Phase Behavior and Structure of Ternary Amphiphilic Block

The phase behavior of amphiphilic copolymer−alkanol−water ternary systems was investigated for triblock copolymers of similar molecular weight and...
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Langmuir 1997, 13, 2471-2479

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Phase Behavior and Structure of Ternary Amphiphilic Block Copolymer-Alkanol-Water Systems: Comparison of Poly(ethylene oxide)/Poly(propylene oxide) to Poly(ethylene oxide)/Poly(tetrahydrofuran) Copolymers Peter Holmqvist,* Paschalis Alexandridis, and Bjo¨rn Lindman Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received August 16, 1996. In Final Form: December 30, 1996X The phase behavior of amphiphilic copolymer-alkanol-water ternary systems was investigated for triblock copolymers of similar molecular weight and the same hydrophilic block [poly(ethylene oxide), E] but having different hydrophobic blocks [poly(propylene oxide), P, or poly(n-butylene oxide) ) poly(tetrahydrofuran), T]. The alkanol used (butan-1-ol) was comparable in terms of chemical composition to the hydrophobic segments. A rich phase behavior was obtained for the polymer with the P middle block (Pluronic F127, E100P70E100): five different one-phase regions, i.e., micellar (L1) and reverse micellar (L2) solutions, and (micellar) cubic (I1), hexagonal (H1), and lamellar (LR) lyotropic liquid crystalline regions, were detected. The microstructure in the liquid crystalline regions was established from small-angle X-ray measurements; I1 was found to be primitive cubic. The alkanol molecules are most likely anchored with their OH- group at the E-P interface, increasing the apparent volume of the P blocks relative to that of the E blocks, and thus causing a decrease in the polymer layer curvature from spherical to cylindrical. Only a single one-phase region, extending from the water to the alkanol corner, was observed in the E100T27E100 ternary phase diagram. The extent of the one-phase regions decreased for both E100T27E100 and E100P70E100 systems when the alkanol molecular weight increased.

Introduction Amphiphilic copolymers, and in particular triblock copolymers consisting of a poly(propylene oxide) (P) middle block and poly(ethylene oxide) (E) end blocks (commercially available under the Pluronic and Synperonic trade names and also known as poloxamers), exhibit a very interesting solution behavior and have been the subject of many recent studies (see refs 1-4 for reviews). Block copolymers of the EnPxEn type behave in aqueous solutions in many respects like typical non-ionic surfactants: they are surface active,5 P being hydrophobic and E hydrophilic, and can form different self-assembled structures (micellar solutions6,7 as well as cubic, hexagonal, and lamellar lyotropic liquid crystals8-11 ) depending on the polymer concentration and/or temperature. The characteristic length and time scales related to selfassembly are usually larger than those of typical surfactants, since the amphiphilic block copolymers are 10 to 30 times bigger. An advantage of this class of “surfactants”, when compared to typical nonionic surfactants, is the great control over the amphiphilic properties afforded by the variation (during synthesis) of the size and the ratio between the hydrophobic and the hydrophilic blocks;12 X

this is especially important in the various applications.4,13,14 Moreover, poloxamer-type block copolymers exhibit low toxicity15 and are thus used in pharmaceutical products.14 Block copolymers consisting of poly(ethylene oxide) and poly(1,2-butylene oxide) (B)16,17 or poly(nbutylene oxide) (also called polytetrahydrofuran, T, because of the monomer used for their synthesis)18 are also surface active and self-assemble in solution but are less studied than the EnPxEn copolymers; EnBxEn copolymers became commercially available only recently.16 Although the aqueous solution behavior of EnPxEn-type amphiphilic block copolymers is well studied1-3 relatively little is known about their ternary systems with water and organic solvents.19-22 The hydrophobic blocks of the EnPxEn, EnBxEn, and EnTxEn amphiphilic copolymers differ in polarity from those commonly found in nonionic surfactants: in the P, B, and T blocks there is a lot of ether oxygen present which imparts polarity, while the hydrophobic part of nonionic surfactants is usually an alkyl chain. Because of this polar character, it is not possible to dissolve aliphatic hydrocarbons (which are often encountered in mixtures with nonionic surfactants23 ) in

Abstract published in Advance ACS Abstracts, March 1, 1997.

(1) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1. (2) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (3) Chu, B.; Zhou, Z. Surfactant Sci. Ser. 1996, 60, 67. (4) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers: Self-Assembly and Applications; Elsevier Science B.V.: Amsterdam, 1997. (5) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604. (6) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5440. (7) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (8) Malmsten, M.; Lindman, B. Macromolecules 1993, 26, 1282. (9) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (10) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 2690. (11) Zhou, D.; Alexandridis, P.; Khan, A. J. Colloid Interface Sci. 1996, 183, 339. (12) Whitmarsh, R. H. Surfactant Sci. Ser. 1996, 60, 1.

S0743-7463(96)00819-0 CCC: $14.00

(13) Edens, M. W. Surfactant Sci. Ser. 1996, 60, 185. (14) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490. (15) Rodriguez, S. C.; Singer, E. J. Surfactant Sci. Ser. 1996, 60, 211. (16) Yang, Y.-W.; Deng, N.-J.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (17) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1996, 12, 1419. (18) Holmqvist, P. ; Nilsson, S ; Tiberg, F Colloid Polym. Sci., in press. (19) Chu, B. Langmuir 1995, 11, 414. (20) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (21) Alexandridis, P.; Olsson, U.; Lindman, B. J. Phys. Chem. 1996, 100, 280. (22) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (23) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994.

