Oil-Induced Aggregation of Block Copolymer in Aqueous Solution

Oil-Induced Aggregation of Block Copolymer in Aqueous Solution

Jun-he Ma, Yun Wang, Chen Guo,* and Hui-zhou Liu*Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Instituteof Process Engineering, Chinese Academy of Sciences. Graduate UniVersity of Chinese Academy of Sciences.Beijing 100080, P. R. China

Ya-lin TangInstitute of Chemistry, Chinese Academy of Sciences. Beijing 100080, P. R. China

Pratap BahadurDepartment of Chemistry, V.N. South Gujarat UniVersity, Surat 395007, Gujarat, India

ReceiVed: April 25, 2007; In Final Form: July 22, 2007

The oil-induced aggregation behavior of PEO-PPO-PEO Pluronic P84 [(EO)19(PO)39(EO)19] in aqueoussolutions has been systematically investigated by1H NMR spectroscopy, freeze-fracture transmission electronmicroscopy (FF-TEM), and dynamic light scattering (DLS). The critical micellization temperature (CMT)for P84 in the presence of oils decreases with increasing oil concentration. The effectiveness of various oilsin decreasing the CMT of block copolymer follows the orderm-xylene (C8H10) > toluene (C7H8) > benzene(C6H6) > n-octane (C8H18) > n-hexane (C6H14) ≈ cyclohexane (C6H12). It was found that the amount ofanhydrous PO methyl groups increases whereas the amount of hydrated PO methyl groups decreases uponthe addition of oils. At low oil concentration, the oil molecules are entrapped by the micellar core, but as theoil concentration increases above a certain value, the micellar core swells significantly as a result of thepenetrated oil molecules, and much larger aggregates are formed. Intermolecular rotating-frame nuclearOverhauser effect (ROE) measurements between P84 and benzene were performed at 10 and 40°C. Thespecific interaction between benzene and the methyl groups of PPO was determined, and it was observed thatthe interaction site remained unchanged as the temperature was increased.

Introduction

Currently, there is a great deal of research interest in poly-(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO) triblock copolymers (commercially availableunder the trade names Poloxamers or Pluronics), as they arewidely used for various applications in the nanotechnology,pharmaceutical, bioprocessing, and detergent industries.1-4

Block copolymers consisting of a central PPO block flankedby two PEO blocks display thermosensitive amphiphilic proper-ties. The interesting features of PEO-PPO-PEO block co-polymers in aqueous solution are their temperature-dependentself-association and rich phase behavior.5-12 The process of self-association can be induced by increasing the temperature toexceed the critical micellization temperature (CMT); however,this effect is different for different copolymers.13,14

The aggregation properties of PEO-PPO-PEO block co-polymers in aqueous solutions are very sensitive to the cosolutesadded.15 The effects of salts have been discussed in terms of“salting-in” and “salting-out” effects and follow the Hofmeisferseries.16-20 However, studies of the effects of various organicsubstances on the phase behavior of PEO-PPO-PEO blockcopolymers have been quite limited. The experimental inves-tigations revealed that short-chain alcohols, urea, and formamideprevent the onset of micellization of Pluronic polymers in water,whereas long-chain alcohols (butanol, pentanol, etc.) andhydrazine favor micelle formation.21-26

In fact, oil can also be used to adjust the properties of blockcopolymers in aqueous solution. The most important use of oilis in the modulation of microemulsions, systems consisted ofwater, oil, and amphiphiles that are characterized as opticallyisotropic and thermodynamically stable liquid solutions.27,28Oil-in-water microemulsions are very promising for use as drugdelivery vehicles.29-31 Among the numerous microemulsionsystems, Pluronic-based microemulsions have attracted greatattention mainly because of two advantages: the diverseinterfacial properties of these surfactants and their high doseuptake by patients without any apparent side effects.32 However,most experimental studies of oil-swollen Pluronic micelles, eventhose utilizing powerful scattering techniques, such as small-angle neutron scattering (SANS)33,34 and small-angle X-rayscattering (SAXS),35 have taken simplified views of the oil-polymer aggregates by treating them as homogeneous hardspheres or cylinders and have not attempted to reveal the controland interaction mechanism of oil toward Pluronic-based mi-croemulsion systems at the molecular level.

