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Polymer Degradation and Stability xxx (2014) 1e10

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Degradation reactions during sulphonation of poly(styrene-co-acrylicacid) used as membranes

L. Melo a, R. Benavides a, *, G. Martínez a, L. Da Silva b, M.M.S. Paula b

a Centro de Investigaci�on en Química Aplicada, Blvd. Enrique Reyna 140, Saltillo, Coah. 25294, Mexicob Laboratorio de Sintesis de Complexos Multifuncionais, Universidade do Extremo Sul Catarinense, Criciuma, SC, Brazil

a r t i c l e i n f o

Article history:Received 28 February 2014Received in revised form30 May 2014Accepted 2 June 2014Available online xxx

Keywords:Poly(styrene-co-acrylic acid)SulphonationCrosslinkingPolymer matrix decomposition

* Corresponding author. Tel.: þ52 844 4389830x13E-mail addresses: roberto.benavides@ciqa.ed

(R. Benavides).

http://dx.doi.org/10.1016/j.polymdegradstab.2014.06.00141-3910/© 2014 Elsevier Ltd. All rights reserved.

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a b s t r a c t

In this study, a random copolymer of poly(styrene-co-acrylic acid) (PS-AA) was synthesized in solutionby radical polymerization and partially crosslinked with divinyl benzene to improve mechanical resis-tance. The copolymer (PS-AA) was sulphonated with different theoretical molar quantities (20e60%) ofsulphuric acid (H2SO4) and acetyl sulphate (CH3COOSO3H). The sulphonated PS-AA materials werecharacterized with infrared spectroscopy (FTIR), insoluble material percentage by soxhlet extraction,molar mass by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). FTIRindicated the presence of sulphonic groups bound into the copolymer matrix and some changes inspecific bands. Gel percentage was considerably increased when H2SO4 was used as the sulphonationagent while a drop in molecular weight was detected by GPC with acetyl sulphate as the agent. Theformer effect is due to linkages through the sulphone groups and the latter as a consequence of apolymer matrix destructive side reaction. DSC thermograms show that sulphonation with H2SO4 de-creases the Tg value, while CH3COOSO3H increases the transition, in comparison with the neat PS-AA.The latter could be due to the presence of ionic interactions in the copolymer. Side reactions duringsulphonation of polymers are very important for the final physical properties.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

There is a worldwide major need for alternative sources of en-ergy to reduce the increasing environmental pollution problem. Anappropriate option is the use of fuel cells (FC), however its wide-spread use is limited due to the wear or degradation of their moreexpensive components, namely the catalysts and the electrolyte.Such an electrolyte is a polymeric proton exchange membrane(PEM) for the PEMFC type of cells, which is a relatively simple to useand replaceable membrane for low temperature systems.

The ideal characteristics of a PEM membrane are: chemical andmechanical stability under normal operation conditions, highproton conductivity, low permeability to the fuel, modest economiccost, as well as feasibility to be dissolved, melted and/or extruded touse them in fuel cells with non flat geometries [1e3].

The materials used must comply with some of the characteris-tics to be able to be used as fuel cell membranes, like proton

22; fax: þ52 844 4389839.u.mx, robciqa@gmail.com

02

l., Degradation reactions durttp://dx.doi.org/10.1016/j.pol

conductivity which depends on the water content, it has to supportthe fuel cell operating temperature [1,4] and have the type andamount of acid groups. The presence of weak acid groups controlsthe swelling in water while stronger acidic groups provide theneeded conductivity due to its strong ionic characteristics [5].

