Journal of Membrane Science 449 (2014) 109–118
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Gas transport properties of interfacially polymerized polyamide
composite membranes under different pre-treatments
and temperatures
Jonathan Albo, Jinhui Wang, Toshinori Tsuru n
Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagayami-yama, Higashi-Hiroshima 739-8527, Japan
art ic l e i nf o
a b s t r a c t
Article history:
Received 10 July 2013
Accepted 21 August 2013
Available online 27 August 2013
Thin-film composite reverse osmosis membranes were dried under different membrane pre-treatment
procedures and evaluated at increased temperatures by gas separation tests. The obtained permeance
and selectivity values indicated the presence of highly-permeable regions in the dry samples of the
commercial membranes.
Treatment with ethanol–hexane in a solvent exchange process, as well as membrane immersion in
t-butanol followed by freeze drying, increased the gas permeance by a factor of 1.8 to 9, and from 1.6 to
3.2, respectively, by comparison with room temperature and oven drying. Nevertheless, a Knudsendiffusion transport mechanism was dominant after both pre-treatments.
The permeation temperature remarkably influenced gas selectivity and permeance, and a maximum
He/N2 selectivity occurred at 150 1C with considerable high permeance results, which may suggest the
use of polyamide membranes as alternative materials for high-temperature separation processes. The
temperature-induced changes in the polymer structure and in the transport of compounds can be
explained by Knudsen and activated diffusion mechanisms throughout a highly-permeable regions and a
dense polyamide matrix, respectively.
& 2013 Elsevier B.V. All rights reserved.
Keywords:
Gas separation
RO membranes
Polyamide
Membrane pre-treatments
Temperature influence
1. Introduction
The reverse osmosis (RO) process, which uses polymeric
semipermeable membranes to achieve molecular separation, is
now an economic and universally accepted technique, with the
major breakthrough in this field being achieved with the development of thin-film composites membranes (TFC) [1]. These TFC
materials generally consist of three layers: a polyamide barrier
skin, a porous support layer (often polysulfone), and a non-woven
polyester. Both the polysulfone and polyester layers provide
mechanical support for the polyamide layer, which generally
provides selectivity to the membrane. The thin selective upper
layer and porous support can be separately optimized to give high
permeability and selectivity.
Today, most commercial RO TFC membranes are formed in-situ
by the interfacial polymerization of an aromatic polyamine such as
m-phenylenediamine (MPD) with one or more aromatic polyacyl
halides (for example, trimesoyl chloride (TMC)). These aromaticbased membranes exhibit good performance in many desalination
and water purification applications and are already in mass
n
Corresponding author. Tel.: þ 81 824 24 7714; fax: þ 81 824 22 7191.
E-mail address: tsuru@hiroshima-u.ac.jp (T. Tsuru).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.memsci.2013.08.026
production [2,3] because of the high flux and high rejection
provided by the thin separating layer in the composite structure.
Aromatic polyamide materials, however, have not merited comparatively special attention for gas separation either in dense
membranes [4–6] or composite structures [6–12] despite the
mechanical and chemical properties that make them attractive
for this separation process [13].
Since the active layer is extremely thin in TFC membranes,
treatment prior to their use for gas separation is a very sensitive
process. Usually this pre-treatment involves two general steps:
(1) cleaning of the membrane surface by a solvent, and (2) drying
of the residual solvent from within the membrane structure. The
water-swollen hydrogel that fills the pores in the support membrane becomes a rigid glass when dried for use in gas separation,
giving very low gas permeability, although relatively high selectivities may yet be achieved [2]. Therefore, membrane pretreatment can cause shrinkage and swelling, but can also cause
the removal of residual monomers or additives and morphological
changes that effectively varied membrane properties [7,8,14–21].
Additionally, RO membranes are generally perceived to have a
non-porous separating layer where transport occurs via a solution–
diffusion process, but an examination of the literature indicated that
some dry RO membranes exhibits highly-permeable regions that are
usually attributed to the presence of membrane defects [7,8,16–19]
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J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
formed either during the membrane preparation or during drying
treatments. For instance, Louie et al. [7] identified defects in
commercial polyamide RO membranes that were eliminated when
the membrane surface was coated with a polyether–polyamide block
copolymer, suggesting that using n-butanol as a solvent for applying
coatings negatively affects water and gas permeation. Kuehne et al.
[19] studied the effect of varying processing conditions during TFC
RO membrane fabrication and reported an enhanced flux after
washing the membrane with glycerol and organic salts. They
attributed rejection differences between membrane samples to
membrane imperfections. Jezowska et al. tested RO polyamide
membranes after a cleaning step concluding that membrane homogeneities could be partially caused by improper pre-treatment of the
membrane material [16]. However, Sridhar et al. [11] tested interfacial polymerized TFC membranes with an ultrathin defect-free
polyamide skin layer for gas separation. The limited selectivities
obtained, in comparison to that of a dense film, suggested consideration of the presence of less cross-linked regions in the polyamide separation layer. Li et al. [12] recently compared defect and
defect-free interfacially-polymerized TFC membranes, attributing the
deterioration of gas selectivity not only to membrane defects, but to
cross-linked polyamide layer density. As a consequence, a clear
understanding of the TFC RO polyamide membrane pore structure
has not yet been achieved.
