Reporte Verano de La Ciencia

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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:

[email protected],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
mailto:[email protected]:[email protected]


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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

Reporte Verano de La Ciencia

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