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Swelling Behavior of Chitosan Hydrogels in Ionic Liquid-Water Binary Systems Geoffrey M. Spinks,† Chang Kee Lee,‡ Gordon G. Wallace,† Sun I. Kim,‡ and Seon Jeong Kim*,‡ ARC Centre of Excellence in Electromaterials Science, Intelligent Polymer Research Institute, UniVersity of Wollongong, NSW, Australia, and Center for Bio-Artificial Muscle and Department of Biomedical Engineering, Hanyang UniVersity, Seoul 133-791, Korea ReceiVed June 2, 2006. In Final Form: August 18, 2006 The swelling behavior of chitosan hydrogels in ionic liquid-water binary systems was studied using hydrophilic room-temperature ionic liquids (RTILs) to elucidate the swelling mechanism of chitosan hydrogels. No penetration of RTIL into a dry chitosan material was observed. Swelling was achieved by soaking in water-RTIL binary mixtures, with larger swelling observed at higher water contents. In one instance, the binary mixture was acidic and produced larger than expected swelling due to the dissociation of the amine groups in the chitosan. The equilibrium binary system content behavior of the chitosan hydrogels depended upon the amount of free water, which is a measure of the number of water molecules that do not interact with the ionic liquid. After evaporation of water, remnant RTIL remained in the chitosan network and hardness testing indicated a plasticization effect, suggesting that the RTIL molecularly mixed with the chitosan. Chitosan hydrogels containing only RTIL were prepared by dropping pure RTIL onto a fully preswollen hydrogel followed by water evaporation. This method may be a useful means for preparing air-stable swollen chitosan gels.
* Corresponding author. Address: Seongdong P.O. Box 55, Seoul 133605, Korea. Tel: +82-2-2220-2321. Fax: +82-2-2291-2320. E-mail:
[email protected]. † University of Wollongong. ‡ Hanyang University.
gel electrolytes for use in soft actuators,13-15 because hydrogels can be electrolytes themselves, and can also house an electrolyte by dissolving the electrolyte in the hydrogel network. Chitosan is used as a component in hydrogels, and is a polymer electrolyte16-18 because of its ease in forming a positive charge in a low pH environment. Chitosan is biocompatible, and is a plentiful material with a long and well-established technological base. However, hydrogels have a serious lack of electrical stability with regard to electrolytes in ambient conditions, caused by the high volatility of water. This means that there is a limit to the application lifetime of hydrogels in polymeric conducting systems.19 Room-temperature ionic liquids (RTILs) are interesting liquid electrolytes for electrochemical applications. Previous workers have shown the utility of RTILs as electrolytes in electrochemical systems to achieve unprecedented environmental and electrochemical stability.20-23 We have reported19,24 on the electrochemistry of conducting polymers in RTILs, and shown that it is well defined and more stable than in other organic electrolytes.25 Major reasons for the interest in ionic liquids are their
(1) Plunkett, K. N.; Kraft, M. L.; Yu, Q.; Moore, J. S. Macromolecules 2003, 36, 3960. (2) Stubbe, B. G.; Braeckmans, K.; Horkay, F.; Hennink, W. E.; De Smedt, S. C.; Demeester, J. Macromolecules 2002, 35, 2501. (3) Chiu, H.-C.; Lin, Y.-F.; Hung, S.-H. Macromolecules 2002, 35, 5235. (4) Kuckling, D.; Harmon, M. E.; Frank, C. W. Macromolecules 2002, 35, 6377. (5) Kim, S. J.; Shin, S. R.; Lee, S. M.; Kim, I. Y.; Kim, S. I. Smart Mater. Struct. 2004, 13, 1036. (6) Dawlee, S.; Sugandhi, A.; Balakrishnan, B.; Labarre, D.; Jayakrishnan, A. Biomacromolecules 2005, 6, 2040. (7) Jeong, W. J.; Kim, J. Y.; Kim, S. J.; Lee, S. H.; Mensing, G.; Beebe, D. J. Lap Chip 2004, 4, 576. (8) Murthy, N.; Thng, Y. X.; Schuck, S.; Xu, M. C.; Frechet, J. M. J. J. Am. Chem. Soc. 2002, 124, 12398. (9) Martens, P. J.; Bryant, S. J.; Anseth, K. S. Biomacromolecules 2003, 4, 283. (10) Shi, J.; Guo, Z.-X.; Zhan, B.; Luo, H.; Li, Y.; Zhu, D. J. Phys. Chem. B 2005, 109, 14789. (11) Moschou, E. A.; Peteu, S. F.; Bachas, L. G.; Madou, M. J.; Daunert, S. Chem. Mater. 2004, 16, 2499. (12) Kim, S. J.; Kim, H. I.; Park, S. J.; Kim, S. I. Sens. Actuators, A 2004, 115, 146.
