interaction of urea with pluronic block copolymers by 1 h nmr...

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 ofSciences, 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 establishedto interact selectively with the PEO blocks of the block copolymer, and the interaction sites were found notto change with increasing temperature. Such interactions influence the self-assembly properties of the blockcopolymer in solution by increasing the hydration of the block copolymers and stabilizing the gaucheconformation of the PPO chain. Therefore, urea increases the critical micellization temperature (CMT) valuesof PEO-PPO-PEO copolymers, and the effect of urea on the CMT is more pronounced for copolymerswith higher PEO contents and lower for those with increased contents of PPO segments.

Introduction

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethyleneoxide) (PEO-PPO-PEO) triblock copolymers, commerciallyavailable as Pluronics (BASF) and Poloxamers (ICI), are oneof the most well-known high-molecular-weight nonionic sur-factants.1 Both the ratio between the EO and PO blocks andtheir sequence can be readily altered in synthesis, so that thesematerials are widely used in such varied fields as detergency,wetting, emulsification, and lubrication, as well as cosmetics,bioprocessing, and pharmaceuticals.2 Most applications of thesecopolymers arise from their unique solution and associativeproperties as a consequence of the differences between the PEOand PPO blocks in selective solvents. In aqueous media, theinteresting features of PEO-PPO-PEO block copolymers aretheir temperature-dependent self-association and their rich phasebehavior.3-10 The process of self-association can be inducedby increasing the block copolymer concentration to be abovethe critical micellization concentration (CMC) and/or adjustingthe 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 beenshown to have a strong effect on the aggregation behavior ofPEO-PPO-PEO triblock copolymers in aqueous solution.Among these additives, urea has proven to be an efficientmodifier in changing the properties of PEO-PPO-PEO mi-cellar solutions.27

Urea and its derivatives are well-known denaturants ofproteins,28 because of their ability to weaken hydrophobicinteractions in aqueous solution.29 For the same reason, oneexpects that ureas could be used to alter the properties of

micellar solutions by a delicate modulation of the balance ofhydrophobic/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 hydro-dynamic radii of ionic micelles.

Two different mechanisms for urea action in aqueous micellarsolutions have been proposed: an indirect mechanism wherebyurea changes the “structure” of water to facilitate the solvationof a hydrocarbon chain of nonpolar solute33 and a directmechanism whereby urea replaces some of the water moleculesin the hydration shell of the solute but has almost no effect onthe water structure.34 The indirect mechanism has received themost attention and is widely accepted; in the past, manyexperimental results showed that urea acts as a “water-structurebreaker”, destroying the long-range order of pure water andreducing the degree of water-water hydrogen bonding.35-37

However, most of the experimental techniques used in thesestudies did not provide information at the molecular level, andconflicting interpretations of urea action have been proposed.38

On the other hand, computer simulations seem to indicate thaturea has a negligible effect on water structure;39-41 at the sametime, some studies using electron-spin resonance spectroscopy42

have shown that urea mainly replaces some water molecules inthe hydration shell around the solute. These findings seem tosupport the direct mechanism of urea action.

This article reports the interaction of urea with PEO-PPO-PEO micellar solutions as detected through1H nuclear magneticresonance (NMR) spectroscopy. The purpose of this work wasto obtain information concerning the detailed interaction sitesbetween urea and different moieties of the triblock copolymerspecies and, thus, deduce a clear molecular-level mechanismof the effect of urea on the micellization of PEO-PPO-PEOblock copolymers.

2. Experimental Section

Materials. The PEO-PPO-PEO triblock copolymersPluronic F88, P84, and P123 were obtained as a gift fromBASF (Parsippany, NJ) and were used as received. The

* To whom correspondence should be addressed. Phone:+86-10-62555005. Fax: +86-10-62554264. E-mail: [email protected] [email protected].

† Graduate School of Chinese Academy of Sciences, Chinese Academyof Sciences.

‡ Institute of Chemistry, Chinese Academy of Sciences.§ V. N. South Gujarat University.

