A novel semi-aromatic polyamide TFC reverse osmosis membrane fabricated from a dendritic molecule of trimesoylamidoamine through a two-step amine-immersion mode

Article abstract of DOI:10.1039/c7ra07298h

In this work, a novel semi-aromatic polyamide RO membrane was fabricated by using a new self-made dendritic molecule trimesoylamidoamine (TMAAM) as a key functional amine monomer that combined 1,3-diamino-2-propanol (DAP) to react with trimesoyl chloride (TMC) through interfacial polymerization technology. By adjusting the TMAAM concentration and amine-immersion mode, this new TMAAM-based semi-aromatic polyamide RO membrane exhibits simultaneously improved water permeability, antifouling and chlorine-tolerant properties. The introduced TMAAM units in polyamide chains can enhance the TMAAM-based membrane's water flux due to its dendritic structure as well as rich hydrophilic groups, and the DAP-TMAAM-TMC membrane prepared via the new two-step amine-immersion mode has 1.9 times more water flux without loss of salt rejection than the pristine DAP-TMC membrane and also shows higher water flux than the hand-cast conventional aromatic polyamide MPD-TMC membrane, respectively. At the same time, the regularly distributed hydroxyl groups and aliphatic amide bonds in TMAAM units contribute to the improved hydrophilicity and chlorine-tolerant property of the resultant DAP-TMAAM-TMC membrane, respectively. It is also demonstrated that the new two-step amine-immersion mode leads to a much smoother surface which endows the resultant DAP-TMAAM-TMC membrane with a favorable antifouling ability. This research provides us with a promising functional amine monomer and a new membrane formation method to fabricate a high performance RO membrane.

Full text of DOI:10.1039/c7ra07298h

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A novel semi-aromatic polyamide TFC reverse
osmosis membrane fabricated from a dendritic
molecule of trimesoylamidoamine through a two-
step amine-immersion mode†
Cite this: RSC Adv., 2017, 7, 39127
ac
Hao Wu,‡ab Xiao-Lin Chen,‡ab Xiang Huang,ab Hui-Min Ruan,ac Yan-Li Ji,
*
ac
Li-Fen Liu
and Cong-Jie Gaoac
*
In this work, a novel semi-aromatic polyamide RO membrane was fabricated by using a new self-made
dendritic molecule trimesoylamidoamine (TMAAM) as a key functional amine monomer that combined
1,3-diamino-2-propanol (DAP) to react with trimesoyl chloride (TMC) through interfacial polymerization
technology. By adjusting the TMAAM concentration and amine-immersion mode, this new TMAAM-
based semi-aromatic polyamide RO membrane exhibits simultaneously improved water permeability,
antifouling and chlorine-tolerant properties. The introduced TMAAM units in polyamide chains can
enhance the TMAAM-based membrane's water flux due to its dendritic structure as well as rich
hydrophilic groups, and the DAP–TMAAM–TMC membrane prepared via the new two-step amine-
immersion mode has 1.9 times more water flux without loss of salt rejection than the pristine DAP–TMC
membrane and also shows higher water flux than the hand-cast conventional aromatic polyamide MPD–
TMC membrane, respectively. At the same time, the regularly distributed hydroxyl groups and aliphatic
amide bonds in TMAAM units contribute to the improved hydrophilicity and chlorine-tolerant property of
the resultant DAP–TMAAM–TMC membrane, respectively. It is also demonstrated that the new two-step
amine-immersion mode leads to a much smoother surface which endows the resultant DAP–TMAAM–
TMC membrane with a favorable antifouling ability. This research provides us with a promising functional
amine monomer and a new membrane formation method to fabricate a high performance RO membrane.
