Effect of Glycols on the Self-Assembly of Amphiphilic Block

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Effect of Glycols on the Self-Assembly of Amphiphilic Block Copolymers in Water. 2. Glycol Location in the Microstructure Paschalis Alexandridis,*,†,‡ Rouja Ivanova,†,§ and Bjo¨rn Lindman† Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, S-221 00 Lund, Sweden, and Department of Chemical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200 Received September 17, 1999. In Final Form: January 17, 2000 The effects of cosolvents on the microstructure of the lyotropic liquid crystals formed by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in water were examined in ternary isothermal systems consisting of a representative PEO-PPO-PEO copolymer (Pluronic P105, (EO)37(PO)58(EO)37), water, and a polar cosolvent (glycerol, propylene glycol, ethanol, or glucose, referred to as “glycols”). The characteristic length scales of the lyotropic liquid crystals were established using small-angle X-ray scattering (SAXS). The SAXS data are analyzed and discussed with respect to (i) influence of the different glycols and the glycol content on the characteristic length scales, (ii) area occupied by one macromolecule at the polar/apolar interface, (iii) location of the different glycols in the microstructure, and (iv) glycol ability to modify the interfacial curvature through swelling the PEOPPO-PEO macromolecule to different extents. The trends in the glycol effects observed in the ternary phase diagrams (“macroscopic” information) are confirmed and reinforced by the detailed microstructural information obtained here by SAXS. A correlation is found between the glycol effect on the microstructure of the lyotropic liquid crystals (the degree of relative swelling) and the glycol polarity (expressed in terms of octanol/water partition coefficient, dielectric constant or solubility parameter).

Introduction Amphiphiles, i.e., substances such as surfactants, lipids, or block copolymers that combine moieties with different affinities, hydrophilic (or more generally solvophilic) and hydrophobic (or solvophobic), find many diverse industrial applications1-6 due to their unique property to selfassemble in solution or at interfaces. In many of these applications various polar, apolar, or mixed solvents are used in addition to water. Therefore, the fundamental understanding of the mechanism of the solvent effects on the amphiphile self-assembly is essential. Although intermolecular interactions of the same nature are the driving force for the self-assembling behavior of the low-molecular-weight surfactants and the macromolecular amphiphiles, the latter show much richer structural polymorphism. While in the surfactant systems simple geometrical constraints fix the preferred curvature, and thus limit the variety of the microstructures formed, in the macromolecular amphiphiles the preferred curvature is much more flexible due to the ability of the different blocks of the macromolecule to swell to different extents with selective solvents.7-11 Amphiphilic block copolymers, * To whom correspondence should be addressed at the State University of New York. E-mail: [email protected]. † Lund University. ‡ The State University of New York. § On leave from the Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria. (1) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490501. (2) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1997, 2, 478489. (3) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain, 2nd ed.; Wiley-VCH: New York, 1999. (4) Lawrence, M. Chem. Soc. Rev. 1994, 417-424. (5) Edens, M. Surf. Sci. Ser. 1996, 60, 185-210. (6) Paavola, A.; Yliruusi, J.; Rosenberg, P. J. Controlled Release 1998, 52, 169-178.

such as poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), are able to selfassemble in the presence of solvents selective for the copolymer blocks into a large variety of lyotropic liquid crystalline phases with lamellar, hexagonal, or (micellar or bicontinuous) cubic structure with either oil-in-water (“normal”) or water-in-oil (“reverse”) morphology.7,12 The ability to utilize, modify, and predict the solvent effects is very important for the systems of macromolecular amphiphiles. This task, however, turns out to be complex. The problem of the relationship between the properties of the solvents and their physicochemical parameters has attracted many theoretical and computer simulation investigations.13-21 Despite the significant advances that (7) Alexandridis, P.; Olsson, U.; Lindman B. Langmuir 1998, 14, 2627-2638. (8) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700-7710. (9) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 26902700. (10) Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788-6797. (11) Svensson, B.; Alexandridis, P.; Olsson, U. J. Phys. Chem. B 1998, 102, 7541-7548. (12) Olsson, U.; Wennerstro¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113. (13) Abraham, M.; Grellier, P.; McGill, A. J. Chem. Soc., Perkin Trans. 2 1988, 339-345. (14) Dutkiewicz, M. J. Chem Soc., Faraday Trans. 1996, 92, 637640. (15) Giesen, D. J.; Hawkins, G. D.; Liotard, D. A.; Cramer, C. J.; Truhlar, D. G. Theor. Chem. Acc. 1997, 98, 85-109. (16) Huibers, P.; Katritzky, A. J. Colloid Interface Sci. 1997, 193, 132-136. (17) Huibers, P.; Katritzky, A. J. Chem. Inf. Comput. Sci. 1998, 38, 283-292. (18) Katritzky, A.; Tamm, T.; Wang, Y.; Karelson, M. J. Chem. Inf. Comput. Sci. 1999, 39, 692-698. (19) Svensson, M.; Alexandridis, P.; Linse, P. Macromolecules 1999, 32, 637-645. (20) Svensson, M.; Alexandridis, P.; Linse, P. Macromolecules 1999, 32, 5435-5443.