© 1997 American Chemical Society

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the hydrophobic part of the EnPxEn-type copolymers.24,25 Polar organic solvents, such as alkanols, are more similar to the hydrophobic part of these copolymers and should be suitable as (selective) solvents. We present here detailed information on the phase behavior and structure of ternary systems consisting of an amphiphilic block copolymer, alkanol, and water, where the alkanol (butan-1-ol or hexan-1-ol) is chosen to be similar in chemical nature to the monomers making up the hydrophobic block of the copolymer. The block copolymers studied, E100P70E100, E100T27E100, and E17T27E17, allow us to examine the effects of the hydrophobic block type (P versus T) as well as the hydrophilic block size (E ) 100 and E ) 17) on the phase behavior. The E100P70E100 block copolymer is of similar molecular weight and hydrophilic/hydrophobic block weight ratio to the E100T27E100 block copolymer, but it has a less hydrophobic (middle) block. The dilute aqueous solution behavior of E100T27E100 and E17T27E17 has been recently studied;18 a binary E100P70E100-water phase diagram has also been reported.9 The organization of the paper is the following: The isothermal (25 °C) phase behavior of the E100P70E100butanol-water system is first presented, and information on the structure of the various lyotropic liquid crystalline phases (characterized by small-angle X-ray scattering) is reported; the location of the alkanol in the microstructures and its role on the phase behavior are deduced. The pronounced influence of the hydrophobic block type on the ternary phase behavior is demonstrated by contrasting the E100T27E100-butanol-water system to the E100P70E100butanol-water system. The E17T27E17-butanol-water phase diagram is then presented, allowing the examination of the hydrophilic (E) block size effect on the phase behavior. Finally, the effect of the alkanol type on the phase behavior is elucidated by comparing the ternary systems of E100P70E100 and E100T27E100 with water and butanol or hexanol. Materials and Methods Materials. Poly(ethylene oxide)-b-poly(tetrahydrofuran)-bpoly(ethylene oxide) copolymers, having the composition EnT27En (n ) 17 and 100), were synthesized by Akzo Nobel Surface Chemistry, Stenungsund, Sweden. The poly(ethylene oxide)b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic F127) copolymer, denoted here E100P70E100 (based on its molecular weight and composition), was kindly supplied by BASF Svenska AB, Go¨teborg, Sweden. The molecular weights of the polymers are 12500 g mol-1 for E100T27E100, 3500 g mol-1 for E17T27E17, and 13300 g mol-1 for E100P70E100. The EnT27En polymers were dialyzed against Millipore-filtered water (dialyzing membrane: Ultrasette from Filtron Technology Corp. with molecular weight cut-off of 6000) and then freeze-dried; E100P70E100 was used as received. The two alkanols, butan-1-ol and hexan-1-ol, were purchased from BDH Chemicals Ltd. (Poole, England) and used as received. Millipore-filtered water was used for all preparations. The bulk density for the E100P70E100 copolymer is 1.05 g/mL at 25 °C according to the manufacturer. The bulk densities of butanol and water are 0.81 and 1 g/mL, respectively. Phase Diagram Determination. Over a hundred samples were individually prepared in glass-tubes, flame-sealed, homogenized, and left to equilibrate at 25 °C for at least 2 weeks in order to determine the boundaries of the different phases in the ternary polymer-alkanol-water systems. Following equilibration, the samples were checked for phase separation and birefringence. More samples were prepared in the vicinity of the phase boundaries in order to accurately determine their location. In some two-phase regions, the tie-lines and the phase (24) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210. (25) Hurter, P. N.; Alexandridis, P.; Hatton, T. A. Surfactant Sci. Ser. 1995, 55, 191.

Holmqvist et al. boundaries were determined by separating (using centrifugation) the two-phase samples and analyzing the composition of each phase. The alkanol and polymer contents in the phase-separated samples were determined by chromatographic (HPLC) and refractive index measurements, respectively. For the HPLC analysis, the polymer-alkanol-water sample was dissolved in acetonitrile (to prevent the block copolymer from associating) and eluted through a Nucleosil 100-5 C18 HPLC column (Macherey-Nagel) with acetonitrile + 5% water as the mobile phase. A HPLC 2150 pump from Pharmacia LKB was used to pump the mobile phase through the column. The alkanol separated from the other components in the HPLC column and it was thus possible to determine its weight fraction; the water and copolymer were not resolved by the HPLC column. A fraction of the same sample was diluted with water and the refractive index was measured at 950 nm with a 2142 differential refractometer (LKB, Bromma, Sweden). The refractive index, n, of a sample containing two or more dissolved components can be described in dilute systems by the following equation:

nsample ) nsolvent + (dn/dc)1c1 + (dn/dc)2c2 + ...

(1)

where cn is the concentration and (dn/dc)n is the refractive index increment for the dissolved compounds. Since the weight fraction of the alkanol was obtained from the HPLC analysis, it was possible to determine the weight fraction of the copolymer from the refractive index measurements. In order to determine the structure of the birefringent phases, both deuterium splitting NMR in D2O10 and microscopy under polarized light9 were tried. The results obtained from these two methods were not very useful for the E100P70E100-butanol-water system. The deuterium splitting give just one broad peak, and no textures (typical of lyotropic mesophases) were revealed by polarized microscopy. Instead, small-angle X-ray scattering was successfully employed for the structural characterization of the birefringent as well as the isotropic lyotropic liquid crystalline phases. Small-Angle X-ray Scattering (SAXS). The measurements were performed on a Kratky compact small-angle system equipped with a position sensitive detector (OED 50M from M Braun, Graz, Austria) containing 1024 channels of 51.3 µm width. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID300 X-ray generator, operating at 50 kV and 40 mA. A 10 µm thick nickel filter was used to remove the Kβ radiation and a 1.5 mm tungsten filter put in to protect the detector from the primary beam. The sample-to-detector distance was 277 mm. The volume between the sample and the detector was kept under vacuum during data collection in order to minimize the background scattering. The temperature was kept on 25 °C (( 0.1 °C) with a Peltier element. The obtained Bragg peaks are relatively sharp, in which case the peak position can be evaluated from the slitsmeared SAXS data.10,20,21 All samples tested with SAXS were one-phase and had been equilibrated for at least a month before the measurement. Structural Characterization. The relative position of the SAXS peaks on the scattering vector (q) axis was used to determine the structure [i.e., lamellar (smectic) or hexagonal (cylindrical assemblies crystallized in a hexagonal lattice)] of the birefringent lyotropic liquid crystalline phases. For the lamellar structure, the relative position of the peaks should obey the relationship 1:2:3 ..., while for the hexagonal structure the relationship 1:x3:2:x7:3 ... The following formulas were used to determine the characteristic lengths (lattice parameters) of the different structures from the position (q*) of the first (and most intense) peak