It is well known that sulphonated aromatic polymers are me-chanically, thermally and chemically stable, hence these materialsmay be useful in fuel cell applications. Aromatic polymers studiedinclude sulphonated poly(p-phenylene)s, sulphonated poly-sulfones, sulphonated poly(ether ether ketone)s, sulphonated pol-yimides (SPIs), sulphonated polyphosphazenes, and sulphonatedpolybenzimidazoles, among others [1]. Such aromatic polymershave been readily sulphonated using various sulphonating agents:chlorosulphonic acid [6e11], sulphur trioxide [12], sulphurtrioxide-based complex [13e15], trimethylsilyl chlorosulphonate[16e20], H2SO4eAg2SO4 [21] and hexanoyl sulphate [22]. The mostpopular sulphonating agents are: H2SO4 [7,10,23e26] and acetylsulphate (CH3COOSO3H) [14,27e30]. However, homogeneity of thereaction media is totally different due to the lack of miscibility ofH2SO4 with organic solvents as dichloromethane, while acetylsulphate has total miscibility. Such conditions creates considerabledifferences in the sulfonated product. Post-sulphonation reaction

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

Table 1Chemical reagents employed during the acetyl sulphate preparation for sulphona-tion reactions.

Theoreticalsulphonationdegree (% mol)

H2SO4

(mol)Aceticanhydride(mol)

Acetyl sulphateformed (mol)

20 0.2 0.22 0.240 0.4 0.45 0.450 0.5 0.56 0.560 0.6 0.67 0.6

L. Melo et al. / Polymer Degradation and Stability xxx (2014) 1e102

conditions suffer from a lack of control over the degree and locationof functionalization, with the possibility of polymer degradationinvolving undesirable modifications in the in-use properties of thematerial. Among the degradation effects we can mention thepolymer chain breakage into fragments through a variety of ways,resulting in low molecular weight products. There is also a possi-bility of thermochemical degradation, which is induced by thecombined action of chemical agents and heat; often both effectscould occur simultaneously, complicating the whole process[1,31,32].

Taking into account the characteristics needed in a PEM and theprofitability of synthesizing a polymeric material with low costchemical reagents, in this work is described the synthesis of apoly(styrene-co-acrylic acid), with subsequent sulphonation withH2SO4 or CH3COOSO3H. The only expected result from those sul-phonation reactions is the incorporation of the eSO3H groupneeded to provide proton conductivity to the copolymer.

2. Experimental

2.1. Materials

Styrene (St, 99%, Aldrich) was purified by washing thoroughlywith aqueous 20% NaOH and with distilled water to remove in-hibitors; it was also dried for several hours with CaCl2 [33] anddistilled at theminimum temperature applying reduced pressure ina nitrogen atmosphere. Acrylic acid (AA, 99%, Aldrich) was left incontact with phenothiazine to inhibit polymerization [34] duringits distillation, also at the minimum temperature applying reducedpressure in a nitrogen atmosphere. Benzoyl peroxide initiator (BPO)was dissolved in dichloromethane (CH2Cl2) at room temperatureand then precipitated by adding an equal volume of methanol(MeOH) [33]. Formed crystals were filtered and dried at roomtemperature under vacuum during 24 h. BPO, St and AAwere storedin dark conditions at approximately 4 �C before use. Divinylben-zene (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 reactionThe poly(styrene-co-acrylic acid) (PS-AA) copolymers were

synthesized with 94 %mol of St and 6 %mol of AA. The reactionswere carried out by conventional solution free radical polymeri-zation, using DEB as solvent. BPO was used as radical initiator at0.045 %mol, and DVB was employed as crosslinking agent at0.25 mol%. The initiator and crosslinking agent concentrations usedwere selected from previous experiments made in our researchgroup to synthesize a random PS-AA copolymer with Mn ¼ 68,012,Mw ¼ 259,095 and Ð ¼ 3.8, which is soluble in THF and allows filmformation by casting. The copolymerization reaction was carriedout mixing and stirring vigorously at 200 rpm the monomers,initiator, crosslinking agent and solvent, during 120 min at 90 �Cunder a nitrogen atmosphere. A four-necked jacketed glass reactorequipped with a condenser was used as a reactor. The final productwas precipitated in an excess of methanol and the copolymer pu-rified by dissolving it in THF and recovering by precipitation inmethanol. The copolymer was dried in a vacuum oven at 65e70 �Cduring 48 h.

