Ionic Liquid Modifies the Lower Critical Solution Temperature (LCST

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Ionic Liquid Modifies the Lower Critical Solution Temperature (LCST) of Poly(N-isopropylacrylamide) in Aqueous Solution P. Madhusudhana Reddy and P. Venkatesu* Department of Chemistry, University of Delhi, Delhi-110 007, India ABSTRACT:

The effect of the imidazolium based ionic liquid (IL) 1-benzyl-3-methylimidazolium tetrafluoroborate ([Bzmim][BF4]) was investigated on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution by using fluorescence, viscometric, and dynamic light scattering (DLS) techniques. The measurements were performed at four different [Bmim][BF4] concentrations (1
4 mg/mL) in PNIPAM aqueous solution. Our experimental results elucidate that the IL induces the collapsed globular structure of polymer, facilitated by the weakening of hydrogen bonds between the amide group of the polymer and water molecules; therefore, IL destabilizes the hydrated macromolecule structure. We observed that the phase transition of PNIPAM aqueous solution abruptly shifts down with increasing IL concentration mainly due to hydrophobic collapse and aggregation of a macromolecule. These results unambiguously reveal that the imidazolium based IL significantly affected the phase transition of PNIPAM and ruptured the hydrogen bonding between polymer and water molecules, and eventually the hydrated macromolecule structure was destabilized.

’ INTRODUCTION Water-soluble amphiphilic polymers have received great attention and popularity in recent years due to their potential applications in biotechnology, medical diagnostics, drug screening, and other areas of actual interest.1
5 Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermo-responsive amphiphilic polymer, which is often used as a simple model for biomacromolecules.6,7 The schematic chemical structure of this macromolecule, which bears both a hydrophobic isopropyl group and a hydrophilic amide group, is provided in Figure 1. Apparently, PNIPAM is a water-soluble polymer whose aqueous solution shows a lower critical solution temperature (LCST) around 32
33 °C, mainly due to its interand intramolecular hydrogen bonds.1,8 PNIPAM exists in an expanded coil conformation in water, contributing to the homogeneous liquid phase at below its LCST. By increasing the temperature, hydrophobic collapse of polymer occurs, which leads to a change in conformation from coil to globular conformation. This property of PNIPAM has attracted considerable attention and has been extensively studied for the past three decades.1,8
11 It was found that the phase transition is independent of the molecular weight and concentration of PNIPAM.9 Several studies were performed on the effects of surfactants,12
16 salts,17
21 and cosolvents22
24 on the phase transition of PNIPAM in r 2011 American Chemical Society

aqueous medium. The existing results explicitly elucidate how the third component essentially affected the LCST of the PNIPAM solution. Nevertheless, the influence of ionic liquid (IL) on the LCST of PNIPAM in water, which is a new field of attractive and active research, has been rather scarcely investigated up to now. Currently, not only polymer chemists but also biologists are paying much attention toward ionic liquids (ILs) due to their specific applications such as negligible vapor pressure, nonflammability, and higher ionic conductivity.25
31 On the other hand, ILs are considered to be possible replacements of conventional organic solvents and solutes to reduce the environmental and economic costs in chemical industry.25,26 Presumably, the LCST behavior of PNIPAM in water is in marked contrast with its UCST behavior in IL.32 Imidazolium based IL has the ability to form hydrogen bonds with drugs and proteins.33 These ILs have been extensively and essentially used as sensors in electrochemical analysis for detecting organic and inorganic molecules.34
36 1-Benzyl-3-methyl imidazolium tetrafluoroborate [Bzmim][BF4] is a room temperature IL, and its Received: February 24, 2011 Revised: March 28, 2011 Published: April 05, 2011 4752

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Figure 1. Chemical structures of poly(N-isopropylacrylamide) and 1-benzyl-3-methylimidazolium tetrafluoroborate.

chemical structure is also included in Figure 1. The structure of [Bzmim][BF4] is constituted of a tetrafluoroborate inorganic anion and an imidazolium organic cation, which bears a positive charge on the nitrogen atom attached to the methyl group. Indeed, there is a lag of conclusions on the phase transition of PNIPAM aqueous solution with IL as an additive. On the experimental side, to the best of our knowledge, this is the first direct measurement of IL as an additive that weakened the hydrogen bonds between the amide group of the polymer and water molecules. The present study illustrates the effect of IL ([Bzmim][BF4]) on the LCST of PNIPAM in water by using fluorescence, viscometri,c and dynamic light scattering (DLS) techniques as a function of IL concentration at various temperatures under atmospheric pressure.

