Langmuir 1993,9, 3324-3326
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Swelling of Polyacrylamide Gels with Pendant Poly(ethy1ene oxide) Chains in Water and in Ionic Surfactant Solutions Lennart Piculell,’J Dominique Hourdet: and Ilias Iliopoulos’ Physical Chemistry 1, Chemical Center, University of Lund, Box 124, S-221 00 Lund, Sweden, and Laboratoire de Physico-Chimie Macromolkculaire, Universitl Pierre et Marie Curie, CNRS URA-278, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received June 2, 1993. In Final Form: August 24,1993
Introduction The swelling of polymer gels in aqueous solution continues to attract much research effort. From the point of view of application, the studies are largely motivated by the desire to design efficient absorbents of water and aqueous solutions. The best absorption capacity (the largest swelling) is generally found for ionic gels, owing to the large contribution to the osmotic pressure exerted by the mobile counterions confined to the swollen ge1.’r2 The swelling of ionic gels is decreased considerably, however, in solutions containing electrolyte, especiallyif the latter includes multivalent ions of a sign opposite that of the charged groups of the gel.3 Recent studies have pointed to new possibilities of creating ionic gels by using a neutral, but slightly hydrophobic, gel in ionic surfactant solutions, where the surfactant micelles bind to the gel network.P6 Inomata et aL4 demonstrated a swelling of N-isopropylacrylamide gels in solutions of sodium dodecyl sulfate (SDS)at concentrations below the bulk critical micelle concentration (cmc). Later, studies on the same combination of gel and surfactant a t high surfactant concentrations (ca. 70 mM) were reported by Zhang et a L 5 who also showed that the swelling behavior was remarkably insensitive to additionsof even very high concentrations (I1M) of NaC1. A very recent study by Wada et aL6 compares the swelling of N-(n-propyl)-, N-cyclopropyl-,and N-isopropylacrylamide gels in solutions of a range of anionic and cationic surfactants of varying alkyl chain lengths. The above studies also clearly demonstrate the usefulness of gel swelling experiments as a means to study polymersurfactant interactions, an area which is of considerable current i n t e r e ~ t . ~ The present study reports swellingstudies, in water and in SDS solutions, of a novel type of polyacrylamide-based copolymer gel, incorporating a small molar fraction of “macromonomers” with pendant poly(ethy1ene oxide) (PEO) chains. The polyacrylamide backbone is inert to SDS, whereas the PEO chains bind SDS micelles.8 An additional interesting feature, which could affect the swelling behavior of this type of copolymer gel, is that
PEO and polyacrylamide constitute an incompatible + University of
Lund. Universit.6 Pierre et Marie Curie.
(1)Flory, P. J. Principles of Polymer Chemistry; Cornel1 University Press: Ithaca, NY 1953; Chapter XIII:3c. (2) Tanaka, T.; Fillmore, D.; Sun,S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980,45, 1636. (3) Ohmine, I.; Tanaka, T. J . Chem. Phys. 1982, 77, 5725. (4) Inomata, H.; Goto, S.; Saito, S. Langmuir 1992,8, 1030. (5) Zhang, Y.-Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (6) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H . ; Saito, S. Langmuir 1993, 9, 46. (7) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants With Polymers And Proteins; CRC Press: Ann Arbor, MI, 1993. (8)Cf. Goddard, E. D. Reference 7, p 158.
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polymer pair, as revealed by the segregative phase separation occurringin aqueousmixturesof the two polymers! The aim of the work is to study the swelling of copolymer gels with varying contents of the surfactant-binding macromonomer in water and in SDS solutions covering a wide range of surfactant concentration.
