Polyelectrolyte Gel Transitions: Experimental ... - ACS Publications

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Langmuir 2005, 21, 10004-10015

Polyelectrolyte Gel Transitions: Experimental Aspects of Charge Inhomogeneity in the Swelling and Segmental Attractions in the Shrinking† Etsuo Kokufuta* Graduate School of Life and Environmental Sciences and Institute of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan Received February 28, 2005. In Final Form: May 31, 2005 This paper aims to provide a systematic discussion based on our experimental results both previously published and unpublished, to promote better understanding of volume-phase transitions in polyelectrolyte gels. Special attention was paid to the distribution of network charges as well as to the attractive interaction among polymer segments. From looking at how these effects appear in the swelling curves, an exploration of the nature of polyelectrolyte gel transitions was attempted . Two sorts of polyelectrolyte gels, temperatureresponsive ionic gels based on N-isopropylacrylamide (NIPA) and cationic poly(ethyleneimine) (PEI) gels, were mainly employed with various modifications. The charge inhomogeneity within the gel phase was created by surfactant binding, immobilized enzyme reaction and physical entrapment of polyions. The attractive interactions holding the gel in a collapsed state were studied in comparison with phase separations of the corresponding linear polyelectrolyte. The main conclusions are summarized as follows: (i) The charge inhomogeneity exhibits a large influence on the volume transition in ionic gels. (ii) Hydrogen bonding and hydrophobic association, other than electrostatic attraction, can be considered to play an important role in the segmental association. (iii) Stably associated segments via one or more of these attractive interactions causes a large hysteresis in the swelling process, in which the repulsive interaction among the fixed charges on the network is dominant as shown in the Katchalsky’s model. (iv) A distribution of “neutral but hydrophilic” moieties (e.g., ion pair or salt-linkage formed between the opposite charged groups) within the gel shows a marked effect on the temperature-induced volume collapse, the aspect of which is similar to that observed in the gels with a charge inhomogeneity.

I. Introduction In the formation of gels through the cross-linking of polymers, various properties of individual polymers become visible on a macroscopic scale. Among these, the volume phase transition (VPT) in gels is one of the most important phenomena that allow us to explore the principles underlying the molecular interactions in synthetic and biological polymers. From a historical viewpoint, the first paper on a VPT was published in 1968 by Dusek and Patterson,1 who showed theoretically that a transition in swollen polymer networks is induced by intramolecular condensation. After a decade of this publication, Tanaka2 demonstrated that an abrupt volume collapse occurs in poly(acrylamide) (PAAm) gels in an acetone-water system upon lowering the temperature or upon increasing the acetone concentration. When limiting to the VPT in polyelectrolyte gels, two experiments3,4 by Tanaka and co-workers would be of historical importance. One was carried out using partially hydrolyzed PAAm gels3 and demonstrated that an increase in the charge density causes an increase of gel volume at the transition as well as a change of the transition threshold. The other using ionic gels consisting of N-isopropylacrylamide (NIPA) and acrylic acid (AAc)4 showed that the transition temperature is increased with †

Part of the Bob Rowell Festschrift special issue. * To whom correspondence should be addressed. Fax: 81-298-53-4605. E-mail: [email protected]. (1) Dusek, K.; Patterson, D. J. Polym. Sci., A-2 1968, 6, 1209. (2) Tanaka, T. Phys. Rev Lett. 1978, 40, 820. (3) Tanaka, T.; Fillmore, D. J.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636. (4) Hirotsu, S.; Hirokawa, Y.; Tanaka T. J. Chem. Phys. 1987, 87, 1392.

increasing the charge density (i.e., moles of carboxylate ions per pregel solution). The ionic gels based on NIPA are now often referred to as the “temperature-sensitive” or “temperature-responsive” polyelectrolyte gel and extensively employed in many studies. After the above1-4 and several other pioneering works5-12 in the early 1980s, numerous studies have focused on the VPT in gels of synthetic and natural polymers. The nature of VPT has become somewhat more transparent to us. For example, it has been well-known that almost all of the ionic gels undergo a VPT in response to small variations in the solution conditions surrounding the gel. Variables that trigger the transition include pH, concentrations of inorganic and organic chemicals, electric fields, and light intensities, as well as temperature when the gels contain temperature-responsive comonomers such as NIPA. All of the gels have the potential of entrapping and holding macromolecular compounds within their network. We have used this capability in combination with a VPT in polyelectrolyte gels and created a novel immobilized enzyme system that allows the energy arising from biochemical changes to be converted into mechanical work through the VPT,13-18 i.e., “biochemo-mechanical system”. To apply the gels to such a technological field, it was needed (5) Ilavsky, M. Macromolecules 1982, 15, 782. (6) Hrouz, J.; Ilavsky, M.; Ulbrich, K.; Kopecek, J. Eur. Polym. J. 1981, 17, 361. (7) Ilavsky, M. Macromolecules 1982, 15, 782. (8) Ilavsky, M.; Hrouz, J. Polym. Bull. 1982, 8, 387. (9) Tanaka, T.; Nishio, I.; Sun, S.-T.; Ueno-Nishio, S. Science 1982, 218, 467. (10) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (11) Irie, M.; Kungwatchakum, D. Makromol. Chem., Rapid Commun. 1984, 5, 829. (12) Osada, Y.; Hasebe, M. Chem. Lett. 1985, 1285.

10.1021/la050530e CCC: $30.25 © 2005 American Chemical Society Published on Web 07/16/2005

Polyelectrolyte Gel Transitions

to establish a simple but universal model for understanding the VPT on molecular grounds. We have therefore tried to account for the VPT by hypothesizing a balance between the repulsive and attractive forces within the cross-linked polymers in the networks which arise from a combination of four intermolecular forces:19,20 ionic, hydrophobic, van der Waals, and hydrogen bonding. When a repulsive force, usually electrostatic in nature, overcomes an attractive force such as hydrogen bonding or the hydrophobic interaction between the network chains, gel volume should increase discontinuously in some cases and continuously in others. The variables triggering the transition influence these intermolecular forces and thereby the balanced state of the attractive and repulsive forces. The concept of this sort is generally accepted in the biochemical field to understand the structuring (or selfassembling) of biomolecules such as proteins.21 However, its qualitative nature causes difficulty in penetrating into the field in which many efforts have been devoted to the understanding of VPT with mathematical means. Almost all of the studies in this field have employed the “concept” of osmotic pressure arising from mobile counterions as the chief driving force for swelling of ionic gels, although the expressions of the other three contributionssrubber elasticity, interactions among polymer segments and solvents, and mixing entropysto the free energy have varied from theory to theory (e.g., see refs 20 and 22). Taking these into account, it is quite natural to attempt the establishment of good experimental systems that allow a simple but a big question to be answered about what is the nature of swelling forces in ionic gels. We have therefore paid special attention to polyelectrolyte gels in which a charge inhomogeneity is created by physical, chemical, and biochemical ways. Also, our attention was paid to a resemblance between linear and cross-linked polyions that may provide a key for the understanding of the main attractive force in the gel collapse. The author aims in this paper to recall the experimental results of our previous studies both published and unpublished and to look at the basic aspects of polyelectrolyte gel transitions on molecular grounds. Through this, it is intended to stimulate further developments in the research of polyelectrolyte gels, in particular of gel-based “smart” or “intelligent” materials that usually undergo a VPT in response to a small change in their ionization states. II. Results and Discussion II.1. Effects of Charge Distributions. Swelling Forces in Polyelectrolyte Gels. Prior to the great development in the study of ionic gel transitions in 1990s, the solution properties of polyelectrolytes had been studied (13) Kokufuta, E.; Zhang, Y.-Q.; Tanaka, T. Nature 1991, 351, 302. Note that the author name in this reference should have read Kokufuta, E. not Kokufata, E. (14) Kokufuta, E.; Tanaka, T. Macromolecules 1991, 24, 1605. (15) Kokufuta, E. Prog. Polym. Sci. 1992, 17, 647. (16) Kokufuta, E. Adv. Polym. Sci. 1993, 110, 159. (17) Kokufuta, E.; Matsukawa, K.; Ebihara, T.; Matsuda, K. In Macroion Characterization: From Dilute Solutions to Complex Fluids; Schmitz, K. S., Ed.; American Chemical Society: Washington, DC, 1994; Chapter 39. (18) Ogawa, K.; Wang, B.; Kokufuta, E. Langmuir 2001, 17, 4704. (19) Ilmain, F.; Tanaka, T.; Kokufuta, E. Nature 1991, 349, 400. (20) Kokufuta, E. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Marcel Dekker: New York, 2000; Chapter 17. (21) (a) Anfinsen, C. B. The Molecular Basis of Evolution, John Wiley & Sons: New York, 1959; p 102. (b) Conn, E. E.; Stumpf, P. K. Outlines of Biochemistry, 4th ed.; John Wiley & Sons: New York, 1976; pp 9899. (22) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163.