10.1021/la9912343 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/02/2000

Self-Assembly of Block Copolymers

have been made in the last years in predicting solvent properties (such as cloud point and solubility16,17) using molecular descriptors (parameters based on the molecular structure and/or basic physicochemical parameters), none of the existing theories can predict the cosolvent effects on the amphiphile self-assembly. Usually in order to obtain a good correlation between the solvent property and its molecular structure, a large number of molecular descriptors needs to be involved even for simple systems. However, the systems used in practical applications are rarely simple two- or three-component systems, but more often complicated multicomponent systems. This raises even further the complexity of the problem. Systematic experiments carried out in simple binary and ternary systems should be useful in laying down the basis for a more thorough understanding of the role of the cosolvents on the amphiphile self-assembly. The results of recent studies on the phase behavior and microstructure of a representative PEO-PPO-PEO block copolymer in the presence of a nonaqueous polar solvent (formamide)22,23 and of the few available related investigations of surfactant systems in nonaqueous polar solvents24-27 show that in both cases micelles start to form at considerably higher amphiphile concentrations and are considerably smaller than those formed in water. However, contrary to the surfactant systems, where fewer microstructures within narrower regions have been obtained in nonaqueous polar solvents, the variety of the microstructures formed by the PEO-PPO-PEO block copolymer in water has been preserved and even enhanced in formamide.22 In general, the effects of cosolvents (organic polar or apolar solvents) and cosolutes (such as inorganic salts) on the critical micellization concentration or cloud point for ionic and nonionic surfactants24,25 as well as amphiphilic copolymers28-35 have been attributed to changes in the water solvent quality. The effects of polyols (e.g., glycerol, propylene glycol, 1,3-butanediol) on the phase behavior and microstructure of alkyl-oligo(ethylene oxide) ether surfactants in water have been studied using small-angle X-ray scattering.36,37 The observed changes (increase or decrease) of the lattice spacing in the hexagonal liquid crystalline phase upon the addition of polyols have been explained by a dehydration/hydration of the poly(ethylene oxide) chain of the surfactants and consecutive decrease/ increase of the surfactant hydrophilicity. (21) Huuskonen, J.; Villa, A.; Tetko, I. J. Pharm. Sci. 1999, 88, 229233. (22) Alexandridis, P. Macromolecules 1998, 31, 6935-6942. (23) Alexandridis, P.; Yang, L. Langmuir, in press. (24) Sjo¨berg, M.; Wa¨rnheim, T. Surf. Sci. Ser. 1997, 67, 179-205. (25) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424-431. (26) Martino, A.; Kaler, E. Colloids Surf., A 1995, 99, 91-99. (27) Friberg, S. E.; Liang, Y. C.; Lockwood F. E. J. Dispersion Sci. Technol. 1987, 8, 407-422. (28) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442-2450. (29) Cheng, Y.; Jolicoeur, C. Macromolecules 1995, 28, 2665-2672. (30) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 60746082. (31) Armstrong, J.; Chowdhry, B.; Mitchell, J.; Beezer, A.; Leharne, S. J. Phys. Chem. 1996, 100, 1738-1745. (32) Jo¨rgensen, E.; Hvidt, S.; Brown, W.; Schille´n, K. Macromolecules 1997, 30, 2355-2364. (33) Alexandridis, P.; Andersson, K. J. Colloid Interface Sci. 1997, 194, 166-173. (34) Alexandridis, P.; Spontak, R. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 130-139. (35) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657-667. (36) Iwanaga, T.; Suzuki, M.; Kunieda, H. Langmuir 1998, 14, 57755781. (37) Aramaki, K.; Olsson, U.; Yamaguchi, Y.; Kunieda, H. Langmuir 1999, 15, 6226-6232.

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A suitable system to probe in a systematic way the effect of cosolvents is offered by the PEO-PPO-PEO macromolecular amphiphiles due to the great potential for modification of their preferred curvature by solvents. The present study extends our previous investigation10 on the role of apolar cosolvents on the phase behavior of PEOPPO-PEO block copolymers to address the effects of various polar cosolvents. The polar cosolvents examined here are glycerol, propylene glycol, ethanol, and glucose, which will be referred to for simplicity as “glycols”. The objective is to study the effect of glycols on the PEOPPO-PEO block copolymer phase behavior and microstructure and to quantify it in terms of the glycol physicochemical parameters. The ultimate goal is to be able to predict the effects of cosolvents. In the first part of this investigation, the results on the general isothermal ternary phase behavior (“macroscopic” information) were presented.38 The present report is an in-depth analysis of the role of glycols on the microstructure (“microscopic” information). The glycol effects on the microstructure are analyzed and related to changes in the interfacial area, the preference of the different glycols to locate in different microdomains, and the glycol ability to modify the interfacial curvature by swelling different blocks of the macromolecule to different extents. It is shown that the microscopic information obtained from the analysis of the small-angle X-ray scattering (SAXS) data confirm and reinforce the macroscopic information about the changes in the interfacial curvature and the swelling of the copolymer blocks obtained on the basis of the general phase behavior in the first part of this study.38 A correlation is made between the glycol effect on the microstructure of the lyotropic liquid crystals and the glycol polarity. Materials and Methods Materials. The poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer, Pluronic P105, was obtained as a gift from BASF Corp. and used as received. According to the manufacturer39 it has molecular weight of 6500 and 50 wt % content of poly(ethylene oxide). On the basis of these data, Pluronic P105 can be represented as (EO)37(PO)58(EO)37. Glycerol (p.a.) was purchased from Prolabo, Fontenay, France. Propylene glycol (1,2-propanediol) (p.a.) and D(+)-glucose monohydrate (g99%) were purchased from Fluka Chemie AG, Buchs, Switzerland. Ethanol (spectrographic grade, g99.5%) was purchased from Kemetyl AB, Haninge, Sweden. The structural formula of Pluronic P105 as well as of the glycols studied are shown in Figure 1. Some basic physicochemical properties of the glycols are given in Table 1.40-43 Judging from their dielectric constants and dipole moments, glycerol, propylene glycol, ethanol, and glucose are all hydrophilic. The notation “glycols”, introduced to refer to all three polar cosolvents (glycerol, propylene glycol, and ethanol) and glucose, emphasizes their polarity and chemical classification as polyols. The ternary isothermal phase diagram for each system studied was drawn using first inspection under polarized light to distinguish between the one-phase regions with different (38) Ivanova, R.; Lindman B.; Alexandridis, P. Langmuir 2000, 16, 3660. (39) Pluronic and Tetronic Block Copolymer Surfactants; Technical Brochure; BASF Corp., 1989. (40) Handbook of Chemistry and Physics, Weast, R., Ed.; CRC Press: Cleveland, OH, 1974. (41) McClellan, A. L. Tables of Experimental Dipole Moments; Rahara Enterprises: El Cerrito, 1974. (42) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-616. (43) Hansen, Ch. Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: New York, 1997; Chapter 10.

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Figure 1. Chemical structure of Pluronic P105 and the glycols used in this study.

microstructures and then SAXS to verify the phase boundaries. Details in the sample preparation procedure, the determination of the phase diagrams, and the SAXS measurements are presented in the companion paper.38 Characteristic Parameters of the Microstructure. The characteristic length scales of the liquid crystalline phases were established through analysis of the SAXS diffraction patterns. The positions of the Bragg diffraction peaks were evaluated from the desmeared spectra (the desmearing was done according to the direct method of beam-height correction44). Characteristics of the microstructure are the lattice parameter, d, in the lamellar structure and the distance between the planes of the centers of two adjacent rows of cylinders, d, in the hexagonal structure (see Figure 2). In the lamellar and the hexagonal structure the lattice parameter (lattice spacing) is given directly by the position, q*, of the first and most intense diffraction peak:

