Hexagonal-to-Lamellar Phase Transition Induced by Addition of a

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Hexagonal-to-Lamellar Phase Transition Induced by Addition of a Random Heteropolymer to the Surfactant-Water System Ewa Z. Radlinska,*,†,‡,§ Tadeusz Gulik-Krzywicki,| Dominique Langevin,⊥ and Franc¸ oise Lafuma‡ Laboratoire de Physique Statistique, CNRS URA 1306, ENS, 24, rue Lhomond, 75231 Paris Cedex, France, Laboratoire de Physico-Chimie Macromole´ culaire, Universite´ Pierre et Marie Curie, CNRS URA 278, ESPCI, 10, rue Vauquelin, 75231 Paris Cedex, France, Centre de Ge´ ne´ tique Mole´ culaire, CNRS UPRA 2420, 91198 Gif-sur-Yvette, France, and Centre de Recherche Paul Pascal, avenue A. Schweitzer, 33600 Pessac, France Received February 18, 1998. In Final Form: June 22, 1998 We report on the microstructural study in a cross section of the ternary phase diagram of pentaethylene glycol mono-n-dodecyl ether (C12E5)/water/random heteropolymer defined by the surfactant-to-water ratio equal to unity and constant temperature of 21 °C. The transformation from a normal hexagonal to a lamellar phase is observed as a function of increasing polymer content. The small-angle X-ray scattering results demonstrate the presence of a well-defined hexagonal structure up to about 1 wt % of polymer content that transforms into a lamellar structure at 7.7 wt % of polymer, as evidenced by two harmonics present in the scattering spectrum. There is no two-phase region observed. Freeze fracture electron microscopy images demonstrate the presence of a hexagonal phase up to the polymer concentration of 1.0 wt % and a lamellar phase for higher polymer concentrations (1.6 wt % and above). The polymer molecules are confined inside the surfactant bilayers of a lamellar phase. The textures seen using polarizing microscopy provide a strong supportive argument for a gradual change of topology with the addition of polymer.

Introduction Phase transformations in lyotropic liquid crystals, their mechanisms, triggering factors, and epitaxial relations between the reticular planes have captured a great deal of scientific effort. Perhaps the most important reason for this interest has been the close relation to processes encountered in living matter.1 The unifying concept of succession of phases in concentrated surfactant solutions has emerged after years of sustained experimental and theoretical effort by many groups (for example, see a recent review by Gelbart and Ben-Shaul2 and references therein). In particular, the decrease of the interaction free energy in complex systems, crucial at high concentrations of amphiphiles, is achieved by the decrease of the curvature of colloidal aggregates. Less curved aggregates are able to pack more efficiently, thus resulting in the decrease of excluded volume. This path leads to a transition from the hexagonal phase of infinite cylinders to the lamellar phase of a stack of bilayers as the surfactant volume fraction increases (as commonly encountered in surfactant/ water systems), often with a bicontinuous cubic phase at the intermediate stage of the interface curvature.3 How* To whom correspondence may be addressed. E-mail: [email protected]. † Laboratoire de Physique Statistique, CNRS URA 1306. ‡ Laboratoire de Physico-Chimie Macromole ´ culaire, Universite´ Pierre et Marie Curie, CNRS URA 278. § Present address: Department of Applied Mathematics, Research School of Physical Sciences and Engineering, The Australian National University, GPO Box 4, Canberra, ACT 0200, Australia. | Centre de Ge ´ ne´tique Mole´culaire, CNRS UPRA 2420. ⊥ Centre de Recherche Paul Pascal. (1) Brown, G. H.; Wolken, J. J. Liquid Crystals and Biological Structures; Academic Press: New York, 1979. (2) Gelbart, W. M.; Ben-Shaul, A. J. Phys. Chem. 1996, 100, 1316913189. (3) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1-46.

ever, the self-energy and entropy requirements counteract the above tendency. Therefore, if the surfactant is such that a high local curvature is preferred, cylindrical defects form spontaneously in the lamellar phase.4,5 Multicomponent systems exhibit a more complex phase behavior at various length scales due to the relaxed structural restrictions.2 For example, mixtures of two surfactants of different spontaneous curvatures in water exhibit a first-order phase transition driven by competing curvatures.6 Another avenue to affect the membrane topology has been recently opened by demonstrating that a certain class of random heteropolymers, namely, poly(styrene-r-sodium styrenesulfonate), may become completely embedded in the hydrophobic interior of the surfactant bilayer.7-9 The addition of polymer moleculessa third componentsinto the water-lipid system adds extra flexibility to the shape and possible deformation of interfaces. Thus, when the bilayers (or monolayers in some phases) contain a variety of molecules such as in the naturally occurring biological systems, the interactions between the hydrocarbon chains and the hydrophilic headgroups are altered. It is wellknown that interactions between the lipids and integral membrane proteins, which are coupled to the orderdisorder transitions within the lipidic matrix, alter both (4) Allain, M.; Kle´man, M. J. Phys. (Paris) 1987, 48, 1799-1807. (5) Siegel, D. P. Biophys. J. 1986, 49, 1155-1170. (6) Andelman, D.; Kozlov, M. M.; Helfrich, W. Europhys. Lett. 1994, 25, 231-236. (7) Radlinska, E. Z.; Gulik-Krzywicki, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C. E.; Ober, R. Phys. Rev. Lett. 1995, 74, 42374240. (8) Radlinska, E. Z.; Gulik-Krzywicki, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C. E.; Ober, R. In Short and Long Chains at Interfaces; Daillant, J., Guenoun, P., Marques, C., Muller, P., Tran Thanh Van, J. Eds.; Editions Frontieres: Gif sur Yvette, 1995. (9) Radlinska, E. Z.; Gulik-Krzywicki, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C. E. J. Phys. II 1997, 7, 1393-1416.

