Lyotropic Liquid Crystallinity in Amphiphilic Block Copolymers

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Lyotropic Liquid Crystallinity in Amphiphilic Block Copolymers: Temperature Effects on Phase Behavior and Structure for Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) Copolymers of Different Composition Paschalis Alexandridis,* Dali Zhou,† and Ali Khan Physical Chemistry 1, Center for Chemistry and Chemical Engineering, University of Lund, P.O. Box 124, Lund S 22100, Sweden Received November 9, 1995. In Final Form: February 9, 1996X The phase behavior and structure of binary amphiphilic polymer-water systems have been studied as a function of polymer concentration and temperature for three poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) (PEO-PPO-PEO) copolymers of different composition, (EO)6(PO)34(EO)6 (L62), (EO)13(PO)30(EO)13 (L64), and (EO)37(PO)58(EO)37 (P105), by using 2H-NMR and small-angle X-ray scattering (SAXS). The number of lyotropic liquid crystalline (LLC) phases formed increases with the poly(ethylene oxide) content and the molecular weight of the polymers in the order L62 < L64 < P105. Only a lamellar LLC phase is formed by L62, while P105 is capable of self-assembling with increasing polymer concentration into (body-centered close-packed) cubic, hexagonal, and lamellar LLC phases. Upon heating, the LLC phases of the L62-water and L64-water systems swell with water; no such swelling is detected for the P105-water system. The thermal stability of the LLC regions increases in the order cubic < hexagonal < lamellar and L62 < L64 < P105. An increase of the temperature results in a decrease in the interfacial area and an increase in the periodicity of the L62 and L64 lamellae. In the P105-water system, the structural dimensions in the lamellar and hexagonal LLC regions are not much affected by temperature. Both the lamellar periodicity and the block copolymer interfacial area decrease with increasing polymer content, for all polymers. The factors influencing the self-assembly mode of amphiphilic copolymers are discussed, and the phase behavior of the PEO-PPO-PEO copolymers in water is compared to that of nonionic surfactants.

I. Introduction Amphiphilic copolymers of the poly(ethylene oxide)-bpoly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPOPEO) type are available commercially as Poloxamer or Pluronic polyglycols and find widespread use as surface active agents. The hydrophilic-lipophilic character of these polymers can be readily altered by varying their molecular weight and chemical composition. Thus, amphiphiles of the PEO-PPO-PEO family can meet the specific requirements in different applications, such as detergency, emulsification, dispersion stabilization, foaming, and lubrication.1,2 A number of PEO-PPO-PEO copolymers have been shown to associate in aqueous solutions to form micelles consisting of a (mainly) PPO core and a corona dominated by hydrated PEO segments.3 The formation of micelles and their structure have been investigated over the last few years with numerous techniques,4,5 and the micellization process is now relatively well understood (see ref 3 for a recent review). In addition to their association into micelles, some PEO-PPO-PEO copolymers have been known to form “gels” in water.2,3 Some of the “gel” phases * To whom correspondence should be addressed. Fax: +46-462224413. † On leave from: Ultra Fine Powder Materials Institute, Chemical Engineering School, Sichuan Union University, Moziqiao, Chengdu, Sichuan 610065, People’s Republic of China. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Schmolka, I. R. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; Chapter 10. (2) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110. (3) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (4) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (5) Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Langmuir 1995, 11, 1468.

S0743-7463(95)01025-0 CCC: $12.00

reported in the literature are of cubic structure, but most others still lack proper structural identification; inappropriate terms such as “hard gel” and “soft gel” are sometimes used to describe the various lyotropic10 liquid crystalline (LLC) phases. Such LLC “gel” phases are frequently encountered during the processing and formulation of amphiphiles; however, they are much less studied than the micellar solutions and insufficiently understood. The structural polymorphism afforded by the PEO-PPO-PEO polymers has only recently been recognized;6-9 depending on the polymer chemical composition, cubic, hexagonal, and/or lamellar LLC phases can be formed with increasing polymer content in a mixture with water (and oil).6-9 The types of LLC structures formed by the PEO-PPOPEO copolymers in water were recently related8 to the copolymer cloud point (which, in turn, is a function of the polymer molecular weight and chemical composition1,3). For polymers with sufficiently high PEO content and molecular weight (high cloud point), association into spherical domains (micelles) and only a cubic LLC phase is expected, exhibiting an extensive concentration and temperature stability range. For polymers having sufficiently low PEO content (low cloud point), bilayer structures and the lamellar LLC phase are observed, while for polymers of intermediate PEO content and molecular (6) Mortensen, K. Europhys. Lett. 1992, 19, 599. (7) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (8) Zhang, K.; Khan, A. Macromolecules 1995, 28, 3807. (9) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (10) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (11) Halle, B.; Wennerstro¨m, H. J. Chem. Phys. 1981, 75, 1928.