(13) MacCallum, J. R., Vincent, C. A., Eds. Polymer Electrolyte ReViews; Elsevier: London, 1987 and 1989; Vols. 1 and 2. (14) Sudipto, K. D.; Aluru, N. R. Mech. Mater. 2004, 36, 395. (15) Kim, S. J.; Park, S. J.; Kim, S. I. Smart Mater. Struct. 2004, 13, 317. (16) Yi, H.; Wu, L.-Q.; Ghodssi, R.; Rubloff, G. W.; Payne, G. F.; Bentley, W. E. Anal. Chem. 2004, 76, 365. (17) Wu, L.-Q.; Gadre, A. P.; Yi, H.; Kastantin, M. J.; Rubloff, G. W.; Bentley, W. E.; Payne, G. F.; Ghodssi, R. Langmuir 2002, 18, 8620. (18) Wu, S.; Zeng, F.; Zhu, H.; Tong, Z. J. Am. Chem. Soc. 2005, 127, 2048. (19) Lu, W.; Fadeev, A. G.; Qi, B.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewicz, J.; Zhou, D.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297, 983. (20) Susan, M. A. B. H.; Kaneko, T.; Noda, A.; Watanabe, M. J. Am. Chem. Soc. 2005, 127, 4976. (21) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881. (22) McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687. (23) Koch, V. R.; Dominey, L. A.; Nanjundiah, C. J. Electrochem. Soc. 1996, 143, 798. (24) Zhou, D.; Spinks, G. M.; Wallace, G. G.; Tiyapiboonchaiya, C.; MacFarlane, D. R.; Forsyth, M.; Sun, J. Electrochimica Acta 2003, 48, 2355.
Introduction The swelling of hydrogels is an important characteristic in their applications. Hydrogels show a large degree of swelling in aqueous environments, and a differential shrink-swell response to different solvents, including organics and salt solutions.1-5 The determination of the swelling range and swelling behavior of a hydrogel is necessary to investigate their applicability in many biological applications, such as wound dressings, drug and protein delivery systems, and as tissue matrixes.6-9 Recently, an attractive field for hydrogels has been in artificial muscles and actuators because they provide a mechanical response to an electrical stimulation.10-12 Hydrogels can control their degree of swelling in response to a specific electrical stimulation, and this is of interest because of its similarity in mimicking the functions of biomuscles. Some hydrogels have been developed as polymer
10.1021/la061586r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/23/2006
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Table 1. List of Several Physical Parametersa,b of RTILs that Have Been Employed as Components in Ionic Liquid-Water Binary Systems
[C4mim]BF4 [C6mim]Cl [C2mim]SCN [C2mim]atf [C3mim]I
melting temperature (°C)
decomposition temperature (°C)
density (g/cm3 at 20 °C)
viscosity (mPa/s)
conductivity (ms/cm)
-71 -85 -50 -50 n.m.c
300 210 220 130 n.m.c
1.263 1.04 1.11 1.29 n.m.c
119.20 (at 25 °C) 7751.74 (at 20 °C) 25.59 (at 20 °C) 37.95 (at 20 °C) 880 (at 20 °C)
3.53 n.m.c n.m.c n.m.c 0.5
a Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Humphry-Baker, R.; Gratzel, M. J. Am. Chem. Soc. 2004, 126, 7164. b Taken from the manufacturer’s safety data sheet of the product that was obtained from Merck KGaA (Darmstadt, Germany) and Solvent-Innovation Co. (Koln, Germany) and the Ionic Liquids Database in Merck (http://ildb.merck.de/ionicliquids/en/startpage.htm). c nm: not measured.
