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Hydration and Conformation of Temperature-Dependent Micellization of PEO-PPO-PEO Block Copolymers in Aqueous Solutions by FT-Raman Chen Guo, Jing Wang, Hui-zhou Liu,* and Jia-yong Chen Young Scientist Laboratory of Separation Science and Engineering, Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China Received August 14, 1998. In Final Form: January 21, 1999 The micellization of four triblock copolymers of the poly(ethylene oxide)x-poly(propylene oxide)y-poly(ethylene oxide)x type (Pluronic P103, P104, P105, and F88) in aqueous solutions versus temperature is followed by Fouier transform-(FT-) Raman spectroscopy. The frequencies and relative intensities of C-H stretching bands in FT-Raman spectra are sensitive to the local polarity and conformation of block copolymer chains, and their variations with temperature are indicators of micellization. Therefore, the critical micellization temperatures of these copolymers of 10% concentration in aqueous solutions are determined. The deconvolution method is used to resolve the overlapping bands in the C-H stretching region, and the deconvoluted spectra provide information about the structural and microenvironmental changes of ethylene oxide (EO) and propylene oxide (PO) blocks, respectively. The hydration of the EO chains in the corona is found to diminish with increasing temperature, and the conformation changes to a more disordered structure. The structure of PO chains changes from a more polar, gauche conformation at low temperatures to a less polar, stretching conformation at high temperatures. The micellization is explained by the change in structure and microenvironment of the EO and PO units. The results confirm the leading role of the hydrophobic PO units in micellization, and reveal a favorable contribution of the hydrophilic EO units.
Introduction Water-soluble poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO- PEO) triblock copolymers are high-molecular-weight nonionic surfactants. They are widely used in detergency, dispersion stabilization, foaming, emulsification, lubrication, etc.1,2 Because of their amphiphilic nature and low toxicity, they also serve as controlled drug delivery systems and protect microorganisms against mechanical damages in bioreactors.3, 4 Several aspects of the structural and dynamic properties of micellar assemblies of the PEO-PPO-PEO block copolymers have been investigated by a variety of experimental methods, including viscometry, light scattering, nuclear magnetic resonance (NMR), UV-vis absorption of selected probes, surface tension, electron spin resonance (ESR), small angle neutron scattering (SANS) and fluorescence.5-15 The most characteristic property of * To whom correspondence should be addressed. (1) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110. (2) Bahadur, P.; Riess, G. Tenside Surfactants Deterg. 1991, 28, 173. (3) Yokoyama, M. Crit. Rev. Ther. Drug Carrier Syst. 1992, 9, 213. (4) Murhammer, D. W.; Goochee, C. F. Biotechnol. Prog. 1990, 6, 142. (5) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (6) Ed Nace, V. M. Nonionic Surfactants; Marcel Dekker: New York, 1997. (7) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (8) Prasad, K. N.; Luong, T. T.; Florence, A. T.; Paris, J.; Vaution, C.; Seiller, M.; Puisieux, F. J. Colloid Interface Sci. 1979, 69, 225. (9) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (10) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (11) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 266, 101. (12) Amstrong, J. K.; Parsonage, J.; Chowdhry, B.; Leharne, S.; Mitchell, J.; Beezer, K.; Lohner, A.; Laggner, P. J. Phys. Chem. 1993, 97, 3904. (13) Zhou, D.; Alexandridis, P.; Khan, A. J. Colloid Interface Sci. 1996, 183, 339. (14) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 2690.