2.2.2. Sulphonation procedures2.2.2.1. Acetyl sulphate preparation. The acetyl sulphate was pre-pared by mixing a measured amount of acetic anhydride in

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dichloromethane under an inert atmosphere (N2). The solutionwascooled down to 0 �C and kept during 10 min, then 98% sulphuricacid in stoichiometric amount with respect to the desired theo-retical %mol of sulphonation in the polymer, was carefully addedunder a nitrogen flow; once the addition was finished, the mixturewas stirred during 10 min more, until the reaction mixture becamea clear and homogeneous solution. The molar amount of aceticanhydride was always in a slight excess with respect to sulphuricacid, in order to scavenge the undesirable water, converting it toacetic acid. The acetyl sulphatewas always freshly prepared prior toeach sulphonation reaction. Table 1 shows the quantities of re-agents employed to have the different theoretical amounts of sul-phonating agent for one mol of aromatic ring from thepoly(styrene-co-acrylic acid).

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 nitrogenatmosphere into a jacketed glass reactor equipped with acondenser and mechanical stirring. The reactor was stirred vigor-ously at 200 rpm and heated to 40 �C with reflux condensation by40 min in order to obtain total solubilisation of the copolymer. Thedesired theoretical amount of sulphonating agent (20, 40, 50 or 60%mol of H2SO4 or CH3COOSO3H) was syringed into the reactor andthe sulphonation reactionwas left to proceed during 2, 10, 30, 60 or120 min under stirring. The reaction was interrupted by adding anexcess of freezing distilled water. The sulfonated copolymer wasfiltered, washed with room temperature distilled water untilreaching the pH of water and then filtered again. Finally, thepolymer was dried at room temperature with an airstream by 24 h.Thematerials were named according to the sulphonating agent (“s”for sulphuric acid and “as” for acetyl sulphate), the sulphonationtime and %mol of theoretical sulphonation.

2.2.3. Casting proceduresMaterials (neat and sulphonated copolymer) were dissolved

separately with tetrahydrofuran at room temperature and thepolymer solutions were poured onto square glass plates of 16 cm2.The ratio copolymer/THF employed was always 0.5 g/3mL/16 cm2.Evaporation of the solvent proceeded very gradually at roomtemperature during 3 days, keeping it covered with another glassplate and leaving only small spaces for the solvent vapour to escape.The membranes obtained were removed from the mould andplaced in a vacuum oven at 60 �C by 3 h to dry them completely.

2.3. Characterization

FTIR spectra were obtained from neat and sulphonated copol-ymer over the wavenumber range of 4000e400 cm�1 using aNicolet Avatar 320 FT-IR Spectrophotometer, with a resolution of4 cm�1 through 32 scans. The polystyrene spectrum included in theinstrument's software OMNIC 5.2 software package was used toconfirm the incorporation of acrylic acid units in the copolymer.Data processing included automatic baseline correction, and the

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

Table 2Physical performance of the neat and sulphonated polymers subjected to the castingprocedure (original sulphonation conditions).

Experiment Membrane code SulphonatingTime (min)

Theoretical %mol ofsulphonating agent

Filmformation

1 PS-AA 0 0 yes2 PS-AA/as 60020% 60 20 fragile3 PS-AA/as 120020% 120 20 fragile4 PS-AA/as 60040% 60 40 no (fragile)5 PS-AA/as 120040% 120 40 no (fragile)6 PS-AA/as 60060% 60 60 no (fragile)7 PS-AA/as 120050% 120 50 no (fragile)8 PS-AA/s 60020% 60 20 no (gel)9 PS-AA/s 120020% 120 20 no (gel)10 PS-AA/s 60040% 60 40 no (gel)11 PS-AA/s 120040% 120 40 no (gel)12 PS-AA/s 60060% 60 60 no (gel)13 PS-AA/s 120060% 120 60 no (gel)

Table 3Physical performance of the sulphonated polymers subjected to the casting proce-dure (less aggressive sulphonation conditions).

Experiment Membrane code SulphonatingTime (min)

Theoretical %mol ofsulphonating agent

Filmformation

14 PS-AA/as 2020% 2 20 yes15 PS-AA/as 10020% 10 20 yes16 PS-AA/as 30020% 30 20 yes17 PS-AA/s 2020% 2 20 yes18 PS-AA/s 10020% 10 20 yes19 PS-AA/s 30020% 30 20 yes

L. Melo et al. / Polymer Degradation and Stability xxx (2014) 1e10 3

semi-quantitative comparisons determined by using an internalreference peak (symmetric vibration of the aromatic ring at1602 cm�1 [35]), considering these bonds are chemically stable andexpected to remain after sulphonation reactions.