nonvolatility, high conductivity, and large electrochemical window. Their nonvolatility diminishes any risk to the electrical stability caused by loss of the solvent to the atmosphere. One aspect of RTILs is their ability to control water miscibility by simply changing the anion. Interest in RTILs has grown since the development of air-stable imidazolium-based RTILs.26 This is even true for “hydrophilic” RTILs that are totally miscible in water. Several researchers have reported observations of the uptake of water into various imidazolium-based RTILs as a function of time.26-29 Their utility in the field of polymer hydrogel electrolytes is not necessarily new, but there have been no reports on the interaction between hydrophilic RTIL-water binary systems and chitosan hydrogels (only between water and RTILs, and various solvents and ionic liquid-polymer hydrogels). Thus, an understanding of the swelling behavior of chitosan hydrogels with an RTIL-water binary system is important for their application. In this paper, we report on the elucidation of the swelling mechanism of a chitosan hydrogel in a hydrophilic RTIL-water binary system, which results in the preparation of a chitosan hydrogel containing an RTIL. Experimental Section Materials. The chitosan used, with an average molecular weight of 2.0 × 105 and a 76% degree of deacetylation, was obtained from the Jakwang Co. (Ansung, Korea). The 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) and 1-hexyl-3-methylimidazolium chloride ([C6mim]Cl) used were purchased from the SolventInnovation Co. (Koln, Germany). The 1-ethyl-3-methylimidazolium thiocyanate ([C2mim]SCN), 1-ethyl-3-methylimidazolium trifluoroacetate ([C2mim]atf), and 1-propyl-3-methylimidazolium iodide ([C3mim]I) used were purchased from the Merck KGaA (Darmstadt, Germany). The properties of these RTILs are listed in Table 1. All RTILs are totally miscible in water. Preparation of Chitosan Hydrogels. Chitosan was completely dissolved in a 2 wt % acetic acid aqueous solution. The concentration of the chitosan in the solution was 2 wt %. The solution was then poured into a Petri dish, and dried for 48 h in a convection oven at 40 °C. Then, the Petri dish was dipped into an aqueous 0.1 M NaOH solution to remove any remaining acetic acid. The chitosan film was then washed several times with deionized water and then dried for a period of one week at 40 °C in a convection oven. Characterizations. To measure the equilibrium ionic liquidwater binary system content (EBC), preweighed dry samples were immersed in various binary systems of different compositions. The weight of the swollen samples was measured after any excessive surface solvent (i.e., binary system) was removed with filter paper. (25) Janiszewska, L.; Osteryoung, R. A. J. Electrochem. Soc. 1987, 1344, 2787. (26) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (27) Gaillon, L.; Sirieix-Plenet, J.; Letellier, P. J. Solution Chem. 2004, 33, 1333. (28) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (29) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275.
This procedure was repeated five times until there was no further weight increase. The EBC was calculated using the following equation EBC(%) )
(
)
Ws - Wd × 100 Ws
where Ws and Wd denote the weight of the samples in the swollen and dry states, respectively. Drying of the swollen chitosan hydrogels was performed in a convection oven at 40 °C. This procedure was also repeated five times until there was no further weight decrease. The remnant of the difference between the two weights is the RTIL of the chitosan hydrogel, determined using the above equation, where Wi was used instead of Ws as the initial weight of the swollen chitosan hydrogel. Wi was measured before placing the swollen chitosan hydrogel into the convection oven. Differential scanning calorimetry (DSC, TA Instruments Q100,) was used to examine the state of water in the swollen hydrogels and ionic liquid. The samples were sealed in aluminum pans, cooled to -25 °C, and then heated to 25 °C using a heating rate of 5 °C/min under nitrogen gas flowing at a rate of 60 cm3/min. For the pH measurements in the RTIL-water binary systems, the pH meter (inoLab Level 1) was used, and the electrode (SenTix Mic) was calibrated with two standard buffers: technical buffer solutions (pH 4.0 and 7.0, from WTW Inc.). Corrections to the readings of the pH meter were made following the indications given in the manual for the pH meter. The pH measurements were replicated five times, and the data presented are the averages of the replicas. Hardness (durometer - Shore A) tests were all performed following standard American Society for Testing and Materials test procedures and were measured using a minimum of five specimens for each chitosan sample.
Results and Discussion The interaction of the chitosan with pure RTILs was considered first. In Figure 1, it is interesting that the dried chitosan film exhibited a decrease in its EBC value in pure RTILs (i.e., the dried chitosan film lost weight after it was placed in the pure RTIL). It can be also seen from Table 2 that the dried chitosan film shrank in pure RTILs, and that the deswelling behavior was independent of temperature. It is expected that bound water in the chitosan polymer networks will leach into the pure RTIL. This bound water represents water molecules that interact with the polymer network in a physical manner. The dried chitosan films can contain bound water because bound water is not removed by a conventional drying procedure in a convection oven.30 Explaining this deswelling mechanism can be complex, because the swelling equilibrium of hydrogels is primarily governed by the balance between the internal osmotic pressure within the polymer network and the internal electrostatic repulsion of the network structure,31,32 owing to the presence of an interaction between water and the anions and/or cations of the RTIL. (30) Kim, S. J.; Lee, C. K.; Kim, S. I. J. Appl. Polym. Sci. 2004, 92, 14671472. (31) Chiu, H.-C.; Lin, Y.-F.; Hung, S.-H. Macromolecules 2002, 35, 5235. (32) Ito, T.; Yamaguchi, T. J. Am. Chem. Soc. 2004, 126, 6202.
