Lower Critical Solution Temperature Phase Behavior of Linear

The solubility and phase behavior of linear polymethacrylate polymers, primarily poly(phenylalkyl methacrylate)s, in imidazolium-based ionic liquids (...
10 downloads 4 Views 170KB Size
Download PDF
3820

Langmuir 2009, 25, 3820-3824

Lower Critical Solution Temperature Phase Behavior of Linear Polymers in Imidazolium-Based Ionic Liquids: Effects of Structural Modifications Koichi Kodama, Hideyuki Nanashima, Takeshi Ueki, Hisashi Kokubo, and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed NoVember 29, 2008. ReVised Manuscript ReceiVed January 14, 2009 The solubility and phase behavior of linear polymethacrylate polymers, primarily poly(phenylalkyl methacrylate)s, in imidazolium-based ionic liquids (ILs) were systematically studied by changing the structure of each component. Solutions of polymethacrylates in 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Cnmim][NTf2]) showed lower critical solution temperature (LCST) phase behavior, and the phase separation temperature (Tc) could be varied by selecting an appropriate combination of a polymer and an IL. An increase in alkyl chain length between the phenyl and ester groups in the polymer side chain decreased the Tc; alternatively, substitution of the imidazolium cation with a longer alkyl chain increased the Tc. When the same anion was used, the miscibility of the polymer/IL system was mainly determined by the alkyl chain length. Tc could also be varied by mixing two ILs in an appropriate ratio. In addition, the kinetics of the reversible phase transition phenomena exhibited by these polymers were examined. Redissolution kinetics were largely controlled by the magnitude of the difference between Tc and the glass transition temperature (Tg) of the polymer (Tc - Tg), in addition to the mutual affinity between the polymer and the IL.

Introduction Much attention has been paid to ionic liquids (ILs) because of their unique and advantageous properties, including negligible volatility, nonflammability, and thermal and electrochemical stability. ILs have been applied as catalyst and reaction media for chemical reactions, electrolytes in electrochemical devices, separation technologies, and many other uses.1 A distinct characteristic of ILs is that they are entirely composed of ions while liquid at room temperature, and this characteristic gives them unique properties as solvents. Another advantage of ILs is the ease of varying their properties by changing the combination of cations and anions. A variety of cations and anions have already been shown to afford ILs with low melting points.2 These ILs are considered to be a new class of green solvents that are actually nonvolatile in contrast with conventional organic solvents, and therefore, it is important to gain a detailed understanding of their properties. In particular, the solubility of various compounds in ILs is an important area of research. Many studies on liquid-liquid phase equilibrium for ILs/ water or ILs/common organic solvents have been carried out,3 which have successfully demonstrated the possibility of using * To whom correspondence should be addressed. Tel/Fax: +81-45-3393955. E-mail: [email protected]. (1) (a) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (b) Sheldon, R. Chem. Commun. 2001, 2399. (c) Ueki, T.; Watanabe, M. Macromolecules 2008, 41, 3739. (d) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123. (2) (a) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.;WileyVCH Verlag, GmbH & Co, KgaA: Weinheim, 2003. (b) Welton, T. Chem. ReV. 1999, 99, 2071. (3) (a) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; J. D.; Rogers, R. D. Chem. Commun. 1998, 1765. (b) Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Chem. Commun. 2001, 2070. (c) Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z. P.; Nunes da Ponte, M.; Szydlowski, J.; Cerdeirina, C. A.; Troncoso, J.; Romani, L.; Esperanca, J. M. S. S.; Guedes, H. J. R.; Sousa, H. C. Green Chem. 2004, 6, 369. (d) Fukumoto, K.; Ohno, H. Angew. Chem., Int. Ed. 2007, 46, 1852. (e) Fukaya, Y.; Sekikawa, K.; Murata, K.; Nakamura, N.; Ohno, H. Chem. Commun. 2007, 3089. (f) Trindade, J. R.; Visak, Z. P.; Blesic, M.; Marrucho, I. M.; Coutinho, J. A. P.; Lopes, J. N. C.; Rebelo, L. P. N. J. Phys. Chem. B 2007, 111, 4737.