lamellar:

q* ) 2π/D

(2)

hexagonal:

q* ) 2π/d

(3)

where D is the spacing between the lamellar layers and d is the repetition distance (between rows of cylinders) in the hexagonal structure. For the lamellar phase it is straightforward to calculate from the lamellar spacing the effective area per poly(ethylene oxide) block (area per polymer molecule at the E-P junction), ap, using

Phase Behavior of Ternary Systems

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Figure 1. Phase diagram of the E100P70E100-butanol-water system at 25 °C. The tie-lines are represented by full straight lines. L1 denotes the water-rich (micellar) solution region, I1 the normal (“oil”-in-water) micellar cubic liquid crystalline region, H1 the normal hexagonal liquid crystalline region, LR the lamellar liquid crystalline region, and L2 the alkanol-rich solution region. the following expression:

ap )

1 νa D φi

(4)

where νa is the volume of one polymer molecule and φi is the volume fraction of the molecules siting at the polar/apolar interface. The formula used to calculate the area per poly(ethylene oxide) block in the normal (oil-in-water) hexagonal structures is20,21

ap )

xfπx3 νa dx2 φi

(5)

where f is the volume fraction of the apolar components. In the case of a reverse (water-in-oil) hexagonal structure, f in eq 5 is replaced by the polar volume fraction, 1 - f. Typically, φi ) φp since the amphiphilic molecules (in our case the block polymers) are responsible for creating the interface between polar/apolar domains and the ensuing microstructure (φp is the polymer volume fraction); it may also be possible that φi ) φp + φo when the “oil” (in our case alkanol) is part of the interface and (φo is the “oil” volume fraction). The relative volume of the polar and apolar regions must be defined in order to calculate the area per poly(ethylene oxide) block in the hexagonal structures. The apolar volume, f, should contain the hydrophobic poly(propylene oxide) block (which in the case of E100P70E100 constitutes ≈30% of the total block copolymer volume) and (possibly) the alkanol. However, butanol is rather polar and, to some extent, soluble in water. The exact location of the alkanol in the different microstructures will affect f and the value for the area per poly(ethylene oxide) block molecule extracted from the SAXS data through eq 5. It turns out that, as discussed below, the area per poly(ethylene oxide) block values provide a strong indication on the alkanol location and on how the alkanol affects the phase behavior. The polar volume, 1 f, consists of the water and the poly(ethylene oxide) part of the block copolymers (the later constituting ≈70% of the total block copolymer volume for E100P70E100).

Results and Discussion E100P70E100-Butanol-Water System: Phase Behavior. The phase diagram for the ternary E100P70E100butanol-water system at 25 °C is presented in Figure 1. An isotropic solution region, denoted L1, was found along

the polymer-water axis up to 20 wt % E100P70E100; up to 10 wt % butanol could be accommodated in this region. The L1 samples were optically transparent and fluid and their viscosity increased when the phase boundary with the liquid crystalline phase (I1) was approached (although the accuracy of the phase boundary is as good as 2 to 3 wt %, no two-phase region was found between the L1 and the I1 phases). An isotropic liquid crystalline region of cubic structure (I1) superseded the L1 solution region along the water-polymer axis and extended up to 65 wt % E100P70E100; its ability to swell with butanol decreased as the upper concentration boundary was approached. A birefringent (anisotropic) region (H1) was formed along the polymer-water axis from 70 to 80 wt % E100P70E100. This region swelled with butanol and extended between the cubic (I1) and the alkanol-rich solution region (L2) down to approximately 20 wt % copolymer. The structure of this region was established from SAXS measurements as hexagonal (as discussed below). The birefringent liquid crystalline region stable in the 20-30 wt % E100P70E100 and 25-30 wt % butanol ranges was identified as having lamellar (smectic) structure (LR). A large one-phase isotropic solution region (L2) extended from the butanolrich corner down to just 20 wt % butanol. The samples in this region were similar in fluidity to butanol down to 50 wt % butanol, but they became more viscous at butanol concentrations below 50 wt %. We note that the boundary between the isotropic solution and the cubic phase on the water-polymer axis reported in this work is consistent with previously published results.9 However, we found the cubic phase to terminate at 65 wt % copolymer instead of 80 wt %, which was reported in ref 9. Although the samples below and above 65 wt % copolymer appeared similar macroscopically, the ones above 65 wt % were birefringent and thus not cubic. SAXS measurements unequivocally established that the structure between 70 and 80 wt % E100P70E100 was that of a hexagonal liquid crystalline array. A detailed characterization of the structure of the various lyotropic liquid crystalline phases follows. E100P70E100-Butanol-Water System: Structural Characterization. Numerous SAXS measurements were performed in the E100P70E100-butanol-water phase diagram since SAXS proved to be the best method available for determining the structure as well as the boundaries of the different liquid crystalline phases occurring in this phase diagram. Water-Rich (Micellar) Solution Phase (L1). SAXS measurements were made in this region at different polymer concentrations keeping constant (5 wt %) the butanol content. No primary peak was found at polymer concentrations below 10 wt %, while a broad peak appeared in the SAXS diffraction patterns (not shown) at copolymer concentrations between 10 and 20 wt %. This peak became sharper, and its position moved to higher q values, with increasing polymer concentration. The critical micellization concentration (the copolymer concentration where micelles start forming) of E100P70E100 in water at 25 °C is ≈1 wt %,7,26 but significant association of unimers (unassociated molecules) into micelles should occur at higher concentrations (≈90% of the polymer associated at 10 wt %).1 This, together with the possible suppression of the copolymer micellization by the presence of butanol, is consistent with the appearance of a broad SAXS peak at copolymer concentrations higher than 10 wt % as the number of micelles increased and the solution became more ordered; the samples were still fluid and isotropic in this region. (26) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Am. Oil Chem. Soc. 1995, 72, 823.

