Melt, Hydration, and Micellization of the PEO–PPO–PEO Block Copolymer Studied by FTIR Spectroscopy

Journal of Colloid and Interface Science 251, 417–423 (2002) doi:10.1006/jcis.2002.8435

Melt, Hydration, and Micellization of the PEO–PPO–PEO Block Copolymer Studied by FTIR Spectroscopy Yan-lei Su, Jing Wang, and Hui-zhou Liu1 Young Scientist Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Processing Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, People’s Republic of China Received November 6, 2001; accepted April 17, 2002; published online June 18, 2002

Fourier transform infrared (FTIR) spectroscopy was used to study the conformational changes of the polyethylene oxide– polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) block copolymer, Pluronic P104, in a large concentration range in a polymer–water system as a function of temperature. The melt in which the conformational transition of the PEO blocks occurs gives remarkable changes in the spectral behavior. A small amount of water in Pluronic P104 can induce the PEO block amorphism. The addition of more water only swells the PEO dominant region and gives no significant difference in the conformational structure of the block copolymer in the ordered phases of Pluronic P104–water mixtures. The PPO blocks of Pluronic P104 are hydrated only in a condition of lower temperature and higher water content. The temperature dependent micellization of Pluronic P104 in water was analyzed by a FTIR spectroscopic method. The appearance of the symmetric deformation band of the anhydrous methyl groups at temperature below the CMT indicates the existence of a hydrophobic microenvironment. The appearance of the symmetric deformation band of the hydrated methyl groups at higher temperatures indicates that the micellar core must contain some amount of water. The results of FTIR data show that the proportion of the anhydrous methyl groups increases and water content in the micellar core decreases during the micellization process. C 2002 Elsevier Science (USA) Key Words: PEO–PPO–PEO block copolymer; melt; hydration; micellization; FTIR.

INTRODUCTION

Polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) block copolymers, commercially available as Poloxamers or Pluronics, are high molecular weight nonionic surfactants. The most interesting features of Pluronic polymers are their self-association in aqueous solutions and their rich phase behavior (1–12). At low polymer concentration and low temperature, the block copolymers dissolve in water as unimers. With increasing concentration or temperature, the phenomenon of critical micellization concentration (CMC) or critical micellization temperature (CMT) is observed, while the aggregation 1 To whom correspondence should be addressed. Fax: +86-10-62554264. E-mail: [email protected].

417

of PEO–PPO–PEO block copolymers occurs leading to the formation of intermolecular micelles. It is generally accepted that the micelle is spherical with a dense core consisting mainly of PPO and a hydrated PEO swollen corona (1–10). At even higher concentrations of the PEO–PPO–PEO block copolymer in water, the ordered phase, such as cubic, hexagonal, and lamellar phases, can be formed. The formation of the cubic phase can be understood by hard-sphere interaction between the micelles (1). At high temperature well above the CMT, a cloud point is reached and the copolymer solutions become opaque due to phase separation (11). PEO–PPO–PEO block copolymer surfactants have high solubility in water in a broad temperature range. The self-assembly and phase behaviors of Pluronic polymers depend on the total molecular weight and on the composition of the copolymers. Variation of the molecular characteristics (PPO/PEO ratio, molecular weight, and polymer architecture), which can be adjusted during the synthesis, can alter the hydrophilic– hydrophobic property of PEO–PPO–PEO block copolymers so that they are widely used in detergency, wetting, foaming– defoaming, emulsification, lubrication, and solubilization, as well as in cosmetics, bioprocessing, and pharmaceutical applications (13, 14). Though the general micellization and gelation behavior of PEO–PPO–PEO block copolymers has been extensively studied, the mechanism at molecular level remains unclear. Hurter (15, 16) and Linse (17, 18) have modeled the micellization with a mean-field lattice theory. In this theory the polymer segments are allowed to have both polar and nonpolar conformations, corresponding to the gauche conformer of the C–C skeleton which is favored in a polar environment and the trans conformer which dominates in nonpolar environment. The segment density profiles of the predicted micelles indicate that there is a finite concentration of water in the micellar core. A small-angle neutron scattering technique has been used to investigate the temperature dependence of the structure of PEO–PPO–PEO block copolymer micelles (19–24). The results showed that the micellar core cannot consist of PPO only but must contain significant quantities of water. The micellar core water content decreases with increasing temperature, with a corresponding increase in the micelle aggregation number (19–21). 0021-9797/02 $35.00

