Interaction of Urea with Pluronic Block Copolymers by 1H NMR

Apr 17, 2007 - Beijing 100080, People's Republic of China, and Department of Chemistry, V. N. South Gujarat UniVersity,. Surat 395007, Gujarat, India...
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J. Phys. Chem. B 2007, 111, 5155-5161

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Interaction of Urea with Pluronic Block Copolymers by 1H NMR Spectroscopy Jun-he Ma,† Chen Guo,*,† Ya-lin Tang,‡ Lin Chen,† P. Bahadur,§ and Hui-zhou Liu*,† Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Department of Chemistry, V. N. South Gujarat UniVersity, Surat 395007, Gujarat, India ReceiVed: February 1, 2007; In Final Form: March 14, 2007

Solution 1H NMR techniques were used to characterize the interaction of urea with poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The urea was established to interact selectively with the PEO blocks of the block copolymer, and the interaction sites were found not to change with increasing temperature. Such interactions influence the self-assembly properties of the block copolymer in solution by increasing the hydration of the block copolymers and stabilizing the gauche conformation of the PPO chain. Therefore, urea increases the critical micellization temperature (CMT) values of PEO-PPO-PEO copolymers, and the effect of urea on the CMT is more pronounced for copolymers with higher PEO contents and lower for those with increased contents of PPO segments.

Introduction Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, commercially available as Pluronics (BASF) and Poloxamers (ICI), are one of the most well-known high-molecular-weight nonionic surfactants.1 Both the ratio between the EO and PO blocks and their sequence can be readily altered in synthesis, so that these materials are widely used in such varied fields as detergency, wetting, emulsification, and lubrication, as well as cosmetics, bioprocessing, and pharmaceuticals.2 Most applications of these copolymers arise from their unique solution and associative properties as a consequence of the differences between the PEO and PPO blocks in selective solvents. In aqueous media, the interesting features of PEO-PPO-PEO block copolymers are their temperature-dependent self-association and their rich phase behavior.3-10 The process of self-association can be induced by increasing the block copolymer concentration to be above the critical micellization concentration (CMC) and/or adjusting the temperature to exceed the critical micellization temperature (CMT).11,12 The inclusion of various additives, such as various salts,13-19 short-chain alcohols,20-25 formamide,26 and others, has been shown to have a strong effect on the aggregation behavior of PEO-PPO-PEO triblock copolymers in aqueous solution. Among these additives, urea has proven to be an efficient modifier in changing the properties of PEO-PPO-PEO micellar solutions.27 Urea and its derivatives are well-known denaturants of proteins,28 because of their ability to weaken hydrophobic interactions in aqueous solution.29 For the same reason, one expects that ureas could be used to alter the properties of * To whom correspondence should be addressed. Phone: +86-1062555005. Fax: +86-10-62554264. E-mail: [email protected] or [email protected]. † Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences. ‡ Institute of Chemistry, Chinese Academy of Sciences. § V. N. South Gujarat University.

micellar solutions by a delicate modulation of the balance of hydrophobic/hydrophilic interactions of surfactants with water.30 Indeed, urea has been shown to increase the CMCs of nonionic31 and ionic32 surfactants and decrease the mean micellar hydrodynamic radii of ionic micelles. Two different mechanisms for urea action in aqueous micellar solutions have been proposed: an indirect mechanism whereby urea changes the “structure” of water to facilitate the solvation of a hydrocarbon chain of nonpolar solute33 and a direct mechanism whereby urea replaces some of the water molecules in the hydration shell of the solute but has almost no effect on the water structure.34 The indirect mechanism has received the most attention and is widely accepted; in the past, many experimental results showed that urea acts as a “water-structure breaker”, destroying the long-range order of pure water and reducing the degree of water-water hydrogen bonding.35-37 However, most of the experimental techniques used in these studies did not provide information at the molecular level, and conflicting interpretations of urea action have been proposed.38 On the other hand, computer simulations seem to indicate that urea has a negligible effect on water structure;39-41 at the same time, some studies using electron-spin resonance spectroscopy42 have shown that urea mainly replaces some water molecules in the hydration shell around the solute. These findings seem to support the direct mechanism of urea action. This article reports the interaction of urea with PEO-PPOPEO micellar solutions as detected through 1H nuclear magnetic resonance (NMR) spectroscopy. The purpose of this work was to obtain information concerning the detailed interaction sites between urea and different moieties of the triblock copolymer species and, thus, deduce a clear molecular-level mechanism of the effect of urea on the micellization of PEO-PPO-PEO block copolymers. 2. Experimental Section Materials. The PEO-PPO-PEO triblock copolymers Pluronic F88, P84, and P123 were obtained as a gift from BASF (Parsippany, NJ) and were used as received. The