ILs as reaction or extraction media. It has been shown that representative hydrophobic ILs, such as 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Cnmim][NTf2]), can be useful in extracting aromatic compounds from aliphatic hydrocarbons.4 Brennecke et al. systematically studied the phase behavior of ILs/alcohols and demonstrated their upper critical solution temperature (UCST) phase behavior.5 Other related studies involving ILs/haloalkanes,6 ILs/nicotine,7 and IL/IL equilibrium systems8 have shown that various ILs can potentially be applied to the separation process. All of these studies have contributed to a fundamental understanding of ILs. The authors previously reported a novel phase transition behavior of linear polymers in ILs. Poly(N-isopropyl acrylamide) (PNIPAm), a representative thermo-responsive polymer that exhibits a lower critical solution temperature (LCST) phase behavior in an aqueous solution, showed the opposite UCST phase behavior in [C2mim][NTf2].9 However, poly(benzyl methacrylate) (PBnMA, 1a) and its copolymers with styrene or methyl methacrylate showed LCST phase behavior in (4) (a) Zhang, S.; Zhang, Z. C. Green Chem. 2002, 4, 376. (b) Su, B. M.; Zhang, S.; Zhang, Z. C. J. Phys. Chem. B 2004, 108, 19510. (c) Domanska, U.; Marciniak, A. J. Chem. Thermodyn. 2005, 37, 577. (d) Lachwa, J.; Szydlowski, J.; Makowska, A.; Seddon, K. R.; Esperanca, J. M. S. S.; Guedes, H. J. R.; Rebelo, L. P. N. Green Chem. 2006, 8, 262. (e) Domanska, U.; Casas, L. M. J. Phys. Chem. B 2007, 111, 4109. (f) Arce, A.; Earle, M. J.; Rodriguez, H.; Seddon, K. R. J. Phys. Chem. B 2007, 111, 4732. (5) (a) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2004, 108, 5113. (b) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. Fluid Phase Equilib. 2005, 228-229, 303. (c) Crosthwaite, J. M.; Muldoon, M. J.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2006, 110, 9354. (6) (a) Lachwa, J.; Szydlowski, J.; Najdanovic-Visak, V.; Rebelo, L. P. N.; Seddon, K. R.; Ponte, M. N.; Esperancu¨a, J. M. S. S.; Guedes, H. J. R. J. Am. Chem. Soc. 2005, 127, 6542. (b) Shiflett, M. B.; Yokozeki, A. J. Chem. Eng. Data 2007, 52, 2007. (c) Shiflett, M. B.; Yokozeki, A. Fluid Phase Equilib. 2007, 259, 210. (7) Visak, Z. P.; Yague, S. L.; Lopes, J. C.; Rebelo, L. P. N. J. Phys. Chem. B 2007, 111, 7934. (8) Arce, A.; Earle, M. J.; Katdare, S. P.; Rodriguez, H.; Seddon, K. R. Chem. Commun. 2006, 2548. (9) Ueki, T.; Watanabe, M. Chem. Lett. 2006, 35, 964.

10.1021/la803945n CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

LCST Phase BehaVior of Polymers in Ionic Liquids

[C2mim][NTf2], exhibiting low-temperature solution phases and high-temperature phase separations.10 These systems were successfully developed to produce thermo-responsive polymer gels with a volume phase transition, using [C2mim][NTf2] as a solvent. The polymer gels were swollen in ILs at low temperatures and shrunken at temperatures higher than the phase separation temperature (Tc) in a reversible manner.10 Recently, the authors have also reported full LCST phase diagrams for poly(ethyl glycidyl ether)s (PEGEs) in [C2mim][NTf2], which undergo liquid-liquid phase separations.11 Several recent reports have identified structure-making interactions between aromatic compounds and IL cations.12 It is expected that the aromatic side chain of PBnMA plays a crucial role in LCST behavior; however, the details of this mechanism are unknown. In this study, to understand this phase behavior with an eccentric entropy change, several PBnMA derivatives were prepared, and the structural effects of polymers and ILs on the phase transition were systematically investigated. How Tc can be varied by mixing two ILs in differing ratios was also explored. In addition, the kinetics of the reversible phase transition phenomena between transparent, homogeneous phases and turbid, separated phases were examined. Finally, the redissolution kinetics of polymers in ILs is discussed in terms of the difference between Tc and the glass transition temperature (Tg) (Tc - Tg) for each polymer, as well as their mutual affinities.

