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SINTESIS DE MEMBRANAS POLIMERICAS ALTERNATIVAS PARA CELDAS DECOMBUSTIBLE
Ocampo Medina Augusto Brasil1
y Benavides Cant Roberto2
RESUMENUn copolimero aleatorio de poli(estireno-co-cido acrlico) (PS-AA) fue sintetizado en solucin por
polimerizacin por radicales y sulfonado con diferentes cantidades molares tericas (20-60%) de cido
sulfrico (H2SO4) y sulfato de acetilo (CH3COOSO3H). Los materiales (PS-AA) sulfonados fueron
caracterizado con espectroscopia infrarroja (FTIR, indicando la presencia de grupos sulfonicos), porcentaje de
material insoluble por extraccin soxhlet, masa molar por cromatografa de permeacion en gel (GPC,
considerable incrementado cuando H2SO4 fue usado como agente sulfonante y una cada en el peso molecular
con sulfato de acetilo como agente), y calorimetra diferencial de barrido (DSC) mostraron que en la
sulfonacion con H2SO4 disminuyo la Tg, mientras que con el CH3COOSO3H incremento la transicin, en
comparacin con el PS-AA sin sulfonar.
ABSTRACTA random copolymer of poly(styrene-co-acrylic acid) (PS-AA) was synthesized in solution by radicalpolymerization and was sulphonated with different theoretical molar quantities (20-60%) of sulfuric acid
(H2SO4) and acetyl sulfate (CH3COOSO3H). The sulphonated PS-AA materials were characterized with
infrared spectroscopy (FTIR, indicating the presence of sulphonic groups), percentage of insoluble material
by soxhlet extraction, molar mass by Gel Permeation Chromatography (GPC, considerably increased when
H2SO4was used as sulfonating agent and a decrease in molecular weight with acetyl sulfate as the agent), and
Differential Scanning Calorimetry (DSC) showed that in the sulfonation with H2SO4 Tg decreased, while
with the transition CH3COOSO3H increase compared to the neat PS-AA.
PALABRAS CLAVEpoli(stireno-co-cido acrlico), sulfonacion, entrecruzamiento, matriz polimrica.
INTRODUCCINEl hombre ha estado cambiando de fuente de energa desde el tiempo de las cavernas. Comenzamos
quemando madera, de ah pasamos a quemar carbn; posteriormente a la quema de petrleo, y ahora estamos
comenzando a quemar gas natural. Cada uno de esos pasos a significado menos carbono y ms hidrogeno. De
manera que de continuar con la tendencia se podra decir que la conclusin lgica para la humanidad es nada
de carbono, solamente hidrogeno. La dramtica disminucin de las reservas mundiales de petrleo nos llevara
en pocos aos, si no se encuentra una solucin, a una crisis energtica sin precedentes que obligara a cambiar
drsticamente el actual modo de vida. Todo indica que el futuro de la energa pasa por el hidrogeno, el
combustible ms limpio que existe, es verstil y muy eficaz. Un combustible revolucionario que transformar
las relaciones sociales y econmicas en todo el mundo. Tambin supone una esperanza en la conquista de una
economa energtica sostenida. Una sociedad ideal de energa renovable es prcticamente imposible sin el
hidrogeno, es un almacn de energa porttil. El hidrogeno no es una fuente de energa principal, es un
sistema en s mismo para transportar y almacenar energa y por lo tanto, el problema sera que la existencia de
yacimientos de hidrogeno; este, se encuentra en la madera, carbn, petrleo, gas, pero sobre todo en el agua,
el componente ms abundante en la superficie terrestre.
1Departamento de Ingeniera Qumica y Bioqumica, Instituto Tecnolgico de Aguascalientes, Av. Lpez Mateos No. 1801 Ote., Fracc.