d ) 2π/q*

(ii) the volume fraction of the apolar domain. The definition of these parameters turns out not always straightforward, and hence, assumptions are necessary to be made. In the following analysis we assume that (i) the copolymer is the only surface-active component in all ternary systems and, therefore, the interfacial volume fraction is equal to the copolymer volume fraction and (ii) the polar domains consist of the PEO blocks, water and glycol, while the apolar domains consist only of the PPO blocks of the copolymer, and therefore, the apolar volume fraction is equal to the volume fraction of the PPO block of the copolymer. The interfacial volume fraction is equal to the volume fraction of the copolymer (as it was assumed) only in the case that the only amphiphilic (i.e., surface-active) component in the system is the copolymer itself. If some of the other components present exhibit surface-active behavior, this assumption will not hold. The second surface-active component can participate in the formation of the interface together with the copolymer, as our earlier study10 indicates and (as will be shown in the discussion section) the present results suggest. The effective volume of one macromolecule can also vary, if in the system are present components interacting strongly with the copolymer and thus changing the volume of the different blocks. The assumption made above concerning the components that constitute the polar and apolar domains is equivalent to strong segregation. An alternative and more realistic assumption, with the same consequence in our analysis, is that of weak segregation with the amount of PEO in the apolar domains comparable to the amount of PPO in the polar domains. As will be shown in the discussion section, the assumption that the entire glycol amount participates in the polar domains does not hold for all of the glycols. The results obtained within these assumptions can serve as criteria to prove the assumptions true as well as an indication for the location of the glycols in the microstructure. In the lamellar structure, the calculation of ap involves only the assumption concerning the interfacial volume fraction. In the normal hexagonal structure, this calculation involves also the second assumption, concerning the components constituting the polar and apolar domains. According to the assumptions made above, the apolar volume fraction, f, is given by

(1)

The interfacial area per PEO block, ap, or the area that a PEO block of the PEO-PPO-PEO block copolymer occupies at the interface between polar and apolar domains, is another characteristic parameter of the microstructure providing valuable information on the packing of the macromolecules. The interfacial area in each microstructure is related to the lattice parameter through simple expressions which can be easily derived using geometrical considerations for the volume occupied by the micelles that constitute the microstructure based on the micellar geometric form (spheres, cylinders, or lamellae). Here only the final equations will be given (for details see refs 8 and 45). To calculate the interfacial area, parameters other than the lattice parameter have to be known, including (i) the interfacial volume fraction, i.e., the volume fraction of the components constituting the polar/apolar interface, and (44) Singh, M.; Ghosh, S.; Shannon, R. J. Appl. Crystallogr. 1993, 26, 787. (45) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23-34.

f ) 0.54Φp

(2)

where Φp is the volume fraction of the copolymer in the ternary system and 0.54 is the volume fraction of the PPO block in the P105 macromolecule (the PPO weight fraction is 0.50, which makes up ∼0.54 volume fraction). The volume of one macromolecule is υp ≈ 10 300 Å3 for P105. To convert the weight fractions of the components into volume fractions, the bulk densities of the copolymer (1.05 g/cm3) and water and glycols (given in Table 1) are used. The interfacial area can be calculated from

lamellar:

hexagonal:

ap )

υp dΦp

ap )

( )

υp πx3 f dΦp 2

1/2

(3)

The dimensions of the apolar domains, i.e., the apolar layer thickness, δ, in the lamellar structure and the radius


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Table 1. Physicochemical Parameters of the Glycols Examined Here solvent

MWa

mpb

bpc

water glycerol glucose propylene glycol ethanol

18.01 92.10 180.16 76.10 46.07

0 18 150 -60 -114

100 290 188 78

densityd (g/cm3)

dielectric constantd

dipole momente

0.9982 1.2567 1.5620 1.0327 0.7873

78.5 40.1

3.11 2.68 14.1

32.0 24.3

1.69

log Pf -2.55 -3.29 -1.41 -0.32

solubility parameterg (MPa1/2) 47.8 36.2 30.2 26.5

a Molecular weight. b Melting point. c Boiling point. d The density and the dielectric constant of solvents are given at 25 °C. The data are taken from ref 40. e The data are taken from ref 41. f Logarithm of the octanol/water partition coefficient. The data are taken from ref 42. g The data are taken from ref 43.

Figure 2. Schematic of the lattice parameter, d, in the lamellar structure (LR) and the distance between the planes of the centers of two adjacent rows of cylinders, d, in the hexagonal structure (H1) obtained from SAXS measurements. Denoted are also the radius of the apolar domains in the hexagonal structure, R, and the apolar layer thickness in the lamellar structure, δ. The shaded areas represent the apolar domains. For clarity, only copolymer molecules at the cross section of the planar and cylindrical micelles are shown.

of the apolar domains, R, in the hexagonal structure, can be determined using

lamellar: hexagonal:

δ ) df

( )

R)d

2 f πx3

1/2

(4)

The analysis and discussion of the trends in the glycol effects on the microstructure presented here are based on the characteristic length scales calculated in the hexagonal and lamellar structures. The calculations of the lattice parameter and the interfacial area in the cubic liquid crystalline phases and the establishment of the crystallographic space group of the cubic lattice (the values of both parameters depend on the crystallographic space group) are given in detail in the companion paper.38 Results A. Effect of Glycols on the Phase Behavior of PEOPPO-PEO Block Copolymers. In the first part of this study, “macroscopic” information on the role of glycols as cosolvents on the self-assembly of the PEO-PPO-PEO block copolymers has been obtained from a study of the ternary isothermal phase diagrams of a representative copolymer (Pluronic P105), water, and each of the four glycols (glycerol, propylene glycol, glucose, and ethanol).38 A pronounced effect of the glycol type on the concentration range of stability of the different lyotropic liquid crystalline structures was found in the systems studied. The ternary isothermal (25 °C) phase diagrams of Pluronic P105 in the presence of water and glycol are presented in Figure 3. A similarity is observed between the effects of glycerol and glucose (Figure 3, left column) in terms of swelling of the hexagonal structure to lower copolymer contents with increasing the glycol content, and in terms of the L1-I1 boundary occurring at constant copolymer content (parallel to the water-glycol axis). Another similarity is observed between propylene glycol and ethanol (Figure 3,

right column) in terms of deswelling of the micellar cubic structure to higher copolymer contents with increasing the glycol content (a bend of the L1-I1 boundary toward the glycol-copolymer axis). These opposing effects are reinforced by the appearance of a bicontinuous cubic phase only in the cases of propylene glycol and ethanol and by the different effects of the glycols on the solution region as well. The trends in the glycol effects, observed on the basis of the phase boundaries, can be considered as indications for the location of the glycols in the microstructure and the degree of swelling of the copolymer blocks, since they reflect changes of the interfacial curvature in the system. The interfacial curvature is defined as positive when the interface bends toward the apolar domains and as negative when the interface bends toward the polar domains.3 The interfacial curvature is zero in the lamellar structure. The transition from lamellar to normal hexagonal to normal micellar cubic structure, observed along the Pluronic P105-water axis with increasing the water content is a result of increasing of the interfacial curvature: It changes from zero (planar micelles) to positive (cylindrical micelles) to highly positive (spherical micelles). This reflects changes in the swelling of the PEO block: An increase in the interfacial curvature is a result of increased swelling of the PEO block compared to the PPO block, and vice versa. Therefore, only from the general appearance of the ternary phase diagrams (“macroscopic” information) a conclusion about the changes in the interfacial curvature, the degree of swelling of the different copolymer blocks, and, hence, the location of the glycols in the microstructure can be drawn. The bend of the hexagonal structure toward the water-glycol axis at higher glycol contents in the cases of glycerol and glucose (Figure 3), i.e., H1 takes preference over I1 (the micellar geometry changes from spheres to cylinders) at constant copolymer content, is an indication of decreasing the interfacial curvature due to decreasing the swelling of the PEO block. On the contrary, in the case of propylene glycol the phase boundaries of the