’ EXPERIMENTAL SECTION Materials. PNIPAM (Mn = 20000
25000) was obtained from Sigma-Aldrich, and [Bzmim][BF4] was obtained from Fluka Biochemical Company. Double distilled deionized water at 18.3 MΩ resistivity was used for sample preparation. Sample Preparation. All mixture samples were prepared gravimetrically using a Mettler Toledo balance with a precision of (0.0001 g. For fluorescence studies, samples were prepared with 12% PNIPAM (W/V), 0
15 mM [Bzmim][BF4], and 2  10
5 M ANS to obtain clear spectra. For the rest of the techniques, five samples were prepared with 0.66 mg/mL PNIPAM and varying the concentration of [Bzmim][BF4] from 0 to 4 mg/mL. All resulting sample solutions were incubated for 24 h at room temperature. Sample solutions were coded as I-0, I-1, I-2, I-3, and I-4, which indicate 0, 1, 2, 3, and 4 mg/mL of IL in PNIPAM aqueous solution, respectively. Before performing the measurements, each sample was filtered with 0.45 μm disposable filters (Millipore, Millex-GS) through a syringe. Instruments and Methods. Fluorescence Studies. All fluorescence measurements were performed using a Cary Eclipse fluorescence spectrophotometer (Varian optical spectroscopy instruments, Mulgrave, Victoria, Australia) with an intense Xenon flash lamp as the light source. Emission spectra were recorded with a slit width of 2.5/2.5 nm and a PMT voltage of 720 V. The scan speed was kept at 1200 nm min
1. The sample solution was introduced into the quartz cuvette with the help of a micropipette. The sample containing quartz cuvette (QC) was placed in a multicell holder, which was electro-thermally controlled at precise temperature regulated by peltiers. The temperature control of the peltier thermostatted cell holders is extremely stable over time, with precision of (0.05 °C. Intensity measurements of the sample were taken at a heating

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rate of 1 °C/15 min and were kept at constant temperature for achieving thermodynamic equilibrium. Viscosity Measurements. The viscosity experiment was performed by using a sine-wave vibro viscometer (model: SV-10 A&D Company Limited, Japan). The instrument has been provided with two sensor plates of gold coating. The two gold coated sensor plates were vibrating with electromagnetic force at the same frequency. These two sensor plates were immersed in the sample cell which is mounted on the adjustable base. The measured viscosity values were displayed in a digital display unit which is attached to the SV-Viscometer. Viscosity measurements of the sample were taken at a heating rate of 1 °C/15 min for achieving thermodynamic equilibrium. Typically, the uncertainty in viscosities is to be 1%. The experiment was performed at the temperature range from 25 to 40 °C by using the circulating temperature control water bath (LAUDA alpha 6, Japan) with accuracy of temperature of (0.02 °C. DLS Measurements. The light scattering intensities were performed by the quasi elastic light scattering instrument (QELS, Photocor-FC, Model-1135P, USA) with an open modular architecture goniometer. An air-cooled He
Ne laser light was used at 633 nm and 20 mW as the light source, and is equipped with a thermostatted sample chamber for maintaining the desired temperature within a temperature range of 2
90 °C. A bubble free sample of around 1.5 mL was introduced in a square glass cuvette with a round aperture (PCS8501) sample cell through a syringe. A Teflon-coated screw cap was placed at the mouth of the cell to secure it from dust. Then, the airtight sample cell was placed in the sample chamber of the DLS instrument. The Brownian motion of particles was detected by DLS and correlated to the particle size. The relationship between the size of a particle and its speed is defined by the Stokes
Einstein equation as given below. dH ¼

kT 3πηD

ð1Þ

where k is Boltzmann’s constant (1.3806503  10
23 m2 kg s
2 K
1), T is the absolute temperature (K), η is the viscosity (mPa 3 s), and D is the diffusion coefficient (m2 s
1). All data were obtained from the instrumental software.

