Oil-Induced Aggregation of Block Copolymer in Aqueous Solution

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J. Phys. Chem. B 2007, 111, 11140-11148

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, Institute of Process Engineering, Chinese Academy of Sciences. Graduate UniVersity of Chinese Academy of Sciences. Beijing 100080, P. R. China

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

Pratap Bahadur Department 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 aqueous solutions has been systematically investigated by 1H NMR spectroscopy, freeze-fracture transmission electron microscopy (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 oils in decreasing the CMT of block copolymer follows the order m-xylene (C8H10) > toluene (C7H8) > benzene (C6H6) > n-octane (C8H18) > n-hexane (C6H14) ≈ cyclohexane (C6H12). It was found that the amount of anhydrous PO methyl groups increases whereas the amount of hydrated PO methyl groups decreases upon the addition of oils. At low oil concentration, the oil molecules are entrapped by the micellar core, but as the oil concentration increases above a certain value, the micellar core swells significantly as a result of the penetrated oil molecules, and much larger aggregates are formed. Intermolecular rotating-frame nuclear Overhauser effect (ROE) measurements between P84 and benzene were performed at 10 and 40 °C. The specific interaction between benzene and the methyl groups of PPO was determined, and it was observed that the 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 available under the trade names Poloxamers or Pluronics), as they are widely used for various applications in the nanotechnology, pharmaceutical, bioprocessing, and detergent industries.1-4 Block copolymers consisting of a central PPO block flanked by two PEO blocks display thermosensitive amphiphilic properties. The interesting features of PEO-PPO-PEO block copolymers in aqueous solution are their temperature-dependent self-association and rich phase behavior.5-12 The process of selfassociation can be induced by increasing the temperature to exceed the critical micellization temperature (CMT); however, this effect is different for different copolymers.13,14 The aggregation properties of PEO-PPO-PEO block copolymers in aqueous solutions are very sensitive to the cosolutes added.15 The effects of salts have been discussed in terms of “salting-in” and “salting-out” effects and follow the Hofmeisfer series.16-20 However, studies of the effects of various organic substances on the phase behavior of PEO-PPO-PEO block copolymers have been quite limited. The experimental investigations revealed that short-chain alcohols, urea, and formamide prevent the onset of micellization of Pluronic polymers in water, whereas long-chain alcohols (butanol, pentanol, etc.) and hydrazine favor micelle formation.21-26 * To whom correspondence should be addressed. Phone: +86-1062555005. Fax: +86-10-62554264. E-mail: [email protected] (C.G.), [email protected] (H.-z.L.).

In fact, oil can also be used to adjust the properties of block copolymers in aqueous solution. The most important use of oil is in the modulation of microemulsions, systems consisted of water, oil, and amphiphiles that are characterized as optically isotropic and thermodynamically stable liquid solutions.27,28 Oilin-water microemulsions are very promising for use as drug delivery vehicles.29-31 Among the numerous microemulsion systems, Pluronic-based microemulsions have attracted great attention mainly because of two advantages: the diverse interfacial properties of these surfactants and their high dose uptake by patients without any apparent side effects.32 However, most experimental studies of oil-swollen Pluronic micelles, even those utilizing powerful scattering techniques, such as smallangle neutron scattering (SANS)33,34 and small-angle X-ray scattering (SAXS),35 have taken simplified views of the oilpolymer aggregates by treating them as homogeneous hard spheres or cylinders and have not attempted to reveal the control and interaction mechanism of oil toward Pluronic-based microemulsion systems at the molecular level. In this work, the effect of oil on the aggregation behavior of Pluronic P84 is investigated using 1H nuclear magnetic resonance (NMR) spectroscopy, freeze-fracture transmission electron microscopy (FF-TEM), and dynamic light scattering (DLS). The molecular-level mechanism of the oil-Pluronic interaction is discussed. 2. Experimental Section Materials. The PEO-PPO-PEO triblock copolymer Pluronic P84 was obtained from BASF (Parsippany, NJ) and was

