Synthesis and formation of a supramolecular nematic liquid crystal in poly(p-phenylene-sulfoterephthalamide)-H2O

Article abstract of DOI:10.1021/ma0475636

The formation of lyotropic solutions in water has yet to be reported for polyaramides. In this work, we investigate the synthesis and properties of a water-soluble polyaramid via the inclusion of a sulfonated monomer in the backbone of the polymer chain. Using sulfonated terephthalic acid (sTA) together with a diamine and a diisocyanate enables us to produce a lyotropic water-soluble polyaramid poly(p-phenylene-sulfoterephthalamide). Two different synthetic routes are presented: (1) the reaction between sTA and p-phenylene diisocyanate (PPDI) and (2) the reaction of activated sTA using triphenyl phosphite and p-phenylenediamine (PPD). Polymers with various molar masses up to 15 000 g mol^-1 using both methods have been obtained. We have examined the optimal reaction conditions for the synthesis of these polymers, in particular the effect of reaction temperature, concentration of monomers and reactants, type of counterions, and solvents. The polymers obtained are mostly soluble in water, up to high concentration (10 wt % without an alkali counterion on the sulfonic acid group), and surprisingly lyotropic nematic behavior has been observed independent of the molar mass of the polymer. The phase diagram of these materials in water has been investigated, and the lyotropic phase has been characterized using optical polarized microscopy.

Full text of DOI:10.1021/ma0475636

Macromolecules 2005, 38, 3647-3652
3647
Synthesis and Formation of a Supramolecular Nematic Liquid Crystal
in Poly(p-phenylene-sulfoterephthalamide)-H2O
Sebastien Viale,†,§ Nan Li,† Anton H. M. Schotman,‡ Adam S. Best,†,§ and
Stephen J. Picken*,†,§
Delft University of Technology, Polymer Materials and Engineering, Julianalaan 136,
2628 BL Delft, The Netherlands; Teijin Twaron B.V., Research Institute, P.O. Box 9300,
6800 SB Arnhem, The Netherlands; and Dutch Polymer Institute (DPI), P.O. Box 902,
NL-5600 AX, Eindhoven, The Netherlands
Received November 25, 2004; Revised Manuscript Received February 10, 2005
ABSTRACT: The formation of lyotropic solutions in water has yet to be reported for polyaramides. In
this work, we investigate the synthesis and properties of a water-soluble polyaramid via the inclusion of
a sulfonated monomer in the backbone of the polymer chain. Using sulfonated terephthalic acid (sTA)
together with a diamine and a diisocyanate enables us to produce a lyotropic water-soluble polyaramid
poly(p-phenylene-sulfoterephthalamide). Two different synthetic routes are presented: (1) the reaction
between sTA and p-phenylene diisocyanate (PPDI) and (2) the reaction of activated sTA using triphenyl
phosphite and p-phenylenediamine (PPD). Polymers with various molar masses up to 15 000 g mol-1
using both methods have been obtained. We have examined the optimal reaction conditions for the
synthesis of these polymers, in particular the effect of reaction temperature, concentration of monomers
and reactants, type of counterions, and solvents. The polymers obtained are mostly soluble in water, up
to high concentration (10 wt % without an alkali counterion on the sulfonic acid group), and surprisingly
lyotropic nematic behavior has been observed independent of the molar mass of the polymer. The phase
diagram of these materials in water has been investigated, and the lyotropic phase has been characterized
using optical polarized microscopy.
Introduction
behavior. In this approach the polymer backbone stiff-
ness is not disturbed. The bulky substituents used by
Kricheldorf et al.,5-8 such as aromatic rings, have been
shown to increase the solubility of the final materials,
but no clear liquid crystalline behavior has been ob-
tained in solution.
The enhancement in solubility of polyaramides has
been an important area of research in the past few
years. To reveal the liquid crystalline properties of
polyaramides such as Kevlar and Twaron, sulfuric acid
is the solvent of choice. This is due to the hydrophobic
nature of the aromatic backbone in combination with
strong intermolecular hydrogen bonding between the
chains that reduces the solubility of polymer and leads
to a high melting point.1 Several papers have reported
structural modifications of PPTA via three main strate-
gies: (1) disturbing the stiffness of the polymers chains,
(2) increasing the distance between the polymer chains,
and (3) improving the solubility of the polymer backbone
in water through the introduction of ionizable moieties.
