gel films prepared by coating thesol solutions on silicon wafer, followed by heating the films at 150 °C for 10 min. The
broad and strong absorption bands centered at 1065 cm-1 are attributed to the stretchingfrequencies of Si–O–Si linkages in the PSQ networks. Although the Si-O bands of SiOH groups are also seen at 956 cm-1, their intensities are low, indicating that the Si-OH
groups still remain after calcination at 150 °C, but only to a small extent. Peaks thatappear at 2800–2970 cm-1 are attributed to C-H stretching. That the siloxane bands
appear sharper in the IR spectra of TTESP2A and TTESP3A-based films than others is
likely due to higher rigidity of the gel network, as observed for a BTESE3-based film.
An acetylene stretching band is also seen at 2189 cm-1 for the TTESP2A-based film.


澳

19

130

(a)

120

Transmittance [a.u.]

110
100
90
80
TTESPA70

TDEMSPA60

TTESMA

50
40

4000

3500

3000

2500
2000
Wavenumber [cm-1]

1500

1000

500



105

(b)

Transmittance [a.u.]

100
95
90
TTESP2A
TTESP3A

85
80

3750

3250

2750
2250
1750
Wavenumber [cm-1]

1250

750



Figure 2.3. FT-IR spectra of (a) TTESPA, TDEMSPA and TTESMA (b) TTESP2A andTTESP3A gel films prepared by heating three precursor sols coated on silicon wafer at
150 °C.
澳
Cross-section topography澳
The cross-section topography of these membranes obtained by the SG process andthe IP process was examined by SEM (Figure 2.4). The SEM images demonstrate that
dense PSQ separation layers are formed on the polysulfone support membranes by boththe SG process and the IP process. Membranes prepared by the SG process of TTESPA,TDEMSPA, TTESMA, TTESP2A, and TTESP3A exhibit the formation of finehomogeneous separation layers with thicknesses ranging from 60 to 150 nm (Figure 2.4(a) to (e)). This is attributed to the successful control of the sol particle size and thecalcination temperature. In contrast, membranes prepared from TTESPA, TDEMSPA,
and TTESMA via IP process exhibit thicknesses ranging from 900 to 1,000 nm (Figure澳20

2.4 (f) to (h)). Membrane thickness could be tuned by controlling precursor
concentration during IP process. To investigate the thickness dependence of RO
performance for membranes prepared by the IP process, the author carried out TTESPA
interfacial polymerization at a lower concentration to obtain a membrane with a similar
thickness to that prepared by the SG method (Figure 2.4 (i)). This thinner IP membrane
showed a slight increase in water permeability and a decrease in NaCl rejection (seebelow). In contrast to TTESPA- and TDEMSPA-based membranes, the TTESMAbased membrane shows cracks in and peeling of the separation layer. This deformation
may be one reason for its inferior performance compared with the other membranes.
In addition, the cross-sectional SEM images of the TTESP2A- and TTESP3A-based
membranes showed ambiguous interfaces between the support membrane and the PSQ
layer when prepared via the IP process (Figure 2.4 (j) and (k)), in contrast to those of
TTESPA, TTESMA, and TDEMSPA. It is likely that the introduction of polar ethylene
and acetylene bonds increased the hydrophilicity of the precursors and their PSQ
oligomers to permit their permeation to the aqueous layer in the IP process. This seems
to affect the RO performance of the membranes (vide infra).

澳

21

澳
Figure 2.4. Cross-sectional SEM images of membranes prepared by SG process using(a) TTESPA, (b) TDEMSPA, (c) TTESMA, (d) TTESP2A, and I TTSEP3A, and those
prepared by IP process of (f) TTESPA, (g) TDEMSPA, and (h) TTESMA, (i) TESPA(1 wt%), (j) TTESP2A, and (k) TTESP3A.

Mechanical strength and porous properties
To further study the process dependence of membrane RO properties, the author
investigated the pencil hardness test to know the mechanical strength of the membranesurface. TTESPA-based membranes prepared by the IP process of 1 wt% solution and
the SG process and having similar thicknesses exhibited pencil hardness indexes of 5B
and 3H, respectively. This seems to indicate that the SG process can form a denser film
than the IP process. This is consistent with the results obtained from RO experiments,澳22

that is, the membrane prepared by the IP process is thicker than that by the SG process,but has higher water permeability.
Membrane porosity significantly affects water permeability. To investigate theporosity of TTESPA-, TTESMA, TTESP2A, and TTESP3A-based gels, nitrogen
adsorption and desorption measurements were performed. Their gel powders wereprepared by drying the sols at 60 °C in the air for 2 days followed by calcination at
150 °C in nitrogen for 12 h. The results are presented in Figure 2.5, indicating the nonporous properties of these gel powders, although they exhibit favorable water
permeability in RO experiments. This suggests that the water separation properties ofthe present membranes are likely based on the diffusion-controlled mechanism rather
than molecular sieving effects [7]. It seems likely that polymers still had many reactive
sites and the structures were fluxional and mobile in the sol states, thus providing nonporous films upon calcination.
澳澳
Figure 2.5. N2 adsorption (solid plot)/desorption (open plot) isotherms of TTESPA
and TTESMA (top) and TTESP2A and TTESP3A (bottom) gel powders.

澳

23

RO performance
RO measurements of these membranes prepared by the SG process and the IP
process were performed with 2,000 ppm NaCl aqueous solution under the feed pressureof 1.5 MPa at room temperature. The RO performance is presented in Table 2.1 andFigure 2.6 exhibits the trade-off plots of water permeability and salt rejection. TTESPAbased membranes prepared by the SG process and the IP process exhibit favorable salt
rejection of 96.6% and 95.6%, respectively. In contrast to the SG process, themembrane prepared by the IP process has approximately five times higher water
permeability (7.3 × 10-13 m3/m2sPa) than that prepared by the SG process (1.7 × 10-13
m3/m2sPa), although the membrane prepared by the IP process is thicker than that
prepared by the SG process (Figure 2.4 (a) and (f)). This is likely due to the growth
pattern difference of the membranes during the SG process and the IP process. Although
no obviously evident differences in membrane morphologies depending on the process
in the FE-SEM images, the fact that the TTESPA-based membrane prepared by the SG
process has higher mechanical strength than that prepared by the IP process seems toreflect the difference in membrane denseness. The membrane prepared by the IP
process using a lower concentration of TTESPA solution shows lower salt rejection andhigher water permeability than the membrane prepared by the SG process, which has
almost the same separation layer thickness. In addition, the RO performance of
membranes prepared by the copolymerization of TTESPA and BTESE is also shown in
Table 2.1 (TT-BT1 and TT-BT3). Compared with the BTESPA and BTESE copolymerbased membrane (BT-BT3 in Table 2.1), TT-BT1 and TT-BT3 do not demonstrate thesame degree of increase in water permeability, probably because TTESPA has threereactive ethoxysilyl units that can form a denser film than BTESPA.
The TDEMSPA-based membranes prepared by the SG process and the IP process
exhibit water permeability of 5.3 × 10-14 m3/m2sPa and 6.0 × 10- 14 m3/m2sPa andsalt rejection of 94.3% and 92.5%, respectively. The TDEMSPA- and TTESPA-based
membranes exhibit similar salt rejection values. However, the TDEMSPA-based
membranes show lower water permeability values than the TTESPA- based membranes,澳

24which is probably due to the methyl groups in the TDEMSPA-based membranes
hindering the passage of water molecules through the membrane. In addition, thehydrophobicity of the methyl groups decreased water permeability, as evidenced by thehigh water contact angles of the TDEMSPA-based membrane surface (Figure 2.6).

澳Figure 2.6. Water contact angles of membrane surfaces (SG: SG process; IP: IP process).
澳
The TTESMA-based membrane prepared by the SG process exhibits high water
permeability of 2.6 × 10-12 m3/m2sPa but low salt rejection of 70.2%. In contrast, theTTESMA-based membrane prepared by the IP process only exhibits approximately 30%
salt rejection. This result is consistent with the result obtained by SEM analysis of themembrane that shows cracks in the separation layer (Figure 2.4(h)). This may be dueto the short methylene chains that have little flexibility. It seems that the membrane
prepared by the SG process also has some deformations, including small cracks and/or
pinholes, which cannot be observed from its cross-sectional SEM image (Figure 2.4(c)).
Comparing the RO performance with our previous work for organically bridged
PSQ-RO membranes, which was reported in the literature, (Figure 2.7), the water
permeability of the TTESPA-based membrane prepared by the SG process is higher
than those of BTESE- and BTESPA-based membranes, and slightly lower than those of
BTESE2- (bis(triethoxysilyl)ethane) and BTESE3-(bis(triethoxysilyl)ethyne) based
membranes, and the TTESPA-based membrane prepared by the IP process exhibits
澳

25

favorable water permeability even though a slight decrease of salt rejection is observed.
The favorable water permeability and high NaCl rejection of the TTESPA-based
membrane would be ascribed to the three propylene chains of TTESPA, which can form
a dense separation layer with moderate flexibility.
It was reported that membranes prepared from BTESE2 and BTESE3, which withrigid and polar ethylene and acetylene units showed higher RO performance than the
BTESE-derived membranes. For further improve the RO performance of TTESPAderived membranes, the author introduced ethylene and acetylene units in precursors
TTESP2A and TTESP3A. As expected, the RO performance of the SG membranes of
TTESP2A and TTESP3A showed higher water permeability than and similar NaCl
rejection to those of the TTESPA-membrane (Table 2.1). However, the water
permeability and NaCl rejection of the membranes prepared by IP process of TTESP2A
and TTESP3A are lower than those of TTESPA. Permeation of the precursors to theaqueous layers forming the gels in the support membrane suggested by the crosssectional SEM images would be a reason for the lower permeability (Figure 2.4 (j) and
(k)).


澳

26

Table 2.1. Summary of RO performance of TTESPA-, TTESMA-, TDEMSPA-,TTESP2A-, and TTESP3A-based membranes.
LiquidMembrane
Processa
NaCl rejection/%
permeability/(m3/m2sPa)
TTESPASG
1.7 × 10-13
96.6-13
TTESPAIP
7.3 × 1095.6
-13
TTESPAIP (1 wt%) 7.9 × 10
87.2
-14
TDEMSPASG
5.3 × 10
94.3
-14
TDEMSPAIP
6.0 × 1092.5
TTESMASG
2.6 × 10-1270.2-12TTESMAIP
9.7 × 1033.0b
-13TT-BT1IP
2.1 × 1091.2
b
-13
TT-BT3IP
2.9 × 1092.0c
-12BT-BT3IP
1.3 × 1092.0
-13TTESP2ASG
3.3 × 1096.5
-14
TTESP2AIP
4.4 × 1094.2
-13
TTESP3ASG
5.5 × 1094.1
-13
TTESP3AIP
2.1× 1092.5
d-11
NTR7450
2.4 × 1013.1
a
IP: IP process; SG: SG process.
b
TT-BT1: TTESPA copolymerized with BTESE in 1:1 ratio; TT-BT3: TTESPA
copolymerized with BTESE in 1:3 ratio.
c
BT-BT3: BTESPA copolymerized with BTESE in 1:3 ratio; data from literature [8].
dPolysulfone support membrane.

Figure 2.7. Summary of RO performance of membranes, together with those of +BTESE (SG), − BTESE2 (SG), h BTESE3 (SG), and - BTESPA (SG) from theliterature.

澳

27

Stability towards chlorine exposure was examined for the TTESPA-based
membrane. As shown in Figure 2.8, when the membrane was immersed in a 1,000 ppm
NaOCl aqueous solution, liquid permeability and NaCl rejection were almost
unchanged up to the exposure time of 10 h, indicating high stability of the membrane.

Figure 2.8. Effects of chlorine exposure on liquid permeability and NaCl rejection of
TTESPA-based membrane, using 1,000 ppm NaOCl solution.
Summary
In conclusion, the author examined TTESPA, TTESMA, TDEMSPA, TTESP2A,
and TTESP3A as new precursors for PSQ-RO membrane preparation by both IP process
and the SG process. The TTESPA-based membrane exhibited good RO performance of
7.3 × 10-13 m3/m2sPa water permeability and 95.6% NaCl rejection (IP process) and1.7 × 10-13 m3/m2sPa and 96.6% NaCl rejection (sol- gel process). These data aremuch higher than those of the membrane prepared by homopolymerization of BTESA,
suggesting the potential of TTESPA. They are comparable to those of the membrane
prepared by SG process of BTESE3, which are the best data so far obtained for PSQ
membranes. As high performance BTESE3 membranes can be obtained only by SG
process, the use of TTESPA that can form good membranes by simpler IP process seems
to be advantageous.
The nonporous structure of the membrane was revealed by nitrogen adsorption anddesorption measurements, suggesting that the diffusion-controlled mechanism isoperative for water separation. The TDEMSPA-based membranes also demonstrated
favorable NaCl rejection, although water permeability was slightly low. Evidence澳

