Reverse osmosis (RO) is a common method to remove dissolved
salts from seawater and brackish water . Water flux
and salt rejection are two key parameters for RO membranes,
and efficient desalination relies on both high water flux and
salt rejection of the membrane. The thin-film composite
(TFC) membranes are widely used in commercial single pass
seawater desalination plants because they exhibit high water
flux and salt and organic rejections, a wide operating range of
temperature and pH, and high stability to biological attacks
[2-5]. The TFC membranes consist of a highly-selective thin
aromatic polyamide (PA) layer formed via in situ polycondensation
on a reinforced porous sublayer. A great advantage
of TFC technology is that the ultra-thin barrier layer and the
porous support can be independently optimized with respect
to structure, stability, and performance .
Porous support layers are commonly prepared by the phase
separation of polymer solution . Numerous studies have
been carried out to investigate the effects of polymer concentration
and additives on the performance of this porous support
membrane. It is well known that the polymer concentration
in the casting solution has a great effect on the porosity
of the final membrane . A higher polymer concentration
will lead to a lower porosity, because increasing the polymer
concentration in solution results in a higher viscosity, thus reducing
transport rates and slowing the demixing process .
A higher polymer concentration has also shown to increase
the membrane top layer thickness and decrease formation of
macrovoid . Another important parameter is the additive
in the casting solution, which can affect the final membrane
characteristics either by changing solvent capacity or
by changing phase separation kinetics and thermodynamic
properties . The most important effects of the additives
are the increase of hydrophilicity of the membrane surface,
suppression of macrovoid formation, and enhancement of
pore interconnectivity . Polyvinylpyrrolidone(PVP) was
usually used as the additive in the preparation of porous
polysulfone (PSf) or polyethersulfone (PES) membranes
for ultrafiltration (UF). The addition of PVP showed various
effects on porous membrane structure originating from
different dope solution compositions. For example, the work
of Boom showed that PVP suppressed the formation of macrovoid
in the substrate layer in PES/N-methylpyrrolidone
(NMP)/PVP solution . In another study, Yoo and et al. indicated that PVP enlarged the macrovoid structure
rather than suppressed the pore structure in PSf/ Dimethylformamide
(DMF)/PVP solutions. All these effects could further
influence the performance of final TFC membranes.
Fabrication of the thin barrier layer is based on interfacial
polymerization (IP), i.e., a polymerization reaction that takes
place at the interface of porous support layer between two immiscible
phases . Usually, the thin-film active layer consists
of aromatic PA formed by the IP of m-phenylenediamine
(MPD) in the aqueous phase and trimesoyl chloride (TMC)
in the organic phase. Two structural moieties may exist in the
derived PA. One is the crosslinked portion (m) and the other
is the linear moiety (n), which has an unreacted acid chloride
groups that subsequently hydrolyzes to form a carboxylic acid
There are many studies reported in the literature on the impact
of polymer concentration and additive to the structure
and performance of UF membranes. It is not clear, however,
how the changing characteristics of the support layer, including
those induced by using different polymer concentrations
and additive during the fabrication process, could affect the
performance of the TFC membranes. In this work, we aimed
to evaluate the effects of polymer concentration and additive
on the support layer formation and in particular, the performance
of the TFC membranes fabricated using support layers
of varying characteristics.
PSf beads (35,000 Da), PVP powder (10,000 Da and 40,000
Da), DMF (anhydrous, 99.8%), TMC (98%), MPD (99.8%),
Red MX-5B (615.33 Da), and Bovine serum albumin (BSA,
66,000 Da) were purchased from Sigma-Aldrich (St. Louis,
MO) and used as received.
Preparation of PSf porous support layers
A measured amount of PSf was added to airtight bottles and
then a specified amount of DMF was added to dissolve the PSf.
When the effects of the PVP additive were studied, the polymer-
solvent mixture was also spiked with a specified amount
of PVP. This casting solution was ultrasonicated for 1 h and
stirred at 45 0C for 6 h. And then, the casting solution was
kept still overnight at room temperature for degassing. The
membrane was formed by spreading the polymer solution
over a clean glass plate by a casting knife with a gap of 100μm
followed by immediately immersing the glass plate into the
deionized (DI) water at room temperature. The precipitated
membrane was washed thoroughly and stored in DI water at
5 °C prior to test.