© 1996 American Chemical Society

Liquid Crystallinity in Amphiphilic Copolymers

weight (intermediate cloud point), the mode of selfassembly depends on the polymer concentration and can be easily modulated by the solvent. The formation of various LLC phases in the presence of water can also be addressed from the side of polymer science:9 in the absence of water the block copolymer is in a disordered state (“melt”); the addition of water enhances the segregation of the “hydrophilic” PEO and “hydrophobic” PPO blocks and thus facilitates the ordering of the copolymer molecules in various LLC structures. The study of binary amphiphile-water systems is of great fundamental value, since they form the basis of the understanding of the more complicated, multicomponent, systems which are always encountered in practice. We report here on the binary phase diagrams of three PEOPPO-PEO amphiphilic copolymers in water, over the whole concentration range and the 20-80 °C temperature range. The emphasis is on thermodynamics (phase behavior) and on structure; the copolymers studied were chosen so as to observe the effects of both the polymer molecular weight and chemical composition on the self-assembly. 2H-NMR and small-angle X-ray scattering (SAXS) were the main tools employed in determining the boundaries and the structure, respectively, of the various phases formed. The organization of the paper is the following: An overview of the phase behavior of the three binary polymer-water systems is given first. The structure of the nonisotropic (lamellar and hexagonal) LLC phases is then discussed, and the dependence of the structural parameters (determined from SAXS) on the polymer content and temperature is presented. Complementary information (obtained from 2H-NMR) on hydration and order in these LLC phases is also provided. The isotropic cubic LLC and micellar solution regions are then examined, and the cubic structure is resolved with the help of SAXS. Finally, the factors influencing the self-assembly of amphiphilic copolymers are discussed in the context of the findings presented here, and the binary phase behavior of the PEOPPO-PEO copolymers in water is compared to that of nonionic alkyl-oligo(ethylene oxide) surfactants. II. Materials and Methods A. Materials. The Pluronic L62, L64, and P105 poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers were obtained as a gift from BASF Corp., Parsippany, NJ, and were used as received (PLURONIC is a registered trademark of BASF Corporation). The L62, L64, and P105 polymers have nominal molecular weights of 2500, 2900, and 6500, respectively, and PEO wt % contents of ∼20, 40, and 50, respectively (according to the manufacturer). On the basis of their molecular weights and chemical compositions, L62, L64, and P105 can be represented by the formulas (EO)6(PO)34(EO)6, (EO)13(PO)30(EO)13, and (EO)37(PO)58(EO)37, respectively. The bulk polymer density value of 1.05 g/mL is used here in the analysis of the SAXS data. 2H O (99.80 atom % 2H) was purchased from Dr. Glaser, AG, 2 Basel, Switzerland; its density at 25 °C is 1.104 g/mL. B. Determination of Phase Diagrams. Samples were prepared individually by weighing appropriate amounts of polymer and heavy water into 8 mm (i.d.) glass tubes which were flame-sealed immediately. The glass tubes with the samples were centrifuged repeatedly in both directions over a few days to facilitate mixing. During the time period required for homogenization, the samples were kept in a temperaturecontrolled room that was maintained at 25 ( 0.5 °C. Subsequently, they were left at a given temperature in a temperaturecontrolled bath or oven for several days to attain equilibrium. The samples were first examined for homogeneity and birefringence by ocular inspection, against scattered light, and between crossed polaroids, in order to obtain an indication of the boundaries between isotropic (non-birefringent) and nonisotropic (birefringent) phases, as well as between one- (transparent) and two-phase (nontransparent) regions. The samples were then studied with 2H-NMR. From the appearance of the 2H-NMR

Langmuir, Vol. 12, No. 11, 1996 2691 spectra it was established whether a certain sample consisted of a single homogeneous phase or of two phases. The presence of an isotropic phase is directly noted from a sharp singlet, and that of an anisotropic phase from a doublet of equal intensity peaks (see section II.D). Finally, the structure of the various LLC phases was established by the reflections identified in SAXS spectra (see sections II.F and II.G). It was thus possible from a systematic variation of composition and temperature, and using the techniques outlined above, to determine with good precision ((0.5%) the phase diagrams. The phase diagrams presented in this manuscript were constructed by a careful observation of the samples during the course of 8 months. We have no indication that the initial centrifugation of the samples affected the final equilibrium state. After the initial mixing, the samples were kept standing for about 4 weeks before the 2H-NMR measurements were carried out; 2H-NMR measurements were repeated several times for representative samples over a period of 8 monthssno changes were observed after the initial mixing/ solvation/equilibration period of 2-4 weeks. C. 2H-NMR. 2H-NMR experiments were performed at a resonance frequency of 15.371 MHz (2.3 T) on a Bruker MSL100 pulsed superconducting spectrometer working in the Fourier transform mode. The sample temperature was controlled during the NMR measurement by passing air of the desired temperature through the sample holder. The quadrupole splittings ∆(2H) were measured as the peak-to-peak distance and are given in frequency units (Hz). As discussed below, 2H-NMR of deuterated water can be conveniently used to study the phase equilibria of amphiphile-water systems, as well as to provide information of the hydration state of the amphiphiles.8,9,12,13 D. Phase Equilibria and Hydration in Lyotropic Liquid Crystals Using 2H-NMR. The NMR spectrum of deuterated water is dominated by the interaction of the deuteron quadrupole moment with the electric field gradients in the nucleus. For an anisotropic LLC sample, this quadrupole interaction generates a spectrum with two peaks of equal intensity (as shown in Figure 1c and e). In an isotropic solution or LLC phase, on the other hand, this interaction is averaged to zero, as a result of rapid and isotropic molecular motions, and the NMR spectrum will consist of a sharp singlet (see Figure 1a). The observed quadrupole splitting ∆(2H) depends on the fraction of deuterons in one or more anisotropic sites, the quadrupole coupling constant, and the average molecular ordering of water molecules in the sites. A detailed theoretical treatment of ∆(2H) quadrupole splitting in LLC phases has been reported elsewhere.11 For a heterogeneous system consisting of two or more phases, one expects a superposition of the 2H-NMR spectra representative of each phase, provided that the deuteron exchange between the phases is slow (Figure 1b and d). Thus for a system containing a mixture of a lamellar and a hexagonal LLC phase, two quadrupole splittings will be observed, as shown in Figure 1d. Theoretically, in such a mixture, the splitting of the lamellar phase should be twice that of the hexagonal phase,11 provided that (i) the lamellae have no appreciable curvature, (ii) the local interactions and molecular ordering are the same for the two phases, and (iii) translational diffusion around cylinders in the hexagonal structure occurs in a time shorter than that for the inverse splitting. However, the existence of two or more isotropic phases can normally not be established by the 2H-NMR method. A practical problem in the phase behavior studies of amphiphiles can be the separation of the LLC phases when present in twophase samples, especially if they have similar densities and/or are very viscous (as was often the case in our system).13 The 2H-NMR technique employed here does not require a macroscopic separation of the individual phases of a mixture. The magnitude of the deuteron quadrupole splitting contains information on the hydration of the amphiphile aggregates (i.e., cylinders, lamellae) in terms of the fraction of water molecules appreciably oriented by the aggregate surfaces and their average degree of orientation (as described by an order parameter).13 The quadrupole splitting, ∆(2H), can be expressed in the context of the conventional “two-site” model,8 according to which (i) water is either “free” or “bound”, (ii) there is a fast exchange between (12) Persson, N.-O.; Fontell, K.; Lindman, B.; Tiddy, G. I. T. J. Colloid Interface Sci. 1975, 53, 461. (13) Khan, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1982, 86, 4266.