By introducing defects along the polymer chains, such
as kinked and double kinked monomers and nonplanar
monomers, Preston et al.2-4 reduced the chain stiffness,
but also the lyotropic behavior was lost. These modifica-
tions improved the solubility in aprotic polar amide
solvents such as dimethylacetamide (DMAc) or N-
methylpyrrolidone (NMP) after the addition of salt
(CaCl2 and LiCl). However, it appears that this ap-
proach is not successful in terms of formation of a liquid
crystalline phase in organic solvents; solubility is im-
proved at the expense of the lyotropic liquid crystal
formation.
In a recent paper,9 we reported on the upgraded
synthesis of a sulfonated water-soluble PPTA, poly(p-
sulfophenylene-terephthalamide), P1, previously de-
scribed by Vandenberg et al.10 Unfortunately, again no
clear liquid crystalline phase was observed due to gel
formation. Small-angle neutron experiments revealed
the formation of molecular aggregates upon increasing
counterion concentration in solution.11 Similar results
have been reported by Chu et al.12,13
In a recent communication,14 we presented some
preliminary results on the synthesis and the solution
properties of a novel polymer, poly(p-phenylene-sulfo-
terephthalamide or “Sulfo Invert PPTA”: P2 (see Figure
1). In the present paper, we will present the polymer-
ization conditions using two different, but well-known,
synthetic routes (see Figure 2): (1) a diisocyanate
route15-19 and (2) a direct phosphorylation route.20-22
We will also discuss the critical role played by the
sulfonic acid group on the gel formation of P1 observed
by others and ourselves. The observation of a free-
flowing supramolecular lyotropic nematic phase in P2
provides some insight into the underlying mechanism
for the gel formation in P1. The phase diagram and the
liquid crystalline behavior of P2 will be thoroughly
investigated in this work.
Increasing the distance between the polymer chains
may reduce the formation of hydrogen bonds and
increase the solubility while preserving the lyotropic
† Delft University of Technology.
‡ Teijin Twaron B.V., Research Institute.
§ Dutch Polymer Institute.
* Corresponding author: Fax +31 15 2787415; e-mail
s.j.picken@tnw.tudelft.nll.
Experimental Section
General Methods. 1H and 13C NMR were recorded in D2O
and DMSO-d6 on a 300 MHz UR-300S Varian. Size exclusion
chromatography (SEC) was performed by dissolving the
10.1021/ma0475636 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/30/2005

3648 Viale et al.
Macromolecules, Vol. 38, No. 9, 2005
(DMSO-d6): δ 8.33 (s, 1H), 8 (dd, 1H, J ) 1.8 Hz, J ) 8.1 Hz),
7.69 (d, 1H, J ) 7.8 Hz).13C NMR (DMSO-d6): δ 169, 167
(COOH), 145, 136.1, 132.6, 130.7, 130.3, 128.6.
Polymerization. The polymerization and all transfer op-
erations were carried out in flame-dried glassware and under
argon due to the high moisture sensitivity of the reactants.
After assembly, the equipment was heated and flushed with
argon to remove any absorbed moisture. During the reaction,
argon was slowly flushed through the reactor. The reactor
consists of a 250 mL three-necked flask, equipped with a
cooler, argon inlet and outlet, and a thermometer. The reactor
was heated with a thermo-regulated oil bath. Because of the
batch sizes used in this work, a magnetic stirrer was used
during the polymerization. A mechanical stirrer was consid-
ered to be inappropriate for the present batch size; however,
the use of such a device is acknowledged to assist in obtaining
higher molar mass in analogous pPPTA polymerization reac-
tions.
Figure 1. Structures of Sulfo PPTA, P1 (top), vs Sulfo Invert
PPTA, P2 (bottom).
Diisocyanate Route, Optimum Conditions. 1.74 g (10.09
mmol) of 3 and 3.9 g (20.31 mmol) of phospholene catalyst 5
were dissolved in NMP with 2.63 g of LiCl. The solution was
heated at 75 °C for 1 h, and then 2.67 g (10.09 mmol) of 2
dissolved in NMP was added. The reaction mixture is heated
at 115 °C for 3 h. The yellow viscous solution was precipitated
in 400 mL of methanol. A yellow precipitate was obtained
which was filtered and washed with methanol (500 mL) and
with diethyl ether (500 mL). The goldish powder is dried under
vacuum at 80 °C overnight. The yield was 4 g (91%). 1H NMR
(DMSO-d6): δ 11.5 ppm (1H, s, NHCO), 10.5 ppm (1H, s,
NHCO), 9.9 ppm (t, J ) 7.8 Hz, 2H), 8.6 ppm (t, J ) 7.8 Hz,
2H), 8.1 ppm (t, J ) 8.1 Hz, 3H). 13C NMR (DMSO-d6): 164
ppm, 162 ppm CdO, δ 145.2, 142.8, 138.3, 135.8, 129.7, 129.2,
128.2, 127.8, 127.6, 127.4, 127.1, 126.8.