28suggests that the IP process can form a spacer but thicker membrane than the SG process,and as a result, water permeability is improved but salt rejection is slightly lowered.
Although the RO performance of the TTESPA-based membrane still has room for
improvement, these results offer important information for PSQ precursor design forimproving water permeability in future work. Studies on other applications of themembranes such as for gas separation and liquid separation are also in progress.
Experimental
GeneralAll reactions were carried out in a dry argon atmosphere. TTESMA was purchasedfrom Gelest Inc. and used as obtained. Toluene and diethyl ether used as the reaction
solvents were distilled from P2O5 and CaH2, respectively, and stored over activatedmolecular sieves in dark until use. NMR spectra were recorded on Varian 400–MR and
500 spectrometers. MS spectra were measured on a Shimadzu QP2021SE spectrometer.
In the NMR spectra of TTESPA, TDEMSPA, TTESP2A, and TTESP3A, small
unidentified signals are seen (Figure 2.10-2.13), but they were used for the following
experiments without further purification, because further attempted purification bysilica gel chromatography lowered their purity.
澳Preparation of TTESPA and TDEMSPA
A solution of 1.00 g (7.28 mmol) of triallylamine in 30 mL of toluene was placedin a 200 mL two-necked flask equipped with a magnetic stirrer and a reflux condenser.
To this were added 3.59 g (21.8 mmol) of triethoxysilane and two drops of Karstedt’s
catalyst (19.0–21.5% as Pt in THF). The reaction mixture was heated to reflux for 20 h.
Trace amounts of reactants remaining in the reaction mixture and the solvent were
removed under vacuum to give a yellow liquid, which was then purified by preparative
gel permeation chromatography (GPC) using toluene as the eluent to yield TTESPA asa colorless liquid (4.10 g, 89% yield). 1H NMR (500 MHz, chloroform-d) δ 3.81 (q, J= 7.0 Hz, 18H), 2.45–2.33 (m, 6H), 1.52 (dtd, J = 9.9, 8.0, 5.7 Hz, 6H), 1.22 (t, J = 7.0澳29Hz, 27H), 0.62–0.51 (m, 6H). 13C NMR (126 MHz, chloroform-d) δ 58.28, 57.15, 20.27,
18.28, 8.02. MS m/z 627 (M+).
Tris[3-(diethoxymethylsilyl)propyl]amine was prepared in a fashion similar to
TTESPA. Into a 200 mL two-necked flask equipped with a magnetic stirrer and a reflux
condenser were added 1.00 g (7.28 mmol) of triallylamine and 2.94 g (21.8 mmol) of
diethoxymethylsilane in 30 mL of toluene. To this was added two drops of Karstedt’s
catalyst (19.0-21.5 % as Pt in THF), and the reaction mixture was heated to reflux for
20 h. Excess reactants and the solvent were removed under vacuum to give a yellow
liquid that mainly contained TDEMSPA. After purification by GPC, TDEMSPA was
obtained as a colorless oil (4.10 g, 85% yield). 1H NMR (500 MHz, chloroform-d) δ
3.75 (q, J = 7.0, 0.8 Hz, 12H), 2.48–2.36 (m, 6H), 1.53–1.43 (m, 6H), 1.20 (td, J = 7.0,
0.8 Hz, 18H), 0.59–0.50 (m, 6H), 0.11 (s, 9H). 13C NMR (125 MHz, chloroform-d) δ
58.04, 57.54, 20.56, 18.38, 11.48, -4.91. MS m/z 538 (M+).
Preparation of TTESP2A
A mixture of 1.00 g (7.62 mmol) of tripropargylamine and 3.76 g (23.0 mmol) oftriethoxysilane in 30 mL of toluene was placed in a 200 mL two-necked flask equippedwith a magnetic stirrer and a reflux condenser. To this was added two drops of
Karstedt’s catalyst (19.0-21.5 % as Pt in THF), and the reaction mixture was heated to
85 ℃ for 6 h. Excess reactants and the solvent were removed under vacuum to give ayellow liquid. Kugelrohr distillation of the residue (1 Torr, 200 ℃) gave a mixture oftris[3-(triethoxysilyl)prop-2-enyl]amine and its isomers as a colorless oil (2.1 g, 44 %yield) (unit ratio of trans- and gem-(triethoxysilyl)prop-2-enyl was calculated to be 5 :3 based on 1H NMR integrations).

1

H NMR (400 MHz, chloroform-d) δ 6.48-6.37 (m,1.88H), 6.21 (dt, J = 3.7, 2.1 Hz, 0.29H), 6.14 (dt, J = 3.7, 1.8 Hz, 0.41H), 5.97 (dt, J =3.5, 1.7 Hz, 0.42H), 5.77-5.73 (m, 1.12H), 5.64, 5.63, 5.61 (3 br d, J = 18.8 Hz, 1.88H),
3.83-3.77 (m, 18H), 3.16-3.06 (m, 6H), 1.26-1.18 (m, 27H). 13C NMR (126 MHz,
chloroform-d) δ 150.47, 149.80, 128.99, 128.19, 122.26, 121.51, 58.71, 58.40, 18.18.
MS m/z 623 (M+).
澳30

Preparation of TTESP3A
A solution of 8.51 mL of 2.69 M n-BuLi in hexane (22.9 mmol) and 80 mL ofdiethyl ether was placed in a 500 mL three-necked flask and a magnetic stirrer and theflask was cooled at −78 °C.

To this was slowly added 1 g (7.63 mmol) oftripropargylamine in 30 mL of diethyl ether within 1 h. The mixture was then stirred
for 1 h at this temperature and further stirred for 12 h at room temperature. The
mixture was again cooled to −78 °C and added to a mixture of chlorotriethoxysilane(5.44 g, 27.5 mmol) in 30 mL of diethyl ether cooled at −78 °C within 1 h. The mixture
was further stirred for 1 h at −78 °C and for 12 hours at room temperature. The
resultant precipitates were filtered, and solvents were removed under vacuum. The
residue was subjected to Kugelrohr distillation (1 Torr, 225 ℃) to give 2.00 g (43 %)of tris[(triethoxysilyl)propynyl]amine as a colorless oil. 1H NMR (500 MHz,
Chloroform-d) δ 3.84 (q, J = 7.0 Hz, 18H), 3.51 (s, 6H), 1.22 (t, J = 7.0 Hz, 27H). 13C
NMR (126 MHz, chloroform-d) δ 99.27, 82.25, 59.01, 42.72, 17.96. MS m/z 616 (M+)
SG process for membrane preparation
The SG process was carried out as reported in the literature [9]. A solution of 5 wt%
of precursor in EtOH containing different ratios of distilled water (the ratio depends on
the reactivity of precursor) was stirred at room temperature until the sol particle sizes
determined by dynamic light scattering (DLS) measurement reached 1.6 to 2 nm (2–12h). The reaction mixture was then diluted with ethanol to 1 wt% and preserved in arefrigerator at 4 °C until use.
A polysulfone support membrane (Nitto Denko Corporation NTR-7450 pore size:
1 nm) was washed with ethanol and air-dried in a drying oven at 60 °C for 10 min. After
that, the prepared sol solution was poured onto the support membrane surface at room
temperature. After 30 sec, excess sol solution was removed by decantation and themembrane was air-dried at room temperature for 10 min. After repeating this process
twice, the membrane was calcined at 150 °C for 10 min to facilitate gelation. After澳

31

cooling to room temperature, the membrane was washed with distilled water and usedin the following RO experiments.
澳IP process for membrane preparation
The IP process for the preparation of PSQ membranes was performed as reported
previously [4]. A polysulfone support membrane (Nitto Denko NTR-7450 pore size: 1nm) was washed with ethanol and dried in a drying oven at 60 ℃ for 10 min. Sodium
dodecyl sulfate (SDS) aqueous solution was then poured onto the support membranesurface at room temperature to allow permeation of the SDS aqueous solution. After 5min, excess SDS aqueous solution was removed by decantation and a solution of 5 wt%
of precursor in hexane was poured onto the wet support membrane surface at room
temperature. After 5 min, the hexane solution was removed by decantation and themembrane was calcined at 150 ℃ for 10 min, to complete the polymerization. Aftercooling to room temperature, the membrane was washed with distilled water and usedin the following RO experiments.
Evaluation of RO performance
RO experiments were performed using 2,000 ppm NaCl aqueous solution on a
device shown in Figure 2.9. The feed pressure of 1.5 MPa was applied to the solution
to promote permeation through the membrane at room temperature. RO performanceparameters, water permeability (Lp) and NaCl rejection (R), were determined byequations (1) and (2) [10], respectively, in which ∆P and ∆π are differences in appliedand osmotic pressures and Jv is the permeate water flux. Cf and Cp are NaCl
concentrations of feed and permeate determined by their conductivities, which wereobtained using a conductivity meter (HORIBA, ES-51).
(1)‫ܮ‬௣ ൌ ‫ܬ‬௩ Ȁሺο െ οɎሻܴ ൌ ൬ͳ െ

஼೛
஼೑

(2)

൰ ൈ ͳͲͲΨ澳澳

32

Pressure gauge

澳Pressure regulator

Salt water
MembraneN2 cylinder

Flux out

Figure 2.9. Apparatus for RO experiment.


澳

33





Figure 2.10. 1H NMR (tip) and 13C NMR (bottom) spectra of TTESPA.

澳

34




Figure 2.11.

澳

1

H NMR (tip) and 13C NMR (bottom) spectra of TDEMSPA.

35

Figure 2.12.

澳

1

H NMR (tip) and 13C NMR (bottom) spectra of TTESP2A.

36

澳
㻌
Figure 2.13.

1

H NMR (tip) and 13C NMR (bottom) spectra of TTESP3A.

澳

澳

37

References[1] M.R.N. Monton, E.M. Forsberg, J.D. Brennan, Tailoring sol–gel-derived silica
materials for optical biosensing, Chem. Mater., 24 (2012), 796-811.
[2] L.L. Hench, J.K. West, The SG process, Chem. Rev., 90 (1990), 33-72.
[3] A. Larbot, A. Julbe, C. Guizard, L. Cot, Silica membranes by the SG process, J.
Membr. Sci., 44 (1989), 289-303.
[4] F.-T. Zheng, K. Yamamoto, M. Kanezashi, T. Gunji, T. Tsuru, J.J.C.L. Ohshita,
Preparation of Hybrid Organosilica Reverse Osmosis Membranes by IP process ofBis[(trialkoxysilyl) propyl] amine, Chem. Lett., 47 (2018), 1210-1212.
[5] R. Xu, S.M. Ibrahim, M. Kanezashi, T. Yoshioka, K. Ito, J. Ohshita, T. Tsuru, Newinsights into the microstructure-separation properties of organosilica membranes withethane, ethylene, and acetylene bridges, ACS Appl. Mater. Interfaces, 6 (2014), 93579364.
[6] K. Yamamoto, H. Muragishi, T. Mizumo, T. Gunji, M. Kanezashi, T. Tsuru, J.
Ohshita, Diethylenedioxane-bridged microporous organosilica membrane for gas andwater separation, Sep. Purif. Technol., 207 (2018), 370-376.
[7] K. Yamamoto, J. Ohshita, Bridged polysilsesquioxane membranes for water
desalination, Polym. J., 51 (2019), 1103-1116.
[8] D. Zhang, J. Ohshita, Preparation of robust RO membranes for water desalinationbyinterfacial

copolymerizationofbis[(triethoxysilyl)propyl]amineandbis(triethoxysilyl)ethane, Polym. J., 51 (2019), 1231-1234.
[9] T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka, K. Ito, T. Tsuru, Preparation of
BTESE-derived organosilica membranes for catalytic membrane reactors of
methylcyclohexane dehydrogenation, J. Membr. Sci., 455 (2014), 375-383.
[10] M.L. Lind, D. Eumine Suk, T.-V. Nguyen, E.M. Hoek, Tailoring the structure ofthin film nanocomposite membranes to achieve seawater RO membrane performance,
Environ. Sci. Technol. Lett., 44 (2010), 8230-8235.

澳38

Chapter 3 Preparation of PSQ-RO membranes with hydroxyl groups bycopolymerizationofbis[3-(triethoxysilyl)propyl]amine
andtriethoxy(3glycidyloxypropyl)silane
Introduction
In this chapter, the author demonstrates a strategy for controlling the permeabilityof PSQ-RO membranes by using the ring-opening reaction of epoxy-containing
precursor. In general, the introduction of hydrophilic groups, such as alcohol, ethylene
glycol, and chitosan, is a simple and common method to enhance membranehydrophilicity [1-3]. In PSQ-based materials, these hydrophilic groups can be
introduced easily by modification of the bridging units, or more simply by
copolymerization with trialkoxysilanes that have a hydrophilic group [4, 5]. In thischapter, to improve water permeability, bis[3-(triethoxysilyl)propyl]amine (BTESPA)based membranes were copolymerized with triethoxy(3-glycidyloxypropyl)silane(TEGPS) (Scheme 3.1), expecting that the epoxy groups would improve thehydrophilicity of RO membranes. The author also anticipated that the epoxy groups
would undergo ring opening with an amine unit of BTESPA and form hydroxy (C-OH)
groups, further improving membrane hydrophilicity.

澳
Scheme 3.1. Precursors for organically bridged RO membranes in present studies.
澳

澳

39

Results and discussion
Preparation of membranes by the SG process
Stirring the mixtures of BTESPA/TEGPS (structures are shown in Scheme 3.1) in 2:1
and 5:1 weight ratios with 20 eq water and BTESPA/TEGPS in 10:1 weight ratio with
40 eq water in ethanol for 2 h at room temperature yielded PA-TEG2, PA-TEG5 andPA-TEG10 sols, respectively. Figure 3.1 shows the particle size distribution of PATEG2 and PA-TEG5 sols determined by dynamic light scattering (DLS). Particle size
should be as small as possible to improve water permeability, as large particles tend toincrease membrane thickness and induce membrane cracking and peeling [6], butshould be larger than the pore size of the support membrane to avoid penetration of thesols into the support membrane as already mentioned in Chapter 1. The particle sizesof PA-TEG2, PA-TEG5 and PA-TEG10 are around 1.5 nm, 2.3 nm and 2.0 nm, as
shown in Figure 3.1, which are suitable to prepare RO membranes on the polysulfone
support (Nitto Denko NTR-7450 with 1 nm pore size ).

