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Electrokinetics of the contraction of a polyelectrolyte hydrogel under the influence of constant electric current
Polymer
ELSEVIER
Gels and Networks 3 (1995) 387-393 Elsevier Science Limited Printed in Northern Ireland 0967822/95/$09.50
0%6-7822(94)00031-X
Electrokinetics of the Contraction of a Polyelectrolyte Hydrogel Under the Influence of Constant Electric Current T. Budtova,* Institute
of Macromolecular
I. Suleimenov
dz S. Frenkel
Compounds, Russian Academy of Sciences, Bolshoi prosp. 31, 199004 St. Petersburg, Russia
(Received 30 June 1993; revised 10 August 1994; accepted 10 August 1994)
ABSTRACT The kinetics of the contraction of a highly swollen polyelectrolyte hydrogel under the influence of electric current was investigated. From the results obtained, different mechanisms of the contraction may be supposed to predominate for different degrees of swelling. In the case of a rather small contraction, it depends only on the magnitude of electric charge transported through the sample and is described by the Faraday law with the correction coefficient. The second mechanism may be connected with electro-osmotic release of water.
INTRODUCTION Highly swollen polyelectrolyte hydrogels have different unique physical and chemical properties: they can swell in water to thousands of times their volume and change their characteristics drastically under the influence of different external forces. One of the most interesting properties of hydrogels is their capability for reversible contraction under the influence of external electric fields.‘” Many investigations were undertaken about the contractile phenomena in detail, for instance, by Khokhlov et aL6 They attributed the phenomena of water release to an electro-osmotic mechanism, which was considered to be the main reason for the contraction. In Refs 1-5, 7 and 8, voltage alone was taken as the factor influencing the hydrogel contraction. The degree of sample collapse with time was measured as a function of the voltage. It was assumed that the hydrogel contraction under an electric field is due to * Author to whom all correspondence
should be addressed. 387
388
T. Budtova, I. Suleimenov,
S. Frenkel
electrochemical reactions eliminating near electrodes. De Rossi et ~2.~ associated the contraction with a change of pH changes in the fluid, thereby changing the ionic state of the cross-linked poly(viny1 alcohol)poly(acrylic acid) membranes. In some studies,“*” authors found non-linear transport behavior of ions in the gel. In Ref. 11, the explanation of the contractile behavior is given in terms of ion transport of counter-microions in the electric field. Different aspects of practical applications (electrically activated membranes, release control systems for drugs, etc) were discussed in Refs 2-5. The practical significance of studies of electric field influence on hydrogel behavior is determined not only by their utilisation in chemomechanical devices, but also in purifying and enriching technologies as well, where hydrogels can be used in cyclic contractionswelling regimes.l* However, electrokinetic regularities describing hydrogel collapse under the influence of an electric field are still not fully understood. It seems of interest to carry out investigations where the current or electric charge transported though the hydrogel are the main electrical parameters influencing its behavior. According to the Faraday law, these values determine the character and the course of electrochemical reactions. The aim of this paper is to clarify the relations between electric strength and the hydrogel collapse.
EXPERIMENTAL The hydrogel based on sodium polyacrylate was synthesized by radical polymerization of acrylamide in aqueous solution (monomer concentration during polymerization was 20%). The process of the hydrogel synthesis is described in details in Ref. 13. Water soluble allylcarboxymethylcellulose (degree of substitution in respect to ally1 groups was 0.4, its weight fraction in the reaction solution was 0.2%) was used as a cross-linking agent. Afterwards, the samples were immersed in distilled water in order to remove impurities. They were then subjected to hydrolysis in a 0.1 M solution of NaOH. The degree of swelling of the gel in water was 1000 g/g. The synthesized hydrogel has improved elastic and mechanical properties13 because polymer chains of allylcarboxymethylcellulose, the cross-linking agent used in this study, are stiff and play a role of “rigid cross-bars”. The gel can sustain its shape by itself even if the degree of swelling is very high. Thus it was convenient to use these samples for the study of its contractile behavior under the electric current.
Electrokinetics of a polyelectrolyte hydrogel
Fig. 1.