Swelling BehaVior of Chitosan Hydrogels in IL-H2O
Figure 1. EBC values of chitosan hydrogels according to the amount of water in various RTIL-water binary systems. Table 2. EBC of Chitosan Film in Various Pure RTILs EBC (%) RTIL
35 °C
50 °C
[C4mim]BF4 [C6mim]Cl [C2mim]SCN [C2mim]atf [C3mim]I
-0.15 -0.13 -0.11 -0.14 -0.10
-0.15 -0.13 -0.11 -0.14 -0.10
However, this phenomenon can be simplified in the case where water is absent in the pure RTIL. A pure RTIL is composed of dissociated salts, which can be regarded as free ions. When the chitosan film is immersed in the pure RTIL, the free ions can distribute themselves on the outside of the chitosan hydrogel (i.e., a Donnan equilibrium is established), and a difference in ion concentration between the internal and external solutions of the chitosan film is generated. The internal solution is bound water within the chitosan film, which has 0% ion concentration, and the external solution is pure RTIL, which has a 100% ion concentration. A large osmotic pressure is generated by the concentration gradient generated by a solute such as an RTIL. The osmotic pressure is the main driving force along with the hydrostatic pressure, and this drives the volume flux of the solvent, such as water. Therefore, the pure RTIL leads to a shrinking of the chitosan film. The DSC thermograms depicted in Figure 2 show that a pure RTIL extracts water. Curves a and b are from the RTIL, with curve a being from pure [C4mim]BF4, and curve b from [C4mim]BF4 that contained a fully swollen hydrated chitosan hydrogel. The chitosan hydrogel was removed before the DSC examination shown in curve b. Curve c is the fully swollen hydrated chitosan hydrogel sample initially used in curve b after having been soaked in a pure [C4mim]BF4 solution and then removed. Curve d is from a chitosan hydrogel swollen with pure water only. In Figure 2, the peaks occurring around -1 °C denote the melting point of water. The peak from water existing inside the chitosan hydrogel was obtained from curve d. The peak from water existing inside the chitosan hydrogel disappeared in curve c, while the peak from water was also obtained from the [C4mim]BF4 in curve b. These results are in good agreement with the swelling data shown in Table 1 and the decrease in EBC shown in Figure 1, meaning that the RTILs are capable of extracting the water contained within a chitosan hydrogel network when in contact with hydrogels containing water.
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Figure 2. DSC thermograms for water in a [C4mim]BF4 and chitosan hydrogel: pure [C4mim]BF4 (a) and [C4mim]BF4 (b) containing water from a fully water-swollen chitosan hydrogel, a chitosan hydrogel soaked in [C4mim]BF4 (c), and a chitosan hydrogel swollen with pure water (d).
The swelling behavior of chitosan hydrogels in ionic liquidwater binary systems was studied next. All the EBC values increased with increasing water content in the binary mixtures, as shown in Figure 1. In the case of [C4mim]BF4-water and [C6mim]Cl-water binary systems, the final EBC value of the chitosan hydrogel was higher than the equilibrium water content (EWC) of the chitosan hydrogel, and dissociation of the chitosan hydrogel began at a water content of 80 wt %. An EBC value at 90% water content was not determined because these chitosan hydrogels were fully dissociated and dissolved in the binary solvent. The pH of each binary system was measured at 90% water content to interpret the swelling behavior of chitosan hydrogels in the binary systems. Although measuring pH using a pH meter may have difficulties caused by the high ion concentration in binary mixtures with a high content of ionic liquid, at 90% water, the ion concentration is 0.01 molar and should not adversely interfere with the pH measurement. In ionic liquid-water binary systems, ionic liquids form acidic or basic solutions when they are in contact with water.33 The chitosan hydrogels have a pHdependent swelling behavior caused by the dissociation of -NH3+ ions, with a dissociation rate constant, pKa, of 6.3.34,35 The measured pH values for the 90% water binary systems were 3.4, 3.9, 8.1, 8.7, and 8.2 for [C4mim]BF4-water, [C6mim]Cl-water, [C2mim]SCN-water, [C2mim]atf-water, and [C3mim]I-water binary systems, respectively. The pH of the [C4mim]BF4-water or [C6mim]Cl-water binary systems was below the dissociation pKa of the chitosan hydrogels. Thus, the value of EBC in these systems increased to a value higher than the EWC of chitosan. All binary systems with pH > pKa achieved approximately the same maximum EBC as that found with pure water. The pH behavior of the [C4mim]BF4-water binary system was studied in more detail because of the high swelling achieved with this system. The pH of all binary mixtures was determined using the standard pH meter, although it is acknowledged that some interference may occur at the low water contents due to the high ion concentration. Figure 3 shows that the overall pH (33) MacFarlane, D. R.; Forsyth, S. A. ACS Symp. Ser. 2003, 856, 264. (34) Rinaudo, M.; Pavlov, G.; Desbrieres, J. Polymer 1999, 40, 7029. (35) Sorlier, P.; Denuziere, A.; Viton, C.; Domard, A. Biomacromolecules 2001, 2, 765.
