Synthesis of copolyamides based on PA 66 bearing lithium sulfonate groups and having unique thermal properties

Article abstract of DOI:10.1002/pola.24971

Copolyamides of PA 66/6 lithium 5-sulfoisophthalic acid (LiSIPA) containing up to 40 mol % of LiSIPA were prepared in a 1L-pilot reactor operating at high pressures and high temperatures. Interestingly, the presence of lithium sulfonate moieties highly impacted the glass transition temperature of the polyamide. The T_g increased from 59 °C for PA 66 to 155 °C for a copolymer containing about 40 mol % of LiSIPA. 1,3- Dihexylbenzenedicarboxamide and lithium p-toluenesulfonate were synthesized as model compounds to investigate the interaction of lithium sulfonate moieties and amide functions. Infrared spectroscopy using ATR technology performed on mixture of both compounds showed that the carbonyl group of amide functions interacts with the lithium cation of lithium sulfonate moieties. Similar S-O and C-O adsorption bands were observed in copolyamides PA 66/6LiSIPA and in mixture of model compounds, which strongly suggest the formation in the copolyamides of physical cross-linking points centered on lithium cations coordinated by carbonyl groups of amide functions.

Full text of DOI:10.1002/pola.24971

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ARTICLE
Synthesis of Copolyamides Based on PA 66 Bearing Lithium Sulfonate
Groups and Having Unique Thermal Properties
1
1
1
2
´
Geoffroy Cammage, Roger Spitz, Christophe Boisson, Rene Rossin,
2
2
´
´
Stephane Jeol, Franck Touraud
1Universite´ de Lyon, Universite´ de Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polyme`res et Proce´de´s
(C2P2), Equipe LCPP, F-69616 Villeurbanne, France
2Rhodia Ope´rations, Centre de Recherches et Technologies de Lyon, 69192 Saint-Fons, France
Correspondence to: C. Boisson (E-mail: christophe.boisson@univ-lyon1.fr)
Received 24 May 2011; accepted 22 August 2011; published online 12 September 2011
DOI: 10.1002/pola.24971
ABSTRACT: Copolyamides of PA 66/6 lithium 5-sulfoisophthalic
acid (LiSIPA) containing up to 40 mol % of LiSIPA were pre-
pared in a 1L-pilot reactor operating at high pressures and
high temperatures. Interestingly, the presence of lithium sulfo-
nate moieties highly impacted the glass transition temperature
of the polyamide. The Tg increased from 59 ꢀC for PA 66 to 155
ꢀC for a copolymer containing about 40 mol % of LiSIPA. 1,3-
Dihexylbenzenedicarboxamide and lithium p-toluenesulfonate
were synthesized as model compounds to investigate the inter-
action of lithium sulfonate moieties and amide functions. Infra-
red spectroscopy using ATR technology performed on mixture
of both compounds showed that the carbonyl group of amide
functions interacts with the lithium cation of lithium sulfonate
moieties. Similar SAO and CAO adsorption bands were
observed in copolyamides PA 66/6LiSIPA and in mixture of
model compounds, which strongly suggest the formation in
the copolyamides of physical cross-linking points centered on
lithium cations coordinated by carbonyl groups of amide func-
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tions. 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 49: 5057–5062, 2011
KEYWORDS: copolymer; copolymerization; infrared spectros-
copy; polyamides; polycondensation
INTRODUCTION Polyamide 66 (PA 66) is well known as a
high performance material; thanks to the combination of
high thermal and mechanical properties as well as good
processability. Strong interchain hydrogen bonds between
amide groups give to PA 66 a high melting point (about 260
ꢀC) and a crystallinity around 40%, which leads to high ri-
gidity and resistance to heat deꢀformation. The glass transi-
tion temperature, which is ꢁ60 C for dried PA 66, is sensi-
tive to humidity, because water has a plasticizing effect on
polyamide.1,2 In some applications, requiring a high rigidity
at elevated temperatures, an increase in the glass transition
temperature (Tg) without lost of polymer crystallinity can be
an efficient approach. Copolymerization is a logical route to
reach these objectives, and aromatic comonomers such as
terephthalic and isophthalic acids have been used in order to
increase the Tg.3 However, a large content of comonomers is
necessary to obtain significant improvements. For example,
the commercial semicrystalline polyphthalamide PA 6T/66
(55/45 mol %), which displays a high melting transition
close to 310 ꢀC due to an isomorphism of adipic acid and
terephthalic acid with hexamethylenediamine (HMD), has
only a Tg of 95 ꢀC. If more 6T units are added, the melting
temperature is too high and the copolyamide cannot be melt
processed without strong degradations.2,4 The use of iso-
phthalic acid as a comonomer can also increase the Tg of PA
66, but tends to decrease strongly the melting temperature
and the crystallinity of the polyamide. For instance, PA 6I is
ꢀ
amorphous with a Tg of about 125 C, which corresponds to
a limit value when using this comonomer.1,2 The introduction
of functions in the PA 66, which can display molecular inter-
actions in order to decrease the chain mobility of the amor-
phous part of the polymer,5 appears as a major goal in the
area of high performance polyamides.