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Vibrational spectroscopy is a very powerful technique which has been extensively used to study the conformational states of the normal low molecular weight surfactant micelles in aqueous solution (25–31). Many spectral features of the low molecular weight surfactants are sensitive to the conformational changes, the inter- and intrachain interactions, and the chain mobility. A convenient way to detect the temperature-induced phase behavior and self-assembly is to monitor the temperature dependence of the infrared key bands. It is evident that a drastic change of the infrared spectrum occurs over a given temperature interval, with small changes before and after the temperature interval. The starting temperature of the interval can be taken as the phase transition temperature and the CMT (25, 26). The C–H stretching bands of aqueous PEO–PPO–PEO block copolymer solutions in FT–Raman and FTIR spectra are sensitive to the local polarity and the conformation of block copolymer chains, and their variations with temperature are indicators of micellization (32, 33). The purpose of this work was to explore the effects of temperature and hydration on the conformational structure of the PEO–PPO–PEO block copolymer in a large concentration range where the molecular events were observed by FTIR spectroscopy. MATERIALS

A PEO–PPO–PEO block copolymer, Pluronic P104, was obtained from BASF Corp. and used as received. Pluronic P104 can be represented by the formula EO27 PO61 EO27 , based on its molecular weight of 5900 and 40% PEO content. Polypropylene oxide, PPO1000 (MW = 1000), and polyethylene oxide, PEO400 (MW = 400), were kindly donated by Shanghai Surfactant Factory (China). Polyethylene glycol, PEG4000 (MW = 4000), was purchased from Beijing Chemical Reagent Corp. (China). These homopolymers were also used as received. Polymer–water mixtures were prepared individually by weighing appropriate amounts of polymers and distilled water. The mixtures were centrifuged to facilitate mixing and then stored in a refrigerator.

of FTIR spectra as a function of temperature in the range from 4 to 50◦ C, the samples were first cooled to 4◦ C by liquid nitrogen and then heated stepwise with a 2◦ C temperature interval. The OUPS spectroscopic software was used for data handling. RESULTS AND DISCUSSION

Melting Behavior Block copolymers with one crystallizable block and one noncrystallizable block have attracted great attention. One example of such a copolymer is the block copolymer of PEO and PPO where PEO is crystalline at ambient temperature if the molecular weight is larger than appropriately 1000, while PPO is amorphous at temperatures above its glass transition temperature, Tg , which is 270 K. It is known that crystallization of PEO leads to the block copolymeric system forming well-ordered lamellar domains (34–37). FTIR spectra of Pluronic P104 and its related polymers, PPO1000, PEO400, and PEG4000, were recorded; the frequencies and assignments of the FTIR absorption bands in the range from 1400 to 900 cm−1 were summarized in Table 1. These materials have the same skeleton. PPO1000 and PEO400 are liquid at room temperature, but Pluronic P104 is paste and PEG4000 is solid. The band positions of Pluronic P104 are in good agreement with those reported in the literature for the PEO crystalline phase (38–42). It can be concluded that the PEO blocks of Pluronic P104 at 26◦ C are in the crystalline state. There is a great difference between the spectral profiles of PEO400 and those of PEG4000 and Pluronic P104, because PEO400 is amorphous. The existence of methyl groups is confirmed by the band at 1374 cm−1 in the spectrum of Pluronic P104. The symmetric deformation band of methyl groups also appears in the FTIR

TABLE 1 Assignments of FTIR Bands of PPO1000, PEO400, PEG4000, and Pluronic P104 at 26◦ C PPO1000