’ RESULTS AND DISCUSSION To obtain a more detailed and new depiction of the role of the IL on the LCST of PNIPAM aqueous solution, we have investigated the fluorescence intensities, viscosities (η), and particle size (dH) for the phase transition of PNIPAM aqueous solution with and without IL, as a function of the temperature (T). In this study, the effect of the imidazolium group based IL on the phase transition of PNIPAM solution was explicitly exploited. Influence of IL on PNIPAM Aqueous Solution by Fluorescence Studies. In the present study, 8-anilino-1-naphthalene-

sulfonic acid (ANS) is used as a fluorescence probe. ANS gives a very weak intensity at 510 nm in aqueous solution. However, ANS shows either a blue shift or red shift depending upon the decrease or increase in local polarity and mobility.37,38 Experiments involving temperature dependence of the fluorescence measurements of the probe at different IL concentrations were carried out in the temperature range 25
40 °C, and results are summarized in Figure 2. For the current study, the profile of the fluorescence emission spectrum of the probe was recorded upon the temperature of 4753

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Figure 2. Fluorescence intensity of ANS with increasing [Bzmim][BF4] concentration in PNIPAM solutions at different temperatures. λex, 360 nm; PNIPAM, 12% (W/V); [Bzmim][BF4], 0
15 mM; ANS, 2  10
5 M. The colored lines represent the following: red = 0 mM IL, green = 1 mM IL, magenta = 2 mM IL, blue = 5 mM IL, dark yellow = 10 mM IL, and orange = 15 mM IL.

PNIPAM aqueous solution. As shown in Figure 2, at higher temperature, the intensity of the probe was higher than that at lower temperature. It symbolizes that the polymer adopts a relatively compact globule conformation at higher temperature, whereas below the LCST the polymer exhibits a coil conformation. By inclusion of IL in the sample, the fluorescence intensity was clearly affected and the intensity increases with increasing concentration of IL, with the higher intensity attained even at lower temperature. From Figure 2, one can easily understand that the free IL sample exhibits a higher intensity around 32.8 °C, whereas the 15 mM concentration of IL sample shows a higher intensity approximately at 27.1 °C. Clearly, it indicates that the LCST values decrease with increasing concentration of IL in polymer solution. The changes in intensities were caused from the modulations in conformations of polymer. By increasing the concentration of IL, the intensity maximum moves toward lower temperature. At the gelation temperature, due to the coil-to-globule transition of the polymer, the hydrophobicity increases on globular surfaces.39 ANS is well-known for the preferential distribution on hydrophobic
hydrophilic interfaces.40 In this region of hydrophobic
hydrophilic interfaces, the mobility of probe molecules is reduced. Because of this, probe molecules exhibit high intensity at lower temperatures in the presence of IL. Influence of IL on PNIPAM Aqueous Solution by Viscosity Measurements. The experimental η values of the PNIPAM aqueous solution and with four different [Bzmim][BF4] concentrations are summarized in Figure 3 as a function of temperature. Below the polymer’s LCST, the solution has a larger viscosity due to the hydrated coil conformation. On the other hand, above its LCST, the polymer becomes dehydrated and it would collapse and takes to the compact globule structure; therefore, the solution has a lower viscosity.41 This indicates that the hydrogen bonds between the amide group of the polymer and water molecules are ruptured, allowing hydrophobic interactions that occur between the segments of the PNIPAM chains. From Figure 3, it can be seen that the viscosity of pure PNIPAM in aqueous solution was 1.02 mPa 3 s at 25 °C. By increasing the temperature, the viscosity was decreased gradually up to 33 °C. After reaching the LCST value, the viscosity decreases abruptly

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Figure 3. Viscosity (η) of aqueous PNIPAM polymer solution with various concentrations of IL, i.e., 0 mg/mL (O), 1 mg/mL (4), 2 mg/ mL (0), 3 mg/mL (b), and 4 mg/mL (2), as a function of temperature.