Bona Gens, C.P: 20256, Aguascalientes, Ags., Telfono (449) 910 5002.2Centro de Investigacin en Qumica Aplicada, Blvd. Enrique Reyna 140, Saltillo, Coah., Mxico. 25294. correspondance:
roberto.benavides@ciqa.edu.mx,tel. +52(844)4389830 xt. 1322, Fax. +52(844)4389839
Por lo tanto, una solucin de alternativa apropiada sera el uso de celdas de combustible (full cells).
Actualmente este tipo de tecnologa no es de uso comn, debido la degradacin y deterioro de sus
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componentes ms costosos, el electrolito y catalizadores. Tal electrolito sera una Membrana de Intercambio
Protnico (PEM).Este tipo de celdas de combustible tiene las caractersticas de ser relativamente simple de
usar y dicha membrana es reemplazable para sistemas de bajas temperaturas.
MTODOS Y MATERIALES2.Experimental
2.1. Materials
Styrene (St, 99%, Aldrich) was purified by washing thoroughly with aqueous 20 % NaOH and with distilled
water to remove inhibitors; it was also dried for several hours with CaCl 2[33] and distilled at the minimum
temperature applying reduced pressure in a nitrogen atmosphere. Acrylic acid (AA, 99%, Aldrich) was left
in contact with phenothiazine to inhibit polymerization [34] during its distillation, also at the minimum
temperature applying reduced pressure in a nitrogen atmosphere. Benzoyl peroxide initiator (BPO) was
dissolved in dichloromethane (CH2Cl2) at room temperature and then precipitated by adding an equal
volume of methanol (MeOH) [33]. Formed crystals were filtered and dried at room temperature under
vacuum during 24 hours. BPO, St and AA were stored in dark conditions at approximately 4C before use.
Divinylbenzene (DVB, Aldrich), diethylbenzene (DEB, Aldrich), H2SO4 (J.T. Baker), acetic anhydride
(Aldrich), tetrahydrofuran (THF, Aldrich) and dichloromethane (Aldrich) were used as received withoutfurther purification.
2.2. Methods
2.2.1. Copolymerization reaction
The poly(styrene-co-acrylic acid) (PS-AA) copolymers were synthesized with 94 %mol of St and 6 %mol of
AA. The reactions were carried out by conventional solution free radical polymerization, using DEB as
solvent. BPO was used as radical initiator at 0.045 %mol, and DVB was employed as crosslinking agent at
0.25 mol%. The initiator and crosslinking agent concentrations used were selected from previous experiments
made in our research group to synthesize a random PS-AA copolymer with Mn = 68,012, Mw = 259,095 and
= 3.8, which is soluble in THF and allows film formation by casting. The copolymerization reaction was
carried out mixing and stirring vigorously at 200 rpm the monomers, initiator, crosslinking agent and solvent,during 120 min at 90C under a nitrogen atmosphere. A four-necked jacketed glass reactor equipped with a
condenser was used as a reactor. The final product was precipitated in an excess of methanol and the
copolymer purified by dissolving it in THF and recovering by precipitation in methanol. The copolymer was
dried in a vacuum oven at 65-70 C during 48 hours.
2.2.2. Sulphonation procedures
2.2.2.1. Acetyl sulfate preparation
The acetyl sulfate was prepared by mixing a measured amount of acetic anhydride in dichloromethane
under an inert atmosphere (N2). The solution was cooled down to 0 C and kept during 10 minutes, then
98% sulphuric acid in stoichiometric amount with respect to the desired theoretical %mol of
sulphonation in the polymer, was carefully added under a nitrogen flow; once the addition was finished,
the mixture was stirred during 10 minutes more, until the reaction mixture became a clear andhomogeneous solution. The molar amount of acetic anhydride was always in a slight excess with respect
to sulphuric acid, in order to scavenge the undesirable water, converting it to acetic acid. The acetyl
sulphate was always freshly prepared prior to each sulphonation reaction. Table 1 shows the quantities
of reagents employed to have the different theoretical amounts of sulphonating agent for one mol of
aromatic ring from the poly(styrene-co-acrylic acid).
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Table 1. Chemical reagents employed during the acetyl sulphate preparation for sulphonation reactions.