Number [%]30

20

PA-TEG2
PA-TEG5

10

PA-TEG10
PA-TEG2-12

0
0.1

1

10

100

Particle diameter [nm]澳Figure 3.1. Particle size distribution of sols determined by DLS.
Thermogravimetric (TG) analysis provides information on the thermal stability of
PSQ gels. For PSQ membranes preparHG by the SG process, an exceedingly highcalcination temperature leads to the thermal decomposition of organic units, but thetemperature must be sufficiently high to facilitate gelation for the formation of a dense
membrane. Overall, the calcination temperature should be as high as possible but lower
than the decomposition temperature of the organic units. TG analysis was performed at澳40

the heating rate of 10 ℃/min under a nitrogen atmosphere for PA-TEG2, PA-TEG5,and PA-TEG10 gel powders obtained by drying their sol solutions in a dry oven at 60 °C
for 2 days, and the results are shown in Figure 3.2. The gel powder that had a higher
TEGPS ratio tended to exhibit higher weight loss. Specifically, the first slight weightloss observed at temperatures up to 250 °C was due to the desorption of adsorbed water
and water liberation by the condensation reaction between Si-OH groups, forming SiO-Si linkages. The greater weight loss at higher temperatures could be ascribed to thedecomposition of organic units. Based on these results, the calcination temperature was
set to 150 °C, which is the upper limit of the operation temperature of the support
membrane.

澳Figure 3.2. TG curves of gel powders in a nitrogen atmosphere澳at the heating rate of
10 ℃/min.
澳The FT-IR spectra of films were prepared by coating these sol solutions on thesilicon wafer, and calcination the coated wafer at 150 °C in a dry oven in air, as shownin Figure 3.3 and Figure 3.4. The absorption bands centered at 1065 cm-1 were attributed
to the stretching frequencies of Si–O–Si. In addition, Si–O stretching bands of Si-OH
were observed from 920 cm-1 to 980 cm-1. However, their intensities were low, and only
a weak absorption around 3200 cm-1 was observed for O-H vibration, indicating the lowcontents of Si-OH groups even though they were present.
澳

澳

41

澳
Figure 3.3. FT-IR spectra of PA-TEG2, PA-TEG5, and PA-TEG2-12 films preparedby heating precursor sols coated on silicon wafer at 150 °C, and those of PA-TEG2
and PA-TEG2-12 films after soaking in water and drying at 100 °C.
澳澳Figure 3.4. FT-IR spectra of PA-TEG10 film prepared by heating precursor sols
coated on silicon.
A nitrogen adsorption/desorption isotherm is used to evaluate membrane porosity
due to membrane porosity considerably affects its permeability. Figure 3.5 shows theisotherms of PA-TEG2 and PA-TEG5 gels that were prepared by drying their sol
solutions at 60 ℃ in the air for two days, followed by calcination at 150 ℃ in a nitrogen
atmosphere for 12 h, similar to the conditions used to prepare the gel films for IR
spectrometry. The adsorption/desorption isotherms of BTESPA are also presented in
Figure 3.5 for comparison [7]. In contrast to the BTESPA-based gel powder, the PATEG2- and PA-TEG5-based gel powder exhibited no volume adsorption, suggestingthat the gel powder was non-porous. In the water separation process of the PA-TEG2-澳

42and PA-TEG5-based membranes, therefore, the solution-diffusion mechanism isoperative rather than the molecular sieving effects for water separation.
澳澳
Figure 3.5. N2 adsorption/desorption isotherms of PA-TEG2, PA-TEG5, and BTESPA
gel powder.
The surface and cross-sectional morphologies of membranes of PA-TEG2 and PATEG5 were examined by SEM. Generally, water permeability of RO membranes isinversely proportional to its thickness, that is, the thicker the PSQ layer tends to exhibit
lower water permeability. However, too thin PSQ layer will lead to decreases in salt
rejection. Figures 3.6a and 3.6b show the cross-sectional morphologic SEM images of
the PA-TEG2- and PA-TEG5-based membranes, respectively. The PSQ separation layer
could be clearly seen as the top layer whose thickness ranged from 100 to 120 nm. In
addition, surface topography was also characterized by SEM (Figure 3.6c). The smoothand defect-free surface can be observed, indicating the formation of a homogeneous
separation layer.
澳澳Figure 3.6. Cross-sectional SEM images of (a) PA-TEG2- and (b) PA-TEG5-based
membranes. (c) SEM image of PA-TEG2-based membrane surface.

澳43

Reversible hydrolytic C-O-Si bond cleavage
To obtain the molecular structure information of these gels, the author prepared
TEGPS homo-based gel powder by homopolymerization of TEGPS as the control
sample under the same conditions as those for the preparation of PA-TEG2 and PATEG5-based membranes. The 13C solid-state NMR spectra of the gel powders preparedfrom TEGPS homopolymer and PA-TEG2 copolymer are shown in Figure 3.7(a) and3.7(b), respectively. These gel powders were obtained by the same process as that for
TG analysis. For the PA-TEG2-based gel powder, broad peaks centered at
approximately 9, 20, and 55 ppm were clearly observed, and these were attributed tothe propyl group carbons from BTESPA and TEGPS. Peaks (1) to (3) attributed to thecarbons of glycidyloxy group in Figure 3.7(a) disappeared in Figure 3.7(b), indicatingthat the ring opening reaction occurred by interaction with BTESPA (Scheme 3.2). The
broad peak centered at 50 ppm observed in Figure 3.7(b) is likely due to a small amountof glycidyloxy group remaining. To further promote the ring-opening reaction, theauthor prepared PA-TEG2-12-based gel powder by reacting PA-TEG2 sol with 20 eq
of water for an additional 12 h and drying the resultant gel by the same process. The
particle size distribution of PA-TEG2-12 sol determined by DLS is shown in Figure 3.1.
The PA-TEG2-12-based gel powder showed no clear signal around 50 ppm in Figure
3.7(c). As expected, the longer reaction time resulted in the smaller amount ofglycidyloxy group remaining in PA-TEG2-12-based gel. When the PA-TEG2-12-based
gel powder was soaked in water for 12 h and then dried, the peak observed at 15 ppm
in Figure 3.7(b) and 3.7(c) disappeared, as shown in Figure 3.7(d). This peak is likely
due to the C-O-Si group undergoing hydrolysis to form C-OH and Si-OH groups upon
immersion of the gel powder in water. It is known that the C-O-Si bond is less stabletowards hydrolysis than the Si-O-Si bond. In the gelation process, the C-OH groups
produced by the ring opening of epoxy units condense with Si-OH in PA-TEG2 andPA-TEG2-12 sols to form C-O-Si bonds. However, the C-OH groups in much lessmobile gel cannot undergo condensation under the same conditions (Scheme 3.3).
澳
澳

44

澳
Scheme 3.2. Schematic diagram of generation of C-OH bonds by hydrolyzing C-O-Si
bonds in the membranes.

Figure 3.7. 13C DD/MAS NMR spectra of (a) TEGPS homopolymer-based gel
powder, (b) PA-TEG2-based gel powder, (c) PA-TEG2-12-based gel powder before
soaking, and (d) PA-TEG2-12-based gel powder after soaking/drying.

澳

45

澳
Scheme 3.3. Hydrolysis and condensation of copolymers of TEGPS and BTESPA.
To further characterize the structures of the PA-TEG2- and PA-TEG2-12-based
membranes, the author carried out FT-IR analysis of their gel films, which wereprepared by drop casting the sol solutions on a silicon wafer, followed by the calcination
at 150 °C in air. The PA-TEG2 and PA-TEG2-12 gel films were subjected to an
additional immersion experiment. These gel films were soaked in water at room
temperature for 12 h and then dried at 100 °C in air for 1 h. Figure 3.3 exhibits the FTIR spectra of PA-TEG2 and PA-TEG2-12 before and after soaking experiments in water.
A broad peak from 3100 cm-1 to 3600 cm-1 was observed in the spectra of both gel films
after soaking, which was due to the presence of OH groups. In contrast, the IR spectrumof the PA-TEG2 gel film before soaking showed no clear peak in this wavelength range.
This may be explained by assuming that the C-OH groups arising from the hydrolysisof the epoxy units underwent condensation to form C-O-Si bonds in PA-TEG2, asshown in Scheme 3.2. Additional treatment with water resulted in the hydrolysis of
epoxy or C-O-Si bonds to form C-OH groups. The water contact angles of the gel film
surface also provided evidence of increased hydrophilicity, that is, after soaking/drying,
the contact angles of PA-TEG2 and PA-TEG2-12 gel films prepared on glass plates
were decreased from 55.8° to 41.1° and from 60.5° to 43.8°, respectively (Table 3.1 and澳

46

Figure 3.8). Increasing hydrophilicity improved the water permeability of themembranes (vide infra).
澳澳澳Figure 3.8. Water contact angles of the membranes of (a)PA-TEG2, (b) PA-TEG2-12
based membrane before and after soaking in water.
Table 3.1. Contact angles of PA-TEG2-, PA-TEG5-, PA-TEG10 and PA-TEG2-12based membranes.
Membrane

PA-TEG2

PA-TEG5

PA-TEG10

CA

55.8

47.4

56.6

PA-TEG2
soaking 12 hthen dried
41.1

BTESPAa
61.4

RO performance
RO experiments of PA-TEG2-, PA-TEG5-, PA-TEG10- and PA-TEG2-12-based
membranes were performed by using a 2000 ppm NaCl aqueous solution under the feed
pressure of 1.5 MPa at room temperature. A summary of RO performance is presented
in Figure 3.9(a). For the PA-TEG2-based membranes, the author examined their
preparation five times and the data of the membrane that showed the highest salt
rejection are presented in Table 3.2. The membrane exhibited excellent water
permeability of 9.6 × 10-13 m3/m2sPa, which is approximately nine times higher thanthat of the BTESPA homopolymer-based membrane which was reported in the literature(1.1 × 10-13 m3/m2sPa) [8]. In addition, compared to the BTESPA homo-based
membranes which exhibited salt rejection of 96 %, the PA-TEG2-based membrane澳

47

exhibited a slightly higher salt rejection of 97.5%. RO performance of the PA-TEG2based membrane was a little lower than but close to that of commercially available
organic RO membrane SW30HR (water permeability of 1.1 × 10–12 m3/m2sPa and salt
rejection of 98.5%). In addition, the PA-TEG5- and PA-TEG-10 based membranes also
exhibited high water permeability of 1.1 × 10-12 and 7.4× 10-13 m3/m2sPa and good salt
rejection of 94.4 and 94.1 %, respectively. A summary of the RO performance of themembranes examined herein is presented

in Figure 3.9(c), together with thosepreviously reported for BTESE-, BTESE2-, BTESE3-, and BTESPA-based PSQ
membranes [9, 10]. The PA-TEG2-based membrane exhibited higher water
permeability and salt rejection than the membranes prepared in the previous studies.
The introduction of the epoxy-containing comonomer significantly improved water
permeability, which is likely attributed to the ring opening of the epoxy units that
resulted in the formation of C-OH groups during sol formation, which was supportedby small water contact angles of the PA-TEG-based membrane surface (Table 3.1). In
agreement with the observation that C-O-Si bonds were cleaved by soaking in water,
the PA-TEG2-based membrane showed a slight increase in water permeability from 8.9
× 10-13 m3/m2sPa to 1.0 × 10-12 m3/m2sPa during the first two h of the RO experiment,(Figure 3.9(b)), which stabilized with time. The long-term stability of this membrane
was also investigated. As described in Figure 3.10, although the salt rejection is lowered
and the water permeability was increased after the 100 h RO experiment, the changes
were slight showing good durability.
More pronounced changes in water permeability were seen in the PA-TEG2-12based membrane. The PA-TEG2-12-based membrane exhibited water permeability of
5.5 × 10-13 m3/m2sPa and salt rejection of 93.3% at the early stage of the RO experimentand then reached a stable state after 10 h to give the water permeability of 9.3 × 10-13
m3/m2sPa and a slight decrease of salt rejection from 93.3% to 93.0% (Figure 3.9(b)).
Although the water permeability is lower than that of the PA-TEG2-based membrane,
it is higher than that of the BTESPA-based membrane.
澳澳

48

Table 3.2. Summary of RO performance of PA-TEG2, PA-TEG5 and PA-TEG212-based membranes.
MembraneLiquid permeability [(m3/m2s·Pa)]

NaCl rejection [%]PA-TEG2

9.6 × 10-13

97.5

PA-TEG5

1.1 × 10-12

94.4

PA-TEG10

7.4 × 10-13

94.1

PA-TEG2-12

9.3 × 10-13

93.0

TEGPS

1.3 × 10-12

78.0

BTESPA[a]1.2 × 10-13

97.0

1.0 × 10

-13

98.0

2.0 × 10

-13

98.0

8.5 × 10-13

92.0

BTESE

[b]

BTESE2[b]

BTESE3[b]abData from literature
Data from literature澳澳Figure 3.9. (a) Summary of RO performance in this work. (b) RO measurement of
PA-TEG2- and PA-TEG2-12-based membranes by using 2000 ppm NaCl aqueous
solution. (c) Summary of RO performance of the present membranes, together withthose of × BTESPA (SG), + BTESE (SG), ᶭ BTESE2 (SG), and տ BTESE3 (SG)from the literature [10].