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Alexandridis et al. free and bound water within the NMR time scale, and (iii) the ordering of free water molecules is negligible:

∆(2H) ) pνQS ) nνQS(XP/XW)

(1)

where p is the fraction of “bound” water molecules, νQ is the quadrupole splitting constant (ca. 220 Hz), n is the average hydration number of the polymer, XP is the mole fraction of polymer, XW is the mole fraction of water, and S is the order parameter of bound water molecules. E. Small-Angle X-ray Scattering (SAXS). SAXS measurements were performed on a Kratky compact small-angle system equipped with a position sensitive detector (OED 50M from MBraun, Graz, Austria) containing 1024 channels of width 51.3 µm. Cu KR radiation of wavelength 1.524 Å was provided by a Seifert ID-300 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 was used to protect the detector from the primary beam. The X-ray beam width (at half the maximum intensity) was 0.52 mm. This value is low and would lead to negligible smearing effects on the SAXS profiles over the q range of interest; we thus used the peak positions of the smeared spectra in the structural analysis of the SAXS data presented here. The sample-to-detector distance was 277 mm. The samples for SAXS measurements were filled into a quartz capillary using a syringe. Temperature control was achieved by using a Peltier element. The space between the sample and the detector was under vacuum during the measurements in order to minimize scattering from the air. F. Structural Parameters of the Lamellar Phase. The periodicity, d, of the lamellae is determined from the inverse film area (A) per unit volume (V) ratio:

(VA)

-1

d)

(2)

The effective area of the copolymer molecules at the interface between polar and apolar domains can thus be calculated from the SAXS data without any assumptions concerning the degree of segregation or the local structure of the copolymer film (other than the fact that all the amphiphilic polymer participates in the lamellae). The area per unit volume of the copolymer film is given by A/V ) 2Φpas/vp, where Φp is the total volume fraction of polymer, υp is the volume of one copolymer molecule (4000 Å3 for L62, 4600 Å3 for L64, and 10300 Å3 for P105), and as is the effective interfacial area per PEO block. For a lamellar structure the position of the first-order Bragg peak (q: scattering vector) corresponds to

q1 )

2π πΦpas ) d vp

(3)

Assuming a sharp interface between the polar (PEO and water) and apolar (PPO) domains, we can also calculate the thickness, δ, of the apolar domain of the lamellae according to δ ) fd, where f is the volume fraction of PPO in the polymer-water mixture. The PPO blocks make up approximately 83, 62, and 52% of the copolymer volume of L62, L64, and P105, respectively. The corresponding thickness of the polar domains is then given by d - δ. G. Structural Parameters of the Hexagonal Phase. For a two-dimensional hexagonal symmetry the diffraction peaks at qhk, where h and k are the Miller indices, correspond to

qhk ) Figure 1. Typical 2H-NMR spectra obtained from amphiphilic polymer-water samples at different self-assembly states: (a) sharp singlet indicative of an isotropic solution; (b) singlet plus splitting suggesting a two-phase, isotropic solution-nonisotropic (in this case, hexagonal) LLC, sample; (c) one splitting from a hexagonal LLC sample; (d) two splittings obtained from a two-phase sample containing a mixture of a lamellar and a hexagonal LLC phase; (e) one splitting from a lamellar LLC sample.