PEO400

PEG4000

P104

Assignmenta

1360

1374 1360

CH3 symmetric deformation CH2 wag, C–C stretch

1343

1343

CH2 wag

1280

1280

CH2 twist

1242

1242 1235 1149 1104 1060 1014 963 945

CH2 twist

METHODS 1373

FTIR spectra were recorded on a Bruker Vector 22 FTIR spectrometer with a resolution of 2 cm−1 using a DTGS (deuterotriglycine sulfate) detector. The temperature of the sample was measured by a thermocouple inserted into a stainless steel block containing the sample cell. This system comprised a Graseby–Specac temperature cell (P/N 21525), and the temperature measurement was accurate to 0.1◦ C. FTIR spectra were recorded by scanning 64 times. The equilibration time for each temperature measurement was 2 min (32, 33). For FTIR measurements, liquid PPO1000, liquid PEO400, Pluronic P104, and polymer–water mixtures were sandwiched between two BaF2 windows of IR cell, and a spacer was also nipped to prevent evaporation during the measurement. FTIR spectra of PEG4000 were obtained using the KBr pellet method. In the measurement

1350 1345 1298

1325 1297 1249

1108

1108

1149 1108 1060

1014 942 a Assignment

949

962 946

based on Refs. (41) and (42).

C–O–C stretch, C–C stretch C–O–C stretch C–O–C stretch, CH2 rock CH2 rock CH2 rock, C–O–C stretch

419

PEO–PPO–PEO BLOCK COPOLYMER

curs in the temperature interval from 32 to 40◦ C: the bands shift abruptly from 1343 to 1348 cm−1 and from 1242 to 1250 cm−1 . The wavenumbers of 1343 and 1242 cm−1 bands change slightly when the temperature is below 32◦ C and above 40◦ C. The melting point of Pluronic P104 can be denoted 32◦ C, which is the onset of the drastic wavenumber changes of 1343 and 1242 cm−1 bands (the melting point is indicated in Fig. 2). The obtained melting point is consistent with the value reported in the literature (13).

o

6 C o

Absorbance

16 C o

26 c o

36 c o

40 c o

46 C

Effect of Hydration 1300

1200

1100

1000

900

-1

W avenum ber (cm ) FIG. 1. FTIR spectra of Pluronic P104 at various temperatures in the range from 1400 to 900 cm−1 .

1348

1254

1347

1252

1346

1250

1343 cm -1 band

1345

1248

1242 cm -1 band

1344

1246

1343 1244

80 wt %

W a veumber (cm-1)

W a venumber (cm-1)

spectrum of liquid PPO1000, which is at 1373 cm−1 . The weak bands at 1298 and 1014 cm−1 of amorphous PPO1000 also appear in the spectrum of Pluronic P104. The 1280 cm−1 band of Pluronic P104 has a shoulder at 1298 cm−1 . The spectral profiles of Pluronic P104 in the range from 1400 to 900 cm−1 are shown in Fig. 1, where the temperature dependent changes in spectral features can be seen clearly. Bands at 1361, 1280, 1235, 1149, 1060, and 963 cm−1 which are attributed to modes in the crystalline state of PEO disappear when the temperature rises above 40◦ C. In addition, the strong bands at 1343, 1242, and 947 cm−1 , which are also assigned to the crystalline phase, give rise to broad bands at higher temperatures. The vibrational multiplet structure of the crystalline phase collapsing into spectral features associated with the amorphous phase demonstrates that the conformational state of the PEO blocks changes from an ordered helical structure to a disordered structure. The crystalline–amorphous phase transition temperature can be obtained from the temperature dependent shifts of the bands at 1343 and 1242 cm−1 , which are presented in Fig. 2. A change oc-