PEO-PPO-PEO block copolymer is the inverse temperature dependence of micellization. In dilute solutions and at low temperatures, block copolymers dissolve in aqueous solution as unimers (the molecular copolymers), which form a PPO hydrophobic core surrounded by the hydrophilic PEO blocks.8,16 With increasing concentration or temperature, critical micellization concentration (CMC) or critical micellization temperature (CMT) is observed while the aggregation of the PEO-PPO-PEO block copolymers occurs and leads to the formation of intermolecular micelles. The micellization of PEO-PPO-PEO block copolymers closely resembles the behavior of conventional surfactants, which follow a closed association mechanism where there is equilibrium between the unimers and micelles.17,18 One point that remains unclear concerns the “molecule-level” mechanism of micellization, especially the role of ethylene oxide (EO) units.10 There is some disagreement as to the structural and environmental change of EO and propylene oxide (PO) chains from unimer to micellar phase. Karlstrom,19 Linse,20 and Hurter21,22 have modeled this behavior on the basis of the analysis of the chain conformations expected for polymers containing -CH2CH2O- groups. However, few experimental measurements have probed chain conformational changes (especially EO chains) with the micellization of PEO-PPO-PEO block copolymers.23,24 (15) Malka, K.; Schlick, S. Macromolecules 1997, 30, 456. (16) Al-Saden, A. A.; Whateley, T. L.; Florence, A. T. J. Colloid Interface Sci. 1982, 90, 303. (17) Price, C. Pure Appl. Chem. 1983, 55, 1563. (18) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6, 201. (19) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (20) Linse, P. J. Phys. Chem. 1993, 97, 13896. (21) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993, 26, 5030. (22) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993, 26, 5592. (23) Guo, C.; Liu, H.-z.; Chen, J.-y. Colloid Polym. Sci., in press. (24) Caragheorgheopol, A.; Caldararu, H.; Dragutan, I.; Joela, H.; Brown, W. Langmuir 1997, 13, 6912.
10.1021/la981036w CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
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Table 1. Composition of the Pluronic PEO-PPO-PEO Copolymers Studied polymer P103 P104 P105 F88
mol wt
PPO segm wt
no. of PO units
no. of EO units
PPO/PEO
4950 5900 4500 11400
3465 3540 3250 2280
60 61 56 39
2 × 17 2 × 27 2 × 37 2 × 103
1.79 1.15 0.76 0.19
Raman spectroscopy is one of the most powerful techniques, and it provides not only macroscopic but also microscopic information on the structure and dynamics of whole molecule as well as various functional groups. Although water does not impair spectroscopic measurements using the Raman technique, Raman spectroscopy has been extensively used to study conformational states of the hydrocarbon chains in the normal low-molecularweight surfactant micelles in aqueous solution.25,26 Many Raman spectral features of low-molecular-weight surfactants are sensitive to the conformation of various functional groups, the interchain packing, the inter- and intrachain interactions, and the chain mobility. However, there are few reports on PEO-PPO-PEO block copolymers by Fourier transform- (FT) Raman spectroscopy.27,28 On the basis of the findings summarized above, we believe that this quite complex micellization process is not fully understood yet. The purpose of this work is to explore with FT-Raman spectroscopy the structural and microenvironmental changes caused by the micellization of triblock copolymer surfactants PEO-PPO-PEO. Experimental Section Materials. The Pluronic PEO-PPO-PEO triblock copolymers, available in a range of molecular weights and PPO/PEO ratios, are manufactured by BASF Corp., and were used as received. Table 1 identifies the molecular weight and composition of these Pluronic polymers studied. The homopolymers poly(ethylene oxide) (MW ≈ 1500) and poly(propylene oxide) (MW ≈ 1000) were kindly donated by Shanghai surfactant factory and used as received. Aqueous solutions of Pluronics with concentrations of 10% (wt %) are prepared by dissolving the Pluronic powder in distilled water under gentle agitation. The solutions are then studied by means of FT-Raman spectroscopy. FT-Raman Measurements. The Raman spectra with a resolution of 4 cm-1 are recorded on a Bruker RFS100 FT-Raman spectrometer equipped with a liquid N2 Ge detector and a 421N-II-OEM laser operating at a wavelength of 1064 nm. Scattering light is collected at 180° to the incoming laser light. Spectra of various anhydrous samples of PEO-PPO-PEO block copolymers are averaged over 200 scans with a laser power of 50 mW at the sample in the temperature range 5-70 °C. The 10% Pluronic aqueous solutions are contained in 5-mm tubes, and measured in the temperature range of 5-50 °C with 1000 scans and 500-mW laser power at the samples. The temperature of the sample is measured by a thermocouple inserted into the stainless steel block containing the sample tube. The system comprises of a Graseby-Specac temperature cell P/N 21525. The temperature measurement is accurate to (0.1 °C. The equilibration time at each temperature is 3 min. The spectra of block copolymers in aqueous solutions were smoothed. The deconvolution of spectra was performed using the OPUS deconvolution function. The wavenumbers and intensities of the bands in the C-H stretching region were obtained from the deconvoluted spectra using the peak pick function. (25) Wang, P. T. T.; Siminovitch, D. J.; Mantsch, H. H. Biochim. Biophys. Acta 1988, 947, 139. (26) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842. (27) Wartewig, S.; Alig, I.; Hergeth, W.-D.; Lange, J.; Lochmann, R.; Scherzer, T. J. Mol. Struct. 1990, 219, 365. (28) Guo, C.; Liu, H.-z.; Wang, J.; Chen, J.-y. J. Colloid Interface Sci. 1999, 209, 368.