Experimental Section Materials. Acrylamide (h, Merck), Np-methylenebisacrylamide (BIS;Aldrich),ammoniumpersulfate (APS;Prolab), and N,N,iV’,”-tetramethylethylenediamine (TEMED;Aldrich) were used without further purification. Poly(ethy1ene oxide) monoacrylate (PEOMA) macromonomers with a relative molecular mass of 5OOO were prepared according to the functionalization method of o-hydroxypoly(ethy1eneoxide) described’O by Gnanov and Rempp. Sodium dodecyl sulfate (SDS) was obtained from Eastman and used without further purification. Millipore fiitered water was used throughout. Gel Preparation. Gels were prepared by standard radical polymerization with APS (0.44 mg/mL of HzO) aa initiator, BIS (2.5 mg/mL of HzO) as cross-linker, and TEMED (5 mg/mL of HzO) as accelerator. Using a “standard” polyacrylamide gel of 50 mg of AAm/mL of H2O (0.70 mol& of H2O) as a reference, two series of copolymer gels were prepared, where the molar ratio PE0MA:AAm in the reaction bath was varied between 0% and 1%. In the H series, the concentration of AAm was kept constant at 0.70 mol/kg of HzO; in the L series, the total maas of bifunctional monomers (AAm + PEOMA) was kept constant at 50 mg/mL of HzO in the reaction bath. Owing to the large differencein molar mass of the two monomers, the totalmonomer concentration in the reaction bath of the L series varied from 0.70 mol& of HzO (at 0% PEOMA) to 0.44 mollkg of H2O (at 1% PEOMA). After mixing with stirring, the reaction bath was aspired into graded 1-mL pipets with internal diameters & = 2.66 mm, where gelation was allowed to proceed for 1d. All gels thus produced were optically clear. Gel Analysis. To estimate the fraction of PEOMA incorporated in the network, an 0.8-mL sample of each gel was equilibrated in 4 mL of water during 8 d, after which the dialysis medium waa analyzed by gel permeation chromatography. A peak revealed the presence of unreacted PEOMA, the concentration of which was estimated from calibration curves of peak area us PEOMA concentration. The total mass of unreacted PEOMA was then calculated, assuming a homogeneous distribution of unreacted PEOMA in the entire sample volume (gel water), and compared to the known amount of PEOMA in the reaction bath. An average reaction yield of 22% PEOMA incorporated in the network was thus obtained for all gels, with no significant variation with PEOMA concentration or between the L and H series. This value was used to calculate the molar percentage of macromonomers, phi, incorporated in the gels, according to the formula p~ = ~ ~ ~ P E o M , ~ ( O . ~+~ n~ bP )E ,O ~ U where n, is the number of moles of monomer a in the reaction bath. Swelling Experiments. Prior to equilibration in aqueous SDS solutions, the gels were washed for 1d in a large excess of water with two changes. Cylindrical gel pieces of about 8-mm length were then immersed in 10 mL of water or SDS solutions and allowed to swell at room temperature during one week, with four changes of the dialysis medium. The diameters, d, of the swollen gels were then measured with a vernier caliper, and the swelling ratios, V,Vo, were obtained as VIVO= (d/do)s. (The volume of the unswollen gel, as prepared in the reaction bath, was thus chosen as the reference volume VO. This choice was found natural, since we wished to express the effects on the swelling ratio of both the variables p~ and SDS concentration.) Repeat measurements a month after preparation showed a slight additional swelling of all gels, possibly indicating a contribution to the gel swelling from ionic carboxylate groups created by slow hydrolysis of polyacrylamide.2J1
+
(9) Perrau, M. B.; Iliopoulos, I.; Audebert, R. Polymer 1989,30,2112. (10) Gnanov, Y.; Rempp, P. Makromol. Chem. 1987,188, 2111.
0 1993 American Chemical Society
Notes 5
3.5
, . . . ) . . . , , . . . . I , . . . , . . . .
I
ne e m
3 -
VIVO
2.5
e
0
-
a
e
2 -
e
e-
e ee
0.00
0.05
0.10
0.15
0.20
0.25
1.5
Pd% Figure 1. Swellingratio of L series (open symbols)and H series (filled symbols)gels in water (circles)and in 7.5 mM SDS (squares) as functions of the molar percentage of macromonomers.
