Change of Chemical Composition and Hydrogen
Bonding Behavior due to Chlorination of
Crosslinked Polyamide Membranes
Young-Nam Kwon, Chuyang Y. Tang, James O. Leckie
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020
Received 20 March 2006; accepted 18 October 2006
DOI 10.1002/app.25657
Published online 22 January 2008 in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The effect of membrane exposure to hypochlorite oxidant on property changes (chemical composition and hydrogen bonding behavior) of four FilmTecÓ
thin film composite crosslinked polyamide membranes
has been investigated. Crosslinking densities of the membranes were about 25–35%, with about 3–4 chlorines
bound to the repeating unit of the polyamide membranes.
This was equivalent to 39% of all nitrogens being
chlorinated in the polyamide membranes assuming the
amide nitrogen is the dominant reaction site with chlorine. FTIR spectra showed the amide I band (C¼
¼O
stretching peak at 1663 cm 1) of polyamide membranes
shifted to higher wave-numbers and the peak intensity of
the amide II band (N H bending peak at 1541 cm 1)
decreased after chlorination. The peak shift and decrease
of peak intensity resulted from breakage of hydrogen
bonds between C¼
¼O and N H groups within the polymers. The XPS and FTIR analytical analysis showed that
there is no difference in the chlorine attack of polyamide
membranes of higher or lower crosslinking density, and
that the chlorination breaks and weakens hydrogen
bonding. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108:
INTRODUCTION
Use of membrane technology to treat water or wastewater is limited by the gradual deterioration of performance due to membrane fouling. Fouling results
from accumulation of substances on the membrane
surface or within membrane pores and is manifested
by a decline in product water flux.1 Controlling membrane fouling has always been a major challenge in
the membrane processes. Reducing fouling would
increase membrane life, increase run times between
cleanings, and thus decrease operation/maintenance
costs. Among techniques used to reduce fouling, disinfection and chemical cleaning are most prevalent
and a large number of chemical agents are available.2–6 The methods use chemicals to reduce active
microorganisms and remove chemically or physically
absorbed foulants such as sparingly soluble inorganics, dissolved organics, colloids, and microorganisms.7,1 Although these chemicals are effective for
reducing scales and deposits, they are chemically
aggressive to many commercial membrane polymers,
eventually resulting in performance change.
Several studies have investigated the cause of performance decline and the degradation mechanism of
linear polyamide membranes caused by chlorination.8–11 The reason for performance decline has been
identified as likely due to loss of structural integrity
of constituent polymers. Previous studies of polyamide membrane chlorination focused primarily on
investigation of linear polyamide membranes because
of the difficulties of isolating the crosslinked polyamide layer for instrumental analysis. To the best of our
knowledge, chlorination of crosslinked polyamide
membranes has not been systematically investigated.
In this article, the effect of crosslinking density on
the chlorination of four FilmTec' crosslinked polyamide membranes was evaluated by estimating the
degree of chlorine bound to the repeating unit of the
membranes. In addition, the change of hydrogen
bonding behavior of the crosslinked polyamide
membranes caused by chlorination was investigated
by FTIR analysis.
Correspondence to: Y.-N. Kwon (kwonyn@stanford.edu).
Contract grant sponsors: Nanyang Technological University; National Science Foundation; contract grant number:
CTS-0120978.
EXPERIMENTAL
Polyamide membrane
Journal of Applied Polymer Science, Vol. 108, 2061–2066 (2008)
C 2008 Wiley Periodicals, Inc.
V
We have used four commercially available FilmTec'
membranes (BW30, NF90, LE, and XLE) as
2061–2066, 2008
Key words: polyamides; membranes; crosslinking; chlorine; hydrogen bonding
2062
KWON, TANG, AND LECKIE
representative polyamide membranes. The membranes
are thin film composite crosslinked aromatic polyamide membranes produced by interfacial polymerization
of 1,3-phenylenediamine and 1,3,5-benzentricarbonyl
chloride, having amide bonds ( CONH ) and crosslinked/noncrosslinked portions of the structures. Each
virgin membrane sample was prepared by thorough
rinsing with flowing deionized (DI) water for 6 h, sonicating in a Milli-Q water bath for 30 min, and then
drying at room temperature. Membrane chlorination
experiments were carried out using soaking baths. The
membranes were exposed to 2000 ppm chlorine solutions for 1 h at pH 4. The total amount of polyamide
exposure to chlorine was expressed as ppm h. The
soaking tests were performed in Pyrex glass bottles
covered with PTFE (polytetrafluoroethylene) caps, and
the contents were mixed on a shaker. A chlorine
standard was prepared from commercial 6% NaOCl
solution. Exact concentrations were determined by
titration with a sodium thiosulfate standard. Final pH
of the soaking solutions, which were prepared by spiking specific amounts of sodium hypochlorite into the
buffer solution, was adjusted with concentrated HCl or
10N NaOH.