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Figure 2. SAXS diffraction patterns from samples in the cubic liquid crystalline region of the E100P70E100-butanol-water phase diagram. The vertical lines mark the positions of the reflections afforded by the P... crystallographic space group. The different SAXS patterns are drawn on the same scale but are offset vertically for clarity.

Cubic Lyotropic Liquid Crystalline Phase (I1). A number of peaks were observed in the SAXS diffraction patterns obtained from samples in the I1 region. Slitsmeared SAXS diffraction patterns for samples with copolymer concentrations 25, 30, 35, and 40 wt % (at constant butanol concentration of 5 wt %) are presented in Figure 2. The first peak appeared at a q value of ≈0.026 1/Å but the most intense peak was found at a q value of ≈0.043 1/Å, at the same scattering vector value as the broad peak found at lower polymer concentrations within the L1 solution phase. The position of the peaks in the cubic samples did not change significantly with increasing block copolymer concentration. There are three main families of cubic structures:27 primitive (P...), body-centered (I...), and face-centered (F...); within each family there are many crystallographic space groups with different symmetries. While it is difficult to check all possible structures against the experimental SAXS diffraction patterns, we can exclude certain crystallographic space groups based on the relative positions of the first few SAXS peaks (reflections). We have thus ruled out the face-centered (F...) cubic structures from consideration because the peaks in the SAXS patterns of Figure 2 do not follow the relative positions expected for the F... symmetry. Choosing between the I... and P... cubic structures is more difficult because the reflections afforded by the P... structure include the ones from the I... structure as a subset (the I... structures are of higher symmetry and thus result in the lowest number of reflections). However, we can exclude the possibility of an I... structure based on the relative intensity of the peaks: in the discrete I... cubic structures the first peak observed, hkl ) 110, is usually the most intense because it originates from the plane of the highest scattering density; in the P... cubic structures, the most intense is the third, hkl ) 111, peak. This is indeed the case in the diffraction patterns shown in Figure 2 (note that the peak at q ≈ 0.043 1/Å is also observed in the L1 samples) and we have thus indexed the peaks to a primitive (P...) cubic structure. The cubic cell should consist of normal or alkanol-in-water (on the basis of the I1 location in the phase diagram28 ) copolymer (27) Hahn, T., Ed. International Tables for Crystallography; Reidel: Dortrecht, 1983. (28) Fontell, K. Colloid Polym. Sci. 1990, 268, 264.

Holmqvist et al.

Figure 3. Plot of the reciprocal spacings (1/dhkl) of the reflections marked in the SAXS diffraction patterns of Figure 2, plotted versus m ) (h2 + k2 + l2)1/2. The straight line passing through the origin indicates the good fit of the SAXS reflections to the P... cubic structure.

micelles which have crystallized when their effective (including water) volume fraction reached that (0.53) required for hard sphere crystallization.29 Although the structural elements (i.e., micelles) in the I1 cubic phase are generally assumed spherical, there is no definite proof for this. It has been suggested that the unit cell of an I1 cubic phase of crystallographic space group Pm3n is composed of eight rodlike aggregates with an axial ratio around 2.30 Whenever it has been checked, surfactant cubic phases consist of nonspherical micelles. It is thus probable that the micelles in the I1 region elongate as the polymer concentration increases and the H1 region is approached. A total of eight Bragg peaks are marked in Figure 2, indexed as the 100, 110, 111, 200, 210, 211, 220, and 221 reflections of a primitive cubic structure (we note that the Bragg reflections always tend to become very weak at high hkl indexes due to the long-range disorder inherent in liquid crystalline materials). This cubic structure is characterized by Bragg reflections whose reciprocal d spacings follow the relationship 1:x2:x3:2:x5:x6:x8:3 .... The indexing of the diffra data was assessed by plotting the reciprocal spacings (1/dhkl) of the reflections marked in Figure 2 versus m ) (h2 + k2 + l2)1/2. For a cubic structure, such a plot should pass through the origin and be linear with a slope of 1/a, where a is the cubic cell lattice parameter (see Figure 3). The line in Figure 3 indicates the good fit of the data to the P... structure. The value of a, estimated from the data of Figure 3, is 250 Å. In a very recent small-angle neutron scattering study,31 the structure of a shear-oriented 20% (in water) Pluronic F127 (E100P70E100) sample was found body-centered (I...) cubic. A lattice parameter of 280 Å was obtained by considering the first Bragg reflection at q ) 0.032 1/Å as hkl ) 110. While the crystallographic group (I...) reported in ref 31 is different than the one (P...) obtained above, both are possible. The cell lattice parameter of the micellar cubic structures can be related to the block copolymer dimensions (νa, ap) and the sample composition (φi, f) through simple geometrical arguments for the volume occupied by the (spherical) micelles which make up one unit cell.22 The I... cubic cell contains two micelles and thus (for the same νa, ap, φi, f) has a larger lattice parameter than the P... cell with one micelle. Taken together, our (29) Mortensen, K.; Brown, W.; Norden, B. Phys. Rev. Lett. 1992, 68, 2340. (30) Fontell, K.; Fox, K. K.; Hansson, E. Mol. Cryst. Liq. Cryst. 1985, 1, 9. (31) Mortensen, K.; Talmon, Y. Macromolecules 1995, 28, 8829.