Theoretical sulphonation
degree (% mol)H2SO4(mol)
Acetic
anhydride (mol)
Acetyl sulphate
formed (mol)
20 0.2 0.22 0.240 0.4 0.45 0.4
50 0.5 0.56 0.5
60 0.6 0.67 0.6
2.2.2.2. Sulphonation reaction of copolymer
110 g of poly(styrene-co-acrylic acid) was dissolved in 330 ml of DCM under a nitrogen atmosphere into a
jacketed glass reactor equipped with a condenser and mechanical stirring. The reactor was stirred vigorously
at 200 rpm and heated to 40C with reflux condensation by 40 minutes in order to obtain total solubilisation
of the copolymer. The desired theoretical amount of sulphonating agent (20, 40, 50 or 60 %mol of H2SO4or
CH3COOSO3H) was syringed into the reactor and the sulphonation reaction was left to proceed during 2, 10,
30, 60 or 120 minutes under stirring. The reaction was interrupted by adding an excess of freezing distilled
water. The sulfonated copolymer was filtered, washed with room temperature distilled water until reaching
the pH of water and then filtered again. Finally, the polymer was dried at room temperature with an airstream
by 24 hours. The materials were named according to the sulphonating agent (s for sulphuric acid and as
for acetyl sulphate), the sulphonation time and %mol of theoretical sulphonation.
2.2.3. Casting procedures
Materials (neat and sulphonated copolymer) were dissolved separately with tetrahydrofuran at room
temperature and the polymer solutions were poured onto square glass plates of 16 cm 2. The ratio
copolymer/THF employed was always 0.5g/3mL/16cm2. Evaporation of the solvent proceeded very gradually
at room temperature during 3 days, keeping it covered with another glass plate and leaving only small spaces
for the solvent vapour to escape. The membranes obtained were removed from the mould and placed in a
vacuum oven at 60 C by 3 hours to dry them completely.
2.3. Characterization
FTIR spectra were obtained from neat and sulphonated copolymer over the wavenumber range of 4000 - 400
cm-1using a Nicolet Avatar 320 FT-IR Spectrophotometer, with a resolution of 4 cm -1 through 32 scans. The
polystyrene spectrum included in the instrument's software OMNIC 5.2 software package was used to
confirm the incorporation of acrylic acid units in the copolymer. Data processing included automatic baseline
correction, and the semi-quantitative comparisons determined by using an internal reference peak (symmetric
vibration of the aromatic ring at 1602 cm-1[35]), considering these bonds are chemically stable and expected
to remain after sulphonation reactions.
The degree of crosslinking of the copolymers was measured in terms of the gel percent content, namely the
insoluble residue remaining after 12 hours of soxhlet extraction in tetrahydrofuran. A sample of
approximately 0.5 g (w1) was placed inside the filter paper thimble of known mass (w2) and submitted to THF
reflux for 12 hr. The filter paper thimble was then vacuum dried at 80 C for 12 hr (w 3). The percent gelcontent (weight fraction) was calculated by using the equation:
Gel content (%) = {(w3w2)/w1}100 which indicates the degree of crosslinking [36].
The molecular weight of copolymers sulphonated with CH3COOSO3H were measured in a ALLIANCE 2695
Waters Gel Permeation Chromatograph (GPC) equipped with a Waters 2414 refractive index detector. HPLC-
grade tetrahydrofuran (THF) was used as mobile phase at 30C, which was pumped at 1.0 mL/min by two
lineal mixed C columns. The GPC was calibrated using 10 polystyrene standards with molecular weights
ranging 580 to 2.6 106 g/mol, and the analysis time was of 28 min. Samples consisted of the polymer
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solution at 1 mg/mL concentration, filtered through a PTFE filter (pore size 0.45 m).
Differential Scanning Calorimetry (DSC) measurements were performed in a TA Instruments 2920 thermal
analyzer, at the temperature range of 30 to 200 C, and a heating rate of 10 C/min, under N 2atmosphere and
using approximately 10 mg of sample. All samples were submitted to a heating-cooling-heating cycle (30 to200 C) to evaluate the glass transition temperature (Tg).