Figure 1. C-H stretching regions of original Raman spectra of various anhydrous PEO-PPO-PEO block copolymers and anhydrous homopolymer PEO and PPO. A: 5 °C B: 70 °C.
Results and Discussion FT-Raman Spectra of Various Block Copolymers. Chain conformations of solid and liquid long-chain hydrocarbons have been examined extensively by Raman spectroscopy.25,26 Several regions in the Raman spectra are sensitive to conformational changes, such as (1) the 100-300 cm-1 region which corresponds to the lowfrequency longitudinal acoustical mode, (2) the 10001200 cm-1 region which corresponds to the skeletal C-C stretching modes and methylene rocking modes, and (3) the 2800-3000 cm-1 region which corresponds to the C-H stretching modes. Because the C-H stretching modes provide the most intense spectral feature in the Raman spectrum and the spectral intensities and frequencies of the hydrocarbon C-H stretching modes are sensitive to environmental and conformational change, the 2800-3000 cm-1 region is naturally used to monitor changes in the temperature-dependent micellization. Figure 1 presents Raman spectra in the C-H stretching regions of anhydrous samples of four PEO-PPO-PEO block copolymers and two homopolymers PEO and PPO at 5 °C (Figure 1A) and 70 °C (Figure 1B). The spectra of these block copolymers show broad and overlapping bands in the C-H stretching region. The spectral features are different from those of hydrocarbons. Although the vibrational assignments have been discussed previously in hydrocarbons, we briefly summarize here the observed vibrational bands. As shown in Figure 1A, the 2935 and 2970 cm-1 peaks represent the methyl C-H symmetric and asymmetric stretching modes, respectively. The symmetric methylene C-H stretching modes near 2850 cm-1 are difficult to see. The asymmetric methylene C-H stretching modes appear as intense, partially overlapped features near 2880 cm-1.25,26 The four block copolymers and two homopolymers show different spectral features in the C-H stretching region. The spectrum of Pluronic F88, which has the highest PEO content among the block copolymers studied, is similar to that of homopolymer PEO. The spectrum of Pluronic P103, which has the highest PPO content, is similar to that of homopolymer PPO. The spectra of Pluronic P104 and P105, which have less PEO content than F88 and less PPO content than P103, occupy the intermediate states. During the chain
Micellization of PEO-PPO-PEO Block Copolymers
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Figure 2. C-H stretching regions of deconvoluted Raman spectra of various anhydrous PEO-PPO-PEO block copolymers. A: 5 °C. B: 70 °C.
Figure 3. C-H stretching regions of various PEO-PPO-PEO block copolymers in 10% aqueous solutions at 10 and 50 °C. A: original spectra. B: deconvoluted spectra.
disordering process, which occurs when the polymers are heated, the intensities in the 2935 cm-1 region increase, and the spectra of the four block copolymers are very similar to that of homopolymer PPO. The spectra of Pluronic F88 and homopolymer PEO show large changes from low to high temperature, but those of Pluronic P103 and homopolymer PPO show small changes. Pluronics P104 and P105 also occupy the intermediate positions. To fix the number of bands, the deconvolution method is used to obtain independent information about the number of possible components under a complex band profile. Figure 2 shows the deconvoluted spectra in the 2800-3000 cm-1 region of various PEO-PPO-PEO block copolymers. In deconvoluted spectra, the band near 2880 cm-1 can be separated into two components, one around 2875 cm-1 and the other around 2885 cm-1. The intensities of the 2875 cm-1 bands, which increase with increasing PO block size, are strong in the spectrum of PPO and disappear in that of PEO. The bands around 2885 cm-1, which increase in intensity with the increase of EO block size, are strong in the spectrum of PEO and disappear in that of PPO. When the block copolymers are heated, the 2885 cm-1 bands decrease, and the 2875 cm-1 bands increase in intensity. According to Wang et al.25 and Hill and Levin,26 along with our studies, the bands around 2875 and 2885 cm-1 are assigned to the CH2 asymmetric stretching modes of PO block and EO block, respectively. The presence of well-resolved bands in the FT-Raman deconvoluted spectra of block copolymers in aqueous solutions allows us to monitor conveniently the structure and microenvironment of each block as a function of the temperature. The C-H stretching regions of various PEO-PPOPEO block copolymers in aqueous solutions are analyzed in the range of 5-50 °C. The original and deconvoluted spectra of various block copolymers at 10 and 50 °C in aqueous solutions are shown in Figure 3A and B. The Raman spectra of various block copolymers in aqueous solutions are changed considerably as compared with those of anhydrous samples. Although they are composed of the same number of bands in the C-H stretching region as those in anhydrous samples, all of the bands have broadened, lost intensities, and are shifted to higher frequencies. These indicate that block copolymers in