Experimental
389
electric scheme: (1) the cell, (2) constant current (made in Russia), (3) voltmeter, (4) ammeter.
source B5-49
The experiments were carried out in a cell with two platinum electrodes with a mobile anode (Fig. 1). While other workers measure in water (see, for example Refs 10 and 11) or in different water solutions,‘*’ we carried out our measurements in air. This method ensures the current passes through the sample and that the sample does not swell in water immediately after the current is switched off. The mobile anode electrode moved during measurements for closing the circuit. The stability of the current source was within 1%. On the whole, the current was independent of the sample’s resistance during contraction. RESULTS
AND DISCUSSION
The hydrogel contraction under application of a current proceeds anisotropically in the total volume (Fig. 2) (the same phenomenon was recorded by Tanaka et al.’ and Osada et aL2). The strongest collapse occurs near the anode where a layer with a low degree of swelling is formed. The next layer, or volume, is more transient, and the area near
0
2
4 Minutes
Fig. 2.
6
8
J 4.2mA
Schematic picture of hydrogel sample contracting in time under the influence of electric current J = 4.3 mA.
390
T. Budtova, I. Suleimenov,
0
5
IO t
I5
S. Frenkel
20
(min)
Fig. 3.
Kinetic dependences of sample weight change under constant electric current: (1) 1.4 mA, (23) 4.3 mA, (4,5) 8.3 mA. The values of the initial weight m, are given by dashed lines.
the cathode corresponds to the less collapsed hydrogel. The inhomogeneities within the gel density lead to large strains in the sample. This causes destruction of the hydrogel in the case of high currents, because of rapid decrease of its volume. Figure 3 shows the timing of relative weight changes of the sample (weight loss, Am), for various magnitudes of the current [la4 mA (curve l), 4.3 mA (curves 2 and 3) and 8.3 mA (curves 4 and 5)]. The value of Am can be obtained with following equation: Am = m, -m(t) where m, is the initial weight of the gel at equilibrated swelling states and m(t) is the weight of the collapsing sample as a function of time. By horizontal dashed lines l’-5’, the initial weight of the studied samples is marked (it is obvious, that when t+ ~0, m(t) decreases and Am + m,). Figure 3 shows that initially for several minutes, Am values increase linearly for each magnitude of the current. Moreover, on initial linear parts of the graphs, the sample weight change Am does not depend on the value of m, for the fixed current magnitude (in Fig. 3, for example, curves 1 and 2 correspond to the same current magnitude of 83 mA but to different values of mo). Besides, the slope of the linear region on curves l-5 becomes steeper as the magnitude of the current increases.
391
Electrokinetics of a polyelectrolyte hydrogel
This indicates that the hydrogel contraction may be induced by electrochemical reactions. Thus, correspondence between obtained data and the Faraday law are discussed as follows. Let us assume that the occurrence of one (or several) electrochemical reactions leads to a suppression of one carboxylate group dissociation. Then, according to the Faraday law, the sample mass change must be proportional to the value of electric charge passing through the hydrogel and must not depend on the initial weight m, and current magnitude: Aml(QM)
- Jt
(1)
where Q = 103 g/g is the initial degree of swelling in water of the hydrogel, M = 94 g/mole is the molar weight of the sodium polyacrylate and Jt is the electric charge transported through the sample. In Fig. 4 the values of Am/(QM) are plotted against Jt (we included the data for current magnitude 7.2 mA in addition to the above mentioned ones given in Fig. 3). For all given values of current strength, the experimental points fall on the same linear segment at the beginning of the curve Am/(QM) vs Jt. This fact means that the beginning of the hydrogel contraction depends only on the magnitude of the electric charge transported through the sample, but not on the initial weight value or/and current magnitude. Thus the hydrogel behavior may be described by the regularity analogous to the Faraday law. Note that the coefficient of proportionality in eqn (1) is not equal to
A I .o
2.0 Jt
Fig. 4. Dependences hydrogel for different
3.0
(Coulomb)
of Am/QM on charge magnitude Jt transported through the values of current: 1.4mA (+), 4.3 mA (0), 7.2 mA (A) and 8.3 mA (A).