X-ray photoelectron spectroscopy
All XPS analyses were performed using an SSI SProbe Monochromatized XPS Spectrometer, using
aluminum Ka X-ray source (hn ¼ 1486.6 eV). The
electron flood gun was operated with an energy setting of 3 eV to compensate for membrane surface
charging. Wide survey spectra were scanned five
sweeps in the range of 0–1000 eV with a resolution
of 1 eV.
ATR-FTIR spectroscopy
Attenuated total reflection-Fourier transform infrared
(ATR-FTIR) spectra were recorded on a Nicolet
Nexus 470 spectrometer. As an internal reflection
element, a flat plate Ge crystal at an incident angle
of 458 was used to get a higher contribution of the
polyamide skin top layer of the membrane. A minimum of 200 scans at a resolution of 1.0 cm 1 were
signal-averaged. The membranes were placed on the
ATR crystal and pressed onto the surface with a
plate press. The instrument was covered and continuously purged with dry air to prevent interference
of atmospheric moisture with the spectra.
RESULTS AND DISCUSSION
XPS
The chemical compositions (atomic concentration
percentages of oxygen, nitrogen, and carbon) of virJournal of Applied Polymer Science DOI 10.1002/app
TABLE I
Element by Atomic Percent of Virgin
Polyamide Membranes
Type
BW30
LE
XLE
NF90
O (%)
31.41
16.15
15.84
17.39
6
6
6
6
0.59
2.75
2.30
0.77
N (%)
2.09
11.76
11.54
11.22
6
6
6
6
0.96
0.91
0.73
0.67
C (%)
66.50
72.10
72.62
71.39
6
6
6
6
0.69
2.17
2.47
0.69
Hydrogen atom is excluded because hydrogen cannot be
measured by XPS.
gin crosslinked polyamide membranes were examined by XPS and the results are shown in Table I.
All of the polyamide skin top layers of the virgin
FilmTec membranes were composed of carbon, nitrogen, oxygen, and hydrogen. The membranes could
be categorized into two groups according to the
nitrogen content. The atomic percent of nitrogen on
the BW30 membranes was about 2.09. On the other
hand, LE, XLE, and NF90 contained about 11–12%
nitrogen content. The nitrogen percent of totally linear polyamide material, based on the chemical structure of typical polyamides (Fig. 1), would be about
10%, and the nitrogen percent of totally crosslinked
polyamide material would be about 15%. The nitrogen content of LE, XLE, and NF90 membranes were
within the range of 10–15%; however, the nitrogen
content of BW30 was much less than 10%. Furthermore, the atomic percent of oxygen on BW30 was
much larger than other polyamide membranes. The
gaps of nitrogen content and oxygen content among
the membranes suggest that the BW30 membrane
may have an additional source of carbon and oxygen
on the surface of the membrane.
Using the atomic percent of oxygen, nitrogen, and
carbon in Table I, ratios between the composing
atoms were calculated (Table II). As discussed by
Koo et al.,12 the ratios and chemical formula of the
polyamide membrane made it possible to roughly
estimate crosslinking density (degree of crosslinking,
Fig. 1). The crosslinking density, in this study, was
defined as the ratio of crosslinked amide form (X) to
total amide form (X þ Y) in the polymer, and was
calculated by solving X and Y values of the LE, XLE,
and NF90 membranes. LE and XLE had a crosslinking density of 34.11% and NF90 had a crosslinking
density of 25.60% in the polymer chains. The crosslinking density of BW30 could not be calculated
because the membrane had another unknown source
of carbon and oxygen on the surface of the membrane.
Table III shows the atomic percent of oxygen,
nitrogen, carbon, and chlorine after chlorination at
2000 ppm h Cl and pH 4. LE, XLE, and NF90 membranes had about 10% chlorine content after chlorination, but the BW30 membrane had 2.57%. The
CHLORINATION OF CROSSLINKED POLYAMIDE MEMBRANES
2063
TABLE III
Element by Atomic Percent of Polyamide
Membranes Chlorinated under the Condition
of 2000 ppm h Cl and pH 4
O (%)
BW30
LE
XLE
NF90
30.55
16.01
15.46
15.61
6
6
6
6
N (%)
0.79 2.25 6 1.05
1.19 10.03 6 0.64
0.46 10.23 6 0.49
0.83 10.08 6 0.58
C (%)
64.63
62.99
63.51
63.71
6
6
6
6
Cl (%)
1.07 2.57 6 0.75
1.35 10.98 6 0.26
1.11 10.80 6 0.39
0.78 10.59 6 0.33
percent of the virgin membranes (Table I and Fig. 1).