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Figure 3. Isothermal (25 °C) ternary phase diagrams of Pluronic 105, water, and glycol: (top left) P105-water-glycerol system; (top right) P105-water-propylene glycol system; (bottom left) P105-water-glucose system; (bottom right) P105-water-ethanol system. L1 denotes the ternary composition region of liquid clear isotropic solution. I1 denotes the region where the samples are clear isotropic gels and the microstructure is that of spherical micelles arranged in a cubic lattice. H1 denotes the region where the samples are clear birefringent gels and the microstructure is that of cylindrical micelles arranged in a hexagonal lattice. V1 denotes the region where the samples are clear isotropic (hard) gels and the microstructure is that of interconnected (bicontinuous) cylindrical micelles arranged in a cubic lattice. LR denotes the region where the samples are clear birefringent (soft) gels and the microstructure is that of lamellae-planar micelles. The composition is given in weight fractions. The boundaries of the one-phase regions are drawn with solid lines. The lattice parameter obtained by SAXS is given at the corresponding compositions.

hexagonal structure are at constant copolymer content (preserving the same liquid crystalline microstructure at constant copolymer) which implies constant interfacial curvature. B. Influence of Glycols on the SAXS Lattice Spacing. To study the mechanism of the effect of the glycols on the PEO-PPO-PEO block copolymer phase behavior at a molecular level, in addition to the “macroscopic” information obtained from the ternary phase diagrams, “microscopic” information for the glycol effects on the lyotropic liquid crystalline microstructures formed was obtained based on the SAXS characteristic length scales. Systematic SAXS measurements were performed on a series of samples in the liquid crystalline phases in each Pluronic P105-water-glycol system. The lattice

parameter for each sample measured in the hexagonal and lamellar structures was obtained directly from the relative position of the first peak of the SAXS diffraction patterns using eq 1. The values of the lattice parameter for the corresponding compositions are presented in Figure 3 for the four P105-water-glycol systems studied. The trends in the lattice parameter will be discussed in the following directions: (i) at constant glycol content, i.e., probing the effect of the copolymer content, and (ii) at constant copolymer content, i.e., probing the effect of the glycols. Effect of the Copolymer Content at Constant Glycol Content. Figure 3 shows that at constant glycol content (along lines parallel to the copolymer-water axis) the lattice parameter decreases with increasing the


Figure 4. Lattice parameter, d, at 0% glycol plotted versus the copolymer volume fraction, Φp (top), and log d versus -log Φp (bottom). The different symbols correspond to different liquid crystalline structures: circles to hexagonal structure, H1, and squares to lamellar structure, LR. The line in the bottom graph represents the expected dependence for an ideal swelling (d ∼ Φp-1).

copolymer concentration. The data for the lattice parameter in the absence of glycol as a function of the copolymer volume fraction are shown in Figure 4 (top). At 30 and 40 wt % P105 in the hexagonal structure and 70 wt % P105 in the lamellar structure, information about the lattice parameter in the absence of glycol could not be obtained directly because these samples were not within the corresponding one-phase regions. At these copolymer concentrations, an extrapolation using the linear fit of the data at higher glycol contents to zero glycol content was done (see the discussion of Figure 5). The estimated values at 0% glycol from the fits for the different glycols have shown very good agreement and were further used to calculate the relative lattice spacings given in Figures 6 and 7. The trend of decreasing d with increasing the copolymer concentration at constant glycol content is general. It is observed throughout the whole phase diagram regardless of crossing the boundaries of the different liquid crystalline phases and is preserved when a part of the water is replaced by any of the four glycols (see Figure 3). This effect is expected, and it originates from the need the increased copolymer content to be accommodated in the microdomains of the liquid crystalline structures. Hence, more structural elements (i.e., micelles, cylinders, or lamellae) are formed, and as a consequence, the distance between these structural elements (the lattice parameter) decreases. The scaling law describing the swelling of the liquid crystalline structures was tested by plotting log d versus

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log Φp for the lattice parameter in the absence of glycol (Figure 4, bottom). The experimental values are given with points. The line represents the dependence expected for an ideal (one-dimensional) swelling: the ratio of the lattice parameter to the amphiphile volume fraction remains constant in the swelling process (the exponent in the function log d ) f(-log Φp) is 1), and only the interdomain spacing is affected. Such “ideal” swelling behavior has been reported for low-molecular-weight surfactants.3 Figure 4 shows that the behavior of the PEOPPO-PEO block copolymers, even in simple binary system with water, is far from the ideal swelling and indicates that both interdomain and intradomain spacings vary in the swelling, which implies strong interaction between the copolymer blocks and water. Effect of the Glycols at Constant Copolymer Content. The lattice parameter plotted as a function of the glycol content in the hexagonal and lamellar liquid crystalline phases at fixed copolymer content is shown in Figure 5. Data for four different glycols (glycerol, propylene glycol, glucose, and ethanol) are shown on the same graph. The graphs on the left column correspond to the lamellar, LR, structure and the graphs on the right column correspond to the hexagonal, H1, structure. The following trends are observed in Figure 5: Increasing glycerol or glucose content leads to increasing the lattice parameter, whereas propylene glycol and ethanol act toward decreasing the lattice parameter. Moreover, for one and the same ternary composition, the effect of glucose in increasing the lattice parameter is stronger than that of glycerol. The same is true for ethanol, which shows a stronger effect than propylene glycol. The increase (decrease) of the lattice parameter appears to be a linear function of the glycol content. The lines represent the best linear fit passing through the value of the lattice parameter measured in the absence of glycol. An extrapolation of the data to 0% glycol content at 40 and 70 wt % copolymer is done to estimate the lattice parameter in the absence of glycol. The tendency of increasing the lattice parameter with increasing glycerol or glucose content and the opposite trend in the cases of propylene glycol and ethanol are observed in all liquid crystalline phases and at all copolymer concentrations. At a given glycol content, the glycol effect is stronger at lower copolymer concentrations. For example, in the lamellar structure, replacement of part of the water with glycerol or glucose only slightly affects the lattice parameter, while in the hexagonal structure a more definite increase of the lattice parameter (glucose more so than glycerol) is observed. This observation holds for the absolute values of the lattice parameter (Figure 5) as well as for the “double normalized” plots presented below (Figure 6). As shown above, two different effects influence the lattice parameter: the effect of the copolymer content and that of the glycols. The effect of the glycols, as presented in Figure 5, though informative for the absolute values of the lattice parameter, masks the effect of the copolymer content and is combined with a side effect originating from the different glycol densities. The copolymer volume fraction changes upon replacing water by glycol, although the copolymer weight fraction is kept constant, due to the higher than water density of glycerol and glucose (lower than water density of ethanol). However, these variations are small, within few percent. The replacement of water by propylene glycol practically does not change the copolymer volume fraction. To decouple the glycol effects from those of the copolymer content, the effects of the different glycols are