Langmuir, Vol. 25, No. 6, 2009 3821 Chart 1. Structures of poly(benzyl methacrylate) derivatives and ILs used in this study

Experimental Section Preparation and Characterization of Materials. All of the methacrylate monomers other than benzyl methacrylate were synthesized from methacryloyl chloride and the corresponding alcohols. Benzyl methacrylate was purchased from Wako Pure Chemicals. Polymethacrylates (Chart 1, 1a-i) were prepared by the free radical polymerization method (see Supporting Information, SI). Chemical identification of the obtained polymers was verified using 1H NMR (JEOL AL400). Number average molecular weight (Mn) and polydispersity index (Mw/Mn) of the obtained polymers were estimated using gel permeation chromatography (GPC; Shimadzu) with Tosoh columns using tetrahydrofuran (THF) as the eluent and polystyrene standard samples as a reference. Tg’s of the polymers were determined using differential scanning calorimetry (DSC 220C; Seiko Instruments) measurements. The ILs (Chart 1) [Cnmim][NTf2], 1-butyl-2,3-dimethylimidazolium bis(trifluoromethane sulfonyl)imide ([C4dmim][NTf2]), and 1-butylmethylpyrrolidinium bis(trifluoromethane sulfonyl)imide ([bmpro][NTf2]) were prepared and characterized according to previously reported procedures.13 Measurement of Transmittance of Polymer/IL Solutions. A drop of the polymer solution (3 wt%) was placed on a concave slide glass and covered with a cover glass. The slide glass was placed on a hot stage (Imoto, Japan) that enabled temperature control up to 400 °C. The temperature was increased from the ambient temperature to 250 °C by 1 °C/min, and the transmittance of the PBnMA/IL solution was monitored at 500 nm with a USB 2000 fiber optic spectrometer (Ocean Optics). The upper-limit temperature was determined taking into account the possibility of decomposition of the polymethacrylates. In this study, Tc was defined as the temperature at which the transmittance became 50%. To evaluate the rate of (10) (a) Ueki, T.; Watanabe, M. Langmuir 2007, 23, 988. (b) Ueki, T.; Karino, T.; Kobayashi, Y.; Shibayama, M.; Watanabe, M. J. Phys. Chem. B 2007, 111, 4750. (11) Tsuda, R.; Kodama, K.; Ueki, T.; Kokubo, H.; Imabayashi, S.; Watanabe, M. Chem. Commun. 2008, 4939. (12) (a) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Chem. Commun. 2003, 476. (b) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.; Sheppard, O.; Soper, A. K. J. Phys. Chem. B 2005, 109, 1593. (c) Lachwa, J.; Bento, I.; Duarte, M. T.; Lopes, J. N. C.; Rebelo, L. P. N. Chem. Commun. 2006, 2445. (13) (a) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593. (b) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103.

Table 1. Mn, Mw/Mn, and Tg of the Poly(methacrylate) Derivatives Used in This Study Mna 1a 1b 1c 1d 1e 1f 1g 1h 1i

2.3 × 10 1.9 × 104 7.8 × 104 7.5 × 104 7.3 × 104 7.4 × 104 7.6 × 104 5.1 × 104 6.4 × 104 4

Mw/Mn

Tg (°C)

2.19 2.38 2.33 1.74 2.28 1.94 1.91 1.93 1.73

66 68 45 41 33 50 38 -57 51

a Mn and Mw/Mn were determined using GPC calibrated with polystyrene standards.

redissolution, transmittance spectra of the sample were periodically recorded while decreasing the temperature from (Tc + 10) °C to (Tc - 10) °C. First, the sample was heated to (Tc + 10) °C and let to stand for 5 min at that temperature, then the phase-separated solution was cooled to (Tc - 10) °C.

Results and Discussion To investigate the effects of the structures of polymers and ILs on the phase behavior of their mixtures, 9 different polymethacrylates 1a-i were prepared with different substituents on their side chains (Chart 1). Mn, Mw/Mn, and Tg of the synthesized polymers are summarized in Table 1. Six different ILs were also used in this study, selected for their low melting points, high stabilities, and ability to vary their polarity by changing the alkyl chain length on the imidazolium cation (Chart 1). Influence of Polymer and Ionic Liquid Structures on Tc. The solubility and Tc of the 3-wt% polymethacrylate/[Cnmim][NTf2] (n ) 2, 4, 6, and 8) solutions were examined, and the results are summarized in Table 2. Polymers 1b-d with solvatophobic substituents on the m-position of the phenyl group

3822 Langmuir, Vol. 25, No. 6, 2009

Kodama et al.

