Nanofiltration membranes with modified active layer using aromatic polyamide dendrimers

Article abstract of DOI:10.1002/adfm.201201004

The modification of a commercial nanofiltration (NF) membrane (TFC-S) with shape-persistent dendritic molecules is reported. Amphiphilic aromatic polyamide dendrimers (G1-G3) are synthesized via a divergent approach and used for membrane active layer modi

Full text of DOI:10.1002/adfm.201201004

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Nanofiltration Membranes with Modified Active Layer
Using Aromatic Polyamide Dendrimers
Yuan Gao, Ana M. Saenz de Jubera, Benito J. Mariñas,* and Jeffrey S. Moore*
to remove various contaminants from
water in a single treatment step with low
operating pressure (typically 0.3–3 MPa)
and relatively high solute rejection as
well as high water permeation.[2] Com-
mercially-available NF membranes are
typically a composite structure with a ca.
50 μm asymmetric porous support layer
and a ca. 100–200-nm-thin active layer; the
latter serves as a barrier to water contami-
nants.[3] The active layers are usually made
by the method of interfacial polymeriza-
tion resulting in a thin film of crosslinked
polyamide with incompletely crosslinked
carboxylic groups providing a negative
charge.[4] Most commercial membranes
face similar challenges such as a propen-
sity to undergo fouling[5] and inability to
adequately reject certain common water
contaminants, e.g., arsenic(III), pre-
dominantly in the form of arsenious acid
(H3AsO3) in natural waters.[6] Therefore,
there is a need to develop a new genera-
tion of NF membranes having active layers
with adjustable chemistry and structure
to achieve fouling resistance and desired
water/solute selectivity for the broad range
The modification of a commercial nanofiltration (NF) membrane (TFC-S)
with shape-persistent dendritic molecules is reported. Amphiphilic aromatic
polyamide dendrimers (G1–G3) are synthesized via a divergent approach
and used for membrane active layer modification by direct percolation. The
permeate samples collected from the percolation experiments are analyzed by
UV-visible spectroscopy to monitor the influence of dendrimer generations on
percolation behavior and active layer modification. Further characterization of
modified membranes by Rutherford backscattering spectrometry and atomic
force microscopy techniques reveals a relatively low-level accumulation of
dendrimers inside the original TFC-S NF membrane active layer and subse-
quent formation of a coating of pure aramide dendrimers on top of the active
layer. A PES-PVA ultrafiltration membrane is used as a control membrane
support (without an NF active layer) showing that structural compatibility
between the dendrimers and support plays an important role in the mem-
brane modification process. The performance of the modified TFC-S mem-
brane is evaluated on the basis of the rejection abilities for a variety of water
contaminants having a range of molecular size and chemistry. As the water
flux is inversely proportional to the thickness of the active layer, the amount
of dendrimers deposited for specific contaminants are optimized to improve
the solute rejection while maintaining high water flux.
1. Introduction
of conditions encountered in water quality control applications.
Many approaches to improve commercial NF membrane per-
formance have been tried, including coating, plasma treatment,
chemical treatment, grafting polymerization, and UV irradia-
tion.[7] The resulting membranes have generally demonstrated
increased hydrophilicity as well as improved water permeability
and antifouling ability.[8] However, decreased salt rejection was
observed for certain modified polyamide membranes.[8b] Fur-
thermore, increased contaminant rejection is usually accompa-
nied with significant decrease in water flux.[9]
A versatile approach to membrane modification that uti-
lizes well-defined macromolecular architectures was described
recently.[10] In that study, shape-persistent macromolecules
are synthesized and directly percolated through a support
film to fabricate a new generation of NF membranes. Rigid
star amphiphiles (RSAs) with 1–2 nm hydrophobic cores and
hydrophilic side chains were coated onto polyethersulfone (PES)
ultrafiltration (UF) membrane supports to create NF mem-
branes.[10] Characterization of these membranes revealed that
the RSAs produced a uniform active layer atop the PES support,
but also that some of the macromolecules penetrated deeper
into the support, blocking pores and reducing the water flux.
One of the most severe problems worldwide is inadequate
access to sufficient clean water, which motivates research on
new materials for water purification and reuse.[1] In the last
decade, nanofiltration (NF) has emerged as an efficient and eco-
nomical drinking water treatment process, because of its ability
Y. Gao, Prof. J. S. Moore
Departments of Chemistry and Materials Science
and Engineering and Center of Advanced Materials