5155J. Phys. Chem. B2007,111,5155-5161

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

molecular weights and compositions of these Pluronic polymersstudied are listed in Table 1. Urea (analytical grade) wasobtained from Beijing Chemical Reagent Corp., Beijing, China,and was used as received. The reference 2,2-dimethyl-2-silapentane-5-sulfonate sodium urea (DSS,g97%) was pur-chased 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 inD2O solution with gentle agitation. Urea solutions were preparedby dissolving urea in the aqueous Pluronic polymer solutions.The final copolymer solutions contained 0.01 M DSS. Thesolutions were facilitated by mixing with gentle agitation andthen were each transferred to a 5-mm NMR sample tube thatwas sealed immediately with laboratory film. After 15 min ofsonication to remove dissolved paramagnetic dioxygen, thesample tubes were stored in a refrigerator before use.

NMR Methods. All NMR experiments were conducted ona Bruker Avance 600 spectrometer at a Larmor frequency of600.13 MHz for the proton; the spectrometer was equipped witha microprocessor-controlled gradient unit and an inverse-detection multinuclear BBI probe with an actively shieldedz-gradient coil. The sample temperature was kept constant towithin (0.1°C by use of a Bruker BCU-05 temperature controlunit. Temperature was calibrated separately for each probe usinga capillary containing methanol (lowT) or ethylene glycol (highT).43 For all 1H NMR experiments, the samples were allowedto equilibrate at the desired temperature for at least 15 min priorto measurement. Experiments that were repeated at the sametemperature, but were reached by a temperature change in theopposite direction, yielded identical results. DSS was directlyadded into the sample solutions as an internal reference toeliminate temperature-induced shifts. Here, rotating-frame nuclearOverhauser effect (ROE)1H NMR spectra were acquired byusing 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, the1H NMRspectra of 2.5% (w/v) Pluronic F88 in D2O solution in theabsence and presence of urea were measured at varioustemperatures, and the local expanded regions of the HDO, EO-CH2-, PO -CH2-, and PO-CH3 signals are presented inFigure 1A-D, respectively. According to previous assign-ments,45 the triplet at∼1.18 ppm is attributed to the protons ofthe PO-CH3 groups, the broad peaks from about 3.65 to 3.45ppm belong to the PO-CH2- protons, the sharp singlet at∼3.7ppm belongs to the EO-CH2- protons, the signal at∼4.8ppm 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, distinctsignals for all protons can be clearly observed at low temper-atures. The PO-CH2- signals show a hyperfine structure, andthe PO-CH3 signal exhibits a triplet. The presence of distinctmultiplets at lower temperature is because of efficient motionalnarrowing.5 It indicates that the copolymer dissolved in wateras unimers and that all segments of the solvated polymer can

move freely. When the temperature is increased above a certainvalue, the hyperfine structure of the PO-CH2- signals andthe triplet of the PO-CH3 signals disappear in a smalltemperature interval, and both the PO-CH2- and-CH3 signalsbroaden. The line-shape changes of the PO groups are the resultof conformational changes in the PPO chain,45 whereas theobserved line broadening of the PO groups indicates a reducedmobility of the PO segments.47 The onset temperature at whichthe spectral profiles of the PO segments show dramatic changescan be determined as the CMT. It should be noted that a newresonance signal, labeled g, appears at∼3.42 ppm and growsprogressively larger as the temperature increases above theCMT. This new resonance has also been attributed to PO-CH2- protons on the basis of 2D heteronuclear single-quantumcoherence (HSQC)-resolved1H{13C} NMR spectra,45

the appearance of which is attributable to the breakdown of theintramolecular attraction between the PO-CH2- protons andthe nearest ether oxygens during micellization.

For the Pluronic solution in the presense of urea, all of theresonance peaks show downfield shifts with increasing ureaconcentration, and the changes in the spectral profile associatedwith micelle formation move to higher temperature uponaddition of urea. This shows that the addition of urea canstabilize the block copolymer in the premicellar state and preventthe occurrence of micellization at otherwise the same conditionsin the absence of urea.