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

Oil-Induced Aggregation of Block Copolymer

J. Phys. Chem. B, Vol. 111, No. 38, 2007 11141

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

Figure 2. 1H NMR spectra of 5% (w/v) Pluronic P84 in D2O solution with 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 was fixed at 0.06 M.

used as received. P84 can be represented by the formula EO19PO39EO19. Cyclohexane (C6H12), n-hexane (C6H14), n-octane (C8H18), benzene (C6H6), toluene (C7H8), and m-xylene (C8H10)

were supplied by Merck, all with purity greater than 99.0%. All chemicals were used without further purification. The reference 2,2-dimethyl-2-silapentane-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. A heavy-water solution of Pluronic P84 was prepared by weighing appropriate amounts of polymers in D2O solution with gentle agitation. During the experiments, the polymer solutions were further diluted with D2O, and different oils were added to their proposed concentration. A stock solution of 0.6 M DSS in D2O was prepared. For 1H NMR measurements, 1 µL of the stock solution of DSS was injected into 600 µL of aqueous polymer solution with a syringe, so that the final copolymer solution contained 0.001 M DSS. The solution was transferred to a 5-mm NMR sample tube, and the tube was

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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 concentration of the oils was fixed at 0.06 M.

TABLE 1: Literature Values of the Critical Micellization Temperatures (CMTs in °C) of P84 and P85 (similar to P84) Solutions in H2O and D2O Determined by Different Techniquesa H2O/D2O P84 P85

technique

5% conc (w/v)

solubilized DPH6 1 H NMR spectroscopy FTIR spectroscopy42 solubilized DPH6 DLS43 SANS44

23 /26

15% conc (wt)

/22 25 25/25 25/25

a CMT values given in ref 6 for H2O seem to be systematically lower by 2-3 °C than those determined by other techniques (DSC, LS, ultrasonics, SANS, etc).45,46 If the 2-3 °C correction were added, the CMT values determined by 1H NMR spectroscopy would be in good agreement with the literature.

sealed immediately with laboratory film. After 15 min of sonication to remove dissolved paramagnetic dioxygen, the sample tubes were stored in a refrigerator until use.

NMR Spectroscopy. All NMR experiments were conducted on a Bruker Avance 600 spectrometer at a Larmor frequency of 600.13 MHz for protons. 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 the 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).36 For all 1H NMR experiments, the samples were allowed to equilibrate at the desired temperature for at least 15 min prior to measurement. DSS was directly added to 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.37 FF-TEM. The specimens were plunged into ethane cooled by liquid nitrogen. The samples were freeze-fractured at 153 K and 10-4 Pa in a Balzer BAF 400D freeze-etching apparatus and shadowed by platinum/carbon at an angle of 45°. The

Oil-Induced Aggregation of Block Copolymer

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 various temperatures and (bottom) temperature-dependent fractions of hydrated and anhydrous methyl groups of P84 obtained from Figure 6b.

replicas were cleaned to remove sample residuals and examined with Philips Tecnai 20 and Jeal JEM-100cx electron microscopes. Dynamic Light Scattering (DLS). The mean diameters and polydispersities of Pluronic P84 aggregates were determined by dynamic light scattering (DLS) using a Brookhaven 90Plus Nanoparticle Size Analyzer (Brookhaven Instruments Corp., United States) with a 15-mW solid-state laser at room temperature. All analyses were run in triplicate, and the results are reported as average values. 3. Results and Discussion 1H

NMR Spectra of P84 in the Presence of Oil. To investigate the effects of oil on the aggregation behavior of block copolymer, the 1H NMR spectra of 5% (w/v) Pluronic P84 in D2O solution, in the absence and presence of different concentrations of benzene, were acquired at various temperatures. Before investigating the interactions between benzene and triblock copolymer in solution, spectroscopic analyses and