392
T. Budtova,
I. Suleimenov,
S. Frenkel
the Faraday constant F, but is (1.5 + 0.1) F. This factor shows that electrochemical reactions which lead to hydrogel collapse may have a more complicated character (they can be multistage) than in simple electrolysis reactions of ions H,O’ and OH- mentioned in previous study? The linear relation between Am and t is disturbed when the contraction is high. A substantial dependence of Am on the initial weight of the sample m, appears when the hydrogel is collapsing for more than S-10 min for the studied values of current (in Fig. 3 see, for example, curves 1 and 2-they correspond to the same current magnitude and to samples with different initial weight). When the hydrogel contraction is high enough the whole network becomes weakly charged. Thus it can be assumed that the linear areas on the curves Am vs t are connected by the fact that the chains forming the gel are charged. The nonlinear phenomena in these coordinates occur when the hydrogel becomes a weakly or noncharged polymer network, where the swelling due to electrostatic forces is not pronounced. For the clear description of contractile behavior in this second region, the dependence of hydrogel degree of collapse (Y= m(t)/m, on Jt may be used. The results are given in Fig. 5. For Jt > 1 all the experimental points fall on the same curve which approaches the limiting value Ly= 0.03. Contractile behavior of hydrogel with small degrees of swelling or low values of the network charge under electric current may be deduced from the electro-osmotic mechanism described in Ref. 6 for weakly charged hydrogels. However, the determination of the actual mechanism of the gel collapse in this area needs further additional experiments. 1.0
8 0.5
OAA AO
l
OA
Osi ~~ A* 0
A~ 0 0
Fig. 5.
4
I
I
I .o
2.0 Jt (Coulomb)
0410
oA
AAA
1.0
Dependence of hydrogel degree of contraction a on charge Jt transported through the sample. The symbols are the same as in Fig. 4.
Electrokinetics of a polyelectrolyte hydrogel
393
CONCLUSIONS As a result of kinetic studies of polyelectrolyte hydrogel behavior under the influence of a constant electric current, two mechanisms of contraction may be assumed. The first one takes place in the area where the gel is still highly swollen. It is connected to electrochemical processes and it obeys the Faraday law with a correction coefficient. The second mechanism occurs in the area of small amounts of hydrogel swelling (i.e. in weakly charged networks). The mechanism of such a destruction can be connected with electro-osmotic release of water.
ACKNOWLEDGMENT The authors are grateful to Dr A. Buyanov Macromolecular Compounds, St. Petersburg, synthesis.
of the Institute of Russia for sample
REFERENCES 1. Tanaka, T., Nishio, I., Sun, S. T. & Ueno-Nishio, S., Science, 218 (1982) 467. 2. Osada, Y., Adu. Polym. Sci., 82 (1987) 1. 3. Osada, Y. & Gong, J., Prog. Polym. Sci., 18 (1993) 187. 4. Osada, Y., Okuzak, H. & Gong, J. P., TRIP, 2(2) (1994) 61. 5. Osada, Y. & Ross-Murphy, S. B., Scientific American, 268 (1993) 42. 6. Makhaeva, E. S., Starodubtsev, S. G. & Khokhlov, A. R., Fundamental problems of modern polymer science, Abstracts, Leningrad, 27-30 November 1990, p. 55. 7. Shiga, T., Hirose, Y., Okada, A. & Kurauchi, T., J. Appl. Pal. Sci., 44,249 (1992). 8. Osada, Y. and Hasebe,
9. DeRossi,
M. Chem. Lett. 1285 (1985). D. E., Chiarell, P., Buzzigoli, G. & Domenci,
C., Trans. Am.
Sot. Artif Intern. Organs., 32 (1986) 157.
10. Osada, Y., Umezawa, K. & Yamauchi, A., Makronzol. Chern., 189 (1987) 597.
11. Kishi, R. & Osada, Y., J. Chem. Sot., Faraday Trans. I, 85(3) (1989) 655. 12. Budtova, T. V., Suleimenov, I. E., Ya. Frenkel, S. & Suleimenov, E. N., Complexnoye Ispol’sovanie Mineral’nogo Syr’ya, 2 (1992) 48. 13. Buyanov, A. L., Revelskaya, L. G., Petropavlovsky, G. A., Lebedeva, M. F., Zakharov, S. K., Petrova, V. A. & Nud’ga, L. A., Vysokomolec. Soed., 31B (1989) 883.