10.1021/jp070887m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007

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TABLE 1: Composition of PEO-PPO-PEO Block Copolymers polymer

mol wt

PPO segment wt

no. of PO units

no. of EO units

PPO/PEO

P123 P84 F88

5750 4200 11 400

4025 2250 2250

69 39 39

2 × 19 2 × 19 2 × 103

1.79 1.15 0.19

molecular weights and compositions of these Pluronic polymers studied are listed in Table 1. Urea (analytical grade) was obtained from Beijing Chemical Reagent Corp., Beijing, China, and was used as received. The reference 2,2-dimethyl-2silapentane-5-sulfonate sodium urea (DSS, g97%) was purchased from Sigma Aldrich Chemical Corp. D2O (g99.9 atom % 2H) was purchased from CIL Corp. (Andover, MA). Sample Preparation. Heavy-water solutions of Pluronic F88, P84, and P123 were prepared by dissolving the polymers in D2O solution with gentle agitation. Urea solutions were prepared by dissolving urea in the aqueous Pluronic polymer solutions. The final copolymer solutions contained 0.01 M DSS. The solutions were facilitated by mixing with gentle agitation and then were each transferred to a 5-mm NMR sample tube that was sealed immediately with laboratory film. After 15 min of sonication to remove dissolved paramagnetic dioxygen, the sample tubes were stored in a refrigerator before use. NMR Methods. All NMR experiments were conducted on a Bruker Avance 600 spectrometer at a Larmor frequency of 600.13 MHz for the proton; the spectrometer was equipped with a microprocessor-controlled gradient unit and an inversedetection multinuclear BBI probe with an actively shielded z-gradient coil. The sample temperature was kept constant to within (0.1 °C by use of a Bruker BCU-05 temperature control unit. Temperature was calibrated separately for each probe using a capillary containing methanol (low T) or ethylene glycol (high T).43 For all 1H NMR experiments, the samples were allowed to equilibrate at the desired temperature for at least 15 min prior to measurement. Experiments that were repeated at the same temperature, but were reached by a temperature change in the opposite direction, yielded identical results. DSS was directly added into the sample solutions as an internal reference to eliminate temperature-induced shifts. Here, rotating-frame nuclear Overhauser effect (ROE) 1H NMR spectra were acquired by using a selectively excited gradient-selected pulse sequence.44 3. Results and Discussion Micellization of Block Copolymer in the Presence of Urea. To investigate the effect of urea on micellization, the 1H NMR spectra of 2.5% (w/v) Pluronic F88 in D2O solution in the absence and presence of urea were measured at various temperatures, and the local expanded regions of the HDO, EO -CH2-, PO -CH2-, and PO -CH3 signals are presented in Figure 1A-D, respectively. According to previous assignments,45 the triplet at ∼1.18 ppm is attributed to the protons of the PO -CH3 groups, the broad peaks from about 3.65 to 3.45 ppm belong to the PO -CH2- protons, the sharp singlet at ∼3.7 ppm belongs to the EO -CH2- protons, the signal at ∼4.8 ppm is the residual signal of HDO, and the remaining signal at ∼5.8 ppm is the proton resonance of urea. For the Pluronic solution in the absence of urea, distinct signals for all protons can be clearly observed at low temperatures. The PO -CH2- signals show a hyperfine structure, and the PO -CH3 signal exhibits a triplet. The presence of distinct multiplets at lower temperature is because of efficient motional narrowing.5 It indicates that the copolymer dissolved in water as unimers and that all segments of the solvated polymer can