J. Phys. Chem. B, Vol. 111, No. 38, 2007 11143 assignments of the observed 1H NMR resonances were first carried out over the temperature range 10-45 °C. The 1H NMR spectra of the separate and mixed solutions are shown in Figure 1. The triblock copolymer species dissolved in D2O (Figure 1C and D) gives rise to three resolved signal regions that were assigned according to our previous work.38 Specifically, 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 are assigned to the PO -CH2- protons, the intense resonance observed at around 3.7 ppm is assigned to the EO -CH2protons (see Figure S2 in the Supporting Information), the signal at ∼4.8 ppm is the residual signal of HDO, and the remaining signal 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 presence of 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 certain value, the spectral profiles show two characteristic changes in the line shape of the PPO segments: (1) the disappearance of the hyperfine structure of the PO -CH2- signals and the triplet of the PO -CH3 signals as well as the broadening of the signals in a narrow temperature interval and (2) the emergence of a new resonance signal labeled g (∼3.4 ppm, denoted in Figure 1C) that grows progressively larger with increasing temperature. (The particular change of peak g with increasing temperature is shown in Figure S1 in the Supporting Information.) The observed line broadening of the PO groups can be attributed to the reduced mobility of the PO segments, resulting from temperature-induced micellization,40 whereas the emergence of the new resonance is because of the breakdown of the intramolecular (C-H)‚‚‚O hydrogen bond between the PO -CH2protons and the ether oxygen during micellization.38 It has been validated that such changes can be used to emphasize the temperature-dependent micellization of Pluronic polymers in aqueous solution.41 For the P84 solution in the presence of benzene, the spectral profiles of the PO -CH2- and the PO -CH3 signals (especially the PO -CH3 signal) showed a significant broadening with increasing benzene concentration, and the changes in the spectral profile associated with aggregation moved to lower temperature. This indicates that the addition of benzene will destroy the thermal equilibrium of the block copolymer in the unimer state and facilitate the aggregation at otherwise the same conditions in the absence of benzene. A comparison of different oils [cyclohexane (C6H12), nhexane (C6H14), n-octane (C8H18), benzene (C6H6), toluene (C7H8), m-xylene (C8H10)] is presented in Figure 2. The spectra are for a fixed oil concentration of 0.06 M and varying temperatures. It can be seen that the PO signals show only a gentle change upon addition of alkanes, whereas the spectra change significantly upon addition of aromatic hydrocarbons. Effects of Oil on the CMTs of Pluronic P84 in Aqueous Solution. Oil Concentration Effect. To characterize the oilinduced aggregation, the 1H NMR spectra of 5% (w/v) Pluronic P84 in D2O solution at 25 °C as a function of benzene and xylene concentration were acquired and are presented in Figure 3. The insets in Figure 4A and B present the temperature dependence of the integral values of peak g and the width at half-height of the PO -CH3 signal of 5% aqueous P84 solutions as a function of benzene concentration, respectively. Because the CMTs of Pluronic polymers can be determined accurately from the first inflection point of the integral values of peak g

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

Figure 8. Profiles of 1H NMR rotating-frame nuclear Overhauser effect (ROE) spectra of 5% P84 in the presence of 0.1 M benzene. Spectra were recorded at 10 and 40 °C by selectively exciting the (a,d) benzene, (b,e) PPO -CH3, and (c,f) PEO -CH2- signals.

or the width at half-height of the PO -CH3 signal against temperature sigmoid curves41 (the point at which the curve starts deviating from linear behavior, denoted in Figures 4 and 5), the results presented in the figures indicate that the addition of benzene significantly decreases the CMT of Pluronic P84 when the benzene concentration is lower than 0.06 M; however, at concentrations higher than 0.06 M, this effect becomes less apparent. It is probably because the supersaturation of benzene, and the solution will separate into two phases. Literature values of 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 the same concentration], determined by other techniques, are listed in Table 1 for comparison.