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in detail both theoretically and experimentally.23-25 For example, the electrostatic free energy of polyelectrolyte solutions was studied by Hermans and Overbeek,26 Kimball, Cutler and Samelson,27 and Katchalsky et al.28 using different molecular models and methods of calculation. Katchalsky and Michaeli29 then published a theory that allows the behavior of polyelectrolyte gels in salt solutions to be described, through a combination of their treatments28 for the electrostatic free energy (as the contribution due to the fixed charges of the polymer) with (well-known) Flory’s equation30 for the contractile free energy of the ‘neutral’ polymer network. Flory also published his theory31 for polyelectrolyte gels by introduction of the ionic term into his equation for neutral gels; thus, we have had two theories for understanding swelling behaviors of polyelectrolyte gels. In both, however, there is a big difference between the swelling forces arising from the fixed charges at the molecular level. Flory assumed that (i) the equilibrium between the swollen ionic gel and its surroundings closely resembles Donnan membrane equilibria, (ii) the polymer acts as its own membrane which prevents diffusion of the charged substituents inside the gel into the outer solution, and hence (iii) the swelling force resulting from the presence of the fixed charges may be identified with the swelling pressure, or net osmotic pressure, across the semipermeable membrane in a typical Donnan equilibrium. On the other hand, Katchalsky considered the electrostatic free energy (∂Fel/∂V) as the swelling force to result “directly” from the presence of the fixed charges. Then, he assumed that ∂Fel/∂V is the sum of (i) the interaction between the small ions and the polyelectrolyte, which favors deswelling, and (ii) the mechanochemical stretch due to the repulsive forces among the fixed charges on the polyelectrolyte network, which favors the swelling of the gel. We have used the Flory’s model in the design of a biochemo-mechanical system composed of neutral NIPA gels, into which a sugar-binding protein (concanavalin A; Con A) was immobilized by physical gel-entrapping method.13 Then, we predicted that the binding of an ionic oligosaccharide (dextran sulfate sodium; DSS) to the immobilized protein will generate the osmotic pressure owing to sodium ions as the counterion, causing the gel to swell as well as the transition temperature (Tv; ∼32 °C for neutral NIPA gels) to raise. Both predictions were confirmed by the experiments, but the magnitudes in the swelling degree and the Tv change were far smaller than those observed in the NIPA-based polyelectrolyte gels having the same amount of COONa groups. Con A is a globular protein consisting of four subunits, each of which has dimensions 4.4 × 4.0 × 3.9 nm. The immobilized Con A would be surrounded by (or wrapped around) the NIPA network; thus the binding to it of the DSS anions looks to generate many of a negatively charged (23) Rice, S. A.; Nagasawa, M.; Morawetz, H. Polyelectrolyte Solutions: A Theoretical Introduction; Academic Press: New York, 1961. (24) Tanford, C. Physical Chemistry of Macromolecules; John Wiley & Sons: New York, 1961; Chapter 7. (25) Morawetz, H. Macromolecules in Solutions; John Wiley-Interscience Publishers: New York, 1965; pp 348-356. (26) Hermans, J. J.; Overbeek, J. Th. Rec. Trav. Chim. 1948, 67, 761. (27) Kimball, G. E.; Cutler, N.; Samelson, H. J. Phys. Chem. 1952, 56, 57. (28) (a) Kuhn, W.; Ku¨nzle, O.; Katchalsky, A. Helv. Chmi. Acta 1948, 31, 1994. (b) Katchalsky, A.; Ku¨nzle, O.; Kuhn, W. J. Polym. Sci. 1950, 5, 283. (c) Katchalsky, A. J. Polym. Sci. 1951, 7, 393; 1953, 11, 409. (d) Katchalsky, A.; Lifson, S.; Eisenberg, H. J. Polym. Sci. 1951, 7, 571. (29) Katchalsky, A.; Michaeli, I. J. Polym. Sci. 1955, 15, 69. (30) (a) Flory, P. J.; Rehner, J. J. Chem. Phys. 1943, 11, 512. (b) Flory, P. J. Chem. Revs. 1944, 35, 51. (31) Flory, P. J. Principle of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953; pp 576-593.

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local domain of several nanometers in size within the gel. Consequently, it is reasonable to assume that the repulsive electrostatic interaction among the charged domains expands the network to increase the overall gel volume, the magnitude of which is smaller than those of ionic gels composed of AAc-copolymerized NIPA chains. Another possible assumption is the condensation of sodium ions within the charged domain, through which the osmotic pressure could be reduced. Nevertheless, it is difficult to choose either of these assumptions, as well as to say simply that both are the case. As a result, it becomes important to learn whether the swelling force results mainly from the osmotic pressure due to counterions or from repulsive forces among the fixed charges. This would be a general question not only in the development in gel-based technologies, but also in the understanding of the nature of ionic gels. Swelling Behavior of Surfactant-Bound NIPA Gels. It has been known that ionic surfactants such as sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (NaDBS) are bound to neutral NIPA polymers and gels, by which these are converted into a polyelectrolyte (for example, see refs 22 and 32-34). To the polymer, the binding takes place at a critical aggregation concentration (CAC). This causes an elevation of lower critical solution temperature (LCST) as a result of “electrostatic repulsion” between the charges of the polymer-bound surfactant micelles. Similarly, an increase in Tv as well as in the swelling ratio at Tv can be observed in surfactant-bound NIPA gel systems. A few studies35-37 have attempted to describe this VPT behavior in connection with the formation of surfactant micelles in the gel. As can be seen from Figure 1, however, the quantitative analysis for the amount of bound surfactants, followed by a careful comparison of the surfactant-bound NIPA gel and the AAccopolymerized NIPA gel, both of which have almost the same charge density (0.16 ( 0.02), has raised the questions as to38,39 (i) why the gel volumes of both surfactant-bound gels are much smaller than that of the copolymer gel and (ii) why the Tv of the former is higher than that of the latter. Although a rise in temperature causes a slight increase in the network charges that can be accurately determined from surfactant uptake by the gel,38 the differences between both gels, mentioned in (i) and (ii), are not to be found in the common knowledge of ionic gel transitions. In addition, questions (i) and (ii) are essentially the same as that described in the above section; therefore, the answer would help to clarify the nature of swelling forces in ionic gels. To reply to questions (i) and (ii), we now assume that surfactant molecules bind only to the polymer networks within a layer of the gel near the surface, but not to all of the networks in the body of the gel. The reason is that (32) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665. (33) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 1030. (34) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418. (35) Zhang, Y.-Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. Note that Tv (∼85 °C) of an ionic NIPA gel with sodium acrylate (NaAAc; ∼9 mol %) reported in this reference is strangely higher than those (20 mM. The normalization of each equilibrium diameter (d) was performed with the inner diameter (d0) of the capillary used in the gel preparation. This drawing is originally produced using swelling data saved in computers.