In this work, the effect of oil on the aggregation behavior ofPluronic P84 is investigated using1H nuclear magnetic reso-nance (NMR) spectroscopy, freeze-fracture transmission electronmicroscopy (FF-TEM), and dynamic light scattering (DLS). Themolecular-level mechanism of the oil-Pluronic interaction isdiscussed.

2. Experimental Section

Materials. The PEO-PPO-PEO triblock copolymer Plu-ronic P84 was obtained from BASF (Parsippany, NJ) and was

* To whom correspondence should be addressed. Phone:+86-10-62555005. Fax:+86-10-62554264. E-mail: [email protected] (C.G.),[email protected] (H.-z.L.).

11140 J. Phys. Chem. B2007,111,11140-11148

10.1021/jp073192u CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 09/01/2007

used as received. P84 can be represented by the formula EO19-PO39EO19. Cyclohexane (C6H12), n-hexane (C6H14), n-octane(C8H18), benzene (C6H6), toluene (C7H8), andm-xylene (C8H10)

were supplied by Merck, all with purity greater than 99.0%.All chemicals were used without further purification. Thereference 2,2-dimethyl-2-silapentane-5-sulfonate sodium urea(DSS, g97%) was purchased from Sigma Aldrich ChemicalCorp. D2O (g99.9 atom %2H) was purchased from CIL Corp.(Andover, MA).

Sample Preparation.A heavy-water solution of Pluronic P84was prepared by weighing appropriate amounts of polymers inD2O solution with gentle agitation. During the experiments, thepolymer solutions were further diluted with D2O, and differentoils were added to their proposed concentration. A stock solutionof 0.6 M DSS in D2O was prepared. For1H NMR measure-ments, 1µL of the stock solution of DSS was injected into 600µL of aqueous polymer solution with a syringe, so that the finalcopolymer solution contained 0.001 M DSS. The solution wastransferred to a 5-mm NMR sample tube, and the tube was

Figure 1. 1H NMR spectra of 5% (w/v) Pluronic P84 dissolved in benzene/water mixtures with benzene concentrations of (a) 0, (b) 0.04, (c) 0.08,and (d) 0.10 M at various temperatures, showing the (A) benzene, (B) HDO, (C) PO-CH2-, and (D) PO-CH3 signals.

Figure 2. 1H NMR spectra of 5% (w/v) Pluronic P84 in D2O solutionwith the oils (a)n-hexane, (b)n-octane, (c) benzene, (d) toluene, and(e) xylene recorded at various temperatures, showing the (A) HDO,(B) PO-CH2-, and (C) PO-CH3 signals. The oil concentration wasfixed at 0.06 M.

Figure 3. 1H NMR spectra of 5% (w/v) Pluronic P84 in the presenceof different concentrations of (left) benzene and (right) xylene measuredat 25°C, showing the (a,c) PO-CH2- and (b,d) PO-CH3 signals.

Oil-Induced Aggregation of Block Copolymer J. Phys. Chem. B, Vol. 111, No. 38, 200711141

sealed immediately with laboratory film. After 15 min ofsonication to remove dissolved paramagnetic dioxygen, thesample tubes were stored in a refrigerator until use.