Various porous support membranes were produced by systematically
changing the casting solutions. The effect of PSf concentration
was studied by varying PSf from 11 to 19 wt% in
DMF solvent, while the influence of additive PVP was studied
by keeping PSf concentration fix at 15 wt% and varying PVP
from 1 to 10 wt% in the total dope solution.
TFC membrane fabrication
The PA thin-film layer was formed on the top of the PSf support
membranes via IP process. Briefly, 2 % MPD in DI water
and 0.15 % TMC in hexane were used. The PSf substrate layer,
placed on a glass plate, was immersed in 2 % MPD solution.
After 5 min, the excess MPD was removed by using a rubber
roller and then 0.15 % TMC hexane solution was dripped on
the surface to react with MPD for 120 s, which led to the formation
of an active skin layer over the PSf support. The membrane
was then post-treated at 80 °C for 5 min and stored in DI
water at 5 °C prior to the performance testing.
Characterizations and performance assessment of
The hydrophilicity of substrate membrane can be evaluated by
using water contact angle as a proxy. In the present study, the
water contact angles of membranes were measured by the sessile
drop method on a video contact angle system (VCA-2500
XE, AST products, Billerica, MA).
Equilibrium water content (EWC) is related to the porosity
of a membrane. It is an important parameter as it indirectly
indicates the degree of hydrophilicity or hydrophobicity of a
membrane . Membranes were weighed in an electronic
balance in a wet state after mopping the surface water with a
clean tissue paper. The wet membranes were dried in an oven
for 2 h at 60 0C and weighed. The EWC at room temperature
was calculated as follows: EWC=(W_W-W_d)/W_w ×100% (1)
where Ww is weight of wet membranes (g) and Wd weight of
dry membranes (g).
The scanning electron microscope (SEM) images were collected
for the top surfaces as well as cross-sections of membranes
by using Quanta FEG 600 (FEI Company, Hillsboro, OR).
The specimen was coated with platinum by a sputter coater
(K575x, Emitech Ltd., Kent, England) at 20 mA for 1 min to
increase conductivity. To obtain the cross section images, the
wet membranes were cut into pieces and were immersed in
liquid nitrogen. The frozen membranes were then broken by
tweezers to avoid the structure damage in the cross-section.
To assess the support membrane, a dead-end test system was
used for measuring pure water flux and molecular weight cut
off (MWCO). The filtration set-up was described in our previous
study , where the membrane holder (Stirred cell 8200,
Millipore Corp.) had an effective membrane area of 28.7 cm2.
To eliminate the influence of membrane compaction, membranes
were pre-compacted at 20 psi for 2 h, by which time a
steady-state flux was observed. The water flux was calculated
as the following equation: J_w=Q/A∆t(2) where Jw is the pure water flux (L/m2h), Q the volume of water
permeated (L), A the effective membrane area (m2), and Δt the
permeation time (h).
The MWCO of a membrane is generally defined as the molecular
weight of a solute at which above 90% of the solute is
retained. In the present study, chemicals of different molecular weights including Red MX-5B (615.33 Da), PVP (10 kDa),
PVP (40 kDa) and BSA (66 kDa) were chosen to test solute rejection
and MWCO. The BSA was prepared at a concentration
of 1000 mg/L in PBS buffer (pH 7.4) and the others were prepared
at a concentration of 1000 mg/L in the ultrapure water.
The membrane holder was filled with solution and pressurized
at a constant pressure of 10 psi and stirred at 200 rpm throughout
the experiments to minimize concentration polarization.
During the filtration, the permeate solutions were collected
over a period of time. Red MX-5B and BSA concentrations
were measured by using a UV-visible spectrophotometer at a
wavelength of 530 nm and 280 nm, respectively. PVP (10 kDa
or 40 kDa) concentrations were analyzed by a TOC analyzer
(TOC-5000, Shimadzu Corp., Japan). The solute rejection was
calculated by using Eq. 3. R=1-(C_P/C_f )×100% (3) where Cp and Cf are permeate concentration and feed concentration,
Characterizations and performance assessment of
The surface morphology of TFC membrane was analyzed by
SEM. The sample preparation procedure was the same as that
of the substrate membrane. The functional groups of membrane
surface were identified by attenuated total reflection
Fourier transform infrared (ATR FT-IR) spectroscopy. Nicolet
4700 FT-IR (Thermo Electron Corporation, Waltham, MA)
equipped with multi-reflection Smart Performers ATR accessory
was used for this analysis. All spectra included the wave
numbers from 650 to 4000 cm-1 with 64 scans at a resolution
of 2.0 cm-1.