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Figure 5. Effect of glycols on the lattice parameter plotted versus the glycol content at fixed copolymer content for four different glycols (glucose, glycerol, propylene glycol, and ethanol). The graphs on the left column correspond to the lamellar, LR, structure, at different copolymer contents: (top) 80%; (middle) 75%; (bottom) 70%. The graphs on the right column correspond to the hexagonal, H1, structure, at different copolymer contents: (top) 60%; (middle) 50%; (bottom) 40%. The lines represent the best linear fit by the least-squares method of the data points (the expected linear dependence in the cases with only one data point is denoted by a dashed line).

presented in Figure 6 as “double normalized” plots: The variation of the lattice parameter at different glycol contents with respect to that in the absence of glycol is plotted versus the glycol volume fraction relative to the volume fraction of the glycol + water solvent mixture. This presentation helps to account for the side effect of the density differences and allows comparison of the net effects of the different glycols. The data in Figure 6 are presented in the same layout as in Figure 5. In addition to the observation that glucose and ethanol have a much stronger effect on increasing and decreasing the lattice parameter than glycerol and propylene glycol,

respectively, Figure 6 shows that the lattice parameter in the presence of glycol can vary no more than about +20% or -20% from that in the absence of glycol (copolymerwater binary system). These values represent the approximate limits within which the lattice parameter can vary while preserving the same microstructure and reveal the extent to which the macromolecule can deform under external stresses imposed by changes in the solvent conditions. Therefore, glycols which influence the lattice parameter stronger (here, glucose and ethanol) will be able to maintain the microstructure up to lower glycol/ solvent mixture ratios. This finding correlates very well with the macroscopic observations in the ternary phase


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Figure 6. Percent variation of the lattice parameter at different glycol contents with respect to that at 0% glycol, plotted versus the percent glycol volume fraction relative to the volume fraction of the glycol + water solvent mixture. Data at constant copolymer content for four different glycols (glucose, glycerol, propylene glycol, and ethanol) are shown on the same graph. The graphs on the left column correspond to the lamellar, LR, structure, at different copolymer contents: (top) 80%; (middle) 75%; (bottom) 70%. The graphs on the right column correspond to the hexagonal, H1, structure, at different copolymer contents: (top) 60%; (middle) 50%; (bottom) 40%. The lines represent the best linear fit by the least-squares method of the data points (the expected linear dependence in the cases with only one data point is denoted by a dashed line).

diagrams: The liquid crystalline structures break up at approximately the same value of the lattice parameter, although at different glycol contents for the different glycols (Figure 3, see also ref 38). The microstructure is destabilized when the changes in the lattice parameter exceed (20%. The values of the lattice parameter measured in the micellar cubic structure at 30 wt % P105 are in good agreement to the data at the same copolymer concentration in the hexagonal structure (both measured and estimated) (Figure 7). Again, the lines represent the best linear fit.

An extrapolation of the data to 0% glycol content in the hexagonal structure was done in a similar way as in Figure 5. The data in the micellar cubic phase follow the trends observed in the hexagonal and lamellar structures. In fact, this agreement in the lattice parameter and especially in the interfacial area per PEO block (see the discussion section, Figure 8) has been the main argument in favor of the assignment of the normal micellar cubic lattice in the cases of glycerol, glucose, and propylene glycol to primitive structure (for the crystallographic analysis see ref 38).

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Figure 7. Comparison between the lattice parameter in the micellar cubic (left column) and hexagonal (right column) liquid crystalline structures at 30 wt % P105. Both absolute values (as in Figure 5) and normalized (as in Figure 6) are given. The lines represent the best linear fit by the least-squares method of the data points (the expected linear dependence in the cases with only one data point is denoted by a dashed line).

Figure 8. Interfacial area per PEO block (at 0% glycol content) plotted as a function of the copolymer volume fraction. The different symbols correspond to different liquid crystalline structures: diamonds to (primitive) micellar cubic structure, I1, circles to hexagonal structure, H1, and squares to lamellar structure, LR. The line represents the best fit (by the leastsquares method) of all data points for the interfacial area as a linear function of the copolymer volume fraction.

Discussion A. The Role of Glycols on the Block Copolymer Microstructure: Interfacial Area and Location of the Glycols. The similarity between the effects of glycerol and glucose opposing those of propylene glycol and ethanol observed on the basis of the “macroscopic” information on the general phase behavior obtained from the ternary phase diagrams38 is confirmed and supported by the trends discussed on the basis of the microstructural information obtained from the SAXS data. Glycerol and glucose

increase the lattice spacing of the liquid crystalline structures with increasing the glycol content, while propylene glycol and ethanol decrease it. The increase of the lattice parameter resulting from the introduction of glycerol or glucose can be attributed to a decrease of the interfacial area per PEO block copolymer related to a contraction (deswelling) of the PEO blocks by glycerol or glucose. Macroscopically, this is expressed as a bend in the hexagonal structure along the direction of the waterglycol axis and preference of H1 over I1 at higher glycol contents (see Figure 3). The decrease in the lattice parameter resulting from the introduction of propylene glycol or ethanol can be attributed to an increase of the interfacial area per block copolymer related to swelling of the PEO block or both the PEO and PPO blocks by propylene glycol or ethanol. As alluded to above, in addition to the lattice parameter, the interfacial area per PEO block can provide valuable information on the packing of the macromolecules and gives a basis for a deeper understanding of the cosolvent effects at a molecular level. The interfacial area is connected to the lattice parameter through simple geometrical relations. However, while the lattice parameter in the hexagonal and lamellar structure is obtained directly from the experimental data (i.e., without any assumptions), the calculation of the interfacial area involves some assumptions (which were discussed in the “materials and methods” section). Despite these assumptions or rather due to them, the interfacial area has proved useful in the establishment of the microstructure22,38 and in the interpretation of the location of the cosolvents.10,22 It has been already emphasized that the interfacial area can be used as a criterion for the accuracy of the