Interaction between Urea and Pluronic Polymer.ROEmeasurements were used to confirm the interactions betweenurea and the different moieties of the Pluronic polymers.Intermolecular1H cross-relaxation processes by NOE can, ingeneral, occur for molecules that are in close spatial contact(within 0.5 nm), being mediated by through-space dipole-dipolecouplings, and can be used to probe molecular proximity.46 Ina typical experiment, the urea protons and PPO-CH3 protonswere selectively excited in the unimer and micellar regions,respectively, and cross-relaxation processes to other protonmoieties were monitored. Figure 2 shows the1H cross-relaxationROE spectra for the entire spectral region at 25°C (umimerregion) and 50°C (micelle region). When the urea protons wereexcited, proton cross-relaxation peaks for HDO and PEO blocksare observed, whereas when the PPO-CH3 protons wereexcited, no cross-relaxation peaks were observed. The spectradid not change when the temperature was increased from 25 to50 °C. These proton cross-relaxation ROE results thus confirmthat the direct interactions of urea molecules with watermolecules and PEO blocks and the locus of interaction do notchange with increasing temperature.

Effect of Urea on Water Structure.Figure 1A shows thechanges in the1H NMR spectra of the residual HDO signalwith increasing concentration of urea at different temperatures.The temperature-dependent chemical shift (δ) of the HDO signalas a function urea concentration is plotted in Figure 3. Thechemical shift shows a linear upfield shift with increasingtemperature, whereas it exhibits a large downfield shift withaddition of urea. Because the upfield shift of the HDO signalis due to the breakdown of the hydrogen-bonding structure inwater,45 these results indicate that the hydrogen bonds betweenwater molecules are progressively weakened by heat. Theobservation that urea increases the chemical shifts of HDOsuggests that the addition of urea has an effect opposite to thatof temperature in changing the hydrogen-bonding structure ofwater. It seems that urea acts as a structure-maker for waterand facilitates hydrogen bonding among water molecules orbetween water and urea molecules. Although urea causes a

TABLE 1: Composition of PEO-PPO-PEO BlockCopolymers

polymer mol wtPPO

segment wtno. of

PO unitsno. of

EO units PPO/PEO

P123 5750 4025 69 2× 19 1.79P84 4200 2250 39 2× 19 1.15F88 11 400 2250 39 2× 103 0.19

5156 J. Phys. Chem. B, Vol. 111, No. 19, 2007 Ma et al.

breakdown of long-range order, the water in concentrated ureasolutions is nevertheless extensively hydrogen-bonded, asevidenced by the fact that the chemical shift of the HDO signalis still strongly temperature-dependent. The high solubility ofurea suggests the existence of some urea-water interactions.These findings appear to support the ROE results that ureadirectly interacts with water.

Effects of Urea on PEO.The temperature-dependent chemicalshift (δ) of the EO -CH2- protons as a function of ureaconcentration is plotted in Figure 4. The chemical shift of theEO -CH2- protons shows a linear decrease with increasingtemperature in the absence of urea, whereas the chemical shiftof the EO-CH2- protons becomes independent of temperaturein the presence of 2 M urea and shows a linear increase in thepresence of higher urea concentrations. The slight upfield shiftindicates that the PEO blocks experience a small degree ofdehydration with increasing temperature.45 When urea is added,the chemical shift of the EO-CH2- protons undergoes a largedownfield shift. The higher the urea concentration, the moreextensive the downfield the chemical shift. The observeddownfield shift of the PEO protons is a manifestation of theincreased hydration of the PEO segments. The reverse trend ofthe chemical shift of the EO-CH2- protons in the presenceof higher urea concentrations also suggests the direct interactionof urea with the hydrated PEO corona of the block copolymermicelles. When the urea is dissolved in water, the polar ureamolecules enter the polar region of the micellar shell, replacethe water molecules around the PEO blocks, and directly formhydrogen bonds with the PEO blocks.

