Effect of Salt Content on the Rheological Properties of Hydrogel Based

Effect of Salt Content on the Rheological Properties of Hydrogel Based on ... the excessive network formation resulting in collapse of the network str...
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J. Phys. Chem. B 2010, 114, 1541–1547

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Effect of Salt Content on the Rheological Properties of Hydrogel Based on Oligomeric Electrolyte Shyamal Kumar Kundu, Masaru Yoshida,† and Mitsuhiro Shibayama* Institute for Solid State Physics, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan ReceiVed: July 3, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009

Dynamic light scattering and oscillatory rheology experiments were performed to study the effects of various salts on the hydrogel consisting of an oligomeric electrolyte gelator, poly(pyridinium-1,4-diyliminocarbonyl1,4-phenylenemethylene chloride) (1-Cl). Sol-gel transition temperature increased with increasing salt concentration that suggested the salt-in behavior. The concentration dependence of the dynamic shear moduli showed power-law scaling behavior and was compared with the predictions made by the fractal gel model. The brittleness was increased by increasing salt concentration, indicating that 1-Cl hydrogel became better packed into stronger networks in ionic solutions. After certain salt concentrations, 1-Cl hydrogel started precipitation that might be due to the excessive network formation resulting in collapse of the network structure. The recovery of the mechanical properties of 1-Cl hydrogel was completely reduced in the presence of salts. 1. Introduction Many hydrogels are sensitive to different environmental conditions, such as temperature,1,2 solvent,3 coexisting solutes including ions,4 pH,5 electrical or magnetic field,6 and so forth. Among these conditions, the effects of salts or ions on the sol-gel transition of hydrogels are of great interest and importance because salts are commonly present in many cases such as biological systems. A potential important issue in the use of hydrogels derived from synthetic compound for the applications, including hygiene, cosmetics, agriculture, medicine, biotechnology, etc., is their compatibility with buffers and salts. A variety of organo- and hydrogelators capable of immobilizing organic fluids and/or water have been proposed, in which various types of intermolecular interactions, such as hydrogen (H)-bond, π-π, cation-π, and electrostatic interactions, play a significant role. Note that the organo- and hydrogelators form a transparent and thermoreversible physical gel mainly via direct H-bonding between the functional groups and/or by helix assembly.7,8 Recently, we reported sol-gel transitions and hysteresis behaviors of an ionic gelator, a low-molecular weight organogelator, poly(pyridinium-1,4-diyliminocarbonyl-1,4-phenylenemethylene chloride) (1-Cl) (Scheme 1).9 1-Cl is one of the rare examples as a main-chain polyelectrolyte synthesized by “self-condensation” and it has some notable characteristics, for example, (i) very simple one-pot preparation including condensation and intermolecular quaternization reaction, (ii) relatively high ionic conductivity of the ionic liquid gel, (iii) a fast recovery of its rheological properties, and (iv) resistance to acid in contrast to the some natural gelator (e.g., agarose and gelatin), etc.10 On the basis of the experimental results, we proposed a new type of cross-linking mechanism, i.e., formation of network structure via chlorine-ion mediated hydrogen bond (Scheme 2).9 Based on the properties of polyelectrolyte hydrogels,11,12 the electrostatic interaction is quite crucial for such gel systems with * To whom correspondence should be addressed. E-mail: sibayama@ issp.u-tokai.ac.jp. † Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 3058565, Japan.

SCHEME 1: Chemical Structure of Poly(pyridinium-1, 4-diyliminocarbonyl-1,4-phenylenemethylene chloride) (1-Cl)

SCHEME 2: One of the Possible Forms for the Chlorine Ion Mediated H-Bonding Structure of 1-Cl in Water

many charges. Indeed it has been known that the gels would collapse in the presence of added salts. The added salt generally acts to screen the many like charges along the chains, weakening the forces responsible for chain stretching and interchain repulsion that typically support these gel networks. Such behavior is seen in chemically cross-linked poly(acrylic acid) gels.12 Many studies have shown that the magnitude of various salt effects varies according to the lyotropic or Hofmeister series.13-17 A typical Hofmeister order for anions was reported as SO42- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCN-.18 Ions on the left hand, called kosmotropes, can be strongly hydrated, exhibiting strong interactions with water molecules. As a result, they tend to cause “salt-out” or to enhance hydrophobicity of a solute in water. In contrast, ions on the right-hand (called chaotropes) can be weakly hydrated, tending to cause “salt-in”, which increases the solubility of a nonpolar