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Figure 3. Change in pH of the [C4mim]BF4-water binary system and the EBC with increasing water content.
of the binary system was acidic, with pH < 3.4 for all mixtures. The pH tended to increase with increasing water content above 40%, as shown in Figure 3. In general, the EWC value of a chitosan hydrogel depends on the pH and decreases with increasing pH36 because chitosan hydrogels contain pH-responsive groups, such as NH3+ ions, which dissociate in acidic solutions. The increase in the pH of a solution inevitably leads to a change in the background ionic strength of the chitosan. Since all the counterions associated with the ionized units are released from inside the polymeric network by the weak electrostatic attraction, the lost charge comes from the chitosan’s own backbone (NH3+ f NH2), and the background ionic strength of the chitosan becomes stronger (i.e., the local electrostatic repulsion becomes weaker). The increase in pH of the solution leads to a decrease in the EWC of the chitosan hydrogel. Figure 3 shows that the EBC of the chitosan hydrogels apparently increased with increasing pH of the solution above a water content of 40%. It should be noted, however, that the pH of all binary mixtures in this system is well below the pKa of chitosan. The swelling behavior of chitosan hydrogels depends on the amount of water in the binary system. Conversely, as the amount of water in the surrounding liquid increases, the difference in ionic concentration also increases, leading to more swelling due to the higher osmotic pressure. As shown in Figure 3, the chitosan approaches the maximum amount of swelling in a 20% [C4mim]BF4-80% water mixture, and full dissolution of the polymer occurs in the 90% mixture. Figure 4 shows that the [C4mim]BF4 imbibed into the chitosan by soaking in [C4mim]BF4-water mixtures remained in the fully dried chitosan films because of the nonvolatility of the RTIL. The amount of [C4mim]BF4 remaining in the dry chitosan film increased with increasing water content of the soaking solution. These results suggest that the water had interacted with the RTIL in the binary system. Seddon et al.29 reported that water is accommodated in the ionic liquid structure in the ionic liquidrich region (i.e., where the mole fraction of the ionic liquid (xs) in the binary system is 0.5 < xs e1.0), possibly by forming hydrogen bonds with both the anions and the cations of the ionic liquid. The accommodated water molecules reduce the electrostatic attraction between the ions of the ionic liquid, and then the viscosity of the binary system decreases markedly. The decrease in viscosity supports easier swelling in the hydrogel. Upon further addition of water (xs < 0.5, i.e., the water-rich region), the free (36) Kim, S. J.; Park, S. J.; Kim, S. I. React. Funct. Polym. 2003, 55, 53.
Figure 4. Remnant [C4mim]BF4 in a chitosan film after drying a chitosan hydrogel swollen with a binary liquid system.