Received 2nd July 2017
Accepted 30th July 2017
DOI: 10.1039/c7ra07298h
rsc.li/rsc-advances
the RO process, which deteriorates the RO membrane's sepa-
ration performances and shortens its lifetime, and ultimately
limits the widespread application of RO technology.8–13
1. Introduction
Nowadays, reverse osmosis (RO) technology has a wide range of
applications in the areas of desalination of seawater and
brackish water, production of pure water, concentration of
solutions, and wastewater treatment.1–5 The core of RO tech-
nology is the high performance RO membrane, and at present
the commercial aromatic polyamide RO membranes used
widely in RO process are known for their ideal separation
performances including high salt rejection rates and moderate
water ux.6,7 However, these conventional aromatic polyamide
materials always suffer from fouling and chlorination during
Many strategies have been reported to settle the fouling and
chlorination problems of RO membranes such as pretreating
the RO feed solution, developing new membrane materials,
modifying the conventional aromatic polyamide RO membrane
and periodic cleaning etc.14–38 However, there are still some
limitations such as tedious pretreatment process, complicated
modication conditions. Especially, some strategies normally
lead to a trade-off effect among the antifouling, chlorine-
tolerance and/or reasonable separation performance of the
RO membrane, which faces a great challenge to synergistically
improve the RO membrane performances.14,39,40
It is generally considered that the chlorination of conven-
tional aromatic polyamide results from the N-chlorination by
attacking of active chlorine and the following benzene ring-
chlorination via Orton rearrangement, so the aliphatic poly-
amide structure without aromatic ring has a great potential for
improving the chlorine resistance of the RO membrane.41–43 At
the same time, the surface properties including hydrophilicity,
morphology, roughness and charge have signicant impacts on
aCenter for Membrane and Water Science and Technology, Ocean College, Zhejiang
University of Technology, Hangzhou 310014, China. E-mail: lifenliu@zjut.edu.cn;
yanliji@zjut.edu.cn
bCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310014, China
cCollaborative Innovation Center of Membrane Separation and Water Treatment of
Zhejiang Province, Hangzhou 310014, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra07298h
‡ These authors contributed equally.
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the antifouling ability of the RO membrane.6–12 Recently, Hoek resistant properties and reasonable separation performance
et al. reported a new polyamide RO membrane with antifouling through new interfacial polymerization mode.
and chlorine-tolerant properties fabricated from an aliphatic
amine of 1,3-diamino-2-propanol (DAP), but this membrane
2. Experimental section
2.1. Synthesis of the key functional monomer TMAAM
exhibited low water permeability.44 In the same year, Sum used
the dendritic ethylenediamine cored poly(amidoamine)
(PAMAM) as a amine monomer to enhance the water perme-
ability of the TFC membrane through interfacial polymerization
due to its dendritic and hydrophilic nature, and this membrane
presented favorable nanoltration performance owing to the
introduction of the co-polymerized monomer piperazine as
a void-ller in the membrane skin layer during interfacial
polymerization,45 which suggested that the dendritic macro-
molecule has an advantage to improve the membrane water ux
due to its large free volume.46,47 However, to our knowledge, the
use of dendritic macromolecule in RO membranes is still quite
limited.48,49
2.1.1. Materials and reagents. Trimesoyl chloride (TMC)
and 1,3-diamino-2-propanol (DAP) were purchased from the
Shang Hai Bangcheng Co. Ltd., China. Triethylamine (TEA),
chloroform, methyl alcohol, ethyl alcohol, DMF were purchased
from Aladdin Reagent Co. Ltd (Shanghai, China). All reagents
used in monomer synthesis are of analytic grade unless other-
wise specied.
2.1.2. Synthesis procedure. The functional monomer tri-
mesoylamidoamine (TMAAM) was synthesized via three-step
method including esterication, amidation and ester ami-
nolysis reactions (Scheme 1). First, 4.00 g (0.015 mol) TMC was
dissolved into 30 ml chloroform in the round-bottom ask and
then 1.0 g (0.033 mol) methyl alcohol suspended in 10 ml
chloroform was slowly added. The mixture was stirred for
a while at room temperature, then heated to 60 ꢀC and reuxed
for 4 h. Aerwards the mixture was cooled (2–4 ꢀC cooling bath
temperature) and then a solution of 0.69 g (0.0075 mol) DAP,
5.00 g (0.05 mol) TEA in 30 ml chloroform and 1 ml DMF was
slowly added in this cooled mixture. Aer warming up to room
temperature, the mixture was stirred to keep reaction for 2.5 h.