for the Purification of Water with Systems
University of Illinois at Urbana-Champaign
Urbana, IL 61801, USA
E-mail: jsmoore@illinois.edu
A. M. S. de Jubera, Prof. B. J. Mariñas
Department of Civil and Environmental Engineering
and Center of Advanced Materials for
the Purification of Water with Systems
University of Illinois at Urbana-Champaign
Urbana, IL 61801, USA
E-mail: marinas@illinois.edu
DOI: 10.1002/adfm.201201004
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Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201004
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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The rejection of arsenic(III) achieved with the RSA membranes
was comparable to the rejection obtained by the commercial NF
membranes ESNA and TFC-S.[11] These findings encourage us
to extend this molecular deposition method to the modification
of commercial polyamide NF membranes. The small pore size
distribution of NF membranes prevents building block mol-
ecules from breaking through, thus minimizing internal pore
blocking and associated water flux decrease as a result of mem-
brane modification.[12] Here, we survey the use of aromatic
polyamide dendrimers with varied compositions and structures
for membrane modification.
Dendrimers are regularly branched, three dimensional macr-
omolecules. Many of their characteristics, for example the size
and shape of the molecule and the position of functional groups,
can be well controlled by systematic changes in their synthesis.
Thus, they are designed to exhibit unique properties, such as
good solubility, low viscosity, multivalence, and encapsulation
effects, leading to a variety of applications.[13] In recent years,
aromatic polyamide (aramide) dendrimers have been prepared
by both convergent and divergent synthetic approaches.[14] They
maintain the good thermal, chemical, and mechanical proper-
ties of linear aromatic polyamides, but with improved solubility,
functionality, and processability. From a structural point of view,
there is high similarity between these aramide dendrimers and
the interfacial polymerized polyamides in NF membrane active
layers, which provides similar chemical properties and strong
interactions between aramide dendrimers and membrane active
layers, making them good candidates for polyamide NF mem-
brane modification. Moreover, unlike the highly crosslinked
and cyclized polyamides in active layers, aramide dendrimers
have sufficient solubility in organic solvents. Therefore, they
have the potential to be deposited on the support membrane
by percolation. By increasing the dendrimer generation and tai-
loring the peripheral group chemistry, it is possible to use these
structurally well-defined aramide dendrimers to systematically
investigate membrane-performance relationships.
structures for the modification of polyamide active layers of
NF membranes by direct percolation onto the support. A series
of bidendron aramide dendrimers were chosen as target com-
pounds (Figure 1).[15] Since the repeating unit has all substitutes
in the m-position, the aramide dendrimers are good mimics
to the polyamide networks used in commercial membranes,
but with a perfectly defined molecular structure. p-Phenylen-
ediamine (PDA) as the core structural unit reduces the steric
hindrance between the two dendritic segments. Oligoethylene
glycol (OEG) side chains were attached to the dendrimer’s
periphery through amide bond linkages[16] for the dual purpose
of overcoming the dendrimer’s limited solubility in methanol
and providing greater hydrophilicity for membrane fouling
resistance. These dendrimers were then deposited onto a com-
mercial polyamide NF membrane (FLUID SYSTEMS TFC-S,
Koch Membrane Systems, Wilmington, Massachusetts) for
membrane modification. In addition, similar procedures were
performed with PES membranes as a support layer (without
active layer) control experiment. The quality of the aramide
dendrimer modified active layers was characterized by UV-vis
spectroscopy, atomic force microscopy (AFM) and Rutherford
backscattering spectrometry (RBS). Membrane performance
was evaluated on the basis of rejection capabilities of organic
contaminant surrogate Rhodamine WT (R-WT), and sodium
chloride, barium chloride and arsenic(III).
2. Results and Discussion
2.1. Synthesis and Characterization of Aramide Dendrimers
The synthesis of aramide dendrimers G1–G3 followed a facile
divergent approach recently reported by Ueda and coworkers
(Scheme 1).[15] Amine-terminated NH2-Gn (n = 1–3) were syn-
thesized by using 3,5-bis(trifluoroacetamido)benzoyl chloride as
an AB2 building block with all reactions carried out in a single
flask. 2-[2-(2-Methoxyethoxy)ethoxy]acetyl chloride, which was
This article describes the synthesis and characterization of
amphiphilic aramide dendrimers with well-defined branched
Figure 1. Aramide dendrimers with an OEG periphery (G1–G3); R = -CH2O(CH2CH2O)2CH3.
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DOI: 10.1002/adfm.201201004