water molecules that do not form hydrogen bonds with the ionic liquid appear as surplus water molecules that dissolve the RTIL ions in the binary system. The water molecules accommodated in the ionic liquid initiate swelling of the chitosan hydrogel, and the increase in the amount of free water leads to the promotion of swelling in the chitosan hydrogels because it leads to an additional local electrostatic repulsion of the chitosan. In Figure 4, the water content (wt %) is represented as the mole fraction of [C4mim]BF4 (xs), that is, 0.44, 0.26, 0.17, 0.12, 0.08, 0.06, 0.04, 0.02, and 0.01 as the water content increased from 10 to 90% in steps of 10%, respectively. Remnant [C4mim]BF4 was not observed at 10% water content, where xs < 0.5, and there was no free water. However, for all other binary mixtures, remnant [C4mim]BF4 was found in the hydrogel after drying. In these systems, xs > 0.5, so that these binary systems were in the water-rich region. The RTILs formed a symmetrical complex by interacting with water molecules via hydrogen bonds when the ionic liquid was accommodated by the water molecules (i.e., in the water-rich region).26,37,38 It is expected that the size of this symmetrical complex is too large to penetrate into the chitosan hydrogel network because the EBC of the chitosan hydrogels is around 10% at a water content of 10%. This means that the pores of the chitosan hydrogel networks are small. At a water content of 40% and above, the sudden increase in remnant [C4mim]BF4 means that enough free water was present to increase the pore size in the chitosan hydrogel and allow a greater ingress of RTIL complexes. These results are in good agreement with the data shown in Figures 1 and 3, where a water content of 40% is a point of inflection in the EBC and pH curves. In addition, it was confirmed from the mole fraction of [C4mim]BF4 that the EBC behavior of the chitosan hydrogels did not depend on the pH; that is, the EBC behavior of the chitosan hydrogels mainly depended on the amount of free water in the binary system. Consequently, the dissolved ions of the RTIL (i.e., [BMI]+ and [BF4]-) form a symmetrical complex by interacting with water molecules via hydrogen bonds. When the amount of free water molecules is not high enough to swell the hydrogel in the binary system, the symmetrical complex cannot penetrate into (37) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (38) Neve, F.; Francescangeli, O.; Crispin, A.; Charmant, J. Chem. Mater. 2001, 13, 2032.
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the chitosan hydrogel network sufficiently because the chitosan hydrogel network’s pore size is too small. The narrow paths of the chitosan hydrogel network are also an obstruction to the penetration of cations, such as imidazolium salts, because the imidazolium ion is larger than the RTIL anions, which aggregate to form micelles when dissolved in water in the water-rich region. The degree of swelling of the chitosan hydrogels increased with increasing free water content. The increase in the degree of swelling of the chitosan hydrogels induces a strong repulsion between the chitosan backbones, which results in an extended pathway. Therefore, a symmetrical complex or micelle of the cations can directly fully penetrate into the chitosan hydrogel networks due to the free water. The penetration of pure [C4mim]BF4 was studied by dropping pure RTIL onto the surface of a preswollen chitosan hydrogel with water only. We found that the pure RTIL can penetrate into the preswollen chitosan hydrogels with water only. After pure RTIL was dropped onto a surface of chitosan hydrogel, the chitosan hydrogel was dried, and then the remnant RTIL in the chitosan networks was compared to the results shown in Figure 4. The amount of remnant RTIL was almost the same as that shown in Figure 4. This suggests that the RTIL can penetrate into the swollen chitosan hydrogel network directly by itself, which means that the RTIL ions can have mobility in the swollen chitosan networks. By using the direct dropping RTIL effect, the chitosan hydrogels containing the RTIL were prepared easily. The hardness of the dried samples was also determined to investigate the degree of mixing between the RTIL and the chitosan. The initial dry chitosan film had a Shore A hardness of 95. After soaking in an 20% [C4mim]BF4-80% water binary
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system, the hardness decreased to 55 Shore A. Upon subsequent removal of the water, the hardness increased to 73 Shore A, which is significantly lower than the original value. The results show that the remnant RTIL is molecularly mixed with the chitosan and does not form separate phases within the glassy chitosan matrix. In other words, the RTIL, once imbibed into the chitosan, acts as an effective plasticizer through weak interactions between the chitosan and RTIL ions.
Conclusion We have elucidated the effect of RTILs on the swelling properties of chitosan hydrogels: (1) Pure RTILs change the osmotic pressure outside the chitosan hydrogel, leading to a deswelling of the chitosan hydrogel. (2) The swelling behavior of chitosan hydrogels in a binary system mainly depends on the free water, even when the pH of the binary system is changed by the RTIL. (3) Chitosan hydrogels containing an RTIL can be made by direct addition of the pure ionic liquid onto fully waterswollen hydrogels, which means that the RTIL ions are mobile in the swollen chitosan networks. By elucidating the swelling mechanism of chitosan hydrogels, we confirm the possibility of using chitosan hydrogels containing an RTIL in polymeric conducting systems. Acknowledgment. This work was supported by Creative Research Initiative Center for Bio-Artificial Muscle of the Ministry of Science & Technology (MOST)/the Korea Science and Engineering Foundation (KOSEF) in Korea. LA061586R
