FTIR Studies of Ionic Interactions in Blends
Fourier-transform infrared (FTIR) spectroscopy has the potential to demonstrate and enumerate ionic interactions in polymer blends because of its sensitivity to changes in chemical and
physical environment. We describe here a blend of p-sulfonated polystyrene (SPS), with 4.5 mol%
sulfonation (synthesis described previously by Fitzgerald and Weiss’), and an amine-terminated
poly(alky1ene oxide) (APAO) of molecular weight 900 (Jeffamine ED-900, Texaco Chemical
Company). Such a polymer pair forms a miscible blend over a limited concentration range, as has
been described elsewhere?
The spectroscopic demonstration of ionic interactions confirms the earlier results of Tannenbaum et al.,3 who studied blends of SPS and a polyurethane with tertiary amine functionality.
However, we differ from earlier workers in the important point of the assignment of the
characteristic stretching vibration of the N+-H moiety. We demonstrate here that the broad
absorption in our spectra a t about 3450 cm-’, which by analogy with the interpretation of
polyurethane-SPS blends would be interpreted as being caused by N+-H moieties, is in fact due
to absorbed water, and that the N+-H stretching mode is in the range 3200-2800 cm-’,
depending on the amine. The correct assignment is important when quantifying the extent of
ionic interactions and because of the well-known dependence of the properties and IR spectra of
ionomers on absorbed water.4
Figure 1 shows the 1400-800 cm-’ spectral region for SPS and the difference spectrum for the
blend (23 : 73 APAO/SPS) minus APAO. The difference spectrum therefore represents the sum of
the SPS contribution to the blend spectrum and any spectral changes associated with interactions. The most noticeable features in the difference spectrum are the absence of the 1101 cm
absorption characteristic of p-sulfonic acid groups and the appearance of strong absorptions at
1009, 1128, and 1228 cm-’ charactesistic of the sulfonate salt.’-3
Figure 2 shows the 4000-2200 cm-’ absorbance spectrum of APAO and the difference spectrum
for the blend (23 : 73 APAO/SPS) minus SPS. The difference spectrum therefore represents the
sum of the APAO contribution and any spectral changes associated with interactions. The most
noticeable features of the spectrum not associated with APAO are broad absorptions at about
3450 and 3050 cm-’. By analogy with the spectra of other tertiary amine salts,5 we tentatively
assigned the lower frequency absorption (about 3050 cm-’) to the N+-H stretching mode and
the higher frequency absorption (about 3450 cm-’) to adsorbed water. Figure 2c shows the
“double-difference” spectrum produced when both SPS and APAO are subtracted from the blend.
A complex combination of overlapping absorptions is produced, but the spectrum is consistent
with the occurrence of “negative absorptions” associated with loss of amine functionality (about
3500, 3300 cm- ’) and positive absorptions at 3450 and about 3050 cm- associated with absorbed
water and N+-H functionality.
In order to confirm this hypothesis, we examined model systems comprising SPS with
somewhat higher degrees of sulfonation (8.5 and 24 mol%) neutralized with tributylamine (TBA).
TBA was chosen because it is a tertiary amine (as in the blends of Ref. 3), and any unreacted
amine could be removed in vacuum, thus simplifying the interpretation of the spectra.
Figure 3a shows the 4000-2000 cm-’ spectral region of SPS (8.5 mol% sulfonation) in the form
of a thin film cast from tetrahydrofuran (THF) on to a sodium chloride window. An SPS/TBA
’
’
H
I
H
I
H
I
SPS
H
I
/H H
\
/H H \
IH H
\
APAO
Journal of Polymer Science: Part B: Polymer Physics, Vol. 26, 1545-1548 (1988)
0 1988 John Wiley & Sons, Inc.