The performance of TFC membranes were evaluated by a high
pressure filtration system as presented in our pervious study
. The membrane holder (Model: XX4504700, stainless
steel, Millipore Corp., Billerica, MA) had an effective membrane
area of 9.6 cm2. Prior to test, each membrane was compressed
by DI water at 300 psi for 5 h. After pure water flux
test, salt solution (final concentration of 2000 mg/L of NaCl)
was added and the conductivity of feed and permeate solutions
was measured by a conductivity/TDS meter (HACH Company,
Loveland, CO). The measurement was conducted at 25±1
°C, which was controlled by a water circulator (Isotemp 6200
R20F, FisherScientific, Inc., Pittsburgh, PA). The pure water
flux and salt rejection were calculated with Eq. (4) and Eq. (5),
R=(1-C_P/C_f )×100%(5)where J is the water flux (L/m2h), M is the weight of permeate
(g), A is the effective membrane area (m2), t is the test time (s),
P is the applied pressure (psi), R is the rejection ratio and Cp
and Cf are the concentration of permeate and feed solution,
Results and discussion
Effects of PSf concentration on support layers
Cross-sectional and surface morphologies of support layers
and surfaces of TFC membranes were obtained through SEM
analysis and presented in Figure 1. In the cross-sectional images,
a typical asymmetric structure was observed. The fingerlike
macrovoids were suppressed with increasing PSf concentration.
Furthermore, when PSf concentration reached 19%,
the structure became sponge-like and appeared much denser
compared with support layer with 11% PSf. In general, increasing
the polymer concentration in casting solution would
result in a higher viscosity, which tends to reduce transport
rates thereby produce a slower demixing . Furthermore, the
precipitation path will cross the binodal at a higher polymer
concentration. These factors could contribute to a thicker top
layer, lower porosity and diminished macrovoid formation
The pore information for each support layer calculated by software
Image J was presented in Table 1 (images with higher resolution
were presented in Figure S1), including the average pore
size and pore area fraction of the support layers.
The pore area fraction decreased with increasing PSf concentration.
This is consistent with the previous result that the substrate
membrane became denser and less porous when the PSf
concentration was high. For TFC membrane surface, a typical "ridge and valley" morphology was observed. There is no obvious
difference between PA layers formed on different support
Water contact angle can be used as a method to test the hydrophilicity
of substrate membranes. As indicated by Figure 2, there was no significant change in the water contact angle
among membranes prepared with different PSf concentrations.
The contact angle was around 83° for all support layers.
It appeared that while there were some structural changes in
morphology of the membranes, the membrane hydrophilicity
as indicated by the contact angle measurements remained the
same, suggesting that here the hydrophilicity was mainly controlled
by the polymer chemistry.
EWC of the membrane, however, decreased significantly with
increasing PSf concentration (Figure 2). The decrease in EWC
confirmed the change of porosity in the support layer with increasing
PSf concentration, because the surface pores as well
as cavities inside the support layer are responsible for accommodating
water in the membranes .
The support layers were further assessed for their pure water flux and MWCO. Figure 3 indicated that the pure water
flux, measured under the pressure of 10 psi, decreased with
increasing PSf concentration. The water flux was minimal for
membranes prepared from 17% and 19% PSf. This is consistent
with the SEM results of membrane surfaces and crosssections,
where higher PSf concentration led to a denser layer
with thicker skin layer and smaller surface pores.
The membrane MWCOs were assessed by measuring rejections
of solutes with different molecular weights, using the dead-end system under a trans-membrane pressure of 10 psi.
As shown in Figure 4, the MWCO decreased with increasing
PSf concentration. This is consistent with our general understanding
that the support layer became denser and less porous
when the PSf concentration was high, which would lead to a
high solute rejection.
Figure 5 presents the SEM images of support layers and TFC
membranes prepared with different PVP concentrations.
