Comparative study of electrostatic binding vs. complexation of Cu2+ ions with water-soluble polymers...

Comparative Study of Electrostatic Binding vs.Complexation of Cu21 Ions With Water-SolublePolymers Containing Styrene Sulphonic Acid and/orMaleic Acid Units or Their Sodium Salt Forms

EUDOKIA K. OIKONOMOU,1,2 GEORGIOS BOKIAS,1 JOANNIS K. KALLITSIS1,2

1Department of Chemistry, University of Patras, GR-26504 Patras, Greece

2Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High-TemperatureChemical Processes (ICE/HT FORTH), P.O. Box 1414, GR-26504 Patras, Greece

Received 25 January 2008; revised 12 March 2008; accepted 17 March 2008DOI: 10.1002/polb.21448Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The interaction of Cu21 ions with the homopolymer poly(styrene sulfonicacid) (PSSH), as well as with the copolymers of maleic acid (MAc) with styrenesulfonic acid (SSH) or vinyl acetate (VAc), was investigated in dilute aqueous solutionthrough turbidimetry, potentiometry, viscometry, and spectrophotometry in the visibleregion. Cu21 ions were introduced either through neutralization with Cu(OH)2 of theacid form of the (co)polymers (PSSH, P(SSH-co-MAc) and P(VAc-co-MAc)) or throughmixing of the sodium salt form of the (co)polymers (PSSNa, P(SSNa-co-MANa) andP(VAc-co-MANa)) with CuSO4. Turbidimetry, potentiometry, and spectrophotometryrevealed that the first carboxylic group of MAc or both carboxylate groups of MANaare involved in the complexation with Cu21 ions when neutralization with Cu(OH)2or mixing with CuSO4 are applied, respectively. The increased values of the reducedviscosity observed mainly at the first stages of neutralization of P(VAc-co-MAc) withCu(OH)2 indicate that interchain polymer-Cu21 complexation takes possibly place.Finally, the spectrophotometric behavior observed upon neutralization of P(SSH-co-MAc) with Cu(OH)2 or mixing of P(SSNa-co-MANa) with CuSO4 revealed that thestrength of counterion binding by the sulfonate groups is, in fact, comparable withthe complexation of Cu21 ions with the carboxylate groups of MAc. VVC 2008 Wiley Periodicals,

Inc. J Polym Sci Part B: Polym Phys 46: 1149–1158, 2008

Keywords: counterion binding; maleic acid; metal-polymer complexes; polyelectro-lytes; water-soluble polymers

INTRODUCTION

The scientific interest in the synthesis andinvestigation of metal-containing polymers iscontinuously increasing, as a result of the widevariety of their potential applications.1 The

metal ions may be bound to the polymer eitherthrough electrostatic attractions, being simplythe counterions of a strong polyanion, like poly(-styrene sulfonate), or through coordination withadequate ligands attached to the polymer back-bone, like carboxylate groups, forming the so-called polymer-metal complexes. Both classes ofmaterials are very prominent for possible practi-cal applications in several technological fields.Thus, the first class of materials has been exten-sively studied because of their ion-exchange

Correspondence to: G. Bokias (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 46, 1149–1158 (2008)VVC 2008 Wiley Periodicals, Inc.

1149

properties2–4 finding applications in ion-exchange resins5 or in water purification frommetal contamination.6 On the other hand, thepolymer-metal complexes offer many advantagesover nonpolymeric metal complexes in a varietyof chemical reactions.7 In addition, they presentunique characteristics in absorption spectra,coordination structures, stability, catalytic activ-ities and other, offering several possibilities fornumerous applications, for instance, in pollutioncontrol8–10 hydrometallurgy,10 catalytic reac-tions11 or in medicine and in biochemistry.12

The formation of coordination complexes ofCu21 with carboxyl-containing polymers hasbeen investigated mainly with polymers bearingonly one carboxylate group per repeating unitlike poly(acrylic acid), but also with polymersbearing two carboxylate groups per repeatingunit, like the (co)polymers of maleic acid (MAc).1

