Swelling, Shrinking, Deformation, and Oscillation of Polyampholyte

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Swelling, Shrinking, Deformation, and Oscillation of Polyampholyte Gels Based on Vinyl 2-Aminoethyl Ether and Sodium Acrylate† Sarkyt E. Kudaibergenov*,‡ and Vladimir B. Sigitov Department of Polymer Chemistry, Kazakh State National University, Karasai Batyra Str. 95, 480012, Almaty, Republic of Kazakhstan Received August 20, 1998. In Final Form: January 21, 1999 The behavior of amphoteric hydrogels based on vinyl 2-aminoethyl ether and sodium acrylate under the influence of pH, ionic and solvent composition, temperature, and dc electric field has been studied. The excess of positive or negative charges causes the swelling of polyampholyte networks. At the isoelectric point (IEP) the polyampholyte gel is in shrunken state due to the formation of intraionic contacts. An “antipolyelectrolyte” effect is observed at the IEP: the gel considerably swells in the presence of neutral salt. The condensation of bulky anions to positively charged groups of polyampholyte gels enhances the shrinking rate. The shrinking process can be described by apparent first-order kinetics. Polyampholyte gel shrinks with the increasing of temperature when the overall charge is neutral (IEP), while it shrinks effectively with addition of acetone when the overall charge is negative. In dependence of the network net charge, the ionic strength of the external solution, and the direction of dc electric field, the polyampholyte specimen can bend, shrink, swell, and oscillate. Under the same conditions, positively and negatively charged precursors of polyampholytes only swell or shrink. Shrinking and swelling of amphoteric gels are determined by the concentration of mobile ions inside and outside of gel. To interpret the oscillation phenomenon the Donnan equilibrium and water hydrolysis are utilized. The realization of “antipolyelectrolyte” or polyampholyte and polyelectrolyte effects is probably the driving force of gel behavior. The oscillation-relaxation regimes are characterized by numerical value of 1/τ (where τ is the relaxation period). The phase portraits of all oscillation-relaxation curves are in good agreement with the Faraday law.

Introduction It is well-known that polymer gels undergo continuous or discontinuous volume phase transition in response to temperature,1 solvent composition,2 pH,3 ionic composition,4 and dc electric field.5 A number of studies on the volume phase transition of anionic, cationic, and nonionic hydrogels have been reviewed.6,7 Little attention, however, has been paid to the behavior of amphoteric polyelectrolyte hydrogels.8-11 Amphoteric gels show a minimal swelling degree when the amounts of anionic and cationic monomers are equal. As the concentration of the anionic and cationic parts deviates from the equimolar ones, the swelling ratio increases rapidly. A reasonable explanation for these results is that the number of osmotically active ions in the hydrogel phase increases as the molar ratio of cationic to anionic groups diverges from unity according to the Donnan equilibrium. Recently English et al.12 have examined polyampholyte hydrogel swelling over a wide range of bath salt concentrations. A sequence of theoretical approximations based on the Mayer ionic solution theory † Presented at Polyelectrolytes ‘98, Inuyama, Japan, May 31June 3, 1998. ‡ E-mail: [email protected].

(1) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820. (2) Ilavsky, M. Macromolecules 1980, 15, 782. (3) Ohmine, I.; Tanaka, T. J. Chem. Phys. II 1982, 5725. (4) Ricka, J.; Tanaka, T. Macromolecules 1985, 18, 83. (5) Osada, Y. Adv. Polym. Sci. 1987, 82, 3. (6) Adv. Polym. Sci. 1992, 109, 1. (7) Adv. Polym. Sci. 1993, 110, 1. (8) Starodubtzev, S. G.; Ryabina, V. R.; Khokhlov, A. R. Vysokomol. Soedin., Ser. A 1987, 29, 2281. (9) Myoga, A.; Katayama, S. Polym. Prepr. Jpn. 1987, 36, 2852. (10) Myoga, A.; Katayama, S. Kobunshi Kagaku 1988, 37, 530, 757. (11) Katayama, S.; Myoga, A.; Akahori, Y. J. Phys. Chem. 1992, 96, 4698. (12) English, A. E.; Tanaka, T.; Edelman, E. R. Macromolecules 1988, 21, 1989.