In general, the transport of molecules at different temperatures is
dependent upon the available free volume in the polymer matrix, as
well as sufficient energy of the molecules to overcome attractive
forces between chains. In this sense, analysis of the change in
membrane permeation due to polymer flexibility with temperature
is useful for characterization of the TFC RO polyamide membrane
pore structure. Because RO membrane pores are equal to, or only a
few times the size of gas molecules, transport across these membranes can be expected to occur in the intermediate regions from
Knudsen diffusion and activated diffusion. Ideally, this mechanism
enables gas molecules to be separated, while the separation of those
gas molecules is difficult if their diameter sizes are similar [22].
Since different conditions can influence membrane heterogeneity and the transport of compounds through the selective
polyamide layer, the main objective of this work was to systematically explore whether drying pre-treatments and temperature
can result in improvements in asymmetric RO membranes for gas
separation. To the best of our knowledge, no studies of the effect of
increasing permeation temperature on the separation performance of TFC RO polyamide membranes have been reported.
The effects are discussed in terms of permeance and separation
factor variations. An additional part of this work was devoted to
determining whether such effects also produce the transformation
of inhomogeneous regions into denser structures, thus defining
the transport mechanism through the membrane.
A clearer understanding of the effects of these processes on
membrane properties will aid in the development of improved
membranes with better separation performance for hightemperature processes.
Table 1
Water permeability and salt rejection of RO membranes as listed by the provider.
Membrane
Test conditions
Water permeability
[L/(m2 h bar)]
Salt rejection
[%]
SWC5
ESPA2
CPA5
32,000 ppm; 5.5 MPa; 25 1C
1500 ppm; 1.05 MPa; 25 1C
1500 ppm; 1.55 MPa; 25 1C
1.38
4.15
3.28
99.8
99.6
99.7
meta-bisulfite solution. Table 1 shows the water permeability
and salt rejection in the RO membranes under the conditions
listed in the provider information data sheet.
The membranes consisted of a thin-film-composite with a topskin aromatic polyamide, PA, layer ( 200 nm), a middle microporous polysulfone, PSF ( 40 mm), and a bottom polyethylene
terephthalate, PET, layer ( 120 mm). The specific chemical composition of the PA layer is proprietary information of the supplier.
PA membranes were additionally formed in-situ by interfacial
polymerization [23] and applied in some sections of this work for
comparison. Briefly, 1 ml of aqueous solution of 1,3-phenylenediamine (m-phenylenediamine, MPD) (2 wt%) and sodium lauryl sulfate
(0.15 wt%) was poured on a PSF membrane, and then the excess of
the solution was removed softly with filter paper. Subsequently, a
1 ml hexane solution of 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride, TMC) (0.1 wt%) was poured on the support. After a 1min polymerization reaction, the excess solution was drained, and
the membrane was dried in air for 15 min. Finally, the membrane
was rinsed with deionized water.
All high purity chemicals of analytical grade applied in this
study were provided by Sigma Aldrich (Japan).
2.2. Membrane pre-treatment methods
Based on a review of the available literature, three different
common membrane pre-treatments were selected [11,24,25]. The
procedures were adapted and applied to membrane samples prior
to their use in gas separation:
Room Temperature–Oven (RTO): Membranes were washed several times in a pure-water bath, then dried at room temperature for 24 h, and finally placed in an oven at 120 1C for 30 min
[11].
Ethanol–Hexane (EH): Membranes were washed several times in
a pure-water bath, then immersed in ethanol for 5 min and
afterwards soaked in a hexane bath for 1 min. Finally, the solvent
was evaporated at room temperature for 15 min [11,24].
Freeze Drying (FD): Membranes were washed several times in a
pure-water bath, then immersed in 50, 75, 90, 95, and 100 wt%
t-butanol aqueous solutions for 15 min. Then, membrane
samples were placed in pure t-butanol in freeze-dried equipment under vacuum for 2 h [25].
2.3. Gas separation experiments
2. Experimental
2.1. Materials
Three commercial RO membranes were provided by Niito
Denko (Japan) and applied in this study: SWC5 (seawater
membrane), ESPA2 (energy-saving RO membrane), and CPA5
(high-rejection RO membrane). Membrane samples were vacuum
sealed in a polyethylene bag containing less than 1% sodium
Membrane samples (2.21 cm2) were tested in a stainless
permeation cell using He, H2, CO2, O2, N2, C3H8, and SF6 at
temperatures that ranged from ambient (room temperature,
16 73 1C) to 200 1C in an oven. A schematic drawing of the
experimental apparatus appears in Fig. 1. The feed gas pressure
was set at 2.5 bar, and the permeate was at atmospheric pressure.
The flow rate of the permeating gas was measured using a bubble
flow meter. Prior to testing, membranes were under vacuum with
a He flow for 1 h. Additionally, samples were under vacuum for
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J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
He
H2 CO2 O2
N2
C 3 H8
SF6
Fig. 1. Schematic drawing of the gas experimental setup.
5 min between different gas measurements in order to remove the
gas from within the gas permeation equipment.