In the latter case, the polymer-Cu21 complexa-tion may be complicated by the two-step dissoci-ation of MAc,13,14 accompanied by a conforma-tional chain transition from a compact globuleto an extended coil.15 Nevertheless, the complex-ation of metal ions with MAc units is of great in-terest, due to the structural characteristics ofthe (co)polymers, enabling the variation of thecharge density along the chain or controlling thehydrophobic/hydrophilic balance using adequatecomonomers, hydrophobic such as styrene16–20

ethylene,17,21,22 olefin23 and other functionalcomonomers like N-vinylimidazole,24 thiopheneand furan25 or more hydrophilic ones such asacrylic acid,26,27 styrene sulfonic acid (SSH)27,28

and acrylamide.29 In many cases, the complexa-tion of the carboxylate groups of MAc with Cu21

ions causes the contraction of the polymerchains, dehydration, decrease in solubility and,finally, precipitation. As a consequence, the com-plexation has been investigated by a large vari-ety of techniques, both in the solid state18,23,26,28

but also in solution.16,17,19,21,22,27,30 Among others,an important question raised in many of thesestudies is the stoichiometry of the interaction. Forexample, it has been shown that in the case ofpoly(maleic acid) about 66% of the carboxylategroups can coordinate to copper ions.31 Never-theless, for copolymers of MAc with styrene,18

acrylic acid,26 or SSH28 the participation of onlythe first carboxylate group in the complex isclaimed, when the complexation takes place inthe acidic pH-region.

When multivalent metal ions are bound to astrong polyelectrolyte like poly(styrene sulfonic

acid) (PSSH), the driving force is electrostaticand the metal ions are simply the counterions ofthe negatively charged chain.32,33 As polystyrenesulfonic acid is a strong acid, it presents an im-portant retention ability already at pH ¼ 3, incontrast to carboxyl-containing polymers, pre-senting significant retention abilities at higherpH values.34 The degree of binding between thecounterions and the polyelectrolyte has beenproved by polarography, conductometry and dial-ysis equilibrium to depend on the ionic strength,the degree of neutralization of the polyacid, thenature of the neutralization agent, the polymerconcentration, the molecular weight, the densityof ionogenic groups along the chain, and thechemical structure of the counterion.3 Thus, ithas been shown that in the case of copper ions,the retention ability increases when the poly-mer/metal ratio increases34 or when the copperconcentration decreases at constant polymer/metalratio.6 Moreover, the interaction is influenced bythe presence of other species in the solution.35

The introduction of Cu21 ions in the polymeris usually achieved through an ion exchangeprocess between a copper(II) salt and the chargedpolymer. Nevertheless, a neutralization processusing Cu(OH)2 may also be applicable when thepolymer contains strong acid groups. As the firstcarboxylic group of MAc is a rather strong acid,13

both processes may be applied, giving complemen-tary information on the polymer/metal interaction.However, the use of the neutralization process israre36 and it has not been reported, to the best ofour knowledge, for the introduction of Cu21 ionsto water-soluble polymers.

In the present work, we apply these two proc-esses on the study of the interaction of Cu21

ions with copolymers of MAc either with a strongacid, namely SSH or with an inactive hydrophobicmonomer namely vinyl acetate (VAc). Dependingon the process applied, the two copolymers wereused either in the acid form, denoted as P(SSH-co-MAc) and P(VAc-co-MAc), or in the sodium saltform, denoted as P(SSNa-co-MANa) and P(VAc-co-MANa), respectively (Scheme 1). The homopoly-mer, PSSH, as well as its sodium salt form poly(sodium styrene sulfonate), PSSNa, were alsoinvestigated under the same conditions, for rea-sons of comparison. We mostly focus on the com-prehension of the influence of the neighboringcomonomers (competition for Cu21 uptake, hydro-philic/hydrophobic balance) on the polymer-Cu21

interaction in dilute aqueous solution, when thesetwo ‘‘inverse’’ processes are applied.

1150 OIKONOMOU, BOKIAS, AND KALLITSIS

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

EXPERIMENTAL

Materials

The monomers, maleic acid (MAc) and VAc, thesolvents tetrahydrofuran (THF) and diethylether, and the Cu(II) reagents, copper hydroxide(Cu(OH)2) and copper sulfate (CuSO4), werepurchased from Aldrich. The initiator a0-a0 azobisisobutyronitrile (AIBN) was purchased fromMerck. Water was purified by means of a Seral-pur Pro 90C apparatus combined with an USFElga water purification unit.