has been used to interpret the swelling behavior of polyampholyte gel. Synthetic polymer gels are known to exist in two phases, swollen and collapsed. Annaka and Tanaka13-15 have reported the existence of more than two phases in randomly distributed polyampholytes.The temperature dependence of the swelling ratios of two N-isopropylacrylamide (NIPA)-based amphoteric gels was measured.16,17 When the content of anionic and cationic groups is equal in NIPA-based gel, its swelling behavior is similar to NIPA gel itself. When the ratio of cationic and anionic groups deviated from 50/50, the transition temperature and swelling ratio increased. These results were explained by considering that the ionic contacts formed between the cationic and anionic units affect the swelling pressure of the gel. It was also found that the NIPA/betaine amphoteric gels containing equal constituents in pendant chains exhibit a swelling behavior similar to that of the integral type polyampholytes. Equimolar polyampholyte gel18 undergoes a gradual volume change with an increase in acetone composition in acetone-water mixture. When the content of the cationic groups increases, collapse of gel is observed. On the other hand, the same sample immersed in acetone-water mixtures of 60 and 65% acetone swells in a low-temperature region and collapses in a hightemperature region, while the gel in mixtures of 70% acetone or above is observed to maintain a collapsed state (13) Hirokawa, Y.; Tanaka, T.; Sato, E. Macromolecules 1985, 18, 2784. (14) Annaka, M.; Tanaka, T. Nature 1992, 365, 430. (15) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (16) Wada, N.; Yagi, Y.; Inomata, H.; Saito, S. J. Polym. Sci. Polym. Chem. Ed. 1993, 31, 2647. (17) Yu, H.; Grainger, D. W. Polym. Prepr. 1993, 34, 829. (18) Baker, J. P.; Stephens, D. R.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1992, 25, 1955.

10.1021/la981070a CCC: $18.00 © 1999 American Chemical Society Published on Web 04/08/1999

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over a wide temperature interval. It is concluded that although the amphoteric gel consists of various proportions of cationic and anionic groups it exhibits the same pattern of volume phase transition, as did monoionic gels. Polyelectrolyte gels swell, shrink, or bend when dc is applied.19-22 Tanaka et al.23 were the first who observed the deformation of polyelectrolyte gel on application of an electric field. The authors21 proposed a semiquantitative theory which describes the swelling and shrinking behavior of polyelectrolyte gel under an electric field. Osada et al.24 have constructed an eellike gel actuator on the basis of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and studied its chemomechanical properties. Novel gel systems demonstrating rhythmically pulsatile motion similar to that of a heartbeat were developed by the authors.25 The pH oscillations were also observed for the system consisting of iodate-sulfite-thiosulfate and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) when the polymeric acid substituted for sulfuric acid.26 Recently27 an analytically tractable model for the description of polyampholytes in an external electric field has been presented. Since positive and negative charges of both polyampholyte and counterions in general have different distributions and stoichiometries, one can expect that the phase transition induced by external factors will also be different. Moreover the behavior of amphoteric gels near the isoelectric point has not been considered thoroughly. Besides the outstanding characteristics of ionic gels for water absorption and retention, their swelling and shrinking kinetics are of interest for purification of wastewater and to construct drug delivery systems. Experimental Section Water-swelling copolymers based on vinyl 2-aminoethyl ether (VAEE) and sodium acrylate (SA) were synthesized by γ-irradiation polymerization at various ratios of monomer mixtures in the presence of cross-linking agent divinyl ether of diethylene glycol (DVEDEG) in a water-ethanol mixture (volume ratio 1:1). (19) Shiga, T.; Kurauchi, T. J. Appl. Polym. Sci. 1990, 39, 2305. (20) Shiga, T.; Hirose, Y.; Okada, A.; Kurauchi, T. J. Appl. Polym. Sci. 1992, 44, 249. (21) Doi, M.; Matsumoto, M.; Hirose, Y. Macromolecules 1992, 25, 5504. (22) Choi, O. S.; Yuk, S. H.; Lee, H. B.; Jhon, M. S. J. Appl. Polym. Sci. 1994, 51, 375. (23) Tanaka, T.; Nishio, I.; Sun, S.; Ueno-Nishio, S. Science 1982, 218, 476. (24) Ueoka, Y.; Isogai, N.; Nitta, T.; Okuzaki, H.; Gong, J. P.; Osada, Y. Preprints of Sapporo Symposium on Intelligent Polymer Gels; Hokkaido University: Sapporo, 1994. (25) Yoshida, R.; lchijo, H.; Hakuta, T.; Yamaguchi, T. Macromol. Rapid Commun. 1995, 16, 305. (26) Giannos, S. A.; Dinh, S. M.; Brener, B. Macromol. Rapid Commun. 1995, 16, 527. (27) Schiessel, H.; Biumen, A. Macromol. Theory Simul. 1997, 6, 103.