assumptions made: obtaining self-consistent values for the interfacial area justifies the assumptions made. The interfacial area in the lamellar and hexagonal microstructure was calculated using eq 3; for the calculation of the interfacial area in the cubic microstructures, see ref 38. The interfacial area in the simpler case of nonionic ethylene oxide containing low-molecular-weight surfactants is determined by the ethylene oxide chain.12 The interfacial area is found not to vary significantly, and the changes are commonly attributed only to changes in the degree of hydration/dehydration (swelling) of the ethylene oxide chain, e.g.36 In the case of amphiphilic PEO-PPOPEO copolymers, the interfacial area depends on the ratio between the polar (PEO containing) and the apolar (PPO containing) domains, rather than on the PEO block alone. However, it is expected that the interfacial area in copolymer systems will also be invariant provided the assignment made of the polar and apolar domains is correct. Indeed, the interfacial area has been shown to vary only slightly (within 10%) in many systems of PEOPPO-PEO block copolymers, water, and xylene, e.g.7,10,11 Such invariance of the calculated interfacial area proves correct the definition of the interface and the apolar volume fraction. In the cases where a more significant (up to 200%) change in the interfacial area has been found, a redefinition of these parameters had to be made.10 The interfacial area per PEO block in the absence of glycol is presented in Figure 8 as a function of the copolymer volume fraction. The figure shows a decrease in the interfacial area, i.e., at constant glycol content in addition to the decrease in the lattice parameter needed to accommodate the increased copolymer concentration, further increase takes place in the packing of the macromolecules by decreasing the area per PEO block. The interfacial area throughout all liquid crystalline structures appears to be a linear function of the copolymer volume fraction. However, the variations of the interfacial area within each liquid crystalline microstructure are small (12% in H1 and 8% in LR). This confirms that in the binary copolymer-water system the assumptions made in the calculation of ap are correct; i.e., only the PPO block participates in the apolar volume fraction and only the copolymer is interfacially active. This result is not unexpected, since water is a solvent selective for the PEO blocks of the copolymer but not for the PPO blocks: The increase of the copolymer content on the expense of the water content leads to decreasing the swelling of the PEO block and, hence, decreasing the interfacial area. The data for the interfacial area in the absence of glycol presented in Figure 8 give the necessary basis for a better understanding of the more complex trends observed in the interfacial area in the presence of glycols. Figure 9 shows the results for the interfacial area per PEO block as a function of the percent glycol volume fraction relative to the volume fraction of the glycol + water solvent mixture. Increasing glycerol or glucose volume fraction in the solvent mixture decreases the interfacial area, while increasing propylene glycol or ethanol content increases it. As was noted, the variations in the interfacial area per PEO block are expected not to be large. While this is true for glycerol and glucose (the interfacial area varies no more than 12% within each microstructure), in the cases of propylene glycol and ethanol the area per PEO block changes more significantly. Therefore in the cases of glycerol and glucose the assignment of the polar/apolar domains is correct. Glycerol and glucose are located inside the polar microdomains, do not participate in the formation of the interface, and do not

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contribute to the swelling of the PEO blocks. In fact, the closed areas in the graphs for glycerol and glucose in Figure 9 can be considered as another representation of the H1 and LR phase boundaries of their phase diagrams. They represent the limits regarding the values of the interfacial area and the volume ratios of the glycol/solvent mixture within which the hexagonal and the lamellar microstructures are preserved. The slight decrease of the interfacial area upon replacing water with glycerol or glucose in the solvent mixture is due to a decrease of the swelling of the PEO blocks, as was inferred from the preference of the hexagonal over the micellar cubic structure at higher glycol content. Ethanol and propylene glycol show opposite and somewhat stronger effects than glycerol or glucose: The interfacial area varies more significantly (Figure 9). Therefore, the assignment of the apolar domains as including only the PPO block may not be correct in these two cases, and the glycols may have preference to locate in the apolar domains. However, an attempt to revise the definition of the apolar domains and to include part of the glycol in the apolar volume fraction leads to even higher interfacial areas per PEO block. A possible explanation is that the introduction of ethanol or propylene glycol counterbalances the deswelling of the PEO blocks upon decrease of the water content in the solvent mixture and even further increases the interfacial area by swelling both PEO and PPO blocks. In these two cases, in fact, the assumption that the interfacial volume fraction is equal to the copolymer volume fraction does not hold anymore since part of the glycol (that swelling the two copolymer blocks) will participate in the formation of the interface as well. For a correct calculation of the interfacial area, a higher apolar volume fraction (including part of the glycol) and a higher “effective” copolymer volume fraction are necessary (see ref 10 for such a case). Another indication of the swelling effect of propylene glycol and ethanol is the formation of bicontinuous cubic structure. It is known that such bicontinuous structures are favored close to the order-disorder transition (weak segregation state).46 The appearance of the bicontinuous cubic structure only in the cases of propylene glycol and ethanol suggests that these glycols weaken the degree of block segregation, which is initially induced by water, by swelling both PEO and PPO blocks. The effects of propylene glycol and ethanol on the lattice parameter and the interfacial area found here are similar to those obtained for PEO-PPO-PEO block copolymers in a binary system with formamide22 and in a ternary system with water and butanol.10 The conclusions for the locations of the glycols, made on the basis of the analysis of the interfacial area, are supported by the results for the radius of the apolar domains and apolar layer thickness calculated using eq 4 (i.e., within the assumptions made, see the “materials and methods” section). The radius of the apolar domains and the apolar layer thickness at 0% glycol content as well as in the presence of glycerol, glucose, propylene glycol, and ethanol are shown in Figure 10. In the absence of glycol (solid circles), the apolar domains increase in size as the copolymer volume fraction increases, as expected provided the assumptions made are correct. From this reference point the addition of glycerol or glucose leads to an increase of the apolar domains (look at the points above the line). However, this increase is accounted (46) Bates, F. S.; Schultz, M. F.; Khandpur, A. K.; Fo¨rster, S.; Rosedale, J. H.; Almdal, K.; Mortensen, K. Faraday Discuss. 1994, 98, 7-18.