from 0.87 (at 33 °C) to 0.76 mPa 3 s at 34 °C due to the change in conformation from coil to globule. The phase transition viscosity was 0.87 mPa 3 s. On further increase of temperature, the viscosity was found to decrease slightly and finally approached a constant value of 0.70 mPa 3 s. These results indicate that, for the present sample, the polymer underwent a completely aggregated state after the water molecules were squeezed out from the polymer. The IL as an additive presumably exerts its effect on the LCST of the polymer through the combined effects of its ions. To explore the importance of these contributions, the effects of different concentrations of IL are investigated at temperatures ranging from 25 to 40 °C. Figure 3 depicts that the phase transition curves shift down with increasing IL concentration in PNIPAM solution. A minute amount of IL is enough to affect the LCST of PNIPAM solution by preferentially adsorbing the water molecules and weakening the hydrogen bonds between the amide group of the polymer and water molecules. The aforesaid hypothesis is quite consistent with the conclusion of Wamser42 and Freier et al.43 According to their results, the anion was hydrolyzed to boron trifluoride hydroxide, [BF3OH]
, and hydrogen fluoride even at environmental conditions. The phase transition region of PNIPAM aqueous solution (Tc) decreases (from 33 to 32, 28, and 27 °C) with increasing concentration of IL (from 0 to 1, 2, and 3 mg/mL, respectively). Further, it can be seen from Figure 3 that the coil region of the single homogeneous phase was observed at 24 °C for the higher concentration (4 mg/mL) of IL. Then, with increasing temperature (even around 25
26 °C), turbidity was formed and the viscosities of the macromolecule linearly decreased with increasing temperature up to 40 °C. The IL may alter the polymer conformation at all concentrations of IL, mainly due to the weakening of hydrogen bonds between amide and water molecules. Influence of IL on PNIPAM Aqueous Solution by DLS Measurements. To ascertain the contribution of IL on the phase transition of PNIPAM aqueous solution, we have further exploited DLS measurements as a function of IL concentration at various temperatures. Figure 4 reveals the intensity distribution graphs of the free-IL PNIPAM and various concentrations of IL (1
4 mg/mL) solutions, at their respective LCST values. At the phase transition region, we observed two peaks. One of these 4754

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Figure 4. DLS spectra of the intensity distribution graph of aqueous PNIPAM polymer solution with various concentrations of IL, i.e., 0 mg/ mL (O), 1 mg/mL (4), 2 mg/mL (0), 3 mg/mL (b), and 4 mg/mL (2), at their respective LCST values.

Figure 5. The hydrodynamic diameter (dH) of aqueous PNIPAM polymer solution with various concentrations of IL, i.e., 0 mg/mL (O), 1 mg/mL (4), 2 mg/mL (0), 3 mg/mL (b), and 4 mg/mL (2), as a function of temperature.

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peaks was obtained from a major population (75 nm) of low intensity. This peak shows that the PNIPAM molecules are highly aggregated in the solution. The other population (the second peak, 3 nm) corresponds to a higher percentage of intensity. Such a result leads to the idea that intermolecular cross-linking is dominant. For the sake of clarity, we do not present all the distribution graphs at each temperature. The aggregate size of PNIPAM in IL solutions is expected to be different from that in IL-free solution, because we have found that PNIPAM interacts with IL. Again, the size distribution curve (Figure 4) for PINPAM in (1
4 mg/mL) IL shows a similar trend as in pure PNIPAM solution. However, as can be seen from Figure 4, the dH and intensity values of PNIPAM in IL are higher and lower than those in aqueous solution. Since these measurements were made at the phase transition temperature, the changes in the size particles should not be due to intra- or intermolecular hydrogen bonding between polymer segments. Therefore, this change in the size particles arises from the interaction between the polymer and IL molecules. It is also noted that, in the intensity distribution graph, the peak area for high aggregation will appear at least 106 times larger than that of the first peak for smaller particles. An automatic temperature scan of the sample chamber allows observation of both the size and scattering intensity as a function of temperature. The marked point where both the size and intensity start to increase significantly is called the LCST. Figure 5 depicts the values of dH as a function of temperature for IL in 0, 1, 2, 3, and 4 mg/mL IL solutions. At temperatures below the LCST, we observed dH was small (11.2
15.5 nm) at 20
33 °C for PNIPAM in water, which can be attributed to the polymer in coil conformation. It is remarkable that dH (75.1 nm) rapidly increases around 33
34 °C. Moreover, the increase of the size occurs in a quite narrow temperature range (1
2 °C) due to polymer starts globule state from coil conformation, revealing that phase transition of PNIPAM occurs in this region. Figure 5 shows that the value of dH gradually decreases ultimately not changed as temperatures above the LCST. Clearly, above the LCST, the macromolecule undergoes a collapsed globule compact structure, since hydrogen bonds between PNIPAM and water molecules are ruptured, with the polymer converting to the