Following the completion of the reaction, the mixture was
washed by 2 ꢁ 50 ml water and the resulting organic phase was
then dried with sodium sulfate, ltered and concentrated
successively. At last, the residue was mixed with 40 g of silica gel
and evaporated to dryness and chromatographed with gradient
elution (petroleum ether/ethyl acetate 1/1 to ethyl acetate) to
In this work, a novel dendrimer of trimesoylamidoamine
(TMAAM) was designed and synthesized as a key functional
amine monomer. Different from the above reported amine
monomers, the TMAAM monomer not only exhibits dendritic
structure but also contains rich aliphatic amine and hydroxyl
groups (Scheme 1), which has some potential advantages for
fabrication of the TMAAM-based RO membranes. For example,
TMAAM units can enhance the membrane's water permeability
due to the existence of hydrophilic groups and its dendritic
structure generating more space between polymeric chains. The
rich hydrophilic groups in the TMAAM units are also benecial
for the membrane's antifouling. Furthermore, the aliphatic
amide structure in polyamide chains contributes to chlorine
resistance of the membrane. However, when TMAAM was used
as a single amine monomer to react with TMC via interfacial
polymerization to fabricate the RO membrane, the resulting
membrane showed relatively low salt rejection which was also
due to the more space between polyamide chains caused by the
TMAAM's dendritic structure. Based on above advantages and
disadvantages of both dendritic TMAAM and aliphatic amine
DAP, therefore, the TMAAM was combined with DAP as the key
function monomer in aqueous phase to prepare the novel semi-
aromatic polyamide RO membrane with antifouling, chlorine-
obtain 2.30
g white powdery intermediate of 1,3-bis-
[30,50-bis(methyloxycarbonyl)phenylcarbonylamido]-propane-2-
ol (MOCPCAP).
A solution of 2.00 g DAP and 2.10 g MOCPCAP in 60 ml
methyl alcohol was added in the round-bottom ask at room
temperature and then reuxed for 24 h at 60 ꢀC. Aer the
completion of the reaction, the mixture was transferred from
ask into a Rotavapor under vacuum to remove the solvent.
Precipitate was get in ethanol, ltered and washed twice with
ethanol, and nally dried to get 3.0 g (yield 99%) of a white
powdery TMAAM.
2.1.3. Characterization. The product TMAAM was identi-
ed by 1H NMR (Brucker-400 M) and 13C NMR (Brucker-400 M),
GC-MS (capillary: Trace GC 2000 Series CE Instrument Thermo
Quest; MS: Trace C MS Thermo Quest), IR (Nicolet IR200), GC
(GC-2010A, Shimadzu Corp., Japan) and melting-point
measurement (WRS-1B, Cany instruments Co. Ltd., China).
2.2. Fabrication of TFC reverse osmosis membrane
2.2.1. Materials and reagents. The polysulfone (PSf)
microporous supporting lm (molecular weight cut-off of about
100 000 g molꢂ1) was supplied by the Hangzhou Development
Center of Water Treatment Technology, China. The key func-
tional monomer TMAAM used to fabricate the RO membrane
Scheme 1 Synthesis route of trimesoylamidoamine.
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was synthesized by ourselves. The other used key monomers Fig. 1(a)) with TEA (3.0 wt%), SDS (0.15 wt%) and CSA (4.0 wt%)
include DAP and TMC (Shanghai Bangcheng Co. Ltd., China), was poured into the top surface of the PSf support membrane
m-phenylenediamine (MPD) as well as aqueous phase additives (clamped between two Teon frames) for soaking 2–10 minutes.
including TEA, sodium dodecyl sulfate (SDS) and (+)-10-cham- Aer removing the excess solution and air-drying until no
phor sulfonic acid (CSA) (J&K Scientic Ltd.). Active chlorine remaining liquids, the TMC (0.15 wt%) organic phase solution
solution used in chlorination experiment was NaClO (Aladdin was poured rapidly on the dip-coated surface to react with the
Reagent Co. Ltd., 5 wt% free chlorine). All other reagents such adhered amine for 40–60 s, and then the frame with the nascent
ꢀ
as n-hexane and sodium chloride (NaCl) etc. are used as received polyamide skin layer was heated to 60 C (8–10 min) in the air
without further purication unless otherwise specied. Deion- dryer for further polymerization to obtain the RO membranes
ized (DI) water (#5 mS cmꢂ1) was produced by a two-stage including MPD–TMC, DAP–TMC, DAP/TMAAM–TMC and
reverse osmosis system.