Oil Type Effect. The effects of oil type on the integral values of peak g and the width at half-height of the PO -CH3 signal of Pluronic P84 in aqueous solutions are presented in Figure 5A and B, respectively. The effectiveness of the oils in decreasing the CMTs of Pluronic P84 follows the order m-xylene (C8H10) > toluene (C7H8) > benzene (C6H6) > n-octane (C8H18) > n-hexane (C6H14) ≈ cyclohexane (C6H12). On a mole concentration basis, the aromatic hydrocarbons show a much stronger trend in CMT reduction than the alkanes. Because the increase of the integral area of peak g can be directly correlated to the decrease of gauche conformers in the PPO chain,41 it appears that both the type of oil introduced and its concentration can affect the conformational state of the PPO chain.

Oil-Induced Aggregation of Block Copolymer

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

Although the dehydration of the hydrophobic PO units plays a leading role in micellization, the dehydration of the hydrophilic EO units also strongly influences the process.10 To reveal the effect of oils on the hydration state of the micellar corona, the 1H NMR spectra and the chemical shift of the EO signal under different concentrations of benzene and various oils at different temperatures are presented in Figures S2-S4 (see the Supporting Information). The chemical shift of the EO -CH2- protons shows a linear decrease in chemical shift values with increasing temperature in the absence of oil, whereas the chemical shift of the EO -CH2- protons increases slightly with increasing oil concentration or addition of aromatic hydrocarbons. The slight upfield shift indicates that the PEO blocks experience a small degree of dehydration with increasing temperature. The downfield shift in the presence of oils is a manifestation of the increased hydration of the PEO segments, which might be

J. Phys. Chem. B, Vol. 111, No. 38, 2007 11145 caused by the hydrophobic effect of the oils. This result also suggests the indirect interaction between the oil molecules and the PEO segments. Interaction between Oil and Pluronic P84. The changes in line width and position effectively demonstrate that the oil species interact with the triblock copolymer in solution. It is interesting to note that a new resonance signal of the PO -CH3 signals at approximately 0.87 ppm appears at lower temperatures upon the addition of oil. The signal could not be observed in the absence of oil and is unobvious in the presence of alkanes, but it is clearly resolved in the presence of aromatic hydrocarbons (see Figure 6b for the results obtained in the presence of 0.0 6M toluene) and grows larger with increasing oil concentration. A further increase in temperature leads to a downfield shift of this new resonance and results in an overlapping of this new signal with the original PO -CH3 signal at about 20 °C. We deduce that the two signals of the PO -CH3 groups of P84 can be correlated to the two states: one is a hydrated state corresponding to the peak around 1.17 ppm (surrounded by water), and the other is an anhydrous state associated with the peak near 0.9 ppm. A similar phenomenon of the splitting of the symmetric deformation band of methyl groups of Pluronic polymers in water has already been observed by FTIR spectroscopy.11,12 With the assumption of separate hydrated and anhydrous signals, we can determine the fraction of anhydrous PPO methyl groups, because the areas under the resonances are proportional to the concentrations of methyl protons in the respective states. The areas under the different signals were determined by fitting a sum of Lorentz functions to the signals and then calculating the relative area under each signal. The results from this analysis are presented in Figure 6 (bottom) as a function of temperature. As shown, the percentage of anhydrous PO -CH3 groups increases with increasing temperature at the expense of that of the hydrated PO -CH3 signal when the temperature is above the CMT. However, at 45 °C, ca. 10% of the PPO methyl groups still remain in the hydrated state. Of course, the relative peak area fraction is not the exact proportion of the anhydrous or the hydrated methyl groups. Nevertheless, it can be used as a criterion in that its increase or decrease unambiguously shows an increase or decrease of the proportion of the anhydrous or hydrated methyl groups. It is known that the main reason for the temperature-induced aggregation of Pluronic micelles is the dehydration of the PPO segments.2,6-8 The results presented here show that the addition of oils obviously increases the amount of the anhydrous PO -CH3 groups and thus will increase the hydrophobicity of the Pluronic polymer, which will probably decrease the energy barrier for the aggregation of P84 molecules at the same temperature. Interaction Sites Determination between Benzene and P84. When two moieties are in close spatial proximity, the local electronic environments of either or both species can be perturbed, altering the local magnetic fields and leading to changes in their observed isotropic chemical shifts. To date, the 1H NMR chemical shift is a well-established indicator for determining the loci of interactions between the different species in solution.47 Figure 7A and B show the temperature-dependent chemical shifts (δ) of benzene and the residual HDO signals as a function of benzene concentration, respectively. The chemical shift of benzene shows an upfield shift with increasing temperature at lower concentration, whereas it exhibits an opposite downfield shift when the benzene concentration exceeds 0.06 M. It is well-known that the inclusion of a species in a highly hydrophobic environment would result in upfield shift of the