Wanka and Hoffmann have reported a phase diagram of a Pluronic P104–water system in the temperature range from 0 to 100◦ C (1). At ambient temperature, the polymer–water mixture with 80 wt% Pluronic P104 is lamellar phase, 60 wt% Pluronic P104 is hexagonal phase, 40 wt% Pluronic P104 is cubic phase, and 22 and 10 wt% Pluronic P104 are isotropic solutions (1). However, knowledge about the exact mechanism of phase changes induced by hydration is still incomplete. Vibrational spectroscopy is an ideal tool to analyze the chain conformation of polymers which has been used to study the structural changes accompanying hydration of a model polypeptide (43). For characterizing the effect of hydration on the properties of the PEO–PPO–PEO block copolymer, FTIR spectra of Pluronic P104–water mixtures at various concentrations were measured; a part of FTIR spectral profiles are shown in Fig. 3. Bands at 1361, 1149, 1060, and 963 cm−1 associated with crystalline PEO do not appear in the spectra of Pluronic P104–water mixtures. The positions of the ether CH2 wagging (1350 cm−1 ), twisting (1298 and 1250 cm−1 ), and rocking (948 cm−1 ) modes are all characteristic for amorphous PEO (the PPO blocks also contribute to these bands). On the whole, the spectral profiles of Pluronic P104–water mixtures are similar to those of PEO400 and melting Pluronic P104. It was deduced that the PEO blocks in Pluronic P104–water mixtures are amorphous.

Absorbance

1400

60 wt % 40 wt % 22 wt % 10 wt %

1342 1242 1341 0

10

20

30

40

50

o

Temperature ( C ) FIG. 2. The temperature dependence of the wavenumbers of 1343 and 1242 cm−1 bands of Pluronic P104; the arrows indicate the melting point.

1400

1300

1200

1100

1000

900

Wavenumber (cm-1) FIG. 3. FTIR spectra of polymer–water mixtures with various Pluronic P104 contents in the range from 1400 to 900 cm−1 at 26◦ C.

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SU, WANG, AND LIU

Wavenumber (cm-1)

1382

1380

80 60 40 22 10

1378

wt % wt % wt % wt % wt %

1376

1374

0

10

20

30

40

50

o

Temperature ( C) FIG. 4. The temperature dependence of the wavenumbers of the symmetric deformation band of methyl groups of polymer–water mixtures with various Pluronic P104 contents; the arrows indicate the CMT values.

1110

Wavenumber (cm-1)

The hydrogen-bond model has been extensively used to interpret the mechanism of the solubility of water in oxyethylene type surfactants and polymers (44–47). An amorphous structure is more favorable than a helical structure for the PEO blocks in Pluronic P104–water mixtures, which may be attributed to the effect of H2 O molecules bound to the PEO segments. It is likely that it is difficult for the PEO chain with bound H2 O molecules to take a helical structure because of steric restriction, and hence it is forced to have a disordered structure (47). In Fig. 3, the band positions and shapes in the spectra of Pluronic P104–water mixtures do not change with decreasing polymer content from 80 to 40 wt%. Even in 22 and 10 wt% Pluronic P104 aqueous solution, the bands to the amorphous PEO blocks are similar to those in higher concentration mixtures. The number of bound H2 O molecules per PEO repeat unit was estimated to be one from NMR relaxation time measurements using a model of fast exchange of water molecules between a bound and an unbound water fraction (44) and differential scanning calorimetry (45). In the polymer–water mixture with 80 wt% Pluronic P104, there are enough H2 O molecules to bind all PEO segments. When the water content is higher than the amount of water needed to bind PEO segments, the increase of water only swells the shrunk-bulky PEO blocks (48) (the PPO blocks remain anhydrous). The swell of the PEO blocks, depending on the degree of hydration, gives an increased interface area per PEO block, which would alter the interface curvature (48– 50) so that the Pluronic P104 aggregate shapes generally appear in a sequence of lamella–hexagon–sphere. When PPO1000 is dissolved in water, the symmetric deformation band of methyl groups in PPO1000 shifts from 1373 to 1381 cm−1 . The solubility of PPO is influenced by temperature, so that the temperature dependent wavenumbers of the symmetric deformation bands of methyl groups of Pluronic P104–water mixtures are presented in Fig. 4. At concentrations of 80, 60, and 40 wt%, the wavenumbers of the symmetric deformation bands of methyl groups remain constant and are the same as that observed for neat Pluronic P104, which suggests that the PPO

