Swelling Behavior of Anionic Acrylamide-Based Hydrogels in Aqueous Salt Solutions: Comparison of Experiment with Theory

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1 Swelling Behavior of Anionic Acrylamide-Baed Hydrogel in Aqueou Salt Solution: Comparion of Experiment with Theory OĞUZ OKAY, 1,2 SAFIYE B. SARIIŞIK, 1 SIBEL D. ZOR 1 1 Department of Chemitry, Kocaeli Univerity, Izmit, Kocaeli, Turkey 2 Department of Chemitry, TUBITAK Marmara Reearch Center, P.O. Box 21, Gebze, Kocaeli, Turkey Received 17 November 1997; accepted 2 March 1998 ABSTRACT: A erie of hydrogel were prepared from acrylamide and 2-acrylamido-2- methylpropaneulfonic acid (AMPS) monomer with 0 80 mol % AMPS and uing N,N -methylenebi(acrylamide) a the crolinker. The welling capacitie of hydrogel were meaured in water and in aqueou NaCl olution. The volume welling ratio q v of hydrogel in water increae harply when the mole fraction f c of AMPS increae from 0 to At higher value of f c from 0.06 up to 0.18, no change in the welling capacitie of hydrogel wa oberved; in thi range of f c, q v become nearly contant at 750. However, a f c further increae, q v tart to increae again monotonically over the entire range of f c. At a fixed value of f c, the welling ratio of hydrogel decreae with increaing alt concentration in the external olution. The reult of the welling meaurement in aqueou alt olution were compared with the prediction of the Flory Rehner theory of welling equilibrium. It wa hown that the theory correctly predict the welling behavior of hydrogel up to 80 mol % charge denitie. The method of etimation of the network parameter wa found to be unimportant in the prediction of the experimental welling data. The network parameter ued in the imulation only correct the deficiency of the welling theory John Wiley & Son, Inc. J Appl Polym Sci 70: , 1998 Key word: anionic hydrogel; polyacrylamide; 2-acrylamido-2-methylpropaneulfonic acid; welling; welling theory INTRODUCTION In recent year, hydrophilic gel called hydrogel have received coniderable attention for ue a pecific orbent and a upport carrier in biomedical engineering. Hydrogel are mainly prepared by free-radical crolinking copolymerization of acrylamide (AAm) and a divinyl monomer Correpondence to: O. Okay, Itanbul Technical Univerity, Department of Chemitry, Malak, Itanbul, Turkey. Contract grant ponor: Scientific and Technical Reearch Council of Turkey; contract grant number: TBAG Journal of Applied Polymer Science, Vol. 70, (1998) 1998 John Wiley & Son, Inc. CCC /98/ (crolinker) in aqueou olution. To increae the welling capacity of hydrogel, an ionic monomer i alo included in the monomer mixture. The deired property of hydrogel, uch a their welling capacity, i obtained by adjuting the concentration a well a the compoition of the initial monomer mixture. Invetigation of the welling behavior of hydrogel have been reported in the lat 4 decade. Mot of thee tudie have been concentrated on hydrogel containing weakly ionizable group uch a the carboxylate group. 1 7 The welling propertie of thee weak polyelectrolyte gel trongly vary depending on the ph of the olution. 567

2 568 OKAY, SARIIŞIK, AND ZOR However, hydrogel with trong electrolyte diociate completely in the overall ph range and, therefore, exhibit ph independent welling behavior. Sulfonic acid containing hydrogel repreent a cla of trong polyelectrolyte gel with a high degree of ionization. It wa hown that the linear polymer with ulfonate group derived from 2-acrylamido-2-methylpropaneulfonic acid (AMPS) exhibit extenive coil expanion in aqueou olution; 7 even in 5M NaCl olution, the expanion of polymer coil due to charge repulion cannot be totally creened. 