The degassed and sealed glass test tubes with diameter 5-10 mm containing VAEE, SA, and DVEDEG (4 mol %) were irradiated with the help of 60Co “RXM-γ-25M” at an irradiation dose 170 rad/s during 3-5 h. The gels were removed, cut into cylinders and washed by distilled water during 2 or 3 weeks to remove the sol fraction. The repeating unit of amphoteric gel and the ratio of anionic to cationic groups in copolymers determined by potentiometric titration and elemental analysis as well as the isoelectric points found from the swelling degree are shown in Table 1. The content of VAEE in copolymers did not exceed 40 mol % owing to its low activity in radical copolymerization. The swelling degree of samples in water calculated by the formula R ) (W - W0)/W0 (W0, weight of dry gel; W, weight of gel at equilibrium-swollen state) varied from 80 to 110 g/g. The kinetics of shrinking was detected by periodically weighing of gel samples with time. The swelling, shrinking, and deformation experiments under the action of dc electric field were carried out in the cell provided with platinum electrodes. A cylindrical gel rod with diameter 1 cm and length 5 cm was placed between two platinum electrodes in the center of cell so that the axis of the cylinder was parallel or perpendicular to the direction of electric current, not touching the electrodes. All measurements were carried out at constant voltage 10 V during 1 h. The weight and dimension of gels were detected by both weighing and use of a cathetometer “B-630” or photographically with the help of a “Zenit-ET” camera. pH and ionic strength of the external solution were adjusted by phthalate buffer and neutral salt KCl. The ionic strength of the external solution was kept between 5 × 10-3 and 2.5 × 10-2.

Results and Discussion pH and Ionic Composition Sensitivity. pH-sensitive hydrogels usually contain pendant acidic or basic groups which change the ionization state in response to pH variation. The balance of negative and positive charges in polyampholytes can be reached by either synthetic control of polyampholyte stoichiometry keeping an equimolar concentration of acidic and basic monomers in the final product (quenched polyampholytes) or at the isoelectric point (IEP) adjusting the electroneutrality of the macromolecular chain by addition of acid or base (annealed polyampholytes). The annealed polyampholytes can easily be transformed into unbalanced ones by changing pH. Figure 1 shows the influence of pH and ionic strength on the swelling degree of PA-1 in aqueous and aqueous-salt solutions. When the pH of the external solution deviates from the isoelectric pH, polyampholyte gels behave as polycations and polyanions, respectively, and the swelling degree increases. Near the isoelectric point (pHIEP ) 4.8) the intraionic attraction between opposite charges makes polymer chains collapse. The addition of a neutral salt suppresses the relative volume (V/V0) change of gel from both sides of the IEP. An “antipolyelectrolyte” effect is observed at the IEP: the gel is in swollen state in the

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Figure 1. Dependence of W/W0 of PA-1 on pH in pure water (1) and in the presence of 0.1 M KCl (2).