1104 1098 80 wt % 60 wt % 40 wt % 22 wt % 10 wt %

1092 1086 1080

0

10

20

30

40

50

o

Temperature ( C) FIG. 5. The temperature dependence of the wavenumbers of the C–O–C stretching vibration of polymer–water mixtures with various Pluronic P104 contents; the arrows indicate the CMT values.

blocks are not hydrated. However, the frequencies of the symmetric deformation bands of methyl groups of aqueous Pluronic P104 solutions at concentrations of 22 and 10 wt% change dramatically when heated from 4 to 50◦ C. At low temperature, the bands of methyl groups are at 1381 cm−1 , which means that the methyl groups are hydrated and the microenvironment around the methyl groups is similar to that of the aqueous PPO1000 solution. The wavenumbers of the bands of methyl groups exhibit an abrupt shift from 1381 to 1375 cm−1 with increasing temperature and remain at 1375 cm−1 at higher temperatures. An alternation of the methyl groups in PPO blocks from a hydrated to an anhydrous microenvironment (micellization process) would result in the wavenumber changes (33). The influence of temperature on the C–O–C stretching vibrations of Pluronic P104–water mixtures, the strongest band in the studied frequency ranges, is plotted in Fig. 5. Those bands are a combination of the C–O–C stretching vibrations of both PPO blocks and PEO blocks. The changes of the wavenumbers of polymer–water mixtures with 80, 60, and 40 wt% Pluronic P104 are small with an increase of temperature, only a slight shift toward low frequencies at higher temperatures. However, the wavenumbers of the C–O–C stretching bands of 22 and 10 wt% aqueous Pluronic P104 solutions move significantly toward higher wavenumbers with an increase of temperature. Since the C–O–C stretching band of the pure sample is at 1104 cm−1 , the frequency shift toward a higher wavenumber may be related to the dehydration of the ether backbone. It is known that the hydrophobic PPO blocks apparently reduce contact with water during the micellization process (1–8). Another explanation of the frequency changes with temperature are associated with conformational changes of the ether backbone of PPO blocks from more polar, gauche conformation of unimers to less polar, trans conformation of micelles during the micellization process (15–18). Based on above results, the PEO blocks of Pluronic P104 are amorphous in a large concentration range, while the PPO blocks are hydrated only in a condition of lower temperature and lower

421

PEO–PPO–PEO BLOCK COPOLYMER

1375 cm

-1

o

6 C

A b sorbance

polymer concentration. This provides an opportunity to suggest herein that the hydration of the PEO–PPO–PEO block copolymer be in the following process. First, water penetrates into the most easily accessible PEO block region where H2 O molecules are bound to PEO segments. Further hydration would swell the disordered PEO blocks. Finally, when the PEO blocks are saturated with water, additional solvent would hydrate the PPO blocks of Pluronic polymer. Water preferentially condenses into the PEO-rich domain (51), perhaps because the PEO chains are accommodated with an ice-like structure of water. Despite the fact that PPO has the same backbone structure as PEO, an optimal water structure cannot be formed in the PPO hydrous shell, since the methyl groups of PPO constitute a sterical hindrance (52).

o

16 C o

26 C o

36 C o

46 C

1400

1350

1300

1250

1200

-1

W a venumber (cm ) FIG. 7. FTIR spectra of 10 wt% aqueous Pluronic P104 solution at various temperatures in the range from 1400 to 1200 cm−1 .