Gas permeances were calculated using the following equation:
PðiÞ ¼
jðiÞ
ð1Þ
2
where P(i) is the permeance of i (mol/(m s Pa)), J(i) is the
permeate flux (mol/(m2 s)), and Δp(i) is the partial pressure
difference (Pa). The ability of the membrane to separate different
gases, depending on the separating layer properties, was quantified by the membrane gas selectivity:
PðiÞ
PðjÞ
10-8
SWC5
ESPA2
CPA5
10
-9
2
3
4
5
6
Kinetic diameter [Å]
ΔpðiÞ
αði=jÞ ¼
Permeance [mol/(m2·s·Pa)]
10-7
Fig. 2. Single-gas permeation of the three commercial PA membranes after drying
at RTO. Permeance was measured at room temperature (167 3 1C).
3. Results and discussion
3.1. Gas permeation properties of RO samples
ð2Þ
where α is the membrane selectivity for gas i relative to gas j.
It is generally recognized that the gas transport mechanism
through polymeric membranes is controlled by the solution and
diffusion processes of the permeating gases, where the temperature dependence of permeance is decided by the relative magnitude of two counteracting contributions: kinetic (diffusion) and
equilibrium (sorption). The number of molecules with kinetic
energy larger than the activation energy for permeation, Ep, is
proportional to exp( Ep/RT), assuming a Maxwellian velocity
distribution of molecules. Therefore, the permeance P(i) for the
activated diffusion mechanism can be expressed by an Arrheniustype relationship:
EpðiÞ
ð3Þ
PðiÞ ¼ CðiÞexp
RT
where C(i) is a constant depending on the system and Ep(i) (kJ/mol)
is the activation energy for the permeation of gas, i, which is the
difference between the sorption energy and substantial activation
for diffusion.
2.4. Characterization
a) Infrared spectroscopy
The top surfaces of the membrane samples were pressed down
against a 451 single-reflection internal element in a JASCO FTIR4100 Fourier Transform Infrared (FITR) with Attenuated Total
Reflection (ATR-IR) spectrometer. Results were treated using a
JASCO Spectra Manager Version 2.1.
b) Thermal stability
The polymer thermal stability was characterized by simultaneous thermogravimetric analysis (TGA) and by differential
thermal analysis (DTA). Measurements were carried out in a
Shimadzu TG/DTA-60 apparatus in nitrogen for a single heating
cycle between room temperature and 500 1C at a constant
heating rate of 10 1C/min. For TGA, the mass loss of the sample
during heating was recorded.
a) Scanning electron microscopy
Surface images of membrane samples were obtained with a
Hitachi S-4800 scanning electron microscope (SEM) using an
accelerating voltage of 4 kV. Samples were sputter-coated with
palladium/platinum to minimize charging in a JEOL JFC-1300
Auto Fine Coater.
Fig. 2 shows the single-gas permeance, P, of all gases tested at
room temperature as a function of kinetic diameter for three
membrane types (SWC5, ESPA2 and CPA5) when pre-treated with
RTO procedure. Samples were cut from the same membrane sheet,
which minimized the membrane heterogeneity effect, and three
samples were measured for each membrane type. Gas permeation
variability for membrane samples of the same type varied by as
much as 16.2%; these differences were attributed to uneven
membrane density and to heterogeneities in the separating layer.
N2 permeance, P N2 , in the SWC5 membrane resulted in
P N2 ¼1.64 70.16 10 8 mol/(m2 s Pa), which was within the value
range reported in the literature for the same membrane type,
P N2 ¼1.6 10 8–3.4 10 8 mol/(m2 s Pa), and corresponded to
the high permeance of 50–100 GPU (1 GPU ¼3.4 10 10 mol/
(m2 s Pa)) [7].
The membrane average selectivity (α), calculated using Eq. (2),
for each gas with respect to nitrogen permeance is presented in
Table 2.
The gas selectivity value for all membranes under RTO pretreatment was consistent with Knudsen characteristics for CO2, O2,
C3H8, and SF6 gases. Nevertheless, when compared with Knudsen
diffusion, a slightly higher selectivity value was obtained for He
and H2 permeance, which may indicate a mixed transport
mechanism.
The performance of these commercial membranes for the He/
N2 pair resulted in an unfavorable result when compared with the
performance for the same pair in pure aromatic PA dense membranes synthesized from monomers bearing methyl substituents
and p-phenylene hinge-like connecting linkages (α(He/N2) ¼
67.39) [4] and for PA membranes bearing a pendent phenyl group,
a hexafluoroisopropylidene (6F) linkage (α(He/N2)¼ 40) and a
sulfonyl (SO2) linkage (α(He/N2) ¼ 100) [26]. Additionally, the
result was far from that of H2/N2 selectivity achieved with similar
dense materials, such as poly(amide-imide), in Robeson's upper
bound correlation, (α(H2/N2)¼72) [27].
The lower selectivities reported for the commercial membranes
could be ascribed either to membrane defects or to the presence of
a less crosslinked PA layer, where the higher polymer motion led
to a decrease in gas separation. Moreover, N2 permeance in the
PA membrane formed in-situ in this work resulted in
P N2 ¼9.46 70.34 10 10 mol/(m2 s Pa), which was about a twoorder of magnitude lower permeance than that obtained by the
TFC membranes. The relatively high permeances obtained may
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J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
He
Table 2
Gas selectivity values for three commercial RO membranes under RTO.