NMR Spectroscopy.All NMR experiments were conductedon a Bruker Avance 600 spectrometer at a Larmor frequencyof 600.13 MHz for protons. The spectrometer was equippedwith a 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 the use of a Bruker BCU-05 temperaturecontrol unit. Temperature was calibrated separately for eachprobe using a capillary containing methanol (lowT) or ethyleneglycol (high T).36 For all 1H NMR experiments, the sampleswere allowed to equilibrate at the desired temperature for atleast 15 min prior to measurement. DSS was directly added tothe sample solutions as an internal reference to eliminatetemperature-induced shifts. Here, rotating-frame nuclear Over-hauser effect (ROE)1H NMR spectra were acquired by usinga selectively excited gradient-selected pulse sequence.37

FF-TEM. The specimens were plunged into ethane cooledby liquid nitrogen. The samples were freeze-fractured at 153 Kand 10-4 Pa in a Balzer BAF 400D freeze-etching apparatusand shadowed by platinum/carbon at an angle of 45°. The

Figure 4. (A) Temperature-dependent integral values of peak g (see Figure 1), with the integral area of the PO-CH3 signal calibrated to 117, and(B) width at half-height of the PO-CH3 signal of 5% aqueous Pluronic P84 solutions in the presence of different concentrations of benzene.

Figure 5. (A) Temperature-dependent integral values of peak g (see Figure 2), with the integral area of the PO-CH3 signal calibrated to 117, and(B) width at half-height of the PO-CH3 signal of 5% aqueous Pluronic P84 solutions in the presence of different kinds of oils. The concentrationof the oils was fixed at 0.06 M.

TABLE 1: Literature Values of the Critical MicellizationTemperatures (CMTs in °C) of P84 and P85 (similar to P84)Solutions in H2O and D2O Determined by DifferentTechniquesa

H2O/D2O

technique 5% conc (w/v) 15% conc (wt)

P84 solubilized DPH6 231H NMR spectroscopy /26FTIR spectroscopy42 /22

P85 solubilized DPH6 25DLS43 25/25SANS44 25/25

a CMT values given in ref 6 for H2O seem to be systematically lowerby 2-3 °C than those determined by other techniques (DSC, LS,ultrasonics, SANS, etc).45,46 If the 2-3 °C correction were added, theCMT values determined by1H NMR spectroscopy would be in goodagreement with the literature.

11142 J. Phys. Chem. B, Vol. 111, No. 38, 2007 Ma et al.

replicas were cleaned to remove sample residuals and examinedwith Philips Tecnai 20 and Jeal JEM-100cx electron micro-scopes.

Dynamic Light Scattering (DLS). The mean diameters andpolydispersities of Pluronic P84 aggregates were determinedby dynamic light scattering (DLS) using a Brookhaven 90PlusNanoparticle Size Analyzer (Brookhaven Instruments Corp.,United States) with a 15-mW solid-state laser at room temper-ature. All analyses were run in triplicate, and the results arereported as average values.

3. Results and Discussion

1H NMR Spectra of P84 in the Presence of Oil.Toinvestigate the effects of oil on the aggregation behavior of blockcopolymer, the1H NMR spectra of 5% (w/v) Pluronic P84 inD2O solution, in the absence and presence of different concen-trations of benzene, were acquired at various temperatures.Before investigating the interactions between benzene andtriblock copolymer in solution, spectroscopic analyses and

assignments of the observed1H NMR resonances were firstcarried out over the temperature range 10-45 °C. The1H NMRspectra of the separate and mixed solutions are shown in Figure1. The triblock copolymer species dissolved in D2O (Figure 1Cand D) gives rise to three resolved signal regions that wereassigned according to our previous work.38 Specifically, thetriplet 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 areassigned to the PO-CH2- protons, the intense resonanceobserved at around 3.7 ppm is assigned to the EO-CH2-protons (see Figure S2 in the Supporting Information), the signalat ∼4.8 ppm is the residual signal of HDO, and the remainingsignal at∼7.3 ppm is the proton resonance of benzene.