Phase Behavior of Ternary Systems

Figure 4. SAXS diffraction patterns from samples in the hexagonal liquid crystalline region of the E100P70E100-butanolwater phase diagram. The figure shows SAXS patterns of samples with 25/20/55, 30/15/55, 45/10/45, 50/10/40, and 60/ 5/35 wt % E100P70E100/butanol/water compositions. The different SAXS patterns are drawn on the same scale but are offset vertically for clarity.

results and that of ref 31 suggest that the cubic structure changes from body-centered along the polymer-water axis to primitive in the presence of butanol. A similar behavior has been observed in a ternary system containing two Pluronic copolymers (P105, E37P58E37; L64, E13P30E13) and water:11 a body-centered cubic structure was found along the E37P58E37-water axis and a primitive structure when half (wt) of E37P58E37 was replaced by E13P30E13. Hexagonal Lyotropic Liquid Crystalline Phase (H1). Numerous SAXS measurements were made throughout this region; some slit-smeared SAXS diffraction patterns, typical of samples in the H1 region, are shown in Figure 4. The SAXS diffraction patterns, together with the observation that the samples were birefringent and stiff, indicate that this one-phase region has hexagonal structure. At least four peaks (and sometimes even a fifth one) could be distinguished in the SAXS diffraction patterns, with peak positions which follow the relationship 1:x3:2:x7:3, characteristic of a hexagonal structure. The question arose as to whether the morphology was water-continuous (alkanol-in-water or “normal” hexagonal) or alkanol-continuous (water-inalkanol or “reverse” hexagonal). In order to address this issue, we calculated the characteristic length using eq 3 (no assumptions required) and then the interfacial area per PPO-chain using eq 5 (assuming either φi ) φp or φi ) φp + φo and f ) φp + 1/2φo due the interfacial location of the butanol). The characteristic length values are listed in Table 1 and are also indicated in Figure 5 in the appropriate location (composition) of the ternary phase diagram. As seen in Figure 5, the repetition distance decreased both when the polymer concentration increased and (to a lesser extent) when the butanol concentration increased (these trends also held in the lamellar region as discussed below). The decrease of the characteristic length with increasing polymer concentration follows from the fact that, as more polymer has to fit in the hexagonal structure, the number of cylinders (consisting of polymer) increases and the cylinders get closer to each other. In order to interpret the second observation (decreasing characteristic length with increasing butanol concentration), we have to find out the location of butanol in the microstructure. The majority of the butanol should be present either in the apolar volume, i.e., mixed with the poly(propylene oxide), or at the interface between the P (apolar) and the

Langmuir, Vol. 13, No. 9, 1997 2475

E (polar) domains. A small part of the butanol will be dissolved in water (up to ∼7.5 wt % butanol is soluble in water at 25 °C32 ), but we will neglect this in the ensuing calculations. Values for the area per poly(ethylene oxide) block at the polar-apolar interface were calculated using the appropriate volume fractions for the normal and the reverse hexagonal structures and considering the two possible locations for the butanol; the interfacial area values are listed in Table 1. If the structure were reverse hexagonal, the estimated interfacial area per poly(ethylene oxide) block would be much larger than that for the lamellar structure; furthermore, it would depend a lot on the polymer concentration (see Table 1, column rev hex area/PEO). This is probably not the case, since we would expect the area per polymer molecule at the polar/apolar interface to be approximately the same in the different structures and regions of the phase diagram.20,21 This constitutes strong evidence that the structure is not reverse hexagonal. If, on the other hand, the structure were normal hexagonal with all the butanol located in the apolar volume, then the estimated interfacial area per poly(ethylene oxide) block values would become more reasonable (see column with the heading nor hex area/PEO 1 in Table 1). Still, the interfacial area values estimated under this assumption vary considerably throughout the hexagonal region and are much higher than those in the lamellar region at low polymer concentrations; this finding suggests that the assumption of the alkanol being part of the apolar volume is not correct. Indeed, when the interfacial area is calculated assuming that the butanol is located at the polar/apolar interface (see column with the heading nor hex area/PEO 2 in Table 1), we obtain from eq 5 area per poly(ethylene oxide) block values which do not vary much throughout the hexagonal region (the 20% increase in the interfacial area when butanol increased from 5 to 20% may be an indication that not all the butanol is located at the interface, but still this assumption is much better than that considering butanol part of the apolar volume). We have thus concluded that the majority of the butanol is located at the polar/apolar interface, presumably acting as cosurfactant and modifying (decreasing) the curvature of the polymer from spherical (micellar/cubic structure along the water-polymer axis) to cylindrical (hexagonal structure) in the presence of ≈10 wt % alkanol. The relative size of the different blocks in the copolymer play an important role in determining the phase behavior by affecting the curvature and packing symmetry in the ordered microstructures. When the apparent volume of the soluble blocks (E) is much larger than that of the insoluble blocks (P), spherical assemblies are favored;10 this appears to be the case for the E100P70E100-water system (up to 65 wt % copolymer). The alkanol molecules are most likely anchored with their OH- group at the E-P interface, increasing in the apparent volume of the P blocks relative to that of the E blocks and causing a decrease in the curvature (from spherical to cylindrical). The decrease in the characteristic lengths with increasing butanol concentration can be explained in the same way as that of increasing polymer content. Lamellar Lyotropic Liquid Crystalline Phase (Lr). The lamellar structure in this region was established from SAXS diffraction patterns. A typical SAXS scattering pattern in the LR region is shown in Figure 6. At least two peaks could be resolved with the relative position of 1 and 2. Scattering patterns from both the hexagonal and the lamellar regions (with samples that have an increasing polymer concentration) are presented in Figure 7. Here (32) von Erichsen, L. Brennst.-Chem. 1952, 33, 166.