Polymers

The homopolymer poly(styrene sulfonate) so-dium salt, PSSNa, and the copolymer poly(sty-rene sulfonate-co-maleate) sodium salt 1:1(mol:mol), P(SSNa-co-MANa), were purchasedfrom Aldrich. The acid forms of these (co)poly-mers (PSSH and P(SSH-co-MAc)) were obtainedby dissolving the sodium salt form in HCl 1 Msolution, purification by dialysis (SIGMA, cutoff¼ 12000), followed by freeze-drying.

The copolymer poly(vinyl acetate-co maleicacid), P(VAc-co-MAc), was synthesized throughfree radical polymerization in THF, using AIBNas initiator. The synthesis was performed in a50 mL flask using 4.0 g of MAc (35 mmol), 3.0 gVAc (35 mmol), 0.34 g (2.1mmol) AIBN, and10 mL THF at oil bath temperature 80 8C. Thecopolymer was precipitated in cold diethyl etherand was dried at 50 8C in a vacuum oven. Thecomposition of the polymer was determined by1H NMR spectroscopy in D2O and it was found1:1 (mol:mol). To study the sodium salt form ofthe copolymer, P(VAc-co-MANa), the stoichio-metric amount of a 0.1 M NaOH aqueous solu-tion was added in the desired solution of P(VAc-co-MAc) at room temperature.

Equipment and Methods

1H NMR Spectroscopy

1H NMR spectra were obtained on a BrukerAdvance DPX 400 MHz spectrometer. The sam-ple was dissolved in deuterated water (D2O).

UV–Vis Spectrophotometry

UV–Vis spectra were recorded using a Hitatchispectrophotometer model U 1800. All spectro-scopic measurements were performed in quartzglass cuvettes (1 cm). The UV–Vis spectra wereobtained from 190 to 1100 nm, but we focusedon the absorption band in the visible region.

Turbidimetry

The optical density of dilute aqueous polymer-Cu21 solutions was monitored at 500 nm usinga Hitachi spectrophotometer, model U 2001.

Viscometry

The reduced viscosity, gred, studies were carriedout at 25 8C, using an automated viscositymeasuring system (Schott-Geraete AVS 310,Germany), equipped with an Ubbelohde viscom-eter, type I (the flow time of water at 25 8C was� 85 s).

Preparation of Solutions

Concentrated aqueous solutions of all polymers,PSSNa, PSSH, P(SSNa-co-MANa), P(SSH-co-MAc), and P(VAc-co-MAc) were prepared by gen-tle stirring overnight. The concentrated solutionof P(VAc-MANa) was prepared from the solutionof P(VAc-co-MAc), followed by stoichiometric

Scheme 1. Chemical structures of the copolymers used in the present study in theacid form (P(SSH-co-MAc) and P(VAc-co-MAc)).

ELECTROSTATIC BINDING VS. COMPLEXATION OF Cu2þ IONS 1151


neutralization with NaOH at room temperature,as described previously.

The solution of the sodium salt form of the(co)polymers were mixed with an aqueousCuSO4 solution and diluted to the desired con-centration. Alternatively, the solutions of theacid form of the (co)polymers were diluted to thedesired concentration, followed by stepwise addi-tion of solid Cu(OH)2 under stirring for severalhours. The polymer concentration was fixed at20 mM of each active monomer unit, SSH(SSNa) and/or MAc (MANa).

RESULTS AND DISCUSSION

As mentioned previously, a major aim of thepresent study was to compare the two ‘‘inverse’’processes for the introduction of Cu21 ions inpolymers containing sulfonate and/or carboxy-late groups, namely titration of the acid form ofthe polymers with Cu(OH)2 or ion exchangebetween CuSO4 and the sodium salt of the poly-mers. Regardless of the process, Cu21 ions areincorporated in the polymer either through elec-trostatic interactions as counterions of the sulfo-nate groups or through complexation with thecarboxylate groups. As all studies were per-formed at constant polymer concentration (20mM of each active monomer unit, SSH (SSNa)and/or MAc (MANa)), the concentration of activegroups changes from 20 mM in the case ofPSSH or PSSNa, where the active groups arethe sulfonate groups, to 40 mM in the case ofP(VAc-co-MAc) or P(VAc-co-MANa), where theactive groups are the carboxylate groups, and,finally, to 60 mM in the case of P(SSH-co-MAc)or P(SSNa-co-MANa), where the active groupsare both the sulfonate and the carboxylategroups. As a consequence, for reasons of compar-ison, the results will be presented in terms ofthe mixing ratio r, defined as