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

affected protons.48 Such is the case here. In addition, the penetration of benzene into the core of the micelles is temperature-dependent. However, when the concentration of benzene is higher than 0.06 M, the chemical shift of benzene shows a downfield shift, the same trend as observed for neat benzene in D2O solution with increasing temperature (see Figure S5 in the Supporting Information). When the temperature increases above 25 °C, the chemical shift of benzene becomes independent of temperature. However, the chemical shift of HDO seems to be unaffected upon the addition of benzene in the entire temperature range investigated, which otherwise validates that oil molecules interact directly with the block copolymer and have almost no effect on the water structure. Selective ROE measurements were carried out to confirm the interaction sites between benzene and the different moieties of Pluronic polymer. Intermolecular 1H cross-relaxation processes by the NOE can, in general, occur for molecules that are in close spatial contact (within 0.5 nm), which is mediated by through-space dipole-dipole couplings and can be used to probe molecular proximity.49 In a typical experiment, the benzene, PO -CH3, and EO -CH2- signals were selectively excited at 10 (unimer region) and 40 °C (micellar region). Figure 8 shows the 1H cross-relaxation ROE spectra for the entire spectral region. When the benzene or PO -CH3 protons were excited, only the proton cross-relaxation peaks for the PO -CH3 groups and benzene were observed, which suggests that there exists a strong interaction between the PO block and the benzene molecules. However, when excited for the EO -CH2- peak, only the residual water signal is retained in the spectra. The spectra do not change when the temperature is increased from 10 to 40 °C. These proton cross-relaxation ROE results confirm that the benzene molecules directly interact with the PPO methyl groups, even if Pluronic P84 stays as unimer. Morphology and Size of P84 Micelles under Different Concentrations of Xylene. FF-TEM can yield direct imaging of the sizes, aggregates, and shapes of the liquid sample. The FF-TEM micrographs of aqueous Pluronic P84 solution in the absence and presence of m-xylene with fixed polymer concentration (5%) and varying m-xylene concentrations are shown in Figure 9. It can be seen from this figure that a small amount of m-xylene has clear effects on the Pluronic P84 micelles. The size of the Pluronic P84 micelles increases with increasing m-xylene concentration. When the concentration of xylene reaches 0.06 M, much larger aggregates are formed, the diameter of which increases abruptly, and the granularity decreases.

TABLE 2: Mean Diameter of Pluronic P184 and m-Xylene in Aqueous Solution with the Concentration of Pluronic P84 Fixed at 5% (w/v) and the Concentration of Xylene Varied xylene concentration (M) 0 0.01 0.02 0.04 0.06 0.08 0.10