Similarly, the amount of chlorine bound to the
repeating units of the polymeric membranes could
be estimated using the crosslinking density and
atomic percent of chlorinated membranes (Table III).
The repeating unit of LE and XLE membranes was
assumed to be
[(C21O3N4)1 þ (C15O4N2)1.93]n
based on the crosslinking density (ratio of 34.11%
crosslinked form to 65.89% noncrosslinked forms is
the same as 1–1.93). The 10.98% of chlorine content
on the LE membrane is equivalent to 3.07 chlorines
for each repeating unit, and 10.80% of chlorine content on the XLE membrane is equivalent to 3.01
chlorines per repeating unit. On the other hand, the
repeating unit for NF90 was assumed to be
[(C21O3N4)1 þ (C15O4N2)2.91]n , and, thus, the
repeating unit of the NF90 had 3.84 chlorines bound
per unit. NF90 membranes, with about 20% more
nitrogen per repeating unit, had about 20% more
chlorines bound to each repeating unit compared to
LE and XLE, and this result shows that nitrogen in
the polyamide membrane is the important reaction
site for chlorine. When amide nitrogen is assumed
as the dominant reaction site with chlorine, it is
interesting to note that regardless of crosslinking
density almost the same percent of nitrogens in their
repeating units are chlorinated: 38% for XLE and
39% for LE and NF90.
Figure 1 Calculation of crosslinking density of the polyamide membranes.
membranes having higher nitrogen content had
more atomic percent of chlorine attached to the
membrane (Fig. 2). Crosslinking densities of LE,
XLE, and NF90 were calculated using the atomic
TABLE II
Ratio between Atomic Percent of Oxygen, Nitrogen,
and Carbon on the Polyamide Membranes
O/N
O/C
N/C
LE
XLE
NF90
BW30
1.37
0.22
0.16
1.37
0.22
0.16
1.55
0.24
0.16
15.03
0.47
0.03
Figure 2 Atomic percent of chlorine and nitrogen of
degraded PA membrane surfaces after chlorination at 2000
ppm h and pH 4.
Journal of Applied Polymer Science DOI 10.1002/app
2064
KWON, TANG, AND LECKIE
was because the FilmTec membranes started from
the same monomers (1,3-phenylenediamine and
1,3,5-benzentricarbonyl chloride).
Figures 4 and 5 show the FTIR spectra of amide I
and amide II bands of virgin FilmTec membranes
and the membranes chlorinated at 2000 ppm h Cl at
pH 4, respectively.
All the virgin membranes showed amide I bands
near 1663 cm 1, and the peak shift slightly to higher
wave-number after chlorination (Fig. 4). Since the
amide I band has contributions from several
motions, including C¼
¼O stretching, C N stretching,
and C C N deformation vibration,13 the change of
Figure 3 ATR-FTIR absorption spectra of FilmTec polyamide membranes (BW30, NF90, LE, and XLE). [Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
ATR-FTIR spectroscopy
The FTIR of FilmTec membranes showed very similar spectra across all membrane types (Fig. 3). This
Figure 4 ATR-FTIR absorption spectra of amide I band
for virgin and degraded polyamide membranes at 2000
ppm h chlorine and pH 4. [Color figure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]
Journal of Applied Polymer Science DOI 10.1002/app
Figure 5 ATR-FTIR absorption spectra of amide II band
for virgin and degraded polyamide membranes at 2000
ppm h chlorine and pH 4. [Color figure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]
CHLORINATION OF CROSSLINKED POLYAMIDE MEMBRANES
band intensity and the peak shift may be explained
by the change of subpeaks composing the amide
I band. The amide I band is mainly due to the
C¼
¼O stretching motion and, on the surface of the
polyamide membranes, there are two types of C¼
¼O
stretching motions due to the stretching of hydrogen
bonded carbonyl groups and the stretching of nonhydrogen bonded carbonyl groups [Fig. 6(A)]. The
intensity change and peak shift of the amide I band
caused by chlorination was likely due to the intensity changes of the two subcarbonyl peaks resulting
from the change of concentration of the hydrogen
bonded and nonhydrogen bonded carbonyl groups
on the membrane. When the amide I band was
deconvoluted into two symmetric Gaussian peaks,
the peak at lower wave-number was assigned to
hydrogen bonded carbonyl groups, since the hydrogen bonded C¼
¼O group has a decreased doublebond character of the C¼
¼O moiety, shifting the
absorption band to lower frequency.14 Chlorination
2065
breaks hydrogen bonds between C¼
¼O and N H
groups, and thus increases the number of nonhydrogen bonded carbonyl groups. Combinations of the
decreased peak intensity of lower wave-number
(hydrogen bonded) carbonyl groups and increased
peak intensity of higher wave-number (nonhydrogen
bonded) carbonyl groups shifted the amide I band to
higher wave-number.