RESULTADOS3. Results and Discussion
3.1. Physical performance of the polymers.
Considering that the obtained materials must have possibilities for using them as membranes, ability to form
films was the main characteristic needed to be able to continue experimenting with them. The materials
obtained from the sulphonation reaction of the PS-AA copolymer during 60 and 120 minutes with 20, 40, 50
and 60 %mol of sulphonating agent were all prepared by casting. The copolymers PS-AA/s 60 and PS-
AA/s120 were
partially soluble in THF and by consequence their films had an heterogeneous thickness. In
contrast, copolymers PS-AA/as60 and PS-AA/as120 dissolved easily in THF, but once the solvent was
evaporated, the films were fragile and impossible to unmold (Table 2). To understand their undesirablephysical performance, materials were further characterized by soxhlet extraction and GPC analysis.
Table 2. Physical performance of the neat and sulphonated polymers subjected to the casting procedure
(original sulphonation conditions).
Experiment Membrane codeSulphonating
time (min)
Theoretical %mol
of sulphonating
agent
Film
formation
1 PS-AA 0 0 yes
2 PS-AA/as 6020% 60 20 fragile
3 PS-AA/as 12020% 120 20 fragile
4 PS-AA/as 6040% 60 40 no (fragile)
5 PS-AA/as 12040% 120 40 no (fragile)6 PS-AA/as 6060% 60 60 no (fragile)
7 PS-AA/as 12050% 120 50 no (fragile)
8 PS-AA/s 6020% 60 20 no (gel)
9 PS-AA/s 12020% 120 20 no (gel)
10 PS-AA/s 6040% 60 40 no (gel)
11 PS-AA/s 12040% 120 40 no (gel)
12 PS-AA/s 6060% 60 60 no (gel)
13 PS-AA/s 12060% 120 60 no (gel)
Taking into account earlier results, six further sulphonation reactions were carried with the copolymer, but
with less aggressive sulphonation conditions: shorter sulphonation times (2, 10 and 30 minutes) and only 20
%mol of sulphonating agent. These experiments are described in table 3 (14-19). Those sulphonatedcopolymers were totally soluble in THF, able to prepare by casting procedure and their films easy to unmold.
These results suggest the convenience of using mild sulphonation conditions for such copolymer.
Table 3. Physical performance of the sulphonated polymers subjected to the casting procedure (less
aggressive sulphonation conditions).
Experiment Membrane codeSulphonating
time (min)
Theoretical %mol of
sulphonating agent
Film
formation
14 PS-AA/as 220% 2 20 yes
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15 PS-AA/as 1020% 10 20 yes
16 PS-AA/as 3020% 30 20 yes
17 PS-AA/s 220% 2 20 yes
18 PS-AA/s 1020% 10 20 yes
19 PS-AA/s 3020% 30 20 yes
3.2. Gel content by soxhlet extraction
As already mentioned, gel content is a way to understand the physical performance of the first set of
sulphonated copolymers (experiments 2-13, Table 1). Gel results shown in Figures 1a and 1b indicate that
when H2SO4is employed as sulphonating agent the gel content of the PS-AA/s60 and PS-AA/s120 materials
is higher than the neat copolymer (only 1.6 %gel). Such increments in the gel content are responsible for the
partial solubility of the materials, which in turn forms heterogeneous films with irregular thickness and rough
surface. On the other hand, the sulphonated materials PS-AA/as60 and PS-AA/as120 (Figures 1c and 1d)
have similar gel content than the neat copolymer, which in turn explains why those materials are easily
dissolved in THF.
Figure 1. Insolubility degree of the copolymers measured in terms of gel content remaining after 12 hours ofsoxhlet extraction in THF.
Sulphonation reactions reported in this work were carried out at 40 C, so the chemical changes observed in
our PS-AA should have happened at that temperature. In the literature there is no specific information
regarding PS-AA copolymer thermal stability studies below 90-100 C, probably because this type of study
usually aims at effects of degradation during pyrolysis. Moreover, there is not enough information about the
change of properties in such copolymers when they undergo sulphonation reactions.