Polyamide-bearing sulfonate groups should be an interesting
way to create electrostatic interaction, because metal sulfo-
nate groups are already used in numerous ionomers.6–8 Until
now, literature describes different ways to obtain polyamide
bearing sulfonate groups,9,10 either by direct sulfonation of
polyamide or via polymerization of sulfonated aromatic acid
with aliphatic or aromatic diamines. Such sulfonated copolya-
mides could be used as ion exchange membranes in fuel
cells.11–13 In textile applications, polyamide-bearing sulfonate
groups have been obtained via copolymerization of adipic
Additional Supporting Information may be found in the online version of this article.
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acid and lithium salt of 5-sulfoisophthalic acid (LiSIPA) with
HMD in order to improve dyeing process and stain resist-
ance.14–19 These properties showed that molecular interac-
tions are possible in copolyamides. Interactions involving
amide functions and metal sulfonate groups have not been
directly studied in copolyamide but only in polyamide/ion-
omer blends, where ionic groups along a polymer backbone
allow compatibilization.20–25
Synthesis of 1,3-Dihexylbenzenedicarboxamide (A)
In a clave, 7.0 g of isophthalic acid (0.042 mol) and 5 equiv
of hexylamine (21.3 g, 0.209 mol) were introduced. The
clave was progressively heated to 250 ꢀC, and the reaction
mixture was stirred during 2 h 30 min. The solid (A) was
removed from the clave and then dried under vacuum at
ꢀ
180 C.
1
3
ꢀ
H NMR (d6-DMSO, 25 C, 250 MHz) d (ppm): 0.86 (t, J ¼ 7
In a blend of poly(N,N0-dimethylethylene sebacamide) and
sulfonated polystyrene, Feng and coworkers26 have observed
coordination of the oxygen atom of the amide carbonyl
group with different metal cations. In a PA 6/lithium sulfo-
nated polystyrene blend, Rajagopalan and coworkers27 have
proposed a model that describes the local interactional
behavior of the ion pair ‘‘SO3ꢂ…Liþ’’ according to its environ-
ment. This model implies the formation of ionic aggregates
of lithium sulfonate moieties when ion pairs are closely asso-
ciated and interactions between lithium cation and carbonyl
groups as well as hydrogen bonds between NH amide and
SO sulfonate.28–30
Hz, 6H, ACH3), 1.28 (m, 3J ¼ 7 Hz, 12H, ACH2ACH2A
CH2ACH2ACH3), 1.52 (m, 3J ¼ 7 Hz, 4H, ACONHACH2A
CH2ACH2A), 3.26 (q, 3J ¼ 6 Hz, 4H, ACONHACH2ACH2),
7.52 (t, 3J ¼ 8 Hz, 1H, HArom5), 7.93 (dd, 3J ¼ 8 Hz, 4J ¼ 2
Hz, 2H, HArom4,6), 8.27 (t, 4J ¼ 2 Hz, 1H, HArom2), 8.52 (t, 3J
¼ 6 Hz, 2H, ACONHA). 13C NMR (d6-DMSO, 25 ꢀC, 250
MHz) d (ppm): 14.4 (ACH3), 22.6 (ACH2ACH2ACH3), 26.7
(ACONHACH2ACH2ACH2ACH2A), 29.6 (ACONHACH2A
CH2A), 31.5 (ACONHACH2ACH2ACH2ACH2A), 39.8 (ACONHA
CH2A), 126.7 (CArom2), 128.6 (CArom5), 130.0 (CArom4,6), 135.4
(CArom1,3), and 166.3 (ACONHA).Anal. calculated for C20H32N2O2:
C, 72.25; H, 9.70; N, 8.43. Found: C, 71.54; H, 9.38; N, 8.25.