Table 2. LCST Solubility Behavior of 3-wt% Solutions of Polymethacrylates (1a-i) in [Cnmim][NTf2] (n ) 2, 4, 6, 8) from 30 to 250 °C [C2mim][NTf2] [C4mim][NTf2] [C6mim][NTf2] [C8mim][NTf2] 1a 1b 1c 1d 1e 1f 1g 1h 1i a

105 °C insoluble insoluble insoluble 54 °C 87 °C 42 °C insoluble soluble

158 °C insoluble insoluble insoluble 130 °C 168 °C 118 °C insoluble soluble

225 °C insoluble insoluble insoluble soluble soluble 218 °C 144 °C

a

soluble insoluble insoluble 162 °C soluble soluble soluble 242 °C

a

Not determined.

Figure 1. Temperature dependence of transmittance at 500 nm for 3-wt% solutions of polymethacrylates/[C2mim][NTf2] in the heating process. Black triangle: 1a/[C2mim][NTf2]; black square: 1e/[C2mim][NTf2]; black diamond: 1f/[C2mim][NTf2]; black circle: 1g/[C2mim][NTf2].

of the side chain of 1a were insoluble in [C2mim][NTf2] in the measured temperature range and had lower solubility than 1a. Alternatively, polymers 1e and 1f with methoxy- and fluorogroups on the benzene ring showed LCST phase behavior, and their Tc’s were 54 and 87 °C, respectively. The temperature dependence of their transmittance at 500 nm is shown in Figure 1. The Tc’s of 1e and 1f were much lower than that of 1a. It is also interesting to note that polymers 1g and 1h, with longer alkyl spacers between the phenyl and the ester groups in their side chains, had lower Tc’s than 1a. The Tc of the 1g/ [C2mim][NTf2] solution was 42 °C, which is close to ambient temperature; 1h and [C2mim][NTf2] were immiscible in the measured temperature range. In contrast, a polar polymethacrylate, poly(2-pyridylmethyl methacrylate) (1i), was dissolved in [C2mim][NTf2] from 30 to 250 °C, indicating a much higher solubility than 1a. The LCST phase behavior of these methacrylate polymers in ILs phenomenologically resembles that of PNIPAm derivatives in aqueous solutions. The Tc’s of aqueous solutions of PNIPAm derivatives are known to be strongly dependent on the hydrophilic and hydrophobic balance of the polymers. In PNIPAm, the amide group is hydrophilic, whereas the isopropyl group and the hydrocarbon main chain are hydrophobic. In aqueous solutions of PNIPAm, the introduction of hydrophobic substituents into PNIPAm results in a lower LCST.14 It has been shown that many common acrylate and methacrylate polymers are soluble in [C2mim][NTf2], whereas hydrophobic nonpolar and low-polar polymers, such as polyethylene, polypropylene, and polystyrene, are not soluble.9 Thus, PBnMA can be recognized as an amphiphilic polymer toward [C2mim][NTf2], whose ester group (14) Taylor, L. D.; Cerankowski, L. D. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 2551.