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Table 1. Results from SAXS Experiments Performed on Various Compositions in the F127-Water-Butanol Systema wt % wt % vol frac vol frac wt % vol frac SAXS apolar apolar lamellar nor hex nor hex rev hex E100P70E100 H2O C4H9OH E100P70E100 H 2O C4H9OH dist (Å) volume 1 volume 2 area/PEO area/PEO 1 area/PEO 2 area/PEO 20 25 20 25 55 60 70 40 45 50 55 65 70 25 30 35 40 44 50 55 60 65 25 30 35 40 45 50 a

55 50 50 45 40 35 25 50 45 40 35 25 20 60 55 50 45 41 35 30 25 20 55 50 45 40 35 30

25 25 30 30 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20

0.182 0.231 0.182 0.228 0.536 0.586 0.686 0.384 0.432 0.481 0.530 0.628 0.678 0.236 0.284 0.332 0.379 0.418 0.476 0.524 0.572 0.621 0.233 0.280 0.328 0.375 0.423 0.470

0.524 0.476 0.470 0.423 0.402 0.352 0.252 0.494 0.445 0.396 0.347 0.249 0.199 0.584 0.536 0.488 0.440 0.401 0.343 0.294 0.246 0.197 0.529 0.482 0.434 0.386 0.338 0.291

0.294 0.293 0.348 0.348 0.062 0.062 0.062 0.122 0.123 0.123 0.123 0.123 0.123 0.180 0.180 0.180 0.181 0.181 0.181 0.182 0.182 0.182 0.238 0.238 0.238 0.239 0.239 0.239

146.4 158.2 174.4 158.2 149.2 143.7 134.8 143.7 141.1 138.6 138.6 129.8 127.9 154.6 146.4 141.4 136.5 136.5 131.0 131.1 127.9 123.9 153.9 144.7 137.7 130.3 126.1 130.3

0.349 0.362 0.403 0.434 0.228 0.244 0.275 0.241 0.257 0.272 0.287 0.318 0.333 0.253 0.268 0.283 0.298 0.311 0.329 0.344 0.359 0.375 0.310 0.325 0.340 0.355 0.370 0.385

0.055 0.069 0.055 0.068 0.166 0.182 0.213 0.119 0.134 0.149 0.164 0.195 0.210 0.073 0.088 0.103 0.117 0.130 0.148 0.162 0.177 0.193 0.072 0.087 0.102 0.116 0.131 0.146

287.2 241.1 216.2 219.2 196.0 192.3 185.8 294.5 274.1 257.9 240.4 227.4 218.9 459.2 414.8 377.2 350.0 323.9 304.6 282.3 270.4 262.5 516.8 467.7 429.7 404.9 378.8 335.7

169.1 167.8 164.9 207.5 199.4 192.6 183.5 179.9 175.7 233.9 229.1 221.9 216.5 207.2 203.2 194.0 190.5 189.0 227.8 226.4 223.8 223.7 219.7 202.8

366.9 345.0 307.4 528.8 472.9 428.0 384.1 338.1 314.3 793.8 690.1 604.9 541.4 487.1 439.6 394.0 365.1 343.0 775.4 678.4 603.0 550.1 498.4 428.0

The different apolar volumes and area per PEO chain is explained in the text.

Figure 5. Characteristic lengths (calculated from eqs 2 and 3 and given in angstro¨ms) shown on the phase diagram for various samples in the lamellar and hexagonal lyotropic liquid crystalline regions.

Figure 7. Differences in the SAXS diffraction patterns between the lamellar and hexagonal liquid crystalline regions in the E100P70E100-butanol-water phase diagram. The figure shows SAXS patterns of samples with 25/30/45, 30/25/45, 35/20/45, 44/15/41, and 55/10/35 wt % E100P70E100-butanol-water compositions.

Figure 6. A slit-smeared SAXS diffraction pattern from a sample in the lamellar liquid crystalline region of the E100P70E100-butanol-water phase diagram (E100P70E100butanol-water: 25/30/45 wt %). The scattering curves are also shown on an expanded intensity scale (right-hand side axis) to expose the second-order peak.

it is possible to observe that the second highest peak changes from the relative position of 2 in the lamellar region to that of x3 in the hexagonal region. The lattice spacing was calculated for the measured samples in this region with eq 2; these values are presented in Table 1;

they are also indicated in the phase diagram of Figure 5. The lamellar spacing increased with decreasing block copolymer concentration. The values for the interfacial area per poly(ethylene oxide) block were calculated from eq 4, assuming φi ) φp + φo, and are presented in Table 1 (see column denoted lamellar area/PEO). The areas per poly(ethylene oxide) block in the LR region are up to 50% higher than those in the H1 region. Alkanol-Rich Solution Phase (L2). X-ray measurements were also used to probe the structure in this region (typical SAXS diffraction patterns are shown in Figure 8). No primary peak was found in the SAXS spectra at butanol concentrations higher than 55 wt %, suggesting that there is no order in the system. However, at butanol concentrations below 55 wt %, a primary peak appeared in the SAXS scattering patterns which got sharper and more intense as the butanol concentration decreased. This indicated the formation of some sort of order in the system,

Phase Behavior of Ternary Systems

Figure 8. SAXS diffraction patterns from samples in the reverse micellar solution region of the E100P70E100-butanolwater phase diagram. The figure shows diffraction patterns of samples with 25/55/20, 35/40/25, and 45/25/30 wt % E100P70E100butanol-water compositions.