Only sulfonated copolyamides with low content amount of
comonomer, known as lithium salt of 5-sulfoisophthalic acid,
has been synthesized, because less than 2 mol % of a sulfo-
nated comonomer are enough to obtain tinctorial properties.
The present work aims at preparing PA 66/6LiSIPA with
higher content of lithium sulfonate moieties and at investi-
gating their interactions with the polyamide skeleton and
their influence on the thermal properties of the polyamide.
Preparation of Lithium p-Toluenesulfonate
A 250-mL two-necked flask was charged with 10.0 g of 4-
methylbenzenesulfonic acid hydrate (0.058 mol) and 1.4 g of
lithium hydroxide (0.058 mol). One hundred milliliters of
distilled water was added to dissolve the two reactants. The
flask was purged with argon, and the mixture was stirred
for 4 h under an argon stream. The water was evaporated
off under vacuum at 90 ꢀC. The product was dissolved in a
toluene/ethanol mixture (90/10 vol/vol) and was filtrated.
Finally,ꢀlithium p-toluenesulfonate was dried under vacuum
at 110 C during 12 h.
EXPERIMENTAL
Materials
Anal. calculated for C7H7LiO3S: C, 47.2; H, 3.96; Li, 3.90
Found: C, 45.35; H, 4.30; Li, 3.60
Nylon salt (equimolar mixture of adipic acid and HMD) and
aqueous solution of HMD were supplied by Rhodia. Lithium
salt of 5-sulfoisophthalic acid (LiSIPA), isophthalic acid, and
hexylamine were purchased from Acros Organics. Lithium
hydroxide and 4-methylbenzenesulfonic acid hydrate were
purchased from Aldrich.
Characterizations
High-resolution liquid NMR spectroscopy was carried out
with a Bruker spectrometer operating at 250 MHz for 1H.
Samples were examined using 8 mg of copolyamide in 0.7
ꢀ
mL of deuterated formic acid at 25 C. Chemical shift values
(d) are given in ppm in reference to formic acid resonance at
8.50 ppm.
Polyamide Synthesis
Syntheses of PA 66 and copolyamides with LiSIPA were per-
formed in a 1 L-pilot reactor for high temperature and pres-
sure reactions. The Nylon salt, aqueous solution of an equi-
molar mixture of HMD and LiSIPA, and distilled water were
introduced in the reactor. Quantities of monomers were cal-
culated so as to obtain 260 g of copolyamide with the
desired lithium sulfonate content, and water was added to
start process with a monomer weight concentration in water
of 52 wt%. The reactor was first purged using nitrogen and
was then heated to the desired temperature and the water
was progressively distilled. The polymerization finished in
bulk. Finally, copolyamide was removed from the reactor by
casting, using a bottom drain valve. Reactor temperature
must be above melting temperature of polyamide, but no
desulfonation is expected, because LiSIPA degradation occurs
The end group concentration of copolyamides was deter-
mined by potentiometric back titration using a Metrohm 736
titrator. Samples were examined at a concentration of 1 g of
copolyamide in 50 mL in trifluoroethanol/chloroform (77/23
vol/vol) containing 9 mL of tetrabutylammonium hydroxide.
Amine end concentration (GTA) and acid end concentration
(GTC) were given in mmol per kilogram. The average molar
mass in number is calculated using following equation:
Mn ¼ 2 ꢃ 106=ðGTA ꢄ GTCÞ:
Differential scanning calorimetry (DSC) measurements were
performed on a Mettler Toledo DSC 1 STARe System appara-
tus. Equipment was calibrated with indium, and all samples
were dried under vacuum at 90 ꢀC during 16 h, accurately
ꢀ
beyond 360 C (see Supporting Information Fig. S1 in ESI).