Figure 2. Schematic representation for a cylindrical gel with inhomogeneously bound surfactant molecules. A horizontal sectional view of a cylindrical gel after attainment of a saturated level of surfactant binding is illustrated together with a concentration profile of bound surfactant: a, part with a saturated level of bound surfactants; b, part in which the surfactant concentration gradually decreases toward the center; and c, part in which there is no bound surfactant (i.e., corresponding to neutral gel). This drawing is reproduced from ref 39 with several modifications. Note that a similar model has been applied to understand size changes of NIPA-based ionic microgels (or nanogels)40 into which charges were introduced via the surfactant binding (see ref 40a) and the copolymerization of vinylacetic acid (see ref 40b).

charges of the surfactant-bound layer seem to hinder the further diffusion into internal parts of the gel of the surfactant molecules with charges of the same sign. Thus, a NIPA gel with bound surfactants seems to resemble a sort of “composite” polyelectrolyte gel consisting of crosslinked polymer chains with different amounts of ionizable groups, as shown in Figure 2. Since the Tv (∼32 °C) of the pure NIPA gel is lower than those of three ionic NIPA gels (curves b-d in Figure 1), part c in Figure 2 should collapse at temperatures >32 °C, but parts a and b remain in the swollen state. We usually define Tv as the temperature at which a gel discontinuously and completely collapses; therefore, the experimentally determined Tv of a surfactant-bound gel is primarily governed by the degree of


Figure 3. Position dependence of transmittance (PDT) for cylindrical wet gel disk (a) and dry gel disk (b) with a saturated level of NaDBS binding. The diameter (d0) before the surfactant binding was 2 mm each for both gels. Wavelengths used for the measurements: 261 nm (open circles) and 400 nm (filled circles). This drawing was reproduced from ref 39 with several modifications.

ionization in part a, but not in parts b and c. However, the swelling ratio as the overall gel volume is strongly affected by changes in the degree of ionization in part b. Even if part a is maintained at the same level of ionization in both the SDS and NaDBS-bound gels, this is a case that the level in part b for the former is higher than that for the latter. Then, we can see the features in Figure 1; that is, (i) the SDS-bound gel exhibits a high swelling degree compared with the NaDBS-bound gel and (ii) the total change in the gel volume at Tv for both surfactant-bound gels is smaller than that for the copolymer gel with a homogeneous charge distribution as a result of random copolymerization of AAc monomers. The measurement of the spatial distribution of surfactant concentration within the gel phase should provide direct evidence for the local surfactant binding as assumed above. NaDBS is an appropriate surfactant for this purpose because it carries a phenyl group which exhibits an absorption band in the UV range. Figure 3 shows typical examples of the position dependence of transmittance (PDT) for the sample gel disks with bound NaDBS. The scanning was performed from the right-hand to the lefthand end of each gel disk. Two wavelengths, 261 and 400 nm, were used to measure the PDT. Since NaDBS has no absorption at 400 nm, the transmittance at this wavelength varies depending on the sample thickness. Therefore, the PDT at 400 nm should be used as a control. PDTs (a) and (b) are the results obtained using wet and dry gels with an initial diameter (d0) of 2 mm, respectively. The wet gel sample was fully swollen in pure water prior to contact with the NaDBS solution, whereas the dry gel was directly treated with the surfactant solution without swelling procedure (for more details, see the Experimental Section and ref 39). In PDT(a), a rapid decrease in transmittance at both ends of each sample was observed when using the wavelength at 261 nm. However, the transmittance increased gradually and became constant when the scanning point approached the central core of the gel disk. On the other hand, the transmittance at 400 nm was almost constant, independent of the position. Therefore, it is evident that the surfactant molecules bound strongly only around the gel walls. This conclusion can be supported by PDT(b) for the dry gel in which we did not observe specific position-dependent changes in transmittance at

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261 nm. In the case of the dry gel, the surfactant solution permeated throughout the gel phase, like the taking up of water by a sponge, allowing the surfactant molecules to bind to the network without distribution, at least within the resolution (30 µm) of our microscopic spectroscopy, the value of which is dependent on the size of the beam going through the gel disks. Thus, a marked difference can be observed in the diameters of the surfactant-bound wet and dry gels, i.e., the distance between the right and left-hand ends which is larger for the dry gel than for the wet gel, although their diameters in the preparation were identical with each other. As a result, it has become apparent that the VPT in surfactant-bound ionic NIPA gels is obviously affected by the inhomogeneously distributed charges. The same conclusion has been obtained from experiments with nanogels of NIPA homopolymer to which NaDBS had been bound (see ref 40a). If the osmotic pressure due to mobile counterions plays an important role in the VPT of polyelectrolyte gels, one would expect that there is little influence of the inhomogeneity of charge distributions within the gel. Thus, the results presented in this section may provide a key to consider whether the VPT of polyelectrolyte gels is governed by osmotic pressure. In my opinion, it seems that the nature of NIPA gels with ionic surfactants is explained well in terms of the Katchalsky’s model not based on the osmotic pressure arising from counterions. Nevertheless, one might argue that condensation of counterions would reduce the osmotic pressure, when the polymer-bound surfactant micelles act as a highly charged object. This allows us to understand why the volume of the surfactant-bound gels is much smaller than that of the corresponding copolymer gels but not to account for the fact that the Tv of the former is much higher than that (>80 °C) of the latter (see Figure 1). Another argument may be raised that the results contain errors due to nonequilibrium effects, in particular on the difference between the dry and wet gels. Our PDT data were obtained with the gel samples which had been incubated in the surfactant solution for 10 days. In addition, the same swelling curves as those of b and c in Figure 1 can be obtained even when the gels were incubated for a month (data not presented). Nevertheless, we cannot “escape” from the criticism that the observations should be performed over months, more strictly over years. Effect of Enzymatically Formed pH Gradients within Polyelectrolyte Gels. Taking the above into account, we have tried to develop other techniques for establishing charge inhomogeneities in the gel. For this purpose, we studied an enzymatic approach for creating a pH gradient within the gel phase.41 Temperature-responsive cationic gels with immobilized urease, in the shape of a small cylinder with diameter 290-640 µm, were then synthesized via gelation of an aqueous monomer solution containing NIPA, 1-vinylimidazole (VI; cationic monomer), Bis, and the enzyme. Diameters at different positions of the cylinder were microscopically measured in a cell through which a substrate solution (i.e., 1 mM urea solution) at pH 4 and at 35 °C was passed at a constant flow rate; therefore, both substrate concentration and pH at the gel surface were maintained at a constant level throughout the experimental period. Figure 4 shows typical time courses of enzymatically and chemically induced diameter changes of a cylindrical (40) (a) Suzuki, H.; Kokufuta, E. Colloids Surf. A 1999, 147, 233. (b) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544. (41) (a) Ogawa, K.; Kokufuta, E. Langmuir 2002, 18, 5561. (b) Ogawa, K.; Kokufuta, E. Macromol. Symp. 2004, 207, 241.

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Figure 4. Time-dependent changes in diameters at the top (d1), center (d2), and bottom (d3) of cylindrical gel with immobilized urease: (a) Enzymatic shrinking, which was made through quick replacement of the outer medium (5 mM maleate buffer not containing the substrate; pH 4) by the substrate solution (5 mM maleate buffer containing 1 mM urea as the substrate; pH 4); (b) Chemical shrinking made through quick replacement of the outer medium (5 mM maleate buffer; pH 4) by another medium (5 mM maleate buffer; pH 6). The same gel was used in measurements (a) and (b); therefore, P-1 denotes the gel before the replacements of the outer medium by the substrate solution or by the substrate-free buffer. P-2 is the gel in the substrate solution after 60 min, and P-3 the gel in the substrate-free medium at pH 6 after 60 min. The measurements were carried out at 35 °C in a flow (1 mL/min) of the outer solutions. This drawing was reproduced from ref 41a with a minor modification.