move freely. When the temperature is increased above a certain value, the hyperfine structure of the PO -CH2- signals and the triplet of the PO -CH3 signals disappear in a small temperature interval, and both the PO -CH2- and -CH3 signals broaden. The line-shape changes of the PO groups are the result of conformational changes in the PPO chain,45 whereas the observed line broadening of the PO groups indicates a reduced mobility of the PO segments.47 The onset temperature at which the spectral profiles of the PO segments show dramatic changes can be determined as the CMT. It should be noted that a new resonance signal, labeled g, appears at ∼3.42 ppm and grows progressively larger as the temperature increases above the CMT. This new resonance has also been attributed to PO -CH2- protons on the basis of 2D heteronuclear singlequantumcoherence (HSQC)-resolved 1H{13C} NMR spectra,45 the appearance of which is attributable to the breakdown of the intramolecular attraction between the PO -CH2- protons and the nearest ether oxygens during micellization. For the Pluronic solution in the presense of urea, all of the resonance peaks show downfield shifts with increasing urea concentration, and the changes in the spectral profile associated with micelle formation move to higher temperature upon addition of urea. This shows that the addition of urea can stabilize the block copolymer in the premicellar state and prevent the occurrence of micellization at otherwise the same conditions in the absence of urea. Interaction between Urea and Pluronic Polymer. ROE measurements were used to confirm the interactions between urea and the different moieties of the Pluronic polymers. Intermolecular 1H cross-relaxation processes by NOE can, in general, occur for molecules that are in close spatial contact (within 0.5 nm), being mediated by through-space dipole-dipole couplings, and can be used to probe molecular proximity.46 In a typical experiment, the urea protons and PPO -CH3 protons were selectively excited in the unimer and micellar regions, respectively, and cross-relaxation processes to other proton moieties were monitored. Figure 2 shows the 1H cross-relaxation ROE spectra for the entire spectral region at 25 °C (umimer region) and 50 °C (micelle region). When the urea protons were excited, proton cross-relaxation peaks for HDO and PEO blocks are observed, whereas when the PPO -CH3 protons were excited, no cross-relaxation peaks were observed. The spectra did not change when the temperature was increased from 25 to 50 °C. These proton cross-relaxation ROE results thus confirm that the direct interactions of urea molecules with water molecules and PEO blocks and the locus of interaction do not change with increasing temperature. Effect of Urea on Water Structure. Figure 1A shows the changes in the 1H NMR spectra of the residual HDO signal with increasing concentration of urea at different temperatures. The temperature-dependent chemical shift (δ) of the HDO signal as a function urea concentration is plotted in Figure 3. The chemical shift shows a linear upfield shift with increasing temperature, whereas it exhibits a large downfield shift with addition of urea. Because the upfield shift of the HDO signal is due to the breakdown of the hydrogen-bonding structure in water,45 these results indicate that the hydrogen bonds between water molecules are progressively weakened by heat. The observation that urea increases the chemical shifts of HDO suggests that the addition of urea has an effect opposite to that of temperature in changing the hydrogen-bonding structure of water. It seems that urea acts as a structure-maker for water and facilitates hydrogen bonding among water molecules or between water and urea molecules. Although urea causes a

Interactions of Urea with Pluronics

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Figure 1. 1H NMR spectra of 2.5% (w/v) Pluronic F88 dissolved in urea/water mixtures at various temperatures: [urea] ) (a) 0, (b) 2, (c) 4, (d) 6, and (e) 8 M. Spectra show the (A) HDO, (B) EO -CH2-, (C) PO -CH2-, and (D) PO -CH3 signals.

breakdown of long-range order, the water in concentrated urea solutions is nevertheless extensively hydrogen-bonded, as evidenced by the fact that the chemical shift of the HDO signal is still strongly temperature-dependent. The high solubility of urea suggests the existence of some urea-water interactions. These findings appear to support the ROE results that urea directly interacts with water. Effects of Urea on PEO. The temperature-dependent chemical shift (δ) of the EO -CH2- protons as a function of urea concentration is plotted in Figure 4. The chemical shift of the EO -CH2- protons shows a linear decrease with increasing temperature in the absence of urea, whereas the chemical shift of the EO -CH2- protons becomes independent of temperature in the presence of 2 M urea and shows a linear increase in the presence of higher urea concentrations. The slight upfield shift indicates that the PEO blocks experience a small degree of dehydration with increasing temperature.45 When urea is added, the chemical shift of the EO -CH2- protons undergoes a large downfield shift. The higher the urea concentration, the more extensive the downfield the chemical shift. The observed downfield shift of the PEO protons is a manifestation of the increased hydration of the PEO segments. The reverse trend of the chemical shift of the EO -CH2- protons in the presence of higher urea concentrations also suggests the direct interaction of urea with the hydrated PEO corona of the block copolymer micelles. When the urea is dissolved in water, the polar urea molecules enter the polar region of the micellar shell, replace the water molecules around the PEO blocks, and directly form hydrogen bonds with the PEO blocks.