澳

49

澳
Figure 3.10. Long-term stability experiment of PA-TEG2-based membrane.
澳Resistance to chlorine and thermal stability
For practical RO applications, RO membranes are expected to exhibit thermal
stability and resistance to chlorine. The PA-TEG2-based membrane demonstrated
excellent resistance to chlorine: the membrane exposed to 1000 ppm NaClO aqueous
solution at room temperature exhibited no obvious changes in water permeability andsalt rejection up to 10 h exposure (10000 ppm‫ڄ‬h), as illustrated in Figure 3.11(a). To
investigate the thermal stability of the membrane, RO measurement was carried out atelevated temperatures. The PA-TEG2-based membrane showed high thermal stability
up to 60 °C. At this temperature, water permeability increased from 9.6 × 10 -13
m3/m2sPa to 1.82 × 10-12 m3/m2sPa, and salt rejection decreased from 97.5% to 95.5%.
Moreover, when the temperature was lowered to room temperature, the original water
permeability and salt rejection were recovered (water permeability of 9.6 × 10-13
m3/m2sPa and salt rejection of 97.5 %), as shown in Figure 3.11(b). On the other hand,when water temperature exceeded 80 °C, salt rejection decreased dramatically and did
not return to its original value (Figure 3.12). Exceedingly high temperatures seemed tocause serious damage to the membrane. Indeed, when the coated film of PA-TEG2 ona silicon wafer was treated with hot water at 90 °C for 2 h, C-H vibration bands around2900 cm-1 were weakened, suggesting that organic units were liberated from the film
to an extent (Figure 3.13). However, thermal stability of the present membrane was澳50superior to that of commercial polyamide-based membranes for which the operating
temperature limit is 45 °C [11].
澳Figure 3.11. (a) Chlorine exposure test by using 1000 ppm NaClO aqueous solution.
(b) Thermal stability test by using 2000 ppm NaCl aqueous solution.
100

95
10-11 0

90

85
10-12

Rejection [%]Liquid permeability [m3 l/m2·s·Pa]

10-10 0

澳

80

0

75
10-13 0

70
0

25瀽C

1

70瀽C

2

80瀽C

3

90瀽C

425瀽C

澳

Temperature [ °C]

Figure 3.12. Thermal stability test for PA-TEG2-based membrane using 2000 ppm
NaCl aqueous solution.
160

140

Transmittance [a.u.]

120

100

80

60

40

PA-TEG2
PA-TEG2 heat treatment 2h then dried

20

0

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber [cm⁻¹]

澔Figure 3.13. FT-IR spectra of PA-TEG2 film prepared by heating precursor sols coatedon silicon wafer at 150 °C, before and after heat treatment in water at 90 °C for 2 h anddrying at 100 °C.

澳51

Summary
In conclusion, the author successfully introduced epoxy groups into PSQ-RO
membranes. The epoxy groups significantly improved membrane hydrophilicity
through the ring opening reaction, thereby increasing membrane permeability.
Although the PA-TEG2-based gel powder exhibited non-porous properties, its
membrane exhibited high water permeability of 9.6 × 10-13 m3/m2sPa and excellent salt
rejection of 97.5%, which were close to those of commercially available organic RO
membrane SW30HR. In addition, this membrane showed high resistance to chlorineand excellent thermal stability up to 60 °C, indicating potential application as a robust
membrane. The author also conducted studies on the condensation/hydrolysis of the CO-Si group. The surface C-OH group content increased as the immersion time in water
increased, resulting in the improvement of membrane permeability.
澳
Experimental
GeneralBis[3-(triethoxysilyl)propyl]amine with 95% purity was purchased from Gelest Inc.
Triethoxy(3-glycidyloxypropyl)silane with 96% purity was purchased from TCI Co.,
Ltd. All chemicals were used without further purification. Ethanol used as the reaction
solvent during the SG process was distilled from Mg and stored over activatedmolecular sieves in the dark until use. DLS measurements were carried out on a
Malvern Zetasizer Nano (Malvern, ZEN3600) analyzer. TGA was conducted using an
SII EXSTAR TG-DTA6200 thermal analyzer in the temperature range of 25 °C to1000 °C in a nitrogen atmosphere. Contact angle measurements were performed with a
Kyowa DM300 contact angle meter.
澳
澳
澳
澳
澳

52

Sol-gel process for membrane preparation and Evaluation of RO performance
The SG process, membrane preparation process was conducted as chapter 1
membrane preparation section.
The evaluation of RO performance method was conducted as reported in chapter 1.
Two RO performance parameters, water permeability (Lp) and salt rejection (R), were
determined by using equations (1) and (2) [12], respectively, in which ∆P and ∆π aredifferences in applied and osmotic pressures, and Jv is the permeate water flux. Cf and
Cp are NaCl concentrations of feed and permeate, which were determined from theirconductivities obtained using a conductivity meter (HORIBA, ES-51).
(1)‫ܮ‬௣ ൌ ‫ܬ‬௩ Ȁሺο െ οɎሻ஼೛(2)ܴ ൌ ൬ͳ െ ஼ ൰ ൈ ͳͲͲΨ
೑

References[1] S. Krishnan, C.J. Weinman, C.K. Ober, Advances in polymers for anti-biofoulingsurfaces, J. Mater. Chem., 18 (2008), 3405-3413.
[2] A.C. Sagle, E.M. Van Wagner, H. Ju, B.D. McCloskey, B.D. Freeman, M.M. Sharma,
PEG-coated reverse osmosis membranes: desalination properties and fouling resistance,
J. Membr. Sci., 340 (2009), 92-108.
[3] L.-Y. Chen, X.-H. Lin, J.-G. Gai, Polyethylenimine linked glycidol surface
antifouling reverse osmosis membrane, Industrial & Engineering Chemistry Research,
57 (2018), 2322-2328.
[4] F.-T. Zheng, K. Yamamoto, M. Kanezashi, T. Tsuru, J. Ohshita, Preparation ofbridgedsilicaRO

membranesfromcopolymerizationofbis(triethoxysilyl)ethene/(hydroxymethyl)triethoxysilane. Effects of ethenylene-bridgeenhancing water permeability, J. Membr. Sci., 546 (2018), 173-178.
[5] K. Yamamoto, J. Ohshita, T. Mizumo, M. Kanezashi, T. Tsuru, Preparation ofhydroxyl group containing bridged organosilica membranes for water desalination, Sep.
Purif. Technol., 156 (2015), 396-402.
澳

53

[6] G. Li, M. Kanezashi, T. Tsuru, Preparation of organic–inorganic hybrid silica
membranes using organoalkoxysilanes: the effect of pendant groups, J. Membr. Sci.,
379 (2011), 287-295.
[7] K. Yamamoto, S. Koge, K. Sasahara, T. Mizumo, Y. Kaneko, M. Kanezashi, T. Tsuru,
J. Ohshita, Preparation of bridged polysilsesquioxane membranes from bis[3(triethoxysilyl)propyl]amine for water desalination, Bull. Chem. Soc. Jpn., 90 (2017),
1035-1040.
[8] E.S. Hatakeyama, C.J. Gabriel, B.R. Wiesenauer, J.L. Lohr, M. Zhou, R.D. Noble,
D.L. Gin, Water filtration performance of a lyotropic liquid crystal polymer membranewith uniform, sub-1-nm pores, J. Membr. Sci., 366 (2011), 62-72.
[9] R. Xu, J. Wang, M. Kanezashi, T. Yoshioka, T. Tsuru, Development of robust
organosilica membranes for reverse osmosis, Langmuir, 27 (2011), 13996-13999.
[10] R. Xu, S.M. Ibrahim, M. Kanezashi, T. Yoshioka, K. Ito, J. Ohshita, T. Tsuru, Newinsights into the microstructure-separation properties of organosilica membranes withethane, ethylene, and acetylene bridges, ACS Appl. Mater. Interfaces, 6 (2014), 93579364.
[11] M. Mänttäri, A. Pihlajamäki, E. Kaipainen, M. Nyström, Effect of temperature andmembrane pre-treatment by pressure on the filtration properties of nanofiltration
membranes, Desalination, 145 (2002), 81-86.
[12] M.L. Lind, D. Eumine Suk, T.-V. Nguyen, E.M. Hoek, Tailoring the structure ofthin film nanocomposite membranes to achieve seawater RO membrane performance,
Environ. Sci. Technol. Lett., 44 (2010), 8230-8235.
澳

澳

54

Chapter 4 Preparation of high water permeable PSQ-RO membranes by introducing
hydroxyethylurea-based hydrophilic water channels
Introduction
In the previous works, several strategies have been reported to increase thepermeability of water of PSQ-RO membranes by introducing hydroxyl groups. For
example, membranes prepared by the copolymerization of bis(triethoxysilyl)ethane(BTESE) with hydroxymethyl(triethoxy)silane (HMTES) exhibited about three times
higher permeability of water than BTESE homopolymer membranes [1]. Similarly, thecopolymerization of bis(triethoxysilyl)ethene (BTESE2) with HMTES resulted inapproximately seven times higher permeability of water than BTESE2 homopolymerbased membranes [2]. In addition, in chapter 2, we proposed a strategy to improve water
permeability by the introduction of epoxy groups via the copolymerization of bis[3(triethoxysilyl)propyl]amine (BTESPA) and triethoxy(3-glycidyloxypropyl)silane.
Although we have obtained similar water permeability to those of commercial
polyamide RO membranes, higher water permeability PSQ-RO membrane should be
exploredIn this chapter, to realize the even higher water permeability of PSQ-based RO
membranes, the author copolymerized BTESPA with N-(2-hydroxyethyl)-N’-[3(triethoxysilyl)propyl]urea (HETESPU). Two other hydroxyurea derivatives were also
examined as the comonomers. It has been reported that ureidosiloxane precursors
tended to self-organize by hydrogen bonding during the sol-gel (SG) process [3], andthe hydroxyl groups in the HETESPU units enhanced the water affinity of the channel
and the membrane surface [4]. The author found that during the preparation of themembranes, aggregation through hydrogen bonding occurred in the PSQ-based
membrane (see Figure 4.1 for schematic illustration), which can be regarded as ahydrophilic water channel, thereby dramatically increasing the permeability. In addition,
the tight packing of urea units through the hydrogen bonding enhanced the robustnessof the resultant membranes.

澳

55

澳
O
O

N
H

Si

O

O

O

OH

N
H

O

O
N
H

Si
O

HETESPUSiOOBTESPAhydrolysis and polycondensation

PSQ layer

Polysulfone support membrane

OH

OH

HN

HN
O

HO

O
R

O
HO

Si

O

Si

:

HO

Si

Si

O
O
Si

OH R

OH

HN

O
HN

R

Si OH
HO
OH O
Si
OH
R
Si
O

:
O

OH

Si

O
O

HO

three-dimensional PSQ network

Si
O

O
O

ureidosiloxane-based periodic structure

Figure 4.1. Schematic explanation of the structure of HETESPU-BTESPA copolymer
membrane.
澳Results and discussion
Membrane preparation
Mixtures of BTESPA: HETESPU = 90:10, 75:25, and 50:50 were subjected tohydrolysis/copolymerization via sol-gel process to yield sols PAHE10, PAHE25, andPAHE50, respectively. The particle size distribution of these sols is presented in Figure
4.2(a). The sol sizes were well controlled into the range of 1.5 to 2 nm, which aresuitable for coating on a polysulfone support membrane as described in Chapter 1.
Thermogravimetric analysis (TGA) was performed to determine the calcination
temperature for membrane formation on the support membrane. The TGA curves ofthese gel powders are presented in Figure 4.2(b). The gradual weight loss below 200 °C
would be ascribed to the liberation of water adsorbed and generated by the dehydrationof Si-OH groups giving Si-O-C and Si-O-Si bonds. At higher temperatures, thedecomposition of organic units would take place, resulting in the larger weight loss.
Based on these results, the calcination temperature of the membranes was set at 150 °C.

澳56

FT-IR spectral analyses were performed to obtain information on polymer gel
formation. Coating the sol solutions on a silicon wafer and heating the sol films at
150 °C in nitrogen gave the gel films. The FT-IR spectra are presented in Figure 4.2(c).
The broad Si-O-Si stretching bands were seen at 1065 cm-1 . With regard to the urea
units, peaks attributed to C-N and C=O stretching appeared at 1265 cm-1 and 1645 to
1635 cm-1, respectively [5]. The band assignable to N-H bending was apparent at 1570
cm-1. Compared with the standard absorption values of the C=O peak (1760 to 1670
cm-1) and the N-H peak (1650 to 1580 cm-1), these peaks appeared at lower
wavenumbers, most likely because of the hydrogen bonding of the N-H---O=C and/or
OH---O=C units [6].

Figure 4.2. (a) Dynamic light scattering (DLS) plots for particle size distribution. (b)TGA traces of gel powders in nitrogen. (c) FT-IR spectra of gel films on silicon wafer.
澳
澳
澳

57

SEM surface morphologies and cross-sectional topographies
The cross-sectional topographies of membranes of PAHE10, PAHE25, PAHE50,
and HETESPU and BTESPA homopolymers were examined by SEM, and the images
are shown in Figures 4.3(a) to (e). The PSQ-based membrane layer could be observedat the top with around 50 nm thickness, although the boundary of the support membrane
and the PSQ-based membrane could not be clearly seen. Typically, the membranethickness is inversely related to the water permeability, a thin PSQ layer tends toincrease the permeability. However, when a PSQ-based membrane layer is too thin, adrastic decrease in NaCl rejection may occur, due to insufficient diffusion control andsieving effect. The SEM surface images of the membranes are shown in Figures 4.3 (f)to (j). The membranes prepared from PAHE10 and PAHE25 exhibited a smooth surface,indicating the formation of fine, homogeneous, and defect-free PSQ-based separation
layers, similar to that exhibited by the BTESPA homopolymer membrane (Figure 4.3(j)),
although particulates were noticeable on the surface, likely due to the aggregation of
HETESPU units during gelation. The excess content of HETESPU led to the formationof pinholes (Figures 4.3(h) and 3(i)). The pinhole depth curve of the PAHE50-based
membrane was examined by AFM and shown in Figure 4.4. Pinhole depth wasapproximately 10 to 12 nm, and pinhole diameter was approximately 500 nm,
consistent with those observed in the SEM images. It seemed that these pinholes
contributed to inferior water separation performance (vide infra).
澳Figure 4.3. Cross-sectional topographies of membranes of (a) PAHE10, (b) PAHE25 (c)PAHE50, and (d) HETESPU and (e) BTESPA homopolymers, and surface
morphologies of membranes of (f) PAHE10, (g) PAHE25, and (h) PAHE50, and (i)HETESPU and (j) BTESPA homopolymers, determined by SEM
澳58

Depth [nm]

45
40
35
30
25
20
15
10
5
0
0

200

400

600

800

1,000

1,200

1,400

Distance [nm]澳Figure 4.4. Pinhole depth curve of PAHE50-based membrane analyzed by AFM.
澳
AFM topographical and phase images of these membranes' surfaces based on
BTESPA, PAHE10, PAHE25, and PAHE50 are shown in Figure 4.5. The BTESPAbased membrane exhibited a smooth surface in both topographical and phase images.
In the phase images, as HETESPU content increased, brightly colored aggregates
gradually appeared (Figures 4.5(e) to (h)), suggesting that HETESPU tended toaggregate during the membrane formation. AFM hardness images also provided
evidence of the aggregate formation on these membrane surfaces (Figure 4.6). AsHETESPU content increased, more bright spots could be seen on the AFM hardness
images which are likely due to aggregates. The average roughness (R a) of thecopolymer membranes was larger than that of the membrane based on BTESPA
homopolymer, in agreement with the formation of aggregates.