For the P84 solution in the absence of oil (see Figure 1A),the PO-CH2- signals show a hyperfine structure, and the PO-CH3 signal exhibits a triplet at low temperatures. The presenceof distinct multiplets is due to an efficient motional narrowing,39

indicating that the copolymer dissolves in water as a unimer.However, when the temperature is increased above a certainvalue, the spectral profiles show two characteristic changes inthe line shape of the PPO segments: (1) the disappearance ofthe hyperfine structure of the PO-CH2- signals and the tripletof the PO-CH3 signals as well as the broadening of the signalsin a narrow temperature interval and (2) the emergence of anew resonance signal labeled g (∼3.4 ppm, denoted in Figure1C) that grows progressively larger with increasing temperature.(The particular change of peak g with increasing temperatureis shown in Figure S1 in the Supporting Information.) Theobserved line broadening of the PO groups can be attributed tothe reduced mobility of the PO segments, resulting fromtemperature-induced micellization,40 whereas the emergence ofthe new resonance is because of the breakdown of the intramo-lecular (C-H)‚‚‚O hydrogen bond between the PO-CH2-protons and the ether oxygen during micellization.38 It has beenvalidated that such changes can be used to emphasize thetemperature-dependent micellization of Pluronic polymers inaqueous solution.41

For the P84 solution in the presence of benzene, the spectralprofiles of the PO-CH2- and the PO-CH3 signals (especiallythe PO-CH3 signal) showed a significant broadening withincreasing benzene concentration, and the changes in the spectralprofile associated with aggregation moved to lower temperature.This indicates that the addition of benzene will destroy thethermal equilibrium of the block copolymer in the unimer stateand facilitate the aggregation at otherwise the same conditionsin the absence of benzene.

A comparison of different oils [cyclohexane (C6H12), n-hexane (C6H14), n-octane (C8H18), benzene (C6H6), toluene(C7H8), m-xylene (C8H10)] is presented in Figure 2. The spectraare for a fixed oil concentration of 0.06 M and varyingtemperatures. It can be seen that the PO signals show only agentle change upon addition of alkanes, whereas the spectrachange significantly upon addition of aromatic hydrocarbons.

Effects of Oil on the CMTs of Pluronic P84 in AqueousSolution. Oil Concentration Effect.To characterize the oil-induced aggregation, the1H NMR spectra of 5% (w/v) PluronicP84 in D2O solution at 25°C as a function of benzene andxylene concentration were acquired and are presented in Figure3. The insets in Figure 4A and B present the temperaturedependence of the integral values of peak g and the width athalf-height of the PO-CH3 signal of 5% aqueous P84 solutionsas a function of benzene concentration, respectively. Becausethe CMTs of Pluronic polymers can be determined accuratelyfrom the first inflection point of the integral values of peak g

Figure 6. (Top) 1H NMR spectra of the (a) PO-CH2-and (b) PO-CH3 regions of 5% P84 in the presence of 0.06 M toluene at varioustemperatures and (bottom) temperature-dependent fractions of hydratedand anhydrous methyl groups of P84 obtained from Figure 6b.


or the width at half-height of the PO-CH3 signal againsttemperature sigmoid curves41 (the point at which the curve startsdeviating from linear behavior, denoted in Figures 4 and 5),the results presented in the figures indicate that the addition ofbenzene significantly decreases the CMT of Pluronic P84 whenthe benzene concentration is lower than 0.06 M; however, atconcentrations higher than 0.06 M, this effect becomes lessapparent. It is probably because the supersaturation of benzene,and the solution will separate into two phases. Literature valuesof the CMTs of P84 and P85 [where P85 is (EO)26(PO)40(EO)26

and the CMT of P85 is 1-2 °C higher than that of P84 at thesame concentration], determined by other techniques, are listedin Table 1 for comparison.

Oil Type Effect.The effects of oil type on the integral valuesof peak g and the width at half-height of the PO-CH3 signalof Pluronic P84 in aqueous solutions are presented in Figure5A and B, respectively. The effectiveness of the oils indecreasing the CMTs of Pluronic P84 follows the orderm-xylene(C8H10) > toluene (C7H8) > benzene (C6H6) > n-octane (C8H18)> n-hexane (C6H14) ≈ cyclohexane (C6H12). On a moleconcentration basis, the aromatic hydrocarbons show a muchstronger trend in CMT reduction than the alkanes. Because theincrease of the integral area of peak g can be directly correlatedto the decrease of gauche conformers in the PPO chain,41 itappears that both the type of oil introduced and its concentrationcan affect the conformational state of the PPO chain.