gel with immobilized urease. The enzymatic and chemical means of shrinking the gel were respectively based on the use of a substrate-containing buffer (pH 4) and of a substrate-free buffer (pH 6 with NH4OH). We measured diameters at the top (d1), center (d2), and bottom (d3) of the cylindrical gel. In the chemical gel collapse, the diameters at these three positions are identical with one another, particularly in an equilibrium state at which the swelling ratio is independent of time. In the enzymatic process (i.e., hydrolysis of urea by immobilized urease), there is no difference in diameters d1 at the top and d3 at the bottom, but these are larger than diameter d2 at the center after the establishment of swelling equilibrium. Thus, the enzymatic shrinking allows the gel to change its shape of a “real” cylinder to that of a “distorted” cylinder with the top and bottom still in a swollen state. The present cationic gels undergo a shrinking change when increasing pH, because the network charges are eliminated via the deprotonation of the imidazole ions: tNH + + OH- f tN + H2O. The immobilized enzyme reaction rises pH within the gel phase; therefore, the gel shrinks. Such an enzymatic increase in pH occurs by diffusion of the substrate from the outer solution, but at the same time, the reaction products (NH4+ plus CO32-) diffuse into the outer solution. These conditions permit a

Kokufuta

pH gradient between the surface and the center of the gel to be generated. Such a pH gradient varies depending on time but does not disappear even in a steady state. In contrast to the enzymatic process, the chemically induced shrinking of the gel takes place only through a pH difference between the gel (pH 4) and the outer medium (pH 6); thus, in an equilibrium state, there is no pH gradient. Consequently, one may say that the position dependence of the swelling ratio which has been observed in the enzymatic process should be due to a pH gradient within the gel phase. Then, it is natural to conclude that the present results cannot be explained by considering osmotic pressure arising from mobile counterions within the gel phase surrounded by the Donnan potential barrier. To support this conclusion, mathematical simulations have been conducted with a reaction-diffusion model.41 The central part of the gel was then taken as an infinitely long circular cylinder. There was a good agreement between the results of simulations and experiments after equilibrium swelling was reached. Effect of AAc Distribution in NIPA Networks. Now it becomes of importance to study at the molecular level the effect of charge inhomogeneity on the VPT. For this purpose, we synthesized four ionic gels composed of NIPA and AAc residues (NIPA:AAc ratio of the feed ∼10:1 by moles the monomers or repeating units) using the following approaches:42 (i) for gel I, the redox polymerization of an aqueous solution containing NIPA, AAc, and Bis which can be initiated by a pair of ammonium persulfate (APS, initiator) and N,N,N′,N′-tetramethylethylenediamine (TMED; accelerator); (ii) for gel II, the physical entrapment of poly(AAc) by an Bis-cross-linked NIPA gel, the performance of which is based on the same method employed in (i) except for the use of poly(AAc) instead of the AAc monomer; (iii) for gel III, the gelation of an aqueous solution containing poly(NIPA) and poly(AAc) by γ-rays from 60Co under conditions where no complexation occurs between poly(NIPA) and poly(AAc); and (iv) for gel IV whose AAc distribution may be expected to be similar to that of gel I, the γ-ray irradiation of an aqueous solution of a copolymer of NIPA and AAc. Figure 5 shows the temperature dependence of the normalized equilibrium diameters (d/d0) at pHs 3 and 10 for gels I and II as the example. At pH 10 at which the COOH groups of the AAc units are fully ionized (COOH + NaOH f COO- + Na+ + H2O), a marked difference due to the AAc distribution was observed between gel I and gel II. The localized COO- ions in gel II are not effective in preventing the thermally induced gel collapse at temperatures above Tv. This clearly indicates the effect of microscopically created charge inhomogeneity on the VPT. Since there is little disagreement between the transition temperatures for the polymers and gels of NIPA, a similar molecular mechanism may be predicted in the phase separation and the volume transition. For gel II composed of ca. 90% of Bis-cross-linked “pure” NIPA chains, it is predictable that its VPT behavior is not so different from those of poly(NIPA) and NIPA gel, even at pH 10 where the incorporated poly(AAc) chains (ca. 10% in unit mole base) are fully ionized. Indeed, at pH 10 gel II underwent a VPT at a temperature near to the Tv (∼32 °C) of the NIPA polymer and gel. The same result has been obtained by a comparison of gels III and IV (see ref 42). In addition, when the swelling curves of gel II were studied at pHs 3 and 10 as a function (42) Kokufuta, E.; Wang, B.; Yoshida, R.; Khokhlov, A. R.; Hirata, M. Macromolecules 1998, 31, 6878.


Figure 5. Temperature dependence of normalized equilibrium diameters (d/d0) at pHs 3 and 10 for gels I (a) and II (b). Poly(AAc) with Mw ) 450 000 was used for the preparation of gel II. This drawing was reproduced from ref 42 with several modifications.

of the molecular weight of the entrapped poly(AAc), the following results have been obtained: (i) The increase in the molecular weight at pH 10 brought about an increase in the gel volume over the temperature range of 2550 °C; and (ii) at pH 3 and at temperatures < Tv, the gel volume decreased with increasing molecular weight of the entrapped poly(AAc). Therefore, it has been clear at the molecular level that the charge distribution has a marked effect on the swelling of NIPA-based polyelectrolyte gels. II.2. Attractive Chain Interactions. Conformation Changes of Polyelectrolytes. Flexible polyacids and polybases undergo a conformation change in response to pH and ionic strength, because both factors directly affect the “net charge” of a polyion via the change in the ionization degree as well as in the interaction among the fixed charges, counterions and byions. For example, when increasing the pH of an aqueous poly(AAc) solution containing moderate concentrations of small salts such as NaCl, intrinsic viscosity ([η]) as an indication of polyion expansion increases gradually, passes through a plateau, and decreases slightly. Such a conformation change can be observed in many polyelectrolytes; however, there are a very few examples from which we may learn an abrupt change in the conformation in response to a small change in pH and salt concentration (see next section). This seems to be the main reason that the VPT in polyelectrolyte gels was little compared with a conformation change of the corresponding polyions, in particular in experimental studies. Indeed, Katchalsky and Michaeli29 made no mention of VPT in their experiments, in which the swelling of a polyelectrolyte gel obtained from methacrylic acid (MAAc) by lightly cross-linking with divinylbenzene was studied as a function of the degree of ionization and the concentration of salts (LiCl, NaCl, and KCl). Although a conformational transition of poly(MAAc) was suggested long ago by potentiometric titrations in the presence and the absence of salts (e.g., see ref 25), VPT has not yet been observed in “pure” poly(MAAc) gels. Polyelectrolyte gels composed of various ionic monomers and NIPA (more generally, N-alkyl substituted acrylamide

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monomers with a suitable alkyl-chain length) undergo a volume collapse with a raise in temperature, under appropriate conditions of pH and ionic strength (salt concentration). It is widely believed that this process is due to aggregation of the NIPA segments via an interpolymer interaction referring to as hydrophobic association. The interaction of this type is opposite to the fact that the usual attractive interactions among polymer segments become weak with a raise in temperature. Thus, we have to consider the polymer-water (solvent) interaction in the collapse of linear or cross-linked poly(NIPA) chains. Although there have been a lot of studies dealing with the hydrophobic interaction in NIPA polymers and gels, recently Ramon et al.43 clearly showed the scenario leading to the phase separation of poly(NIPA) in water using attenuated total reflectance (ATR)/Fourier transform infrared spectroscopy. According to their observations on deconvoluted spectra of the amide I and amide II peaks as well as of the OH stretch envelope of water, there are two steps in the interactions below the phase separation temperature (i.e., LCST): The first step consists of the breaking of intermolecular hydrogen bonds between the amide groups and water molecules (solvent) to yield the free amide groups, and the second step causes an increase in the intramolecular hydrogen bonding, from which a coil-to-globule transition is induced. They observed no changes in the hydrophobic signals below the phase separation temperature, suggesting that the hydrophobic interaction plays a dominant role in the aggregation of the collapsed chains due to exposing hydrophobic moieties. Taking these into account, we may understand why the VPT of NIPA-based gels or the phase separation of NIPAbased polymers take place by heating. From the above two examples, it is evident that the molecular mechanism of VPTs in polyelectrolyte gels should be explored in comparison with solution properties of polyelectrolytes. For this, a good experimental system is needed. Role of Hydrogen Bonding in the Volume Collapse of Poly(ethyleneimine) Gels. Poly(ethyleneimine) (PEI) is a representative polybase with either a linear or a branched polymer structure; the former is more often abbreviated as LPEI and the latter as BPEI. The acid hydrolysis of poly(ethyloxazoline) (PEOX) gives rise to LPEI,44 whereas BPEI can be obtained through the ring-opening polymerization of ethyleneimine.45 Our early study46 with BPEI showed an abrupt change of [η] at pH ∼ 6, suggesting a conformational change (see panel a in Figure 6). Similarly, a configurational change of BPEI was observed by examining the inward permeation of n-propyl alcohol through poly(styrene) (PSt) microcapsules with an outer surface layer onto which BPEI had been adsorbed (see panel b in Figure 1).47,48 There was a good agreement between the pH levels at which the conformational transition in aqueous solutions and the configurational change on the outer surface of the microcapsules take place. These results suggest that a VPT will be observed (43) Ramon, O.; Kesselman, E.; Berkovici, R.; Cohen, Y.; Paz, Y. J. Polym. Sci. B: Polym. Phys. 2001, 39, 1665. (44) (a) Saegusa, T.; Ikeda, H.; Fujii, H. Polym. J. 1972, 3, 35. (b) Warakomski, J. M.; Thill, B. P. J. Polym. Sci., Polym. Chem. Ed. 1990, 28, 3551. (45) Davis L. E. In Water-Soluble Resins; Davidson, R. L., Sittig, M., Eds; Van Nostrand Reinhold Co.: New York, 1968; Chapter 11. (46) Kokufuta, E.; Hirata, M.; Iwai, S. Kobunshi Ronbunshu (Jpn. Ed.) 1974, 31, 234; Kobunshi Ronbunshu (Engl. Ed.) 1974, 3, 11383. (47) Kokufuta, E.; Sodeyama, T.; Katano, T. J. Chem. Soc., Chem. Commun. 1986, 641. (48) Kokufuta, E. In Colloid-Polymer Interactions: Particulate, Amphiphilic, and Biological Surfaces; Dubin, P. L., Tong, P., Eds; American Chemical Society: Washington, DC, 1993; Chapter 8.