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The main reaction expected at our random PS-AA copolymer during sulphonation is the incorporation of
sulphonic acid groups in the aromatic rings; however, results indicate that side reactions are not insignificant
or negligible at the working temperature (40 C). A well-known side reaction is the formation of sulphones,
i.e. chemical crosslinks between the polymer chains [13]; furthermore, the amount of these crosslinks
increases when increasing the temperature at which the process of sulphonation is carried out [28]. Suchcrosslinks can occur at relatively low reaction temperatures using several sulphonating agents, for example, at
40 C with acetyl sulphate [29], at 55 C with H 2SO4[37] or between 30 and 60 C with silica sulphuric acid
[38], to name a few. It is also known that these types of chemical crosslinks (sulphones) are also formed
between structures different to that found in polystyrene, such as PEEK (poly ethyl ether ketone), which
crosslinks by the same type of sulphones when HSO 3Cl at 50 C is used [11]. Thus, the temperature at which
the crosslinking reactions occur between polymer chains are relatively low considering that the sulphonation
reactions described in the literature were carried out over a wide range of temperatures, usually from -20 to
300 C [28].
3.3. Gel Permeation Chromatography
In order to understand the unfortunate chemical changes occurring in the neat copolymer PS-AA when it was
sulphonated with CH3COOSO3H (Table 1, experiments 2-7) is by the molecular weight measurement. The
GPC chromatograms of these materials are shown in Figure 2. There is a clear reduction of the molecular
weight of the sulphonated copolymer, compared with the neat one. In general, sulphonated copolymers with
CH3COOSO3H have a molecular weight lower than 40,000 g/mol, which could explain their brittleness or
lack of plasticity to unmold. It is reported in the literature that polystyrene having a Mw < 150,000 is
generally too brittle to be useful and explains why no general-purpose moulding and extrusion grades of PS
having MW < 180,000 are sold commercially [39].
Figure 2. Molecular weight of neat and sulphonated copolymer with CH 3COOSO3H at 20, 40, 50 and 60 %
mol during 60 and 120 minutes.
From Figure 2 we can also observe differences in dispersity, which increases probably because during the
sulphonation reaction, various events take place during the incorporation of the -SO3H group, such as
crosslinking, chain scission and/or probably degradation of the acrylic acid units.
Considering the fact that degradation studies of copolymers with acrylic acid units reported in the literature
are basically performed under pyrolysis conditions (temperatures of 300 C or higher), the search was
directed to other areas. There is a report of PS-AA copolymer purification, carried out at temperatures under
90 C to avoid dehydration reactions and the formation of anhydride groups in the copolymer [40]. No
degradation effect was mentioned; although it is possible that such degradation reactions of AA units were not
noticed by the authors and indeed may not only occur during the pyrolysis process.
If such thermal degradation of the carboxylic acid groups occurs, the following processes can be expected:
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elimination of water attached to the acid groups, dehydration of neighbouring -COOH groups and formation
of anhydrides with subsequent decarboxylation [41-45] and the formation of unsaturated groups. Even
backbone depolymerization or total destruction of the polymer matrix can occur [46]. Probably the
dehydration reactions and formation of anhydride in the PS-AA copolymer, reported by Wang et al. [40]
correspond to the beginning of a process that could end in the destruction of the polymer matrix. The lattercould be reflected as a decrease of the molecular weight of the copolymer.
There is also a high possibility that previous decomposition processes can be catalyzed when the copolymer is
immersed in an acidic environment. Arthur Ferris [47] published a patent titled "Carboxysulphonic cation-
exchange resins", where styrene and vinyltoluene were copolymerized with acrylic acid, methacrylic acid or
esters. During further sulphonation he observed the formation of cyclic structures after loss of carboxylic
compounds. He also found that such decarboxylation reactions happen more often in AA copolymers. Ferris
also mentioned that if temperature is raised close to 45 C, the loss of carboxylic groups increases rapidly,
reaching 50 % or more at 60 C.