Temperature Dependent Micellization of Pluronic P104 in Aqueous Solution An increase of temperature of PEO–PPO–PEO block copolymer solutions provides the thermodynamic driving force of the transfer of PO groups from the aqueous medium into the hydrophobic microenvironment (micellar core). The progressive loss of the PPO hydrous shell would influence the spectral behavior of the Pluronic polymer. The temperature dependences of FTIR spectral profiles of aqueous Pluroic P104 solutions at concentrations of 22 and 10 wt% in the wavenumber range from 1400 to 1200 cm−1 are presented in Figs. 6 and 7, respectively. The symmetric deformation bands of methyl groups of 22 and 10 wt% aqueous Pluronic P104 solutions are at 1381 cm−1 at low temperature. The weak shoulder at 1375 cm−1 can also be found in the low frequency region of the 1381-cm−1 band at a temperature of 6◦ C. With increasing temperature, the relative peak intensity of the 1381-cm−1 band decreases while the shoulder at 1375 cm−1 increases. It can be seen clearly that the symmetric deformation band of methyl groups splits into two bands, which are at 1381 and 1375 cm−1 , respectively, in the spectral profiles of aqueous Pluronic P104 solutions at a concentration of 22 wt% at 16◦ C and at a concentration of 10 wt%

at 26◦ C. At higher temperatures, the band at 1375 cm−1 dominates the symmetric deformation band of methyl groups, and the band at 1381 cm−1 becomes a weak shoulder. We deduced that the methyl groups of Pluronic P104 in aqueous solutions are composed of two states, one is a hydrated state corresponding to the band around 1381 cm−1 (surrounded by water) and the other is an anhydrous state associated with the band near 1375 cm−1 . A similar phenomenon has been observed by Maeda (53). The microenvironment around the hydrated methyl groups is polar and around the anhydrous methyl groups is nonpolar. In order to obtain quantitative information on the temperature dependent spectroscopic behavior, the spectral pattern in the symmetric deformation vibration peak of methyl groups is resolved into two components which are associated with anhydrous and hydrated methyl groups, respectively. Since there is an overlap of the absorption bands, the curve-fitting method (53, 54) was used to fit the peaks in the range from 1400 to 1320 cm−1 . One representative result is presented in Fig. 8. The

1381 cm -1 band -1

1375 cm band

-1

Absorbance

1375 cm

o

Absorbance

6 C o

16 C o

26 c o

36 C o

46 C

1400 1400

1350

1300

1250

1200

W a venumber (cm-1 ) FIG. 6. FTIR spectra of 22 wt% aqueous Pluronic P104 solution at various temperatures in the range from 1400 to 1200 cm−1 .

1380

1360

1340

1320

-1

W a venumber (cm ) FIG. 8. A representative baseline-subtracted FTIR spectrum of 10 wt% aqueous Pluronic P104 solution at 26◦ C in the range from 1400 to 1320 cm−1 and a best curving-fitting result; the fitting peaks at 1381 and 1375 cm−1 are indicated.

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SU, WANG, AND LIU

Peak Area Fraction

0.6 0.5

22 w t % 10 w t %

0.4 0.3 0.2 0.1 0.0 0

10

20

30

40

50

o

T e mperature ( C ) FIG. 9. The temperature dependence of the relative peak area fraction of the anhydrated methyl groups of 22 and 10 wt% aqueous Pluronic P104 solutions; the arrows indicate the CMT values.

peak frequency values determined by the fittings are the same as those obtained from the observed spectral profiles. On the basis of the fitting results, the integrated peak areas of anhydrous (peak at 1375 cm−1 ) and hydrated methyl groups (peak at 1381 cm−1 ) were calculated. The total integrated peak area of anhydrous and hydrated methyl groups was normalized to 1; the relative peak area fraction of each component corresponds to the fraction of the state. Figure 9 shows the plots of the relative peak area fractions of the anhydrous methyl groups of 22 and 10 wt% aqueous Pluronic P104 solutions estimated in this way as a function of temperature. The relative peak area fractions associated with the anhydrous methyl groups do not change until a characteristic temperature is reached, after which the fraction increases gradually with an increase of temperature. Incorporation of unimers into micelles and dehydration of the micellar core can give a fitting description of the changes versus temperature. The parameter of greater fundamental value for aqueous Pluronic solutions is the CMT, which can be obtained from the temperature dependence of the relative peak area fractions of the anhydrous methyl groups. The CMT is defined as the temperature at which the relative peak area fraction of the anhydrous methyl groups deviates from the baseline contributed only by unimers. In Fig. 9, the CMT values of 22 and 10 wt% aqueous Pluronic P104 solutions can be determined to be 12 and 18◦ C, respectively. The same results can also be obtained from Figs. 4 and 5. In Figs. 6, 7, and 9, it is found that a few methyl groups of aqueous Pluronic P104 solutions exist in the anhydrous state at the temperature below the CMT. The unimolecular micelle hypothesis has been proposed to account for the existence of a nonpolar microenvironment (10). When the temperature is below the CMT, the PPO blocks of Pluronic polymers are collapsed to globulars surrounded by the expanded hydrophilic PEO blocks, in this way forming intramolecular micelles. The anhydrous methyl groups may be in the center of Pluronic unimolecular micelles. The relative peak area fraction of the anhydrous