SWC5
ESPA2
CPA5
Knudsen
Gas selectivity, α [–]
1.647 0.16
1.30 70.14
1.117 0.18
He/
N2
H2/
N2
CO2/
N2
O2/
N2
C3H8/ SF6/
N2
N2
3.96
4.10
4.30
2.64
4.03
4.23
4.55
3.73
0.82
0.95
0.77
0.80
0.96
0.95
0.92
0.94
0.80
0.82
0.80
0.80
0.42
0.46
0.42
0.44
Permeance [mol/(m2·s·Pa)]
Membrane N2 permeance, P N2 108
[mol/(m2 s Pa)]
10-5
10-6
Absorbance
CPA5-PSF
1600
CPA5-RTO
10-7
10-8
2
3
4
5
6
Fig. 4. Single-gas permeation of CPA5-RTO and CPA5-PSF samples at room
temperature (167 3 1C).
(a)
1800
CPA5-PSF
Kinetic diameter [Å]
(d)
(b)
SF6
10-4
10-9
(c)
CPA5-RTO
CO2 N2
10-3
1400
1200
1000
Wavenumber [cm-1]
Fig. 3. IR normalized spectra for the CPA5-RTO sample and membrane exposed to
hypochlorite solutions (CPA5-PSF). (a) Amide bonding, C–N stretching; (b) amide
bonding, C¼ O stretching; (c) PSF, C–O stretching; and, (d) PSF, symmetric SO2
stretching vibration.
indicate that the PA separating layer of the commercial TFC
membranes consists of two different structures: a dense matrix
and highly-permeable regions in which gas flows primarily via
Knudsen diffusion.
3.2. Estimation of highly-permeable regions in the separating layer
Since the PA active layer of RO membranes is extremely
vulnerable to chlorine compounds, a membrane sample was
immersed in a sodium hypochlorite solution for 96 h [28]. This
procedure was intended to chemically remove the PA layer, to
which selectivity membrane characteristics are attributed, and
therefore to evaluate the separation performance of the middle
and support layers (PSF-PET). CPA5 membrane was primarily
applied for the remainder of the present work due to the slightly
higher He/N2 selectivity, as shown in Table 2.
The partial removal of the PA top layer was confirmed in the
ATR-FTIR analysis presented in Fig. 3. The FTIR spectra was
normalized with 1148 cm 1 band absorbance, which can be attributed to the PSF symmetric SO2 stretching vibration [11], since PSF
was expected to remain invariable after hypochlorite exposure.
As observed, the spectra bands attributed to amide bonding at
1581 and 1488 cm 1 due to C–N and ¼O stretching of the amide
group [11] reduced their intensity. The absorbance was reduced to
52.1% and 41.5% for 1581 and 1488 cm 1 bands, respectively, by
comparison with CPA5-RTO, indicating the partial removal of the
PA layer. This can be explained as the modification produced by
chlorine exposure in the hydrogen bonding and ring chlorination
during hypochlorite exposure, which led to the polyamide layer
failure due to alteration [28]. However, bands at 1238 and
1148 cm 1 that are attributed to C–O and symmetric SO2 stretching vibration [11] confirmed that PSF remained after the treatment
due to the lack of active hydrogen that confers a high resistance to
chlorine exposure [29].
CPA5-RTO and CPA5-PSF samples were evaluated in terms of
their permeance, and the results are presented in Fig. 4.
The membrane permeance in CPA5-PSF was increased as high as
three orders of magnitude for the studied gases, with selectivities
below typical Knudsen values (i.e., α(He/N2)¼2.45, α(CO2/N2)¼0.69,
α(SF6/N2)¼0.46). This reflects the increasing contribution of convective gas flow when the PA layer was partially removed, hence, a
reduction in gas separation (gas selectivity) compared with the
CPA5-RTO sample. Therefore, Fig. 4 confirms that the separation
characteristics of the RO commercial membrane for gas separation
can mainly be attributed to the top aromatic PA layer with a dense
matrix and highly-permeable regions.
3.3. Gas permeation under different pre-treatments
Values for the nitrogen permeation and gas selectivity of the
samples treated under different drying procedures (RTO, EH and
FD) are shown in Figs. 5 and 6, respectively.
Gas permeance was increased by factors of 1.8 to 9 for EH
samples, and from 1.6 to 3.2 for FD compared with that in RTO
samples (Fig. 5), showing the degree to which the different pretreatments produced layer alterations with effect in gas permeance. The nitrogen permeance value for the ESPA2-EH sample
was the highest, and was significantly higher than that of the
SWC5-RTO sample. It is remarkable that the permeance was
increased by factors of 9 and 7.1 for ESPA2-EH and CPA5-EH,
respectively, when compared with that of the RTO sample. However, SWC5 showed no significant changes after the three procedures. The small changes in gas permeation indicated that SWC5
consists of a rigid material that it less affected by pre-treatments,
while ESPA2 and CPA5, with higher water permeabilities than the
SWC5 membrane (Table 1), present larger ridge-and-valley structures that cause gas permeance to be highly dependent on the pretreatment methodology applied to these materials.
In the FD procedure membrane morphology is fixed with a
minimized distortion that may occur during normal drying, and
thus low membrane shrinkage is expected [30]. Besides, alcohols
are known to swell polymers to various degrees, influencing the
free volume between polymer chains and, as a consequence, the
separation performance [14], which occurred in the EH pretreatment.
The results showed that highly-permeable regions did not
permanently shrink or collapse when the membrane was dried with
ethanol–hexane or freeze drying, as such changes would have
resulted in lower permeances and gas transport continued to be
dominated by a Knudsen diffusion-based flow after both pretreatments (Fig. 6).