澳

59

(e)
(a)

[nm]
2
0.0

0

0.5

0.5

Ra = 0.18 nm

1.0

100 nm

1.0

[μm]

(b)

(f)

[nm]
5
0.0

0

0.5

0.5

Ra = 0.55 nm

1.0

100 nm

1.0

[μm]

(g)

(c)

[nm]
10
0.0
0

0.5

0.5

Ra = 0.64 nm

1.0

100 nm

1.0

[μm]

(d)

(h)

[nm]
10
0.0
0

0.5

0.5

Ra = 0.46 nm

1.0

100 nm1.0[μm]

Figure 4.5. AFM topographical images of membranes of (a) BTESPA, (b) PAHE10,(c) PAHE25, and (d) PAHE50, and phase images of membranes of (e) BTESPA, (f)PAHE10, (g) PAHE25, and (h) PAHE50.

Figure 4.6. AFM hardness images of membranes (a) BTESPA, (b) PAHE10, (c)PAHE25, and (d) PAHE50.
澳

60

Porous properties
Nitrogen adsorption/desorption measurements of PAHE10, PAHE25, and PAHE50
copolymer and BTESPA homopolymer gels were performed to reveal their porous
properties. In general, the increased porosity is proportional to water permeability. The
nitrogen adsorption/desorption isotherms of these gel powders are shown in Figure 4.7.
Compared with the surface area of 176 m2/g for the reference sample of BTESPA
homopolymer gel, the surface area was 237 m2/g for PAHE10 and 15.0 m2/g for
PAHE25, whereas the PAHE50-derived gel was non-porous. Suggesting that a small
amount of HETESPU increased the porosity, However, an excess amount of HETESPU
made the porosity decrease. Although the reason for porosity changes depending on the
HETESPU content is still unclear, probably due to enhanced aggregation by hydrogenbonding of HETESPU units would increase the porosity, but excessive aggregation
would lead to shrinkage of the gel decreasing the porosity. A similar tendency wasobserved for surface roughness determined by AFM measurements (Figure 4.5), which
increased with increasing the HETESPU content from BTESPA homopolymer to
PAHE25-derived membrane but decreased for PAHE50-derived membrane. These
results contrasted the results for the PAHE25- and PAHE50-derived membranes, which
exhibited extremely high water permeability (see next section). The author presumed
that the enhancement of hydrophilicity by increasing HETESPU content was
responsible for the improvement of water permeability, as described below.

澳61

Figure 4.7. N2 adsorption/desorption isotherms of PAHE10, PAHE25, PAHE50, and
BTESPA gel powder.
Table 4.1. Surface areas and micropore volumes.
SampleBET surface area SBET (m2/g)

Micropore volume VP (cm3/g)

BTESPA
PAHE10PAHE25
PAHE50176
237
15
non-porous

0.17620.20530.0180non-porous澳RO performance
RO performance of PAHE10-, PAHE25-, PAHE50-, and HETESPU homopolymerbased membranes were summarized in Figure 4.8(a). The RO measurement was carriedout under the feed pressure of 1.5 MPa at room temperature with a NaCl aqueous
solution (2000 ppm). The HETESPU homopolymer membrane exhibited the highest
water permeability (2.6 × 10-12 m3/m2sPa) among the membranes, with a NaCl rejectionof 91.1%. However, the HETESPU-homo-based membrane was unstable. After an 8-h
RO experiment, water permeability was increased to 4.7 × 10-12 m3/m2sPa and NaCl
rejection was drastically decreased from 91.1% to 75.8%. The high water permeability
was probably attributed to the high hydrophilicity by the introduction of urea and澳

62hydroxy groups, as shown in the FT-IR spectrum in Figure 4.2(c). In addition, it was
hypothesized that the aggregation of HETESPU units formed a hydrophilic channel forwater permeation as presented in Figure 4.1. However, the HETESPU homopolymerbased membrane possessed pinholes on the surface (Figure 4.3(h)), which was likely
due to excessive aggregation of HETESPU. This seemed to be also responsible for the
relatively high permeability of water and low NaCl rejection, in comparison with the
BTESPA homopolymer membrane (water permeability = 1.1 × 10-13 m3/m2sPa, NaCl
rejection = 96%) [7]. For membranes that were prepared from the copolymerization,
the PAHE25-based membrane exhibited the best RO performance with high water
permeability of 1.86 × 10-12 m3/m2sPa and the NaCl rejection of 95.9%, exceeding the
RO performance of PSQ-based RO membranes, so far reported previously, assummarized in Figure 4.8(c). Indeed, the PAHE25-based membrane exhibited
approximately 19-fold higher water permeability than the BTESPA homopolymer
membrane, whereas their NaCl rejection values were nearly the same. Further increaseof HETESPU content to 50% resulted in a slight increase in water permeability,
although NaCl rejection was decreased to 82.2%. The aggregation of HETESPU units
was expected to increase water permeability through the formation of hydrophilic water
channels, as described above. However, excessive aggregation resulted in the formationof pinholes, which decreased NaCl rejection, as in the case of the HETESPU
homopolymer membrane. Increased porosity by the copolymerization with HETESPU
may be also a reason for improved permeability compared to the BTESPA
homopolymer membrane. However, PAHE50 is non-porous, thereby being difficult to
discuss the porosity-permeability relationship of the present HETESPU-containing
membranes.
Improved water permeability can be achieved by increasing the surfacehydrophilicity of RO membranes. Figure 4.8(b) demonstrates that the surface watercontact angles of these membranes gradually decreased from 61.4° to 53.9° as the
HETESPU content increased from 0 to 50 %, which is consistent with the increase of
membrane permeability in the same order, due to the higher hydrophilicity of澳63

HETESPU with urea and hydroxy units than BTESPA with an amine unit. Membrane
porosity, surface hydrophilicity, and aggregation forming hydrophilic channels always
played important roles in affecting the water permeability of the present membranes.
The porosity of the PAHE10-based membrane seemed to be increased compared to
BTESPA homopolymer membrane, judging from the higher BET surface area of the
PAHE10-based gel powder than that of the BTESPA homopolymer (Table 4.1). In
addition, the smaller water contact angle indicated enhanced surface hydrophilicity. For
the PAHE25-based membranes, their porosity seemed to be suppressed, buthydrophilicity was further enhanced overcoming the suppressed porosity, which lead toan even more enhanced water permeability than that of PAHE10-based membrane. To
understand the effects of BTESPA, another precursor BTESE was also used for the
copolymerization with HETESPU, and the RO performance is summarized in Table 4.2.
As shown in Table 4.3, water permeability was significantly improved as the HETESPU
content increased. However, at the same time, NaCl rejection was markedly lowered,
due to the pinholes on the surface (Figure 4.9). This might be due to the lack ofhydrogen bonding between comonomers BTESE and HETESPU, which leads to poor
miscibility between them. Controlling the hydrogen bonding between and withincomonomers is essential to obtaining high-performance membranes using HETESPU.
Expecting to obtain the urea-derived membranes with even higher performance, theauthor prepared N-[2,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea
andN-[1,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea

as

new

monomers. Trimethylsilyl units were used to protect hydroxyl groups to induce
synthesis to proceed, and these trimethylsilyl units could be easily removed byhydrolysis of the trimethylsiloxy units during the SG process for the formation ofhydroxy units. Membranes were prepared by the copolymerization of these precursors
with BTESPA in the weight ratios of 1 to 3 by the same process as that for PAHE-based
membranes. The sample names and the RO performance are summarized in Table 4.3.
The gel powders prepared from PAHE2-25 and PAHE2’-25 exhibited similar porosity
to the PAHE25-based gel powders (Figure 4.10). The addition of one hydroxyl group澳

64

Table 4.2. Summary of RO performance.
MembraneLiquid permeance/(m/s·Pa)
NaCl rejection [%]-12
HETESPU
2.6 × 1091.1
BTESE[a]1.0 × 10-13
98.0
[b]
-12
BTHE5
4.1 × 10
86.7
BTHE10[b]4.8 × 10-12
82.2
[b]
-12
BTHE25
3.3 × 1082.2
[a] data from a literature [8][b] Membranes prepared from copolymerization of BTESE/HETESPU with theratio of 95:5, 90:10 and 75:25.
did not significantly increase water permeability, namely, the PAHE2-25-based RO
membrane exhibited a slight improvement in water permeability to 1.94 × 10 -12
m3/m2sPa compared with the PAHE25-based membrane (1.86 × 10-12 m3/m2sPa), withincreased NaCl rejection to 96.8% from 95.9%. In contrast, the membrane based on
PAHE2’-25- showed lower permeability and lower rejection than the PAHE25-based
membrane.
澳
Table 4.3. Summary of RO performance of PAHE25-, PAHE2-25-, and PAHE2'-25based membranes.
NaCl
Precursors for copolymerization with Sample Water
rejection
BTESPA in the ratio of 1:3
namepermeability/(m/s·Pa)
[%]
O
N
H

(EtO)3Si

N
H

OH

PAHE2
5

1.86 × 10-12

95.9

OTMS

PAHE2
-25

1.94 × 10-12

96.8

PAHE2
’-25

1.04 × 10-12

88.9

O(EtO)3Si

N
H

N
H
OTMS
OTMS

O

澳
(EtO)3Si

澳

N
H

N
H

OTMS

65

Figure 4.8. (a) RO performance of HETESPU-, PAHE10-, PAHE25-, PAHE50-, and
BTESPA-based membranes. (b) Water contact angles of HETESPU-, PAHE10-,
PAHE25-, PAHE50-, and BTESPA-based membranes. (c) Plots of RO performance ofthe present membranes, and those of BTESE, BTESE2, BTESE3, BTESPA, BT-HM50,
BT2-HM20, 2,5-bis[2-(triethoxysilyl)ethyl]-1,4-dioxane (BTESED) and 1,4-bis(triethoxysilylmethyl)1,2,3-triazole (BTESMAz) from the literature [1, 2, 7, 9, 10]. (d)RO measurement of PAHE25- and HETESPU-based membranes for 24 h.
澳澳澳澳Figure 4.9. Surface morphologies SEM images of membranes (a)HETESPU, (b)BTHE25 and (c) BTESPA
澳

澳

66

澳
Figure 4.10. N2 adsorption/desorption isotherms of PAHE2-25 and PAHE2’-25-based
gel powder.
澳Previously, membrane preparation was carried out by the copolymerization of
BTESE and BTESE2 with (hydroxymethyl)triethoxysilane (HMTES) to enhance thewater permeability of PSQ-based RO membranes (Scheme 4.1) [1, 2]. Figure 4.8(c)shows the RO performance of various membranes with optimum copolymer ratio. The
copolymer membranes with the BTESE/BTESE2:HMTES ratio of 50:50 (BT-HM50
and BT2-HM50) had water permeability of 3.4 × 10-13 m3/m2sPa and 5.0 × 10-13
m3/m2sPa and rejection of 95.5% and 92%, respectively. Thus, the introduction ofhydroxyl groups into BTESE and BTESE2 through the copolymerization with HMTES
improved water permeability by approximately three and seven times, respectively, andslightly decreased NaCl rejection compared with BTESE and BTESE2 homopolymer
membranes. Throughout the RO experiments, these membranes showed a gradualincrease in water permeability and reached constant values, as shown in Figure 4.8(c).
The mechanism is outlined in Scheme 4.1. Dehydration condensation reaction occurred
during gelation, Si-OH groups forming Si-O-Si linkages and Si-OH and C-OH groups
generating Si-O-C bonds. The hydrolysis of Si-O-C bonds occurs more easily than SiO-Si linkages, and the hydrolysis of the Si-O-C bonds took place to form C–OH and
Si-OH bonds during the RO experiment, which was responsible for the enhancementof permeability and the slight decrease of rejection. However, it was found that the澳67water permeability of these HETESPU-derived membranes was stable even when themembranes were immersed in water. As shown in Figure 4.8(d), no clear changes wereobserved in the PAHE25-based membrane in water permeability and NaCl rejection
even after RO experiments for 24 h. The IR spectra of the PAHE25-based gel film also
exhibited no significant changes upon soaking in water for 12 h and then dried at 100 °C(Figure 4.11). This was likely due to the structural rigidity of the HETESPU-derived
gels as a result of hydrogen bonding. The rigid structure hindered the dehydration
condensation of the C-OH units with the Si-OH units to form C-O-Si linkages during
the gelation.澳 However, the HETESPU homopolymer membrane might be highlyhydrophilic and readily underwent hydrolysis changing the membrane performance aspresented in Figure 4.8(d). The author speculated that the high water flux in the pinholes
would accelerate the hydrolysis and liberation of monomeric and/or oligomericmolecules from the membrane.

Scheme 4.1. Formation and dehydration of C-O-Si units in BT-HM and BT2-HM
membranes.