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.

Figure 2. Profiles of1H NMR rotating-frame nuclear Overhauser effect(ROE) spectra of 2.5% F88 in the presence of 2 M urea solution. Spectrawere recorded at 25 and 50°C by selectively exciting (a,c) the ureasignal and (b,d) PPO-CH3 signal.

Interactions of Urea with Pluronics J. Phys. Chem. B, Vol. 111, No. 19, 20075157

Effects of Urea on PPO.The temperature-dependent chemicalshift (δ) of the PO-CH3 signal of aqueous 2.5% Pluronic F88solutions in the presence of different concentrations of urea ispresented in Figure 5. It is clear that an increase in temperatureleads to a marked decrease in the chemical shift of the PO-CH3

protons. The chemical shift of the PO-CH3 protons undergoesa large decrease at certain temperatures; below or above thesetemperatures, a slight linear decrease can be observed. Manyexperimental studies have demonstrated that the PPO segmentsof block copolymers are in a hydrated state at lower temper-atures.8-12 Because the interaction of the PPO protons with waterenhances the deshielding effect of the C-H protons and resultsin 1H downfield shifts, the upfield chemical shift with increasingtemperature is related to the adverse dehydration process. Inthe temperature interval from 36 to 45°C in the absence ofurea solution, the significant upfield shift of the PO-CH3

protons indicates that the PPO blocks apparently reduce thecontact with water and form a hydrophobic microenvironment.At increasing temperature above 45°C, the linear decrease ofthe chemical shift of the PO protons suggests that water iscontinually excluded from the micellar core. It can be inferredthat an increase in temperature increases the hydrophobicity ofPluronic polymer micelles, thus decreasing the water contentin the micellar core. When urea is added, the chemical shift ofthe PO segments exhibits a marked increase; the greater theamount of urea added, the farther downfield the chemical shifts,similar to the change for EO segments. From the above1H NMRROE results, it seems that most PPO segments probably cannotinteract directly with urea; therefore, an indirect interactionbetween PPO and urea molecules is suggested. Because theaddition of urea increases the hydration of PEO and enhancesthe hydrogen-bonding structure in water, the hydration of thehydrophobic groups of PPO increases as well. In the premicellarstate, the dehydration of PPO is the dominating reason for themicellization of block copolymers in aqueous solution. Theincreasing hydration of PPO with the addition of urea willeventually prevent the occurrence of micellization. The CMTvalues determined from the first inflection point on the chemicalshift versus temperature plots for 2.5% Pluronic F88 in D2Osolution 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 themicellar state, all of the hydrophobic groups are buried insidethe micellar core, and hence, interactions between EO segmentsand urea dominate over the hydrophobic group-urea interac-tions.

As shown in Figure 1, the resonance signals of the POsegments in the1H NMR spectra show a clear line broadeningwith increasing temperature and become narrow again uponaddition of urea. To obtain accurate quantitative information,the half-height width (the line width at half-height,∆ν1/2) wasused to characterize the exact changes. For the triplet of thePO -CH3 signal, the∆ν1/2 was measured at the half-heightposition 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 inthe absence of urea solution, the∆ν1/2 value of the PO-CH3

signals remains constant with increasing temperature. A changeoccurs in the temperature interval from 36 to 44°C, and thehalf-width increases abruptly from 12.6 to 19.9 Hz. Becausethe line width is inversely proportional to the mobility of therelated segments,47 this sharp increase in half-height widthindicates an abrupt decrease of chain mobility, which isconsidered to be related to the aggregation of Pluronic F88.Upon micellization, the PO segments are confined in the coreof the micelles. Compacting of the PPO chains limits theirmovement and therefore induces a decrease of chain mobility.When the temperature is above 44°C, the∆ν1/2 value of thePO-CH3 signal decreases with increasing temperature, whichsuggests that the aggregation process has finished and that chain

Figure 3. Temperature-dependent1H NMR chemical shifts observedfor the residual HDO signal of 2.5% (w/v) Pluronic F88 solutions atvarious urea concentrations.