TMAAM–TMC. Finally, the obtained RO membranes were
2.2.2. Fabrication of the RO membranes. The new semi- rinsed thoroughly with DI water (conductivity # 5 mS cmꢂ1) and
aromatic polyamide TMAAM-based RO membranes were fabri- stored in NaHSO3 solution (1.0 wt%) for later use.
cated on the PSf support membrane through interfacial poly-
Since TMAAM molecule is much bigger than DAP molecule,
merization technology as compared to the pristine DAP-based the diffusion speed of TMAAM would be much slower than that of
membrane (DAP–TMC) and conventional aromatic polyamide DAP. As compared to the above DAP/TMAAM–TMC membrane
membrane (MPD–TMC). First, the aqueous solution of MPD prepared via conventional single-step amine-immersion mode,
(2.0 wt%) or composite amine of DAP and TMAAM (Table 1 and the DAP–TMAAM–TMC was fabricated through new two-step
Table 1 Amine composition of the aqueous phase used to prepare the new polyamide membranes
Membrane
Amine composition
Amine-immersion mode
Single-step
DAP–TMC (M1)
TMAAM–TMC (M2)
DAP/TMAAM–TMC (M3)
①
②
③
④
⑤
⑥
⑦
⑧
2 wt% DAP
2 wt% TMAAM
2 wt% DAP + 0.1 wt% TMAAM
2 wt% DAP + 0.2 wt% TMAAM
2 wt% DAP + 0.3 wt% TMAAM
2 wt% DAP + 0.4 wt% TMAAM
2 wt% DAP + 0.5 wt% TMAAM
1 wt% DAP + 1 wt% TMAAM
1 wt% DAP + 1 wt% TMAAM
DAP–TMAAM–TMC (M4)
Two-step
Fig. 1 The amine-immersion modes of RO membrane fabrication.
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amine-immersion mode. Aer the 1 minute immersion of the rst ltration apparatus (Fig. 2). In order to avoid the water perme-
aqueous of 1.0 wt% DAP with TEA, SDS and CSA, the second ability decline due to compaction, the membrane was pre-
aqueous solution of 1.0 wt% TMAAM with TEA, SDS and CSA was pressurized with DI water (1.55 MPa, 25 ꢀC, 2 h) until pure
poured on the air-dried DAP-coated surface for 1 minute immer- water ux reached a steady state. Then, 10 L of the west lake
sion. Subsequently, the interfacial polymerization reaction with water with microltration pretreatment (Table S1†) was added
TMC, heat curing and DI water rinse were carried out in a similar to the feed tank. Aer reaching a steady state again (1.55 MPa,
ꢀ
manner as mentioned above.
25 C), the permeation performance of membranes was tested
for 8 days. In order to decrease the error of fouling test, 3
samples of the same membrane were loaded in parallel in each
fouling test, and the fouling test was also performed for dupli-
cates for 3 times. Finally, the average value was calculated from
these experimental data to obtain the variation of water ux and
salt rejection of the membranes.
2.3. Membrane separation performance test
The separation performance were tested with cross-ow
membrane ltration apparatus (Fig. 2). The membrane coupons
(effective area: 12 cm2) were equilibrated with DI water for at least
30 min (1.55 MPa, 25 ꢀC) before the permeation tests to reach
a steady state. Then, permeation tests were carried out with
different concentration NaCl solution (pH ¼ 7.0). The experi-
mental data including permeate volume and conductivity
(DDS-307, Cany instruments Co. Ltd., China) were recorded
aer reaching a steady-state ux again. Finally, water ux (J)
was calculated from the equation: J ¼ V/(At), where V is the
permeate volume, A denotes the effective membrane area, and t
is the permeation time. Salt rejection (R) was determined as
R ¼ 1 ꢂ Cp/Cf, where Cp and Cf are permeate and feed concen-
trations, respectively, which were derived from conductivity
analysis.
2.6. Characterization of the membranes
Chemical composition and structure of the resultant RO
membranes were characterized with attenuated total reection
Fourier transformed infrared spectroscopy (ATR-FTIR, Nico-
let6700, Thermo Electron Corporation, USA) and X-ray photo-
electronic spectroscopy (XPS, Kratos AXIS Ultra DLD, Shimadzu
Corporation, Japan). Surface and cross-section morphologies
were observed with scanning electronic microscopy (SEM, S-
4700, Hitachi Ltd., Japan) and atomic force microscope (AFM,
Park Instrument Auto Probe CT). Static contact angles were
¨
obtained with DSA10-MK2 contact angle meter (KRUSS GmbH
2.4. Membrane active chlorine exposure experiment
Germany). All membrane samples were washed with DI water
until no remaining NaHSO3 and dried under vacuum at 40 C
ꢀ
To investigate the membrane's chlorine tolerant property, the
membrane coupons were exposed to different concentration
chlorine solutions (2000, 6000, 8000 and 10 000 ppm, respectively)
for 1 h at room temperature which could effectively simulate long-
time exposure to low chlorine concentration, where the active
chlorine solutions were prepared with NaClO in DI water at
constant pH 8.0. Then, the separation performance of these
membranes was tested aer being thoroughly rinsed with DI
water. Finally, experimental data including water ux and salt
rejection were recorded and calculated in a similar manner as
above.
for at least 1 h before measurements.