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Figure 6. Dependence of intrinsic viscosity ([η]) (a) and permeability constant (P) (b) on pH. In (b), open and filled circles refer to P values for the capsules with and without adsorbed BPEI layer, respectively. Presented in (c) is a schematic drawing of configurational change of polyions adsorbed onto outer surface of microcapsule: looped form at pH e 5.5; and flat form at pH g 6.5. This drawing was reproduced from ref 47 with several modifications.

in a BPEI gel. However, our previous study49 showed that although a discontinuous volume collapse takes place at pH ∼11 in the deprotonation process by increasing pH, the protonation by decreasing pH from 12 to 3 led to a “gradual” increase in the gel volume; thus, we turned our attention to LPEI.50 The selection of LPEI brings about the following advantages: (i) The deprotonation was fully studied as a function of pH by means of potentiometric51,52 and calorimetric titrations with strong bases such as NaOH;53 (ii) the polymer precipitates to form crystalline hydrates from alkaline solutions (pH > 9) through hydrogen bonding between the -NH- groups and water molecules;52,54 and (iii) X-ray structure analysis54 has demonstrated that such hydrates consist of alternately stacked layers of polymers and water molecules in the crystallized state. To prepare LPEI gels, the polymer was lightly cross-linked with ethylene glycol diglycidyl ether (EGDGE). A careful elemental analysis showed that 100 monomer units contain 12 cross-linking points composed of the -N< group (or -NHCl< in the salt form). Figure 7 shows pH(49) Hirata, M.; Yamada, K.; Ebihara, T.; Matsuda, K.; Kokufuta, E. In Macro-ion Characterization: From Dilute Solutions to Complex Fluids; Schmitz, K. S., Ed.; American Chemical Society: Washington, DC, 1994; Chapter 37. (50) Kokufuta. E.; Suzuki, H.; Yoshida, R.; Yamada, K.; Hirata, M.; Kaneko, F. Langmuir 1998, 14, 788. (51) Kobayashi, S.; Hiroishi, K.; Tokunoh, M.; Saegusa, T. Macromolecules 1987, 20, 1496. (52) Weyts, K. F.; Goethals, E. J. Makromol. Chem., Rapid Commun. 1989, 10, 299. (53) Lewis, E. A.; Barkley, J.; Pierre, T. S. Macromolecules 1981, 14, 546. (54) Chatani, Y.; Tadokoro, H.; Saegusa, T.; Ikeda, H. Macromolecules 1981, 14, 315.

Kokufuta

Figure 7. pH-induced changes of normalized equilibrium gel diameters (d/d0) at 20 °C in aqueous solutions containing different concentrations of urea: 0 M (circles); 1 M (squares); 3 M (triangles). Open symbols denote an increase in pH from 3 to 12, whereas filled symbols show a decrease in pH from 12 to 3. The values in the parentheses indicate the pH at the phase transition point. The normalization of each observed equilibrium diameter (d) was performed with the inner diameter (d0) of the capillary used in the gel preparation. The attainment of an equilibrium was then evaluated by careful measurements in which the pH change was made to fluctuate several times in (0.2 units around a setting used for determining the diameter. This fluctuation forced the gel to start either swelling or shrinking within several minutes when the setting was the pH leading to a VPT. This drawing was reproduced from ref 50 with a minor modification.

dependent changes in the normalized equilibrium gel diameter (d/d0) in the presence and the absence of urea. During a pH change cycle (3-12 followed by 12-3) in the absence of urea, the gel undergos a VPT near pH 10.7 (collapse) and pH 5.9 (swelling); thus, a large hysteresis appears in the swelling curve. Note that the observation of hysteresis is not due to nonequilibrium effects (see the caption of Figure 7 as well as ref 50 for more details). Let us now try to explain on molecular grounds the observed swelling-deswelling characteristics of the LPEI gel, especially the pH-induced change, in terms of a balance between the repulsive and attractive forces within the cross-linked polymers in the network. Previous studies52,54 have reported that a highly deprotonated LPEI forms crystalline hydrates in its alkaline solutions (pH > 9). The existence of two distinct hydrates, sesquihydrate with a unit cell containing 8 monomeric units and 12 water molecules, and dihydrate with a unit cell containing 4 monomeric units and 8 water molecules, has been reported on the basis of the X-ray structure analysis. The crystals of both hydrates consist of alternatingly stacked layers of polymers and water molecules arranged parallel to the bc plane. As shown in Figure 8, three types of hydrogen bonds (N_H‚‚‚O, O_H‚‚‚O, and O_H‚‚‚N) play a major role in the stabilization of the crystal lattices. The cross-linking of polymers does not allows the formation of such crystalline hydrates all over the gel phase; however, there is a possibility of forming a stable structure (probably not so much different from that in Figure 8) among several of the polymer segments in the network at pH > 10.7 at which both the -NH- and -Nd groups are highly deprotonated. Therefore, it is at this pH that the attractive force overcomes the repulsive electrostatic force. We consider hydrogen bonding to be the attractive force at work and assume that the gels which have collapsed due to hydrogen bonding do not swell again until significant numbers of the -NH- and -Nd groups are protonated as the pH decreases; this may account for the large


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Figure 9. Changes of normalized equilibrium gel diameters (d/d0) in aqueous pH 3 solutions containing different surfactants at 20 and 60 °C. The kinds of surfactants and the temperatures for the measurements are shown in the figure. This drawing was reproduced from ref 50 with a minor modification.

