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Langmuir 1999, 15, 4236-4244
Swelling, Structure, and Elasticity of Polyampholyte Hydrogels† G. Nisato,* J. P. Munch, and S. J. Candau Laboratoire de Dynamique des Fluides Complexes, UMR 7506, CNRSsULP, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France Received August 13, 1998. In Final Form: November 27, 1998 We studied the swelling and elastic behavior of chemically cross-linked [2-(methacryloyloxy)ethyltrimethylammonium chloride-2-(acrylamido)-2-ethylpropanesulfonate] polyampholyte gels in equilibrium with saline solutions. The net charge of the polymer network and the ionic strength of the swelling solvent are crucial parameters governing the properties of the gels. The swelling equilibrium behavior of gels with a strong charge imbalance was analogous to polyelectrolyte systems (monotonic decrease of swelling ratio with rising salt concentration). Globally neutral gels showed an antipolyelectrolyte behavior (collapse at low ionic strength and monotonic increase of swelling ratio with rising salt concentration). Weakly unbalanced polyampholyte gels showed a characteristic minimum in their swelling curves, the same gel reaching the same equilibrium volume in two different ionic conditions. The internal structure was imaged using differential interference contrast microscopy and revealed that morphological modifications of the gel structure accompany volume variations. The evolution of the elastic modulus of polyampholyte gels at equilibrium is quite complex, because the same sample can behave like a polyelectrolyte in low ionic strength conditions and as a balanced polyampholyte at high ionic strength. Thus a polyampholyte gel can reach the same swelling ratio at equilibrium for two different ionic strengths, but is then characterized by two different values of the shear modulus.
Introduction Polyampholytic hydrogels are cross-linked polymer networks containing both positively and negatively charged units. The presence of ionic groups of opposite charges along the backbone of the network chains leads to a complex behavior that is essentially controlled by electrostatics. Long-range Coulombic interactions are still a great challenge to the polymer physicist, and theoretical tools that have been successful in modeling neutral polymer solutions, such as scaling laws,1 are not easily extended to charged systems. Recently, much theoretical effort has been devoted to understanding the conformations of linear polyampholyte chains.2-10 If the net charge is large, the chains are expected to behave like conventional polyelectrolytes in which counterions play a major role, because they ensure the overall neutrality of the solution. In balanced (i.e., globally neutral) polyampholytes the net electrostatic forces are attractive so that, in pure water, the chains have a tendency to collapse into compact globules. Addition of salt, which screens these interactions and weakens the intrachain attractions, induces a swelling of the chain. * Corresponding author. Current address: Polymers Division, National Institute for Standards and Technology, Gaithersburg, MD 20899. E-mail:
[email protected]. † Presented at Polyelectrolytes ’98, Inuyama, Japan, May 31June 3, 1998. (1) deGennes, P.-G. Scaling concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (2) Qian, C.; Kholodenko, A. L. J. Chem. Phys. 1988, 89, 5273. (3) Higgs, P. G.; Joanny, J.-F. J. Chem. Phys. 1991, 94, 1543. (4) Kantor, Y.; Lin, H.; Kardar, M. Phys. Rev. Lett. 1992, 69, 6164. (5) Kantor, Y.; Kardar, M.; Lin, H. Phys. Rev. E 1994, 49, 1383. (6) Kantor, Y.; Kardar, M. Phys. Rev. E 1995, 52, 835. (7) Gutin, A. M.; Shakhnovich, E. I. Phys. Rev. E 1994, 50, R3322. (8) Dobrynin, A. V.; Rubinstein, M. J. Phys. II 1995, 5, 677. (9) Bratko, D.; Chakraborty, A. K. J. Phys. Chem. 1996, 100, 1164. (10) Candau, F.; Joanny, J.-F. Polyampholytes (Properties in Aqueous Solution); Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 7. - P.
The few experimental studies of polyampholyte gels focused essentially on their swelling behavior.11-18 The results are in qualitative agreement with the predictions of single-chain models. In particular, balanced polyampholyte gels collapse in low ionic strength conditions and swell at high salt concentration. When the net charge is increased, polyelectrolyte behavior is observed and the equilibrium swelling degree decreases with increasing salt concentration. For polyampholytes near the balance point, polyelectrolyte effect is dominant at low salt concentration, whereas the polyampholyte behavior appears at high salt concentration. Swelling models solely based on classical Donnan theory cannot predict a polyampholytic swelling behavior because the electrostatic forces between ions are not considered directly. To account for these Coulombic interactions, several approaches have been proposed that describe qualitatively the general swelling behavior, including the characteristic polyampholyte effect (swelling at high salt concentration). However, fitting the existing models to the swelling data requires several adjustable parameters and does not allow to discriminate unambiguously these models. By contrast, the behavior of the shear modulus provides a good signature of a given model. In particular, the study of the variation of the shear modulus as a function of the equilibrium swelling degree reduces the number of adjustable parameters and mini(11) Baker, J. P.; Stephens, D. R.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1992, 25, 1955. (12) Annaka, M.; Tanaka, T. Nature 1992, 355, 430. (13) Baker, J. P.; Blanch, H. W.; Prausnitz, J. M. Polymer 1995, 36, 1061. (14) English, A. E.; Mafe, S.; Manzanares, J. A.; Yu, X. H.; Grosberg, A. Y.; Tanaka, T. J. Chem. Phys. 1996, 104, 8713. (15) Katayama, S.; Myoga, A.; Akahori, Y. J. Phys. Chem. 1996, 96, 4698. (16) Kudaibergenov, S. E. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1079. (17) Than, L. T. M.; Makhaeva, E. E.; Khokhlov, A. R. Polym. Gels Networks 1997, 5, 357. (18) Mafe, S.; Manzanares, J. A.; English, A. E.; Tanaka, T. Phys. Rev. Lett. 1997, 79, 3086.