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weighted (5–10 mg), and sealed in a 40-lL aluminum pans.
Thermal cycle breaks down into five segments:
showed the presence of internal and chain-end LiSIPA units
and possibly unreacted comonomer. Therefore, copolyamide
LiSIPA content (%LiSIPA) can be determined in comparison
with adipic acid content (eq 1), and %LiSIPA match the per-
centage of ‘‘6AISLi’’ repeating units in the copolyamide, if
chain ends are neglected.
• A thermal history erasing was done by heating the sample
ꢂ1
ꢀ
ꢀ
from 30 to 300 C at 10 C min followed by 5 min iso-
ꢀ
therm at 300 C.
• A controlled cooling was done bꢂy1cooling the sample from
300 ꢀC to ꢂ30 ꢀC at 10 ꢀC min followed by 5 min iso-
0
0
0
0
0
%LiSIPA ¼ ½fI6 þ I7 g=3 þ fI6 e þ I7 e1 þ I7 e2Þ=3ꢅ=
ꢀ
therm at ꢂ30 C.
0
0
0
0
0
ꢀ
ꢀ
½fI6 þ I7 g=3 þ fI6 e þ I7 e1 þ I7 e2Þ=3 þ I1=4ꢅ
ð2Þ
• A second heating of the sample from ꢂ30 C to 300 C at
ꢂ1
ꢀ
10 C min
.
Copolyamides containing up to 40 mol % of LiSIPA were
synthesized. Each sample was analyzed by potentiometry
and DSC in order to determine respectively their molar
mass and thermal properties. The results are summarized in
Table 1. It is well known that the polycondensation rate is
directly related to the molar mass of the resulting polymer.
Molar masses of copolyamides PA 66/6LiSIPA decrease
according to the LiSIPA content. Thus, the presence of LiSIPA
in the reaction media decreases the rate of polycondensation.
Moreover, copolyamides PA 66/6LiSIPA have an important
part of LiSIPA units located at the chain end. For the copo-
lyamide bearing 17.9 mol % of LiSIPA, about 70% of chain
ends corresponds to a sulfoisophthalic moeity. As a blank
experiment, the synthesis of a copolyamide PA 66/6I using
18.1 mol % of isophthalic acid was also performed and
showed only a limited decrease of the molar mass compared
to PA 66. These results demonstrate that the reaction
between HMD and LiSIPA is a limitative step and in
Only the second heating run was considered for the determi-
nation of glass transition temperatures, melting tempera-
tures, and enthalpies associated.
Infrared spectroscopy was performed on a ThermoFischer
spectrometer with ATR technology.
RESULTS AND DISCUSSION
Copolyamide Syntheses
Copolyamides bearing sulfonate groups PA 66/6LiSIPA were
prepared in a 1 L reactor, using Nylon salt and an equimolar
mixture of LiSIPA and HMD. The chemical composition of
copolyamides was determined using 1H NMR spectroscopy
(Fig. 1). More details concerning the assignments of the
peaks are given in Supporting Information (13C NMR spec-
1
trum and H–13C correlation in Supporting Information Figs.
S2 and S3). The analysis of the aromatic resonance region
FIGURE 1 1H NMR spectrum of a copolyamide PA 66/6LiSIPA (the starred peak corresponds to the proton of HCOOD coupled
with a 13C). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 1 Macromolecular Properties Evolution Depending on LiSIPA Content
Comonomer Contentb
Thermal Propertiesc
Mn (g molꢂ1
)
Global (%)
Internal (%)
Chain End (%)
Unreacted (%)
Tg (ꢀC)
Tm (ꢀC)
v
a
Copolyamide
PA 66
15,600
13,800
12,700
10,400
6,900
/
/
/
/
59
70
262
37%
29%
34%
22%
PA 66/6I
18.1
4.6
/
/
/
238
PA 66/6LiSIPA
PA 66/6LiSIPA
PA 66/6LiSIPA
PA 66/6LiSIPA
PA 66/6LiSIPA
a
89.5
92.1
87.8
77.9
82.8
10.5
7.9
12.2
20.1
15.8
b
0
77
252
9.7
0
95
238
17.9
26.5
40.3
Traces
2.0
102
137
155
Amorphous
Amorphous
Amorphous
4,300
3,400
1.5
Average molar mass in number was calculated thanks to end group
analysis.