r ¼ C2þCu=Cactive ð1Þ

where CCu21 and Cactive is the molar concentra-

tion of Cu21 ions and active groups of the poly-mer respectively. Using this definition, the stoi-chiometry of polymer-Cu21 association isattained at r ¼ 0.5, i.e., when Cu21 ions areintroduced to all present active groups, consider-ing that both carboxylate groups of MAc(MANa) may be involved in the polymer-Cu21

complexation.

Cu21 Loading of the (co)polymers

As known, the complexation of Cu21 ions withcarboxyl-containing polymers in aqueous solu-tion often leads to turbid solutions and eventu-ally to macroscopic phase separation.37,38 On theother hand, as the solubility of Cu(OH)2 inwater is very low, the appearance of insolublematerial upon titration of strong polyacids withCu(OH)2 can be used to indicate the end-point ofCu21 loading of the polymer. Therefore, we pro-ceeded to a turbidimetric study, to obtain somefirst information on the association of Cu21 ionswith our copolymers, applying the two ‘‘inverse’’processes described above.

The optical density of aqueous solutions ofmixtures of CuSO4 with the sodium salt form ofthe three polymers (PSSNa, P(SSNa-co-MANa)and P(VAc-co-MANa)) is presented in Figure 1as a function of the mixing ratio, r. As expected,the aqueous solutions of the mixtures of PSSNawith CuSO4 remain transparent regardless ofthe value of r. Similarly, no turbidity is observedwhen CuSO4 is mixed with the copolymerP(SSNa-co-MANa), as a result of the high netcharge (due to the high SSNa content of the co-polymer) even if complexation with MANa takesplace. On the contrary, the solutions of CuSO4

with P(VAc-MANa) become abruptly stronglyturbid at r � 0.4, while macroscopic phase sepa-ration is observed at somewhat higher r values(r � 0.5). These r values indicate that both car-boxylate groups of MANa are involved in thecomplexation with Cu21. Apparently, as r

Figure 1. Variation of the optical density with themixing ratio r of aqueous solutions of CuSO4 with pol-ymers PSSNa (l) and P(SSNa-co-MANa) (~), P(VAc-co-MANa) (n).



approaches stoichiometry the polymer-Cu21

complex formed is not soluble anymore in water,due to the lack of net charge and to a strong in-herent hydrophobicity (note that VAc is alsostrongly hydrophobic).

The results of the turbidimetric titration ofthe acid form of the (co)polymers with Cu(OH)2are presented in Figure 2 as a function of themixing ratio r. As it can be seen, at the firststeps of titration (low r), the Cu(OH)2 powderadded is rapidly consumed due to the neutrali-zation reaction with the polyacid and the solu-tions are transparent in all cases. Nevertheless,above a critical r value any additional Cu(OH)2powder remains insoluble, clearly phase sepa-rated from the aqueous solution, while the su-pernatant is still transparent. Under the condi-tions of rapid agitation applied in our study, thisphase separated material is observed as a slightincrease in optical density. As seen in Figure 2,the critical r value for the appearance of insolu-ble Cu(OH)2 powder is � 0.3 for the case of thehomopolymer PSSH. This indicates that underthe conditions of our experiments, the strongpolyacid PSSH can be loaded up to a degree of� 60% with Cu21 ions through neutralizationwith Cu(OH)2. Similarly, the critical r value forthe copolymer P(SSH-co-MAc) is � 0.22, indicat-ing that � 44% of the total acid groups of the co-polymer can be loaded with Cu21 ions, i.e., themajority of the sulfonate groups and just thefirst carboxylic group of each MAc unit can prob-ably be neutralized by Cu(OH)2. The latter is

also confirmed from the corresponding resultsconcerning the copolymer P(VAc-co-MAc). Thecritical r value for this copolymer is � 0.25, indi-cating that half of the total carboxylic acid unitscan be neutralized by Cu(OH)2. Apparently, asthe second carboxylic group of MAc is a veryweak acid, in contrast to the first carboxylicgroup, it can not be neutralized by Cu(OH)2. Asa consequence the polymer-Cu21 complexformed in this last case is uncharged and hydro-phobic, so that it separates out from water, asevidenced by the large turbidity observed in Fig-ure 2.