10.1021/jp906312f  2010 American Chemical Society Published on Web 01/07/2010

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solute.19,20 Several studies suggest that ions directly interact with macromolecules by binding to their hydrophilic groups.21-25 Other studies suggest an indirect interaction with the polymer through perturbation of the water environment or by induction of changes in hydrogen bonding of water to the polar groups of the polymers.26,27 A few studies on electrolyte polymers were focused on the sol-gel transitions and the stability of the gels.28-31 However, detailed analysis of the salt on the electrolyte polymers is still lacking. Since it has been suggested that our hydrogel based on oligomeric electrolyte is supported by a somewhat different mechanism from that of the conventional H-bonding gelators (Scheme 2),10 it will be of great interest to know the salt effect on the gelation behavior especially about the gel tolerance against high ionic strength solutions. In our previous reports,9,32 we studied an aqueous solution and hydrogel of an oligomeric electrolyte, 1-Cl, by rheology and dynamic light scattering and discussed the gelation mechanism and the gel recovery process. We also proposed a possible structure of the supramolecular assembly of 1-Cl in water where the gel formation and the gel recovery process took place by cross-linking of the amide groups via chlorine ions and with water molecules. Here, the chlorine ion mediated H-bonding is responsible for the gelation and also the gel recovery processes. There is an equilibrium between the amide-anion complex and free ones. However, added salt can break this equilibrium. The objective of the present study is to explore the sol-gel transition and the viscoelastic properties of 1-Cl hydrogel with the addition of various salts using dynamic light scattering (DLS) and rheology. We will discuss the salt effects on sol-gel transition, concentration dependence of the dynamic moduli, which gives information on the connectivity of 1-Cl molecules in a gel and the rheological properties of 1-Cl in presence and absence of salts. 2. Experimental Section 2.1. Sample. An oligomeric electrolyte, poly(pyridinium-1,4diyliminocarbonyl-1,4-phenylenemethylene chloride) (1-Cl), was synthesized as an ionic gelator by simple mixing of aminopyridine and chloromethyl benzoyl chloride in dichloromethane with slight excess of triethylamine (Scheme 1).10 The weight average molecular weight (MW) and the degree of polymerization estimated from the MW were 5.12 × 103 Da and 22, respectively. 1-Cl carries a positive charge on the main chain33,34 and its structure was determined by NMR spectroscopy in D2O. 1-Cl is soluble only in water and it produces a translucent gel at concentrations above 7.5 g/L determined by visual evaluation. A powder of 1-Cl (10 g/L) was immersed in distilled water and the mixture was sonicated for 30 min. Then the dispersion was heated at around 97 °C by a heat gun to form a clear isotropic sol phase (1-Cl/water). The monovalent salts of KCl, NaCl, NaBr, and NaI were supplied from Wako Pure Chemical (Japan) and these were used without further purification. 2.2. Dynamic Light Scattering (DLS). DLS experiments were carried out by ALV-5000-F SLS/DLS compact goniometer system (Langen, Germany). A He-Ne laser with the wavelength of 632.8 nm was used as an incident beam. The scattered photons were collected with an avalanche photodiode system, and the scattered intensity was obtained as the counting rate of the photons. The time-intensity correlation function was calculated as a convolution of the scattered intensity. The typical measuring time was 30 s and the scattering angle was 90°. For the sake of determining the gelation points, the scattered light intensity was monitored as a function of temperature. The temperature of the sample was carefully controlled with a water

Kundu et al. circulating system (RTE-111M, Neslab, Co., Ltd., Newton, MA). For observation of the gelation temperature, the experiments were performed by cooling processes with a rate of 0.2 °C/min. Note that the temperature was fixed during measurements with the precision of 0.1 °C. 2.3. Rheology. Rheological measurements were carried out by MCR501, Anton Paar, Austria. An oscillatory rheology experiments were carried our by using a cone plate geometry (plate diameter 50 mm and cone angle 0.994°). The frequency sweep experiments and the gel-recovery process were observed at different fixed temperatures from 25 to 55 °C with the precision of 0.1 °C. Note that we repeated each set of the experiments several times and found good reproducibility. 3. Result and Discussion 3.1. Dynamic Light Scattering (DLS). The instantaneous scattering intensity, I(τ), at wave vectors of magnitude q depends on the special arrangement of the scattering centers at time τ. As molecules move, changing their conformations and the locations in space, the scattering intensity, I(τ), fluctuates in time. The value of the scattering intensity, averaged over a long time interval τ, can be written as35