Considering these findings it is possible to say that, when acetyl sulphate (CH 3COOSO3H) is employed,
decarboxylation reactions occur with further destruction of the polymer matrix in the same way as reported by
Ferris [47], this leads to a molecular weight reduction. The difference in this case is that such decomposition
reactions are happening 5 C below the temperature reported for the initiation of such a degradation.
From Figure 2 we also notice that when sulphonation reactions are carried out at longer periods of time (60
and 120 minutes), some of the polymeric chains decrease in size down to the order of 1000 g/mol. This
phenomenon occurs specifically with the copolymers PS-AA/as6020% and PS-AA/as12020%; as a
consequence, such materials are not able to form films by the casting procedure, since once the solvent
evaporates the polymer films fracture (see table 1).
Taking into account the drastic reduction in the molecular weight of the sulphonated copolymers under such
sulphonation conditions and the results obtained by soxhlet extraction, it was decided to carry out further
sulphonation reactions under milder conditions. Under reaction times of 2, 10 and 30 minutes and with the
theoretical degree of sulphonation of only 20 %mol, sulphonated copolymers were able to form films by
casting and keep the original molecular weight seen for the non sulphonated copolymer (Figure 3).
Figure 3 Molecular weight of neat and sulphonated copolymer at 2, 10 and 30 minutes and 20 %mol with
H2SO4(left) and CH3COOSO3H (right).
It can be seen from Figure 3 that there is a shift in the chain population toward lower molecular weights and
an increase in the polymer chains with molecular weight between 6.6x10 6 - 3.2x106 g/mol. These two
phenomena give an indication that chain-breaking reactions are occurring, decreasing the molecular weight of
the material, as well as chemical crosslinking reactions that increase the molecular weight of a few polymer
chains. However, the predominant overall reactions are those which decrease the molecular weight of the
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copolymer.
3.4. Infrared analysis
The bands at 703, 760, 1453, 1498 cm
-1
and those at 3200 - 3000 cm
-1
are all representative of the vibrationsassociated with the aromatic ring C-H bend [48]. Those bands, specifically at 703 and 760 cm -1are the out-of-
plane skeleton bending vibrations of benzene ring (characteristic bands of PS), and the out-of-plane bending
vibration of the five CH groups, characteristic of the monosubstituted benzene ring. Thus, these two
bands, especially the intense band at 703 cm-1 provide a way to measure PS sulfonation (the lower the
sulfonation degree, the greater the intensities of these bands [49]). Figures 4 and 5 show the infrared spectra
of neat and sulfonated poly(styrene-co-acrylic acid) copolymer at different sulphonation degrees, showing a
greater reduction of bands when CH3COOSO3H was used, comparing with the H2SO4treated copolymer.
The sulfonation can also be verified through the asymmetric (S-O) vibration at 1180 cm -1, it appears as a very
broad band at approximately 1100 cm-11350 cm-1[29]. In Figure 4 and 5 this band is depicted as a dashed
area, where the signals at 1034 and 1156 cm -1represent the symmetric and asymmetric stretching vibrations
of the sulfonate group [30]. Dashed areas and such signals are bigger when CH3COOSO3H was used as a
sulphonating agent comparing with H2SO4treated copolymer.
Particular spectra differences between sulphonated copolymer with H 2SO4 (Figure 4) and acetyl sulphate
(Figure 5) are the bands of the carboxylic acid (1704 cm-1) and the 3450 cm-1 signal. The latter attributed to
the stretching vibration of the sulphonic acid group (-SO3H) [50]. Both are always higher in acetyl sulphate
treated copolymers. Moreover, carbonyl group band of the carboxylic acid undergoes changes in intensity and
position after sulphonation reactions. It is known that ketones, aldehydes, carboxylic acids, carboxylic ester,
lactones, acid halides, anhydrides, amides, and lactams show a strong C=O stretching absorption band in the
region of 18701540 cm-1. Within its range, the position of the C=O stretching band is determined by the
following factors: sample physical state, electronic and mass effects of neighbouring substituent, conjugation,
hydrogen bonding (intermolecular and intramolecular), and ring strain [51].