The loading of the acid form of the (co)poly-mers with Cu21 ions may also be evidenced bythe pH variation observed upon their neutraliza-tion with Cu(OH)2. These potentiometric resultsare presented in Figure 3 as a function of themixing ratio r. For the case of the neutralizationof PSSH with Cu(OH)2, the typical titrationcurve of a strong acid is obtained, i.e., as long asCu(OH)2 is consumed by the strong polyacid (upto a mixing ratio r ¼ 0.3) pH is low and ratherconstant at a value of � 2.3 to 2.7, whereas pHincreases sharply obtaining a new constantvalue of � 6 for higher mixing ratios (r > 0.35).Therefore, the inflection point of this titrationcurve is clearly determined at 0.3 < r < 0.35, ina good agreement with the turbidimetric resultsdiscussed in Figure 2 (the first traces of insolu-ble Cu(OH)2 have been observed at r ¼ 0.3).When SSH is copolymerized with the less strongacid MAc, a gradual pH increase is observedthroughout neutralization (indicating that thispolymer solution presents some buffering ability)

Figure 2. Variation of the optical density with themixing ratio r of aqueous solutions of Cu(OH)2 withpolymers PSSH (*), P(SSH-co-MAc) (D) and P(VAc-co-MAc) (u).

Figure 3. Titration curves of PSSH (*), P(SSH-co-MAc) (D) and P(VAc-co-MAc) (u) with Cu(OH)2 inaqueous solution.



and finally pH levels off, taking a constant valueof � 6 at r > 0.3, in a short delay with theappearance of insoluble Cu(OH)2 in the solution(r � 0.22, see Fig. 2). No clear inflection pointscan be determined in this case, suggesting thatSSH and MAc units (at least the first carboxylicacid group) are neutralized simultaneously. Thebuffering abilities are much more significant inthe case of P(VAc-co-MAc), where a very gradualpH increase is observed upon neutralization.Nevertheless, the phase separation of the poly-mer-Cu21 complex formed together with theappearance of insoluble Cu(OH)2 perturb thetitration curve, so that the final level off is notobserved even at high mixing ratios.

Viscometry

To obtain some first information on the confor-mational changes of the macromolecular chainsupon introduction of Cu21 ions, reduced viscos-ity studies were performed in dilute solution. Tohave a direct comparison of the behavior of all(co)polymers, both in the acid and in the sodiumsalt form, the results are presented in terms ofthe reduced viscosity ratio, rgred, defined as

rgred ¼ gred=ðgredÞ0 ð2Þ

where gred and (gred)0 is the reduced viscosity ofeach polymer in the presence and in the absenceof Cu21 ions, respectively. Therefore, using sucha representation, a value of rgred lower thanunity reflects the shrinkage of the polymerchain in the presence of Cu21 ions due to intra-chain polymer-metal ion association, whereaspossible interchain gel-like structures would beexpressed by rgred values higher than unity.

The variation of the reduced viscosity ratio,rgred, of PSSNa, P(SSNA-co-MANa) and P(VAc-co-MANa) upon addition of CuSO4 is presentedin Figure 4, as a function of the mixing ratio r.A significant decrease of rgred is observed whenCuSO4 is added to the PSSNa solution. Forinstance, at r ¼ 0.5 the value of rgred is � 0.4,indicating an important shrinkage of the poly-mer chain. When Na2SO4 is used instead ofCuSO4 under similar concentration conditions,the value of rgred remains close to unity, verify-ing that the observed behavior is not just anionic strength effect. In fact, this shrinkage ofthe polyelectolyte chain could be the result ofthe strong counterion binding of the divalentCu21 ions as compared to the monovalent Na1