Isc ) 〈I(0)〉 ≡ lim

τf∞

1 τ

∫0τ I(t) dt

(1)

The time correlation function of the scattering light intensity (ICF), g(2)(τ), was obtained from the normalized field correlation function g(1)(τ) by

g(2)(τ) )

〈I(0)I(τ)〉T 〈I(0)〉T2

) B[1 + B'|g(1)(τ)| 2]

(2)

where, B and B′ are respectively constants depending on the instrument optics and to the sample, 〈I(0)〉T means the time average scattering intensity at time τ ) 0 and g(1)(τ) can be written as

g(1)(τ) )

∫ G(Γ) exp(-Γτ) dΓ

(3)

where ∫G(Γ) dΓ ) 1, and Γ is the characteristic decay rate. Sol-gel transition is defined as a point where connectivity correlation diverges. Shibayama et al.36,37 proposed four methods of gel-point determination with dynamic light scattering, DLS. The gel point is determined as a point at which one of the following features is observed: (i) a large fluctuation and a drastic increase in the scattering intensity, (ii) a power-law behavior in ICF, (iii) a characteristic broadening in the distribution function obtained by inverse Laplace transform of ICF, or (iv) a depression of the initial amplitude of ICF. The applicability of these methods was confirmed for chemical gels, physical gels, gelators, and glass-forming systems.38 In our present study, we will demonstrate the first criterion for the gel-point determination. The temperature-dependent gelation measurements were carried out by cooling process. Figure 1 shows that the variations of the intensity of the ICF with temperature for 10 g/L 1-Cl/ water in the presence of different amount of NaCl salt concentrations. The gelation temperatures were determined as the point where the scattering intensity started fluctuations. The gelation temperatures are easily obtained where the dashed lines

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Figure 3. Variations of G′ and G′′ with time during gelation process at 25 °C for 10 g/L 1-Cl/water in the presence of different amounts of NaCl salt concentrations. The strain, γ, and the angular frequency, ω, were chosen to be γ ) 0.5%, ω ) 1 rad/s.

Figure 1. Variations of the scattering intensity with temperature during cooling process for 10 g/L 1-Cl/water in the presence of NaCl salt concentrations (a) 0 mM, (b) 0.005 mM, (c) 0.01 mM, and (d) 0.015 mM. Dashed vertical lines are shown at the sol-gel point.

Figure 2. Variation of the sol-gel transition temperature, Tgel, as a function of salt concentrations. The three salts of NaCl, KCl, and NaBr were used. The solid lines are drawn as a guide for the eye.

are indicated the sol-gel transition points. It is seen from this figure that the sol-gel transition temperature increases with increasing NaCl concentration. Figure 2 shows the variations of the sol-gel transition temperature, Tgel, as a function of various salt concentrations. Here, we studied the different monovalent salts (from 0 to 0.015 mM of KCl, NaCl, and NaBr) and observed that the sol-gel transition temperature increased with salt concentrations. We could not observe the sol-gel transition in the wide range of salt concentrations even for NaI salt because the solution became opaque by adding more salts in the solution. It has been reported that most of the salts in Hofmeister series decrease the solubility of organic solutes those are originally dissolved in water (salting-out phenomenon). On the contrary, some of salts (NaI, NaClO4, NaSCN) show an