Figure 2. Profiles of 1H NMR rotating-frame nuclear Overhauser effect (ROE) spectra of 2.5% F88 in the presence of 2 M urea solution. Spectra were recorded at 25 and 50 °C by selectively exciting (a,c) the urea signal and (b,d) PPO -CH3 signal.

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Figure 3. Temperature-dependent 1H NMR chemical shifts observed for the residual HDO signal of 2.5% (w/v) Pluronic F88 solutions at various urea concentrations.

Figure 4. Temperature-dependent chemical shifts of the EO -CH2signal of 2.5% aqueous Pluronic F88 solutions in the presence of different concentrations of urea.

Figure 5. Temperature-dependent chemical shifts of the PO -CH3 signal of 2.5% aqueous Pluronic F88 solutions in the presence of different concentrations of urea. Arrows denote the CMT.

Effects of Urea on PPO. The temperature-dependent chemical shift (δ) of the PO -CH3 signal of aqueous 2.5% Pluronic F88 solutions in the presence of different concentrations of urea is presented in Figure 5. It is clear that an increase in temperature leads to a marked decrease in the chemical shift of the PO -CH3

Ma et al. protons. The chemical shift of the PO -CH3 protons undergoes a large decrease at certain temperatures; below or above these temperatures, a slight linear decrease can be observed. Many experimental studies have demonstrated that the PPO segments of block copolymers are in a hydrated state at lower temperatures.8-12 Because the interaction of the PPO protons with water enhances the deshielding effect of the C-H protons and results in 1H downfield shifts, the upfield chemical shift with increasing temperature is related to the adverse dehydration process. In the temperature interval from 36 to 45 °C in the absence of urea solution, the significant upfield shift of the PO -CH3 protons indicates that the PPO blocks apparently reduce the contact with water and form a hydrophobic microenvironment. At increasing temperature above 45 °C, the linear decrease of the chemical shift of the PO protons suggests that water is continually excluded from the micellar core. It can be inferred that an increase in temperature increases the hydrophobicity of Pluronic polymer micelles, thus decreasing the water content in the micellar core. When urea is added, the chemical shift of the PO segments exhibits a marked increase; the greater the amount of urea added, the farther downfield the chemical shifts, similar to the change for EO segments. From the above 1H NMR ROE results, it seems that most PPO segments probably cannot interact directly with urea; therefore, an indirect interaction between PPO and urea molecules is suggested. Because the addition of urea increases the hydration of PEO and enhances the hydrogen-bonding structure in water, the hydration of the hydrophobic groups of PPO increases as well. In the premicellar state, the dehydration of PPO is the dominating reason for the micellization of block copolymers in aqueous solution. The increasing hydration of PPO with the addition of urea will eventually prevent the occurrence of micellization. The CMT values determined from the first inflection point on the chemical shift versus temperature plots for 2.5% Pluronic F88 in D2O solution is about 36 °C, shifted to about 39.5, 43, ∼46.5, and >50 °C in 2, 4, 6, and 8 M urea solutions, respectively. In the micellar state, all of the hydrophobic groups are buried inside the micellar core, and hence, interactions between EO segments and urea dominate over the hydrophobic group-urea interactions. As shown in Figure 1, the resonance signals of the PO segments in the 1H NMR spectra show a clear line broadening with increasing temperature and become narrow again upon addition of urea. To obtain accurate quantitative information, the half-height width (the line width at half-height, ∆ν1/2) was used to characterize the exact changes. For the triplet of the PO -CH3 signal, the ∆ν1/2 was measured at the half-height position between the highest point of the signal and the baseline. The temperature-dependent ∆ν1/2 values (Hz) of the PO -CH3 signal are plotted in Figure 6. At temperatures below 36 °C in the absence of urea solution, the ∆ν1/2 value of the PO -CH3 signals remains constant with increasing temperature. A change occurs in the temperature interval from 36 to 44 °C, and the half-width increases abruptly from 12.6 to 19.9 Hz. Because the line width is inversely proportional to the mobility of the related segments,47 this sharp increase in half-height width indicates an abrupt decrease of chain mobility, which is considered to be related to the aggregation of Pluronic F88. Upon micellization, the PO segments are confined in the core of the micelles. Compacting of the PPO chains limits their movement and therefore induces a decrease of chain mobility. When the temperature is above 44 °C, the ∆ν1/2 value of the PO -CH3 signal decreases with increasing temperature, which suggests that the aggregation process has finished and that chain