Figure 7. Temperature-dependent chemical shifts of the (A) benzene and (B) HDO signals of 5% aqueous Pluronic P84 solutions in the presenceof different concentrations of benzene.

Figure 8. Profiles of1H NMR rotating-frame nuclear Overhauser effect (ROE) spectra of 5% P84 in the presence of 0.1 M benzene. Spectra wererecorded at 10 and 40°C by selectively exciting the (a,d) benzene, (b,e) PPO-CH3, and (c,f) PEO-CH2- signals.


Although the dehydration of the hydrophobic PO units playsa leading role in micellization, the dehydration of the hydrophilicEO units also strongly influences the process.10 To reveal theeffect of oils on the hydration state of the micellar corona, the1H NMR spectra and the chemical shift of the EO signal underdifferent concentrations of benzene and various oils at differenttemperatures are presented in Figures S2-S4 (see the SupportingInformation). The chemical shift of the EO-CH2- protonsshows a linear decrease in chemical shift values with increasingtemperature in the absence of oil, whereas the chemical shiftof the EO-CH2- protons increases slightly with increasingoil concentration or addition of aromatic hydrocarbons. Theslight upfield shift indicates that the PEO blocks experience asmall degree of dehydration with increasing temperature. Thedownfield shift in the presence of oils is a manifestation of theincreased hydration of the PEO segments, which might be

caused by the hydrophobic effect of the oils. This result alsosuggests the indirect interaction between the oil molecules andthe PEO segments.

Interaction between Oil and Pluronic P84.The changesin line width and position effectively demonstrate that the oilspecies interact with the triblock copolymer in solution. It isinteresting to note that a new resonance signal of the PO-CH3

signals at approximately 0.87 ppm appears at lower temperaturesupon the addition of oil. The signal could not be observed inthe absence of oil and is unobvious in the presence of alkanes,but it is clearly resolved in the presence of aromatic hydrocar-bons (see Figure 6b for the results obtained in the presence of0.0 6M toluene) and grows larger with increasing oil concentra-tion. A further increase in temperature leads to a downfield shiftof this new resonance and results in an overlapping of this newsignal with the original PO-CH3 signal at about 20°C. Wededuce that the two signals of the PO-CH3 groups of P84 canbe correlated to the two states: one is a hydrated statecorresponding to the peak around 1.17 ppm (surrounded bywater), and the other is an anhydrous state associated with thepeak near 0.9 ppm. A similar phenomenon of the splitting ofthe symmetric deformation band of methyl groups of Pluronicpolymers in water has already been observed by FTIR spec-troscopy.11,12 With the assumption of separate hydrated andanhydrous signals, we can determine the fraction of anhydrousPPO methyl groups, because the areas under the resonances areproportional to the concentrations of methyl protons in therespective states. The areas under the different signals weredetermined by fitting a sum of Lorentz functions to the signalsand then calculating the relative area under each signal. Theresults from this analysis are presented in Figure 6 (bottom) asa function of temperature. As shown, the percentage ofanhydrous PO-CH3 groups increases with increasing temper-ature at the expense of that of the hydrated PO-CH3 signalwhen the temperature is above the CMT. However, at 45°C,ca. 10% of the PPO methyl groups still remain in the hydratedstate. Of course, the relative peak area fraction is not the exactproportion of the anhydrous or the hydrated methyl groups.Nevertheless, it can be used as a criterion in that its increase ordecrease unambiguously shows an increase or decrease of theproportion of the anhydrous or hydrated methyl groups. It isknown that the main reason for the temperature-inducedaggregation of Pluronic micelles is the dehydration of the PPOsegments.2,6-8 The results presented here show that the additionof oils obviously increases the amount of the anhydrous PO-CH3 groups and thus will increase the hydrophobicity of thePluronic polymer, which will probably decrease the energybarrier for the aggregation of P84 molecules at the sametemperature.