opposite action (salting-in) to increase the solubility of them. From Figure 2 it is seen that the sol-gel transition temperature increases with increasing salt concentration that suggests the salt-in behavior,39-41 because the salt-in ions have almost no effect on the water structure but replace some of the water molecules in the hydration shell of the solutes.41 Therefore, on heating, it is difficult for solute molecules to form hydrophobic aggregates at the same temperature. To reach the critical requirement for intermolecular hydrophobic association, more energy is needed, leading to a higher sol-gel transition temperature.39-41 In our study, no significant differences following the Hofmeister series were observed for all salts. 3.2. Rheology. Figure 3 shows the variations of the storage (G′) and loss (G′′) moduli with time during gelation process at 25 °C for 10 g/L 1-Cl/water in the presence of different amounts of NaCl salt concentrations. It is seen from this figure that hydrogels with different salt concentrations show significant differences in their rheological properties. It was observed that, in the absence of salt, initially G′ was smaller than G′′ and then G′ increased gradually. Finally, G′ was observed to be almost 2 orders of magnitude higher than G′′. The crossover point (G′ ) G′′) was observed within 1 min. In the presence of a small amount of salts (e.g., 0.005 mM NaCl), no crossover between G′ and G′′ was found. It was observed that in the beginning G′ . G′′ and then both of the moduli increase slowly. For all gels (Figure 3) the G′ values were more than an order of magnitude greater than G′′. Such a behavior was indicated from the DLS study where Tgel increased with salt concentration. An increase in the ionic strength resulted in the formation of hydrogels much faster with higher storage moduli. After a certain ionic strength, G′ and G′′ decreased, indicating breakdown of the gel network. Similar results were observed for NaBr, NaI, and KCl salts. Now, we will discuss first the variations of G′ and G′′ with 1-Cl concentration and then show the effects of salts on the rheological properties of 1-Cl hydrogel by using a particular concentration e.g., 10 g/L 1-Cl/water. The 1-Cl hydrogel showed a gelation transition in water at the minimum gelation concentration (C0) of 6.3 g/L.32 In Figure 4, a and b, the dynamic moduli, G′ and G′′, are shown as a function of angular frequency, ω, for different concentrations of 1-Cl at 25 °C (above C0) in the absence and presence of NaCl salt, respectively. Note that the frequency dependence of G′ and G′′ was obtained after 90 min from the sample preparation because about 60 min was needed for the structure formation. It is seen from Figure 4a,b that G′ is almost frequency independent. By increasing 1-Cl concentration (Figure 4a), the

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Figure 4. Frequency dependence of G′ and G′′ of 1-Cl hydrogel at different 1-Cl concentrations for (a) 0 mM and (b) 0.04 mM NaCl at 25 °C. Low-amplitude strain γ ) 0.5% was applied.

gel strength became stronger so that G′ value increased. For all gels the G′ values were almost 2 orders of magnitude greater than G′′. By addition of 0.04 mM NaCl salt in each 1-Cl concentration (Figure 4b), both the dynamic moduli, G′ and G′′, were increased by more than an order of magnitude. The variations of G′ and G′′ as a function of 1-Cl concentration, C, are shown in Figure 5, a and b, for 0 and 0.04 mM NaCl, respectively, at 25 °C. These types of variations are often examined for physically cross-linked polymer and biopolymer gels. The data were obtained at ω ) 1 rad/s from all the frequency sweep experiments. It is seen from this figure that the slope of each data sets (with and without NaCl salt) is almost the same within experimental error. Only the magnitudes of both the G′ and G′′ are increased by more than an order of magnitude for each concentration of 1-Cl by addition of, e.g., 0.04 mM NaCl. Now, we can analyze the results (Figure 5) using scaling relationship of G′ and G′′ as a function of 1-Cl concentration, C. Clark and Ross-Murphy42 discussed the relationships between shear modulus (in practice, the storage modulus component, G′) and concentration for protein and polysaccharide gels. They have also provided a theoretical analysis for the power law dependence of G′ on polymer concentration according to which a limiting C2 relationship is predicted at high C/C0 ratios, where C0 is the critical concentration below which no macroscopic gel is formed. In reviewing the rheological data of various gelling biopolymers, these authors pointed out that much higher power law dependence is usually found at concentrations corresponding to C/C0 less than 10. MacKintosh et al.43 proposed a model for density cross-linked actin gels and entangled solutions. They showed that for an entangled solution the plateau modulus scaled with concentration, C, as

G' ∝ CA

(4)

with A ) 11/5, where A is the power law exponent. This prediction was consistent with the concentration dependence

Figure 5. Double-logarithmic plots of G′ and G′′ as a function of 1-Cl concentration for 0 and 0.04 mM NaCl at 25 °C. The data were obtained at ω ) 1 rad/s from all the frequency sweep experiments. The solid lines are the fitting lines with eq 4.