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Figure 9. Interfacial area per PEO block plotted as a function of the percent glycol volume fraction relative to the volume fraction of the glycol + water solvent mixture. Data for different copolymer contents (indicated on the insets as weight percent) in a given P105-water-glycol system are shown on the same graph. The data at 30, 40, 50, and 60 wt % correspond to the hexagonal structure, H1. The data at 70, 75, and 80 wt % correspond to the lamellar structure, LR. The layout of the graphs is the same as that used for the ternary phase diagrams: (top left) P105-water-glycerol system; (top right) P105-water-propylene glycol system; (bottom left) P105-water-glucose system; (bottom right) P105-water-ethanol system. The closed areas in the graphs for glycerol and glucose represent the limits regarding the values of the interfacial area, and the volume ratios of the glycol/solvent mixture within which the hexagonal and lamellar microstructure are preserved.

for by the increase of the copolymer volume fraction upon substitution of water with glycerol or glucose. The values for the dimensions of the apolar domain in the presence of glycol differ insignificantly (up to 5%) from the corresponding values in the absence of glycol. This result is consistent with previously found trends in the dimensions of the apolar domains in PEO-PPO-PEO/water/xylene systems7,10,11 and supports once more the correct assignment of the apolar domains in the cases of glycerol and glucose. On the contrary, the addition of propylene glycol and ethanol leads to a decrease in the apolar domains (look at the points below the line). In the case of propylene glycol, the addition of the glycol does not change the copolymer volume fraction and the decrease in the size of the apolar domains cannot be accounted for by possible density changes. In the case of ethanol, the density difference cannot explain the observed decrease as well

(though the copolymer volume fraction decreases upon replacing water by ethanol, the values of the apolar domain dimensions remain far from the corresponding values in the absence of glycol). The maximum deviation of the values for the apolar domain dimensions in the cases of propylene glycol and ethanol from that in the absence of glycols is more significant, about 10 to 15%. To obtain reasonable values for the apolar domain dimensions in these cases, it would be necessary to increase the apolar volume fraction, including in the apolar domains part of the glycol in addition to the PPO blocks of the copolymer. This is an indication that in the cases of propylene glycol and ethanol, part of the glycol is located in the core of the apolar domains. In summary, the “microscopic” information obtained by the analysis of the SAXS characteristic length scales has proved that glycerol and glucose are highly polar solvents located only and inside the polar microdomains


Figure 10. Radius of the apolar domains (in the hexagonal structure) and the apolar layer thickness (in the lamellar structure) at 0% glycol content as well as for various contents of glycerol, glucose, propylene glycol, or ethanol plotted versus the copolymer volume fraction. The line represents the best fit (by the least-squares method) of the data points at 0% glycol content as a linear function of the copolymer volume fraction.

(do not participate in the formation of the interface) and do not contribute to the swelling of the PEO blocks, as it was suggested.38 Propylene glycol and ethanol show a complex behavior: Part of the glycol is located in the core of the apolar domains while another part participates in the formation of the interface by swelling both PEO and PPO blocks of the copolymer and, hence, decreases the amphiphobicity of the copolymer. B. Correlation between the Glycol Effects on the Block Copolymer Microstructure and the Glycol Relative Polarity. Replacement of part of water with glycerol or glucose tends to increase the lattice parameter, much more so in the hexagonal structure compared to the lamellar (Figure 5 and Figure 6). In fact, the relative increase (slope) increases with decreasing the copolymer concentration. Replacement of part of water with propylene glycol or ethanol tends to decrease the lattice parameter. For ethanol the relative decrease (slope) increases with decreasing the polymer concentration; for propylene glycol, the copolymer concentration does not affect much the relative decrease (slope). This trend is visualized at Figure 11 as fanlike patterns. Figure 11 shows the variation of the lattice parameter at different glycol contents with respect to that at 0% glycol, plotted versus the glycol volume fraction relative to the glycol + water solvent mixture volume fraction. Here as in Figure 6 and Figure 9, the ratio of the glycol volume fraction to the solvent mixture volume fraction is considered in order to enable comparison of the net effect of the different glycols. Data for different copolymer concentrations (indicated on the insets) in the micellar cubic, hexagonal, and lamellar structures for a given P105-water-glycol system are shown on the same graph. The similarity in the trends shown in the glycerol and glucose graphs is striking, as is their difference from the propylene glycol and ethanol graphs. The glycol effects, i.e., the relative increase (decrease) of the lattice parameter, are stronger at lower copolymer concentration (higher glycol concentration). An evaluation of the corresponding relative increase (decrease) proved this dependence true for all glycols, but up to a different degree. The glycol effect represented as the relative swelling per glycol content is given by the slope (R) of the plots shown in Figure 11. In Figure 12 the relative swelling per glycol content is plotted as a function of the copolymer volume fraction. It is notable that this dependence appears to be linear for each glycol (the lines represent the best

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linear fit by the least-squares method). Or in other words, the effect of the glycol is proportional to the copolymer content but is specific for a given glycol. To eliminate the influence of the copolymer on the relative swelling per glycol content and to extract a parameter characteristic only of the glycol, the relative swelling per glycol content per copolymer content is evaluated, which is given by the slope (β) of the plots for the different glycols shown in Figure 12. As shown in the first part of this study,38 the glycol effect on the ternary phase behavior correlates well to the glycol relative polarity. The choice of the parameters used as a characteristic of the relative polarity of glycols, the octanol/water partition coefficient, the dielectric constant, and the solubility parameter (data for the glycols studied are given in Table 1), was discussed in the companion paper.38 The glycol properties related to their polarity are complementary as described by these three parameters. The dielectric constant is a parameter strictly defining a single physicochemical property, and reliable data are available. The use of the octanol/water partition coefficient or the solubility parameter has the advantage of capturing a number of features related to the glycol polarity, but their experimental determination is subject to inaccuracies due to, for example, the presence of specific interactions in the system (e.g., hydrogen bonding).42,47 Correlations between the relative swelling per glycol content per copolymer content and the glycol relative polarity (expressed in terms of the octanol/water partition coefficient, the dielectric constant or the solubility parameter) are shown in Figure 13. In this figure, the complex effect of the glycols as cosolvents on the microstructure of the liquid crystals in ternary systems with PEO-PPO-PEO block copolymer and water is captured first by extracting the relative swelling per glycol content (the slopes in Figure 11) and then the relative swelling per glycol content per copolymer content (the second slopes in Figure 12). The dependence shown in Figure 13 passes through zero, separating the glycols in two groups with opposite effects, those which increase the lattice parameter and decrease the interfacial area and those which decrease the lattice parameter and increase the interfacial area. The correlation between the effect of glycols, expressed as the highest glycol/water ratio able to maintain the stability of the liquid crystalline structures, and the glycol polarity made in the first part of this study38 suggests that there exist “optimal” glycols where a maximum extent of the liquid crystalline phases can be achieved. As shown (Figure 5) the relative lattice spacing can be modified by no more than (20%; therefore glycols which have smaller effect on the lattice parameter will be able to maintain the microstructure up to higher glycol/water ratios. This corroborates very well the dependence shown in Figure 13 as the change in its sign occurs within the same region of glycol polarities to which the “optimal” glycols belong. Propylene glycol possibly belongs to the group of the “optimal” glycols and correspondingly affects only slightly the lattice parameter. These correlations demonstrate the relationship between the effect of glycols as cosolvents on the ternary phase behavior and the liquid crystalline microstructure of PEO-PPO-PEO block copolymers on one hand and the glycol physicochemical properties, in particular their polarity on the other. A hypothesis for the effect of the glycols on the PEOPPO-PEO block copolymer phase behavior in water was proposed38 on the basis of the correlation made between (47) Fried, J. R. Polymer Science and Technology; Prentice Hall: Englewood Cliffs, NJ, 1995; Chapter 3.