Figure 2. Polymerization routes investigated in this study:
top, diisocyanate route; bottom, phosphorylation route.
Phosphorylation Route, Typical Procedure. 1 g (4.06
mmol) of 2, 2.90 g (9.34 mmol) of TPP, 3.28 g of LiCl, and 8.20
mL of pyridine were dissolved in 40 mL of NMP. The solution
was heated 40 °C for 15 min, and then 0.76 g (4.06 mmol) of
4 was added. The reaction mixture was heated at 115 °C for
4 h. The yellow viscous solution was precipitated in 500 mL
of methanol. A yellow precipitate was obtained. The precipitate
was filtered and washed with methanol (500 mL) and diethyl
ether (500 mL). The gold-colored powder was dried under
vacuum at 80 °C overnight; the yield was 1.1 g (77%). 1H NMR
(DMSO-d6): δ 11.32 ppm (1H, s, NHCO), 10.5 ppm (1H, s,
NHCO), 8.85 ppm (s, 1H), 8.48 ppm (s, 1H), 7.5 ppm (q, 4H),
7.34 ppm (s, 1H). 13C NMR (DMSO-d6): 165.26 ppm, 164.37
ppm CdO, δ 144.4, 143.6, 130.7, 129.4, 128.3, 126.5, 125.9,
125.7, 120.9, 120.7, 120, 119.9.
polymeric sample in concentrated sulfuric acid (1 mg/mL) and
separating this using a modified Zorbax column (250 × 6.2
mm) and concentrated sulfuric acid as the mobile phase (0.1
mL/min). A UV detector operating at 340 nm was used. From
the chromatograms, the molar mass values were calculated
using Cirrus version 1.1 GPC software (Polymer Labs). As
standard references for the SEC analysis, a Twaron (PpPTA)
yarn type 1010 (the molar mass which has been previously
determined by DLS as 30 000 g mol-1) and an Aramid trimer
were used. The salt content in our polymers was determined
by flame atomic absorbance spectroscopy (FAAS) using a
Perkin-Elmer Plasma 40 flame atomic absorbance spectrom-
eter. To characterize the phase behavior in aqueous solution,
the polymer samples were dissolved in hot water using
ultrasonic mixing at 60 °C for 2 h prior to analysis. Thermo-
gravimetric analysis (TGA) measurements were performed on
the solid polymer samples in the form of a powder using a
Perkin-Elmer TGA 7a, with the sample under a flow of
nitrogen and with a heating rate of 10 °C/min from 25 to 400
°C.
Materials. N-Methylpyrrolidone (99.99% Aldrich Sure Seal),
dimethylacetamide (99.99% Aldrich Sure Seal), triphenyl
phosphite (99%+, Aldrich), and pyridine (99.99% Aldrich Sure
Seal) were used as received. 2,5-Dimethylbenzenesulfonic acid
(1, Acros 99%), 1,4-phenylene diisocyanate (3, Aldrich 99%),
1,4-phenylenediamine (4, 99% provided by Teijin Twaron), and
3-methyl-1-phenyl-2-phospholene 1-oxide (5, ABCR, 99%) were
used as received. Dowex-50wx4-200, (Sigma-Aldrich), lithium
chloride (Aldrich, 99+%), and calcium chloride were dried
under vacuum in an oven at 250 °C overnight. Other solvents
are of technical grade.
Results and Discussion
1. Diisocyanate Route. To determine the effect of
all the parameters examined in this study, SEC analysis
has been used to obtain a crude estimation of the molar
mass of the polymers prepared. Diisocyanate monomers
together with sTA, 2, produce polyaramides with the
aid of a phospholene catalyst. The catalyst requires time
to react with the diisocyanates in order to form carbo-
diimides prior to the polymerization reaction.16 To
achieve this the monomer should be activated first at a
certain temperature and for a certain time. To deter-
mine the optimal reaction conditions, different reaction
times have been studied, namely, no activation of the
monomers, 1 h, 2 h, and 3 h of activation. Our experi-
mental results show that the best conditions are 1 h at
75 °C.