Figure 4. Temperature-dependent chemical shifts of the EO-CH2-signal of 2.5% aqueous Pluronic F88 solutions in the presence ofdifferent concentrations of urea.

Figure 5. Temperature-dependent chemical shifts of the PO-CH3

signal of 2.5% aqueous Pluronic F88 solutions in the presence ofdifferent concentrations of urea. Arrows denote the CMT.


mobility increases mainly because of the decreasing viscosityof 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 hasincreased. This finding appears to suggest that the addition ofurea provides the driving force for the transfer of water fromthe medium to the micellar core and increase the hydration ofthe PPO block, which will result in the dissociation of the blockcopolymer micelles at higher urea concentration.

As shown in Figure 1, a new resonance signal (denoted byg) appears and grows progressively larger at temperatures abovethe CMT. This new signal arises as a result of the breakdownof the intramolecular (C-H)‚‚‚O hydrogen bonds between thePO -CH2- protons and the nearest ether oxygens withincreasing temperature.45 The presence of the intramolecular(C-H)‚‚‚O attraction has been shown to provide gauche stabilityin the main chain of PPO.48,49On the other hand, the breakdownof the intramolecular hydrogen bonds might result in a decreasein the number of gauche conformers in the PPO chain.Therefore, the increase of the integral area of the peak labeledg can be directly correlated with the decrease in the number ofgauche conformers in the PPO chain. If we calibrate the integralarea of the PO-CH3 signal to 117 (the number of methylprotons in an F88 molecule) at each temperature, quantitativeinformation on the temperature-dependent changes of peak gcan be obtained. The temperature-dependent integral value ofpeak g is plotted as a function of urea concentration in Figure7. It is observed that the integral value increases abruptly at theCMT until the other boundary of transition region is reached,which indicates the breakdown of the intramolecular hydrogenbonds and thus a decrease in the number of gauche conformersin the PPO chain. The addition of urea significantly decreasesthe integral value of peak g but does not affect the features ofthe spectral profiles. This indicates that the addition of ureahas a stabilizing effect on the gauche conformation of the PPOchain and prevents the unimer-to-micelle transition, thusincreasing the CMT. The temperatures at which the micellizationtransition occurs are in good accordance with those presentedin Figures 5 and 6 and show the same increasing trend withincreasing urea concentration. It is generally accepted that, abovethe CMT, there is an equilibrium region referred to as theunimer-to-micelle transition region in which significant amountsof both free and associated copolymer molecules coexist.50,51

It can be determined between the two inflection points in Figures

5-7 (the inflection points were determined from the intersec-tions of two tangent lines, as marked in the figures). Consideringthe determination error, the results are identical and thetemperature range over which this transition takes place appearsto be unaffected by the presence of urea.

Effect of Polymer Composition on Urea-Pluronic Inter-action. To compare the different effects of urea on the PEOand 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 hydrophobicblocks (P84 and P123) in D2O solution were investigated, asshown Figure 8. Although all of the Pluronic polymers studiedshow the same linear dependence of the CMT on the ureaconcentration (for the concentration range investigated), theslopes 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, whereasthe slope decreased to 0.38°C/M (urea) for P123 with increasingnumber of PO units. This would indicate that the effect of ureaon the micellization of block copolymers becomes stronger whenthe number of hydrophilic PEO blocks is increased. Thus, theurea-PEO interaction is a primary factor in the increase of themicellization temperature of block copolymers in aqueous urea

Figure 6. Temperature-dependent half-height widths of the PO-CH3

signal of 2.5% aqueous Pluronic F88 solutions in the presence ofdifferent concentrations of urea. Arrows denote the CMT.

Figure 7. Temperature-dependent integral values of peak g (see Figure2) with the integral area of the PO-CH3 signal calibrated to 117 inthe presence of different concentrations of urea. Arrows denote theCMT.