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J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
Table 3
Gas permeance of CPA5 after RTO, EH and progressive solvent exchange (PSE) pretreatment.
N2 permeance [mol/(m2·s·Pa)]
10-6
Membrane-treatment type Permeance, P 108 [mol/(m2 s Pa)]
10-7
He
10
CPA5-RTO
CPA5-EH
CPA5-PSE
-8
10-9
RTO
EH
SWC5
FD
RTO
EH
ESPA2
FD
RTO
EH
FD
CPA5
Fig. 5. N2 permeance of three commercial PA membranes after different pretreatments.
H2
CO2
N2
4.79 7 0.33 5.077 0.51 0.86 7 0.06 1.03 7 0.14
20.8 7 1.3
30.1 72.2
6.58 7 0.45 7.92 7 0.87
20.6
29.7
6.18
8
mechanisms through which treatment with alcohols increase gas
permeance in polyamide membranes are expected to be affected
by other phenomena, such as the effective removal of plugging
compounds from the membrane surface by pure ethanol or the
interactions related to polymer–solvent solubility producing the
swelling of the cross-linked polymer, on dependence of the ridgeand-valley membrane structures.
6
He / N2
3.4. Correlation of nitrogen and pure water permeability
4
Fig. 7 features a plot of the nitrogen permeance under RTO, EH
and FD pre-treatments versus water permeability values from
provider information (Table 1).
Gas and water permeabilities did not generally correlate for FD
and RTO procedures, except when there was EH pre-treatment. As
discussed above, this could be attributed to preservatives or
other compounds that were present initially in the samples that
were not dissolved in water, but were dissolved during ethanol
immersion, and the avoidance of pore shrinkage during the
ethanol-hexane pre-treatment, probably preserving the original
ridge-and-valley membrane structure and a good gas and water
permeability correlation.
2
0
H2 / N2
6
4
2
0
3.5. Thermal characterization
CO2 / N2
2
1
0
RTO
EH
FD
Fig. 6. Gas selectivity values for RTO, EH and FD procedures in CPA5. Knudsen
diffusion-based selectivity values are denoted by the black horizontal lines.
Based on these results it is also possible that CPA5 and ESPA2
membranes had some permeable regions plugged by compounds
(i.e., residual synthesis monomers or additives) that were removed
only by ethanol immersion, but not in water contact. An enhanced
permeance after treatment with a strong solvent was also reported
in the literature for interfacially polymerized PA composite membranes, explained by the partial dissolution of small molecules
fragments from the surface [31].
Consequently, a CPA5 membrane sample was additionally
immersed in 50, 75, 90, and 95% aqueous solutions, and then in
pure ethanol for 15 min in a progressive solvent exchange pretreatment (CPA5-PSE) to evaluate if the membrane was susceptible
to an ethanol attack during pure ethanol immersion in EH. The
results are presented in Table 3, together with permeances
obtained in the CPA5-RTO sample for comparison.
As shown in Table 3, the EH and PSE treatments showed similar
permeances, indicating that the integrity of the membrane structure was preserved after immersion in pure ethanol. Therefore, the
For the potential application of TFC PA membranes at elevated
temperatures a thermal characterization is required. The glass
transition temperature, Tg, is significant in determining the
mechanical properties of the TFC membrane since structure and
separation properties could sharply change at this point [32]. For
DTA, the Tg is taken at the inflexion point of the heat capacity
change, while the melting point, Tm, refers to the minimum of the
exotherm peaks. The TGA and DTA curves under a nitrogen
atmosphere are shown in Fig. 8 for a CPA5-RTO PA composite
membrane and Table 4 summarizes the four inflexion temperatures detected in the DTA curve. In addition, Tg for different TFC
layers after isolation were obtained from the respective DTA
curves (see Supplementary information) for comparison. A thin
blade was used to carefully remove the PET substrate from the
composite membrane, while the PSF middle layer was isolated
after hypochlorite immersion. Because of the difficulties in isolating the PA layer from the TFC, a thin blade was used to remove the
pure PA from an interfacial polymerization membrane formed insitu and the Tg was included for discussion.
The first inflexion point was related to the PA Tg, followed by
crystallization and a second minimum at the exotherm peak,
Tm ¼211.1 1C. This was in good agreement with the literature
where a Tg and Tm at 190 1C and 200–221 1C, respectively, were
observed for dense PA-6 [33]. In addition, the Tg was similar to that
obtained for the formed pure PA material in the present study,
Tg ¼ 186.6 1C. After isolation, the PSF glass transition temperature,
Tg ¼ 183.3 1C, was in good accordance with the value obtained via
differential scanning calorimeter measurements (Tg ¼ 185 1C) [34].
Note that the influence of the PSF substrate on the Tg value of the
TFC membrane was hidden in the figure due to similar thermal
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J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
10-6
10-7
10-8
SWC5
10-9
0
1
ESPA2
CPA5
2
3
0.5
2
3
-20
4
T = 192.9 ºC
150
200
250
100
200
300
400
He
N
He/N
0
RT
50
100
150
200
Fig. 9. Gas permeance and He/N2 selectivity for CPA5-RTO after increases in the
permeation temperature.
Temperature
cycle
0
500
Temperature [ºC]
Fig. 8. DTA (in black) and TGA (in gray) curves for CPA5-RTO composite membrane
(heating constant rate of 10 1C/min) up to 500 1C.