Similarly, they showed an asymmetric structure consisting of
a dense top layer and a porous sublayer. The sublayer seems
to have finger-like cavities as well as macrovoid structure. For
macrovoid formation, the membrane must have a skin layer to
limit the penetration of a large amount of nonsolvent into the
sublayer and must prevent nuclei formation after a few nuclei
(which were the origins of the macrovoids) formed . PVP
is likely to be leached out from the membrane and the structure
growth rate increased with PVP, leading to the formation
of the porous layer. When the PVP concentration increased,
however, the growth rate became slower, so the top layer became
For the surface of support layers, the membranes were quite porous at 0% PVP and 3% PVP. With further increasing PVP
concentration to 5% and 10%, the surface became denser and
less porous. This could be attributed to the diffusion rate of
solvent. When PVP concentration was low, the polymer diffusion
rate is high, making the surface more porous. However,
when PVP concentration reached 5%, the viscosity played
a dominant role in the diffusion process. Low diffusion rate
made the support layer surface less porous. For TFC membrane
surfaces, there is no obvious difference in polyamide
layer surface morphology among the supports with different
For support layers with different PVP concentrations (Table 2) images with higher resolution were presented in Figure S2),
the area fraction increased when PVP concentration increased
to 3%, then it decreased with further increasing PVP concentration.
This result is consistent with the previous observation
about the trade-off correlation between the thermodynamic
enhancement and rheological diffusion inhibition.
As shown in Figure 6, water contact angle of the support membranes
decreased from 83.6 ± 4.5° to 34.5 ± 10.8° with increasing
PVP concentration, indicating an increased surface hydrophilicity.
The water contact angle of membrane with 10% PVP
was not shown here because after dropping water droplet on membrane surface, it spread too fast to measure a stable water
Furthermore, EMC increased with increasing PVP concentration. Even introducing 1% PVP into the casting solution could
increase EWC significantly. PVP increased the porosities and
enlarged the pore size, which would cause more water entrapped
inside the support layer.
Figure 7 presents the pure water flux for support layers containing
various PVP concentrations. The pure water flux increased
initially with increasing PVP concentration, however,
when the PVP concentration reached 4%, it started to decrease.
The fluxes of membranes with 5% PVP and 10% PVP
were even lower than the one without PVP.
For UF or microfiltration (MF) membranes, the pore size and pore distribution on the dense layer determine the water flux.
Here, the initial increase of water flux can be explained by the
enhanced phase separation induced by the thermodynamic
immiscibility of PVP. The decrease of water flux with a PVP
concentration over 4% could be attributed to the hindered molecular diffusion induced by the increased viscosity after
introducing PVP. At a low PVP concentration, the enhancement
of phase separation could outweigh the hindered diffusion.
The increased demixing rate on the interface induced
the rapid collapse of polymer molecules concurrent with the
formation of gaps between collapsed molecules, leading to a
more porous and permeable membrane [20, 21]. With a further
increase of PVP concentration, diffusion delay due to the
increased viscosity could overcome the enhancement of phase
separation, leading to a dense and thick top layer with low porosity
and low degree of pore interconnectivity . Therefore,
the convex relationship between water flux and PVP concentration
can be explained by the trade-off relationship between
the thermodynamic immiscibility and rheological diffusion
inhibition. Similarly, when the oxidized multi-walled carbon
nanotubes (OMWNTs)/PSf nanocomposite hollow fiber
membranes were developed , we also observed a convex
relationship between water flux and filler concentration. The
hydrophilic OMWNTs played a similar role as PVP during the
phase separation process.
Figure 8 shows the rejections of different solutes by support
membranes prepared with different PVP concentrations.
There was no clear trend for the change of MWCO with increasing
PVP concentration. However, when compared with
the MWCO of support layer containing 0% PVP presented in
Figure 4 (15% PSf), the addition of PVP was found to have
changed the MWCO to some extent.
Performance of TFC membranes
TFC membranes prepared with different support layers were
examined to study the effects of support layer properties on
the permeation behavior of TFC membranes. The membranes
were characterized in terms of water flux and salt rejection.
As presented in Figure 9, the water flux decreased with increasing
PSf concentration, while salt rejection increased at first
and leveled off. Considering the unchanged surface hydrophilicity
of support layers, the changed flux could be attributed to
the porous support layer with more water channels which can
collect permeate from the PA thin-film layer. So the effectiveness
of PA thin-film layer seemed to increase with more open support layer structure. The relatively low salt rejection (87 ±
1.5 L/m2h) for TFC membrane prepared with low PSf concentration
(11 %) could be caused by the existence of small defects
on the loose support layer after IP process. With an increase of
PSf concentration to 15 %, the salt rejection reached 97.5 %,
the water flux decreased from 40 to 30 L/m2h, though.