The degree of crosslinking of the copolymers was measured interms of the gel percent content, namely the insoluble residueremaining after 12 h of soxhlet extraction in tetrahydrofuran. Asample of approximately 0.5 g (w1) was placed inside the filterpaper thimble of known mass (w2) and submitted to THF reflux for12 h. The filter paper thimble was then vacuum dried at 80 �C for12 h (w3). The percent gel content (weight fraction) was calculatedby using the equation:

Gel contentð%Þ ¼ fðw3 �w2Þ=w1g � 100

which indicates the degree of crosslinking [36].The molecular weight of copolymers sulphonated with

CH3COOSO3H were measured in a ALLIANCE 2695 Waters GelPermeation Chromatograph (GPC) equipped with a Waters 2414refractive index detector. HPLC-grade tetrahydrofuran (THF) wasused as mobile phase at 30 �C, which was pumped at 1.0 mL/min bytwo lineal mixed C columns. The GPC was calibrated using 10polystyrene standards with molecular weights ranging 580 to2.6 � 106 g/mol, and the analysis time was of 28 min. Samplesconsisted of the polymer solution at 1 mg/mL concentration,filtered through a PTFE filter (pore size 0.45 mm).

Differential Scanning Calorimetry (DSC) measurements wereperformed in a TA Instruments 2920 thermal analyzer, at thetemperature range of 30e200 �C, and a heating rate of 10 �C/min,under N2 atmosphere and using approximately 10mg of sample. Allsamples were submitted to a heating-cooling-heating cycle(30e200 �C) to evaluate the glass transition temperature (Tg).

3. Results and discussion

3.1. Physical performance of the polymers

Considering that the obtained materials must have possibilitiesfor using them as membranes, ability to form films was the maincharacteristic needed to be able to continue experimenting withthem. The materials obtained from the sulphonation reaction of thePS-AA copolymer during 60 and 120 min with 20, 40, 50 and 60 %mol of sulphonating agent were all prepared by casting. The co-polymers PS-AA/s600 and PS-AA/s1200 were partially soluble in THFand by consequence their films had an heterogeneous thickness. Incontrast, copolymers PS-AA/as600 and PS-AA/as1200 dissolvedeasily in THF, but once the solvent was evaporated, the films werefragile and impossible to unmold (Table 2). To understand theirundesirable physical performance, materials were further charac-terized by soxhlet extraction and GPC analysis.

Taking into account earlier results, six further sulphonation re-actions were carried with the copolymer, but with less aggressivesulphonation conditions: shorter sulphonation times (2, 10 and30 min) and only 20 %mol of sulphonating agent. These experi-ments are described in Table 3 (14e19). Those sulphonated co-polymers were totally soluble in THF, able to prepare by castingprocedure and their films easy to unmold. These results suggest theconvenience of using mild sulphonation conditions for suchcopolymer.

3.2. Gel content by soxhlet extraction

As already mentioned, gel content is a way to understand thephysical performance of the first set of sulphonated copolymers(experiments 2-13, Table 1). Gel results shown in Fig. 1a and b

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indicate that when H2SO4 is employed as sulphonating agent thegel content of the PS-AA/s600 and PS-AA/s1200 materials is higherthan the neat copolymer (only 1.6 %gel). Such increments in the gelcontent are responsible for the partial solubility of the materials,which in turn forms heterogeneous films with irregular thicknessand rough surface. On the other hand, the sulphonated materialsPS-AA/as600 and PS-AA/as1200 (Fig. 1c and d) have similar gelcontent than the neat copolymer, which in turn explains why thosematerials are easily dissolved in THF.

Sulphonation reactions reported in this workwere carried out at40 �C, so the chemical changes observed in our PS-AA should havehappened at that temperature. In the literature there is no specificinformation regarding PS-AA copolymer thermal stability studiesbelow 90e100 �C, probably because this type of study usually aimsat effects of degradation during pyrolysis. Moreover, there is notenough information about the change of properties in such co-polymers when they undergo sulphonation reactions.