Interactions of Urea with Pluronics

Figure 6. Temperature-dependent half-height widths of the PO -CH3 signal of 2.5% aqueous Pluronic F88 solutions in the presence of different concentrations of urea. Arrows denote the CMT.

mobility increases mainly because of the decreasing viscosity of the solvent. It can be seen that the ∆ν1/2 value of the PO -CH3 signal decreases significantly upon addition of urea, indicating that the hydration of the hydrophobic PO groups has increased. This finding appears to suggest that the addition of urea provides the driving force for the transfer of water from the medium to the micellar core and increase the hydration of the PPO block, which will result in the dissociation of the block copolymer micelles at higher urea concentration. As shown in Figure 1, a new resonance signal (denoted by g) appears and grows progressively larger at temperatures above the CMT. This new signal arises as a result of the breakdown of the intramolecular (C-H)‚‚‚O hydrogen bonds between the PO -CH2- protons and the nearest ether oxygens with increasing temperature.45 The presence of the intramolecular (C-H)‚‚‚O attraction has been shown to provide gauche stability in the main chain of PPO.48,49 On the other hand, the breakdown of the intramolecular hydrogen bonds might result in a decrease in the number of gauche conformers in the PPO chain. Therefore, the increase of the integral area of the peak labeled g can be directly correlated with the decrease in the number of gauche conformers in the PPO chain. If we calibrate the integral area of the PO -CH3 signal to 117 (the number of methyl protons in an F88 molecule) at each temperature, quantitative information on the temperature-dependent changes of peak g can be obtained. The temperature-dependent integral value of peak g is plotted as a function of urea concentration in Figure 7. It is observed that the integral value increases abruptly at the CMT until the other boundary of transition region is reached, which indicates the breakdown of the intramolecular hydrogen bonds and thus a decrease in the number of gauche conformers in the PPO chain. The addition of urea significantly decreases the integral value of peak g but does not affect the features of the spectral profiles. This indicates that the addition of urea has a stabilizing effect on the gauche conformation of the PPO chain and prevents the unimer-to-micelle transition, thus increasing the CMT. The temperatures at which the micellization transition occurs are in good accordance with those presented in Figures 5 and 6 and show the same increasing trend with increasing urea concentration. It is generally accepted that, above the CMT, there is an equilibrium region referred to as the unimer-to-micelle transition region in which significant amounts of both free and associated copolymer molecules coexist.50,51 It can be determined between the two inflection points in Figures

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Figure 7. Temperature-dependent integral values of peak g (see Figure 2) with the integral area of the PO -CH3 signal calibrated to 117 in the presence of different concentrations of urea. Arrows denote the CMT.

Figure 8. Effect of urea on the critical micellization temperatures (CMTs) of 2.5% solutions of Pluronic F88, P84, and P123 in D2O.

5-7 (the inflection points were determined from the intersections of two tangent lines, as marked in the figures). Considering the determination error, the results are identical and the temperature range over which this transition takes place appears to be unaffected by the presence of urea. Effect of Polymer Composition on Urea-Pluronic Interaction. To compare the different effects of urea on the PEO and PPO blocks, the effects of urea on the CMTs of 2.5% Pluronic polymers containing the same hydrophobic (PPO) segments with differing hydrophilic (PEO) blocks (F88 and P84) and the same hydrophilic segments with differing hydrophobic blocks (P84 and P123) in D2O solution were investigated, as shown Figure 8. Although all of the Pluronic polymers studied show the same linear dependence of the CMT on the urea concentration (for the concentration range investigated), the slopes of the CMT versus urea concentration lines are different. The slope increased from 1.04 °C/M (urea) for P84 to 1.75 °C/M (urea) for F88 with increasing number of EO units, whereas the slope decreased to 0.38 °C/M (urea) for P123 with increasing number of PO units. This would indicate that the effect of urea on the micellization of block copolymers becomes stronger when the number of hydrophilic PEO blocks is increased. Thus, the urea-PEO interaction is a primary factor in the increase of the micellization temperature of block copolymers in aqueous urea