functions as a solvatophilic group, while the phenyl group and the main chain function as solvatophobic groups. Regarding the phase behavior of PBnMA-derived polymers, chemical modification of the phenyl groups on the side chains of the polymers dramatically changes their solubility, which suggests that the phenyl groups play an important role in determining the Tc of the polymers in IL solution. In the case of binary systems with critical solution temperatures, such as alcohols/ILs and aromatic hydrocarbons/ILs, the introduction of longer alkyl groups to the substrate lowers the miscibility between the substrate and ILs.4,5 As previously reported, the Tcs of PBnMAs/[Cnmim][NTf2] were highly dependent on the alkyl chain lengths of the imidazolium cations, and substitution with a longer alkyl chain increased the Tc.10a The solubilities of various polymethacrylates in several [Cnmim][NTf2] ILs were further examined to determine the effect of the IL structure. The IL anion was fixed as [NTf2], and [Cnmim]s with different alkyl chain lengths were selected as the cations. The solubility and Tc’s of 1e-i in [Cnmim][NTf2] (n ) 2, 4, 6, and 8) are summarized in Table 2. The LCST phase transition behaviors were observed in all of these ILs. Similar to 1a/[Cnmim][NTf2], Tc and mutual solubility increase as the alkyl chain of the imidazolium cation is lengthened from ethyl to butyl, hexyl, and octyl groups. 1d and 1h are immiscible in [C2mim][NTf2]; however, they also exhibit LCST phase behavior in the ILs with longer alkyl chains such as [C8mim][NTf2]. However, 1e-g are soluble in [C8mim][NTf2], which indicates that their Tc’s are higher than the upper limit of the measured temperature range (30-250 °C). A similar trend of increased miscibility with longer alkyl chains in the imidazolium cations was also observed in previously reported substrate/IL binary mixture systems.4-6 The phase behavior of 1a and 1g in 1-butyl-2,3-dimethylimidazolium bis(trifluoromethane sulfonyl)imide ([C4dmim][NTf2]) was also investigated. The most acidic proton at the imidazolium C2 position of [C4mim][NTf2] is replaced with a methyl group in [C4dmim][NTf2]. The Tc’s of 1a/[C4dmim][NTf2] and 1g/ [C4dmim][NTf2] were 176 and 136 °C, respectively and not substantially different from those in [C4mim][NTf2] (SI, Figure S1). The most acidic proton is primarily responsible for hydrogen bonds between the IL and the substrate, therefore, it appears that hydrogen bonding interactions are not crucial to the phase behavior of these polymer/IL mixture systems. This is in contrast with the alcohol/IL and PEGE/IL mixture systems, in which hydrogen bonds play an important role and the Tc is significantly changed by the introduction of a methyl group at the C2 position of the imidazolium cation.5,11 Change in Tc with Polymer and Ionic Liquid Structures. LCST phase separation occurs when both the enthalpy change (∆Hmix) and the entropy change (∆Smix) for mixing become negative and the entropic contribution (∆SmixT) to the Gibbs energy change (∆Gmix) becomes greater than ∆Hmix. This unique phase behavior has primarily been observed in aqueous solutions of polymers, including PNIPAm. Hydrophobic hydration in aqueous solutions is structure-making solvation, and is considered to be a reason for the negative ∆Smix that leads to LCST phase separation. The LCST phase behavior of aqueous PNIPAm solutions is also attributed to entropic changes associated with hydration-dehydration processes of polymer chains.15 It has recently been reported in various molecular simulation studies16 that ILs exhibit medium-range ordering, that is, segregated structures of alkylimidazolium-based ILs, consisting (15) (a) Okada, Y.; Tanaka, F. Macromolecules 2005, 38, 4465. (b) Ono, Y.; Shikata, T. J. Am. Chem. Soc. 2006, 128, 10030. (c) Ono, Y.; Shikata, T. J. Phys. Chem. B 2007, 111, 1511.