TABLE 1: Fitting Parameter, A, Obtained from Figure 5 by Fitting Eq 4 and the Space Fractal Dimension, df, Calculated from Eq 5 G′ 0.0 mM NaCl 0.04 mM NaCl

G′′

A

df

A

df

4.25 3.8

2.77 2.74

4.1 3.8

2.76 2.74

of G′ of F-actin in the range of 3-20 g/L.44 For densely crosslinked gels, the plateau modulus scaled with concentration, C, with A ) 5/2.43 Ozbas et al.45 showed that, for oligopeptide hydrogel, the plateau moduli of the hydrogels scale with the peptide concentration with an exponent of 2.5 (A ) 5/2). The concentration dependences of G′ and G′′ shown in Figure 5 are fitted with eq 4. The fitting values are listed in Table 1. Much higher power law dependence (for all data sets, A ≈ 4.2 within experimental error) than those predicted from the scaling theory was observed at concentrations corresponding to C/C0 less than 10. These results indicate that the scaling behavior observed from the present study is in disagreement with the MacKintosh theory.43 The present results can be analyzed using the fractal gel model.46,47 Shih et al.48 proposed a scaling model by defining two separate regimes: the strong-link regime at low particle concentrations and the weak-link regime at high particle concentrations. In this model, it is assumed that the structure of gels is constituted by fractal flocs, which during gelation aggregate with each other. The elastic properties of a floc are determined by its effective backbone, which can be approximated as a linear chain of springs. In the stronglink regime, where the interfloc links are stronger than the intrafloc links, the macroscopic elasticity of the gel is given by that interlinks. In the weak-link regime, where the flocs are more rigid than the interfloc links, the elasticity of the interfloc links determines the elasticity of the gel. In this case, Shih et al.48 derived the same expression (eq 4) where the power law exponent is related to

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A)

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d-2 d - df

(5)

where d is the space dimension (d ) 3) and df is the space fractal dimension. The power law exponent, A, obtained from Figure 5 using eq 4 can be described by eq 5. The space fractal dimension, df, calculated from eq 5 is listed in Table 1. The value of df is approximately 2.75. The variations of G′ and G′′ as a function of 1-Cl concentration, C, are shown in Figure 6 for different fixed temperatures. The data were obtained at ω ) 1 rad/s from all the frequency sweep experiments. It is seen from this figure that the slope of each data set is almost temperature independent within an experimental error. The solid lines are the fitted lines with eq 4. The values of the power law exponent are listed in Table 2. The space fractal dimension, df, calculated from eq 5 is also listed in Table 2. The values of df is obtained to be about 2.75 and it is almost temperature independent. This value is also the same when the some amount of salt was added to each concentration of 1-Cl (Figure 5). Surprisingly, the space fractal dimension, df, obtained from this study is close to the value of the power law exponent, R, obtained from the time-temperature phase diagram32 by DLS study. Figure 7 shows the frequency dependence of G′ and G′′ for 10 g/L 1-Cl in the presence of salts, e.g., 0.02 mM each of

Figure 7. Frequency dependence of G′ and G′′ for 10 g/L 1-Cl in the presence of, e.g., 0.02 mM of each NaCl, NaBr, and NaI at 25 °C. Low-amplitude strain γ ) 0.5% was applied. X ) Cl, Br, and I.

Figure 8. Semilog plot of the hydrogel strength (G′) for 10 g/L 1-Cl/ water as a function of salt concentration at 25 °C. The four different salts (KCl, NaCl, NaBr, and NaI) were used. The data were obtained at ω ) 1 rad/s from all the frequency sweep experiments. The solid curves are drawn as a guide for the eye.

Figure 6. Double-logarithmic plots of G′ and G′′ as a function of 1-Cl concentration for different fixed temperatures. The data were obtained at ω ) 1 rad/s from all the frequency sweep experiments. The solid lines are the fitting lines with eq 4.