An important issue is the choice of the solvent as the
reaction medium. It has been already reported that
carbodiimides can react with NMP and DMAc.19 To
reduce this side reaction the best solvent is sulfolane,
however, our monomers are insoluble in sulfolane, and
therefore, we decided to use NMP instead. To reduce
the possible side reaction between the carbodiimide and
2-Sulfoterephthalic Acid (2). In a 1 L reactor vessel
equipped with a mechanical stirrer and a cooler, 20.84 g (0.111
mol) of 1 was dissolved in 500 mL of water, and then 82.36 g
(0.52 mol) of KMnO4 was added stepwise for 5 h. The reaction
mixture was heated at reflux for 48 h, followed by filtering to
remove the residual MnO2. The filtrate was ion exchanged with
Dowex (three times) to remove the K+ from the sulfonic acid
group. HCl was then added to the filtrate, and immediate
precipitation of 2 occurred. The yield was 15 g (54.5%).1H NMR

Macromolecules, Vol. 38, No. 9, 2005
Supramolecular Nematic Liquid Crystal 3649
Table 1. Solution Behavior of the Polymers Obtained in
This Study
the solvent, we carefully tuned the monomer concentra-
tion and found that (10 mmol) was the best concentra-
tion to prevent side reactions that can alter the final
molar mass.
Under these carefully chosen conditions, the carbo-
diimides can react further with the sulfonated tereph-
thalic acid. We investigated the time and temperature
dependence of the polymerization reaction. The effect
of temperature on the final molar mass has been
studied, with three different temperatures were used:
75, 115, and 125 °C. The highest molar mass obtained
for the final polymer was at 115 °C.
The effect of reaction time on the reaction has been
followed by SEC and NMR. 2 mL of the reaction mixture
was extracted every 1 h and precipitated in methanol
and washed. These samples have been analyzed using
SEC, and the results clearly show that the reaction
takes place rapidly and the molar mass reaches a
plateau value of 8000 g mol-1 after 3 h of reaction. We
found that 115 °C combined with 3 h of reaction time
produces the highest molar mass polymer with this
method. Using this optimized diisocyanate route, poly-
mers of P2 with reasonable molar masses were obtained
of between 8500 and 13 000 g mol-1. A further increase
of the molar mass would be anticipated on increasing
the batch size.
molar mass
[Li], wt %
water solubilitya
LCb
1400
1900
1800
1700
1300
1500
1600
1800
1900
2000
2700
8500
6000
13000
15000
0.01c
0.01c
1.12
1.01
0.34
0.64
0.30
0.36
0.30
0.40
0.64
0.43
0.40
0.30
0.42
+
no
+
no
+
+
no
no
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
a Polymer powder dissolved in water and boiled for 15 min. b LC
) liquid crystalline behavior, observation based on 6 wt % solution.
c Synthesis with CaCl2; +, inhomogeneous solution; +++, clear
yellow solution.
The solubility problem of polyaramides has been
previously discussed by Otsuki,21 and it has been shown
to have a dramatic effect on the final molar mass of this
class of polymer.22 The use of symmetric monomers may
also affect the final molar mass of the polymer due to
the formation of a linear polyaramide that can precipi-
tate during the polymerization, thereby reducing the
final molar mass.19 Consequently, the amount of NMP
used at the early stage of the polymerization has to be
tuned carefully in order to prevent undesired precipita-
tion. A high-shear mechanical stirring device, to prevent
the precipitation of the polymer, and a larger batch size
can be used to increase the final molar mass.
2. Phosphorylation Route. The more classical
synthesis route is via the use of a phosphorylation
method,20-22 and this has also enabled us to produce
high molar mass polymers of P2. To obtain the optimum
reaction parameters, again a number of conditions have
been carefully investigated.
Figure 3. Phase diagram of P2 as a function of temperature
and polymer concentration, with alkali ion (9) and acid form
(]).
3. Phase Diagram. As mentioned previously in the
Introduction, no clear lyotropic behavior was observed
for P1 in water due to the formation of a gel phase. We
have briefly described the polymer P2, as obtained via
the phosphorylation or the diisocyanate route, is soluble
in water and can show liquid crystalline behavior at
rather low polymer concentration. In Table 1 some
solubility results are summarized where the LC behav-
ior is evaluated at 6 wt % polymer concentration.14
For solutions of P1, when the alkali ion concentration
is high (5 wt %), the polymer is not soluble in water
and no liquid crystalline behavior is observed. This
behavior has been attributed19 to the hydrogen-bonding
interactions between the sulfonic acid group and the
water, which are screened due to the presence of the
alkali metal ion, resulting in a behavior similar to that
of PPTA in water.