aqueous solution exhibit hydration and disorder of chains as compared with anhydrous copolymers. When temperature increases, all of the bands in C-H stretching region increase intensities, and shift to lower frequencies. As shown in deconvoluted spectra, the intensities of 2875 cm-1 bands increase faster than those of 2885 cm-1 bands with increasing temperature. At lower temperatures, the CH2 asymmetric stretching modes of EO blocks around 2885 cm-1 are stronger than those of PO blocks around 2875 cm-1, whereas at higher temperature, the intensities of the bands around 2885 cm-1 are weaker than those of the bands around 2875 cm-1. Phase Transitions. From an inspection of Figure 3, it is clear that an increase in temperature leads to drastic changes in FT-Raman spectra. The most apparent effect on the Raman spectra in the C-H stretching region is the temperature-induced frequency shift of the methylene asymmetric stretching modes and the symmetric and asymmetric stretching modes of the methyl group. For the methylene asymmetric C-H stretching bands of PO and EO blocks, the variation of the band peak frequencies as a function of temperature is displayed in Figure 4. At certain temperature, the bands of PEO-PPO-PEO block copolymers in aqueous solutions undergo large change in frequencies with the change of temperature. Below or above these temperatures, little change can be observed. Similar trends are also found for the bands near 2930 and 2970 cm-1. As evident from Figure 4, transitions corresponding to the critical micelle temperature (CMT) are observed in wavenumber shifts. The CMT values of various block copolymers are obtained from the first break in the wavenumber versus temperature sigmoid curves, as shown in Figure 4 and listed in Table 2. We find that the CMT values for the Pluronic copolymer solutions (at a given copolymer concentration) decrease with the increase of the number of PO segments. These results indicate that copolymers with larger hydrophobic domains form micelles at lower temperatures. As presented in Figure 4, micelle formation of the block copolymer with a lower PPO segment (such as Pluronic F88) is more difficult, but for the Pluronics P103, P104, and P105, which have larger PPO segments, micelle formation is much easier. These results obtained by FT-Raman spectroscopy are in rea-
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Figure 5. Intensities of the CH2 asymmetric stretching modes of EO (2875 cm-1) and PO (2885 cm-1) segments relative to the CH3 symmetric stretching modes (2930 cm-1) of various anhydrous PEO-PPO-PEO block copolymers.
Figure 4. Wavenumber shifts of CH2 asymmetric stretching modes of (A) PO and (B) EO blocks in various PEO-PPO-PEO block copolymers in 10% aqueous solutions with increasing temperature. Table 2. Critical Micellization Temperatures for 10% Pluronic Copolymer Solutions Pluronics CMT (°C)
F88
P105
P104
P103
29.0
16.5
16.0
15.0
sonable agreement with the temperature-dependent micellization of aqueous copolymers investigated with other techniques.9,29-33 Hydration and Conformation of Pluronic Copolymer Chains on Micellization. By using the deconvolution method, the well-resolved bands in the Raman spectra of PEO-PPO-PEO block copolymers show the methylene asymmetric band of PPO blocks near 2875 cm-1 and the methylene asymmetric stretching band of PEO blocks near 2885 cm-1. From the temperature-dependent changes of the methylene asymmetric stretching modes of EO and PO blocks, the EO and PO chains, respectively, could be monitored as functions of temperature. As is evident from Figure 4A, the 2875 cm-1 bands of the four block copolymers in aqueous solutions undergo large frequency changes with increasing temperature, and present reverse sigmoid dependence on temperature. Identical wavenumber shifts of CH2 asymmetric stretching modes of EO blocks near 2885 cm-1 are also observed in Figure 4B. It is known that the dehydration of hydrocarbon chains resulted in a reduction of wavenumber.34 Thus the shifts of the bands from high to lower frequencies in (29) Tontisakis, A.; Hilfiker, R.; Chu, B. J. Colloid Interface Sci. 1990, 135, 427. (30) Almgren, M.; Alsins, J. Langmuir 1991, 7, 446. (31) Bahadur, P.; Almgren, M.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A. J. Colloid Interface Sci. 1992, 151, 157. (32) Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Langmuir 1992, 8, 1903. (33) Linse, P.; Malmsten, M. Macromolecules 1992, 25, 5434. (34) Atsushi, I.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935.