Figure 9. Phase diagram of the E100T27E100-butanol-water system at 25 °C. The tie-lines are represented by full straight lines. The dotted line indicates the region where the viscosity increases. The shaded region was not investigated.

presumably that of reverse micelles. No further investigation was undertaken in the L2 region. Effect of Hydrophobic Block Type: the E100T27E100Butanol-Water System. The phase diagram for the E100T27E100-butanol-water system (presented in Figure 9) allows us to examine the influence of hydrophobic block type on the phase behavior of amphiphilic copolymers. The E100T27E100 block copolymer is of similar molecular weight and hydrophilic/hydrophobic weight ratio to the E100P70E100 block copolymer, but it has a more hydrophobic (less polar) middle block (T versus P). While the phase diagram of Figure 9 is similar to that of E100P70E100 with respect to the location of the water-butanol immiscibility region and the slope of its tie-lines, the E100P70E100butanol-water system exhibits a much greater variety of phases. Only a single one-phase region has been observed for the ternary E100T27E100-butanol-water system. This onephase region is extending from the water-rich corner over a middle region to the alkanol-rich corner. In the waterrich corner of the phase diagram the solution contains spherical micelles.18 It has been shown in previous studies18 that the E100T27E100 block copolymer is surface active. The block copolymer forms micelles in water at

Langmuir, Vol. 13, No. 9, 1997 2477

concentrations below 10 wt % polymer and is soluble in water up to at least 60 wt %; the solution viscosity increased with polymer concentration, but no “gels” or cubic structure has been found.18 The viscosity in the one-phase E100T27E100-butanol-water region increases at higher block copolymer contents; in the middle area of the phase diagram, marked in Figure 9 with dotted lines, the viscosity is increasing even more, but without any observed formation of liquid crystalline phases (e.g., cubic, lamellar, or hexagonal). In the alkanol-rich corner, the solution becomes again more fluid. Although no structural transition was discerned in this one-phase region, the topology should change from “oil”-in-water to water-in“oil” over some sort of intermediate region. This transition was not investigated further. The difference in the phase behavior in water-alkanol mixtures of the ostensibly similar E-P-E and E-T-E copolymers is striking. In trying to rationalize it, we consulted phase diagrams of ternary nonionic surfactantalkanol-water systems (for which there are only a few published reports). The E100T27E100-butanol-water, E100T27E100-hexanol-water, and E17T27E17-butanolwater phase diagrams are similar to that of C6H13(CH2CH2O)6OH-octanol-water, where the alkanol is of comparable length to the surfactant tail.33 In all the aforementioned phase diagrams, a one-phase solution region stretches from the water corner to the alkanol corner and no liquid crystalline phases have been observed. The E100P70E100-butanol-water system is found similar to the ternary Triton X-100-decanol-water system34 (even though Triton-X is soluble in decanol while E100P70E100 is not soluble in butanol). Both phase diagrams exhibit a micellar region stretching from the water corner along the water-amphiphile axis and a hexagonal phase at higher amphiphile concentration. An extended liquid crystalline region is located in the middle of both phase diagrams. In the Triton X-100-decanol-water case this is a lamellar phase, while in the E100P70E100-butanolwater system it is a continuation of the hexagonal region and at low surfactant concentration changes to a lamellar structure. The carbon-chain length of the alkanols in those two system is not comparable to that of the apolar group of the amphiphile. Both E100P70E100 and Triton-X have been observed to form liquid crystalline phases in the absence of alkanol (along the water-amphiphile axis). Similar phase behavior as for the E100P70E100-butanolwater system has also been observed in some ionic surfactant-water-alcohol systems.23 These have isotropic solution regions along the surfactant-water axis, at low surfactant concentrations, and isotropic solution regions extending from the alcohol corner. An anisotropic liquid crystalline region is found between these two isotropic solution regions. These emerge from the surfactant-water axis at high surfactant concentrations. The pronounced effect of the hydrophobic block type on the ternary phase behavior (i.e., absence of liquid crystalline order in the E100T27E100-butanol-water system) can be attributed to the (chemical) similarity of 1-butanol to the tetrahydrofuran (n-butylene oxide) segments of the E100T27E100 copolymer. It seems also that the presence (or absence) of liquid crystalline order along the block copolymer-water axis dictates the phase behavior in the presence of “oil”; it is notable that E100T27E100 forms no liquid crystalline structures in the absence of alkanol (along the water-polymer axis). Effect of Hydrophilic (E) Block Size: the E17T27E17-Butanol-Water System. An E-T-E block (33) Mulley, B. A.; Metcalf, A. D. J. Colloid Sci. 1964, 19, 501. (34) Ekwall, P.; Mandell, L.; Fontell, K. Mol. Crys. Liq. Cryst. 1969, 8, 157.