Table 4
DTA inflexion temperatures for the TFC membrane.
Membrane
T [1C]
Notes
CPA5-RTO
192.9
211.1
250.9
441.4
PA glass transition followed by crystallization
PA melting point
PET melting point
Material decomposition
conductivity that the PA layer. A third minimum exotherm peak for
the TFC was detected at 250.9 1C and was ascribed to PET melting
point [35]. Tg for PET after isolation was found to be Tg ¼61.3 1C.
The final peak at 441.4 1C showed the material decomposition
temperature, since it also brought a sharp decrease in the TGA
curve (69% membrane weight reduction).
Because of the high glass transition, Tg, and decomposition
temperature, these PA-based membranes show promise as highperformance polymeric material for gas separation processes at
elevated temperatures.
3.6. Effect of temperature on gas separation
Temperature-permeance dependency at increasing temperatures (room temperature (RT), 50, 100, 150, and 200 1C) for He and
N2 transport though a CPA5-RTO membrane sample is presented
in Fig. 9. The temperature was progressively increased and controlled by an oven sensor placed on the flat module surface.
As Fig. 9 shows, with an increase in temperature He permeance
was importantly enhanced up to 150 1C, and then decreased. On
the other hand, N2 permeance did not vary during exposure to
temperatures reaching 150 1C. Relaxation is very temperaturedependent for polymeric materials [36] and so, membrane
T [1C] Permeance, P 108
[mol/(m2 s Pa)]
He
Gas selectivity α(He/N2)
[–]
N2
First
RT
50
100
4.86 7 0.6
1.137 0.15
4.3 70.2
5.98 7 0.9 1.03 7 0.12
5.8 70.4
17.8 7 0.21 1.08 7 0.13 16.6 70.3
Second
50
150
6.197 0.63 1.127 0.19
5.5 70.3
31.7 7 4.1
1.117 0.18 28.4 70.5
Third
50
200
50
6.86 7 0.55 1.107 0.13
0.85 7 0.33 0.08 7 0.04
0.737 0.12 0.09 7 0.03
0.25
300
-30
0
Weight [mg]
Heat Flow [µW/mg]
0.75
1
10
10-9
Table 5
Permeance and separation factors for CPA5-RTO with increasing temperatures in
cycles.
1
-10
10-8
Temperature [ºC]
Fig. 7. N2 permeance after different pre-treatments versus pure water permeability
for the three commercial membranes. RTO (○), EH (Δ) and FD (□).
0
20
5
Water permeability [L/(m2·h·bar)]
10
10-7
10-10
4
30
Gas selectivity [-]
Permeance [mol/(m2·s·Pa)]
N2 Permeance [mol/(m2·s·Pa)]
10-6
6.2 70.2
10 70.8
8.1 72.2
polymer chain mobility was enhanced for He permeation but not
to the extent to facilitate the permeation for larger molecule sizes,
such as N2, through the dense matrix. Hence, this phenomenon led
to an enhanced He/N2 gas selectivity at 150 1C. This is in similarity
to the highest salt rejection values for RO membranes heat-treated
in a 120–150 1C temperature range [37].
Further temperature increases to 200 1C, beyond the glass
transition point (Tg ¼192.9 1C), led to crystallization. This material
reorganization brought a sharp decrease in He and N2 permeance,
suggesting that a partial closing of membrane pores had occurred.
To further explore the temperature effect, He and N2 were
evaluated at increasing–decreasing temperatures. The cycles were
defined as follows: (first) RT (16 73 1C), then 50 1C and 100 1C;
(second) 50–150 1C; and (third) 50–200 1C to finally measure gas
permeance at 50 1C. The results are presented in Table 5.
He and N2 permeances remained practically invariable at 50 1C,
in comparison with that at 50 1C after exposures of 100 1C (first
cycle) and 150 1C (second cycle), which meant that CPA5 membrane suffered no irreversible structure alteration at temperatures
up to 150 1C. Conversely, after exposures to 200 1C (third cycle),
membrane performance (permeance) was drastically reduced for
both gases. In fact, reductions of 95 and 92% for He and N2
permeance, respectively, were obtained, which led to an increase
in gas selectivity, α(He/N2), from 5.8 to 8.1 due to the reduced
number of highly-permeable membrane structures. If those
regions had completely shrunk at the applied temperature, there
would have been an important increase in gas selectivity because
gas would have been forced to permeate through the dense
polymer matrix. These results confirmed the irreversibility of
membrane chain alterations beyond the Tg of the material, and
the partial closing of membrane highly-permeable regions. In
Fig. 10, surface images obtained by SEM revealed the roughness
J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
of the membrane skin layer. The samples were placed in an oven
for 1 h at 50, 150 and 200 1C before SEM.
CPA5 images after exposure to 150 1C (c–d) show a rougher
appearance than the membranes treated at 50 1C (a–b). This could
have been related to a membrane densification process taking
place at this temperature, as reported in the literature [37]. In
addition, the appearance may have been related to an enhancement in dehydration and the removal of the preparation residual
groups from the membrane surface after exposure to 150 1C,
leading to a smoother top layer probably without affecting the
core. A further temperature increase beyond the Tg at 200 1C (e–f),
clearly showed physical alterations, in accordance with the aforementioned results. The membrane assumed a flatter surface,
influencing the width and length of the highly permeable regions
of the PA selective layer.