The main reaction expected at our random PS-AA copolymerduring sulphonation is the incorporation of sulphonic acid groupsin the aromatic rings; however, results indicate that side reactionsare 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]; further-more, the amount of these crosslinks increases when increasing thetemperature at which the process of sulphonation is carried out[28]. Such crosslinks can occur at relatively low reaction tempera-tures using several sulphonating agents, for example, at 40 �C withacetyl sulphate [29], at 55 �C with H2SO4 [37] or between 30 and60 �C with silica sulphuric acid [38], to name a few. It is also knownthat these types of chemical crosslinks (sulphones) are also formedbetween structures different to that found in polystyrene, such asPEEK (poly ethyl ether ketone), which crosslinks by the same typeof sulphones when HSO3Cl at 50 �C is used [11]. Thus, the tem-perature at which the crosslinking reactions occur between poly-mer chains are relatively low considering that the sulphonation

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

Fig. 1. Insolubility degree of the copolymers measured in terms of gel content remaining after 12 h of soxhlet extraction in THF.

Fig. 2. Molecular weight of neat and sulphonated copolymer with CH3COOSO3H at 20,40, 50 and 60% mol during 60 and 120 min.

L. Melo et al. / Polymer Degradation and Stability xxx (2014) 1e104

reactions described in the literature were carried out over a widerange of temperatures, usually from �20 to 300 �C [28].

3.3. Gel permeation chromatography

In order to understand the unfortunate chemical changesoccurring in the neat copolymer PS-AA when it was sulphonatedwith CH3COOSO3H (Table 1, experiments 2e7) is by the molecularweight measurement. The GPC chromatograms of these materialsare shown in Fig. 2. There is a clear reduction of the molecularweight of the sulphonated copolymer, compared with the neat one.In general, sulphonated copolymers with CH3COOSO3H have amolecular weight lower than 40,000 g/mol, which could explaintheir brittleness or lack of plasticity to unmold. It is reported in theliterature that polystyrene having a Mw < 150,000 is generally toobrittle to be useful and explains why no general-purpose mouldingand extrusion grades of PS having MW < 180,000 are soldcommercially [39].

From Fig. 2 we can also observe differences in dispersity, whichincreases probably because during the sulphonation reaction,various events take place during the incorporation of the eSO3Hgroup, such as crosslinking, chain scission and/or probably degra-dation of the acrylic acid units.

Considering the fact that degradation studies of copolymerswith acrylic acid units reported in the literature are basically per-formed under pyrolysis conditions (temperatures of 300 �C orhigher), the search was directed to other areas. There is a report ofPS-AA copolymer purification, carried out at temperatures under90 �C to avoid dehydration reactions and the formation of anhy-dride groups in the copolymer [40]. No degradation effect wasmentioned; although it is possible that such degradation reactionsof AA units were not noticed by the authors and indeed may notonly occur during the pyrolysis process.

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If such thermal degradation of the carboxylic acid groups occurs,the following processes can be expected: elimination of waterattached to the acid groups, dehydration of neighbouring eCOOHgroups and formation of anhydrides with subsequent decarboxyl-ation [41e45] and the formation of unsaturated groups. Evenbackbone depolymerization or total destruction of the polymermatrix can occur [46]. Probably the dehydration reactions andformation of anhydride in the PS-AA copolymer, reported by Wanget al. [40] correspond to the beginning of a process that could end inthe destruction of the polymer matrix. The latter could be reflectedas a decrease of the molecular weight of the copolymer.

There is also a high possibility that previous decompositionprocesses can be catalysed when the copolymer is immersed in anacidic environment. Arthur Ferris [47] published a patent titled

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

L. Melo et al. / Polymer Degradation and Stability xxx (2014) 1e10 5

“Carboxysulphonic cation-exchange resins”, where styrene andvinyltoluene were copolymerized with acrylic acid, methacrylicacid or esters. During further sulphonation he observed the for-mation of cyclic structures after loss of carboxylic compounds. Healso found that such decarboxylation reactions happen more oftenin AA copolymers. Ferris also mentioned that if temperature israised 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 acetylsulphate (CH3COOSO3H) is employed, decarboxylation reactionsoccur with further destruction of the polymer matrix in the sameway as reported by Ferris [47], this leads to a molecular weightreduction. The difference in this case is that such decompositionreactions are happening 5 �C below the temperature reported forthe initiation of such a degradation.