Figure 3 shows the phase diagram of Sulfo invert
PPTA with two different samples: in the presence of
alkali ion and in the acid form. With alkali ion (K+, 3.3
wt %) as the counterion, 6 wt % represents the solubility
limit of the polymer; at higher polymer concentrations
the powder absorbs the water and swells. Interestingly
for the samples in the acid form, the solubility limit is
increased to 10 wt %. This observation can be explained
by the alkali ion inducing greater aggregation of the
Our first concern was the reaction temperature, as
already reported in the literature23 this has been found
to play a major role in the final molar mass of aramid-
based polymers. Different temperatures have been
studied: 75, 115, and 125 °C, with the highest molar
mass polymers obtained at 115 °C.
The reaction time has also been studied using offline
SEC to determine the kinetics of the reaction. To
achieve this, 2 mL of the reaction mixture was extracted
every hour and precipitated in methanol and washed.
After 4 h, the molar mass was found to reach a
maximum, and no further increase was observed.
Monomer concentration as well as the monomer feed
has been investigated. As observed in industrial pro-
cesses, the impact of monomer concentration and mono-
mer feed is crucial. We observe a tremendous increase
in the molar mass when the monomer concentration is
high (3.3 mol) and the monomer feed is continuous (total
10 g). No effect on the final molar mass of the polymer
was observed with different choices of salt, namely LiCl
and CaCl2, solvent (NMP or DMAc), and the rate of
addition of the monomers.

3650 Viale et al.
Macromolecules, Vol. 38, No. 9, 2005
Figure 4. Evolution of the fraction of the isotropic phase in the biphasic region as a function of temperature.
polymer chains, and therefore the sizes of the aggregates
in solution are bigger. Consequently, the solubility limit
is higher for the sample without any alkali ions due to
the smaller size of the aggregates, which delays pre-
cipitation.
The phase behavior shown in Figure 3 appears to
consist of two different phase diagrams superimposed
on each other; at low temperature between 10 and 70
°C a more or less athermal biphasic region can be
observed which can be modeled using excluded volume
based theories like Onsager or Khokhlov-Semenov
theory, while the upper limit (70-80 °C) of the nematic
phase seems to be related to the stability of the
supramolecular assemblies themselves, i.e., it repre-
sents a transition from a supramolecular to a molecular
solution. Some preliminary results from XRD indicate
that below 0.5 wt % a transition from a supramolecular
to molecular structure can be observed at approximately
70-80 °C.14
Figure 5. Biphasic solution of sample 190 at 0.5 wt % in
water (magnification 300×).
to form a nematic solution; this result has been con-
firmed in a recent publication.14
Isotropic solutions are observed for low polymer
concentrations in the presence of K+, up to 0.5 wt %
polymer in water and up to 1 wt % in the acid form. A
biphasic system is observed in both cases using optical
polarization microscopy. Nematic droplets with charac-
teristic Maltese crosses are dispersed in surrounding
isotropic solution, as shown in Figure 5.
To explore the biphasic region of the solutions with
potassium as the counterion in more detail, 10 samples
between 0.1 and 1 wt % polymer have been prepared,
and the results are shown in Figure 4.
Upon increasing the polymer concentration, the iso-
tropic phase disappears. A homogeneous nematic phase
is obtained at a concentration of 2-6 wt % in the
presence of alkali ions, and the samples show a char-
acteristic Schlieren texture. In the case of the acid from,
the phase boundary is shifted to 3 wt %, and similar
textures are observed.
It is remarkable to note that the nematic solution is
obtained for a relatively low weight fraction of polymer,
and the birefringence of these solutions is relatively
high, implying the molecules are highly oriented. For
these concentrations, colored nematic textures are
obtained, as shown in Figure 6.
5. Gel Formation. The striking result of this work
is the lack of gel formation in solution when compared
to P1. Further investigation has been done in order to
understand the difference in the solution behavior of
the two polymers. As a first approximation, we can
establish the following: the structures of P1 and P2
differ only by the distance between the sulfonic acid
group and the neighboring amide group, and therefore
P2 is more soluble in water than P1.24
For both systems, with and without potassium coun-
terions, the biphasic region is only slightly affected by
the temperature, revealing that we are dealing with an
athermal liquid crystal phase. The upper limit between
N f I, at approximately 70-75 °C for both samples, is
due to the destruction of the aggregates, which form the
building blocks of the nematic phase.