Interaction Sites Determination between Benzene and P84.When two moieties are in close spatial proximity, the localelectronic environments of either or both species can beperturbed, altering the local magnetic fields and leading tochanges in their observed isotropic chemical shifts. To date,the 1H NMR chemical shift is a well-established indicator fordetermining the loci of interactions between the different speciesin solution.47 Figure 7A and B show the temperature-dependentchemical shifts (δ) of benzene and the residual HDO signals asa function of benzene concentration, respectively. The chemicalshift of benzene shows an upfield shift with increasing tem-perature at lower concentration, whereas it exhibits an oppositedownfield shift when the benzene concentration exceeds 0.06M. It is well-known that the inclusion of a species in a highlyhydrophobic environment would result in upfield shift of the

Figure 9. FF-TEM micrographs of 5% Pluronic P84 micellar solutionswith various concentrations ofm-xylene: (a) 0, (b) 0.02, (c) 0.04, (d)0.06, (e) 0.08, and (f) 0.10 M.


affected protons.48 Such is the case here. In addition, thepenetration of benzene into the core of the micelles is temper-ature-dependent. However, when the concentration of benzeneis higher than 0.06 M, the chemical shift of benzene shows adownfield shift, the same trend as observed for neat benzene inD2O solution with increasing temperature (see Figure S5 in theSupporting Information). When the temperature increases above25 °C, the chemical shift of benzene becomes independent oftemperature. However, the chemical shift of HDO seems to beunaffected upon the addition of benzene in the entire temperaturerange investigated, which otherwise validates that oil moleculesinteract directly with the block copolymer and have almost noeffect on the water structure.

Selective ROE measurements were carried out to confirm theinteraction sites between benzene and the different moieties ofPluronic polymer. Intermolecular1H cross-relaxation processesby the NOE can, in general, occur for molecules that are inclose spatial contact (within 0.5 nm), which is mediated bythrough-space dipole-dipole couplings and can be used to probemolecular proximity.49 In a typical experiment, the benzene,PO-CH3, and EO-CH2- signals were selectively excited at10 (unimer region) and 40°C (micellar region). Figure 8 showsthe 1H cross-relaxation ROE spectra for the entire spectralregion. When the benzene or PO-CH3 protons were excited,only the proton cross-relaxation peaks for the PO-CH3 groupsand benzene were observed, which suggests that there exists astrong interaction between the PO block and the benzenemolecules. However, when excited for the EO-CH2- peak,only the residual water signal is retained in the spectra. Thespectra do not change when the temperature is increased from10 to 40°C. These proton cross-relaxation ROE results confirmthat the benzene molecules directly interact with the PPO methylgroups, even if Pluronic P84 stays as unimer.

Morphology and Size of P84 Micelles under DifferentConcentrations of Xylene.FF-TEM can yield direct imagingof the sizes, aggregates, and shapes of the liquid sample. TheFF-TEM micrographs of aqueous Pluronic P84 solution in theabsence and presence ofm-xylene with fixed polymer concen-tration (5%) and varyingm-xylene concentrations are shownin Figure 9. It can be seen from this figure that a small amountof m-xylene has clear effects on the Pluronic P84 micelles. Thesize of the Pluronic P84 micelles increases with increasingm-xylene concentration. When the concentration of xylenereaches 0.06 M, much larger aggregates are formed, the diameterof which increases abruptly, and the granularity decreases.

The size and distribution of the aggregates in the mixedsystem were further examined by DLS. Figure 10 shows theintensity-weighted size distribution of the hydrodynamic diam-eter (Dh), measured at room temperature, of 5% Pluronic P84in m-xylene aqueous solutions of different concentrations. It isshown that P84 forms micelles inm-xylene aqueous solutionswhen the concentration ofm-xylene is below 0.04 M, and thediameter of the micelle increases only slightly with increasingxylene concentration. Meanwhile, above this concentration, itforms much larger aggregates. The polydispersity of the micellesor aggregates, evaluated through the ratioµ2/Γ2 by cumulantanalysis, is reported in Table 2, whereµ2 is the second momentin the cumulant expansion of the correlation function andΓ isthe decay rate. It has been found that the mean diameter of themicelles increased from 21 to 49 nm and the polydispersity ofmicelles increased from 0.107 to 0.171 when the concentrationof xylene was increased from 0 to 0.04 M, whereas above thisconcentration, the mean diameter increased abruptly to 1127.9nm and the polydispersity increased to 0.313 in 0.1 Mm-xyleneaqueous solutions.