3. Results and discussion
3.1. Characterization of the chemical structure of TMAAM
The synthetic route for obtaining TMAAM is shown in Scheme
1. As can be seen, TMAAM was prepared via three-step method
including esterication, amidation and ester aminolysis reac-
tions. The chemical structure of TMAAM were conrmed by FT-
IR, 13C NMR, H NMR (Scheme 2) and HRMS, and the results
1
are as below.
C33H50N10O11 [762.37] mp: 100–103 ꢀC;
IR (KBr): 582.4, 711.1, 961.2, 1048.3, 1096.3, 1291.4, 1432.0,
2.5. Fouling test
To evaluate the membrane's antifouling property, the dynamic
fouling experiment was carried out with a cross-ow membrane
1542.6, 1651.7, 2930.3, 3077.2 and 3294.6 cmꢂ1
;
Fig. 2 Device for the permeation tests.
39130 | RSC Adv., 2017, 7, 39127–39137
Scheme 2 1H NMR schematic diagram of TMAAM.
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1H NMR (500 MHz, DMSO-d6): d 8.78–8.66 (m, 6H), 8.46 (t, J introduction of TMAAM units has few impact on the chemical
¼ 9.8 Hz, 6H), 3.90 (d, J ¼ 5.3 Hz, 1H), 3.61–3.56 (m, 4H), 3.47– structure of the polyamide skin layer of the pristine membrane.
3.42 (m, 12H), 3.36–3.20 (m, 13H), 2.62–2.47 (m, 8H);
3.2.2. XPS analysis of the membranes. XPS is particularly
13C NMR (126 MHz, DMSO-d6): d 165.90(2C), 165.84(4C), well suited for examining the skin layer of a membrane, because
˚
135.02(6C), 128.66(6C), 70.79(4C), 68.56, 45.48(6C) and it probes only a short distance (10–90 A) into the surface of the
43.51(4C);
solid. The quantitative elemental composition of the top-most
HRMS calculated for C33H51N10O11 [M + 1]: 763.3733; found: layer of the sample can be calculated from the spectrum.
763.3717.
Therefore, the RO composite membranes with thin active layer
(approximately 0.2 mm) were analyzed for the chemical
composition of skin layer through XPS analysis.
When polymerizing on the surface of PSf support, it was
assumed that two kinds of polymer chains had been formed
including DAP–TMC and TMAAM–TMC. The DAP–TMC poly-
mer chain has two possible ideal structures including linear
with pendant –COOH (unit A) and totally cross-linked (unit B) as
shown in Fig. 4. Since TMAAM was derived from the reaction of
DAP and TMC, the totally cross-linked reaction of TMAAM and
TMC is similar with that of DAP and TMC. In other words, the
TMAAM–TMC polymer chain has the same ideal totally cross-
linked structure (unit B), while it has a different linear structure
with pendent –COOH (unit C). Herein, the pendant –COOH
groups in the linear structures is attributed to the hydrolysis of
–COCl groups during interfacial polymerization process.
According to these ideal chemical structures, the relative atomic
concentration (AC%) of characteristic polyamide elements (C, O
and N) and their concentration ratios of the different polymer
chains were calculated (Table 2).
3.2. Characterization of the active layer of RO membranes
3.2.1. ATR-FTIR spectrum. ATR-FTIR was used to analyze
the chemical structure of the skin layer of RO membranes, as
compared IR analyses of the key functional monomers. Fig. 3
presents the main characteristic peaks of the new RO
membranes including DAP/TMAAM–TMC and DAP–TMAAM–
TMC, compared to the pristine DAP–TMC membrane and the
monomers including DAP, TMAAM and TMC. It can be seen
that the characteristic peaks of three monomers such as
1760 cmꢂ1 (–COCl) and 3294 cmꢂ1 (n N–H, primary amine)
disappeared aer interfacial polymerization, and new charac-
teristic peaks appeared correspondingly in the ATR-FTIR
spectra of membranes. The amide I (n C]O, 1645, 1649,
1647, 1652 cmꢂ1), II (d NH, 1502, 1504 cmꢂ1) and III (n C–N,
1241, 1239, 1240 cmꢂ1) peaks of the active layer respectively are
clearly identied as evidence of existing of functional –NHCO–
bond. The peaks are located at 1584, 1585 and 1486, 1487 cmꢂ1
as expected for the benzene ring vibration. Obviously, the
Fig. 3 ATR-FTIR spectrum of TFC RO membranes.