Figure 8. Schematic illustration for three types of hydrogen bonds in a dihydrate of LPEI. The structural formula (top) and the corresponding atomic arrangement (bottom) were originally illustrated by reference to Figures 4, 6, and 7 in ref 54. Each H2O oxygen atom is bound to four hydrogen atoms; two of them are covalently bonded and the remaining two are hydrogen bridged from neighboring water molecules or NH groups. Such a hydrogen bond-stabilized structure would form in highly deprotonated LPEI chain segments in the network. This drawing was reproduced from ref 50 with a minor modification.

hysteresis in the swelling curves. On the other hand, it is predictable that an increase in NaCl concentration can eliminate the positive charges of the network as the repulsive electrostatic force, but it will fail to form such a hydrogen bond-stabilized structure because of the protonated ionizable groups (-NH2+- and -NH+d). Indeed, we have observed that the gel collapses monotonically without VPT when NaCl concentration is increased.50 Urea has been frequently employed in the biochemical field as a means to identify hydrogen bonds since it is generally believed that urea can break up intra- or intermolecular hydrogen bonding of proteins in aqueous systems. We thus studied the effects of urea on the pH dependence of the gel diameter. As can be seen from Figure 7, urea facilitates the swelling of the gels at higher pH levels during the protonation process, even at urea concentrations one-fourth or one-eighth lower than the most commonly employed concentration (8 M) in the denaturation of proteins. It has long been believed that urea disrupts the cluster structure of water molecules,55 i.e., “structure breaking effect.” By this, urea would inhibit the formation of a structure (as shown in Figure 8) stabilized through the hydrogen bonds with water molecules. In the presence of urea, therefore, both the acceleration of the protonation (i.e., ionization) and the weakening of the attractive interaction between the cross(55) For previous studies showing that urea acts as a water structure breaker, see the introduction of the following articles: (a) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 5786. (b) Tanaka, H.; Touhara, H.; Nakanishi, K.; Watanabe, N. J. Chem. Phys. 1984, 80, 5170. However, we should note that both articles deal with a molecular dynamics simulation of a dilute aqueous urea solution and report that urea has little effect on water structure under an infinitely low concentration.

linked chains, due to its structure breaking effect, takes place at the same time during the protonation process. As a result, a pH at which the gel swells discontinuously should shift to an alkaline pH side; in other words, a tendency for the hysteresis to disappear can be observed when adding urea. Surfactant-Induced Hydrophobic Interaction in LPEI Gel Collapse. Our previous study56 demonstrated that NaDBS does not bind homogeneously to all of the charged ammonium ions of EGDGE-cross-linked LPEI gels, but binds inhomogeneously to the charges in the near vicinity of the gel surface. This inhomogeneous surfactant binding was evidenced by the staining of the bound surfactant anions with cationic dyes such as crystal violet and toluidene blue. Thus, an aspect of the surfactant binding observed in the cationic LPEI gel is close to that in the neutral NIPA gel. As can be seen from Figure 9, however, the binding of surfactants to the LPEI gel brings about a dramatic volume collapse due not only to the neutralization of the charges but also to hydrophobic interaction. This is evident by the following results: (i) Sodium butyl sulfonate (SBS) without definitive critical micelle concentration (Cmc) in aqueous solutions exhibited the same effect on the gel diameter as NaCl. (ii) Sodium octyl sulfonate SOS (Cmc ∼ 130 mM) and SDS (Cmc ∼ 8.3 mM) showed remarkable and strong effects on the VPT of the LPEI gel. (iii) The concentration at which the transition takes place is lower for SDS than SOS because the former is stronger than the latter with respect to hydrophobicity. (iv) The SDS concentration bringing about the transition was lower at 60 °C than 20 °C; this agrees with the common knowledge that a rise in temperature enhances the hydrophobic interaction. In addition, (v) the gel diameter was about half compared to the diameter of a fully deprotonated gel at pH > 10.7 (see Figures 7 and 9), clearly indicating that the hydrophobic interaction may act as an attractive force in the gel collapse. When considering an inhomogeneous surfactant-binding, one may expect the formation of a “skin layer” on the gel surface. This layer would act as a barrier for diffusion of the surfactants from the bulk to the gel phase, causing a reduction of the surfactant concentration within the gel. When this is the case, an increase in the hydrophobicity of surfactants, allowing the formation of a much denser (56) Kokufuta, E.; Suzuki, H.; Yoshida, R.; Kaneko, K.; Yamada, K.; Hirata, M. Colloids Surf. A 1999, 147, 179.

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skin layer, should raise the surfactant concentration of the outer solution that brings about VPT. This is however contrary to the results in Figure 9, indicating that the hydrophobic interaction is the main factor rather than the neutralization of the charges. For result (iv), on the other hand, we have to consider the latter effect in addition to the former, because a rise of temperature allows a lowering of the SDS concentration bringing about VPT due to facilitated diffusion of SDS into the gel phase. As a result, the volume collapse in the surfactant-LPEI gel system can be discussed in terms of both charge neutralization and hydrophobic interaction. Electrostatic Attraction in Polyelectrolyte Gel Collapse. Naturally, this subject falls into the category of polyampholytes. In the case of polyampholyte gels, long-range Coulombic interactions between the opposite charges are generally believed to be the chief factor which governs the gel volume.57 For example, the gel volume minimizes at an isoelectric point (pI), defining as the pH at which the electrostatic attraction between the opposite charges becomes a maximum (e.g., see refs 58-60). The gel under a collapsed state at pI, however, swells again upon addition of salts such as NaCl in the outer solution (e.g., see refs 57 and 58). This behavior is a contrast to that of the usual polyelectrolyte gels, thus referring to as “antipolyelectrolyte” behavior. To understand this salt effect, we need to take into consideration the weakening of the electrostatic attraction between the opposite charges via screening with counterions. As a result, it has been pointed out that swelling models solely based on classical Donnan theory cannot predict the behavior of polyampholytic gels because the electrostatic forces are not considered directly.57 Taking this into account, one may expect that polyampholyte gels with an inhomogeneous charge distribution are a good model for studying interactions between the fixed charges at the molecular level, as mentioned in section II.2. For the introduction of charge distributions in the polyampholyte gel network, the following three approaches would be considered: (i) immobilization of polyanions into polycation networks; (ii) immobilization of polycations into polyanion networks; and (iii) immobilization of a polyanion-polycation mixture into neutral polymer networks. Here, we employed approach (i) and immobilized poly(AAc) within a cationic copolymer network consisting of VI and NIPA.61 The gelation was carried out by free radical polymerization of aqueous monomer solutions with the following composition (in mol %): 69% NIPA; 1% Bis, 15% VI; and either of 15% AAc or poly(AAc). We thus obtained two sorts of polyampholyte gels, i.e., G1 with immobilized poly(AAc) and G2 with randomly copolymerized AAc. Figure 10a shows the equilibrium swelling ratio (Qe ∼ d/d0, where d0 is the diameter of the cylindrical gel in preparation) vs pH curves for the immobilized gel (G1) and the copolymer gel (G2). The pH curves of both the gels exhibit a wide pI range in which each gel volume is minimized; thus, there lie two pH regions bringing about gel swelling on either side of pI, demonstrating a typical polyampholyte behavior of both of the gels. When the immobilized gel is compared with the copolymer gel, however, we may notice the following features: (i) The pI range is broader in the immobilized gel (G1) than in the copolymerized gel (G2). (ii) At the pI range, the swelling (57) Nisato, G.; Munch, J. P.; Candau, S. J. Langmuir 1999, 36, 1061. (58) Baker, J. P.; Blanch, H. W.; Prausnitz, J. M. Polymer 1995, 24, 549. (59) Kudaibergenov, S. E.; Sigitov, V. B. Langmuir 1999, 15, 4230. (60) Ogawa, Y.; Ogawa, K.; Wang, B.; Kokufuta, E. Langmuir 2001, 17, 2670. (61) Ogawa, Y.; Ogawa, K.; Kokufuta, E. Langmuir 2004, 20, 2546.