To better understand membrane morphology and performance
at high temperature, a CPA5 sample treated with hypochlorite
solution (CPA5-PSF) was tested for gas permeation at 200 1C.
This determined whether it was the PA membrane or the PSF/
PET support that were mainly responsible for the gas selectivities
after Tg. Fig. 11 shows the obtained results. The values for
permeation through the PSF/PET substrate treated at RT and at
150 1C were included for comparison, although no alterations are
expected below the Tg point.
Similar, or slightly higher, permeance values compared with
those at RT and 150 1C, were obtained for a CPA5-PSF sample
115
treated at 200 1C. Also, the permeance results (103–104 mol/
(m2 s Pa)) were remarkably higher than those obtained for the
CPA5 TFC membrane (108–109 mol/(m2 s Pa)), reflecting the
increasing contribution of convective gas-flow in the PSF/PET
sample. Consequently, the separation performance of CPA5-RTO
after exposure at 200 1C may yet be attributable to the PA layer.
3.7. Transport model at increasing temperatures
In the region of activated diffusion, molecules with size
differences can be effectively separated by molecular sieving.
When the activated diffusion transport mechanism is dominant,
the permeation has a tendency to increase along with the
temperature. Fig. 12 shows an Arrhenius plot (Eq. (3)) of the
permeances observed for He, CO2, O2, N2, and SF6 below Tg of
the TFC.
He and CO2 permeances increased with increasing temperature. Actually, the permeance was enhanced by a factor of 7.4 for
He and 2.2 for CO2 at 150 1C when compared with the performance at room temperature. Alternatively, O2, N2 and SF6 permeances remained practically constant with temperature changes,
which possibly can be explained by the Knudsen transport
mechanism. If the molecules permeate uniquely via Knudsen
diffusion mechanism through the TFC membrane, the dependence
of permeance on temperature should be independent of the gases
species and pore size, and thus the experimental gas permeances,
Fig. 10. SEM images of a CPA5 membrane sample under increasing temperatures. Images on the left side were taken at 10k magnification and on the right at 25k. (a–b) 50 1C,
(c–d) 150 1C, (e–f) 200 1C. The scale bar at the lower right represents 5 mm (10 k magnification) and 2 mm (25k magnification).
116
J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
CO2 N2
SF6
10-4
He
200 ºC
10-4
150 ºC
P · (MRT)0.5 [-]
Permeance [mol/(m2·s·Pa)]
He
RT
O
N
SF
10-5
10-5
10-6
10-6
2
3
4
5
Fig. 11. Single-gas permeation of CPA5-PSF sample at room temperature after RT,
150 and 200 1C temperature treatment.
10-6
He
CO
O
2
SF
N
4
5
6
Kinetic diameter [Å]
Fig. 13. Comparison of permeance, P, multiplied by (MRT)0.5 observed at RT, 50,
100, and 150 1C for the studied gases in CPA5-RTO. Larger markers indicate higher
temperatures.
Table 6
Activation energies for permeation, Ep, in CPA5-RTO.
CPA5-RTO
10-7
Ep [kJ/mol]
10
3
6
Kinetic diameter [A°]
Permeance [mol/(m2·s·Pa)]
CO
Gas
He
CO2
O2
N2
SF6
15.96
6.15
1.21
0.07
0.06
-8
10-9
2
2.5
3
3.5
1000/T [K-1]
Fig. 12. Arrhenius plot for studied gases through CPA5-RTO sample. The results
from three replicates were within 15.7% of the values shown.
P, multiplied by (MRT)0.5 should be identical across all temperature range [38]. Fig. 13 shows the values of P(MRT)0.5 for all gases
at the experimental temperatures.
The highest P(MRT)0.5 values obtained for He and CO2 confirmed an activated diffusion transport mechanism. Aromatic PA
materials are considered to have a high degree of chain rigidity
provided by the presence of aromatic ring and the strong interchain forces, which accounts for an elevated cohesive energy
density and molecular packing [4]. Thus, the limited increase in
PA chain mobility with temperature also produced a limited
increase in membrane free volume, resulting in sufficient energy
for small gases to overcome the attractive forces between chains.
Table 6 summarizes the activation energies for permeation, Ep (Eq.
(3)). As expected, the values for He and CO2 were the highest
among the studied gases species.
The higher permeance at increasing temperature reported for
He (2.6 Å) and CO2 (3.3 Å), when compared with that for O2
(3.46 Å), N2 (3.64 Å) and SF6 (5.5 Å), denoted an effective molecular sieving separation in the PA-dense region. Simultaneously,
CO2 has a relatively large affinity to PA materials due to reactive
functional groups of amine moieties [39–41]. Therefore, chemical
reactions between permeating species and pores may also cause
the separation performance to vary at increasing temperatures, in
comparison with the increases observed in He permeation.
The results confirmed that the dry PA layer consisted of two
different pore structures. First, a dense matrix where chain
mobility with temperature enabled permeation by the activated
diffusion of small gases, such as He, and second, highly-permeable
regions where larger species, such as N2, could permeate exclusively via a Knudsen mechanism.