Taking the above experimental results into consideration, thefollowing mechanism for the oil-induced aggregation of PluronicP84 is proposed: When oils are added to P84 aqueous solution,the entropy is decreased because of the ordering of water inthe presence of this substance.50 The oil molecules replace thehydrated shell of PPO segments and interact directly with thePPO methyl groups as a result of the driving force of entropy.This leads to a decrease of the free energy of the water aroundPPO because of unfavorable entropy contributions33 and thusresults in the micellization of block copolymer at lowertemperatures. When the oil concentration is low, the oilmolecules become encapsulated in the micellar core. However,when the oil concentration is above a certain value, more andmore oil molecules are embedded in the micelles, causing an

Figure 10. Intensity-weighted size distribution of Pluronic P84 inm-xylene solutions of different concentrations.

TABLE 2: Mean Diameter of Pluronic P184 and m-Xylenein Aqueous Solution with the Concentration of Pluronic P84Fixed at 5% (w/v) and the Concentration of Xylene Varied

xyleneconcentration (M)

effectivediameter (nm)

polydispersity

0 21.8 0.1070.01 23.1 0.1280.02 29.1 0.1520.04 49.2 0.1710.06 255.0 0.2070.08 422.7 0.2290.10 1127.9 0.313


abrupt swelling of the micellar core. The microemulsion particlesformed at higher oil concentration or temperature seem to havea quite stable chemical nature, because the chemical shift ofoil molecules in that state remains quite constant, as is evidentin Figure 7A.

Conclusion

The oil-induced aggregation behavior of Pluronic P84 inaqueous solution was studied by1H NMR spectroscopy, FF-TEM, and DLS. NMR spectroscopy gave valuable informationon the interaction sites between the oil molecules and thetriblock copolymer species. It was shown that the oil moleculesinteract directly with the PPO methyl groups of P84 in bothunimer and micellar regions, whereas the oil molecules appearnot to directly interact with the PEO blocks and the solventwater.

Oil can play an important role in modifying the propertiesof aqueous copolymer solutions. The CMT values of PluronicP84 were found to decrease upon addition of oil. Our studyexamined both alkanes and aromatic hydrocarbons. It was shownthat the presence of different oils decreases the CMTs of P84in the orderm-xylene (C8H10) > toluene (C7H8) > benzene(C6H6) > n-octane (C8H18) > n-hexane (C6H14) ≈ cyclohexane(C6H12).

The PO-CH3 resonance signal of P84 splits in the presenceof oil solutions into two peaks that are associated with thehydrated and anhydrous methyl groups. The relative peakproportion of the anhydrous PO-CH3 groups increases withincreasing temperature or oil concentration at the expense ofthe hydrated PO-CH3 groups. It can be deduced that theaddition of oils will increase the amount of anhydrous PO-CH3

groups and thus increase the hydrophobicity of the Pluronicmicelles.

The FF-TEM micrographs and DLS results show that the sizeof the aggregates increases significantly when the oil concentra-tion is above a certain value. The reason might be the swellingof the micellar core caused by the addition of oil mole-cules.

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 the Major Aspect ofKnowledge Innovation Project of the Chinese Academy ofSciences (No. KSCX2-YW-G-019).

Supporting Information Available: 1H NMR spectra ofthe PO-CH2- groups of P84 at various temperatures,1H NMRspectra and chemical shifts of the EO-CH2- groups of P84with varying temperature and changing oil concentration andtype, temperature-dependent chemical shifts of benzene in D2Osolution. This material is available free of charge via the Internetat http://pubs.acs.org.

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