4πxh2 + k2 + hk ax3

(4)

where a is the lattice parameter (nearest neighbor distance). In the case of infinite cylinders of cross section radius Rcyl and volume fraction Φcyl, we have the relation

(

Rcyl ) a

)

x3 Φ 2π cyl

1/2

(5)

Assuming a normal hexagonal structure and associating the PPO


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chains with the apolar domain of the cylinders (Φcyl ) f), we have:

Rcyl )

fvp asΦp

a

(6)

This corresponds to defining the polar-apolar interface at the interface separating the PEO and PPO blocks of the copolymer. Substituting eqs 5 and 6 into eq 4, we then obtain

qhk )

(

)

h2 + k2 + hk (x3/2π)f

1/22a Φ s p

vp

(7)

III. Results and Analysis A. Overview of the Phase Behavior. The phase diagrams of the L62-water, L64-water, and P105-water binary systems are presented in Figure 2; the boundaries of the one- and two-phase regions are indicated. The effects of temperature on the copolymer self-assembly were explored over the whole composition range and over a wide temperature range (20-80 °C). The copolymer molecules can self-organize in different topologies depending both on the polymer concentration (lyotropic behavior) and on the temperature (thermotropic behavior). At high polymer (low water) content, isotropic solution regions (liquid, L2, and pastelike, P) are observed in all three polymer-water systems. We anticipate that in the L2 and P regions water is molecularly dissolved in the polymer; apart from the delineation of the phase boundaries, however, no further study was undertaken in the L2 and P regions. As alluded to in the Introduction, the presence of water increases the segregation between the PEO and PPO blocks and induces order in the block copolymer systems. This is evident in the formation of extended lamellar (D) LLC regions at sufficiently high (∼20%) water content for all three polymers. The thermal stability of the lamellar structure increases in the order L62 < L64 < P105. The lamellar phase is the only LLC phase formed by L62, while L64, which has a similar size PPO block to that of L62 but longer PEO blocks, forms both hexagonal (E) and lamellar LLC phases, separated by a narrow isotropic solution phase. An extended hexagonal LLC phase is formed by P105 (of similar chemical composition to that of L64 but of higher molecular weight), in equilibrium with the lamellar region; a micellar cubic LLC phase (I) supersedes the hexagonal region in the P105-water system at higher water contents. Isotropic solutions (L1) dominate the high-water-content side of all the binary phase diagrams; the extent of the L1 regions is limited at high temperatures by the cloud point of the polymers. The structural parameters of the LLC phases were determined from SAXS as a function of polymer concentration and temperature, while additional information was provided by 2H-NMR. The results on the various phases are presented below in the order D, E, I, L1. B. Lamellar (D) Phases. The samples in the lamellar LLC regions are transparent, are birefringent, can flow under their own weight, and exhibit splittings in the 2HNMR spectrum, the values of which can provide information on the hydration of the copolymers (see Figure 8 and related discussion). The lamellar region in the L62-water system is stable at 20 °C for the 63-76 wt % polymer concentration range. Upon heating, the lamellae swell with water and the stability of the lamellar region shifts to lower polymer concentrations, e.g., 51-69 wt % polymer at 55 °C. At temperatures >55 °C the extent of the lamellar structure decreases and, finally, the lamellae melt at ∼65 °C. The lamellar region in the L64-water

b

c

Figure 2. Phase diagrams (composition versus temperature) of the (a) L62-water, (b) L64-water, and (c) P105-water binary systems. The following notation is used for the various regions: L1 ) isotropic water-rich (micellar) solution phase, I ) cubic lyotropic liquid crystalline (LLC) phase, E ) hexagonal LLC phase, D ) lamellar LLC phase, L2 ) isotropic polymer-rich solution phase, P ) pastelike polymer-rich phase. Two-phase regions are also marked. Dotted lines indicate some uncertainty to the phase boundaries.

systems is stable at 20 °C for the 64-81 wt % polymer concentration range. Similarly to the L62-water systems, the stability of the L64-water lamellar region shifts upon heating to lower polymer concentrations, e.g., 55-72 wt % at 55 °C; the L64-water lamellae melt at ∼85 °C. P105, having double the molecular weight of L62 or L64, forms lamellae at higher concentrations (e.g., 73-87 wt % at 20 °C), stable over a larger temperature range (>85 °C) than L62 and L64. The P105 lamellae do not swell much with increasing temperature.

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Figure 3. Slit-smeared SAXS spectra obtained at four different temperatures for (a) a L62-water sample of 64.0:36.0 wt % composition, (b) a L64-water sample of 64.5:35.5 wt % composition, and (c) a P105-water sample of 75.0:25.0 wt % composition. The SAXS spectra are drawn on the same scale but are offset vertically for clarity.

The one-dimensional lamellar structure was established by SAXS experiments. Slit-smeared SAXS spectra obtained at different temperatures (over the 25-65 °C temperature range) for L62-water, L64-water, and P105-water lamellar samples are shown in Figure 3. Second-order peaks can be observed following the 1:2:.. pattern. The lamellae are stable over the whole temperature range examined. The relative intensity and width of the peaks were not affected by temperature; the position of the peak maximum shifted with increasing temperature, reflecting a change in the structural dimensions (see also Figure 5). SAXS measurements were also performed as a function of polymer concentration at a constant temperature (spectra not shown). The effects of polymer concentration on the lattice parameter (lamellar periodicity), d, as well as on the apolar film thickness, δ, and on the interfacial area per PEO block (derived from eq 3) in the L62, L64, and P105 lamellae are presented in Figure 4; data were obtained at a constant temperature (25 °C). Similar trends in the characteristic dimensions were observed for all three polymers. The characteristic lamellar lattice parameter decreased with increasing polymer content; the apolar film thickness, on the other hand, increased with increasing polymer content, a reflection of the increase in the PPO volume fraction, f. The interfacial area per PEO block was found to decrease with increasing polymer content. The lamellae, and in particular their polar part, “contract” as water is removed; the effects, however, are rather small (∼10% change). The periodicity was similar for the L62 and L64 lamellae, ∼70-80 Å, since the molecular weights of L62 and L64 are comparable. The interfacial area per PEO block was also similar (∼80 Å2) for L62 and L64; this is despite the fact that the PEO blocks of L62 and L64 are of different size. P105, with a higher molecular weight, exhibits a larger lamellar periodicity (∼115 Å) and interfacial area (∼120 Å2). The block copolymer molecules have the possibility to arrange themselves at the interface not only around the covalent bond between the different blocks but also in other positions along the polymer chain; however, the interfacial area values obtained from eq 3 suggest that the polymers do not attain such a “train-configuration”. The effects of temperature on the characteristic dimensions for the lamellar liquid crystalline structure are presented in Figure 5 for the L62-water, L64-water, and