All the virgin FilmTec membranes showed amide
II bands (N H bending) near 1541 cm 1,14 and the
peak intensity decreased after chlorination (Fig. 5).
This was due to the decline of the number of N H
group and thus breakage of hydrogen bonds in the
membrane caused by chlorination [Fig. 6(B)].
According to the performance experiments in our
other work,15 the change of hydrogen bonding behavior due to the chlorination of crosslinked polyamide
membranes caused flux changes depending on pH
and concentration of chlorine in the soaking bath.
CONCLUSIONS
Figure 6 Stretching vibration modes for the carbonyl
(C¼
¼O) group of the hydrogen bonded amide bond and
nonhydrogen bonded amide bond (A). Bending vibration
modes for the (N H) group of hydrogen bonded amide
bond and the disappearance of the N H bending mode
after chlorination (B). [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.
com.]
Analytical analysis using XPS and FTIR provided
valuable information about the chemical composition
of crosslinked polyamide membranes and the change
of hydrogen bonding behavior due to chlorination of
the membranes. All of the polyamide skin top layers
of the FilmTec membranes investigated were composed of carbon, nitrogen, oxygen, and hydrogen.
The BW30 membrane likely has another source of carbon and oxygen on the surface of the membrane.
Nitrogen in the polyamide membrane was an important reaction site for chlorine. The crosslinking densities of the membranes, with the exception of the
BW30 membrane having another source of carbon
and nitrogen, were estimated based on the ratios of
the composing atoms (carbon, oxygen, and nitrogen).
The repeating units of the membranes and the number of chlorines bound to the repeating units were
also calculated with the information on crosslinking
densities and the atomic percent of chlorinated membranes. Crosslinking densities of the membranes were
about 25–35%, with about 3–4 chlorines bound to the
repeating unit of the polyamide. This was equivalent
to 39% of all nitrogens being chlorinated in the polyamide membranes, assuming that the amide nitrogen is the dominant reaction site with chlorine. FTIR
spectra showed that the amide I band (C¼
¼O stretching peak at 1663 cm 1) of polyamide membranes
shifted to a higher wave-number and the peak intensity of amide II band (N H bending at 1541 cm 1)
decreased after chlorination. The changes of the peaks
resulted from breakage of hydrogen bonds between
C¼
¼O and N H groups of the membranes.
Membranes used for this study were generously donated
by FilmTec'.
Journal of Applied Polymer Science DOI 10.1002/app
2066
References
1. Speth, T. F.; Summers, R. S.; Gusses, A. M. Environ Sci Technol 1998, 32, 3612.
2. Bartels, C. R.; Wilf, M.; Andes, K.; Iong, J. Water Sci Technol
2005, 51, 473.
3. Ebrahim, S. Desalination 1994, 96, 225.
4. Ebrahim, S. Eldessouky, H. Desalination 1994, 99, 169.
5. Gabelich, C. J.; Yun, T. I.; Coffey, B. M.; Suffet, I. H. Desalination 2003, 154, 207.
6. Tragardh, G. Desalination 1989, 71, 325.
7. Potts, D. E.; Ahlert, R. C.; Wang, S. S. Desalination 1981, 36,
235.
Journal of Applied Polymer Science DOI 10.1002/app
KWON, TANG, AND LECKIE
8. Avlonitis, S.; Hanbury, W. T.; Hodgkiess, T. Desalination 1992,
85, 321.
9. Glater, J.; Zachariah, M. R. ACS Symp Ser 1985, 345.
10. Kawaguchi, T.; Tamura, H. J Appl Polym Sci 1984, 29, 3359.
11. Singh, R. J Membr Sci 1994, 88, 285.
12. Koo, J.-Y.; Petersen, R. J.; Cadotte, J. E. Polym Prepr Div Polym
Chem Am Chem Soc 1986, 27, 391.
13. Skrovanek, D. J.; Howe, S. E.; Painter, P. C.; Coleman, M. M.
Macromolecules 1985, 18, 1676.
14. Socrates, G. Infrared Characteristic Group Frequencies; WileyInterscience: Chichester, UK, 1994; p 90.
15. Kwon, Y.-N.; Tang, C. Y.; Leckie, J. O. J Appl Polym Sci 2006,
102, 5895.