From Fig. 2 we also notice that when sulphonation reactions arecarried out at longer periods of time (60 and 120 min), some of thepolymeric chains decrease in size down to the order of 1000 g/mol.This phenomenon occurs specifically with the copolymers PS-AA/as60020% and PS-AA/as120020%; as a consequence, such materialsare not able to form films by the casting procedure, since once thesolvent evaporates the polymer films fracture (see Table 1).

Taking into account the drastic reduction in the molecularweight of the sulphonated copolymers under such sulphonationconditions and the results obtained by soxhlet extraction, it wasdecided to carry out further sulphonation reactions under milderconditions. Under reaction times of 2, 10 and 30 min and with thetheoretical degree of sulphonation of only 20 %mol, sulphonatedcopolymers were able to form films by casting and keep theoriginal molecular weight seen for the non sulphonated copol-ymer (Fig. 3).

It can be seen from Fig. 3 that there is a shift in the chain popu-lation toward lower molecular weights and an increase in the poly-mer chains with molecular weight between 6.6 � 106e3.2 � 106 g/mol. These two phenomena give an indication that chain-breakingreactions are occurring, decreasing the molecular weight of thematerial, as well as chemical crosslinking reactions that increase themolecular weight of a few polymer chains. However, the predomi-nant overall reactions are those which decrease the molecularweight of the copolymer.

3.4. Infrared analysis

The bands at 703, 760, 1453, 1498 cm�1 and those at3200e3000 cm�1 are all representative of the vibrations associatedwith the aromatic ring CeH bend [48]. Those bands, specifically at703 and 760 cm�1 are the out-of-plane skeleton bending vibrations

Fig. 3. Molecular weight of neat and sulphonated copolymer at 2, 10 an

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of benzene ring (characteristic bands of PS), and the out-of-planebending vibration of the five eCHe groups, characteristic of themonosubstituted benzene ring. Thus, these two bands, especiallythe intense band at 703 cm�1 provide a way to measure PS sul-phonation (the lower the sulphonation degree, the greater the in-tensities of these bands [49]). Figs. 4 and 5 show the infraredspectra of neat and sulfonated poly(styrene-co-acrylic acid)copolymer at different sulphonation degrees, showing a greaterreduction of bands when CH3COOSO3H was used, comparing withthe H2SO4 treated copolymer.

The sulphonation can also be verified through the asymmetric(SeO) vibration at 1180 cm�1, it appears as a very broad band atapproximately 1100 cm�1e1350 cm�1 [29]. In Figs. 4 and 5 thisband is depicted as a dashed area, where the signals at 1034 and1156 cm�1 represent the symmetric and asymmetric stretchingvibrations of the sulfonate group [30]. Dashed areas and such sig-nals are bigger when CH3COOSO3H was used as a sulphonatingagent comparing with H2SO4 treated copolymer.

Particular spectra differences between sulphonated copolymerwith H2SO4 (Fig. 4) and acetyl sulphate (Fig. 5) are the bands of thecarboxylic acid (1704 cm�1) and the 3450 cm�1 signal. The latterattributed to the stretching vibration of the sulphonic acid group(-SO3H) [50]. Both are always higher in acetyl sulphate treatedcopolymers. Moreover, carbonyl group band of the carboxylic acidundergoes changes in intensity and position after sulphonationreactions. It is known that ketones, aldehydes, carboxylic acids,carboxylic ester, lactones, acid halides, anhydrides, amides, andlactams show a strong C]O stretching absorption band in the re-gion of 1870e1540 cm�1. Within its range, the position of the C]Ostretching band is determined by the following factors: samplephysical state, electronic and mass effects of neighbouring sub-stituent, conjugation, hydrogen bonding (intermolecular andintramolecular), and ring strain [51].