Figure 4. Characteristic dimensions (lattice parameter, apolar film thickness, and interfacial area) for the lamellar LLC structure, plotted at constant temperature (25 °C) as a function of the (a) L62 volume fraction, (b) L64 volume fraction, and (c) P105 volume fraction.

P105-water samples whose SAXS spectra are shown in Figure 3. A common feature of the L62 and L64 lamellae is a decrease in the polymer interfacial area, in conjunction with an increase in the lamellar periodicity (and the apolar film thickness), upon heating. Those changes can be


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Figure 6. (a) Slit-smeared SAXS spectrum obtained at 30 °C for a L64-water sample of 52.9:47.1 wt % composition; the scattering curve is also shown on an expanded intensity scale (right-hand-side axis) to reveal the higher order peaks (1:31/2:2) characteristic of the hexagonal structure. (b) Slit-smeared SAXS spectra obtained at different temperatures for a P105-water sample of 60.0:40.0 wt % composition. The SAXS spectra are drawn on the same scale but are offset vertically for clarity. Figure 5. Characteristic dimensions (lattice parameter, apolar film thickness, and interfacial area) for the lamellar LLC structure plotted as a function of temperature for (a) an L62water sample of 64.0:36.0 wt % composition, (b) an L64-water sample of 64.5:35.5 wt % composition, and (c) a P105-water sample of 75.0:35.0 wt % composition.

attributed to a dehydration of the PEO-PPO-PEO molecules, a phenomenon which is also manifested in other solution properties of these copolymers5,14,15 and is supported by model calculations.16 The situation is different, however, in the case of the P105 lamellae; here the lamellar periodicity exhibits a slight decrease and the interfacial area a slight increase with increasing temperature. The polymer and, more specifically, the PEO concentration in the P105 lamellar region are larger than the corresponding concentrations in the L62 and L64 lamellar regions and are such that all water will be hydrating the EO segments (see Figure 8 and related discussion). As a result, the effects of dehydration with increasing temperature would be less important for the P105 lamellae and would not affect their structural dimensions. We also note that the thermal stability of the P105 lamellar region is higher than that of L62 and L64 (see Figure 2). C. Hexagonal (E) Phases. Normal (“oil-in-water”) hexagonal LLC phases supersede the lamellar LLC phases upon increasing the water content in the L64-water and P105-water binary systems (see Figure 2). The samples in the hexagonal regions are relatively stiff, transparent, and birefringent. They exhibit a splitting in the 2H-NMR (14) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604. (15) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730. (16) Linse, P. J. Phys. Chem. 1993, 97, 13896.

spectrum, and the value of the splitting, ∆(2H), increases linearly with increasing polymer concentration (see Figure 8 and related discussion). The hexagonal structure in the P105-water system is stable at 20 °C for the 47-66 wt % polymer concentration range; this stability range is not affected much by temperature. The stability of the hexagonal region in the L64-water systems, on the other hand, is highly dependent on temperature. Contrary to the P105-water system, where the hexagonal phase is in equilibrium with the lamellar phase, the hexagonal region in the L64-water system is an “island” encircled by an isotropic polymer solution (L1). In fact, an interesting thermotropic behavior is observed: upon heating past 22 °C, a ∼50% polymer solution crystallizes into a hexagonal structure and then melts into an isotropic solution at ∼40 °C and, following a two-phase transition region, crystallizes into a lamellar structure at ∼60 °C. These phase transitions have been followed with both 2H-NMR and SAXS and are fully reversible (the kinetics for the crystallization into the hexagonal order are on the order of a few hours). The hexagonal L64-water region is rather narrow and extends from 54 wt % polymer at 22 °C to 46 wt % polymer at 46 °C; similarly to the L64-water lamellar region, the stability of the L64-water hexagonal region shifts to lower polymer concentrations as the temperature increases. The hexagonal structure is not attained in the L62-water binary system. The two-dimensional hexagonal structure (hexagonallypacked array of cylindrical micelles) was established by SAXS experiments which revealed sharp reflections obeying the ratio 1:x3:2, characteristic of this structure. Figure 6a shows the slit-smeared SAXS pattern (plotted as a function of the scattering vector q) that was obtained

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Figure 7. Characteristic dimensions (lattice parameter, apolar cylinder radius, and interfacial area) for the hexagonal LLC phase plotted (a) as a function of the P105 volume fraction at constant temperature (25 °C) and (b) as a function of temperature for a P105-water sample of 60.0:40.0 wt % composition.