transition region represent the dehydration of the PO and EO chains in micellization. Little dehydration of the PO and EO chains is observed in unimer and micellar regions. Comparing Figure 4A with 4B, we also find that, in micellization, the methylene asymmetric stretching modes of PO blocks undergo larger wavenumber shifts (about 5 cm-1 for Pluronics P103, P104, and P105 and about 1.7 cm-1 for Pluronic F88) than those of EO blocks (about 1.7 cm-1 for Pluronics P103, P104, and P105 and about 0.9 cm-1 for Pluronic F88). It seems that the dehydration of EO blocks is less than that of PO blocks. These results lend support to the idea that the EO chains are in contact with the solvent and around the hydrophobic core. The increase in dehydration of PO and EO chains with increasing temperature means that there are pronounced solute-solvent interactions at low temperatures, mainly due to hydrogen bonding formation between oxygen atoms and water molecules, which becomes weak with increasing temperature. From the dehydration of PEO and PPO blocks in micellization, we can deduce that the unimer is a loose structure. Water molecules penetrated deeply into the unimer, and the micropolarities of the PPO and PEO chains in unimer are high. Above the transition region, the compact micelle core is formed, the PPO chain is nearly anhydrous, and it is located at a nonpolarity site. In micelle states, the PEO chains in the corona are also at a less polarity site. The intensities of the CH2 asymmetric stretching mode relative to the CH3 symmetric stretching mode is an orderdisorder parameter.26 It can provide information on the conformational change of block copolymer chains. The intensities of the CH2 asymmetric stretching mode of EO and PO segments relative to the CH3 symmetric stretching mode (values of I2885/I2930 and I2875/I2930) of various anhydrous PEO-PPO-PEO block copolymer samples at 5 and 70 °C are plotted in Figure 5 as a function of the PPO/PEO ratios of the block copolymers. Both values decrease with increasing PPO block size. These indicate that the PO and EO chains exhibit more disordered structure with increasing PPO/PEO ratio. As is evident from Figure 5, the two peak height ratios decrease with increasing temperature. This represents that both PPO and PEO chains change to more disordered structures in molten states. The two peak height ratios satisfactorily describe the conformational change of PO and EO chains in anhydrous PEO-PPO-PEO block copolymer samples during the chain disordering process by heating the block copolymers. The temperature dependence of I2885/I2930 and I2875/I2930 of the four block copolymers in aqueous solutions with concentration of 10% in the temperature range of 5-50 °C are shown in Figure 6A and 6B. The two parameters
Micellization of PEO-PPO-PEO Block Copolymers
Figure 6. Intensities of the CH2 asymmetric stretching modes of (A) EO and (B) PO segments to the CH3 symmetric stretching modes in various PEO-PPO-PEO block copolymers in 10% aqueous solutions with increasing temperature.
undergo different changes along with increasing temperature. The I2885/I2930 values exhibit a reverse sigmoid curve, and the I2875/I2930 values exhibit a sigmoid curve. Both values show abrupt changes at CMT. We infer from the spectroscopic findings that both PO and EO chains have an abrupt conformational change in micellization. The PO chains change to more ordered structure, and the EO chains to more disordered structure with increasing temperature. All of the block copolymers studied here have similar structural changes with temperature in EO and PO blocks except for Pluronic F88. In the transition region, the I2885/ I2930 value of Pluronic F88 decreases initially with increasing temperature followed by an increase with a further increase of temperature. The deviations that were observed might be mainly due to the enhanced interaction between the hydrated PEO chains belonging to different micelle coronas. As compared with other block copolymers, the high-EO-content block copolymer (Pluronic F88) shows small change in Raman spectral features. It is somewhat difficult to determine the CMT of Pluronic F88 in aqueous solution. In the study of Wartewig et al.,27 no micellization of PEO-PPO-PEO block copolymers was observed by Raman spectroscopy because of the high EO content (80%) block copolymers used. Molecular Mechanism of Temperature-Dependent Micellization. Because of the presence of methyl group, the microenvironment and conformation of the PO chains in block copolymers are rather easy to study. The dehydration and conformational change of PO chains can also be observed by NMR.35 However, the microenvironment and conformation of the EO chains are difficult to study because the groups in EO chains also exist in PO chains. By the deconvolution method, FT-Raman spectroscopy can give satisfactory results about the microenvironment and conformation of both PO and EO chains (35) Cau, F.; Lacelle, S. Macromolecules 1996, 29, 170.