Finally, Table 7 summarizes the separation performance of
polymeric materials in the Robeson's upper bound relationship
for He/N2, H2/N2 and CO2/N2 in a 25–35 1C temperature range [27],
in comparison with the values achieved in this work at 150 1C for a
CPA5-RTO TFC membrane and for a PA membrane formed in-situ
by interfacial polymerization (PA in-situ). The obtained permeance
results for He (2.6 Å) in this work are compared with those for H2
(2.9 Å) in the literature, since they are expected to permeate in a
similar manner by the same transport mechanism. The values are
compared in permeance (mol/(m2 s Pa)).
The gas selectivity values achieved in the present study at
150 1C were comparable to those found in the literature at room
temperature, although the membrane did not outperform the
previously reported highest separation factors for H2/N2 [46] and
CO2/N2 [51]. Nevertheless, the PA membranes had a considerably
high He and CO2 permeance. Actually, the He permeance at
150 1C was PHe ¼36.2 71.6 10 8 mol/(m2 s Pa) and PHe ¼41.8
75.3 10 8 mol/(m2 s Pa) for CPA5-RTO TFC membrane and
for the PA membrane formed in-situ by interfacial polymerization
(PA in-situ), respectively. These high He (2.6 Å) permeances outperformed the target set by the U.S. Department of Energy for
membranes to be used in high purity H2 (2.9 Å) production
P H2 ¼34 10 8 mol/(m2 s Pa) (1000 GPU) [52]. Therefore, these
results endorse the use of PA membranes as alternative materials
for H2 purification, as well as in high-temperature separation
processes.
Consequently, as far as we could ascertain, these results
represent the first report of a bi-modal structure for interfacially
polymerized PA TFC membranes. These structures were clearly
identified after measuring the permeation-temperature dependency of PA-based membranes. The results will be useful in the
development of efficient gas-separating membranes for hightemperature processes.
4. Conclusions
In this work, polyamide-based thin-film composite reverse
osmosis membranes were dried under different pre-treatment
117
J. Albo et al. / Journal of Membrane Science 449 (2014) 109–118
Table 7
Performance of polymeric materials for He, H2 and CO2 with respect to N2.
Membrane type
Permeance, P 108 [mol/(m2 s Pa)]
He
Polyimide (6FDA-6FpDA:DABA (2:1))
Polyarylate (TMHFBPA I/T)
Hyflons AD60X
Poly(trimethylsilylpropyne)
CPA5-RTO TFC, 150 1C
PA in-situ, 150 1C
13.6
0.10
0.28
5.44
36.2
41.8
Gas selectivity,
65
64.8
50.3
1.03
28.4
99.2
H2
Polybenzoxazinone imide (PBOI-2-Cu þ )
Polyimide (1,1-6FDA-DIA)
Polyimide (NTDA-BAPHFDS(H))
PIM-7
PIM-1
Poly(trimethylsilylpropyne cophenylpropyne) (95/5)
0.01
0.03
0.04
1.03
0.95
8.40
960
165
141
20.5
14.1
2.5
CO2
Poly[bis(2-(2-methoxyethoxy)ethoxy) phosphazene]
PIM-7
PIM-1
CPA5-RTO TFC, 150 1C
PA in-situ, 150 1C
0.08
1.32
1.67
1.89
6.02
62.5
26.2
25
17.2
14.3
procedures and were evaluated at increasing temperatures by gas
separation tests.
As a result, the gas permeation properties through the membranes changed depending on the pre-treatment procedures
applied to the membranes and the influence on the swelling/
shrinkage of the selective layer. In particular, an increase by a
factor of 1.8 to 9 and from 1.6 to 3.2, was observed when
the membranes were pre-treated with ethanol–hexane and
freeze-drying, respectively, to samples dried at room temperature
and oven. Additionally, gas and water permeability values did
not correlate for freeze-drying and room-temperature–oven
treatments, but there was correlation for the ethanol–hexane
pre-treatment procedure. This was mainly attributed to the
avoidance of the pore shrinkage occurred during the ethanol–
hexane procedure, as well as the removal of plugging compounds
from within membrane structure during alcohol immersion.
Moreover, the permeation temperature importantly influenced
gas permeability and selectivity. A maximum He/N2 gas selectivity
was obtained at 150 1C, α(He/N2)¼ 28.4, explained by the activated
diffusion of He through the dense polyamide region of the membrane, with an activation energy for permeation of Ep ¼ 15.96 kJ/mol.
Further temperature increase to 200 1C, beyond glass transition
temperature, led to membrane structure reorganization, and caused
a sharp decrease in He and N2 permeance. The results were
confirmed by SEM images.
Consequently, gas permeation tests revealed that the
polyamide layer of the composite membranes was not perfectly
homogenous, but rather consisted of a dense matrix and highlypermeable structures in which gases permeated via Knudsen
diffusion and determine the membrane separation performance
in dry samples.
The results of the present study suggest that the separation
performance of polyamide RO membranes can be optimized by the
application of drying pre-treatments and permeation temperatures,
giving useful information for potential applications of polyamidebased thin-film composite reverse osmosis membranes.
Acknowledgments
The authors gratefully acknowledge the financial support from
the Japan Society for the Promotion of Science, under the Postdoctoral Fellowship for Foreign Researchers FY2012.
α (Gas/N2) [–]
Ref.
[42]
[43]
[44]
[45]
This work
This work
[46]
[47]
[48]
[48]
[49]
[50]
[51]
[49]
[49]
This work
This work
Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.memsci.2013.08.026.
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