Kokufuta

Figure 10. Effects of pH (a) in the absence of NaCl and NaCl concentration (b) at pHs 3, 5, and 10 on normalized equilibrium gel diameters (d/d0) for polyampholyte gels G1 and G2 at 25 °C. This drawing was reproduced from ref 61 with several modifications.

ratio (Qe ∼ 0.8) of G1 is smaller than that (Qe ∼ 1.1) of G2. (iii) A pH < 4 the swelling ratios for both gels having the cationic VI ions attached to the network are close to each other, but (iv) at pH < 8 there is a marked difference in the swelling curves due to the distribution of anionic charges (i.e., either AAc was copolymerized or poly(AAc) was immobilized). In particular, (v) a hysteresis is observed at different pH ranges (note that these results were carefully obtained by fluctuating pH several times around a setting as mentioned in the section of LPEI gels). As a result, we can observe the effect of charge heterogeneity not only in the previous NIPA-AAc gel system but also in the present polyampholyte gel system. Because G1 consists of the physically entrapped poly(AAc) chains within the VI-NIPA copolymer network, most of the cationic charges should have a chance to form a stable salt-linkage or ion pair (i.e., ≡ NH +- COO -). The salt-linkage of this sort would act as a physically cross-linked “strand” but not “point” in the gel; therefore, the G1 with poly(AAc) could be in a fully collapsed state over a wide pH range, the level of which was higher by one unit than that of the copolymer gel (G2) at the alkaline side. Also, the swelling ratio at pI of G1 was smaller than that of G2. Once either a point or a strand of the salt-linkage between the opposite charges is formed, it will be stabilized via other intermolecular interactions such as hydrogen bonding and will not be dissociated easily by changes in pH, as observed in the LPEI gel. This would be responsible for a hysteresis which appeared in the swelling curves of the poly(AAc)-immobilized G1 gel and the AAc-copolymerized G2 gel during the cyclical change of pH. Because


Figure 11. Temperature dependence of normalized equilibrium gel diameters (d/d25) for G1 and G2 at different pH levels in the absence of NaCl. The normalization of each diameter (d) was performed with the diameter (d25) measured at 25 °C. This drawing was reproduced from ref 61 with several modifications.

of a high stability of the strand formed in G1, a pH range leading to the hysteresis should be wider in the result of G1 than in G2. These comparisons on the hysteresis could provide a defense against the criticism that “habitually” forced the swelling behavior of this sort to be discussed in the nonequilibrium regime. Let us next look at salt effects on both polyampholyte gels as a function of pH (see b in Figure 10). At pH 5 lying within the range of pI for both G1 and G2, the addition of NaCl caused the gel to swell. This is an antipolyelectrolyte behavior, generally observed in polyampholyte gels and considered to be a result of screening of electrostatic attraction by counterions. In contrast, at pH 3 and at pH 10, the gel shrinks with increasing the salt concentration because of the screening of electrostatic repulsion. The magnitude of the shrinking (i.e., slope of d/d0(∼Qe) vs [NaCl] curve) was almost the same for G1 and G2 at pH 3 as well as for G2 at pH 10. This means little difference in the screening effect of NaCl on the negative or positive charges which were introduced to the gel via random polymerization of VI and AAc and NIPA. At pH 10 and at NaCl concentrations >0.01 M, however, the G1 gel with immobilized poly(AAc) showed a marked different salt effect, i.e., a slight decrease in the swelling ratio (Qe) with increasing NaCl concentration. Therefore, it is clear that a distribution of charges within the network plays an important role in polyelectrolyte gel collapse caused by screening of electrostatic interactions with neutral salts. In particular, a contracted poly(AAc) ion at high NaCl concentrations seems to shrink cross-linked polymer chains around it as a result of entanglement but not over the whole of the network. This is an answer to the question that arose in the last part of Con A-loaded gels. As a result, one cannot simply say that (i) counterions accumulate close to highly charged objects; thus (ii) their contribution to the osmotic pressure is smaller (sometimes much smaller) than the ideal ln Cs term, where Cs is the average counterion concentration; and consequently (iii) many of the results (e.g., Figure 5) in the previous section can be explained in terms of Flory’s model by taking the effect of “counterion condensation ” into account. Temperature Effect on Gel Collapse via Hydrophobic Interaction. Both G1 and G2 are based on NIPA monomer; thus, it is of importance to study temperature-responsive volume collapse. Figure 11 shows temperature dependence

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of normalized equilibrium swelling ratio (Q25 e ∼ d/d25; d25 ) d at 25 °C) at different pH levels. The normalization seems to be a good approach for examining thermally induced further collapse of G1 and G2 both in a swollen state (pH 3 and pH 10) and in a collapsed state (pH 6 ∼ pI). At pH 3, the positive charges are randomly distributed over the network of G1 and G2, whereas at pH 10 the anionic charges are localized in G1 but randomized in G2. This difference clearly appears in the temperature curves of Q25 e at pH 3 and at pH 10. G2 did not shrink at pH 3 and at pH 10 as the temperature was increased. Also, little shrinking change was observed in G1 at pH 3. At pH 10, however, G1 with an inhomogeneous distribution of negative charges showed a marked collapse with increasing temperature. According to our hypothesizing balance model,19 thermal collapse of NIPA-based polyelectrolyte gels seems to take place when hydrophobic interaction as an attractive force between NIPA residues overcomes ionic repulsion. Such a situation should be considered in G1 at pH 10 because the NIPA-VI network is neutral and the anionic charges of poly(AAc) ions are localized within the network. Another important feature in Figure 11 is that both G1 and G2 undergo a temperature-induced volume collapse at pH 6 within the pI range; the magnitude of the collapse (i.e., shrinking degree) of G1 is larger than that of G2. This may also be interpreted in terms of a distribution of ionic monomers within the gel network. The potentiometric titrations61 showed that in the case of the NIPAbased copolymers of VI and AAc at pH 6, 30% of the VI units have positive charges and 60% of the AAc units have negative charges. Simply assuming a stoichiometric neutralization of positive charges with negative charges, both G1 and G2 contain a 30% excess of negative charges. In the case of G1, however, this excess charge is localized within the network. As a result, hydrophobic aggregation of the NIPA units becomes easy in the G1 network as temperature is increased. To make more clearly the effect of hydrophilic chain segments upon the temperature-induced association among hydrophobic chain segments, two NIPA-based gels were prepared: one consists of the copolymer of NIPA and vinyl pyrrolidone (VP) and the other is the NIPA gel with immobilized poly(VP). The amount of VP monomer or repeating VP unit was varied from 0 to 50 mol %. As can be seen from Figure 12, the copolymerization of a hydrophilic VI monomer causes a rise in Tv, and simultaneously extends a range of temperatures at which a large volume change takes place. By the immobilization of poly(VP), however, there is little change in Tv, although an increase in the gel volume under a fully collapse state is observed. This is the case, even when 50% of poly(VP) was immobilized. These results clearly indicate that the distribution of “neutral but hydrophilic” chain segments within the NIPA network plays an important role in the association of hydrophobic chain segments. Then, it is natural to conclude that the charged segments affect the VPT in polyelectrolyte gels through both the long-range Coulombic interaction and the hydrophilic nature. II.3. General Discussion. The main difference between the Flory’s model and the Katchalsky’s model, mentioned in the first section of II.1, is schematically pictured in Figure 13. Flory considered that a lightly crosslinked ionic gel closely resembles a Donnan membrane system, whereas such a gel in the Katchalsky’s treatment was considered to be an extension of the corresponding polyion. The former was originally employed in the study3 of a VPT of ionic gels by Tanaka et al., who demonstrated

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Figure 12. Temperature dependence of normalized equilibrium diameters (d/d0) for NIPA gels with copolymerized VP (a) as well as with immobilized poly(VP) with Mw ) 360,000 (b). Contents of VP monomer or VP unit in the pregel solution: (rhombus) 0 mol %; (circle) 10 mol %; (right-pointing triangle) 30 mol %; and (square) 50 mol %. The measurements were carried out in pure water.

that the transition is successfully described by the FloryHuggins formula with their own modifications. In particular, their mathematical simulation (Figure 2 in ref 3) showed that the transition changes from a continuous to a discontinuous one as a very slight amount of charge is introduced into the network. It seems that this anecdote was very impressive, perhaps directing us to consider the swelling force of ionic gels on the analogy of a Donnan membrane equilibrium. A model shown in Figure 13b should be considered for understanding and predicting the properties and behavior of ionic gels. Also considered are attractive interactions to facilitate and stabilize the segment association in the network; then we employed the concept of structuring or self-assembling the macromolecule chains in biological systems, the image of which may be given as:

A combination of this concept and the model in Figure

Kokufuta

Figure 13. Schematic pictures of a cationic gel based on Flory’s model (a) and Katchalsky’s model (b). The fixed charge is represented by a square with a plus sign, whereas circles with minus and plus signs denote counterion and byion, respectively.