澳Figure 4.11. FT-IR spectra of PAHE25 based films on repetitive process of calcination
at 250 and 300 °C, and soaking in water and drying at 100 °C.
澳68

Thermal stability
To study the thermal property of the PAHE25-based membrane, the thermal stability
was investigated and presented in Figure 4.12. PAHE25-based membrane exhibited
excellent thermostability up to 60 °C. At 60 °C, NaCl rejection slightly decreased from
95.9% to 92.8%, but lowering the temperature to room temperature recovered NaCl
rejection to 94.5%. Water permeability was recovered as well. However, when theoperating temperature was further raised to 80 °C, the decreased NaCl rejection could
not be recovered even on cooling the temperature to room temperature, indicating
membrane damage partly (Figure 4.13). PAHE25-based membrane exhibited higher
thermal stability than that of commercially available polyamide-based membranes
whose upper operating temperature limit is 45 °C [11]. The high thermal stability was
attributed to the robust Si-O-Si linkage. In addition, strong hydrogen bonding of
HETESPU is also anticipated to stabilize the structure by immobilization.
Unfortunately, the PAHE25-based membrane did not show high chlorine resistanceeven though it contained robust Si-O-Si linkage. The chlorine exposure experiment was
performed at room temperature by using a 1000 ppm NaOCl aqueous solution. On
exposure up to 5000 ppm·h, NaCl rejection slightly decreased from 95.8% to 90.1%
but markedly decreased from 90.1% to 76.0% after further 5000 ppm·h exposure,although water permeability remained stable without significant changes (Figure 4.14).

澳

69

澳
Figure 4.12. Thermal stability test for PAHE25-based membrane by using NaClaqueous solution (2000 ppm).

澳Figure 4.13. Thermal stability measurement by using 2000 ppm NaCl aqueous solution.
澳

澳

70

100
90
80

10-11 070

10-12 0

Rejection [%]Liquid Permeance Lp [m3 l/m2·s·Pa]

10-10 06050

10-13 0
0

2500
5000
7500
Chlorine exposure /ppm‫ڄ‬h

40
10000

Figure 4.14. Chlorine exposure measurement of PAHE25-based membrane by using
1000 ppm NaClO aqueous solution.
澳
Summary
In this chapter, the author have discussed a strategy to introduce hydroxyethyl urea
groups to PSQ-based RO membranes to increase the permeability of water. The
hydroxyethylurea precursor tended to aggregate through hydrogen bonding during thegelation, which can be regarded as a hydrophilic water channel that significantly
increased the permeability of the RO membranes. PAHE25- and PAHE2-25-based
membranes exhibited excellent permeability of water of 1.86 × 10-12 and 1.94 × 10-12
m3/m2sPa, and NaCl rejection of 95.9 and 96.8%, respectively, suggesting that the RO
performance was as good as that of commercial organic RO membrane SW30HR (1.1
× 10-12 m3/m2sPa and 98.5 ± 0.7%). It was also demonstrated that the membrane
exhibited much higher thermal stability and chlorine resistance than polyamide-based
membranes.
Experimental
General澳Ethanolamine (>99% purity) and (3-isocyanatopropyl)triethoxysilane ( 95% purity)
were obtained from TCI Co., Ltd. BTESPA (95% purity) was obtained from Gelest Inc.
These reagents were used as obtained. Dichloromethane employed as the reaction
solvent was distilled from CaH2 and stored over activated molecular sieves in the dark
澳71

until use. Polysulfone support membranes (NTR-7450, pore size: 1 nm) were obtained
from Nitto Denko Corporation. NMR spectra were obtained on Varian System 400MR
and system500 spectrometers (Figure 4.15 to 20). DLS analysis was performed on a
Malvern Zetasizer Nano (ZEN3600) analyzer. TGA was performed on an SII EXSTARTG-DTA6200 thermal analyzer under a gentle nitrogen flow from 25 °C to 1000 °C.
Water contact angles were determined using a Kyowa DM300 contact angle meter.
(SEM and AFM images were obtained on an S-4800 HITACHI scanning electron
microscope and an AFM-5500 HITACHI atomic force microscope, respectively.
澳Synthesis of N-(2-hydroxyethyl)-N’-(3-(triethoxysilyl)propyl)urea 澳
In a two-necked flask fitted with a magnetic stirrer was placed a solution of 0.30 g(4.8 mmol) of ethanolamine in 15 mL of dichloromethane in a dry argon atmosphere.
To

this

was

added

a

solution

of

0.80

g

(3.2

mmol)

of

(3-

isocyanatopropyl)triethoxysilane in 15 mL of dichloromethane at room temperature,and the resultant mixture was stirred for 0.5 h at this temperature. A colorless liquid (1g, > 99% yield) was obtained by evaporation of the solvent and excess ethanolamine invacuo, whose NMR analysis indicated sufficient purity for use in the following reaction
without further purification. 1H NMR (400 MHz in CDCl3) δ 5.04 (s, 1H), 4.90 (s, 1H),
3.81 (q, 6H, J = 7.0 Hz), 3.58 (t, 2H, J = 4.7 Hz), 3.30 (q, J = 5.6 Hz, 2H), 3.15 (q, J =7.0 Hz, 2H), 1.61 (m, 2H), 1.22 (t, J = 7.0 Hz, 9H), 0.68–0.60 (m, 2H). 13C NMR (126
MHz in CDCl3) δ 159.78, 62.52, 58.34, 43.01, 42.82, 23.55, 18.15, 7.59. MS m/z 247(M+ -NHCH2CH2OH).
澳
Synthesis of N-[2,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea andN-[1,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea 澳
In a three-necked flask equipped with a magnetic stirrer 1.00 g (11.0 mmol) of 3amino-1,2-propandiol was dissolved in 30 mL of nitromethane in a dry argon
atmosphere at room temperature. To this was added 0.88 g (5.5 mmol) ofbis(trimethylsilyl)amine and the mixture was stirred for 30 min. After that solvent was澳

72removed under vacuum, 2.71 g (11.0 mmol) of (3-isocyanatopropyl)triethoxysilane in
10 mL dry ethanol was subsequently added and stirred for 2 days at room temperature.
The solvent was removed under vacuum and the residue was purified by preparative
GPC

to

give

a

mixtureofN-[2,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea and its hydrolysis product containing -OTMS groups in
60.3 % yield. 1H NMR (500 MHz, CD3OD) δ 3.81 (q, 6H, J = 7.0 Hz), 3.66-3.43 (m,5H), 3.09 (t, 2H, J = 7.3 Hz), 1.59-1.52 (m, 2H), 1.20 (t, 9H, J = 7.0 Hz), 0.62-0.59 (m,2H), 0.14-0.11 (m, 18H). 13C NMR (100 MHz, CD3OD) δ 161.53, 72.70, 65.72, 59.46,
43.98, 43.67, 24.75, 18.67, 8.42, 0.32. MS m/z 482 (M+).

N-[1,3-bis(trimethylsiloxy)propyl]-N’-[3-(triethoxysilyl)propyl]urea was synthesized
by a similar process using 2-amino-1,3-propandiol. The mixture was purified by
preparative GPC with 80.4 % yield. 1H NMR (400 MHz, CD3OD) δ 3.81 (q, 6H, J =7.0 Hz), 3.73-3.53 (m, 5H), 3.09 (t, 2H, J = 7.0 Hz), 1.62-1.49 (m, 2H), 1.20 (t, 9H, J =7.0 Hz), 0.60 (t, 2H, J = 8.4 Hz), 0.11 (s, 18H). 13C NMR (100 MHz, CD3OD) δ 160.62,
62.00, 59.43, 53.52, 43.51, 24.76, 18.69, 8.44, 0.55. MS m/z 482 (M+).
澳
SG process for membrane preparation and RO experiments澳
The SG process and membrane preparation process was conducted as described inchapter 1, The measurement method of two RO performance parameters, water
permeability (Lp) and salt rejection (R), is the same as described in Chapter 1, whichwere determined by using equations (1) and (2) [12], respectively.
‫ܮ‬௣ ൌ ‫ܬ‬௩ Ȁሺο െ οɎሻ஼೛ܴ ൌ ൬ͳ െ ஼ ൰ ൈ ͳͲͲΨ
೑

澳

(1)
(2)

73

澳
Figure 4.15. 1H NMR spectra of HETESPU濁澳Figure 4.16. 13C NMR spectra of HETESPU濁澳

澳

74

澳
1

Figure 4.17. H NMR spectra of N-[2,3-bis(trimethylsiloxy)propyl]-N’-[3(triethoxysilyl)propyl]urea.澳

澳
13Figure 4.18. C NMR spectra of N-[2,3-bis(trimethylsiloxy)propyl]-N’-[3(triethoxysilyl)propyl]urea.澳

澳75

H NMR
Figure 4.19.
(triethoxysilyl)propyl]urea.

spectra of澳
N-[1,3-bis(trimethylsiloxy)propyl]-N’-[3-Figure 4.20. 13C NMR(triethoxysilyl)propyl]urea.

spectra ofN-[1,3-bis(trimethylsiloxy)propyl]-N’-[3-

1

澳

76

References[1] K. Yamamoto, J. Ohshita, T. Mizumo, M. Kanezashi, T. Tsuru, Preparation ofhydroxyl group containing bridged organosilica membranes for water desalination, Sep.
Purif. Technol., 156 (2015), 396-402.
[2] F.-T. Zheng, K. Yamamoto, M. Kanezashi, T. Tsuru, J. Ohshita, Preparation ofbridgedsilicaRO

membranesfromcopolymerizationofbis(triethoxysilyl)ethene/(hydroxymethyl)triethoxysilane. Effects of ethenylene-bridgeenhancing water permeability, J. Membr. Sci., 546 (2018), 173-178.
[3] E. Licsandru, E. Petit, S. Moldovan, O. Ersen, M. Barboiu, Biomimetic
Autocatalytic Synthesis of Organized Silica Hybrids, Eur. J. Inorg. Chem., 2015 (2015),
3637-3641.
[4] L.-B. Huang, A. Hardiagon, I. Kocsis, C.-A. Jegu, M. Deleanu, A. Gilles, A. van derLee, F. Sterpone, M. Baaden, M. Barboiu, Hydroxy Channels–Adaptive Pathways for
Selective Water Cluster Permeation, Journal of the American Chemical Society, 143(2021), 4224-4233.
[5] T. Kamitani, A. Ishida, H. Imoto, K. Naka, Supramolecular organogel of polyureas
containing POSS units in the main chain: dependence on the POSS and comonomer
structures, Polym. J., (2021), 1-7.
[6] D.S. Bergsman, R.G. Closser, C.J. Tassone, B.M. Clemens, D. Nordlund, S.F. Bent,
Effect of backbone chemistry on the structure of polyurea films deposited by molecularlayer deposition, Chem. Mater., 29 (2017), 1192-1203.
[7] K. Yamamoto, S. Koge, K. Sasahara, T. Mizumo, Y. Kaneko, M. Kanezashi, T. Tsuru,
J. Ohshita, Preparation of bridged polysilsesquioxane membranes from bis[3(triethoxysilyl)propyl]amine for water desalination, Bull. Chem. Soc. Jpn., 90 (2017),
1035-1040.
[8] R. Xu, S.M. Ibrahim, M. Kanezashi, T. Yoshioka, K. Ito, J. Ohshita, T. Tsuru, Newinsights into the microstructure-separation properties of organosilica membranes withethane, ethylene, and acetylene bridges, ACS Appl. Mater. Interfaces, 6 (2014), 93579364.
澳77

[9] K. Yamamoto, H. Muragishi, T. Mizumo, T. Gunji, M. Kanezashi, T. Tsuru, J.
Ohshita, Diethylenedioxane-bridged microporous organosilica membrane for gas andwater separation, Sep. Purif. Technol., 207 (2018), 370-376.
[10] K. Yamamoto, M. Kanezashi, T. Tsuru, J. Ohshita, Preparation of bridged
polysilsesquioxane-based membranes containing 1,2,3-triazole moieties for water
desalination, Polym. J., 49 (2017), 401.
[11] M. Mänttäri, A. Pihlajamäki, E. Kaipainen, M. Nyström, Effect of temperature andmembrane pre-treatment by pressure on the filtration properties of nanofiltration
membranes, Desalination, 145 (2002), 81-86.
[12] M.L. Lind, D. Eumine Suk, T.-V. Nguyen, E.M. Hoek, Tailoring the structure ofthin film nanocomposite membranes to achieve seawater RO membrane performance,
Environ. Sci. Technol. Lett., 44 (2010), 8230-8235.

澳

78

Chapter 5 Preparation of robust and high RO performance PSQ-RO membranes
modified by SiO2 nanoparticles
Introduction
In this chapter, as a new strategy to improve the water permeability of PSQ-RO
membranes,

theauthorintroduced

SiO2

nanoparticles

into

the

bis[3-(triethoxysilyl)propyl]amine (BTESPA)-based RO membrane. The author anticipatedthat the introduction of SiO2 nanoparticles will increase not only surface area but also
micropore volume in the PSQ separation layer by the filler effect, which will lead tothe increase of porosity and thus improve water permeability. In addition, it has beenreported that nanoparticle fillers increase the rigidity of the polymer chain through theinteraction between the nanoparticles and the polymer, which results in the
improvement of thermal stability [1]. The author also anticipated that the introductionof robust and rigid inorganic SiO2 nanoparticles would stabilize the organically bridged
PSQ structure by immobilization, which will increase the thermal stability of the RO
membranes.