10.1021/la981027n CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
Polyampholyte Hydrogels
mizes the effect of structural imperfections. This kind of approach has been successfully applied to the study of neutral gels.19 In this paper we report on measurements of the shear modulus of polyampholyte hydrogels at the swelling equilibrium. The latter was varied by changing the average net charge of the network and the salt concentration of the swelling solutions. Polyampholyte gels with a weak net charge exhibit the unique property of reaching the same degree of equilibrium swelling with widely different salt contents. This is due to screening effects, which play differently on the attractive (polyampholyte) and repulsive (polyelectrolyte) interactions. These systems are thus suited to investigate directly the effect of electrostatic interactions on the elastic properties of gels. Such effects have been experimentally studied in polyelectrolyte gels and are not yet totally understood. Experimental Section Sample Preparation. The sample preparation follows the procedure used for polyelectrolyte gels, which is described in previous papers.20,21 Gels were obtained by radical copolymerization of 2-(methacryloyloxy)ethyltrimethylammonium chloride (MADQUAT) and 2-(acrylamido)-2-ethylpropanesulfonate (NaAMPS); the cross-linking agent was N,N′-methylene(bis)acrylamide (Aldrich). NaAMPS was obtained through titration22 of AMPS(H) (Cassella); MADQUAT (Elf-Atochem) and crosslinker were used as received. The net charge carried by the polymer network is ∆f ) f + - f -, where f + and f - are the molar ratio of cationic and anionic monomers to the total monomer concentration, respectively. The net charge was varied by changing the relative ratios (weight fraction) of anionic and cationic monomers, the total polymer concentration (0.1 g/mL) being constant in all the samples. The cross-linking ratio, defined as the ratio of cross-linking molecules to the total number of monomers, was 2 × 10-2 ( 2 × 10-4 for all the samples. The gelation reaction was initiated by ammonium peroxydisulfate (Aldrich). Before polymerization, the solutions were degassed under vacuum to remove oxygen, a polymerization inhibitor, and nitrogen was bubbled in. The solutions were then immediately sealed in airtight Teflon molds and the polymerization reaction was carried out at 70 °C for at least 10 h. Corpart and Candau showed23 that, in similar systems in the absence of cross-linker, the polymerization leads to quantitative yield after 10 h. Therefore, the global composition of the gel corresponds to the initial monomer composition. The samples in the preparation state are monolithic cylinders with aspect ratio 1 (height ) diameter ) 1 cm). The effective NaCl concentration in the gel at the preparation state, noted Cs°, results from salt added to the solution before polymerization as well as the dissociation of the monomers. The salt concentration due to monomer dissociation is CPA (1 - |∆f |), where CPA is the total monomer concentration. Table 1 summarizes the compositions of the samples. The gels were weighed and their dimensions measured immediately after removal from the Teflon molds and were subsequently placed in polypropylene flasks containing a large excess of either water or brines. The water used for gel synthesis as well as for the preparation of the NaCl solutions was purified using a Millipore MilliQ system and presented a resistivity of 18 MΩ. Brines were changed regularly and gels were allowed to equilibrate for up to 2 weeks. Because the swelling solvent is replaced several times, excess salt and the traces of unreacted moieties are effectively dialyzed from the polymer networks. Therefore, we do not expect any contribution of the unreacted monomers to the observed swelling behavior. Periodic measurement of the dimensions of (19) Bastide, J.; Candau, S. J.; Leibler, L. Macromolecules 1980, 14, 719. (20) Schosseler, F.; Ilmain, F.; Candau, S. J. Macromolecules 1991, 24, 225. (21) Schosseler, F.; Moussaid, A.; Munch, J.-P.; Candau, S. J. J. Phys. II 1991, 1, 1197. (22) Neyret, S.; Candau, F.; Selb, J. Acta Polym. 1996, 47, 323. (23) Corpart, J. M.; Candau, F. Colloid Polym. Sci. 1993, 271, 1064.