In the experiments where H2SO4 is employed (Fig. 4), the bandcorresponding to the stretching of C]O remains in the same po-sition (1704 cm�1) as in the neat copolymer. But in the experimentswhere CH3COOSO3H is employed (Fig. 5), besides such band,another carbonyl signal appears at 1683 cm�1. It is also known thatconjugation with a C]C bond results in delocalization of the pelectrons of both unsaturated groups, which in turn reduces thedouble-bond character of the CeO bond, causing absorption atlower wavenumbers (longer wavelengths). Conjugation with aphenyl group, as in this case, causes absorption in the1685e1666 cm�1 region [51].

Taking into account the previous information, it can be consid-ered that for materials sulphonated with CH3COOSO3H, there is amolecular rearrangement within the copolymer after the

d 30 min and 20 %mol with H2SO4 (left) and CH3COOSO3H (right).

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

Fig. 4. Neat and sulphonated copolymers during 2, 10, 30, 60 and 120 min with 20 %mol of H2SO4.

L. Melo et al. / Polymer Degradation and Stability xxx (2014) 1e106

sulphonation reaction. The signal at 1683 cm�1 corresponds to aC]O stretching vibration from an a,b-unsaturated ketone; whichin turn comes from a decarboxylation reaction [47]. It has beenmentioned [52] that photodegradation occurs for polymers underthe influence of an acidic environment, conducting to chaincrosslinking, oxidation and bond scission. They have alsomentioned that acetophenone type end groups and unsaturationsare formed during such process [53,54].

Fig. 5. Neat and sulphonated copolymers during 2, 10, 3

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The decarboxylation reactions theory is consistent with resultsreported by Ferris [47], who found that in these kind of copolymersthere is a loss of carboxyl groups generated during the sulphona-tion process, resulting in the formation of cyclic structures in thepolymer chains. It was also found that such decarboxylation re-actions happen more often in acrylic acid copolymers than in theester copolymers. Ferris mentions vaguely that during sulphona-tion reactions the formation of some cyclic structures is also

0, 60 and 120 min with 20 %mol of CH3COOSO3H.

ing sulphonation of poly(styrene-co-acrylic acid) used as membranes,ymdegradstab.2014.06.002

Fig. 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 itsformation mechanism. IV) Theoretical PS-AA/s copolymer and its formation mechanism (involving decarboxylation and cyclization reactions with intramolecular, intermolecularcrosslinks through sulphones). V) Formation mechanism of acetophenone type end-groups during copolymer photo-oxidation.

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observed. More recently, some reports [55e60] indicate that co-polymers having units of alternated styrene and acrylic acid andsubjected to sulphonation reactions may undergo partial or com-plete cyclization. It means they could form cyclic ketones or sul-phonated polycyclic structures, which most likely correspond tothose mentioned by Ferris in 1954.

If these polycyclic structures are formed, they will be in verysmall quantities, explaining the weak signal in the spectra.Comparing the overtone signals (2000e1600 cm�1) [51], it can beseen that their profile is not the same for all the spectra: acetylsulphate treated copolymer loose definition. This occurs whenchanging the substituents on the aromatic ring of the initialstructure, either by replacement of an hydrogen by another atom oreven through the formation of polycyclic structures.

Considering FTIR spectra of the copolymers, before and aftersulphonation by both procedures, as well as information found inthe literature regarding cyclic structures formation in the polymerchains after photo-oxidation and loss of carboxylic groups, amechanism is proposed in Fig. 6.

Fig. 6(I) corresponds to a possible representation of the neatcopolymer, which is formed by styrenic and acrylic units and a fewDVB units. Once the copolymer was sulphonated (100%) with anysulphonating agent, the structure (II) is expected, where the prin-cipal effect is the incorporation of sulphonic groups into the styrenerings. However, considering the FTIR spectra of the acetyl sulphatetreated copolymer, chemical structures III and V can be formed,since there is a signal corresponding to an unsaturated carbonyl.

On the other hand, when H2SO4 is employed as sulphonatingagent, the carbonyl signal does not suffer any shift toward lowerwavenumbers, only a reduction in intensity. The latter could becaused by decarboxylation reactions, which precede the cyclizationreactions and form the chemical structure IV. However, when usingthis sulphonating agent, there is also an increasing amount ofinsoluble material from the formation of chemical crosslinksthrough sulphone groups between aromatic rings.