ELSEVIER
Gels and Networks 3 (1995) 387-393 Elsevier Science Limited Printed in Northern Ireland 0967822/95/$09.50
0%6-7822(94)00031-X
Electrokinetics of the Contraction of a Polyelectrolyte Hydrogel Under the Influence of Constant Electric Current T. Budtova,* Institute
of Macromolecular
I. Suleimenov
dz S. Frenkel
Compounds, Russian Academy of Sciences, Bolshoi prosp. 31, 199004 St. Petersburg, Russia
(Received 30 June 1993; revised 10 August 1994; accepted 10 August 1994)
ABSTRACT The kinetics of the contraction of a highly swollen polyelectrolyte hydrogel under the influence of electric current was investigated. From the results obtained, different mechanisms of the contraction may be supposed to predominate for different degrees of swelling. In the case of a rather small contraction, it depends only on the magnitude of electric charge transported through the sample and is described by the Faraday law with the correction coefficient. The second mechanism may be connected with electro-osmotic release of water.
INTRODUCTION Highly swollen polyelectrolyte hydrogels have different unique physical and chemical properties: they can swell in water to thousands of times their volume and change their characteristics drastically under the influence of different external forces. One of the most interesting properties of hydrogels is their capability for reversible contraction under the influence of external electric fields.‘” Many investigations were undertaken about the contractile phenomena in detail, for instance, by Khokhlov et aL6 They attributed the phenomena of water release to an electro-osmotic mechanism, which was considered to be the main reason for the contraction. In Refs 1-5, 7 and 8, voltage alone was taken as the factor influencing the hydrogel contraction. The degree of sample collapse with time was measured as a function of the voltage. It was assumed that the hydrogel contraction under an electric field is due to * Author to whom all correspondence
should be addressed. 387
388
T. Budtova, I. Suleimenov,
S. Frenkel
electrochemical reactions eliminating near electrodes. De Rossi et ~2.~ associated the contraction with a change of pH changes in the fluid, thereby changing the ionic state of the cross-linked poly(viny1 alcohol)poly(acrylic acid) membranes. In some studies,“*” authors found non-linear transport behavior of ions in the gel. In Ref. 11, the explanation of the contractile behavior is given in terms of ion transport of counter-microions in the electric field. Different aspects of practical applications (electrically activated membranes, release control systems for drugs, etc) were discussed in Refs 2-5. The practical significance of studies of electric field influence on hydrogel behavior is determined not only by their utilisation in chemomechanical devices, but also in purifying and enriching technologies as well, where hydrogels can be used in cyclic contractionswelling regimes.l* However, electrokinetic regularities describing hydrogel collapse under the influence of an electric field are still not fully understood. It seems of interest to carry out investigations where the current or electric charge transported though the hydrogel are the main electrical parameters influencing its behavior. According to the Faraday law, these values determine the character and the course of electrochemical reactions. The aim of this paper is to clarify the relations between electric strength and the hydrogel collapse.
EXPERIMENTAL The hydrogel based on sodium polyacrylate was synthesized by radical polymerization of acrylamide in aqueous solution (monomer concentration during polymerization was 20%). The process of the hydrogel synthesis is described in details in Ref. 13. Water soluble allylcarboxymethylcellulose (degree of substitution in respect to ally1 groups was 0.4, its weight fraction in the reaction solution was 0.2%) was used as a cross-linking agent. Afterwards, the samples were immersed in distilled water in order to remove impurities. They were then subjected to hydrolysis in a 0.1 M solution of NaOH. The degree of swelling of the gel in water was 1000 g/g. The synthesized hydrogel has improved elastic and mechanical properties13 because polymer chains of allylcarboxymethylcellulose, the cross-linking agent used in this study, are stiff and play a role of “rigid cross-bars”. The gel can sustain its shape by itself even if the degree of swelling is very high. Thus it was convenient to use these samples for the study of its contractile behavior under the electric current.
Electrokinetics of a polyelectrolyte hydrogel
Fig. 1.