Determined by 1H NMR.
c
Determined by DSC analysis.
particular that the presence of the sulfonate groups
‘‘SO3ꢂ…Liþ’’ slows down polycondensation. Vouyiouka and
coworkers31 have pꢂroposed that it may exist ionic interac-
crystallite growth into the form of lamellae is possible as
soon as macromolecular chains own 3 þ 1/2 or four repeat-
ing units (half repeating unit is a HMD or adipic acid
moiety),32 or in a statistical copolyamide when comonomer
content is less than 22.2 mol %. Statistical repartition of
LiSIPA in copolyamide is not proved in our copolyamide,
but this study is in agreement with the amorphous state of
copolyamides bearing more than 25 mol % of LiSIPA.
þ
tions between ASO3 and the ammonium moiety CH2ANH3
leading to an amine ends’ partial deactivation.
The crystallinity of copolyamides decreases according to the
content of LiSIPA. No melting transition was observed for a
polymer bearing 17.9 mol % of LiSIPA (see Fig. 2). Neverthe-
less, 14% of crystallinity was reached for the same sample
after annealing. The polyamide was heated up to 300 ꢀC,
then cooled down to 140 ꢀC, and kept at 140 ꢀC during
60 h. After cooling down the sample at ꢂ30 ꢀC, a melting
transition was observed by heating the polymer from
ꢂ30 ꢀC to 300 ꢀC at 10 ꢀC minꢂ1. This result demonstrates
that crystallinity is thermodynamically possible but kineti-
cally slow. Using the same heat treatment, no crystallinity
was obtained with the copolymer containing 26.9 mol % of
LiSIPA. It also appeared that the crystal compactness of
copolyamides PA 66/6LiSIPA is impacted by sulfonate
groups, because the use of isophthatic acid as comonomer
resulted in a lower impact on thermal properties than LiSIPA
(Table 1). Studies on PA 66 crystallisation have proved that
Interestingly, the glass transition temperature (Tg) evolution
of polyamide according to LiSIPA content is very impressive
as shown in Figure 2. As little as 4.6 mol % of LiSIPA
increases the Tg of the polyamide of 17 ꢀC and Tg reaches
155 ꢀC for a copolymer bearing 40 mol % of LiSIPA. A pla-
teau is observed between 10 and 20 mol % of LiSIPA, which
corresponds to the structural change in copolyamides, from
a semicrystalline state to an amorphous state. The unique
thermal properties of PA 66/6LiSIPA is principally due to
the presence of ion pair ‘‘SO3ꢂ…Liþ,’’ because a copolymer PA
66/6I showed a Tg being 32 ꢀC lower than the one of the
copolymer PA 66/6LiSIPA having similar content of
comonomer (Table 1).
In summary, the introduction of lithium sulfonate moiety in
PA 66 leads to a large increase of the glass transition tem-
perature. The decrease of the chain mobility may indicate
that new interactions exist between lithium sulfonate groups
and amide functions. These interactions are stronger than
hydrogen bonds usually encountered in polyamide. In addi-
tion, the comonomer LiSIPA hampers the crystallisation
dynamic of the macromolecular chains, which organize dif-
ferently than pure PA 66 by creating crystallite with lower
compactness. Macroscopic effects observed do not allow us
to understand what is the precise nature of the interactions
that occur. That is why model diamides were synthesized in
a second part to study interactions between lithium sulfo-
nate groups and amide functions by infrared spectroscopy.
Molecular Interactions
1,3-Dihexylbenzenedicarboxamide (A) and lithium p-toluene-
sulfonate (see Fig. 3) were synthesized in order to investi-
gate the interactions between lithium sulfonate groups and
FIGURE 2 Thermal properties of copolyamide PA 66/6LiSIPA.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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FIGURE 3 1,3-Dihexylbenzenedicarboxamide (left) and lithium
p-toluenesulfonate (right).
amide functions. Mixtures of these compounds were per-
formed in tetrahydrofuran using various ratio, the resulting
products were analyzed by IR spectroscopy after evaporation
of the solvent. In the following discussion, mixture of reac-
tants is expressed by the ratio ANHACOA/ASO3Li.