From the phase diagram, we conclude that the su-
pramolecular aggregates behave like an athermal liquid
crystal where the formation of the LC phase is mainly
governed by the excluded volume interaction of the
aggregates. When the temperature is increased, the
aggregates disappear, resulting in a molecular solution.
At these elevated temperatures the molecular solution
should become liquid crystalline at higher polymer
concentrations, but we did not succeed in observing this
due to the evaporation of the solvent which resulted in
inhomogeneous samples.
4. Optical Polarized Microscopy (OPM). Consid-
ering the wide range of molar masses of polymers
explored in this work, we can clearly show that the
formation of LC phase is independent of molar mass
and, to a lesser degree, salt concentration. In all cases,
molecular aggregates are formed and become aligned
The probable cause for the decrease in the solubility
of P1 is the formation of an intramolecular hydrogen

Macromolecules, Vol. 38, No. 9, 2005
Supramolecular Nematic Liquid Crystal 3651
Figure 6. Nematic solution of containing alkali ions (molar
mass 2000 g mol-1) at 6 wt % in water (magnification 300×).
Figure 8. TGA of P2 and P1. Note the drop in mass at 225
°C in the case of Sulfo Invert PPTA.
synthetic routes have been pursued: a diisocyanate-
based route and the activation of the monomers using
phosphorylation. Reasonably high molar masses are
obtained, corresponding to a degree of polymerization
of about 30, for both routes.
Importantly, liquid crystalline behavior in water has
been observed and characterized for a modified PPTA.
It appears that this is due to the presence of supramo-
lecular aggregates in the solution, and this is confirmed
by the characteristic athermal lyotropic behavior for the
transition from the isotropic to the nematic phase.
In a molecular liquid crystal system, the concentra-
tion for the IfN transition is would be expected around
12 wt %, whereas in our system the formation of a
nematic phase occurs at very low concentration (below
1 wt %), characteristic of a supramolecular liquid crystal
phase. The effect of the salt on the phase diagram has
been also determined, where the critical concentration
increases when the sample contains no salt.
Finally, no gel formation was observed in Sulfo Invert
PPTA, which is attributed to the lack of intramolecular
hydrogen bonding between the amide and sulfonic acid
group, enhancing the solubility of the final polymer. A
forthcoming paper will discuss the solution behavior of
the supramolecular aggregates using X-ray and SANS
experiments.
Figure 7. FTIR of the sulfonic acid group on P1 (red) and
P2 (blue) in solution (5 wt % in water).
bond between the sulfonic acid group and the hydrogen
from the amide bond. Therefore, the sulfonic acid group
is not fully involved in hydrogen bonding with the water,
which reduces the solubility of the polymer backbone
and enhances gel formation. Figure 7 confirms this
hypothesis, as the FTIR spectra of the region of υs(SO3)
vibration from 1050 to 1000 cm-1 of sulfonic acid group
shows a shoulder for P1; this corresponds to a change
in the polarizability of the bond due to the interaction
with hydrogen of the neighboring amide bond.25,26
A
similar observation of hydrogen bonding between a
sulfonic acid group and water has been reported for
Nafion.27 It is not possible to check the character of the
amide vibration, as this is obscured by the O-H stretch
of water (3200 cm-1).
Acknowledgment. We thank Dr. E. Mendes for
discussions on the structure of the supramolecular
solutions. This work forms part of the research program
of the Dutch Polymer Institute.
Finally, if an additional hydrogen bond is formed
between the sulfonic acid group and the hydrogen of the
amide bond, it is expected that the thermal stability of
the sulfonic acid group should increase. This is con-
firmed by TGA measurement performed on samples
with similar molar mass and counterion concentrations,
and the curves are shown in Figure 8.
A noticeable drop in mass is observed for P2 around
220-280 °C due to the departure of the sulfonic acid
group9 (blue curve).19 This is less pronounced in the case
of P1 (red curve).28
Because of the larger distance between the hydrogen
of the amide bond and the sulfonic acid group in P2, no
intramolecular hydrogen bond is formed; therefore, the
sulfonic acid group can fully play its role as an ionizable
group, and this enhances the water solubility of the
polymer backbone. This effect will be examined further
in an upcoming publication using computational model-
ing.29
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