Scheme 1. Schematic Depiction of Hydrogen Bonds (Black Dashed Lines) between the Anion of the IL and the Water Molecule, Where the Yellow Dashed Lines Indicate Weakening of Hydrogen Bonds between Water and the Amide Group of the Polymer upon Addition of IL below LCSTa

a At phase transition state, these interactions are enhanced in the presence of IL, producing a cloudiness that leads to the formation of intramolecular interactions between the monomers of the polymer. At the globule state, [Bzmim][BF4] induces the hydrophobic collapse/aggregation of the PNIPAM polymer, and the IL destroys the hydrogen bonds between the amide group of the polymer and water molecules. The colors represent the following: pink = cation of ionic liquid, pale green = fluorine, pale pink = boron, red = oxygen, white = hydrogen.

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The Journal of Physical Chemistry B hydrophobic state as a result. Figure 5 explicitly elucidates that the LCST region of the PNIPAM aqueous solution is altered as a function of IL concentration. It can be seen that organic molten salt decreases the LCST value (from 33 °C in the absence of IL to 32, 28, 27, and 26 °C) with increasing IL concentration (from 0 to 1, 2, 3, and 4 mg/mL). Through this, we have explicitly found that the IL decreased the phase transition of PNIPAM solution, due to collapsing and aggregating the macromolecule. Our experimental results clearly reveal that tetrafluoroborate anions are able to strongly attract protons of water molecules from the hydration shell and consequently the rest of the water molecules become more negative. Eventually, these negatively charged ions attracted by the rest of IL this turns to weaken the polymer-associated water molecules of the hydration of the amide groups. The schematic illustration of the influence of IL on PNIPAM was depicted in Scheme 1. These results reveal that temperature as well as IL essentially affected the thermo-responsive polymer aqueous solution. On the other hand, we wish to address one interesting comparison of the phase transition of PNIPAM aqueous solution by IL with the phase transition of PNIPAM aqueous solution by other additives. It appears that the surfactant44
47 enhances while the ILs, salts,20,21,48
50 and biological surfactants19 depress the phase transition of PNIPAM aqueous solution.

’ CONCLUSIONS The modulation of the LCST of PNIPAM aqueous solution was investigated in the presence of IL, [Bzmim][BF4], by using fluorescence, viscometric, and DLS techniques as a function of IL concentration at various temperatures. Our results demonstrate a phase transition of PNIPAM solution, which is obviously dependent not only on the IL concentration but also on the temperature. Apparently, IL decreases the LCST of PNIPAM aqueous solution by the hydrophobic collapse/aggregation of the PNIPAM. The addition of the IL abruptly decreases the viscosity and the size of the particle and increase in intensities of probe in PNIPAM aqueous solution at phase transition region through intramolecular interaction between monomers of polymer and induces the hydrophobic collapse/aggregation of the PNIPAM. Obviously, IL has a significant influence on the LCST value of PNIPAM solution. This may be contributed from the electrostatic interactions between the assertively adsorbed monomers and some effects of steric hindrance. At this point, the interactions between the polymer segments and the IL ions via the water structure are very complicated and the exact nature of these interactions still remains open to debate, although the first results presented here seem to be quite interesting and promising. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: þ91-11-27666646-142. Fax: þ91-11-2766 6605.

’ ACKNOWLEDGMENT Financial support from Department of Science and Technology (DST, New Delhi, India) through grant (SR/SI/PC-54/ 2008) is gratefully acknowledged.

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