methyl groups of unimolecular micelles is about 0.07, which is almost independent of temperature and concentration. In Fig. 9, the relative peak area fraction of the anhydrous methyl groups increases with an increase of temperature when the temperature is above the CMT, which means that the proportion of the anhydrous methyl groups increases and water content in the micellar core decreases. These results are consistent with those obtained by using small-angle neutron scattering studies. The water in the micellar core is replaced gradually by the block copolymer as the temperature increased (19–21). At the same temperature, the relative peak area fraction of the anhydrous methyl groups of 22 wt% aqueous Pluronic P104 solution is higher than that of 10 wt% aqueous Pluronic P104 solution. A increase in concentration would decrease the water content in the micellar core. It should be emphasized that the distinct sharp peaks at 1362 and 1340 cm−1 appear at both sides of the 1350-cm−1 band in the spectra of aqueous Pluronic P104 solutions at concentrations of 22 and 10 wt% (Figs. 6–8). We have deduced that the bands at 1362 and 1340 cm−1 are caused by a part of the PEO segments possessing a crystalline structure (55, 56). The origin of the idea that a part of the PEO segments of aqueous Pluronic P104 solutions is in the crystalline state is unclear. Inoue has found that partial dehydration of the PEO chain of heptaethylene glycol dodecyl ether (C12 E7 ) may facilitate the chain taking a helical structure owing to the release from a steric restriction forced on the PEO chain by the bound H2 O molecules (47). It may be the specific structure of a uni- and multimolecular micelle of the PEO–PPO–PEO block copolymer, which is collapsed or aggregated into a spherical shape, that makes a part of the PEO segments dehydrate and be in the crystalline state. SUMMARY

This study has shown that FTIR spectroscopy is an excellent technique for investigation of the molecular events of the PEO– PPO–PEO block copolymer in the melting behavior, hydration, and micellization process. When the temperature is raised, the vibrational multiplet structure of the crystalline phase of Pluronic P104 collapses into spectral features associated with the amorphous phase, which corresponds to the conformational change of the PEO blocks from an ordered helical structure to a disordered structure occuring at the melting point. FTIR spectra obtained from Pluronic P104–water mixtures of lamellar, hexagonal, and cubic phase are similar to each other. It suggests that there is no essential difference in the conformation of the block copolymer molecules among these ordered phases. Water preferentially penetrates into the most easily accessible PEO block region where H2 O molecules are bound to the PEO segments, and the conformation of the PEO blocks is forced to transform form order to a disordered structure. Further hydration would swell the disordered PEO blocks. The PPO blocks of Pluronic P104 are hydrated only in a condition of lower temperature and higher water concentration.

PEO–PPO–PEO BLOCK COPOLYMER

The symmetric deformation band of methyl groups of Pluronic P104 in water is composed of two components, which are assigned to hydrated and anhydrated methyl species, respectively. At the temperature below the CMT, the proportion of the anhydrous methyl groups of unimolecular micelles is almost independent of temperature and concentration. When the temperature is above the CMT, however, the proportion of the anhydrous methyl groups increases, which means that water is gradually excluded from the micellar core. The higher the polymer concentration, the lower the water content contained in the micellar core. ACKNOWLEDGMENT This work is financially supported by the Outstanding Young Scientist Foundation of China (29925617).

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