ions.39,40 Interestingly, the reduced viscositybehavior of the copolymer P(SSNA-co-MANa)upon addition of CuSO4 is almost identical tothat observed for PSSNa, suggesting that thehydrodynamic volume of the copolymer chain ismostly determined by the counterion binding ofCu21 by the polyelectrolyte (possibly, due to thehigh SSNa content of the copolymer). This, infact, is a first indication that counterion bindingis strong and, at least, competitive to the specificcomplexation of Cu21 with the carboxylateanions of MANa, as it will be also evidenced inthe next section. Contrary to PSSNa andP(SSNA-co-MANa), the viscometric behavior ofP(VAc-co-MANa) upon addition of CuSO4 istotally different. Thus, at mixing ratiosapproaching phase separation, r � 0.3, the valueof rgred is low, indicating that the copolymerchains adopt a compact conformation, as a resultof the complexation of Cu21 ions with the car-boxylate groups of MANa, combined possiblywith the strong hydrophobic character of VAc.On the other hand, at lower mixing ratios, rgredvalues larger than unity are found (rgred � 1.3at r � 0.1), indicating that interchain associa-tion may take place under these conditions,possibly leading to the formation of weakgel-like structures.

These gel-like structures seem to better de-velop when the Cu21 ions are introducedthrough the neutralization of the acid form ofthe copolymer, P(VAc-co-MAc), with Cu(OH)2, asshown in Figure 5. In fact, rgred values up to� 1.8 are now found, while the phenomenon isclearly observed in the whole soluble mixing

Figure 4. Variation of rgred with the mixing ratio rof aqueous solutions of CuSO4 with PSSNa (l),P(SSNa-co-MaNa) (~) and P(VAc-co-MANa) (n).



ratio range, from 0 up to r ¼ 0.3. In addition,when the neutralization process is applied, rgredvalues slightly higher than unity (rgred � 1.2)are observed also in the case of the copolymerP(SSH-co-MAc), as seen in Figure 5. It shouldbe noted that polymer-metal ion complexation isreported to lead to the formation of gels in cer-tain cases.37 In fact, it is worthy to note that wehave observed the formation of stiff hydrogelswhen a similar to P(VAc-co-MAc) copolymer,namely poly(ethylene-co-sodium maleate), wasneutralized with Cu(OH)2 in somewhat moreconcentrated solutions (unpublished results),although this is not the case with the copoly-mers used in the present study.

Spectrophotometric Investigation

The introduction of Cu21 ions in the (co)poly-mers used in the present study, either throughmixing of the sodium salt form with CuSO4 orthrough neutralization of the acid form withCu(OH)2, was investigated by spectrophotometryin the visible region. As an example, some char-acteristic spectra for a Cu21 concentration equalto 0.01 M in the presence of the (co)polymersare presented in Figure 6 and compared to therespective spectrum of an aqueous CuSO4 solu-tion. As seen, when the polymer contains onlysulfonate groups and Cu21 ions may be just thecounterions of the polyanion, the spectrum isalmost identical to that of the pure CuSO4 solu-tion. On the other hand, an intensity enhance-ment and a shift of the position of the absorp-tion maximum to lower wavelengths are

observed for the two copolymers containing MAcunits, as expected when the Cu21 ions are com-plexed with carboxyl-containing polymers.30

Both the shift and the intensity enhancementseem to be more important when P(VAc-co-MAc)is used, whereas the respective changes are in-termediate in the case of P(SSH-co-MAc).

The absorption band in the visible region wasfollowed spectrophotometrically for both proc-esses applied, i.e., neutralization of the acidform of the polymers with Cu(OH)2 or mixing ofthe sodium salt of the polymers with CuSO4.The position of the absorption band as a func-tion of the mixing ratio r is plotted in Figure 7,while the respective changes in maximum ab-sorbance are plotted in Figure 8.

As seen in Figure 7, in the case of PSSH orPSSNa, the absorption band remains practicallyat the same position, regardless of the processapplied and r. Moreover, the absorbance at themaximum is rather low, similar for both proc-esses and it increases linearly with r (Fig. 8).Although it can not be shown here, we haveverified that the behavior observed is almostidentical to that of aqueous pure CuSO4 solu-tions at the corresponding Cu21 concentrations.This, in fact, is expected, as the only possibleassociation mechanism with the polymers is justcounterion binding, not affecting the spectropho-tometric behavior of Cu21 ions.