Figure 5.1. Preparation of PSQ-RO membranes modified by SiO2 nanoparticles.
Results and discussion
Preparation of membranes by the sol-gel process
BTESPA-based membranes containing SiO2 nanoparticles (hereinafter referred toas PASiNP) were prepared by the SG process. Sols were prepared by stirring BTESPA
and hydrophobic SiO2 nanoparticle powders (10 nm, Admatechs Inc) having different
weights in 60 eq water of precursor in ethanol for 2 h at room temperature. Membranes
obtained after coating and calcination were denoted as PASiNP-X, where X represents
the weight percentage of the nanoparticles to the BTESPA precursor. BTESPA澳

79

membrane without SiO2 nanoparticles was also prepared and used as the control
membrane (hereinafter, PA membrane).
For PSQ membranes prepared by the SG process, a high calcination temperaturetends to promote the gelation, leading to a dense PSQ separation layer for highseparation selectivity. However, an excessively high-temperature results in thedecomposition of organic bridges, which leads to the loss of separation properties ofmembranes. To determine the calcination temperature, the author conducted
Thermogravimetric analysis (TGA) of these gel powders, as presented in Figure 5.2.
The initial mass loss was observed up to 100 ℃, which was likely due to the evaporationof adsorbed water in the gels. The second mass loss starting from approximately 300 ℃
to 320 ℃ for BTESPA and 350 ℃ to 400 ℃ for PASiNP gels can be ascribed to thethermal decomposition of organic units. The second mass loss percentages were
calculated based on the mass after the removal of adsorbed water at 100 ℃. BTESPA
gels exhibited approximately 33 % mass loss, and the mass loss values of PASiNP0.5,
PASiNP5, and PASiNP10 gels were 30 %, 29 %, and 25 %, respectively, suggestingsmaller second mass loss for the gels with SiO2 nanoparticles. Compared with BTESPA
gels, the PASiNP gels exhibit higher decomposition temperatures, indicating the filler
effect of SiO2 nanoparticles. On the basis of these observations, the author set thecalcination temperature at 150 ℃ for the membrane preparation, which is the uppertemperature limit for the operation of the polysulfone support membrane [2]. RO
performance of the supporting membrane before and after calcination at 150 ℃ for 10
min is summarized in Table 5.1, suggesting indeed that the calcination at thistemperature did not affect the supporting membrane properties with respect to water
separation. It was reported that calcination of the BTESPA-homopolymer sol at thistemperature showed no decomposition of the organic units and the formation of a PSQ
network composed of T2 (-Si(OSi)2OH) and T3 (-Si(OSi)3) units.
澳
澳

澳

80

Table 5.1. RO performance of supporting membrane (NTR-7450), before andafter calcination at 150 ℃ for 10 min.
NaCl rejection
Liquid permeance/(m3/m2s·Pa)[%]
Before calcination

2.4 × 10-11

13.1

After calcination

2.5 × 10-11

13.1

澳

Figure 5.2. TG curves of gel powders in nitrogen atmosphere at the heating rate of
10 ℃/min.

(b)澳

(a)澳

Figure 5.3. (a) Surface image of membrane of PASiNP1. (b) Cross-sectional
topographies of membranes of PASiNP1
The surface morphology was examined by SEM, and the image is presented in
Figure 5.3(a). A smooth surface on SEM scale was observed without any significant
aggregation, although these membranes exhibited roughness in the Atomic force澳

81

microscopy (AFM) scale. The cross-sectional morphologies of PASiNP membranes
were also examined by SEM, and the image of PASiNP1 membrane is presented in
Figure 5.3(b) as a typical example. The border between the PSQ layer and the support
layer cannot be clearly seen in the image, indicating good adhesion between them. The
AFM structure images and the phase images of the PA and PASiNP membranes areshown in Figure 5.4. Surface roughness (Sa) was calculated on the basis of the AFM
structure images. The PA membrane showed a smooth surface with Sa of 0.18 nm. On
the other hand, the PASiNP0.5 membrane exhibited Sa of 0.82 nm. Further increasing
the SiO2 nanoparticle content led to a slight decrease in Sa and PASiNP10 membrane
exhibited Sa of 0.56 nm. The PA membrane also exhibited a homogeneous surface inthe phase image. However, brightly colored areas gradually appeared in the phase
images when SiO2 nanoparticles were introduced, as shown in Figures 5.4(b)–(f),indicating that SiO2 nanoparticles form hard segments on the surface. As increasing theamount of SiO2 nanoparticles on the surface, the gaps may be reduced, as illustrated in
Figure 5.4(g) through increased coverage of the surface by SiO2 nanoparticles.
However, the roughness was still small with respect to the SiO2 nanoparticle size (10nm), thereby indicating no significant aggregation of SiO2 nanoparticles formed in themembranes probably because of the good miscibility between BTESPA-derived PSQ
and SiO2 nanoparticles. It is presumable that the hydrophobic interaction between theorganic units of BTESPA and hydrophobic SiO2 nanoparticle surface is operative.
Hydrophilic interaction and/or bond formation may be also assumed between the
BTESPA-derived silanol or amine units and trace silanol groups on the hydrophobic
SiO2 nanoparticle surface. The well dispersed SiO2 nanoparticles in PSQ matrix wereobserved by high-resolution Transmission microscope (TEM) images in Figure 5.5.

澳

82

(a)

[nm]
2
0.0

0

200 nm0.5

0.5

Sa = 0.18 nm
1.0

1.0

[μm]

(b)

[nm]
5
0

0

500
500

200 nmSa = 0. 82 nm
[nm]

(c)

[nm]
5
0.0

0

200 nm0.5

0.5

Sa = 0. 73 nm
1.0

1.0

[μm]

(d)

[nm]
5
0.0

0

0.5

0.5

Sa = 0. 69 nm

200 nm1.0

1.0

200 nm

[μm]

(e)

[nm]
5
0.0

0

0.5

0.5

Sa = 0. 65 nm

200 nm
1.0

1.0

[μm]

(f)

[nm]
5
0.0

0

0.5
0.5

Sa = 0.56 nm

1.0

200 nm

1.0

[μm]

(g)

Figure 5.4. AFM structure images (left) and phase images (right) of (a) PA, (b)PASiNP0.5, (c) PASiNP1, (d) PASiNP2.5, (e) PASiNP5, and (f) PASiNP10 membranes,and (g) possible explanation for the decrease of surface roughness as increasing the
SiO2 nanoparticle content. The roughness was calculated for 1 μm × 1 μm area.

澳83

Figure 5.5. TEM images of (a) PASiNP2.5 and (b)PASiNP10 gels.

Porous properties
In general, water permeability of RO membranes was significantly affected bymembrane porosity, and porous membranes exhibit high water permeability. To
investigate the effect of the introduced SiO2 nanoparticles on porosity, nitrogen
adsorption/desorption isotherm measurements of these gel powders of PA and PASiNP
were performed. The respective gel powders were prepared by drying the sols at 60 °C
in air for 2 days, followed by calcination at 150 °C in nitrogen for 12 h. The results areshown in Figure 5.6. All of these gel powders showed the nitrogen isotherms of type II
characteristics. The PASiNP gel powders exhibited interaction with nitrogen even in the
low-pressure region, which was increased as SiO2 nanoparticle content increased,suggesting an increase in micropore volume. The BET surface areas and the micropore
volumes of these gel powders are listed in Table 5.2. The BET surface area of the
BTESPA gel powder was 176 m2/g. On the other hand, the BET surface areas of the
PASiNP gel powders exhibited an increase with increasing SiO2 nanoparticle content
and peaked at 500 m2/g, which was approximately three times higher than that of the
BTESPA gel powder. Although the BET surface area was decreased by the further
introduction of SiO2 nanoparticles (PASiNP10), it was still higher than that of the
BTESPA gel powder. The first increase in BET surface areas was likely due to theintroduction of SiO2 nanoparticles bringing additional micropore volumes between the澳84BTESPA polymer chains and nanoparticles. However, excess nanoparticles will bepacked and leading to a decrease in BET surface area. The results of RO experiments
were in agreement with these observations with respect to water permeability. (videinfra)
300
PASiNP5
PASiNP2.5
PASiNP1

Volume adsorbed [cm3(STP)/g]

250
200

PASiNP0.5
PASiNP10150
BTESPA

100
50
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Relative pressure p / p0

Figure 5.6. N2 adsorption/desorption isotherms of PASiNP0.5, 1, 2.5, 5, and 10 gel
powders and BTESPA gel powder.

Table 5.2. Surface areas and micropore volumes.
SampleBET surface area (m2/g)

Micropore volume (cm3/g)

BTESPA176

0.1762PASiNP0.5385

0.2909PASiNP1485

0.3996

PASiNP2.5498

0.4173PASiNP5500

0.4366

PASiNP10275

0.2924

RO performance
A summary of the RO performance is shown in Figure 5.7. RO experiments of PA
and PASiNP membranes were carried out using a 2000 ppm NaCl aqueous solution澳85

under the feed pressure of 1.5 MPa at room temperature. The PA membrane exhibitedthe lowest water permeability of 1.3 × 10-13 m3/m2sPa among these membranes and thehighest water permeability of 1.2 × 10-12 m3/m2sPa was obtained for the PASiNP2.5
membrane, which is approximately 10 times higher than that of the PA membrane. As
the author used hydrophobic SiO2 nanoparticles, the membrane hydrophilicity would
have been rather suppressed. However, the water contact angles of the PASiNP castfilm surfaces prepared on glass substrates were lower than that of the PA film (Figure5.8). The improved porosity seems to be also a reason for the high water permeability.
In addition, salt rejection exhibited a slight increase as a result of the introduction of
SiO2 nanoparticles. Compared with the PA membrane which exhibited a salt rejectionof 95.8%, the PASiNP0.5 membrane showed a salt rejection of 96.6%. On the other
hand, the PASiNP1, 2.5, and 5 membranes exhibited increased salt rejection of 98.2%,
97.8%, and 98.1%, respectively. However, both liquid permeability and salt rejection
were decreased by the further increase in SiO2 nanoparticle content, namely, the
PASiNP10 membrane exhibited liquid permeability of 6.5 × 10-13 m3/m2sPa and salt
rejection of 97.1%. A similar trend was reported for silica-poly(dimethylsiloxane)
composite membranes with respect to gas separation, which showed an increase of gas
permeance at a low loading ratio of silica up to 15 wt% and then a decrease at 20 wt%.
The author also prepared a membrane with 1wt% of hydrophilic SiO2 nanoparticles
under the same conditions as those for PASiNP1 called PASiNP1’, except for using
methanol/ethanol mixed solvent instead of ethanol for the sol formation. The membranewater permeability was 1.12× 10-12 m3/m2sPa as high as that of PASiNP1. However, thesalt rejection was 94.0 %, which is lower than that of PASiNP1. It seems likely that thelower salt rejection may be due to the poor miscibility of hydrophilic SiO2 nanoparticles,
which cannot form a homogeneous separation layer by aggregation in the PSQ
separation layer.

澳86

Figure 5.7. RO performance of PASiNP0.5, 1, 2.5, 5, and 10 membranes and PA
membrane.

Figure 5.8. Water contact angle of membranes PASiNP0.5, 1, 2.5, 5, 10 and BTESPA.

Resistance to chlorine and thermal stability
For practical seawater separation, sodium hypochlorite (NaOCl) is widely used in
the disinfection process to remove biofilms [3, 4]. To investigate the chlorine resistanceof the PASiNP membranes, the author performed chlorine resistance experiments. The
membranes were immersed in 1000 ppm NaOCl aqueous solution for 10 h at room
temperature. The PA membrane demonstrated excellent resistance to chlorine: no
obvious changes in water permeability and salt rejection were apparent up to 10,000ppm‫ڄ‬h NaClO exposure, as presented in Figure 5.9. The PASiNP membranes also
exhibited favorable resistance to chlorine exposure; salt rejection decreased by only

澳87

0.2–1.8% after 10,000 ppm‫ڄ‬h NaClO exposure, which indicates that the introduction of
SiO2 nanoparticles has no significant influence on chlorine resistance.
In addition, thermal stability measurements of the PA and PASiNP membranes werecarried out with a stepwise increase in feed temperature from 25 to 95 ℃, in which thefeed temperature was kept at each temperature for 3 h. The results are shown in Figure
5.10. Compared with the water permeability and the salt rejection at 25 ℃, water
permeability of the PA membrane was increased by approximately 9-fold at 95 ℃. Thismay be due to the thermal vibration of the polymer network. At the same time, salt
rejection was decreased by 4.7% from 95.8% to 91.1%. Upon lowering the operation
temperature again to room temperature, the performance could not be recovered asshown in Figure 5.13. This seems to indicate that performance changes at elevatedtemperature for PA membrane is due to not only enhanced thermal vibration of thepolymer network, but also degradation of the polymer structure. PASiNP membranes
exhibited much higher thermal stability with smaller changes in the performance,
showing a decrease in salt rejection less than 2.5% and an increase of water
permeability by approximately two times as the operating temperature was elevated to
95 ℃. When the temperature was lowered to room temperature, salt rejection wasrecovered, although the water permeability was decreased to an extent (3.8 × 10-13
m3/m2sPa), as illustrated in Figure 5.11, in contrast to the PA membrane. Compared toother high-performance PSQ-RO membranes reported previously with water
permeability and salt rejection higher than 1 × 10-12 m3/m2sPa and 95%, respectively[5, 6], the PASiNP2.5 membrane exhibits the highest thermal stability. Other highperformance PSQ-RO membranes exhibited thermal stability only up to 70-80 ℃ [5,6]. To further investigate the thermal stability of the membranes, the author examined
the performance changes of PASiNP1 membrane on repetitive heating and coolingcycles from room temperature to 95 ℃. As presented in Figure 5.12, the PASiNP1
membrane exhibited gradual decreases in both water permeability and salt rejection.
However, it is still noteworthy that the membrane showed only slight decreases in theseparameters even after heating at 95 ℃ and reached stable states after the third cycle.
澳88The improvement in thermal stability of these modified membranes seems to be dueto the robust and rigid inorganic SiO2 nanoparticles stabilizing the organically bridged
PSQ structure by immobilization, thus increasing the thermal stability of the RO
membranes. The modified membranes demonstrated high water permeability and highsalt rejection at high temperatures. Compared with commercial polyamide-based
membranes for which the operating temperature limit is 45 ℃ [7], the PASiNP
membranes show high potential for high-temperature operation.
100

15

Permeance before

95

Permeance after

Rejection Before
Rejection After

10

5

90

Rejection [%]Liquid Permeability [ 灤 10-13 m3/m2·s·Pa]

20

85

0

80

PASiNP0.5

PASiNP1

PASiNP2.5

PASiNP5

PASiNP10

BTESPA

Figure 5.9. RO performance of PASiNP0.5, 1, 2.5, 5, and 10 membranes and PA
membrane before and after 10,000 ppm‫ڄ‬h NaClO exposure.