7 Liu and colleague 8 invetigated the welling propertie of the correponding hydrogel and oberved a imilar behavior. Highly concentrated alt olution were required to creen the charge interaction within the hydrogel derived from AMPS and N,Ndimethylacrylamide (DMAA). Tong and Liu 9 oberved a contant welling capacity of AMPS/DMAA hydrogel in the whole ph range and concluded that the AMPS content of the hydrogel correpond to it charge denity. Their calculation reult howed a large dicrepancy between the meaured welling data in water and that predicted by the Flory Rehner theory of welling equilibrium. Hooper and colleague 10 and Baker and colleague 11,12 invetigated the welling propertie of trong polyelectrolyte AAm-baed cationic and ampholytic hydrogel in water and in aqueou alt olution. In contrat to Tong and Liu, they oberved a good agreement between the Flory Rehner theory and experiment. It eem that the approximation of the welling theory cancel each other, o that it correctly predict the welling behavior of their hydrogel with charge denitie below 5 mol %. The cope of the preent work wa to etablih whether it i poible to predict the welling behavior of highly wollen hydrogel with charge denitie up to 80 mol % uing the Flory Rehner theory. It wa alo of interet to clarify the enitivity of the theory prediction to the etimation method of the network parameter. For thi purpoe, we prepared a erie of trong polyelectrolyte AAm-AMPS hydrogel with variou ionic group content and meaured their welling capacitie in water and in aqueou NaCl olution ranging in concentration from 10 5 to 1M. Intead of DMAA, AAm wa ued a a comonomer of AMPS to increae the welling capacity of our hydrogel. The meaured welling data of the hydrogel were compared with that predicted by the Flory Rehner theory. A will be hown, thi imple theory correctly predict the welling behavior of hydrogel with welling capacitie up to 2 L of water per ml of dry hydrogel. EXPERIMENTAL Material AAm (Merck), N,N -methylenebi(aam) (BAAm; Merck), AMPS (AKSA), ammonium perulfate (APS; Merck), N,N,N,N -tetramethylethylenediamine (TEMED; Merck), and NaCl (Merck) were ued a received. Double ditilled and deionized water were ued in hydrogel preparation and in welling meaurement. Hydrogel Synthei Hydrogel were prepared by free-radical crolinking copolymerization of AAm and AMPS with a mall amount of BAAm a a crolinker in aqueou olution. APS and TEMED were, repectively, the initiator and accelerator. Reaction were conducted at room temperature (21 2 C). Gel were prepared according to the following cheme. 5 g AAm-AMPS mixture, NaOH (in the amount correponding to the number of mole of AMPS), 133 mg BAAm, and 40 mg APS were diolved in double ditilled water under cooling to give a total volume of 100 ml. After addition of 0.24 ml of TEMED, the olution wa tranferred to mall tube of 5.8 mm in diameter. Polymerization wa conducted for 24 h. Homologou erie of anionic hydrogel were prepared in thi way, allowing ytematic variation of the AMPS concentration between 0 and 80 mol % (with repect to the monovinyl monomer). The total monomer concentration and the crolinker ratio X (mole ratio of vinyl to divinyl monomer) were fixed at 5 w/v % and 82, repectively. It mut be pointed out that, due to the electrotatic repulion between the AMPS monomer and the growing chain, one may expect a leer reactivity for AMPS compared with AAm in free-radial crolinking copolymerization. Therefore, the microtructure concerning the compoition of the network chain in the hydrogel may be heterogeneou. After polymerization, hydrogel ample were cut into pecimen of 10 mm in length. The ample (uually 10 piece) were then immered in a large exce of ditilled water for at leat 1 week; during thi period, water wa replaced every other day to remove the unreacted pecie. It wa found that the ol fraction in the gel i

3 SWELLING OF HYDROGELS IN AQUEOUS SALT SOLUTIONS % after extraction with water. Half of the gel ample were then ubjected to welling meaurement, wherea the remaining ample were dried according to the following procedure: the wollen gel ample were ucceively wahed with olution whoe compoition were changed gradually from water to pure methanol. Thi olvent exchange proce facilitate final drying of the gel ample. The collaped ample after the treatment with methanol a a final olvent were dried in vacuum at 60 C to contant weight. The weight welling ratio of hydrogel after preparation, q F, wa calculated a q F Swelling Meaurement ma gel after preparation ma dry gel (1) The wollen hydrogel ample from the ame gel (uually five piece) were immered in vial (100 ml) filled with water. The vial were et in a temperature-controlled bath at C. To reach the equilibrium degree of welling, the gel were immered in water at leat for 1 week; the welling equilibrium wa teted by weighing the ample. The gel were then weighed in the wollen