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in block copolymers. These results allow us to deeply analyze the “molecular-level” mechanism of micellization. The homopolymer PEO exists in three types of structure. The thermodynamically more stable structure of PEO is a 7/2 helix, practically the conformational sequence is transgauche-trans. In particular, a planar zigzag structure has been observed, in which the chain is fully extended. In the molten state, the PEO chains represent an open coil with more disordered structure.36 In our study, the EO chains in aqueous solution exhibit an open coil as compared with the structures of anhydrous samples. When the block copolymers dissolve in water, it is assumed that around the EO chains there exists a zone with an increase of structural water.19 At low temperatures, the EO chains present a structure fitting the water shell. With increasing temperature, the conformational change of EO chains is observed. The EO units exist in a highly disordered structure, which is incompatible with an ordered water hydration shell. Thus, the hydration of EO units induced by the conformational change of EO chains leads to weaken solute-solvent interaction. This leads to a weaker screening and stabilization of the hydrophobic PPO blocks from water, and then the unimers aggregate to micelles to stabilize the block copolymer-water system. The PO chains exhibit a disordered structure at lower temperatures and a stretched structure at higher temperatures. The origin of the increasing hydrophobicity of PO units is the results of changing conformations of PO chains. As the temperature of the block copolymer solution is raised, the PO block progressively loses its hydration sphere, resulting in increasing PO-PO interactions, in the way similar to that of the temperature-driven phase separation that occurs in aqueous PPO solutions. The EO units, through dehydration, retain their strong interaction with water. Just as other amphiphilic molecules do, the differing phase preferences of the blocks drive the copolymers to form micelles. Summary The temperature-dependent micellization of PEOPPO-PEO block copolymers with different lengths of EO and PO segments has been studied by FT-Raman spectroscopy in the range of 5-50 °C. When temperature increases, the copolymers undergo three temperatureinduced structural phase transitions as indicated by the wavenumber shift of the CH2 asymmetric stretching mode and CH3 symmetric and asymmetric stretching modes in the C-H stretching region. Therefore, the critical micelle temperatures (CMT) of four Pluronic copolymers in 10% aqueous solution were obtained. The fact that CMT values from FT-Raman spectroscopy agree with those obtained with other techniques indicates that FT-Raman spectroscopy could be used to study the micellization of PEOPPO-PEO block copolymers. By the deconvolution method, the resolved bands in the C-H stretching region are shown. The deconvoluted spectra can satisfactorily provide the information of PEO and PPO blocks in triblock copolymers. The microenvironmental and conformational changes of PEO and PPO blocks could be probed by following the wavenumber shift and peak height ratio along with micellization. In the transition region, PPO chains undergo dehydration accompanied by an increase of trans conformers with increasing temperature. PEO chains also exhibit the dehydration process; however, the conformation of PEO chains changes from ordered to more disordered structures with increasing temperature. Consequently, the molec(36) Marcos, J. I.; Orlandi, E.; Zerbi, G. Polymer 1990, 31, 1899.
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ular-level mechanism behind the temperature dependence observed in the micellization of PEO-PPO-PEO block copolymers in aqueous solutions is recommended because the dehydration of PO and EO blocks accompanied by the conformational change of the two blocks is responsible for micellization. The dehydration and conformational change of PO and EO chains support the polar-nonpolar state model as an explanation of the temperature effect on the solution behavior of PEO and PPO. Acknowledgment. This work was financially supported by the National Nature Science Foundation of
Guo et al.
China (No. 29836130) and a major research project grant from Chinese Academy of Sciences. The authors thank Professor T. A. Hatton of the Massachusetts Institute of Technology for his kindly providing the PEO-PPO-PEO block copolymers. (H.-z. L.) is grateful to the Li Foundation of USA for supporting him as visiting scholar at the Massachusetts Institute of Technology.
LA981036W