S0743-7463(98)00193-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/13/1998

Phase Transition by Polymer Content

the membrane and protein functions and eventually may lead to phase transformations.10-12 To study the role of heteropolymers in the phase transitions of lyotropic liquid crystals, we have chosen water solutions of a nonionic surfactant, pentaethylene glycol dodecyl ether (C12E5). In ambient conditions, this system passes through a sequence of phases: normal hexagonal (HI)-cubic (V)-lamellar (LR) as the volume fraction of the amphiphile is increased. The microstructure evolves from the hexagonal close packing (HI) order of the cylindrical surfactant aggregates of a highly curved interface, through the interconnected structure of cylinders with 3D cubic periodicity (favored by the curvature/ interaction energy factors), to a lamellar phase with flat interfaces, which allow for the most efficient packing of highly concentrated surfactant molecules in solution. The choice of pentaethylene glycol n-dodecyl ether (C12E5) for this study has been motivated by a number of reasons. First, we have recently demonstrated that in the lamellar phase region of the C12E5/water/poly(styreneco-styrenesulfonate) system the polymer molecules remain incorporated inside the surfactant bilayers as isolated entities.7 However, the range of stability for this polymerdoped lamellar phase critically depends on the sufractantto-water ratio and the polymer sulfonation degree. In this work, the polymer sulfonation degree has been chosen at the limit of solubility in water (x ) 0.30) in order to ascertain that the polymer molecules are likely to have strong affinity for the hydrophobic part of the microstructure. Second, the electrostatic interactions between the polymer and the surfactant are absent, which helps to avoid adding extra complexity to the already rather complicated system of interactions. In this work we present experimental evidence that the polymer molecules incorporated into the surfactant/water normal hexagonal phase induce a direct gradual transition to a lamellar structure, with no intermediate cubic phase. This is demonstrated using small-angle X-ray scattering, freeze fracture electron microscopy, and polarizing optical microscopy. Experimental Procedures Pentaethylene glycol n-dodecyl ether (abbreviated as C12E5) was obtained from Nikko Chemicals Co., Ltd., Tokyo, Japan (high purity grade >99%), and used as received. Poly(styrene-r-sodium styrenesulfonate) (abbreviated as PS1-xNaPSSx, x being the degree of sulfonation or charge content) was produced at ESPCI by postsulfonation of polystyrene of weight average molecular weight 250 000 and a polydispersity (Mw/Mn) of 2.13 PS1-xNaPSSx is water soluble for degrees of sulfonation ranging from 30% to 100%.14,15 The water was purified using Millipore filtering. The binary and ternary mixtures were prepared by weight in glass ampules. Immediately after preparation the ampules were flame sealed. They were then heated to 90 °C and shaken, slowly cooled, and centrifuged several times in order to homogenize the sample. This procedure was repeated until the appearance of sample remained stable. To investigate the phase diagram, the samples were stored in a thermostated water bath and visually inspected in transmitted light between crossed polarizers. The phases were further examined using a polarizing microscope. (10) Gulik-Krzywicki, T. Biochim. Biophys. Acta 1975, 415, 1-28. (11) Albertini, G.; Ponzi-Bossi, M. G.; Rustichelli F. In Phase Transitions in Liquid Crystals; Martellucci, S.; Chester, A. N., Eds.; NATO ASI Series B: Physics Vol. 290; Plenum Press: New York, 1992. (12) Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1-69. (13) Makowski, H. S.; Lundberg, R. D.; Singhhal, G. S. US Patent 3870841, EXXON Research and Engineering Company, 1975. (14) Essafi, W.; Lafuma, F.; Williams, C. E. ACS Symp. Ser. 1994, No. 548, 278-286. (15) Essafi, W.; Lafuma, F.; Williams, C. E. J. Phys. II 1995, 5, 12691275. Essafi, W. Ph.D. Thesis, University of Paris VI, 1996.