As it concerns the copolymers P(VAc-co-MANa) or P(VAc-co-MAc), the absorption bandis observed at lower wavelengths and it does not

Figure 6. Absorption spectra of aqueous solution ofCu 21 0,01 M with PSSH (��), PSSNa (. . ..), P(SSNa-co-MANa) (-��-). P(SSH-co-MAc) (-�-�), P(VAc-co-MAc)(. . ..), P(VAc-co-MANa) (_ _ _). The spectrum of a pureaqueous CuSO4 solution (_ _ _) is also presented.

Figure 5. Variation of rgred with the mixing ratio rof aqueous solutions of Cu(OH)2 with PSSH (*),P(SSH-co-MAc) (D) and P(VAc-co-MAc) (u).



practically change with r (Fig. 7), indicatingthat now most Cu21 ions are complexed withthese copolymers in aqueous solution, within thewhole r range studied. This is also verified bythe large absorbance at maximum, changing lin-early with r, as shown in Figure 8. On the otherhand, the position of the absorption band isinfluenced by the process applied and it isobserved at � 760 nm when P(VAc-co-MAc)is neutralized with Cu(OH)2 and at � 740 nmwhen P(VAc-co-MANa) is mixed with CuSO4 inaqueous solution. The larger shift observed inthe second case can possibly be explained by theinvolvement of the second carboxylate group ofMANa in the complexation with Cu21 ions, inagreement with the turbidimetric behavior dis-cussed previously.

Contrary to the behavior observed for thecopolymers containing only SSNa (SSH) orMANa (MAc) units, the behavior of the copoly-mer containing both units in the acid or in thesodium salt form, P(SSH-co-MAc) or P(SSNa-co-MANa), is totally different. Thus, as seen in Fig-ure 7, the absorption band is shifted graduallyfrom � 800 nm (at the limit of r ¼ 0) down to� 760 nm upon neutralization of P(SSH-co-MAc)with Cu(OH)2. Inversely, upon mixing ofP(SSNa-co-MANa) with CuSO4, the absorptionband in the visible region shifts from � 740 nm(at the limit of r ¼ 0) to � 800 nm (at high mix-ing ratios r). In addition to the behavior of theposition of the absorption band, shown in Figure

7, the variation of the maximum absorbance ofthis band applying the two processes, seen inFigure 8, is also interesting. Thus, at the firststages of the introduction of Cu21 ions in the co-polymer (up to r � 0.2), the maximum absorb-ance is similar for both processes applied, neu-tralization or mixing, obtaining intermediatevalues between the two ‘‘extreme’’ cases ofPSSH (PSSNa) and P(VAc-co-MAc) (P(VAc-co-MANa)). However, for r > 0.2, the two curvesdeviate: the curve describing the neutralizationof P(SSH-co-MAc) with Cu(OH)2 tends to followa behavior parallel to that of P(VAc-co-MAc),whereas the curve describing the mixing ofP(SSNa-co-MANa) with CuSO4 tends to follow abehavior parallel to that of PSSNa (or pureCuSO4).

All the above observations can be qualita-tively interpreted considering that in the case ofthe copolymer of SSNa (SSH) with MANa(MAc), the complexation of Cu21 ions with thecarboxylate groups of MANa (MAc) is not verystrong and the strength of the electrostaticinteraction of Cu21 ions with the sulfonateanions (and the consequent Cu21 binding)attached to the copolymer backbone is compara-ble to the strength of Cu21 complexation. Thus,when CuSO4 is mixed with P(SSNa-co-MANa), afraction of Cu21 ions remain uncomplexed, beingthe counterions of sulfate or styrene sulfonateanions. This fraction is very low at the limit of

Figure 8. Variation of the maximum absorbancewith the mixing ratio r of aqueous solutions of CuSO4

with PSSNa (l), P(SSNa-co-MaNa) (~), P(VAc-co-MANa) (u) or Cu(OH)2 with PSSH (*), P(SSH-co-MAc) (D), P(VAc-co-MAc) (u). The correspondingresults for an equimolar mixture of PSSH and P(VAc-co-MAc) (^) are also presented.