Figure 2. Shrinking of PA-3 in phthalate buffer at pH ) 6.2 (1) and 3.8 (2) at µ ) 5 × 10-2 and T ) 298 K.

presence of KCl. This result together with our previous data28,29 entirely confirms the prediction of Tanford30 and the theoretical conclusions of Joanny31 on the unfolding of amphoteric macromolecules at the IEP in the presence of neutral salts. The time dependence of the shrinking ratio for polyampholyte gel measured at various pH values is shown in Figure 2. The shrinking rate of amphoteric gel is high when the overall charge of network is slightly positive (pH ) 3.8). This is probably due to condensation and effective screening of phthalate counterions from the positive charges of polyions. However when the overall charge of network is close to zero (pH ) 4.5) or negative (pH ) 6.2), the effectiveness of shrinking decreases. The semilogarithmic plots of ln(Wmin - W0)/(Wt - W0) (where Wmin is the minimum deswelling volume of the gel at infinite time, W0 is the volume of the gel at time zero, and Wt is the volume of gel at time t) against time show straight lines as illustrated in Figure 3. Therefore, the process of the shrinking may be dealt with by apparent first-order kinetics. As seen from Figure 3, the increase of temperature considerably increases the rate of shrinking. This is connected with enhancement of hydrophobic interactions with increasing of temperature. Temperature and Solvent Sensitivity. We present here briefly the phase transition of the amphoteric gel, having examined temperature and solvent composition (28) Kudaibergenov, S. E.; Shayakhmetov, Sh. Sh.; Bekturov, E. A. Dokl. Akad. Nauk SSSR 1979, 246, 141. (29) Bekturov, E. A.; Kudaibergenov, S. E.; Shayakhmetov, Sh. Sh. Vysokomol. Soedin. Ser. B 1980, 22, 91. (30) Tanford, C. Physical Chemistry of Macromolecules (in Russian); Mir: Moscow, 1965. (31) Higgs, P. G.; Joanny, J.-F. J. Chem. Phys. 1991, 94, 1543.

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Figure 3. Shrinking kinetics of PA-3 in phthalate (1) and acetate (2, 3) buffers at the IEP (pH ) 4.5) (T ) 298 (1, 2) and 323 K (3); µ ) 5 × 10-2).

Figure 4. Influence of temperature on W/W0 of PA-1 at pH ) 4.8 (IEP) (1) and 6.0 (2).

Figure 5. Influence of water-acetone mixture on W/W0 of PA-2 at pH ) 6.7 (1) and 4.5 (IEP) (2).

dependencies. Figure 4 shows the influence of temperature on the relative volume change, W/W0, of PA-1. Negatively and positively charged gel is not influenced by temperature. However at the IEP as the temperature increases, the slightly swollen gel undergoes a volume change at the interval of temperature T ) 308-318 K, falling into the shrunken state. This is probably due to the enhancement of hydrophobic interactions. Figure 5 shows the influence of acetone concentration on W/W0 for PA-2 at the IEP (pH ) 4.5) and in anionic form (pH ) 6.7). The composition of acetone-water mixtures does not influence the volume phase transition of PA-2 at the IEP, while the anionic gel shrinks considerably beginning from 20 vol % up to 60 vol % acetone concentration. In 100 vol % acetone the gel is in shrunken state. These results support the concept of

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curves 1 and 3). The reasonable explanation of this effect is the osmotic pressure difference that occurs in consequence of mobile ion transport to the anode or cathode side. However, the polyampholyte gel swells or oscillates at the IEP (Figure 7, curves 2 and 4). To explain swelling, shrinking, and oscillation (periodically shrinking and swelling) phenomena, the redistribution of fixed and mobile ions under the action of an electric field, the Donnan equilibrium, and water hydrolysis should be taken into account. According to the ref 21 the difference between the concentration of free ions inside and outside of gel is the driving force of swelling and shrinking of polyelectrolyte gels.