Lithium p-toluenesulfonate is characterized by ATR infrared
spectroscopy by the antisymmetric and symmetric stretching
bands of the SAO bonds, one band at 1,130 cmꢂ1 and two
bands at 1,050 and 1,058 cmꢂ1. We suspect that the band at
1,058 cmꢂ1, which is not reported in Ref. 26, is certainly
due to the presence of residual traces of water used for the
synthesis of lithium p-toluenesulfonate. When amide content
increases in the mixture, on one hand, the SAO absorption
bands at 1,050 and 1,130 cmꢂ1 are, respectively, shifted to
1,044 and 1,126 cmꢂ1; on the other hand, the SAO absorp-
tion band at 1,058 cmꢂ1 progressively disappears (see
Fig. 4). The shift of the sulfonate stretching vibrations to
lower frequencies upon the addition of more and more am-
ide indicates that the amide group weakens the electrostatic
field between the metal cation and the sulfonate anion.
FIGURE 5 Infrared spectra of (A) and lithium p-toluenesulfo-
nate mixtures (C–O absorption band). From the top to the bot-
tom, curves are in the same order than the captions. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
the carbonyl group of amide functions. It becomes possible
to take advantage of these observations for the investigation
of PA 66/6LiSIPA by infrared spectroscopy. In copolyamide
PA 66/6LiSIPA containing 4.6–40.7 mol % of LiSIPA, the
ratio ANHACOA/ASO3Li varies in a range from 43 to 5,
which means high amide content versus sulfonate groups.
All copolyamides are characterized by two SAO absorption
bands at 1,040 and 1,110 cmꢂ1, and by a shift of the car-
bonyl absorption band from 1,631 cmꢂ1 in PA 66 and PA
66/6I to 1,636 cmꢂ1 in PA 66/6LiSIPA (see Supporting In-
formation Figs. S4 and S5 in ESI). Similar bands were
observed for mixture of model compounds at high ratio
ANHACOA/ASO3Li. Consequently, the SAO absorption
bands in copolyamide indicate a weakening of the ion pair
‘‘SO3ꢂꢆLiþ’’ electrostatic field, and the shift of CAO absorp-
tion band demonstrates that lithium cation is involved in a
new ion-dipole interaction with several carbonyl groups.
(A) is characterized by ATR infrared spectroscopy by one
single CAO absorption band at 1,630 cmꢂ1. When sulfonate
content increases in the mixture, a new band appears at
1,642 cmꢂ1 next to the first one, and, for larger sulfonate
content, only the absorption band at 1,642 cmꢂ1 remains
(see Fig. 5). This result indicates that the weakening of the
interaction of the sulfonate and the lithium cation is due to
an ion-dipole interaction between the oxygen atom of the
amide carbonyl group and the Liþ of the lithium sulfonate
moiety.
CONCLUSIONS
The investigation of model compounds highlights the pres-
ence of interactions between lithium sulfonate moieties and
The sulfonated copolyamides PA 66/6LiSIPA present a unique
macromolecular organization, due to ion–dipole interactions
involving lithium sulfonate moieties. These interactions make
possible formation of physical cross-linking points centered on
lithium cations, surrounded by several carbonyl groups of am-
ide functions. This physical network decreases significantly
the chains mobility of polyamides leading to a high increase of
the glass transition temperature.
The financial support from the French National Agency for
Research (MATETPRO07_185865) and the competitiveness
cluster Axelera is acknowledged. The laboratory C2P2 is grate-
ful to Mettler Toledo for support concerning thermal analyses.
The authors thank Alain Rousseau for his expert technical help
and Cecile Chamignon for NMR analysis.
REFERENCES AND NOTES
FIGURE 4 Infrared spectra of (A) and lithium p-toluenesulfo-
nate mixtures (S–O absorption region). From the top to the
bottom, curves are in the same order than the captions.
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