3.5. Differential scanning calorimetry

Fig. 7 shows the DSC thermograms in the interval of the glasstransition temperature (Tg) for the copolymers with and withoutsulphonation.

Fig. 7. DSC thermogram

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Changes in the glass transition can be observed for the materialsafter sulphonation procedures. When CH3COOSO3H is employedunder any conditions, all sulphonated copolymers have a Tg higherthan the Tg of the neat material.

The same trend has been observed in other ionomers bydifferent authors [61e63]. The incorporation of ionic groups into apolymer decrease mobility of the chain segments similarly to co-valent cross-links [63] and is attributed to hydrogen bonds andionic interactions, easy to disrupt with heating [64].

Ionic compounds have a tendency to form two types of ag-gregates: multiplets and clusters. Multiplets are considered to bean association of a few ion pairs (<8), completely coated withnonionic chain material. Clusters are suggested to result from theaggregation of multiplets [61]; since the previous are coated,clusters are expected to include chain segments. At a certaincritical temperature clusters decompose back to multiplets. Inamorphous materials, the ions are more efficient raising the glasstransition temperature of the polymers, if they are exclusively inmultiplets. It is expected that each multiplet in our system isacting as a physical cross-link instead of being incorporated inion-rich phase-separated microdomains. As such, ion pairs areeffective in raising the glass transition temperature. The occur-rence of two major peaks in the DMA tangent delta curves in theglass transition region is only associated when phase separationoccurs. Each peak is associated with a transition of one of thephases in the material [61].

For H2SO4 treated copolymer, gel content increase as a result ofcovalent crosslinks; which in turn are a result of sulphones comingfrom the sulphonic groups reaction. Such condition limits thecapability to form hydrogen bonding and ionic interactions to formmultiplets in the polyelectrolytes [61,62].

Besides the latter, it is noteworthy to observe that the Tg intervalfor lower temperatures, when H2SO4 is employed (120.4e112.1 �C),is considerably small than the Tg interval for CH3COOSO3H treatedcopolymer (120.4e154.8 �C). This is consistent with the fact thatthe copolymer composition is predominantly styrene (St/AAratio¼ 94/6); when decarboxylation happens during sulphuric acidsulphonation reactions, the amount of eCOOH ionic interactions isreduced. On the other hand, with acetyl sulphate reactions there isa greater incorporation of sulphonic groups in the styrene rings,enhancing the eSO3H ionic interactions.

s for copolymers.

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4. Conclusion

The poly (styrene-co-acrylic acid) was synthesized and sul-phonated during 60 and 120 min with 20, 40, 50 or 60 %mol ofH2SO4 or CH3COOSO3H; sulphonic groups were incorporated in thearomatic rings, but degradation side reactions also occurredemploying both sulphonating agents. H2SO4 induced the formationof highly crosslinked materials through sulphone groups betweenthe aromatic rings. Sulphonation with CH3COOSO3H inducedpolymer matrix destruction, generating small polymer chains andloosing mechanical stability, but incorporating a larger number ofsulfonic groups in the copolymer.

A first attempt to counteract the degradation reactionsmentioned above was through mild reaction conditions (2, 10 and30min of reaction and 20 %mol of sulphonation agent). Under theseconditions, the molecular weight and the gel content of the sul-phonated copolymers are very similar to the non sulphonatedcopolymer, thus its properties are not reduced and are capable offorming films (by casting) with enough mechanical stability to bemanipulated, which is a physical property essential in order toprepare ion exchange membranes. Further studies will define ifsuch conditions are enough to impart proton exchange ability tothe materials.

Acknowledgements

The authors acknowledge M.C. Silvia Torres Rinc�on assistancewith GPC analysis, Q. Ma. Guadalupe M�endez Padilla for DSCanalysis and M.C. Maria Concepci�on Gonz�alez Cantú for assistancein the laboratory. CONACyTM�exico is also greatly acknowledged forthe PhD. grant given to L. Melo.

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