LCST Phase BehaVior of Polymers in Ionic Liquids

of polar ionic domains and nonpolar alkyl chain domains. The existence of microphase segregation between polar and nonpolar domains changes the way in which polar, nonpolar, and associating solutes are solvated in these imidazolium-based ILs.16c Nonpolar solutes can be solvated in nonpolar domains, polar solutes can be solvated in polar domains, and associating solutes can be solvated between nonpolar and polar domains.16c It is also known that [Cnmim][NTf2] and [Cnmim][PF6] form inclusion crystals or liquid clathrates with small aromatic molecules stabilized by favorable interactions with both cations and anions,12 where the cations interact with π-electrons of the aromatic rings through cation-π interaction, and the anions interact with the C-H bonds of the aromatic rings. These results explain the solubilization of aromatic compounds such as benzene and thiophene in imidazolium-based ILs and also the LCST phase behavior of mixtures of aromatic compounds and ILs. The formation of a liquid clathrate can decrease the mixing entropy and satisfy the conditions for the LCST phase behavior. In the present polymeric cases, it is expected that the side chains of the polymers are preferentially accommodated in the polar regions of the ILs. The negative ∆Hmix and ∆Smix necessary for the LCST behavior may be attributed to the formation of a particular clathrate-like structure of cations around the aromatic groups. The introduction of nonpolar or low-polar substituents on the phenyl groups (1b-e) or the longer alkyl chain between the ester and aromatic groups (1 g-h) may prevent this favorable interaction between the polymer side chain and the polar region of the IL, resulting in decreased miscibility and decreased Tc. The introduction of polarity, while maintaining aromaticity in the polymer side chain (1i), increases miscibility. However, increasing the alkyl chain length on the imidazolium cation increases the extent of microphase segregation of the ILs, which would decrease the entropy of the ILs. Thus, the mixing entropy decrease induced by the particular clathrate-like structure would be reduced, leading to an increase in Tc. It is also possible that more extended microphase separation with longer alkyl chain length promotes preferential interactions of the side chains with the ionic groups and of the main chain with the alkyl chain, which enhances enthalpic stabilization and causes an increase in Tc. On the basis of these results, a subtle difference in the polymer and IL structures significantly changes their miscibility. The authors recently demonstrated that the ∆Hmix and ∆Smix values of 1a in [C1min][NTf2] and [C2min][NTf2] were much smaller than those of PNIPAm in water.17 The striking variation in Tc with only a small change in the polymer and IL structures can be explained by the much smaller magnitudes of ∆Hmix and ∆Smix. Thus, a small change in the magnitudes of of ∆Hmix and ∆Smix caused by the structural changes can lead to a large variation in Tc. Binary Blends of Ionic Liquids. The LCST of an aqueous PNIPAm solution can be altered by end groups, tacticity of PNIPAm, copolymerization with other monomers, addition of inorganic salts, and other factors.14,18 Another method of changing the LCST is the addition of an appropriate organic solvent to water; however, complicated cononsolvency phenomena have been reported.19 (16) (a) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (b) Lopes, J. N. A. C.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 3330. (c) Lopes, J. N. A. C.; Gomes, M. F. C.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 16816. (17) Ueki, T.; Arai, A. A.; Kodama, K.; Kaino, S.; Tanaka, N.; Morita, T.; Nishikawa, K.; Watanabe, M. Pure Appl. Chem., in press. (18) (a) Ray, B.; Isobe, Y.; Matsumoto, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2004, 37, 1702. (b) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505. (c) Furyk, S.; Zhang, Y.; Ortiz-Acosta, D.; Cremer, P. S.; Bergbreiter, D. E. J. Polym. Sci. Part A 2006, 44, 1492.

Langmuir, Vol. 25, No. 6, 2009 3823

Figure 2. Phase transition temperatures of 3-wt% solutions of 1g/ [C2mim][NTf2] + [C4mim][NTf2] with various mixing ratios.

This study focused on binary mixtures of ILs with different alkyl chain lengths in their imidazolium cations and explored the effect of mixing ratios on Tc. Figure 2 shows the measured Tc value of the 3-wt% solutions of 1g in [C2mim][NTf2]/ [C4mim][NTf2] with various mixing ratios. The Tc values changed almost linearly from 42 °C for [C2mim][NTf2] to 118 °C for [C4mim][NTf2] as the weight fraction of [C2mim][NTf2] increased. This result suggests that the phase transition phenomenon and Tc are determined solely by the ratio of the two ILs. A similar phenomenon was observed in the case of [C2mim][NTf2]/ [C6mim][NTf2]. The phase transition temperature of the 1g solution in a 1:1 mixture of [C2mim][NTf2]/[C6mim][NTf2] was 132 °C, close to the calculated average temperature of 130 °C. The phase transition temperature of 1g in [C8mim][NTf2] was not determined because it is soluble throughout the measured temperature range; however, the calculated phase transition temperature of 1h in [C8mim][NTf2] is expected to be around 294 °C, based on that of a 1:1 mixture of [C2mim][NTf2]/ [C8mim][NTf2]. A similar trend was observed for mixtures of two ILs with different cations, [C2mim][NTf2]/1-butylmethylpyrrolidinium bis(trifluoromethane sulfonyl)imide ([bmpro][NTf2]) (SI, Figure S2). This linear change is possibly derived from the unique characteristics of ILs, in that there is only a slight difference between two ILs with a common anion, compared with water/ organic solvent mixtures. It is suggested that Tc can be controlled easily by appropriately adjusting the mixing ratio of two ILs. In addition, the observed linear relationship can be used to estimate a Tc that is too low or too high to be measured directly. Kinetics of the Phase Transition Phenomena. Reversible LCST phase behaviors of aqueous PNIPAm solutions have been widely studied and successfully utilized for a thermo-responsive hydrogel.20 While the present polymethacrylate/IL systems are assumed to be applicable to thermo-responsive ion-gels, it is important to know the kinetics of the phase transition behavior. The phase transition phenomena shown above were reversible, and the turbid solution at high temperature became transparent upon cooling below the Tc. However, the kinetics across the Tc were rather different between the heating and cooling processes. In the case of 3-wt% 1a/[C2mim][NTf2], the phase separation phenomenon occurred immediately for all the polymer/IL combinations. Alternatively, redissolution to give a transparent (19) (a) Winnik, F. M.; Ottavianni, M. F.; Bossmann, S. H.; Pan, W.; GarciaGaribay, M.; Turro, N. J. Macromolecules 1993, 26, 4577. (b) Asano, M.; Winnik, F. M.; Yamashita, T.; Horie, K. Macromolecules 1995, 28, 5861. (c) Dalkas, G.; Pagonis, K.; Bokias, G. Polymer 2006, 47, 243. (20) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163.