In the experiments where H2SO4 is employed (Figure 4), the band corresponding to the stretching of C=O
remains in the same position (1704 cm-1) as in the neat copolymer. But in the experiments where
CH3COOSO3H is employed (Figure 5), besides such band, another carbonyl signal appears at 1683 cm
-1
. It isalso known that conjugation with a C=C bond results in delocalization of the electrons of both unsaturated
groups, which in turn reduces the double-bond character of the C-O bond, causing absorption at lower
wavenumbers (longer wavelengths). Conjugation with a phenyl group, as in this case, causes absorption in the
16851666 cm-1region [51].
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Figure 4. Neat and sulphonated copolymers during 2, 10, 30, 60 and 120 minutes with 20 %mol of H 2SO4.
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Figure 5. Neat and sulphonated copolymers during 2, 10, 30, 60 and 120 minutes with 20 %mol of
CH3COOSO3H.
Taking into account the previous information, it can be considered that for materials sulphonated with
CH3COOSO3H, there is a molecular rearrangement within the copolymer after the sulphonation reaction. Thesignal at 1683 cm-1corresponds to a C=O stretching vibration from an , -unsaturated ketone; which in turn
comes from a decarboxylation reaction [47]. It has been mentioned [52] that photodegradation occurs for
polymers under the influence of an acidic environment, conducting to chain crosslinking, oxidation and bond
scission. They have also mentioned that acetophenone type end groups and unsaturations are formed during
such process [53, 54].
The decarboxylation reactions theory is consistent with results reported by Ferris [47], who found that in these
kind of copolymers there is a loss of carboxyl groups generated during the sulphonation process, resulting in
the formation of cyclic structures in the polymer chains. It was also found that such decarboxylation reactions
happen more often in acrylic acid copolymers than in the ester copolymers. Ferris mentions vaguely that
during sulphonation reactions the formation of some cyclic structures is also observed. More recently, some
reports [55-60] indicate that copolymers having units of alternated styrene and acrylic acid and subjected to
sulphonation reactions may undergo partial or complete cyclization. It means they could form cyclic ketones
or sulphonated polycyclic structures, which most likely correspond to those mentioned by Ferris in 1954.
If these polycyclic structures are formed, they will be in very small quantities, explaining the weak signal in
the spectra. Comparing the overtone signals (2000-1600cm-1) [51], it can be seen that their profile is not the
same for all the spectra: acetyl sulfate treated copolymer loose definition. This occurs when changing the
substituents on the aromatic ring of the initial structure, either by replacement of an hydrogen by another atom
or even through the formation of polycyclic structures.
Considering FTIR spectra of the copolymers, before and after sulphonation by both procedures, as well as
information found in the literature regarding cyclic structures formation in the polymer chains after photo-
oxidation and loss of carboxylic groups, a mechanism is proposed in Figure 6.
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Figure 6. Possible chemical structures of the synthesized copolymers: I) Original PS-AA. II) Ideal
sulphonated copolymer at 100%. III) Theoretical PS-AA/as copolymer structure and its formation
mechanism. IV) Theoretical PS-AA/s copolymer and its formation mechanism (involving decarboxylation
and cyclization reactions with intramolecular, intermolecular crosslinks through sulphones). V) Formation
mechanism of acetophenone type end-groups during copolymer photo-oxidation.
Figure 6 (I) corresponds to a possible representation of the neat copolymer, which is formed by styrenic and
acrylic units and a few DVB units. Once the copolymer was sulphonated (100%) with any sulphonating agent,
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the structure (II) is expected, where the principal effect is the incorporation of sulphonic groups into the
styrene rings. However, considering the FTIR spectra of the acetyl sulphate treated copolymer, chemical
structures III and V can be formed, since there is a signal corresponding to an unsaturated carbonyl.