Figure 5.10. Thermal stability measurement of (a) BTESPA, (b) PASiNP0.5, (c)PASiNP1, (d) PASiNP2.5, (e) PASiNP5, and (f) PASiNP10 membranes using 2000 ppm
NaCl aqueous solution.
澳89

Figure 5.11. Thermal stability measurement of PASiNP2.5 membrane.

Figure 5.12. Thermal stability measurement of PASiNP1 membrane, by heating up to
95 ℃ and cooling down to room temperature for 4 cycles.

澳

90Figure 5.13. Thermal stability measurement of PA membrane.

Conclusions
In conclusion, the author has modified BTESPA-based PSQ-RO membranes with
SiO2 nanoparticles. The introduction of SiO2 nanoparticles significantly increased the
BET surface area, thereby increasing membrane porosity and permeability. The
modified membranes exhibited favorable water permeability and high salt rejection.
The water permeability of 1.2 × 10-12 m3/m2sPa for the PASiNP2.5 membrane isapproximately 10 times higher than the water permeability of the membrane without
SiO2 nanoparticles and similar to that of commercially available organic RO membrane
SW30HR (water permeability 1.1 × 10−12 m3/m2sPa, salt rejection 98.5 ± 0.7%) [8]. In
addition, the PASiNP2.5 membrane exhibited a high salt rejection of 97.8%, which isslightly higher than that of the PA membrane. In addition, the modified membranes
showed excellent performance in terms of chlorine resistance and favorable thermal
stability up to 95 °C, indicating their potential application as robust membranes. This
work is the first on PSQ-RO membranes modified by SiO2 nanoparticles, which may
be applicable to other PSQ-based membranes. Despite our efforts, the author cannot
fully understand the mechanism for improved performance of BTESPA membrane by澳

91

the introduction of SiO2 nanoparticles, although interface structure between PSQ
polymer and SiO2 nanoparticles, void formation, and changes of PSQ morphology may
affect the performance as proposed for mixed matrix membranes that consist ofcomposites of base polymers and nanofillers. However, the increased water
permeability, the high salt rejection, the improved thermal stability, and the excellent
chlorine resistance clearly indicate that this method has great potential for the
preparation of PSQ-RO membranes for practical use.
Experimental
GeneralBis[3-(triethoxysilyl)propyl]amine with 95% purity was purchased from Gelest Inc.
Hydrophobic SiO2 nanoparticle powder (10 nm) was purchased from Admatechs Inc,
while hydrophilic SiO2 nanoparticle (12 nm) was obtained as a 10wt% methanol solfrom Nissan Chemical Corporation. All chemicals were used without further
purification. Ethanol used as the reaction solvent during the SG process was distilledfrom Mg and stored over activated molecular sieves in the dark until use. TGA was
conducted using an SII EXSTAR TG-DTA6200 thermal analyzer in the temperature
range of 50 °C to 1000 °C at the heating rate of 10 °C/min under a gentle nitrogen flow.
Contact angle measurements were performed with a Kyowa DM300 contact angle
meter. AFM measurements were carried out on an AFM-5500 HITACHI atomic force
microscope. Nitrogen adsorption/desorption isotherm measurements were performed
using a BELSORP-MAX Ⅱ 034VP-MZ (MicrotracBEL). TEM images were obtainedby using a JEOL model JEM-2010 microscope (200 kV) at N-BARD Hiroshima
University on samples prepared by coating copper mesh grids with PSQ gel suspensions
and then evaporation of the solvent.
澳Sol-gel process for membrane preparation and evaluation of RO performance
The SG process and membrane preparation process were conducted as described inchapter 1. The measurement method of two RO performance parameters, water澳

92

permeability (Lp) and salt rejection (R), is the same as described in Chapter 1, whichwere determined by using equations (1) and (2) [9], respectively.
(1)‫ܮ‬௣ ൌ ‫ܬ‬௩ Ȁሺο െ οɎሻ஼೛ܴ ൌ ൬ͳ െ ஼ ൰ ൈ ͳͲͲΨ
೑

(2)

References[1] J.-F. Li, Z.-L. Xu, H. Yang, L.-Y. Yu, M. Liu, Effect of TiO2 nanoparticles on thesurface morphology and performance of microporous PES membrane, Appl. Surf. Sci.,
255 (2009), 4725-4732.
[2] K. Yamamoto, S. Koge, K. Sasahara, T. Mizumo, Y. Kaneko, M. Kanezashi, T. Tsuru,
J. Ohshita, Preparation of bridged polysilsesquioxane membranes from bis[3(triethoxysilyl)propyl]amine for water desalination, Bull. Chem. Soc. Jpn., 90 (2017),
1035-1040.
[3] J. Glater, S.-k. Hong, M. Elimelech, The search for a chlorine-resistant reverse
osmosis membrane, Desalination, 95 (1994), 325-345.
[4] R. Wang, Z.-X. Low, S. Liu, Y. Wang, S. Murthy, W. Shen, H. Wang, Thin-film
composite polyamide membrane modified by embedding functionalized boron nitride
nanosheets for reverse osmosis, J. Membr. Sci., 611 (2020), 118389.
[5] D. Zhang, M. Kanezashi, T. Tsuru, K. Yamamoto, T. Gunji, Y. Adachi, J. Ohshita,
Development

of

PSQ-RO

membranes

with

high

water

permeability

bycopolymerization of bis [3-(triethoxysilyl) propyl] amine and triethoxy (3glycidyloxypropyl) silane, J. Membr. Sci., 644 (2022), 120162.
[6] D. Zhang, J. Ohshita, Preparation of robust RO membranes for water desalinationbyinterfacial

copolymerizationofbis[(triethoxysilyl)propyl]amineandbis(triethoxysilyl)ethane, Polym. J., 51 (2019), 1231-1234.
[7] P. Karami, B. Khorshidi, M. McGregor, J.T. Peichel, J.B. Soares, M. Sadrzadeh,
Thermally stable thin film composite polymeric membranes for water treatment: A
review, Journal of Cleaner Production, 250 (2020), 119447.

澳93

[8] E.S. Hatakeyama, C.J. Gabriel, B.R. Wiesenauer, J.L. Lohr, M. Zhou, R.D. Noble,
D.L. Gin, Water filtration performance of a lyotropic liquid crystal polymer membranewith uniform, sub-1-nm pores, J. Membr. Sci., 366 (2011), 62-72.
[9] M.L. Lind, D. Eumine Suk, T.-V. Nguyen, E.M. Hoek, Tailoring the structure of thinfilm nanocomposite membranes to achieve seawater RO membrane performance,
Environ. Sci. Technol. Lett., 44 (2010), 8230-8235.

澳

94

Chapter 6 Conclusions
In conclusion, the author proposed four strategies to improve the water permeabilityof PSQ-RO membranes, including the design and synthesis of new precursors, theintroduction of hydroxyl groups and hydrophilic water channels, and the composite
formation with SiO2 nanoparticles. The author also discussed the membranes
preparation process of the sol-gel process and interfacial polymerization.
In Chapter 1, the author examined TTESPA, TTESMA, TDEMSPA, TTESP2A, and
TTESP3A as new precursors for PSQ-RO membrane preparation by both interfacial
polymerization and the sol-gel process. The TTESPA-based membraneV exhibited good
RO performance of 7.3 × 10-13 m3/m2sPa and 1.7 × 10-13 m3/m2sPa water permeanceand 95.6 % and 96.6 % NaCl rejection for those prepared by the interfacial
polymerization and the sol-gel process, respectively. The difference of RO performanceof membranes prepared from sol-gel process and the interfacial polymerization has also
been discussed. The interfacial polymerization can form a sparser but thicker membranethan the sol–gel process, and as a result, water permeability is improved, but salt
rejection is slightly lowered.
In Chapter 2, the author successfully introduced epoxy groups into PSQ-RO
membranes. The epoxy groups significantly improved membrane hydrophilicity
through the formation of hydroxy groups by the ring opening reactions, thereby
increasing membrane permeability. The author conducted studies on thecondensation/hydrolysis of the C–O–Si group. The surface C–OH group content
increased as the immersion time increased, resulting in the improvement of membrane
permeability. PA-TEG2-based membrane exhibited high water permeability of 9.6 ×
10-13 m3/m2sPa and excellent salt rejection of 97.5%, which were close to those ofcommercially available polyamide RO membrane SW30HR. In addition, thismembrane showed resistance to chlorine and excellent thermal stability up to 60 °C,
indicating potential application as a robust membrane.
In Chapter 3, the author successfully introduced hydroxyethylurea groups to PSQbased RO membranes to increase the permeability of water. The hydroxyethylurea
澳95

groups tended to cause aggregation through hydrogen bonding during the gelation
process, which formed hydrophilic water channels that significantly increased thepermeability of the RO membranes. PAHE25- and PAHE2-25- based membranes
exhibited the highest RO performance so far obtained from PSQ RO membrane withpermeability of water being 1.86 × 10−12 and 1.94 × 10−12 m3/m2sPa and with NaCl
rejection being 95.9% and 96.8 %, respectively. The author also found that themembrane exhibited much higher thermal stability and chlorine resistance thanpolyamide-based membranes. This is a new molecular design of PSQ-based membranes.
The introduction of hydrogen bonding forms a hydrophilic water channel, improving
the water permeability.
In Chapter 4, the author has modified BTESPA-based PSQ-RO membranes with
SiO2 nanoparticles. The BET surface area was significantly increased after the
introduction of SiO2 nanoparticles, thereby increasing membrane porosity andpermeability. The water permeability of 1.2 × 10-12 m3/m2sPa for the PASiNP2.5
membrane is approximately 10 times higher than the water permeability of the
BTESPA-based membrane without SiO2 nanoparticles. In addition, the PASiNP2.5
membrane exhibited a high salt rejection of 97.8%, which is slightly higher than that of
the BTESPA-based membrane. In addition, the modified membranes showed excellent
performance in terms of chlorine resistance and favorable thermal stability up to 95 °C,
indicating their high potential as robust membranes.
On the basis of the above-mentioned results, all of these strategies exhibited
significant improvement in RO performance. The resultant membranes showedfavorable resistance to heat treatment and chlorine exposure, suggesting these PSQ-RO
membranes have high potential application as robust membranes.
澳

澳

96

List of publications
Chapter 2.
Preparation of polysilsesquioxane reverse osmosis membranes for water desalinationfrom tris [(ethoxysilyl) alkyl] amines by sol–gel process and interfacial polymerization.
Dian Zhang, Masakoto Kanezashi, Toshinori Tsuru, Kazuki Yamamoto, Raku Yakuwa,
Takahiro Gunji, Yohei Adachi, Joji Ohshita
Applied Organometallic Chemistry, 2021, 35(11), e6374.
Chapter 3.
Developmentof

copolymerization

PSQ-ROofmembranes

with

high

water

bis[3-(triethoxysilyl)propyl]amine

andpermeability

bytriethoxy(3-glycidyloxypropyl)silane
Dian Zhang, Masakoto Kanezashi, Toshinori Tsuru, Kazuki Yamamoto, Takahiro Gunji,
Yohei Adachi, Joji Ohshita
Journal of Membrane Science, 2022, 644, 120162.
Chapter 4.
Development of Highly Water-Permeable Robust PSQ-Based RO Membranes byIntroducing Hydroxyethylurea-Based Hydrophilic Water Channels
Dian Zhang, Masakoto Kanezashi, Toshinori Tsuru, Kazuki Yamamoto, Takahiro Gunji,
Yohei Adachi, Joji Ohshita
ACS Applied Materials & Interfaces, 2022, 14(18), 21426-21435
Chapter 5.
Development of robust and high-performance polysilsesquioxane reverse osmosis
membranes modified by SiO2 nanoparticles for water desalination
Dian Zhang, Masakoto Kanezashi, Toshinori Tsuru, Kazuki Yamamoto, Takahiro Gunji,
Yohei Adachi, Joji Ohshita
Separation and Purification Technology, 2022, 296, 121421
澳This doctoral thesis includes contents reproduced from the papers listed above withpermission of the publishers.

澳

97Other papers not included in this thesis

澳
澳
澳
澳
澳
澳
Preparation of robust RO membranes for water desalination by interfacial
copolymerization of bis[(triethoxysilyl)propyl]amine and bis(triethoxysilyl)ethane
Dian Zhang, Joji Ohshita
Polymer Journal, 2019, 51(11): 1231-1234.
澳

澳

98

Acknowledgements
This thesis is a summary of my studies from 2019 to 2022 under the direction ofProfessor Joji Ohshita at Hiroshima University.
I would like to express my sincere gratitude to my supervisor Professor Joji Ohshita
for his kind and helpful advice and encouragement. I also would like to thank Professor
Toshinori Tsuru, Associate Professor Masakoto Kanezashi, Assistant Professor Yohei
Adachi, Lecturer Takashi Hamada, Professor Takahiro Gunji, Lecturer Kazuki
Yamamoto for their useful advices and helps.
I would like to thank Mr. Katsuhiro Horata, Mr. Daiji Katsura, Ms. Sakino Takase,
Mr. Tetsuya Maeda, Mr. Sun Weipeng, Ms. Wang Conghuan, Mr. Takashi Kai, Mr.
Keigo Kawakami, Mr. Kei Oshima, Ms. Maho Kurihara, Mr. Mitsuru Sakabe, Mr.
Tetsuya Sugimoto, Mr. Kohei Yamada, Mr. Arata Tanaka, Mr. Takumi Hasegawa, Mr.
Tsubasa Yoshio, Mr. Lin Zhixin, Mr. Shota Terao, Mr. Ryuji Matsuura, Mr. Ryuto
Miyazaki for their kindness and collaboration. We also thank Mr. Toshiyuki Kawashimain Nitto Denko Corporation for performing the SEM measurements in part.

澳澳99