13b has allowed us to illustrate some details of the volume changes observed in different systems of ionic gels; for example, (i) the effects of an inhomogeneous charge distribution created by the surfactant binding, the immobilized enzyme reaction and the physical entrapment of polyions; (ii) a large hysteresis observed in the LPEI and polyampholyte gel systems; and (iii) the differences between surfactant and small salt that were found in the gel collapse. One might argue whether the gel with a charge heterogeneity resembles a Donnan membrane system as considered by Flory. Also argued is that the effect of counterion condensation on the swelling pressure should vary depending on the distribution of charges within the network. Since in the Flory’s model this counterion effect is neglected, a simple and qualitative prospect would force us to see the effect of charge heterogeneity in terms of the counterion condensation. As reported here, however, the swelling curves of the gels with charge heterogeneity cannot be explained solely on the effect of counterion condensation. Thus, rather than relying on the belief that a Donnan equilibrium-based model is generally appropriate for predicting the ionic gel swelling, it would be sensible to consider the effect of charge distributions on the analogy of the behavior in solutions of linear polyions as well as of branched polyions such an ionic graft copolymer. When employing the concept of attractive interactions, we cannot thoroughly account for all of the scenarios for a transition process during which a swollen state turns into a collapsed state. In the shrinking transition of polyampholyte gels, however, we may consider long-range Coulombic attraction as the chief force. In addition, the scenario leading to the phase separation of poly(NIPA)


has been successfully described by infrared spectroscopy.43 Then, one would have a prospect that the structure of hydrated water molecules around the LPEI chain suddenly changes as the degree of deprotonation reaches a certain level (at pH ∼ 10.7), causing a phase separation in the polymer system, as well as an abrupt volume collapse in the gel system. III. Experimental Section Materials. All chemicals were obtained from commercial sources: NIPA from Kojin Chemical Co. (Tokyo, Japan); AAc, Bis (cross-linker), APS (initiator), and TMED (accelerator) from Wako Pure Chemical Co. (Osaka, Japan); VI and VP from Tokyo Chemical Industry Co. (Tokyo, Japan). Poly(AAc) (nominal molecular weight ∼ 4.5 × 105) and poly(VP) (nominal molecular weight ∼3.6 × 105) were also commercially obtained from Aldrich Chemical Co. (Wisconsin, USA). The monomers (NIPA, AAc, VI and VP) were purified according to the usual methods. LPEI was prepared from PEOX (nominal molecular weight ∼ 2.0 × 105; a commercial product from Nihon Shokubai Co., Tokyo, Japan) by the acid hydrolysis with HCl, followed by the deacidification with NaOH.50 Preparation of Gels. All of the pregel solutions containing desired amounts of the cross-linker and of required vinyl monomers, as well as of either polymer42,61 or enzyme41 in case of need, were degassed well and subjected to gelation which was initiated by a pair of APS and TMED. The reaction was allowed to continue for 2 h at 0 °C using a test tube into which glass capillaries with various inner diameters had previously been inserted. In the case of LPEI gels, the gelation was performed by heating an aqueous solution of LPEI and EGDGE (crosslinker) in glass capillaries in a test tube at 60 °C for 12 h.50 After the gelation was completed, the gels were taken out of the capillaries and purified via repetition of swelling and shrinking. All the gel samples were cut into cylinders of approximately 2 mm in length and stored at 3 °C before use. Measurements of Gel Diameter. Our own setup20,50 was used in the measurements of the swelling ratio as a function of pH and concentrations of several solutes (salts, surfactants, and urea), as well as of temperature. Our instrument consists of four different parts: an aqueous phase supply system (APSS) equipped with a pH meter, a conductivity meter, a sampling port and an additive inlet; a microscope (Olympus CK2-TRP-1) with a CCD camera and an image-analyzer; a water-jacketed separable measuring cell with glass meshes; and a temperature control system (TCS). A section of the cylindrical gel was inserted into the glass cell, and an aqueous solution with the desired pH and/ or the solute concentration was continuously supplied around the gel sample from the APSS by means of Ar gas pressure. The conductmetric and pH measurements were used for regulating the pH and the ionic solute concentration of outer medium. The temperature was controlled to within a range of (0.1 °C using TCS with water circulating around both the APSS and the measuring cell. The attainment of an equilibrated gel diameter was evaluated by careful measurements in which the additive concentration was made to fluctuate several times in (3% around a setting used for determining the diameter of the gel sample. Microscopic Spectroscopy. The wet and dry samples were obtained from lyophilized cylindrical gels with and without their swelling in pure water, respectively. The incubation was performed in an aqueous 10 mM NaDBS solution at 25 °C for 10 days. After that, the gels with the adsorbed surfactant were cut with a microtome into disks with a thickness of ca. 0.2 µm. The microscopic spectroscopy was carried out at two wavelengths of 261 nm (UV) and 400 nm (visible) at room temperature using an Olympas microscopic spectrometer (model MMSP-TU). The size of the beam going through the gel disks was fixed to 30 µm, which corresponds to about 1% of the diameters of the disks. Several Other Techniques. Surfactant uptake by a gel was determined by measuring the concentration of surfactant molecules in the outer solution, using either or both of a total organic carbon analyzer and a spectrophotometer. There was a good

Langmuir, Vol. 21, No. 22, 2005 10015 agreement between both analytical methods (within (3%). Elemental analysis was used to determine the degree of crosslinking for LPEI gels, from which the content of -Nd groups was estimated.

IV. Conclusions and Future Topics The following two important aspects of VPT in polyelectrolyte gels have been reported: the effect of charge distributions within the network on the transition and the attractive interactions among polymer segments of the network in the shrinking process. The main conclusions are summarized as follows: (i) The charge distribution of the gel network exhibits a large influence on VPT in ionic gels. (ii) Hydrogen bonding, hydrophobic association, and long-range Coulombic attraction act as the main force to hold the gel in a collapsed sate. (iii) The formation of stably associated segments via one or more of these attractive interactions causes a large hysteresis in the swelling process in which the repulsive interaction among the fixed charges on the network is the main force. (iv) A distribution of neutral but hydrophilic moieties (e.g., an ion-pair or salt-linkage formed between the opposite charged groups) within the gel network shows a marked effect on thermally induced VPTs in which the hydrophobic association dominantly acts to collapse the gel; this aspect is similar to that observed in the gels with a charge inhomogeneity. There are a lot of combinations of monomers and/or polymers in the design and synthesis of polyelectrolyte gel architectures. Then we must consider the repulsive interaction among the fixed charges on the network, instead of the swelling pressure or net osmotic pressure based on counterions within the gel phase, as the chief swelling force. Also considered in the main segmentassociative forces through which gel shrinks are hydrogen bonding, hydrophobic association and long-range Coulombic attraction. It has been pointed out that different polymers can be combined into a single material (such as gels) in many ways, which can lead to a wide range of phase behaviors that directly influence the associated physical properties and ultimate applications; then, four factorsschoice of monomers, molecular architecture, composition, and molecular sizesare considered to control polymer-polymer phase behavior.62 Taking this into account, the present conclusions would be helpful for further developments in the design and synthesis of gelbased “intelligent” materials in which the general requirement is a VPT in response to different stimuli. Acknowledgment. The author would like to acknowledge the late Dr. Toyoichi Tanaka (Professor of Department of Physics and Center for Materials Science and Engineering, MIT, Cambridge) for his useful suggestions and comments. The author also thanks all of the collaborators that appeared in references regarding our previous works, which were supported in part by grants from the Ministry of Education of Japan (Nos. 08558092 and 09875232), the Japan Society for the Promotion Science (No. 15350127), the New Energy and Industrial Technology Development Organization (NEDO), Japan, as well as from University of Tsukuba. Finally, I am particularly grateful to Professor Robert L. Rowell, Senior Editor of Langmuir, for his continuous interest in our manuscripts at submission stage. LA050530E (62) Bates, F. S. Science 1991, 251, 898.

Polyelectrolyte Gel Transitions: Experimental ... - ACS Publications

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