5160 J. Phys. Chem. B, Vol. 111, No. 19, 2007 solutions. The decrease of the slope for the curve of Pluronic P123 with increasing number of PPO blocks suggests that the CMT values are much less influenced by the urea-PPO interaction at higher relative PPO contents and that the effect of urea on the micellization of the block copolymer is lower. According to the determination of interaction sites, the following explanation for the urea-Pluronic interaction has been advanced: When urea is dissolved in water, the urea molecules interact with water molecules through hydrogen bonding. At the same time, many urea molecules penetrate into the PEO hydration shell and interact directly with PEO blocks, leading to an increase of the free energy of water around PEO because of unfavorable entropy contributions. In contrast, the water in the PPO hydration shell is still in a comparatively low-energy state, and thus, water is driven to increase the hydration of the PPO segments. The total effect of urea decreases the dehydration of block copolymers with temperature and stabilizes the conformation of the polymer chains in the premicellar region, which causes the micellization of block copolymers to be difficult so that it can only occur at higher temperatures. This assumption is also supported by the thermodynamic data reported by Alexandridis et al.27 It is known that the micellization of Pluronic polymers in aqueous solution is an entropydriven process,4 whereas the micellization entropy values for Pluronic polymer decreased as the urea concentration was increased.27 At the same time, the enthalpy of micellization was also lowered in the presence of urea and decreased further with increasing urea concentration, in agreement with behavior observed for other nonionic surfactants. Favorable enthalpic interactions between urea, PEO, and water most likely cause the decrease in enthalpy. Conclusion This study has shown that 1H NMR spectroscopy is an excellent technique for the investigation of the interaction of urea with Pluronic polymers. 1H NMR spectroscopy not only presents an accurate means to determine the CMT values of block copolymers in aqueous solution, but also provides valuable information on the interaction sites of urea molecules with the triblock copolymer species. It was shown that the urea molecules interact directly with the PEO moieties of the triblock copolymers and with the solvent water molecules in both unimer and micellar regions, whereas the urea molecules seem not to interact with the PPO blocks directly. Both residual HDO and EO -CH2- signals show a large downfield shift with the addition of urea, indicating that the water molecules and the PEO blocks interact with urea in a similar way. However, the downfield shift of the PPO blocks is possibly the result of an increase in hydration upon addition of urea. When the temperature is above the CMT, the chemical shift of the PO -CH3 signal shows an abrupt upfield shift, indicating the formation of a hydrophobic micellar core. At the same time, the half-height width of the PO -CH3 signal shows a sharp increase at the CMT because of the decreasing segmental mobility in the compact micellar core. The appearance of a new resonance signal due to the breakdown of intramolecular hydrogen bonds between the PO -CH2- protons and the ether oxygens above the CMT suggests a decrease in the number of gauche conformers in the PPO chain. However, upon the addition of urea, all of these changes are shifted to higher temperatures, indicating that the micellization boundary is driven to higher temperatures. It can be concluded that the presence of urea makes Pluronics more hydrophilic (the increase in chemical shift). The integral values of the new signal decrease