TABLE 2: Fitting Parameter, A, Obtained from Figure 6 by Fitting Eq 4 and the Space Fractal Dimension, df, Calculated from Eq 5 G′ 25 °C 40 °C 50 °C

G′′

A

df

A

df

4.25 4.43 4.6

2.77 2.77 2.78

4.1 4.3 4.5

2.76 2.77 2.78

NaCl, NaBr, and NaI at 25 °C. This figure shows that 1-Cl hydrogels with different salt concentrations show significant increase in the G′ value. For all salts, G′ is almost frequency independent. Now we will see the behavior of G′ with different salt concentrations and, for this study, we used four different monovalent salts of KCl, NaCl, NaBr, and NaI. Figure 8 shows the variations of the storage modulus, G′, for 10 g/L 1-Cl/water as a function of salt concentration at 25 °C. It is seen from this figure that initially the storage modulus, G′, increases and then decreases. Addition of halide anions shifts the equilibrium between the amide-anion complex and free anions to the complexation state, showing an apparent increase of G′. Addition of more salt makes excessive aggregation and eventually the gelator molecules start to escape from the gel network by precipitation. It is also seen from this figure that the initial slope decreases for the halide ions and follows the order of I- > Br- > Cl-. It indicates that the salt-in trend decreases. Xu et al.39 observed a similar behavior on aqueous methylcellulose solution by DSC. They studied the effects of monovalent salts (e.g., NaCl, NaBr, NaNO3, and NaI) on the sol-gel transition of aqueous methylcellulose solution and observed that the transition temperatures were dramatically different, resulting in the different slope for each line. The slopes were changed from positive (for NaI) to negative (for NaCl), indicating the change of salt-in to salt-out effects. The shape of the curve on the salts is mainly related to the hydorophilicity of counteranions (Cl- > Br- > I-). The most ‘hydrophobic’ iodide tends to go out from aqueous medium

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Figure 9. Recovery processes of the gel strength as a function of time for 10 g/L 1-Cl/water in the presence of various NaCl salt concentrations at 25 °C.

(always with cation) much earlier than the other halides. Therefore its peak position comes earlier. The hydration energy (∆G) for iodide could be the lowest, i.e., ∆GI < ∆GBr < ∆GCl.49 Therefore, the order of the salts well follows the universal Hofmeister series.50 In water, solvation of a solute by hydrogen bonding is important. In 1-Cl aqueous solutions containing a salt, water molecules solvate the anions of the salt by forming hydrogen bonds with them, which are much stronger than hydrogen bonds formed between water molecules because the water molecules are more attracted by the negative charge. For these three halide ions, as each of them contains the same charge of -1, the inherent electronegativities of the corresponding halogen atoms are not important to the solvation. Instead, the most important factor affecting the solvation strength is the ionic radius of each halide ion. A small ion, because its charge is more concentrated, should be more strongly solvated than a larger one. According to the literature,51 the ionic radii of Cl-, Br-, and I- are 1.81, 1.95, and 2.16 Å, respectively. Therefore, the solvation strength by water for the halide ions follows the order of Cl- > Br- > I-. This explains why the halide ions affected the rheological properties of 1-Cl differently as shown in Figure 8. It is also seen from Figure 8 that the KCl appeared very close to the NaCl one. Note that cations were reported to have less effect on water structure than anions,3,4,26,52-54 which has been once again proved by our experiments. Figure 9 shows the recovery process of the gel strength as a function of time for 10 g/L 1-Cl/water in the presence of various NaCl salt concentrations at 25 °C. Similar to our previous report,9 at first, we applied a low-amplitude oscillation (γ ) 0.5%, ω ) 1 rad/s) for 20 min for the observation of the equilibrium gel strength, and then deformed it by a high shear rate (γ˙ ) 1000 s-1) for 30 min (see the blank portion indicated by the both-headed arrow). Then, we applied a low-amplitude oscillation (the same as the initial values) so as to measure the mechanical response. We reported that 1-Cl hydrogel (without salt) was self-healed and recovered its elastic properties within 220 min after cessation of the large strain.9 For comparison of the recovery process of the 1-Cl hydrogel, we have shown this data in Figure 9. It is seen from Figure 9 that for all the NaCl salt concentrations after cessation of the large strain, both G′ and G′′ dropped down by more than 2 orders of magnitude as a result of fragmentation of the gel. No such recovery of the gel strength was observed. Similar results were observed for NaBr, NaI and KCl salts (data are not shown).