tate and tranferred to vial containing the mot concentrated aqueou NaCl olution. The concentration of NaCl olution ranged from 1.0M to 10 5 M. Gel ample were allowed to well in the olution at leat for 1 week, during which aqueou NaCl wa refrehed to keep the concentration a feeded. After the welling equilibrium i etablihed, the ample were weighed and then tranferred into the next dilute NaCl olution. The welling meaurement in aqueou NaCl were conducted both in direction of decreaing alt concentration from 1M to water and in revere direction from water up to 1M NaCl. No ytematic variation in the recorded welling data wa oberved. The weight welling ratio of hydrogel in water and in aqueou NaCl, q w (ma gel after welling/ma dry gel), were calculated uing the equation ma gel in water q w H 2 O q F ma gel after preparation q w NaCl aq q w (H 2 O) ma gel in aqueou NaCl ma gel in water (2a) (2b) The volume welling ratio of hydrogel, q v, wa calculated a q v 1 q w 1 d (3) where and d are the denitie of polymer and olution, repectively. The value and d ued were 1.35 g ml 1 and1gml 1, repectively. Each welling ratio reported in thi article i an average of at leat five eparate meaurement; tandard deviation of the meaured welling ratio were 4% of the mean. THEORY OF SWELLING EQUILIBRIUM The tate of equilibrium welling of a polymer network immered in a olvent i obtained when the olvent inide the network i in thermodynamic equilibrium with that outide. Thi equilibrium tate i decribed by the equality of the olvent chemical potential 1 in both phae. Thu, at welling equilibrium, we have 1 g 1 (4) where the upercript g and denote the gel and olution phae, repectively. In term of the omotic preure, eq. (4) can alo be written a 1 g 1 0 (5) V 1 where V 1 i the molar volume of olvent. Omotic preure of a gel determine whether the gel tend to expand or to hrink. When nonzero, provide a driving force for gel volume change. Solvent move into or out of the gel until i 0 (i.e., until the force acting on the gel are balanced). During the welling proce, there are at leat three force acting on an ionic gel: thoe due to polymer-olvent mixing (mix), due to deformation of network chain to a more elongated tate (el), and due to the nonuniform ditribution of mobile counterion between the gel and the olution (ion). Within the framework of the Flory Rehner theory, the omotic preure of a gel i the um of thee three contribution 13,14 : mix el ion (6)

4 570 OKAY, SARIIŞIK, AND ZOR According to the Flory Huggin theory, mix i given by 15 mix RT V 1 ln 1 v 2 v 2 v 2 2 (7) where v 2 i the volume fraction of polymer in the hydrogel (i.e., v 2 1/q v ), i the polymer olvent interaction parameter, R i the ga contant, and T i temperature. To decribe the elatic contribution el to the welling preure, everal theorie are available We will ue here the implet affine network model to decribe the behavior of our gel 15 : Uing eq. (4) (11), we obtain the following ytem of equation decribing equilibrium value of v 2 of hydrogel in aqueou alt olution and the ditribution coefficient K of counterion between the gel and olution phae: ln 1 v 2 v 2 v 2 2 N 1 v 2 1/3 v 2 02/3 v 2 /2 2 K 1 V 1 C alt V 1 C fix 0 K K C fix /C alt 1 0 (12a) (12b) where K C g /C alt. Let f be the mole fraction of charged unit in the network and V r the molar volume of a polymer repeat unit, C fix equal to el RT V 1 N 1 v 2 1/3 v 2 02/3 v 2 /2 (8) C fix f V r v 2 (12c) where N i the average number of egment in the network chain and v 0 2 i the volume fraction of polymer network after preparation. Ionic contribution ion to the welling preure i caued by the concentration difference of counterion between the gel and the outer olution. To decribe thi effect completely, one hould conider the ion ion, ion olvent, and ion polymer interaction. The ideal Donnan theory ignore thee interaction and give ion a the preure difference of mobile ion inide and outide the gel 15 : ion RT C g i C i (9) where C i i the mobile ion concentration of pecie i. According to ideal Donnan equilibria, the chemical potential of an ionic pecie i inide the hydrogel mut be equal to that outide. Thu, for aqueou olution of univalent alt, we have the equality C g C g i C 2 C C alt (10) where C alt repreent the alt concentration in the external olution. On