Experimental
389
electric scheme: (1) the cell, (2) constant current (made in Russia), (3) voltmeter, (4) ammeter.
source B5-49
The experiments were carried out in a cell with two platinum electrodes with a mobile anode (Fig. 1). While other workers measure in water (see, for example Refs 10 and 11) or in different water solutions,‘*’ we carried out our measurements in air. This method ensures the current passes through the sample and that the sample does not swell in water immediately after the current is switched off. The mobile anode electrode moved during measurements for closing the circuit. The stability of the current source was within 1%. On the whole, the current was independent of the sample’s resistance during contraction. RESULTS
AND DISCUSSION
The hydrogel contraction under application of a current proceeds anisotropically in the total volume (Fig. 2) (the same phenomenon was recorded by Tanaka et al.’ and Osada et aL2). The strongest collapse occurs near the anode where a layer with a low degree of swelling is formed. The next layer, or volume, is more transient, and the area near
0
2
4 Minutes
Fig. 2.
6
8
J 4.2mA
Schematic picture of hydrogel sample contracting in time under the influence of electric current J = 4.3 mA.
390
T. Budtova, I. Suleimenov,
0
5
IO t
I5
S. Frenkel
20
(min)
Fig. 3.
Kinetic dependences of sample weight change under constant electric current: (1) 1.4 mA, (23) 4.3 mA, (4,5) 8.3 mA. The values of the initial weight m, are given by dashed lines.
the cathode corresponds to the less collapsed hydrogel. The inhomogeneities within the gel density lead to large strains in the sample. This causes destruction of the hydrogel in the case of high currents, because of rapid decrease of its volume. Figure 3 shows the timing of relative weight changes of the sample (weight loss, Am), for various magnitudes of the current [la4 mA (curve l), 4.3 mA (curves 2 and 3) and 8.3 mA (curves 4 and 5)]. The value of Am can be obtained with following equation: Am = m, -m(t) where m, is the initial weight of the gel at equilibrated swelling states and m(t) is the weight of the collapsing sample as a function of time. By horizontal dashed lines l’-5’, the initial weight of the studied samples is marked (it is obvious, that when t+ ~0, m(t) decreases and Am + m,). Figure 3 shows that initially for several minutes, Am values increase linearly for each magnitude of the current. Moreover, on initial linear parts of the graphs, the sample weight change Am does not depend on the value of m, for the fixed current magnitude (in Fig. 3, for example, curves 1 and 2 correspond to the same current magnitude of 83 mA but to different values of mo). Besides, the slope of the linear region on curves l-5 becomes steeper as the magnitude of the current increases.
391
Electrokinetics of a polyelectrolyte hydrogel
This indicates that the hydrogel contraction may be induced by electrochemical reactions. Thus, correspondence between obtained data and the Faraday law are discussed as follows. Let us assume that the occurrence of one (or several) electrochemical reactions leads to a suppression of one carboxylate group dissociation. Then, according to the Faraday law, the sample mass change must be proportional to the value of electric charge passing through the hydrogel and must not depend on the initial weight m, and current magnitude: Aml(QM)
- Jt
(1)
where Q = 103 g/g is the initial degree of swelling in water of the hydrogel, M = 94 g/mole is the molar weight of the sodium polyacrylate and Jt is the electric charge transported through the sample. In Fig. 4 the values of Am/(QM) are plotted against Jt (we included the data for current magnitude 7.2 mA in addition to the above mentioned ones given in Fig. 3). For all given values of current strength, the experimental points fall on the same linear segment at the beginning of the curve Am/(QM) vs Jt. This fact means that the beginning of the hydrogel contraction depends only on the magnitude of the electric charge transported through the sample, but not on the initial weight value or/and current magnitude. Thus the hydrogel behavior may be described by the regularity analogous to the Faraday law. Note that the coefficient of proportionality in eqn (1) is not equal to
A I .o
2.0 Jt
Fig. 4. Dependences hydrogel for different
3.0
(Coulomb)
of Am/QM on charge magnitude Jt transported through the values of current: 1.4mA (+), 4.3 mA (0), 7.2 mA (A) and 8.3 mA (A).