Fig. 4 Possible ideal chemical structures of DAP–TMC and TMAAM–TMC polymer chains.
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Table 2 The element composition of RO membranes derived from the XPS measurement
Item
C%
N%
O%
O/N
Calculated data from ideal structures of
polymer chains
DAP–TMC
Linear with pendant –COOH (A)
Totally cross-linked (B)a
Linear with pendant –COOH (C)
57.14
58.69
57.95
61.31
61.75
64.41
59.77
11.11
15.22
13.26
11.38
12.07
10.88
12.55
31.75
26.09
28.79
27.31
26.18
24.70
27.68
2.85
1.71
2.17
2.40
2.17
2.27
2.21
TMAAM–TMC
DAP–TMC (M1)
TMAAM–TMC (M2)
DAP/TMAAM–TMC (M3)
DAP–TMAAM–TMC (M4)
Actual data of XPS analysis of
membranes
a
The TMAAM–TMC polymer chain has the same ideal cross-linked structure of the DAP–TMC polymer chain. Here M3 denotes that the DAP/
TMAAM–TMC membrane was fabricated from the no. 8 composite amine as shown in Table 1.
Theoretically, the O atom exists in the pendent –COOH, the However, both the actual O/N ratios of 2.27 from the DAP/
cross-linked polyamide bond (–CONH–) and –OH group of DAP TMAAM–TMC membrane and 2.21 from the DAP–TMAAM–
monomer, while the N atom is only in the same cross-linked TMC membrane are between 2.17 from the TMAAM–TMC
polyamide bond. Obviously, the ratio of O/N is closely related membrane and 2.40 from the DAP–TMC membrane, and
to the cross-linking degree of polymeric structure, and the lower meanwhile are far from 1.71 calculated from ideal totally cross-
the O/N ratio is, the higher cross-linking degree the RO linked structure. Obviously, the two membranes of DAP/
membrane has. Since the TMAAM monomer was synthesized TMAAM–TMC and DAP–TMAAM–TMC are mainly composed of
from a given reaction of TMC and DAP (Scheme 1), TMAAM both linear structures of TMAAM–TMC and DAP–TMC polymer
itself has an exact cross-linking degree and its O/N ratio is 1.1. chains, and the DAP–TMAAM–TMC membrane has slightly
Whether TMAAM-based polymer chains exhibit linear structure higher cross-linking degree than the DAP/TMAAM–TMC
and/or totally cross-linked structure, in other words, their O/N membrane. The possible reason is that the two-step amine-
ratios will be lower than 2.85 derived from the ideal linear immersion mode increases the contact chances of TMAAM
DAP–TMC structure (unit A) and higher than 1.71 calculated with TMC which leads to an increase in TMAAM units in the
from the ideal totally cross-linked structure (unit B). In fact, it resultant membrane. Furthermore, it should be noticed that all
can be seen from the Table 2 that the actual O/N ratio of 2.40 of the three TMAAM-based membranes have higher O/N ratios
derived from the DAP–TMC membrane is greatly higher than than the DAP–TMC membrane, in other words, the TMAAM-
1.71 from unit B but near 2.85 from unit A, at the same time, it is based membranes have relative higher cross-linking degree
also higher than 2.17 from unit C as well as greater than all due to the introduction of TMAAM units.
actual O/N ratios of the TMAAM-based RO membranes. These
3.2.3. Surface morphology of the membranes. The combi-
results indicate that the DAP–TMC membrane is mainly nation of SEM and AFM micrograph was used to observe the
composed of the linear DAP–TMC polymer chains and few surface morphologies of the four new semi-aromatic polyamide
totally cross-linked structures. It is further noticed that the RO membranes including DAP–TMC (M1), TMAAM–TMC (M2),
actual O/N ratio (2.17) of the TMAAM–TMC membrane equals to DAP/TMAAM–TMC (M3) and DAP–TMAAM–TMC (M4) and the
the ideal value calculated from unit C, in other word, the conventional polyamide RO membrane TMC–MPD (M0) (Fig. 5).