CCC 0098- 1273/88/071545-@4$04.00
1546
J. POLYM. SCI.: POLYM. PHYS. ED. VOL. 26 (1988)
1000
w
0
z
U
m
U
0
v)
m
a
I
I
I
1300
1100
I
900
WAVENUMBERS c m ’
Fig. 1. Mid-IR spectra of (a) SPS (4 mol% sulfonation) and (b) difference spectrum (23:73
APAO/SPS) minus APAO, scaled to remove the characteristic CH bending absorptions of APAO.
reaction product was prepared by the addition of about 20%TBA to the THF solution of SPS on
the sodium chloride window in the initial stages of the film-casting process. The THF and excess
TBA were removed in vacuum at 100OC. The difference spectrum (Fig. 3b) produced by
subtracting SPS from the SPS/TBA reaction product, using the sharp aromatic C-H stretching
mode of SPS to adjust the scaling, shows a broad absorption a t about 2850 cm-’ superimposed
on the aliphatic C-H stretch absorptions of the TBA. The SPS/TBA reaction product was
w
u
Z
a
m
U
0
m
U
I
PO00
I
3600
I
3200
I
L
2800
ZltOO
WAVENUMBER
Fig. 2. IR spectra of (a) APAO, (b) difference spectrum blend
spectrum blend - (SPS + APAO).
-
SPS, and (c) difference
1547
NOTES
I
w
u
-
t
$000
3600
3200
7.000
2'iOO
i I00
WFIVENUMBER
Fig. 3. IR spectra of (a) SPS (8.5 mo1% sulfonation), (b) difference spectrum SPS/TBA - SPS,
and (c) difference spectrum SPS/TBA exposed - unexposed.
then exposed to the laboratory atmosphere for 15 min, and the difference spectrum, exposed
unexposed, is shown in Figure 3c. The absorption of water from the air produces an appreciable
IR absorption a t 3450 cm-'.
This procedure was repeated with an SPS specimen of a higher degree of sulfonation (24 mol%).
Figure 4a shows the initial spectrum of the SPS/TBA reaction product. Note that transfer of the
specimen from the vacuum oven to the spectrometer took approximately 2 min, and so some
water pickup will already have occurred. Figure 4b shows the difference spectrum produced after
the specimen was exposed in 15 min in the laboratory atmosphere. When the specimen was
redried in a vacuum oven and reexamined, it was shown that water absorption (and hence 3450
cm-' peak production) is a reversible phenomenon. The sodium chloride windows alone had an
insignificant water pickup under identical conditions.
~
100
Fig. 4. IR spectra of (a) SPS (24 molS sulfonation)/TBA product, (b) difference spectrum,
exposed SPS/TBA - unexposed, and (c) difference spectrum, redried SPS/TBA - unexposed.
1548
J. POLYM. SCI.: POLYM. PHYS. ED. VOL. 26 (1988)
We therefore conclude that ionic interactions can occur in SPS/APAO blends, but that the
N + - H stretching mode, which can be used to quantify the number of interactions, is a broad
absorption a t 3200-2800 cm-' and should not be confused with absorbed water a t 3450 cm-'.
References
1. J. J. Fitzgerald and R. A. Weiss, ACS Symp. Series, 302, 35 (1986).
2. C. Beretta and R. A. Weiss, Polym. Mat. Sci. Eng., 56, 566 (1987).
3. R. Tannenbaum, M. Rutkowska, and A. Eisenberg, J. Polym. Sci. Polym. Phys. Ed., 25,
663 (1987).
4. P. C. Painter, B. A. Brozoski, and M. M. Coleman in Fourier Transform Infrared Spectroscopy, H. Ishida, Ed., Plenum, New York, 1987.
5. Aldrich Chemical Company, author, Aldrich Catalog of FTIR Spectra, IMilwaukee, WI,
1985.
ANDREWGARTON*
SHIPINC
WANC
ROBERTA. WEISS
Institute of Materials Science
University of Connecticut
Storrs, C T 06268
Received June 15, 1987
Accepted December 21, 1987
*To whom correspondence should be addressed.