392
T. Budtova,
I. Suleimenov,
S. Frenkel
the Faraday constant F, but is (1.5 + 0.1) F. This factor shows that electrochemical reactions which lead to hydrogel collapse may have a more complicated character (they can be multistage) than in simple electrolysis reactions of ions H,O’ and OH- mentioned in previous study? The linear relation between Am and t is disturbed when the contraction is high. A substantial dependence of Am on the initial weight of the sample m, appears when the hydrogel is collapsing for more than S-10 min for the studied values of current (in Fig. 3 see, for example, curves 1 and 2-they correspond to the same current magnitude and to samples with different initial weight). When the hydrogel contraction is high enough the whole network becomes weakly charged. Thus it can be assumed that the linear areas on the curves Am vs t are connected by the fact that the chains forming the gel are charged. The nonlinear phenomena in these coordinates occur when the hydrogel becomes a weakly or noncharged polymer network, where the swelling due to electrostatic forces is not pronounced. For the clear description of contractile behavior in this second region, the dependence of hydrogel degree of collapse (Y= m(t)/m, on Jt may be used. The results are given in Fig. 5. For Jt > 1 all the experimental points fall on the same curve which approaches the limiting value Ly= 0.03. Contractile behavior of hydrogel with small degrees of swelling or low values of the network charge under electric current may be deduced from the electro-osmotic mechanism described in Ref. 6 for weakly charged hydrogels. However, the determination of the actual mechanism of the gel collapse in this area needs further additional experiments. 1.0
8 0.5
OAA AO
l
OA
Osi ~~ A* 0
A~ 0 0
Fig. 5.
4
I
I
I .o
2.0 Jt (Coulomb)
0410
oA
AAA
1.0
Dependence of hydrogel degree of contraction a on charge Jt transported through the sample. The symbols are the same as in Fig. 4.
Electrokinetics of a polyelectrolyte hydrogel
393
CONCLUSIONS As a result of kinetic studies of polyelectrolyte hydrogel behavior under the influence of a constant electric current, two mechanisms of contraction may be assumed. The first one takes place in the area where the gel is still highly swollen. It is connected to electrochemical processes and it obeys the Faraday law with a correction coefficient. The second mechanism occurs in the area of small amounts of hydrogel swelling (i.e. in weakly charged networks). The mechanism of such a destruction can be connected with electro-osmotic release of water.
ACKNOWLEDGMENT The authors are grateful to Dr A. Buyanov Macromolecular Compounds, St. Petersburg, synthesis.
of the Institute of Russia for sample
REFERENCES 1. Tanaka, T., Nishio, I., Sun, S. T. & Ueno-Nishio, S., Science, 218 (1982) 467. 2. Osada, Y., Adu. Polym. Sci., 82 (1987) 1. 3. Osada, Y. & Gong, J., Prog. Polym. Sci., 18 (1993) 187. 4. Osada, Y., Okuzak, H. & Gong, J. P., TRIP, 2(2) (1994) 61. 5. Osada, Y. & Ross-Murphy, S. B., Scientific American, 268 (1993) 42. 6. Makhaeva, E. S., Starodubtsev, S. G. & Khokhlov, A. R., Fundamental problems of modern polymer science, Abstracts, Leningrad, 27-30 November 1990, p. 55. 7. Shiga, T., Hirose, Y., Okada, A. & Kurauchi, T., J. Appl. Pal. Sci., 44,249 (1992). 8. Osada, Y. and Hasebe,
9. DeRossi,
M. Chem. Lett. 1285 (1985). D. E., Chiarell, P., Buzzigoli, G. & Domenci,
C., Trans. Am.
Sot. Artif Intern. Organs., 32 (1986) 157.
10. Osada, Y., Umezawa, K. & Yamauchi, A., Makronzol. Chern., 189 (1987) 597.
11. Kishi, R. & Osada, Y., J. Chem. Sot., Faraday Trans. I, 85(3) (1989) 655. 12. Budtova, T. V., Suleimenov, I. E., Ya. Frenkel, S. & Suleimenov, E. N., Complexnoye Ispol’sovanie Mineral’nogo Syr’ya, 2 (1992) 48. 13. Buyanov, A. L., Revelskaya, L. G., Petropavlovsky, G. A., Lebedeva, M. F., Zakharov, S. K., Petrova, V. A. & Nud’ga, L. A., Vysokomolec. Soed., 31B (1989) 883.