Results and Discussion Figure 1 shows how the swelling ratios of the gels in pure water and in 7.5 mM SDS (corresponding to maximum swelling; cf. below) vary with the molar percentage of macromonomers. Quite similar results are seen for the L and H series of gels, indicating that, in the range studied, p~ (rather than the total concentration of monomers in the reaction bath) is the variable that primarily governs the swelling ratio. The L gels were, however, found to be loose and sticky in the higher range O f P M , whereas all H gels were firm. The Swellingincreases monotonically w i t h p ~both , in water and in SDS solutions, but the effect is much greater with SDS, owing to the formation of SDS micelles at the PEO side chains of the macromonomers (cf. below). For the reference gel, p~ = 0, no increased swelling was seen in SDS solutions. This is in agreementwith previous findings8that PAm interacts weakly with charged surfactants in aqueous solution. (Actually,the data in Figure 1indicate a slight deswelling in SDS that could be due to an exclusion of SDS micelles from the PAm gels. The magnitude of this effect falls, however, within the uncertainty of the measurements.) The significant increase in the swelling of the copolymer gels with increasing PM in pure water is interesting; we propose that this may be due to the "incompatibility" of PEO and PAm chain^.^ A gel with p~ = 0.13 from the H series was arbitrarily chosen for detailed measurementsof the effect of the SDS concentration, CSDS, on the swelling ratio. The results are shown in Figure 2. No swelling is seen for CSDS I 5 mM, after which V/VOincreases sharply with increasing CSDS. The critical concentration required for the onset of surfactant-induced swelling agrees well with the critical concentration for the formation of polymer-bound micelles, cac, previously reported for mixed SDS-PEO solutions.12 No further swelling is seen when CSDS exceeds the bulk cmc (8 mM),13 when free micelles start to form. (Note that in these experiments, the gel swelling medium was changed several times for each sample, and c4ps is thus equal to the concentration of free SDS, rather than the total SDS concentration.) Instead, a slow decrease in swelling is seen as CSDS increases beyond the cmc. The swelling of the gels in the range cac < CSDS < cmc may be attributed to the osmotic pressure exerted by the (11) Cf. note 13 in Hirokawa, Y.;Tanaka, T. J. Chem. Phys. 1984,81, 6379. (12) Cabane, B.;Duplessix, R. J. Phys. (Paris) 1982,43,1529. (13) Mukerjee, P.;Myeels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC,1971.
counterions of the SDS micelles bound to the gel. Above cmc, all added SDS forms free micelles, the monomer activity stays (roughly)constant, and no further formation of micelles on the PEO chains occurs. The distribution of free micelles between the gel and the surrounding solution is not known, but it may be suspected thalt, if anything, the free micelles will be excluded from the gel network to some extent. With an increasingconcentration of free micelles, the difference in counterion concentration between the gels and the surrounding solution decreases, and so does the swelling ratio. At very high concentrations of SDS, VIVOapproaches the value found at CQDS < cac. From the available data in the literature,P6 it is not clear whether also N-isopropylacrylamide gels show a maximum swelling at CQDS = cmc, although this might be expected from our results. The studies by Inomata and c o - w ~ r k e r sshow ~ * ~an increase in swelling with increasing surfactant concentration below the phase transition temperature, but the data are restricted to surfactant concentrations below the cmc. The swelling studies by Zhang et al.? on the other hand, were performed at a single surfactant concentration (70mM). (Thelatter researchers report that the phase transition temperature of the gel shows no further increase beyond 70 mM SDS, but make no remark on the effect of increasing SDS concentration on the equilibrium swelling.) A complicating factor in the analysis of the swelling of gels with bound surfactant micelles is the possibility that the micellar size varies with parameters such as surfactant concentration, temperature, and added salt. Indeed, Zhang et aL5 attributed the small effects (at all temperatures) of added NaCl on the equilibrium swelling of N-isopropylacrylamide gels to a growth of the polymerbound micelles, which was claimed to compensatefor the screening effect of added salt. While a significant growth of SDS micelles is known to occur at high concentrations no large variations in the micellar of salt and ~urfactant,'~J~ size are expected under the conditions of our study (room temperature, no added salt, comparatively low surfactant c~ncentrations).'~
Conclusions The swelling of cross-linked polymer networks in surfactant solutions provides a sensitive method for the study of associationbetween polymers and ionic surfactant micelles. The absence of association of SDS micelles with (14) Crwnen, Y.;Gelad6, E.;Van der Zegel, M.;Van der Auweraer, M.; Vandendrieseche, H.; De Schryver, F. C.; Almgren, M. J. Phys. Chem. 1983.87. ~ .,-. , 1426. ---(15) S6derman,O.;Jonstr6mer,M.;vanStam, J. J.Chem.Soc.,Fwa&y Trcmns. 1993,89, 1759.
3326 Langmuir, Vol. 9,No. 11,1993
PAm is confirmed by our study, while PEO side chains on the gel give rise to an association when CQDS = cac of mixed PEO/SDS solutions. A maximum swelling is found for CSDS = cmc, and a substantial decrease in the swelling is found for CQDS>> cmc. Even in the absence of surfactant, a swelling of gels with low degrees of PEO modification is seen, presumably due to incompatibility between PEO and PAm in water.
Notes
Acknowledgments. The authors are grateful to Dr. Roland Audebert for stimulating discussions. L.P. wishes to express his gratitude to all friends and colleagues at the Laboratoire de Physico-ChimieMacromol6culaire for their excellent hospitality and for many inspiring exchanges of thoughts and ideas during his extended visit in the summer of 1992,which was supported by a grant from the Swedish Natural Science Research Council.