effective diameter (nm) 21.8 23.1 29.1 49.2 255.0 422.7 1127.9

polydispersity 0.107 0.128 0.152 0.171 0.207 0.229 0.313

The size and distribution of the aggregates in the mixed system were further examined by DLS. Figure 10 shows the intensity-weighted size distribution of the hydrodynamic diameter (Dh), measured at room temperature, of 5% Pluronic P84 in m-xylene aqueous solutions of different concentrations. It is shown that P84 forms micelles in m-xylene aqueous solutions when the concentration of m-xylene is below 0.04 M, and the diameter of the micelle increases only slightly with increasing xylene concentration. Meanwhile, above this concentration, it forms much larger aggregates. The polydispersity of the micelles or aggregates, evaluated through the ratio µ2/Γ2 by cumulant analysis, is reported in Table 2, where µ2 is the second moment in the cumulant expansion of the correlation function and Γ is the decay rate. It has been found that the mean diameter of the micelles increased from 21 to 49 nm and the polydispersity of micelles increased from 0.107 to 0.171 when the concentration of xylene was increased from 0 to 0.04 M, whereas above this concentration, the mean diameter increased abruptly to 1127.9 nm and the polydispersity increased to 0.313 in 0.1 M m-xylene aqueous solutions. Taking the above experimental results into consideration, the following mechanism for the oil-induced aggregation of Pluronic P84 is proposed: When oils are added to P84 aqueous solution, the entropy is decreased because of the ordering of water in the presence of this substance.50 The oil molecules replace the hydrated shell of PPO segments and interact directly with the PPO methyl groups as a result of the driving force of entropy. This leads to a decrease of the free energy of the water around PPO because of unfavorable entropy contributions33 and thus results in the micellization of block copolymer at lower temperatures. When the oil concentration is low, the oil molecules become encapsulated in the micellar core. However, when the oil concentration is above a certain value, more and more oil molecules are embedded in the micelles, causing an

Oil-Induced Aggregation of Block Copolymer abrupt swelling of the micellar core. The microemulsion particles formed at higher oil concentration or temperature seem to have a quite stable chemical nature, because the chemical shift of oil molecules in that state remains quite constant, as is evident in Figure 7A. Conclusion The oil-induced aggregation behavior of Pluronic P84 in aqueous solution was studied by 1H NMR spectroscopy, FFTEM, and DLS. NMR spectroscopy gave valuable information on the interaction sites between the oil molecules and the triblock copolymer species. It was shown that the oil molecules interact directly with the PPO methyl groups of P84 in both unimer and micellar regions, whereas the oil molecules appear not to directly interact with the PEO blocks and the solvent water. Oil can play an important role in modifying the properties of aqueous copolymer solutions. The CMT values of Pluronic P84 were found to decrease upon addition of oil. Our study examined both alkanes and aromatic hydrocarbons. It was shown that the presence of different oils decreases the CMTs of P84 in the order m-xylene (C8H10) > toluene (C7H8) > benzene (C6H6) > n-octane (C8H18) > n-hexane (C6H14) ≈ cyclohexane (C6H12). The PO -CH3 resonance signal of P84 splits in the presence of oil solutions into two peaks that are associated with the hydrated and anhydrous methyl groups. The relative peak proportion of the anhydrous PO -CH3 groups increases with increasing temperature or oil concentration at the expense of the hydrated PO -CH3 groups. It can be deduced that the addition of oils will increase the amount of anhydrous PO -CH3 groups and thus increase the hydrophobicity of the Pluronic micelles. The FF-TEM micrographs and DLS results show that the size of the aggregates increases significantly when the oil concentration is above a certain value. The reason might be the swelling of the micellar core caused by the addition of oil molecules. 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 the Major Aspect of Knowledge Innovation Project of the Chinese Academy of Sciences (No. KSCX2-YW-G-019). Supporting Information Available: 1H NMR spectra of the PO -CH2- groups of P84 at various temperatures, 1H NMR spectra and chemical shifts of the EO -CH2- groups of P84 with varying temperature and changing oil concentration and type, temperature-dependent chemical shifts of benzene in D2O solution. This material is available free of charge via the Internet at http://pubs.acs.org. 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) Hamely, I. W. Block Copolymers in Solution: Fundamentals and Applications; John Wiley & Sons: New York, 2005. (4) Kabanov, A. V.; Batrakova, E. V.; Alakhov, V. Y. AdV. Drug DeliVery ReV. 2002, 54, 759-779.

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