TMAAM–TMC membrane is just composed of linear structures. It can be seen from the SEM images that the four new semi-
Fig. 5 SEM and AFM images of the RO membranes. M0: MPD–TMC; M1: DAP–TMC; M2: TMAAM–TMC; M3: DAP/TMAAM–TMC; M4: DAP–
TMAAM–TMC. Here M3 denotes the DAP/TMAAM–TMC membrane was fabricated from the no. 8 composite amine as shown in Table 1.
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aromatic polyamide RO membranes have different surface
morphologies from the conventional MPD–TMC membrane and
also exhibit much smoother surface. At the same time, the AFM
images are in good accord with the SEM images, and the average
roughness of all the membranes ranks in the following order:
MPD–TMC (36.6 nm) > DAP/TMAAM–TMC (5.8 nm) > TMAAM–
TMC (4.3 nm) > DAP–TMC (4.0 nm) > DAP–TMAAM–TMC (3.3 nm),
which also demonstrates that the four new semi-aromatic poly-
amide membranes are very smoother than the conventional full-
aromatic polyamide MPD–TMC membrane. Furthermore, it is
interesting that a small number of lotus root bulges sporadically
distribute on the surface of DAP/TMAAM–TMC membrane
prepared via conventional single-step amine-immersion mode,
which will easily lead to deposition of foulants on the membrane
surface, but fortunately the DAP–TMAAM–TMC membrane fabri-
cated via the new two-step amine-immersion mode has uniformly
smooth surface. Obviously, the two-step amine-immersion mode
can improve the surface morphology of the RO membrane. The
possible reason is that the large TMAAM molecules dip-coated on
the top-most surface of PSf support rst contacted and reacted
with TMC to form a very loose and rough TMAAM–TMC lm due
to its dendritic structure generating more space between the
polymer chains, and then the small DAP molecules diffused
through the resultant nascent TMAAM–TMC lm to react with
TMC at the interface of two phases and form a smooth DAP–TMC
lm on the surface of TMAAM–TMC lm, as a result of that the
Fig. 7 Schematic illustration of the TMAAM-based polymer chains
facilitating the diffusion of water molecules through the RO
membrane.
DAP–TMAAM–TMC membrane presents smoother surface than
the DAP/TMAAM–TMC membrane.
3.3. Separation performance of the membranes
The separation performance of the resultant new semi-aromatic
polyamide membranes were tested with 2000 ppm NaCl solu-
ꢀ
tion at 1.55 MPa, 25 C as compared to that of the conventional
polyamide MPD–TMC membrane. Fig. 6(a) shows the effects of
different concentration of TMAAM in aqueous phase on the water
ux and salt rejection rate of the membranes as referred the Table
1. It can be seen that the water ux is increased from 14.5 to
32.7 L hꢂ1 mꢂ2 with the rise in TMAAM concentration from 0 to
2 wt%, whereas the salt rejection rate of the membranes is
correspondingly decreased. Especially, the salt rejection of
TMAAM–TMC membrane greatly decreased to 89.8%. Obviously,
the addition of TMAAM in the aqueous phase to modify the DAP–
TMC membrane can improve the water permeability but simul-
taneously cause a loss of salt rejection of the membrane. Fortu-
nately, the DAP–TMAAM–TMC membrane prepared via new two-
step amine-immersion mode has 1.9 and 1.3 times more water
ux without loss of salt rejection rate than the DAP–TMC and
DAP/TMAAM–TMC membranes respectively as shown in Fig. 6(b).
The possible reason is that the TMAAM molecule not only has
a dendritic structure but also contains rich aliphatic amine and
hydroxyl groups regularly along with its molecule chain. Aer
membrane formation, the semi-aromatic polyamide chains based
on TMAAM dendritic units have rich hydrophilic groups and
bonds including –OH, –COOH derived from the hydrolysis of
–COCl group and –C]O from the amide bond, which can facili-
tate water molecules to diffuse through the membrane and thus
improve the membrane's water permeability. On the other hand,
the TMAAM units lead to more space inside or among the polymer
chains, so-called aggregate pore,47,48 which can not only enhance
the water permeability but also reduce the salt selectivity of the
membranes (Fig. 7). Therefore, suitable concentration of TMAAM
is one of the important factors to get an ideal separation perfor-
mance of the new TMAAM-based polyamide RO membrane.