Ma et al. with increasing urea concentration, implying that urea has a stabilizing effect on gauche conformers in the PPO chain. The effect of urea in increasing the CMT values of the block copolymers is more pronounced for copolymers with higher PEO contents and becomes unconspicuous with increasing content of PPO segments. This validates that the interaction between urea and the PEO moieties of the block copolymer play a dominant role in the interaction of urea with Pluronic surfactant species. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20221603, 20676137, and 20490200), the National High Technology Research and Development Program of China (863 Program) (No. 20060102Z2049), and Major Aspect of Knowledge Innovation Project of Chinese Academy of Sciences (No. KSCX2-YW-G-019). References and Notes (1) Nakashima, K; Bahadur, P. AdV. Colloid Interface Sci. 2006, 123126, 75-96. (2) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1-46. (3) Yang, P. D.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Buratto, S. K.; Stucky, G. D. Science 2000, 287, 465-467. (4) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414-2425. (5) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145-4159. (6) Wu, G. W.; Zhou, Z. K.; Chu, B. Macromolecules 1993, 26, 21172125. (7) Armstrong, J. K.; Chowdhry, B. Z.; Snowden, M. J.; Leharne, S. A. Langmuir 1998, 14, 2004-2010. (8) Guo, C.; Wang, J.; Liu, H. J; Chen, J. Y. Langmuir 1999, 15, 27032708. (9) Su, Y.; Wang, J.; Liu, H. J. Phys. Chem. B 2002, 106, 1182311828. (10) Chu, B. Langmuir 1995, 11, 414-421. (11) Su, Y. L.; Wang, J.; Liu, H. Z. Macromolecules 2002, 35, 64266431. (12) Linse, P. J. Phys. Chem. 1993, 97, 13896-13902. (13) Jain, N. J.; Aswal, V. K.; Goyal, P. S.; Bahadur, P. J. Phys. Chem. B 1998, 102, 8452-8458. (14) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5446-5450. (15) Jorgensen, E. B.; Hvidt, S.; Brown, W.; Schillen, K. Macromolecules 1997, 30, 2355-2364. (16) Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Langmuir 1992, 8, 1903-1907. (17) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074-6082. (18) Ganguly, R.; Aswal, V. K.; Hassan, P. A.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys. Chem. B 2005, 109, 5653-5658. (19) Mao, G.; Sukumaran, S.; Beaucage, G.; Saboungi, M. L.; Thiyagarajan, P. Macromolecules 2001, 34, 552-558. (20) Armstrong, J.; Chowdhry, B.; Mitchell, J.; Beezer, A.; Leharne, S. J. Phys. Chem. 1996, 100, 1738-1745. (21) Su, Y. l.; Wei, X. F.; Liu, H. Z. Langmuir 2003, 19, 2995-3000. (22) Alexandridis, P.; Ivanova, R.; Lindman, B. Langmuir 2000, 16, 3676-3689. (23) Kipkemboi, P.; Fogden, A.; Alfredssen, V.; Flodstro¨m, K. Langmuir 2001, 17, 5398-5402. (24) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304-5310. (25) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 3660-3675. (26) Caragheorgheopol, A.; Caldararu, H.; Dragutan, I.; Joela, H.; Brown, W. Langmuir 1997, 13, 6912-6921. (27) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442-2450. (28) Franks, F. Water, A ComprehensiVe Treatise; Plenum Press: New York, 1978; Vol. 4. (29) Tanford, C. J. Am. Chem. Soc. 1964, 86, 2050-2059. (30) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1980. (31) Brieanti, G.; Puwada, S.; Blankschtein, D. J. Phys. Chem. 1991, 95, 8989-8995. (32) Caponetti, E.; Causi, S.; De Lisi, R.; Floriano, M. A.; Milioto, S.; Triolo, R. J. Phys. Chem. 1992, 96, 4950-4960. (33) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218-8222.

Interactions of Urea with Pluronics (34) Roseman, M.; Jencks, W. P. J. Am. Chem. Soc. 1975, 97, 631640. (35) Finer, E. G.; Franks, F.; Tait, M. J. Am. Chem. Soc. 1972, 94, 44244429. (36) Bonner, O. D.; Dednarek, J. M.; Arisman, R. K. J. Am. Chem. Soc. 1977, 99, 2848-2854. (37) Rupley, J. A. J. Phys. Chem. 1964, 68, 2002-2003. (38) Subramanian, S.; Sarma, T. S.; Balasubramanian, D.; Ahuwalia, J. C. J. Phys. Chem. 1971, 75, 815-820. (39) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 57865793. (40) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 57945800. (41) Marchese, R. A.; Mehrota, P. K.; Beveridge, D. L. J. Phys. Chem. 1984, 88, 5692-5702. (42) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218-8222.

J. Phys. Chem. B, Vol. 111, No. 19, 2007 5161 (43) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 92079216. (44) Dalvit, C.; Bovermann, G. Magn. Reson. Chem. 1995, 33, 156159. (45) Ma, J. H.; Guo, C.; Tang, Y. L.; Wang, J.; Zheng, L. L.; Liang, X. F.; Chen, S.; Liu, H. Z. J. Colloid Interface Sci. 2006, 299, 953-961. (46) Schleucher, J.; Quant, J.; Glaser, S. J.; Griesinger, C. J. Magn. Reson. Ser. A 1995, 112, 144-151. (47) Nivaggioli, T.; Tsao, B.; Alexandridis, P.; Hatton, T. A. Langmuir 1995, 11, 119-126. (48) Sasanuma, Y.; Iwata, T.; Kato, Y.; Kato, H. J. Phys. Chem. A 2001, 105, 3277-3283. (49) Law, R. V.; Sasanuma, Y. Macromolecules 1998, 31, 2335-2342. (50) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171-180. (51) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 41284135.