for a L64-water sample of 52.9:47.1 wt % composition at 30 °C. Slit-smeared SAXS spectra obtained at different temperatures (25, 35, 50, and 65 °C) for a P105-water sample of 60.0:40.0 wt % composition are presented in Figure 6b. The characteristic dimensions (lattice parameter, apolar cylinder radius, and interfacial area) for samples in the P105-water hexagonal liquid crystalline phase were determined on the basis of eqs 6 and 7 and are plotted in Figure 7a as a function of the polymer volume fraction. The lattice parameter (i.e., the distance between the centers of two adjacent cylinders) remained almost indifferent to the polymer content (it actually decreased slightly with increasing P105 concentration); the apolar cylinder radius, on the other hand, increased with increasing P105 content, reflecting the increased amount of PPO (comprising the core of the cylinders) in the system. The interfacial area per PEO block decreased with increasing polymer concentration, so that more polymer molecules can be accommodated in the same number of cylinders. The effects of temperature on the structural parameters of the hexagonal phase were studied for a P105-water sample of 60.0:40.0 wt % composition and are presented in Figure 7b. The hexagonal structure in the P105-water systems is stable over a wide temperature range and thus provides a good model system. The lattice parameter increased slightly with increasing temperature; the apolar cylinder radius remained almost constant. The interfacial area per PEO block decreased with increasing temperature, following the corresponding increase in the lattice parameter. We note that the above trends in the lattice parameter and the interfacial area are similar to that observed in the L62 and L64 lamellae but opposite to that of the P105 lamellae (Figure 5); this may be related to the insufficient hydration of the P105 lamellae, as discussed in section III.B. Overall, the structural parameters, as well as the composition range of stability, of the P105water hexagonal and lamellar phases are not much

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influenced by the temperature (for the temperature range investigated here). Information on the hydration of the amphiphilic copolymers and additional evidence on the structural identification have been obtained from 2H-NMR experiments. 2H-NMR splitting values are presented in Figure 8 as a function of both the polymer/water molar ratio (lower X axis) and the EO/water molar ratio (upper X axis) for the L62-water, L64-water, and P105-water binary systems (25 °C). The expression of the deuteron quadrupole splitting values with respect to the EO/molar ratio is appropriate in the case of polymers and facilitates the comparison of polymeric molecules with similar chemical composition but different molecular weight. Inspection of Figure 8 shows that the splitting values in the hexagonal regions of the L64-water and P105-water systems follow the simple “two-site” model17 with constant hydration that eq 7 represents. Moreover, for L64 and P105 samples in the lamellar-hexagonal two-phase regions, the splitting values in the lamellar phase were approximately twice that of the hexagonal phase; this relationship is expected from theoretical considerations11 and confirms the structural assignment done with SAXS. The splitting values in the lamellar L62-water region also follow eq 7, up to a XEO/XW value of 0.2 (i.e., 5 water molecules per EO segment). Deviations from the “two-site” model are observed in the L62-water system at XEO/XW > 0.2, where the splitting values reach a plateau. Such deviations are also found in the case of the L64-water lamellar region at XEO/XW > 0.3 and in the P105-water lamellar region at XEO/XW > 0.5. This can be explained on the basis that almost all of the water in this concentration range is hydrating (“bound” to) the EO segments of the copolymers, thus invalidating the assumption of constant hydration number implied in eq 7. Neutron reflectivity studies of polyethoxylated surfactant monolayers at the air-water interface,18 as well as 17O-NMR relaxation studies,19 showed that each EO segment in the surfactant film is hydrated by 1-3 water molecules. The effects of temperature on the 2H-NMR splitting values are presented in Figure 9 for samples in the L62water, L64-water, and P105-water binary systems. A decrease in the deuteron quadropole splitting values is generally observed upon heating. This can be understood in terms of dehydration of the EO groups and is in agreement with previous results in isotropic solutions20 and liquid crystalline phases8 of PEO-PPO-PEO copolymers in water, as well as with model calculations.16 An exception to the decrease of ∆(2H) with temperature is noted in the L64-water lamellar region, where ∆(2H) appears to increase with increasing temperature up to ∼45 °C. The interpretation of this finding is not straightforward; in the context of the two-site model, ∆(2H) may increase as a result of a change (increase) in the order parameter with increasing temperature. In fact, a change in the order/conformation of the polymer molecules with temperature may be in effect both in the case where ∆(2H) increases with increasing temperature and in the case where ∆(2H) decreases with increasing temperature. This subtle balance between a decrease in hydration and an increase in order in the PEO-PPO-PEO-water systems when the temperature increases can explain the ∆(2H) data shown in Figure 9 and can also reconcile the temperature effects observed in the structural parameters obtained by SAXS (Figures 5 and 7). (17) Persson, N. O.; Lindman, B. J. Phys. Chem. 1975, 79, 1410. (18) Lu, J. R.; Li, Z. X.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2408. (19) Karlstro¨m, G.; Halle, B. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1049. (20) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5446.


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Figure 8. 2H quadrupole splitting values for the (a) L62-water, (b) L64-water, and (c) P105-water binary systems, presented as a function of the polymer/water molar ratio (lower X axis) and the EO/water molar ratio (upper X axis) at 25 °C. Data from the hexagonal LLC regions are denoted with squares, while data from the lamellar LLC regions are denoted with circles; open symbols represent samples in two-phase regions.

Figure 9. 2H quadrupole splitting values plotted as a function of temperature for samples in the (a) L62-water, (b) L64-water, and (c) P105-water systems. Data from the hexagonal LLC regions are denoted with squares, while data from the lamellar LLC regions are denoted with circles.