the other hand, the condition of electroneutrality inide an anionic hydrogel require C g C g C fix (11) where C fix i the concentration of fixed charge in the gel. In the cae of welling of hydrogel in water free of ionic pecie (C alt 0), eq. (12a) reduce to ln 1 v 2 v 2 v 2 2 N 1 v 1/3 2 v 02/3 2 v 2 /2 V 1 C fix 0 (13) Note that, for C alt 10 6 M, the contribution of the alt concentration in the external olution to the welling equilibrium repreented by eq. (12a) and (12b) become negligible; thu, eq. (12a) alo reduce to eq. (13) for C alt 10 6 M. Moreover, for highly wollen hydrogel, ince v 2 1, eq. (12a) and (12b) can alo be written a 0.5 v 2 2 N 1 v 02/3 2 v 1/3 2 2V 1 C alt K 1 V 1 C fix 0 (14) where Calculation 2 C alt K 0.5 C fix 4 C fix (14a) C alt The ytem of eq. (12a) and (12b), in combination with eq. (12c), wa olved numerically to calculate the equilibrium welling ratio of hydrogel in aqueou alt olution (q v 1/v 2 ) and the ditribution coefficient of counterion K. For calculation, the value ued were V 1 18 g ml 1

5 SWELLING OF HYDROGELS IN AQUEOUS SALT SOLUTIONS 571 and V r f (obtained uing the value 71 and 229 g mol 1 for the molecular weight of AAm and AMPS unit, repectively, and uing 1.35 g ml 1 ). The parameter value for polyacrylamide (PAAm) water ytem wa recently evaluated from the welling data for uncharged PAAm hydrogel wollen in water. 10 A bet-fit value for of 0.48 wa obtained. 11 Thi value of the parameter provided a good fit to the experimental welling data of AAm-baed anionic, cationic, and ampholytic hydrogel of variou compoition. 11,12 In the following calculation, wa held contant at thi value. The volume fraction of polymer network after preparation, v 0 2, wa calculated from the experimental q F value a v q 1 F 1 d (15) The experimental value of v 0 2 increaed from to with increaing AMPS content from 0 to 80 mol %. C alt i the independent variable of eq. (12a) and (12b), which wa varied between 10 5 M and 1.0M in our experiment. To olve eq. (12a) and (12b) for the two unknown, q v 1/v 2 and K, the value of the network parameter f and N characterizing the hydrogel mut alo be known. The f and N value of hydrogel can be etimated by two different approache. Chemically Fixed Crolink Denity of Hydrogel Becaue both the crolinker ratio X (mole ratio of monomer to crolinker) and the total monomer concentration were fixed in our experiment, wherea only the mole fraction of AMPS varie between the hydrogel, it i reaonable to aume a contant crolink denity for all the hydrogel. Thu, uing the welling ratio of nonionic gel in water q v 40.9 and uing eq. (13) for C fix 0, we calculated N a , which hould be valid for all other ionic hydrogel. After calibration of the gel, that i, after finding their crolink denitie in term of N, f value of the ionic hydrogel obtained at the ame chemical crolink denity wa calculated from their welling ratio in water and uing eq. (13) for the condition N Note that thi method of etimation of the network parameter aume that the addition of AMPS in the comonomer mixture doe not change the network topology. Chemically Varied Charge Denity of Hydrogel Becaue AMPS i a trong diociating electrolyte, the amount of AMPS ued in the hydrogel preparation hould be equal to it charge denity. Thu, f in eq. (12c) i given by f AMPS mol % 100 (16) From the equilibrium welling ratio of hydrogel in water and uing eq. (13) and (16), one may calculate the crolink denitie of hydrogel in term of N. Note that thi method of etimation of network parameter neglect counterion condenation 6,23 and aume that the ionization degree of AMPS unit equal to unity. Alo, it i aumed that AMPS completely react during the gel formation, and no reaction uch a the hydrolyi reaction of AAm unit take place during the welling meaurement. In the following ection, we will ue both approache to evaluate the network parameter and to compare the prediction of the theory with the experimental data. RESULTS AND DISCUSSION Figure 1 how the volume welling ratio q v of AMPS/AAm hydrogel in water and in aqueou alt olution plotted a a function of f c, the mole fraction of AMPS ued in the gel preparation. A expected, q v increae harply when the mole fraction f c of AMPS increae from 0 to Thi i due to the fact that, a f c increae, the mobile ion (Na ) concentration inide the gel alo increae to maintain the electroneutrality condition. A a reult, the difference between the mobile ion concentration inide and outide the gel increae with increaing f c, which create an additional omotic preure that expand the gel. Alo expected i the decreaed welling ratio of hydrogel with increaing alt concentration in the external olution; thi i due to decreae in the concentration difference of counterion inide and outide the hydrogel. In 1.0M NaCl olution, the welling ratio i almot independent on the AMPS content of hydrogel due to creening of charge interaction within the hydrogel. Interetingly, at higher value of f c from 0.06 up to 0.18, no change in the welling capacitie of hydrogel wa oberved; in thi range of f c, q v become nearly contant at 750. Thi may be a

6 572 OKAY, SARIIŞIK, AND ZOR reult of counterion condenation, a wa oberved previouly in carboxylate-containing AAm hydrogel. 6 However, a f c further increae, q v tart to increae again monotonically over the entire range of f c. Thi welling behavior of hydrogel with f c 0.2 may be a reult of their high welling ratio. The network chain in thee wollen hydrogel are in 9- to 10-fold expanded configuration with repect to dry tate, o that a detruction of the chain may occur during the welling proce. For AMPS/DMAA hydrogel, Tong and Liu alo obtained a imilar welling curve a given in Figure 1. However, due to their inufficient data point in the range of f c 0.20, they interpreted the reult a a monotonic increae of q v over the entire f c range from 0 to However, the preent reult clearly illutrate the plateau in the welling curve between f c 0.06 and Uing the equilibrium welling data q v of hydrogel in water, we can now evaluate the network parameter f and N, decribing the average number of charged unit and of egment in a network chain, repectively. Firt, the value q v 40.9 found by experiment for nonionic gel in water yield N , compared with it chemical value N c 41 calculated from the crolinker ratio X 82. Comparion of thee two value indicate that 90% of the crolinker BAAm ued in the gel ynthei were conumed in cycle and/or in multiple crolink. Thi i in accord with our previou reult. 24,25 Second, to evaluate f and N value of ionic hydrogel, we will ue both approache explained in the previou ection. Chemically Fixed Crolink Denity Auming that N i valid for all other hydrogel, eq. (13) can be olved for f uing the experimental q v data of ionic hydrogel. The calculated value of f are hown in Figure 2(A) a filled ymbol plotted a a function of mole fraction of AMPS in the gel ynthei f c. In the figure, the dotted line repreent the relation f/f c 1 (i.e., in thi line AMPS content of hydrogel equal to their charge denity). It i een that, in up to 1% AMPS content, all AMPS unit act a charged unit in the gel welling. However, a f c further increae, deviation appear from the traight line dependence, and f approache a limiting value of Figure 1 Variation of the volume welling ratio q v of AAm/AMPS hydrogel in water and in aqueou NaCl olution with the mole fraction of AMPS ued in the gel preparation f c. Chemically Varied Charge Denity Auming that f f c, eq. (13) can be olved for N uing the experimental q v data of ionic hydrogel. The calculated value of N are alo hown in Figure 2(B) a open ymbol plotted a a function of f c. N trongly decreae on riing f c and approache it tochiometric value of N c 41, hown in Figure 2(B) a the horizontal dahed line. It i een that the two approache ued in the etimation of the network parameter f and N yield totally different value. We will ue now both approache to predict the welling capacitie of hydrogel in aqueou NaCl olution. The reult are collected in Figure 3 and 4. Herein, the volume welling ratio q v of hydrogel are hown a a function of the NaCl concentration. The experimental data are hown a ymbol. The olid curve were calculated uing eq. (12a) and (12b) and uing f and N value evaluated from the firt approach. The dotted curve were calculated in the ame way; but we ued f and N value evaluated from the econd approach. It i een that both approache provide good agreement with the experimental data up to 1.5 mol % AMPS (Fig. 3). In thi range of AMPS concentration, the firt approach (olid curve, N fixed) provide a lightly better agreement with the experimental