3824 Langmuir, Vol. 25, No. 6, 2009

Figure 3. Time dependence of the transmittance at 500 nm for 3-wt% solutions of 1a/[Cnmim][NTf2] and 1g/[Cnmim][NTf2] (n ) 2, 4, 6) after a temperature decrease from (Tc + 10) °C to (Tc - 10) °C.

solution by cooling occurred slowly. To achieve a more rapid and reversible response to temperature, the kinetics of the redissolution of 1a/[Cnmim][NTf2] and 1g/[Cnmim][NTf2] solutions were studied in detail. Figure 3 shows that the rate of the redissolution process is highly dependent on both the polymer structure and the cation structure of the ILs. For example, the required time for redissolution is shortened from 15 to 7 h when the polymer is changed from 1a to 1g in [C2mim][NTf2]. Moreover, the time is significantly shortened by increasing the alkyl chain length of the imidazolium cation. 1a/[C4mim][NTf2] rapidly redissolves within 3 min, and only 25 s are required in the case of 1a/ [C6mim][NTf2]. Polymer dissolution processes are affected not only by the interaction between the polymer and the solvent but also by swelling of the polymer by the solvent. The latter process appears to be the rate-determining step, which is a function of the mutual diffusivity between the polymer chain and the solvent. The viscosities of the ILs at the phase transition temperature, which determine the solvent diffusivities, were estimated from the Vogel-Fulcher-Tamman (VFT) equation and the reported VTF parameters for [Cnmim][NTf2].13b The dissolution rate of 1g/ [C4mim][NTf2] is much faster than that of 1a/[C2mim][NTf2] (Figure 3), despite their similar Tc’s (118 and 105 °C, respectively) and the similar viscosities of the ILs (4.83 and 5.59 mPa s,

Kodama et al.

respectively). This suggests that the viscosity of the ILs is not a crucial factor affecting the rate of dissolution. The difference between Tc and Tg for each polymer/IL system may be a plausible explanation for differences in the rate of dissolution, since the initial state of dissolution is primarily affected by polymer dynamics, which are governed by Tc - Tg.21 The Tc - Tg term for each combination of polymer/IL decreases in the following order: 1g/[C6mim][NTf2] (180 °C) > 1a/[C6mim][NTf2] (159 °C) > 1a/[C4mim][NTf2] (92 °C) > 1g/[C4mim][NTf2] (80 °C) > 1a/[C2mim][NTf2] (39 °C) > 1g/[C2mim][NTf2] (4 °C). This order roughly agrees with the rate of redissolution of the phaseseparated polymer solutions. A closer look, however, reveals a degree of inconsistency with the magnitude of Tc - Tg, which may be interpreted as the effect of the interaction between the polymer and the IL. Thus, it appears to be largely possible to accelerate the redissolution process of polymers and swelling process of polymer gels in ILs by adjusting the term Tc - Tg.

Conclusions In this study, the effects of systematic modification of the structures of poly(benzyl methacrylate) derivatives and ILs on their LCST phase transition behaviors were demonstrated. The Tc was primarily determined by the polarity of the polymer side chains and the alkyl chain length of the imidazolium cations. In addition, mixture of two ILs changed the Tc of the polymer solution linearly with the ratio of the two ILs, and the Tc was easily adjustable by mixing them in an appropriate ratio. Finally, the kinetics of the reversible phase transition phenomena were examined, and redissolution kinetics were found to be controlled by the magnitude of Tc - Tg. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (No. 452/17073009 and No. B/20350104) from the MEXT of Japan. Supporting Information Available: Detailed experimental procedures and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org. LA803945N (21) (a) Cohen, M. H.; Turnbull, D. J. Chem. Phys. 1959, 31, 1164. (b) Agam, G.; Gibbs, J. H. J. Chem. Phys. 1965, 43, 139.