As presented in Figure 10, salt rejection maintained a high value of 97.5%, however, the water flux decreased with increasing
PVP concentration. Ghosh and Hoek  found that hydrophobic
and rough support membranes produced PA composites
with a higher water permeability because less PA was introduced
to the inside pores so the diffusion pass of water was
not significantly increased. Our observations were consistent
with the literature reports. With an increasing hydrophilicity
of support layer, the water permeation decreased, demonstrating
that the support layer hydrophilicity was an important
factor during the IP process. It was reported that hydrogen
bonding between MPD and hydrophilic substrates limited the
diffusion rate of MPD inside the pores of support layer. In this
situation, some TMC may diffuse into the pores and form PA
deep inside the pores creating a longer effective film thickness
for water permeation [25, 26]. That's why pure water flux could
be higher for PA composites formed over support layers which
were a little bit hydrophobic. However, highly hydrophobic
support layer was not suitable for making PA TFC. To illustrate,
we had to modify the surface of polyvinylidenedifluoride
(PVDF) membrane by plasma treatment to fabricate PVDFsupported
TFC membranes .
In order to further characterize the thickness of PA thin-film
layer, ATR-FTIR spectroscopy technique was used to identify
the main functional groups of the PA thin-film layer and
thereby estimate its thickness based on the calculated depth of
penetration (dp) of infrared beam into the sample material and
absorbance of the carbonyl-stretching characteristic band pertaining
to amide linkage .The spectra of TFC membranes
prepared with different support layers are given in Figure 11a.
The spectra indicated that the IP process had occurred since
a strong band at 1660 cm-1 (amide I) was present which is the
characteristic peak of the C=O band of an amide group. Other
characteristic peaks of PA were also observed at 1547 cm-1 (amide II, C-N stretch) and 1610 cm-1 (aromatic ring breathing)
. By comparing the peak intensity, we were able to
roughly estimate and compare the effective thickness of PA in
different TFC membranes.
During the calculation, all FTIR spectra were adjusted to the same baseline, and then the absorbance intensities of all peaks
located at 1660 cm-1 were recorded and the values are 2.68,
2.98. 2.91. 3.04, 3.22, 3.24, and 3.36 for membranes with PVP
concentrations from 0 to 10%, respectively. Although, the actual
thicknesses of PA thin-film layers could not be obtained
without testing the intensity from a standard PA sample of
known thickness, the normalized thicknesses could be calculated
by dividing all the values with 2.68 as shown in Figure 11b. The thickness increased with increasing PVP concentration.
This is consistent with our previous assumption that the
effective thickness of PA thin-film layer became larger with
increasing support layer hydrophilicity.
In this work, a series of support layers were fabricated by casting
solutions containing different PSf and PVP concentrations
through phase inversion method. And then the performances
of TFC membranes prepared based on these support
layers were systematically studied to elucidate the correlation
between support layer properties and TFC membrane performance.
The results showed that increasing the PSf concentration
in the PSf/DMF system changed the cross-sectional
structure of support layer from an asymmetric whole finger type to sponge type structure. Meanwhile, the support layer
showed a less porous surface, lower water uptake and lower
water permeability. The water permeability of resulting TFC
membranes decreased with increasing PSf concentration because
of low porous support layer structure.
In PSf/PVP/DMF system, PSf concentration was maintained at
15 wt%. The PVP with an average molecular weight of 10 kDa
was used as an additive. Results showed that with an increase
of PVP concentration in casting solution: (i) the cross-sectional
morphology of support layer changed from sponge-like to
finger-like; (ii) the surface hydrophilicity increased; (iii) the
EWC increased which may result from the increased porosity
of support layer; (iv) the water flux of resulting TFC membrane
decreased due to the increased thickness of PA thin-film
layer, while the salt rejection remained relatively high.
We gratefully acknowledge Professor Qingsong Yu in the Department
of Mechanical & Aerospace Engineering at MU for
providing us access to the video contact angle measurement
system. Financial support of this research was partially supported
by the United Geological Survey through Missouri Water
Resources Research Center.