Figure 7. Wavelength of the maximum absorptionof UV–Vis spectra of aqueous solutions of CuSO4 withP(VAc-co-MANa) (n), P(SSNa-co-MaNa) (~), PSSNa(l) or of Cu(OH)2 with P(VAc-co-MAc) (u), PSSH (*)and P(SSH-co-MAc) (D). The corresponding results foran equimolar mixture of PSSH and P(VAc-co-MAc) (^).



r ¼ 0 and the absorption band is observed at� 740 nm. However, upon further addition ofCuSO4 the fraction of uncomplexed Cu21 ionsincreases considerably, explaining the gradualshifting of the absorption band to higher wave-lengths and the concomitant weaker increase inabsorbance, that seems to follow the behavior ofpure CuSO4 at high mixing ratios r. Inversely,when the neutralization process is applied, thevery first Cu21 ions introduced in the solutionseem to be just the counterions of styrene sulfo-nate groups. However, as neutralization pro-ceeds, more and more Cu21 ions are complexedwith the maleate groups, explaining theobserved behavior. An indirect verification of theimportance of counterion binding we also obtainif we follow spectroscopically the neutralizationwith Cu(OH)2 of an equimolar mixture of PSSHand P(VAc-co-MAc) in water. It should bereminded that, in the absence of PSSH, theobserved spectroscopic behavior indicated thatthe complexation of Cu21 ions with the carboxy-late groups of P(VAc-co-MAc) is strong. Never-theless, the behavior of the mixture of the twopolymers upon neutralization with Cu(OH)2,reported also in Figures 7 and 8, is very differ-ent from that observed with pure P(VAc-co-MAc), but it follows rather well the respectivebehavior of P(SSH-co-MAc).

CONCLUSIONS

The incorporation of Cu21 ions either throughneutralization with Cu(OH)2 of the (co)polymersPSSH, P(VAc-co-MAc) and P(SSH-co-MAc), con-taining SSH units and/or MAc units, or throughmixing with CuSO4 of the sodium salt form ofthe respective (co)polymers (PSSNa, P(VAc-co-MANa) and P(SSNa-co-MANa)), was investi-gated in dilute aqueous solution. The drivingforce for Cu21-incorporation is electrostaticinteraction (counterion binding) for the SSH(SSNa)-containing products, whereas the drivingforce for the MAc (MANa) — containing prod-ucts is coordination (complex formation).

Both the process applied (neutralization ormixing) and the nature of the binding groups ofthe (co)polymer (sulfonate or carboxylate) deter-mine the characteristics and the behavior of thefinal polymer-Cu21 product. Thus, turbidimetry,potentiometry and spectrophtometry indicatethat just the first carboxylic group of MAc maybe loaded with Cu21 ions through the neutrali-

zation process. In contrast, when the sodiumsalt form of the MANa-containing copolymers isused, the second carboxylate group of MANa isalso involved in the complexation with the Cu21

ions. For the copolymer containing only MAc(MANa) units, a significant increase of thereduced viscosity at the first stages of Cu21

loading is observed (especially when the neutral-ization process is performed) indicative of theexistence of gel-like structures in dilute aqueoussolution under these conditions, arising possiblyfrom interchain polymer-Cu21 complexation.

Finally, the changes of the absorption band inthe visible region of the spectrum (shift of theband and variation of maximum absorbance)suggest that when both binding units are pres-ent, either in copolymers or in polymer mix-tures, there is a competition between electro-static binding of Cu21 by the sulfonate groupsand complexation of Cu21 with the carboxylategroups of MAc (MANa). As a result, copolymer-ization or polymer blending can be applied tocontrol the form of bound Cu21 ions, either ascounterions or as complexed species, modulatingat will the properties of the final hybrid polymer-Cu21 material depending on the desired application.

Financial support from General Secretariat forResearch and Technology, Greece through the pro-gram ‘Design and Development of new paints withcontrolled release of biocides for submarine applica-tions’ PENED 2003, 03ED825, co-financed by E.U.-European Social Fund (75%) and the Greek Ministryof Development-GSRT, is greatly acknowledged. Par-tial support from Chrotex S.A. is also acknowledged.

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Comparative study of electrostatic binding vs. complexation of Cu2+ ions with water-soluble polymers...

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