∆Π ) RT

Figure 6. Bending and simultaneous contracting of PA-3 with time at the electric field 10 V (µ ) 0.1M KCl). The axis of the gel rod is perpendicular to the dc electric field.

∑(Cig - Cog)

where ∆Π is the osmotic pressure increment, R is the Boltzmann constant, T is the absolute temperature, and Cig and Cog are the concentrations of mobile ions inside and outside the gel, respectively. Since the value of isoelectric pH (pHIEP) of PA-3 is in acidic region (pHIEP ) 4.5), there are equal numbers of NH3+ and COO- ions together with their counterions (sodium and chloride ions) and the excess of undissociated COOH groups. In the absence of an electric field, a uniform potential background is assumed to exist in both gel interior and outer solution. When a dc electric field is applied, the redistribution of fixed charges inside the gel and mobile ions in both the gel interior and bulk will take place (polarization effect). However the electrical neutrality must exist both inside the gel and surrounding bath. The only electrochemical reaction that satisfies this requirement is the electrolysis of water:

2H2O f 4H+ + 4e- + O2 4H2O + 4e- f 4OH- + 2H2

Figure 7. Increment of mass change of PA-3 under the influence of dc electric field at pH ) 3.0 (1), 4.5 (2, 4), and 6.2 (3) at µ ) 1 × 10-2 (1-3) and 2.5 × 10-2 (4).

universality of the phase transition of ionized polymer gels in water-acetone mixtures. Dc Sensitivity. Deformation of polyampholyte gel depends on the net charge of network and the direction of electric field with respect to samples. In anionic or cationic forms the specimens are bent toward the cathode or anode side and simultaneously shrink when the electric field is perpendicular to the long axis of the gel rod (Figure 6). The deflection increases with time and recovers the initial shape after the removal of the electric field. These results are in good agreement with the experimental and theoretical results reported.19,20 However the amplitude of deflection of amphoteric gel gradually decreases near the IEP due to the decrease of the net charge of amphoteric macromolecules.32 Figure 7 represents the increment of mass change of polyampholyte gel in anionic, cationic, and electroneutral forms when the direction of applied electric field is parallel to the axis of the gel rod. In anionic and cationic states the gel specimen shrinks (Figure 7, (32) Kudaibergenov, S. E. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1079.

When the concentration of mobile ions inside of gel is lower than in surrounding bath, e.g. Cig < Cog, and the Donnan effect requires the increase of osmotically active ions inside the gel, the gel swells (Figure 7, curve 4). In this case probably the applied dc causes the “apparent increasing of ionic strength” causing an “antipolyelectrolyte” effect. When the concentration of mobile ions inside of gel is higher than in surrounding bath, e.g. Cig > Cog, at first the gel shrinks due to realization of the polyampholyte effect because the excess of counterions inside the gel will effectively be “dialyzed” from the gel interior to satisfy the Donnan equilibrium thus shrinking the gel (Figure 7, curve 2). However the overall shrinking tendency of amphoteric gel is accompanied by periodic slight swelling and shrinking effects with time. This is probably accounted for the participation of generated H+ and OH- ions in association-dissociation reactions with functional groups of polyampholytes:

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Figure 8. Phase portraits of curves 1, 2, and 4, taken from Figure 7.

Periodic penetrating of H+ and OH- ions is expected to create the pH gradients inside the gel matrix that increase or decrease the osmotic pressure, and the gel shrinks or swells, respectively. Earlier the appearance of a pH gradient inside the gel matrix was also confirmed by researchers.22 The oscillation effect disappears at the end of the experiment, and the pH of the solution deviates from the isoelectric pH and becomes equal to 6.5. Thus under the applied dc electric field depending on the concentration of ions inside and outside of the gel, either the “antipolyelectrolyte” or polyampholyte and polyelectrolyte effects will dominate. The treatment of shrinking, swelling, and oscillation curves in coordinates dm/dt ) f(∆m) was also undertaken. Figure 8 represents the phase portraits of swelling, shrinking, and oscillation taken from Figure 7. All types of curves can be represented as sequential realization of the following: (i) relaxation phenomenon, when ∆m depends on time exponentially and consequently there is a linear dependence between dm/dt and ∆m; (ii) oscillation phenomenon, when the time dependence of ∆m is harmonic, where in this case the phase portrait in coordinates dm/dt ) f(∆m) is ellipsoidal. The oscillation-relaxation regimes are characterized by numerical values of 1/τ (where τ is relaxation period). For relaxation movement, this parameter is equal to tangent of the slope of the linear part; for oscillation movement, it is equal to the ratio of the semiaxis of ellipse. The Faraday law in differential form describes all phase portraits of relaxation-oscillation curves:

dm J ) γ dt F Q(t)M

Figure 10. Mass change of PVAEE with time at µ ) 1 × 10-2 (1) and 2.5 × 10-2 (2).

(1)

Here Q(t) is the weight of equilibrium-swollen gel at time t, M is the mass of repeating units, γ is the correction coefficient close to unit, J is the electric current, and F is the Faraday constant. The solution of eq 1 is the following:

∆m ) ∆mmax [1 - exp(-t/τ)]

Figure 9. Influence of initial specimen mass on oscillation period (1) and frequency (2) of PA-3 gel.

(2)

Experimentally found from eq 2, the time-average values of 1/τ plotted in dependence of the initial mass of the gel specimen are linear, whereas the oscillation-relaxation frequency of the gel changes exponentially on the initial mass of gel (Figure 9). Thus, the oscillation-relaxation processes probably depend on initial gel mass that in turn is controlled by the ionic strength of the external solution. However, the detailed mechanism of this phenomemon is

Figure 11. Mass change of PSA with time at µ ) 5 × 10-3 (1), 1 × 10-2 (2), and 2.5 × 10-2 (3).

not clear and requires further experimental facts and theoretical consideration. Anionic or cationic gels, e.g. cross-linked poly(sodium acrylate) (PSA) and poly(vinyl 2-aminoethyl ether) (PVAEE), in contrast to amphoteric ones only swell or shrink (Figures 10 and 11). The gain or loss of gel weight is the function of time and ionic strength of the external solution. For instance, PVAEE swells at µ ) 1 × 10-2 and shrinks at µ ) 2.5 × 10-2. PSA gel at first swells and then turns back to the original state and shrinks with the length

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of time. These results can be interpreted in the terms of the electric potential difference in gel volume and gelsolution boundary. The difference of the osmotic pressure on the gel-solution interface is also responsible for swelling and shrinking phenomena. At low ionic strength µ ) 5 × 10-3 probably the concentration of mobile ions inside the gel is lower than the outside. As a result, the osmotic pressure increases with time and the gel swells. After 5 min, the gel begins to lose weight due to the decrease of osmotic pressure with time. At relatively higher ionic strength (µ ) 2.5 × 10-2) the difference of concentrations between the gel interior and the outer solution becomes lower and the gel slightly swells and then shrinks. For cationic gel, the osmotic pressure difference increases at µ ) 1 × 10-2, decreases at µ ) 2.5 × 10-2, which causes the swelling and shrinking of the gel, respectively.

of pH, ionic strength of the solution, temperature, wateracetone mixture, and dc application. One can conclude that the “antipolyelectrolyte” effect observed at the IEP is common for both linear and cross-linked polyampholytes. The temperature factor enhances the shrinking of amphoteric gel at the IEP due to strengthening of hydrophobic interactions. The amphoteric gel bends, shrinks, swells, and oscillates when dc electric field is applied. These phenomena can be explained in terms of the osmotic pressure difference caused by the redistribution of fixed and mobile ions and water hydrolysis. Under the applied dc electric field either the “antipolyelectrolyte” effect that leads to swelling of gel or polyampholyte and polyelectrolyte effects that cause shrinking or deswelling will be realized.

Conclusion

Acknowledgment. This work was supported by INTAS-KZ-95-31.

This study has shown a wide spectrum of stimulisensitive properties of polyampholyte gels as a function

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