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Figure 11. Percent variation of the lattice parameter at different glycol contents with respect to that at 0% glycol plotted versus the percent glycol volume fraction relative to the volume fraction of the glycol + water solvent mixture. Data for different copolymer contents (indicated on the insets as weight percent) and microstructures (I1, 30 wt %; H1, 30, 40, 50, and 60 wt %; LR, 70, 75, and 80 wt %) in a given P105-water-glycol system are shown on the same graph. The layout of the graphs is the same as used for the ternary phase diagrams: (top left) P105-water-glycerol system; (top right) P105-water-propylene glycol system; (bottom left) P105-water-glucose system; (bottom right) P105-water-ethanol system.

glycol and higher than that of the propylene glycol, respectively). PEO-resembling glycols would have a preference to locate in the polar domains of the microstructure, the same that the PEO blocks of the copolymer and water occupy. PPO-resembling glycols would have a preference to locate in the apolar microdomains, the same as the PPO segments. Therefore, PEO-resembling glycols (here, glycerol and glucose) and PPO-resembling glycols (here, propylene glycol and ethanol) should exhibit different and opposite effects on the phase behavior and the microstructure of the block copolymer. The microscopic information for the effect of glycols on the lyotropic liquid crystalline structures obtained here by analysis of the SAXS characteristic length scales have been shown to corroborate this hypothesis. Figure 12. Relative swelling per glycol content, R, plotted versus the copolymer volume fraction (the lines represent the best linear fit by the least-squares method). The relative swelling per glycol content is given by the slope of the plots of the variation of the lattice parameter at different glycol contents with respect to that at 0% glycol as a function of the glycol content relative to the glycol + water solvent mixture shown in Figure 11.

the highest glycol/water ratio able to maintain stability of the liquid crystalline phases and the relative polarity of the glycols. The glycols were classified as PEOresembling and PPO-resembling based on their octanol/ water partition coefficients (lower than that of the ethylene

Conclusions The effect of glycols as polar cosolvents on the lyotropic liquid crystalline microstructure of poly(ethylene oxide)poly(propylene oxide) block copolymers (Pluronics) in water has been elucidated. Four glycols, with different relative polarities, are examined: glycerol, propylene glycol, ethanol, and glucose. The pronounced effect on the concentration range of stability of the liquid crystalline phases in the systems studied, observed when the glycol was varied from ethanol (the least polar one) to glucose (the most polar one), has been quantified using small-angle X-ray scattering by an


Figure 13. Correlation between the relative swelling per glycol content per copolymer content, β, and the glycol polarity expressed in terms of (top) the octanol/water partition coefficient, log P, (middle) the dielectric constant, and (bottom) the solubility parameter, δ. The relative swelling per glycol content per copolymer content is given by the slopes of the plots for the different glycols shown in Figure 12 and is characteristic of a given glycol and copolymer. The lines are drawn as a guide to the eye.

analysis of the glycol effect on the characteristic length scales of the lyotropic liquid crystals. At constant glycol content (along lines parallel to the copolymer-water axis in the ternary phase diagram), the lattice parameter decreases with increasing the copolymer content in all systems studied. At constant copolymer content (along lines parallel to the water-glycol axis in the ternary phase diagram), opposite trends are observed for the different

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glycols. Replacement of part of the water with glycerol or glucose increases the lattice parameter (glucose more so than glycerol), whereas propylene glycol and ethanol decrease the lattice parameter (ethanol much more so than propylene glycol). These glycol effects are preserved throughout all lyotropic liquid crystalline phases in the phase diagrams. At one and the same constant copolymer content, the microstructure breaks up at approximately the same values of the lattice parameter in the cases of glycerol and glucose (propylene glycol and ethanol, respectively), although at different glycol content. It has been found that the relative lattice spacing can be modified by no more than (20%. The microstructure is destabilized when the changes in the lattice parameter exceed this value. The effects of the glycols on the microstructure are related to changes in the block copolymer interfacial area, the preference of the different glycols to locate in different microdomains, and the glycol ability to modify the system interfacial curvature by swelling different blocks of the macromolecule to different extents. Replacing water with glycerol or glucose decreases the interfacial area. In these cases the glycols are located only in the polar microdomains and do not contribute to the swelling of the PEO blocks. Ethanol and propylene glycol show opposite and to some degree stronger effects than glycerol or glucose and increase the interfacial area. In these two cases the glycol introduction counterbalances the deswelling of the PEO blocks due to the decrease of the water content and even further increases the interfacial area by swelling both PEO and PPO blocks, thus decreasing the amphiphobicity of the copolymer. A correlation is made between the glycol effect on the lyotropic liquid crystalline microstructures, expressed in terms of the relative swelling per glycol content per copolymer content, and the glycol polarity, expressed in terms of the octanol/water partition coefficient, the dielectric constant, or the solubility parameter. Such correlation agrees very well and reinforces the correlation between the effect of glycols on the general phase behavior of Pluronic block copolymers in water and glycol, expressed as the highest glycol/water ratio able to maintain the stability of the liquid crystalline structures and the glycol polarity. Glycols which are able to maintain the liquid crystalline microstructures up to high glycol/water ratios alter only little the lattice parameter of the microstructure. These correlations demonstrate the relationship between the effect of glycols as cosolvents on the ternary phase behavior and the liquid crystalline microstructure of PEO-PPO-PEO block copolymers, on one hand, and the glycol physicochemical properties, in particular their polarity, on the other. Acknowledgment. This study was supported by the Procter and Gamble Co. University Exploratory Research Program (UERP) including a postdoctoral fellowship for R.I. The research of P.A. at Lund University is supported financially by the Swedish Natural Science Research Council (NFR). The research of P.A. in solvated block copolymers at the State University of New York is sponsored by the Petroleum Research Fund (Grant ACSPRF#33408-G7) and the National Science Foundation (Grant CTS-9875848). The acquisition of the SAXS apparatus was funded by the Swedish Council for Planning and Coordination of Research (FRN). LA9912343

Effect of Glycols on the Self-Assembly of Amphiphilic Block

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