7 SWELLING OF HYDROGELS IN AQUEOUS SALT SOLUTIONS 573 approximation of the Flory Rehner and Flory Huggin theorie, no agreement between theory and experiment can be expected, epecially in cae of highly wollen hydrogel. Thi i, of coure, in the cae, where the actual value of f and N were ued in the calculation. 9,11 The preent reult indicate that the deficiency of the theorie are compenated by the value of the parameter f and N ued in the fit. Thu, it mut be pointed out that the value of f and N generated from the experimental welling data hould only be conidered a fit parameter. Another intereting point that i not viible in the magnification of Figure 3 and 4 i illutrated in Figure 5. The equilibrium welling ratio q v of hydrogel with 3 mol % AMPS content increae again, when the alt concentration further increae from 0.01M to 1.0M. Thi intereting feature, not predicted by the theory, wa alo oberved previouly by Baker and colleague 12 in ampholytic and nonionic AAm-baed hydrogel. Specific interaction between AAm and mobile ion eem to be reponible for thi behavior. Figure 6 how the calculated ditribution coefficient K of mobile counterion Cl between inide and outide the gel phae plotted a a function of the NaCl concentration in the external Figure 2 Variation of the network parameter f and N with the AMPS content of hydrogel f c. In (A), the crolink denity of hydrogel i aumed to be contant (chemically fixed crolink denity), wherea in (B) the charge denity of hydrogel i aumed to be equal to their AMPS content (chemically varied charge denity). The f and N value were evaluated from the welling ratio of hydrogel in pure water and uing eq. (13). data. Agreement between theory and experiment become qualitative at high AMPS content ( 18%; Fig. 4); Herein, the econd approach ( f f c ) provide a better fit to the experimental data. We can conclude that both etimation method of the network parameter can be ued to predict the welling behavior of hydrogel in aqueou alt olution. In fact, due to the everal aumption and Figure 3 Variation of the volume welling ratio q v of hydrogel with the NaCl concentration in the external olution. Experimental data are hown a ymbol. Curve were calculated uing the welling theory. Calculation were for fixed N (olid curve) and for f f c (dotted curve).

8 574 OKAY, SARIIŞIK, AND ZOR Figure 4 Variation of the volume welling ratio q v of hydrogel with the NaCl concentration in the external olution. Experimental data are hown a ymbol. Curve were calculated uing the welling theory. Calculation were for fixed N (olid curve) and for f f c (dotted curve). AMPS mole % 0 (1), 1.5 (2), 3 (3), 6 (4), 18 (5), 60 (6), and 80 (7). olution. Calculation were uing the network parameter generated by the firt approach and for hydrogel with 0 18 mol % AMPS content. Here, K 1 mean that the anion concentration inide the anionic gel i equal to that in the olution, wherea K 0 mean that the anionic gel exclude all the anion. Figure 6 indicate that a large amount of Cl ion penetrate into the gel if the alt concentration in the outer olution i high. Decreaing alt concentration in the olution decreae the K ratio, and it approache 0 for NaCl concentration 10 5 M. Thu, if the concentration of NaCl in the outer olution i ufficiently low, the Cl ion doe not penetrate inide the hydrogel. Figure 5 Variation of the volume welling ratio q v of hydrogel with the NaCl concentration in the external olution. Experimental data are hown a ymbol. Curve only how the trend of data. increae from 0 to At higher value of f c from 0.06 up to 0.18, no change in the welling capacitie of hydrogel wa oberved. However, a CONCLUSIONS A erie of hydrogel from AAm and AMPS monomer were prepared by free-radical crolinking copolymerization uing BAAm a the crolinker. The welling capacitie of the hydrogel were meaured in water and in aqueou alt olution ranging in concentration from 10 5 to 1M. The volume welling ratio q v of hydrogel in water increae harply a the mole fraction f c of AMPS Figure 6 Variation of the calculated ditribution coefficient K of Cl ion between the gel and olution phae a a function of the NaCl concentration. Calculation were for a fixed crolink denity N.

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