D. Cubic (I) Phase. A cubic LLC region is observed in the P105-water binary system between the isotropic micellar solution and the hexagonal phase (see Figure 2). The cubic structure is stable for the 26-44 wt % polymer concentration range at 20 °C. The concentration range of stability decreases with an increase of the temperature, and the cubic structure finally melts at 60 °C. The samples in this region are stiff, transparent, and non-birefringent. They exhibit a sharp singlet in the 2H-NMR spectrum, characteristic of an isotropic structure. The location of the cubic phase in the binary P105-water phase diagram suggests21 that it is composed of micelles which have crystallized in a cubic lattice. Micellar cubic regions in PEO-PPO-PEO-water systems have been recently observed, and their structure has been characterized with SANS.6 Here we assessed the three-dimensional cubic structure using SAXS, which allows much better resolution of the cubic structure reflections than SANS. A slit-smeared SAXS spectrum obtained from the cubic LLC phase at a P105-water wt % composition of 40.0: 60.0 at 25 °C is shown in Figure 10a. The scattering curve is also shown on an expanded intensity scale to expose a large number of higher order Bragg peaks. We note that the Bragg reflections always tend to become very weak at (21) Fontell, K. Colloid Polym. Sci. 1990, 268, 264.

high hkl indexes due to the short-range disorder inherent in LLC materials. The desmeared SAXS data are presented in Figure 10b. It can be seen that desmearing has a very small effect on the information obtained by the SAXS spectra; all the SAXS peaks marked in the slitsmeared spectrum can also be observed in the desmeared spectrum. There are three main families of cubic structures:22 primitive (P...), body-centered (I...), and facecentered (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 spectra, we can exclude certain crystallographic space groups on the basis of 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 spectra of Figure 10 do not follow the relative positions expected for the F... symmetry. Choosing between the I... and P... structures is more difficult. The I... structures are of higher symmetry and thus result in the smallest number of reflections; in fact, the reflections afforded by the P... structure include the ones from the I... structure as a subset. While we cannot exclude the possibility of a P... (22) Hahn, T., Ed. International Tables for Crystallography; Reidel: Dordrecht, Holland, 1983.

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Figure 10. Slit-smeared (a) and desmeared (b) SAXS spectra obtained from the cubic LLC phase at a P105-water weight composition of 40.0:60.0 at 25 °C. The scattering curves are also shown on an expanded intensity scale (right-hand-side axis) to reveal the higher order peaks. The arrows mark the positions of the reflections afforded by the I... crystallographic space group. Filled arrows indicate the observed reflections while empty arrows indicate reflections that are anticipated but not clearly discerned in the SAXS spectra. The desmearing was done according to the direct method of beam-height correction (Singh, M. A.; Ghosh, S. S.; Shannon, R. F., Jr. J. Appl. Crystallogr. 1993, 26, 787).

Alexandridis et al.

to the I... structure. The value of a, estimated from the data of Figure 10, is 200 Å. E. Isotropic (L1) Solution Phases. As seen in Figure 2, isotropic solutions dominate the high-water-content side of all the binary phase diagrams. The L1 region extends up to 58 wt % polymer at 20 °C for both the L62-water and the L64-water systems. This high polymer concentration limit decreases as the temperature increases due to the development with heating of the D region in the L62-water system and the E region in the L64-water system. The extent of the L1 regions is also limited at high temperatures by the cloud point of the polymers. Apart from the delineation of the phase boundary, we have not investigated further the regions denoted as “multiphase” in the phase diagrams of Figure 2. Above the cloud point we expect a coexistence of a polymer-lean and a polymer-rich phase. Recently, a “spongelike” L3 region of narrow temperature stability was identified23 in an aqueous system of a polymer similar to L62 at temperatures and concentrations that fall into the area denoted here as “multiphase”. The situation in the low-polymer-concentration, lowtemperature corner of the L64-water and P105-water phase diagrams is relatively well understood: the block copolymer molecules associate into spherical micelles at the critical micellization concentration (cmc) and temperature (cmt). Cmc and cmt values, as well as information on the micelle structure, have been reported elsewhere.3-5,15,24 An interesting aspect of the phase diagram of the L64-water binary system is the narrow isotropic region observed between the hexagonal and the lamellar phases. The topology in this region should be bicontinuous, since such a structure can attain a mean curvature which is intermediate between those of the hexagonal and the lamellar phases. Bicontinuous cubic phases often occur between the hexagonal and lamellar phases.21 However, the samples that we prepared were of rather low viscosity, which is inconsistent with the normally very viscous LLC cubic arrangement. Furthermore, SAXS spectra presented in Figure 12 show a single broad correlation peak signifying a liquid phase with only short-range order; in fact, the scattering pattern from the sample in the “corridor” between E and D (Figure 12b) is very similar to that from a sample of the same composition but taken at a temperature (20 °C) where E is not stable (Figure 12a). IV. Discussion

Figure 11. Plot of the reciprocal spacings (1/dhkl) of the reflections observed in the SAXS spectra of Figure 10, plotted versus m ) (h2 + k2 + l2)1/2. The line through the origin indicates the good fit of the data to the I cubic structure.

structure, we indexed the peaks of the spectrum shown in Figure 10 to a body-centered (I...) structure. A total of nine Bragg peaks are identified (marked in Figure 10 with the arrows), which can be indexed as the 110, 200, 211, 220, 310, 222, 321, 400, and 411 reflections of a body-centered close-packed cubic structure. This cubic structure is characterized by Bragg reflections whose reciprocal d spacings are in the ratio x2:x4:x6:x8:.... The indexing of the defraction data was assessed by plotting the reciprocal spacings (1/dhkl) of the nine reflections marked in the SAXS spectra of Figure 10 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 11). The 45° line in Figure 11 indicates the good fit of the data