The salt effects on sol-gel transitions and the mechanical properties of hydrogel consisting of an oligomeric electrolyte, 1-Cl, were studied by DLS and rheology. Sol-gel transition temperature increased with increasing salt concentration that suggested the salt-in behavior. An increasing salt concentration resulted in the formation of hydrogels much faster with higher storage moduli. In the presence and absence of salt, the concentration dependence of the dynamic shear moduli showed a power-law scaling behavior. The present results were analyzed using fractal gel model. The space fractal dimension, df, obtained from the present study is close to the value of power law exponent, R, obtained from time-temperature phase diagram by DLS study. After certain salt concentrations, G′ is decreased, resulting in the collapse of the network structure. All salts showed both salt-in and salt-out phenomena and followed the universal Hofmeister series. The nature of G′ on the salts is mainly related to the hydrophilicity of counteranions (Cl- > Br- > I-). Cations were less effective on the water structure than anions. 1-Cl hydrogel recovered its rheological properties after vigorous agitation but the added salt reduced the recovery process. Acknowledgment. This work was supported by the Ministry of Education, Science, Sports and Culture, Japan, for Scientific Research on Priority Areas, 2006-2010, No. 18068004. M.Y. thanks to NEDO for the Industrial Technology Research Grant Program (05A25710a). References and Notes (1) Wanka, G.; Hoffman, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101. (2) Li, L.; Shan, H.; Yue, C. Y.; Lam, Y. C.; Tam, K. C.; Hu, X. Langmuir 2002, 18, 7291. (3) Jeong, B.; Kim, S. W.; Boe, Y. H. AdV. Drug DeliVery ReV. 2002, 54, 37. (4) Starodoubtev, S. G.; Khokhlov, A. R.; Sokolov, E. L.; Chu, B. Macromolecules 1995, 28, 3930. (5) Siegel, R. A.; Firestone, B. A. Macromolecules 1988, 21, 3254. (6) Chin, B. D.; Winter, H. H. Rheol. Acta 2002, 41, 265. (7) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201. (8) Nowak, A. P.; Breedveld, V.; Pakstis, L.; Ozbas, B.; Pine, D. J.; Pochan, D.; Deming, T. J. Nature 2002, 417, 424. (9) Kundu, S. K.; Matsunaga, T.; Yoshida, M.; Shibayama, M. J. Phys. Chem. B 2008, 112, 11537. (10) Yoshida, M.; Koumura, N.; Misawa, Y.; Tamaoki, N.; Matsumoto, H.; Kawanami, H.; Kazaoui, S.; Minami, N. J. Am. Chem. Soc. 2007, 129, 11039. (11) Deming, T. J. Soft Matter 2005, 1, 28. (12) Tanaka, T. Sci. Am. 1981, 244, 110. (13) Nilsson, S.; Piculell, L.; Malmsten, M. J. Phys. Chem. 1990, 94, 5149. (14) Piculell, L.; Nilsson, S. J. Phys. Chem. 1989, 93, 5596. (15) Piculell, L.; Nilsson, S. Prog. Colloid Polym. Sci. 1990, 82, 198. (16) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (17) Melander, W.; Horvath, C. Arch. Biochem. Biophys. 1977, 183, 200. (18) Collins, K. D.; Washabaugh, M. W. Q. ReV. Biophys. 1985, 4, 323. (19) Dougherty, R. C. J. Phys. Chem. B 2001, 105, 4514. (20) Hribar, B.; Southall, N. T.; Vlachy, V.; Dill, K. A. J. Am. Chem. Soc. 2002, 124, 12302. (21) von Hippel, P. H.; Schleich, T. Structure and Stability of Biological Macromolecules; Timasheff, S. N., Fasman, G. D., Eds.; Dekker: New York, 1969. (22) Song, J. D.; Ryoo, R.; John, M. S. Macromolecules 1991, 24, 1727. (23) von Hippel, P. H.; Peticolas, V.; Schack, L.; Karlson, L. Biochemistry 1973, 12, 1256. (24) von Hippel, P. H.; Schleich, T. Acc. Chem. Res. 1969, 2, 257. (25) Takano, M.; Ogata, K.; Kawauchi, S.; Satoh, M.; Komiyama, J. Polym. Gels Networks 1998, 6, 217. (26) Muta, H.; Ishida, K.; Tamaki, E.; Satoh, M. Polymer 2002, 43, 103. (27) Muta, H.; Miwa, H.; Satoh, M. Polymer 2001, 42, 6313.

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