Langmuir, Vol. 15, No. 12, 1999 4237 Table 1. Composition of Polyampholyte Gel Samples in This Studya code pmp43 pmp44 pmp45 pmp56 pmp61 pmp57 pmp46 pmp54
MADQUAT AMPSNa CPA° (mol/L) (mol/L) (mol/L) 0.433 0.382 0.241 0.288 0.2285 0.196 0.0957 0.239
0.0462 0.0866 0.211 0.174 0.2287 0.268 0.348 0.217
0.47 0.47 0.46 0.46 0.46 0.46 0.45 0.46
∆f 0.807 0.630 0.066 0.247 -0.001 -0.156 -0.568 0.048
Cse C s° (mol/L) (mol/L) 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.00
0.54 0.59 0.72 0.68 0.73 0.70 0.60 0.22
a The overall polyampholyte concentration in the preparation state is noted CPA°; ∆f is the average net charge carried by the polymer network; Cse denotes the amount of NaCl added to the solution before copolymerization. The total estimated amount of salt present in the gel at the preparation state is noted Cs°. Crosslinking ratio is 0.02 for all the samples.
the samples ensured that measurements were performed after the gels had reached swelling equilibrium. The swelling ratio at equilibrium Qe with respect to the preparation state is defined as Qe ) m/m0 ) (h/h0)3 ) (d/d0)3 where m, h, d, are the mass, the height, and the diameter of a sample, the subscript being indicative of values in the preparation state. Elastic Modulus. Elastic shear moduli G of the gel samples were determined from uniaxial compression measurements.19 The experimental setup and the protocol used to measure G are described in previous papers.24 The experimental device consisted of a piston driven by a PC-controlled micrometric translation unit (Microcontrole), which imposes a deformation λ, and a force transducer, which measures the nominal stress σ exerted on the gel. The shear modulus G is obtained using: σ/(/λ2 - λ) ) G[1 + A1 (λ2 + 1/λ) + ...], where A1 is a constant and the expansion terms take into account the non-Gaussian elasticity effects that appear at high swelling ratios.24-26 All the measurements were carried out with 0.8 < λ < 1. Each modulus was determined from at least three measurement sets consisting of 50 data points each. Light Scattering. Samples for light-scattering measurements were swollen at equilibrium, then sealed into cylindrical lightscattering cells. The light-scattering experimental setup has been described previously.27 The intensity of light scattered by a gel as well as its autocorrelation function depend on the sample position and the diffusing volume considered, leading to a “nonergodic” situation.28-31 Ensemble averages of the static intensity scattered at each diffusing angle θ were obtained by rotating the sample with continuous current motor. The total measured intensity for each scattering angle results from the average of a hundred different measurements. The scattering data spans a range of q values ranging from 8 × 10-3 to 3.2 × 10-2 nm-1, where q ) 4πn/λ sin(θ/2) (n is the refractive index of the medium, λ ) 488 nm is the wavelength of the laser). Each data set corresponded to at least 200 different scattering angles. For each angle, the total scattered intensity as a function of time I(t) can be written as I(t) ) IF(t) + IG, where IF(t) and IG denote the rapidly fluctuating and the frozen-in (gel) components of the scattered intensity, respectively.32,33 Decomposition into (24) Nisato, G.; Skouri, R.; Schosseler, F.; Munch, J.-P.; Candau, S. J. Faraday Discuss. 1995, 133. (25) Treloar, L. R. G. The Physics of Rubber Elasticity; Clarendon Press: Oxford, 1975. (26) Schro¨der, U. P.; Opperman, W. Properties of Polyelectrolyte Gels; Addad, J. P. C., Ed.; Wiley & Son: Chichester, 1996. (27) Skouri, R.; Schosseler, F.; Munch, J.-P.; Candau, S. J. Europhys. Lett. 1993, 23, 635. (28) Pusey, P. N.; Megen, W. V. Physica A 1989, 157, 705. (29) Joosten, J. G. H.; McCarthy, J.; Pusey, P. N. Macromolecules 1991, 24, 6690. (30) Xue, J.-Z.; Pine, D. J.; Milner, S. T.; Wu, X.-L.; Chaikin, P. M. Phys. Rev. A 1992, 46, 6550. (31) Moussaid, A.; Munch, J.-P.; Schosseler, F.; Candau, S. J. J. Phys. II 1995, 1, 637. (32) Rouf, C.; Bastide, J.; Pujol, J. M.; Schosseler, F.; Munch, J.-P. Phys. Rev. Lett. 1994, 73, 830. (33) Rouf-George, C.; Munch, J.-P.; Schosseler, F.; Pouchelon, A.; Beinert, G.; Boue´, F.; Bastide, J. Macromolecules 1997, 30, 8344.
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Figure 1. Evolution of swelling ratio at equilibrium Qe as a function of the net charge fraction for gels swollen in pure water (O) and 2 mol/L NaCl solutions (b). Solid lines are guides to the eye. Error bars are smaller than the symbols and are not displayed. frozen and fluctuating components was achieved by measuring averages of at least 1000 autocorrelation functions of the scattered intensity corresponding to different positions of the gel. The displacements were performed by a PC-controlled step motor (Microcontrole) allowing 1 µm displacements. The total spatial excursion of the sample is 7 mm. This procedure also allowed us to check experimentally the homogeneity of the gel at a macroscopic scale. Microscopy. Samples were prepared by slicing thin layers (