A. Factors Influencing the Self-Assembly of Amphiphilic Copolymers. The self-organization of A-B block copolymers stems from the segregation between the different blocks in the copolymer. The segregation depends on block-block interactions, described in the context of the Flory-Huggins theory by N(A+B)χAB, and on polymer-solvent interactions, described by NAχAS + NBχBS (where N(A+B) is the number of segments in the polymer, NA and NB are the numbers of segments in the A and B blocks, respectively, χij is the Flory-Huggins interaction parameter, and S denotes the solvent). For a given system, the tendency to segregation increases with increasing polymer molecular weight (proportional to N). The formation of LLC structures by amphiphilic copolymers requires a certain polymer molecular weight in order to satisfy the requirement of a minimum hydrophobic part. In addition to the molecular weight, the relative size of (23) Hecht, E.; Mortensen, K.; Hoffmann, H. Macromolecules 1995, 28, 5465. (24) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Am. Oil Chem. Soc. 1995, 72, 823.


Figure 12. Slit-smeared SAXS spectra obtained from a L64water sample of 52.9:47.1 wt % composition in the L1 region at two different temperatures: (a) 20 °C and (b) 39 °C.

the different blocks in the copolymer and the polymer architecture (position of the blocks) play an important role in determining the phase behavior, by affecting the curvature and packing symmetry of the ordered microstructures. When the apparent volume of the soluble blocks is much larger than that of the insoluble blocks (PPO), spherical or cylindrical structures are favored; the lamellar arrangement is preferred when the relative length of the insoluble blocks is sufficiently large. The presence of the solvent (water) would result in an increase in the apparent volume of the solvent-soluble blocks (PEO) and an enhancement of the repulsion between them. For a given amphiphilic block copolymer, the transition from a spherical to a cylindrical and then to a lamellar arrangement upon increase of the polymer concentration is a consequence of the force balance (interfacial versus stretching energy) in the microdomains. Micelles tend to grow in order to reduce the surface-to-volume ratio and the associated interfacial energy. At the same time, the growth of the micelles is accompanied by an increase in the deformation free energy; the latter originates from the stretching of the polymer chains in the micelle core needed in order to maintain uniform density. The increased degree of freedom available in small curvatures (e.g., cylindrical and bilayer structures) allows more polymer molecules to pack in the aggregates without the packing constraints imposed by the spherical micelles. The stability of the micellar cubic LLC phase is related to the intermicellar interactions. If the repulsion (of steric and hydration type in the case of PEO) between the micelles is strong, then the micelles tend to close-pack into a regular cubic array when their effective volume fraction reaches a sufficiently high value. When the repulsion between the spherical micelles is less strong, the micelle size will increase (the polymer interfacial area will decrease) with increasing polymer concentration and the micelles will elongate without undergoing ordering into a cubic phase. The effects of temperature on the phase behavior of the PEO-PPO-PEO copolymers can also be understood in terms of the polymer molecular weight, relative block size, and concentration arguments advanced in the previous

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paragraph, by invoking the reverse solubility (i.e., decrease in solubility with increasing temperature) of PEO and PPO in water. Upon heating, the PEO-water and PPOwater χ interaction parameters increase16 and the PEOPPO interaction parameter decreases,25 and consequently, the contribution to segregation of the term NAχAS + NBχBS increases and that of the term N(A+B)χAB decreases. Overall, the “hydrophobicity” of the polymer is enhanced and the swelling of the PEO blocks by water is decreased. Thus, lower curvature structures are favored and the stability ot the various LLC regions in the binary polymerwater phase diagrams shifts to lower polymer concentrations as the temperature increases. The above effects appear more pronounced in the case of L64, whose hydrophilic/hydrophobic properties are balanced. B. Comparison of PEO-PPO-PEO Copolymers to Nonionic Surfactants. The PEO-PPO-PEO copolymers are similar (in terms of the PEO headgroup and the block architecture) to the nonionic alkyl-oligo(ethylene oxide) surfactants; in this respect, it is interesting to compare the self-assembly features of these two groups of amphiphiles. The alkyl-oligo(ethylene oxide) surfactants form microdomains with curvature toward oil (“normal” structures) at lower temperatures, while at higher temperatures curvature toward water is favored (“reverse” structures).26,27 In binary systems with water, this property is manifested by the formation of dilute lamellar and L3 (“sponge”) phases at higher temperatures in addition to micellar and the usual liquid crystalline phases at lower temperatures.27 It is instructive to consider the series of C12EO4-C12EO6 surfactants, where the size of the alkyl part is the same and that of the EO part increases (similar to the series L62-L64), and that of C12EO6-C16EO8, where the ratio of the alkyl and the EO part is the same but the molecular weight increases (similar to the series L64-P105). The only LLC phase in the C12EO4water system is the lamellar one; an increase in the number of EO segments allows the formation of hexagonal and bicontinuous cubic LLC phases in the C12EO6-water system, in addition to the lamellar phase.26 The hexagonal structure in the C12EO6-water system melts at ∼40 °C, and the lamellar one melts at ∼75 °C; on the other hand, the hexagonal structure formed by the higher molecular weight C16EO8 melts at ∼60 °C and the lamellae are stable beyond 100 °C. In addition to the hexagonal and lamellar LLC phases, a micellar cubic LLC region is formed in the C16EO8-water system at temperatures

Lyotropic Liquid Crystallinity in Amphiphilic Block Copolymers

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