On the other hand, when H2SO4is employed as sulphonating agent, the carbonyl signal does not suffer anyshift toward lower wavenumbers, only a reduction in intensity. The latter could be caused by decarboxylation
reactions, which precede the cyclization reactions and form the chemical structure IV. However, when using
this sulphonating agent, there is also an increasing amount of insoluble material from the formation of
chemical crosslinks through sulphone groups between aromatic rings.
3.5. Differential Scanning Calorimetry
Figure 7 shows the DSC thermograms in the interval of the glass transition temperature (Tg) for the
copolymers with and without sulphonation.
Figure 7.DSC thermograms for copolymers.
Changes in the glass transition can be observed for the materials after sulphonation procedures. When
CH3COOSO3H is employed under any conditions, all sulphonated copolymers have a Tg higher than the Tg
of the neat material.
The same trend has been observed in other ionomers by different authors [61-63]. The incorporation of ionic
groups into a polymer decrease mobility of the chain segments similarly to covalent cross-links [63] and is
attributed to hydrogen bonds and ionic interactions, easy to disrupt with heating [64].
Ionic compounds have a tendency to form two types of aggregates: multiplets and clusters. Multiplets areconsidered to be an association of a few ion pairs (< 8), completely coated with nonionic chain material.
Clusters are suggested to result from the aggregation of multiplets [61]; since the previous are coated, clusters
are expected to include chain segments. At a certain critical temperature clusters decompose back to
multiplets. In amorphous materials, the ions are more efficient raising the glass transition temperature of the
polymers, if they are exclusively in multiplets. It is expected that each multiplet in our system is acting as a
physical cross-link instead of being incorporated in ion-rich phase-separated microdomains. As such, ion pairs
are effective in raising the glass transition temperature. The occurrence of two major peaks in the DMA
tangent delta curves in the glass transition region is only associated when phase separation occurs. Each peak
is associated with a transition of one of the phases in the material [61].
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For H2SO4treated copolymer, gel content increase as a result of covalent crosslinks; which in turn are a result
of sulphones coming from the sulphonic groups reaction. Such condition limits the capability to form
hydrogen bonding and ionic interactions to form multiplets in the polyelectrolytes [61, 62].
Besides the latter, it is noteworthy to observe that the Tg interval for lower temperatures, when H2SO4 isemployed (120.4-112.1 C), is considerably small than the Tg interval for CH3COOSO3H treated copolymer
(120.4-154.8C). This is consistent with the fact that the copolymer composition is predominantly styrene
(St/AA ratio= 94/6); when decarboxylation happens during sulphuric acid sulphonation reactions, the amount
of -COOH ionic interactions is reduced. On the other hand, with acetyl sulphate reactions there is a greater
incorporation of sulphonic groups in the styrene rings, enhancing the -SO3H ionic interactions.
CONCLUSIONESThe poly (styrene-co-acrylic acid) was synthesized and sulphonated during 60 and 120 minutes with 20, 40,
50 or 60 %mol of H2SO4 or CH3COOSO3H; sulphonic groups were incorporated in the aromatic rings, but
degradation side reactions also occurred employing both sulphonating agents. H2SO4 induced the formation
of highly crosslinked materials through sulphone groups between the aromatic rings. Sulphonation with
CH3COOSO3H induced polymer matrix destruction, generating small polymer chains and loosing
mechanical stability, but incorporating a larger number of sulfonic groups in the copolymer.
A first attempt to counteract the degradation reactions mentioned above was through mild reaction conditions
(2, 10 and 30 minutes of reaction and 20 %mol of sulphonation agent). Under these conditions, the molecular
weight and the gel content of the sulphonated copolymers are very similar to the non sulphonated copolymer,
thus its properties are not reduced and are capable of forming films (by casting) with enough mechanical
stability to be manipulated, which is a physical property essential in order to prepare ion exchange
membranes. Further studies will define if such conditions are enough to impart proton exchange ability to the
materials.
REFERENCIAS