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Figure 10. Phase diagram of the E17T27E17-butanol-water system at 25 °C. The tie-lines are represented by full straight lines. The shaded region was not investigated.

copolymer with shorter poly(ethylene oxide) chains, E17T27E17, was also investigated in a ternary system with butanol and water (see Figure 10), in order to compare it to the E100T27E100-butanol-water system. Contrary to E100T27E100, the E17T27E17 copolymer is soluble in butanol up to approximately 35 wt % but it is not soluble in water at 25 °C. It is, however, possible to dissolve E17T27E17 in water (up to 20 wt %) at 35 °C and then cool it down to 25 °C and obtain a clear solution; these solutions will eventually phase separate (after more than a month). In many aspects the E17T27E17-butanol-water phase diagram is similar to that for E100T27E100-butanol-water: they both exhibit a one-phase region stretching from the water-rich corner to the alkanol-rich corner of the phase diagram and they lack lyotropic liquid crystalline phases. One important difference between the E100T27E100 and the E17T27E17 system is the slope of the tie-lines in the two-phase region. In the E17T27E17-butanol-water system the tie-lines are directed to the water-rich corner while in the E100T27E100-butanol-water system the tie-lines are oriented toward the alkanol-rich corner; the E17T27E17 is enriched in the butanol-rich phase while the E100T27E100 is enriched in the water-rich phase. This confirms that the butanol is a better solvent for the E17T27E17 copolymer than water, while water is a better solvent for the E100T27E100 copolymer than butanol. Effect of Alkanol Molecular Weight: The E100P70E100-Hexanol-Water and E100T27E100-Hexanol-Water Systems. A preliminary ternary phase diagram was prepared for the E100T27E100-hexanol-water system (Figure 11). For the E100P70E100-hexanol-water system we found a micellar phase in the water-rich corner which changes to a cubic phase at higher block copolymer concentration. An isotropic solution region (possibly reverse micellar) is found in the alkanol-rich corner of the phase diagram. Both of these regions are much smaller than the corresponding ones in the E100P70E100-butanolwater system. We were also able to locate a small birefringent area between these phases, but its structure was not investigated. The ternary phase diagram at 25 °C for the E100T27E100hexanol-water system is shown in Figure 12. Similar effects were observed for the E100P70E100 and the E100T27E100 systems when a higher alkanol, hexanol, was introduced: in both cases the two/three-phase region increased and the one-phase region shrank. A single one-phase region

Holmqvist et al.

Figure 11. Phase diagram of the E100P70E100-hexanol-water system at 25 °C. L1 denotes the normal micellar region, I1 the normal micellar cubic region, B a birefringent region, and L2 the alkanol-rich solution region. The shaded region was not investigated.

Figure 12. Phase diagram of the E100T27E100-hexanol-water system at 25 °C. The dotted line indicates the region where the viscosity increases. The shaded region was not investigated.

has been observed for the E100T27E100-hexanol-water system which extended from the water-rich corner to the alkanol-rich corner of the phase diagram. The E100T27E100hexanol-water system showed the same behavior with respect to the change in viscosity at constant copolymer concentration as the butanol system. The change, however, from the fluid region to the more viscous middle region was more pronounced in the E100T27E100-hexanolwater system, and the samples in the middle were more viscous. The samples in this region could still flow, and as for the butanol system, we were not able to identify any liquid crystalline phases. Not surprisingly (given the alkanol-water interactions33), the E100T27E100-hexanolwater system exhibits a two-phase region at low polymer concentrations which is much larger than the E100T27E100butanol-water system. Some of the samples prepared in this two-phase region were milky white. In the E100T27E100-butanol-water system it was possible to get a separation of these samples by means of centrifugation and determine tie-lines. For the E100T27E100-hexanolwater system, though, it has not been possible to separate

Phase Behavior of Ternary Systems

this milky white dispersion either by centrifugation (50000g in 8 h) or by equilibration for more than 12 months at 25 °C. Concluding Remarks A phase behavior and structural investigation of ternary amphiphilic copolymer-alkanol-water systems is presented here. The copolymers are triblocks of similar molecular weight and hydrophilic block [poly(ethylene oxide), E] size but with different hydrophobic blocks [poly(propylene oxide), P, or poly(n-butylene oxide) ) poly(tetrahydrofurane), T]. The alkanol used (butanol) was comparable in terms of chemical composition to the hydrophobic T and P segments. A rich phase diagram was obtained for the polymer with the P middle block, E100P70E100 (Pluronic F127): five onephase regions, normal micellar (L1) and reverse micellar (L2) solutions, and normal micellar cubic (I1), normal hexagonal (H1), and lamellar (LR) lyotropic liquid crystalline regions were detected at 25 °C. The alkanol molecules are most likely anchored with their OH- group at the E-P interface, increasing the apparent volume of the P blocks and thus causing a decrease in the polymer curvature from spherical (along the water-polymer axis) to cylindrical (hexagonal structure) in the presence of ≈10 wt % alkanol. The structure in the I1 region is determined from X-ray diffraction data to be primitive cubic. Only a single one-phase region was observed in the E100T27E100 ternary phase diagram. This region, which extended all the way from the water to the alkanol corner,

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is gradually changing from a micellar solution, at the water corner of the phase diagram, to a structureless solution at the alkanol corner. The pronounced effect of the hydrophobic block type (P versus T) on the ternary phase behavior can be attributed to the chemical similarity between butanol and the T segments and the absence of liquid crystalline order on the water-E100T27E100 axis. The extent of the one-phase regions decreased for both E100T27E100 and E100P70E100 when the alkanol molecular weight increased to hexanol. The E17T27E17-butanol-water phase diagram is similar to that for E100T27E100-butanol-water. In the former, however, the tie-lines are directed to the water-rich corner while in the latter the tie-lines are oriented toward the alkanol-rich corner. This indicates that butanol is a better solvent than water for the E17T27E17 copolymer, while water is a better solvent than butanol for the E100T27E100 copolymer. Acknowledgment. Financial support from the Swedish Natural Science Research Council (NFR) and the Swedish Research Council for Engineering Sciences (TFR) is gratefully acknowledged. The acquisition of the SAXS apparatus was supported from the Swedish Council for Planning and Coordination of Research (FRN). The EnTxEn block copolymers were synthesized and kindly supplied to us by Akzo Nobel Surface Chemistry, Stenungsund, Sweden. LA960819J