Fig. 6 (a) The impact of different concentration of TMAAM in aqueous
phase on the membrane separation performance. (b) Separation
performance of the resultant RO membranes. M0: MPD–TMC; M1:
DAP–TMC; M2: TMAAM–TMC; M3: DAP/TMAAM–TMC; M4: DAP–
TMAAM–TMC. Here M3 denotes the DAP/TMAAM–TMC membrane
was fabricated from the no. 8 composite amine as shown in Table 1.
3.4. Chlorine-tolerant property of the RO membranes
The chlorination experiments were carried out to evaluate the
chlorine-tolerant property of the new semi-aromatic polyamide
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Fig. 8 (a) Separation performance and (b) FTIR variation of the RO membranes after chlorine exposure. (i) MPD–TMC membrane. (ii) DAP–
TMAAM–TMC membrane.
Table 3 Variation of O/N ratios of the RO membranes before and after chlorination
O/N ratio before
chlorination
O/N ratio aer
chlorination
Difference of
O/N ratios
Membrane
Variation%
MPD–TMC
DAP–TMAAM–TMC
1.70
2.17
2.04
2.35
0.74
0.18
20
8.29
DAP–TMAAM–TMC membrane that has relatively ideal separation of N element amount unless decrease. On the other hand, since
performance as demonstrated in above Section 3.3, and compared the TMAAM-based semi-aromatic polyamide chains have
to that of the conventional polyamide MPD–TMC membrane. It aliphatic amide bonds, which can avoid benzene ring-
can be seen from the Fig. 8(a) that the chlorine-tolerant property chlorination via Orton rearrangement and further prevent the
of the DAP–TMAAM–TMC membrane is greatly better than that of
the conventional MPD–TMC membrane. At the chlorine concen-
tration less than 2000 ppm h, both of the salt rejection rate and
water ux of the DAP–TMAAM–TMC membrane just has a slight
increase, while the MPD–TMC membrane exhibits great decrease
in both salt rejection rate and water ux. When the concentration
of the active chlorine is in the range of 2000 to 6000 ppm h, the
salt rejection rate of the DAP–TMAAM–TMC membranes still are
kept stable, but the MPD–TMC membrane's salt rejection
decreases greatly.
It is well known that the attack of active chlorine will cause
a degradation of polyamide chain due to the break of amide
bond,41–43 which mainly presents the variation of amide III (n C–N)
and amide I (n C]O) peaks in the IR spectrum. As shown in
Fig. 8(b), the amide I peaks of the two membranes of DAP–
TMAAM–TMC and MPD–TMC present some changes aer expo-
sure to 6000 ppm h active chlorine solution. However, the amide
III peak of the DAP–TMAAM–TMC membrane just has few
changes, while the conventional MPD–TMC membrane exhibits
great variation. Meanwhile, the XPS analysis also presents the
similar tendency (Fig. S1† and Table 3). The O/N ratios of the skin
layer of both MPT–TMC and the DAP–TMAAM–TMC membranes
increase with the chlorination, but the difference (0.74) of O/N
ratios of the MPD–TMC membrane before and aer chlorina-
tion is much greater than that of the DAP–TMAAM–TMC
membrane (0.18). The possible reason is that the chlorination
leads to increase in O element amount which results from the
Fig. 9 (a) The contact angle and water flux decline of membranes. (b)
The continual fouling test of membranes with west lake water for 8
days.
break of amide bond and the formation of new carboxyl group
(–COOH) and quinone bond (Ph]O), whereas causes few changes
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polyamide chains from break. Obviously, the combination of ATR- Collaborative Innovation Center of Membrane Separation and
FTIR and XPS analyses provides an evidence for the favorable Water Treatment of Zhejiang Province.
chlorine-tolerant ability of the DAP–TMAAM–TMC membrane.
Notes and references
3.5. Antifouling property of the RO membranes
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The contact angle is a measure of the tendency for the water to
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hydrophilicity. As can be seen from the Fig. 9(a), the new semi-
aromatic polyamide DAP–TMAAM–TMC membrane has much
lower contact angle than the conventional polyamide MPD–TMC
membrane, in other words, the former exhibits more hydro-
philic surface than the latter. At the same time, the DAP–
TMAAM–TMC membrane has lower water ux decline than the
MPD–TMC membrane, and the former always shows better
antifouling ability than the latter during the continual fouling
test as shown in Fig. 9(b). One of the possible reasons is that the
DAP–TMAAM–TMC membrane has some –COOH groups as well
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