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


solutions. The decrease of the slope for the curve of PluronicP123 with increasing number of PPO blocks suggests that theCMT values are much less influenced by the urea-PPOinteraction at higher relative PPO contents and that the effectof urea on the micellization of the block copolymer is lower.

According to the determination of interaction sites, thefollowing explanation for the urea-Pluronic interaction has beenadvanced: When urea is dissolved in water, the urea moleculesinteract with water molecules through hydrogen bonding. Atthe same time, many urea molecules penetrate into the PEOhydration shell and interact directly with PEO blocks, leadingto an increase of the free energy of water around PEO becauseof unfavorable entropy contributions. In contrast, the water inthe PPO hydration shell is still in a comparatively low-energystate, and thus, water is driven to increase the hydration of thePPO segments. The total effect of urea decreases the dehydrationof block copolymers with temperature and stabilizes theconformation of the polymer chains in the premicellar region,which causes the micellization of block copolymers to bedifficult so that it can only occur at higher temperatures. Thisassumption is also supported by the thermodynamic datareported by Alexandridis et al.27 It is known that the micelli-zation of Pluronic polymers in aqueous solution is an entropy-driven process,4 whereas the micellization entropy values forPluronic polymer decreased as the urea concentration wasincreased.27 At the same time, the enthalpy of micellization wasalso lowered in the presence of urea and decreased further withincreasing urea concentration, in agreement with behaviorobserved for other nonionic surfactants. Favorable enthalpicinteractions between urea, PEO, and water most likely causethe decrease in enthalpy.

Conclusion

This study has shown that1H NMR spectroscopy is anexcellent technique for the investigation of the interaction ofurea with Pluronic polymers.1H NMR spectroscopy not onlypresents an accurate means to determine the CMT values ofblock copolymers in aqueous solution, but also provides valuableinformation on the interaction sites of urea molecules with thetriblock copolymer species. It was shown that the urea moleculesinteract directly with the PEO moieties of the triblock copoly-mers and with the solvent water molecules in both unimer andmicellar regions, whereas the urea molecules seem not to interactwith the PPO blocks directly.

Both residual HDO and EO-CH2- signals show a largedownfield shift with the addition of urea, indicating that thewater molecules and the PEO blocks interact with urea in asimilar way. However, the downfield shift of the PPO blocksis possibly the result of an increase in hydration upon additionof urea. When the temperature is above the CMT, the chemicalshift of the PO-CH3 signal shows an abrupt upfield shift,indicating the formation of a hydrophobic micellar core. At thesame time, the half-height width of the PO-CH3 signal showsa sharp increase at the CMT because of the decreasing segmentalmobility in the compact micellar core. The appearance of a newresonance signal due to the breakdown of intramolecularhydrogen bonds between the PO-CH2- protons and the etheroxygens above the CMT suggests a decrease in the number ofgauche conformers in the PPO chain. However, upon theaddition of urea, all of these changes are shifted to highertemperatures, indicating that the micellization boundary is drivento higher temperatures. It can be concluded that the presenceof urea makes Pluronics more hydrophilic (the increase inchemical shift). The integral values of the new signal decrease

with increasing urea concentration, implying that urea has astabilizing effect on gauche conformers in the PPO chain. Theeffect of urea in increasing the CMT values of the blockcopolymers is more pronounced for copolymers with higherPEO contents and becomes unconspicuous with increasingcontent of PPO segments. This validates that the interactionbetween urea and the PEO moieties of the block copolymerplay a dominant role in the interaction of urea with Pluronicsurfactant species.

Acknowledgment. This work was financially supported bythe National Natural Science Foundation of China (Nos.20221603, 20676137, and 20490200), the National HighTechnology Research and Development Program of China (863Program) (No. 20060102Z2049), and Major Aspect of Knowl-edge Innovation Project of Chinese Academy of Sciences (No.KSCX2-YW-G-019).

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interaction of urea with pluronic block copolymers by 1 h nmr...

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