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Scheme 1. Synthesis of aramide dendrimers with an OEG periphery (G1–G3).
obtained by reacting 2-[2-(2-methoxyethoxy)ethoxy]acetic acid
with thionyl chloride, was used to functionalize the amine-ter-
minated NH2-Gn (n = 1–3) by amide bond formation.[17] Ara-
mide dendrimers G1–G3 were then obtained in moderate yield
after column chromatography.
backscattering spectrometry technique), heavy elements, such
as iodine, are incorporated into dendrimer molecules.[10] Due to
the divergent feature of the approach, the synthesis is facilitated
by incorporating iodine atoms into the dendrimer core. Thus,
2,6-diiodobenzene-1,4-diamine was synthesized by reduction of
diiodonitroaniline and used for dendrimer preparation in the
same way as for dendrimers G1–G3.[18] Dendrimers 2I-Gn (n =
1–3) were obtained in the yields of 43, 72, and 56%, respectively.
Their structures have been characterized by 1H NMR, 13C NMR,
The structures of dendrimers G1–G3 have been character-
1
ized by H NMR, 13C NMR, MALDI-TOF mass spectrometry
and gel permeation chromatography (GPC). The 1H NMR
spectrum of the dendrimers showed signals in three chemical
shift regions corresponding to amide, aromatic, and ethylene
glycol side chain protons. As shown in Figure 2, the MALDI-
TOF mass spectra of dendrimers G1–G3 gave m/z values of
1039.0, 2215.8, and 4567.5 Da, which are within experimental
error of their calculated m/z values of 1039.5, 2215.9, and
4568.9 Da, respectively. Furthermore, the GPC traces of all
three dendrimers were unimodal and had narrow distributions
(Figure 3). These results confirmed the formation of the desired
dendrimers.
1
and MALDI-TOF mass spectrometry. The H NMR spectrum
of dendrimer 2I-G1 showed peaks corresponding to the amide
protons at chemical shifts of 10.46, 10.28, and 9.87 ppm, and
peaks corresponding to the aromatic protons at 8.38, 8.25, 7.90,
and 7.84 ppm, indicating the asymmetric nature of the diiodo-
substituted compounds. The MALDI-TOF mass spectra showed
signals corresponding to the dendrimer molecular weight peaks
with m/z values at 1271.0, 2467.1, and 4818.9 Da, while the calcu-
lated m/z values are 1268.9, 2467.7, and 4820.7 Da, respectively.
1
In order to quantify the amount of polyamide dendrimers
within a molecular thin film (i.e., using the Rutherford
According the H NMR spectra, the purity of these dendrimers
is over 95%, as no signals from byproducts were observed.
Figure 2. MALDI-TOF MS spectrum of aramide dendrimers G1–G3.
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DOI: 10.1002/adfm.201201004
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G1
G2
G3
dendrimer size on the interaction between
dendrimers and support membranes.
Dendrimers G1–G3 were dissolved in
methanol at approximately 6 mg/L, and per-
colated through the above support film. Per-
colated samples were collected as a function
of time, and the product flux of methanol
(Figure 4a) was measured gravimetrically
with an analytical balance connected to a
computer.[10] In the blank experiment with
pure methanol solvent, an approximately
constant flow rate was maintained over the
whole percolation time. In contrast, all of
the aramide dendrimer solutions underwent
a decrease in percolation flow to various
degrees. Dendrimer G1 had the smallest
Mn
Mw/Mn
1.03
1.03
G1
G2
G3
1800
4000
6200
1.03
20
21
22
23
24
25
Retention Time (min)
Figure 3. GPC traces of aramide dendrimers G1–G3.
effect on the percolation flow, with the flow rate decreasing to
about 67% of the initial value. On the other hand, both den-
drimers G2 and G3 caused more significant reduction in the
percolation flow, leading to only 20% and 28% of the initial
value, respectively. These flow rate changes are consistent with
G1 having less interaction with the membrane meaning that
it likely gets washed through the support. In contrast, G2 and
G3 resulted in greater permeate flux resistance, consistent with
higher pore blockage of the support.[19] Though the pore size
of the PES support is not known, the molecular weight cut-off
reported by the manufacturer is in the range of 0.9 to 2.3 kDa.
2.2. Percolation of Aramide Dendrimers
with the PES UF Support
For the control experiments, aramide dendrimers G1–G3 were
deposited on a previously described polyvinyl alcohol (PVA)
modified PES UF membrane by percolation.[10] The PVA was
used to plug the largest pores of the support for the purpose of
lowering advective permeation. With such support membranes,
we are able to compare the effect of aramide dendrimers with
our previous results with RSAs and to survey the influence of
a)
MeOH
1.5
MeOH+G1
MeOH+G2
MeOH+G3
G1+MeOH
0.6 min
5 min
b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10 min
15 min
24 min
34 min
37 min
50 min
60 min
72 min
87 min
1.0
0.5
0.0
1.5
0.0
0.2
c)
0.4
0.6
0.8
1.0
300
Wavelength (nm)
400
Normalized Time
1.5
G2+MeOH
G3+MeOH
0.6 min
3 min
6 min
9 min
15 min
29 min
43 min
69 min
120 min
d)
2 min
5 min
8 min
19 min
34 min
58 min
86 min
120 min
190 min
247 min
1.0
0.5
0.0
1.0
0.5
0.0
300
Wavelength (nm)
400
300
Wavelength (nm)
400
Figure 4. a) Permeate flow of various dendrimer solutions in methanol and a methanol blank as a function of sampling time for the modified PES-PVA
membranes. b), c) and d) are the UV-vis spectra of permeate versus time for G1, G2, and G3 dendrimer solutions, respectively. The presence of the
dendrimers in the permeate solutions is evident by absorption in the wavelength range of 295 to 350 nm. The lines labeled as Gi+MeOH in the legends
indicate the feed solutions of the dendrimers in methanol.
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2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201004