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Figure 1. SAXS data for a series of samples of the ternary C12E5/water/PS0.7NaPSS0.3 system acquired at the temperature 21 °C: A, no polymer added; B, 1.0 wt % of polymer; C, 3.1 wt % of polymer; D, 4.8 wt % of polymer; E, 7.7 wt % of polymer. For all samples the surfactant-to-water ratio is 1.0. Note a gradual transition from a hexagonal (A, indicated with arrows) to the lamellar (E, indicated with arrows) scattering pattern. Samples for the microscopy studies were placed between a microscope glass and cover slip spaced with a Mylar film and sealed around the edges using Eccobond 286, a general purpose epoxy adhesive. The phases were examined using an Olympus HB-2 polarizing optical microscope equipped with a Mettler central processor (FP80HT) and a Mettler hot stage (FP82HT). The temperature was controlled to (0.1 °C. Samples used for small-angle X-ray scattering (SAXS) work were sealed in a Lindemann glass capillaries of diameter about 1 mm. SAXS measurements were performed at the College de France using a conventional 0.9 m Kratky camera equipped with a rotating cathode Cu KR source and a 1-D detector. The data were corrected for the scattering of empty capillary and detector efficiency based on the scattering from water. For freeze fracture electron microscopy, a layer of liquid sample was deposited on a thin copper holder, squeezed between the holder and a thin copper plate, and then quenched in liquid propane.16 Fracturing was performed by removing the upper plate with a liquid nitrogen cooled knife. The replication was done using unidirectional shadowing at an angle 35° with a platinum-carbon 1-1.5 nm thick metal deposit. The replicas were observed in a Philips EM 410 electron microscope. The contrast in images is related to the depth fluctuation of the metal deposit.

Results and Discussion The SAXS experiments were performed on a set of samples of the ternary C12E5/water/PS0.7NaPSS0.3 system with the constant surfactant-to-water ratio of 1.0 and various amount of polymer. The temperature was maintained at 21 ( 0.1 °C. The results are presented in Figure 1. As a reference, SAXS data were collected for a sample without polymer, and Figure 1A shows a typical pattern for the normal hexagonal phase (HI). A mesophase (16) Aggerbeck, L. P.; Gulik-Krzywicki, T. Methods Ezymol. 1986, 128, 457-472.

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Figure 2. Freeze-fracture electron micrographs of the ternary C12E5/water/PS0.7NaPSS0.3 system for surfactant-to-water ratio 1.0 and increasing polymer concentrations at (A) 1.02 wt %, (B) 1.62 wt %, and (C) 2.14 wt % of the polymer. Note the presence in (A) of characteristic for hexagonal phase striations corresponding to the fractures propagating between the longitudinally oriented cylinders (black arrow). The two spots of optical diffraction (white arrows on the diffraction pattern in the upper right corner) correspond to the mean distance between translationaly ordered cylinders. (B) and (C) are images of a lamellar phase. Note the presence of some particles (black arrows) on the otherwise smooth oblique fracture surfaces in C. The bar length represents 250 nm.

composed of hexagonally packed, infinite cylindrical aggregates with two-dimensional periodicity is characterized in the small-Q range by an array of Bragg reflections originating from the planes of type (10,0), (11,0), (20,0), (21,0), etc.17 The corresponding scattering vectors remain in the ratio 1:x3:x4:x7. Only the first three peaks are detectable in our samples (Figure 1A) owing to large thermal disorder inherent in the liquid-crystalline structure. (17) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, U.K., 1952; Vol. II.

As a third component (the polymer) is gradually added to the system, the structure is initially preserved, as seen for a sample containing 1 wt % of polymer (Figure 1B). The microstructure is hexagonal, as evidenced by the first two reflections and a weak band in place of the third one. However, one can notice a significant broadening of the main peak and varied relative intensities of the Bragg reflections: the second peak becomes larger than that for the polymer-free sample and the third one is washed-out and very weak. As even more polymer is introduced to the system, the first harmonic of the (10,0) reflection

Phase Transition by Polymer Content

becomes very broad with its maximum fixed at a constant Q-value, whereas the other peaks coalescence into a broad band gradually shifting toward larger Q-values. This is shown in spectra C and D of Figure 1 for samples with the polymer concentration 3.1 wt % and 4.8 wt %, respectively. When the polymer concentration is further increased to 7.7 wt %, the X-ray scattering spectrum smoothly evolves and shows two harmonics superimposed on a broad background, which is characteristic of a well-defined lamellar structure (Figure 1E). The large scattering background is typical for a lamellar phase with a deformed layer interface.18 The evolution from the hexagonal phase to the lamellar phase is demonstrated in Figure 2 using freeze fracture electron microscopy (FFEM). The surfaces exposed by cleavage of a rapidly frozen specimen of the normal hexagonal phase propagate along the interface between water and the longitudinally oriented cylinders. The ordered arays of cylinders are visible on the micrograph in Figure 2A, thus demonstrating hexagonal ordering up to a polymer concentration of 1.0 wt %. The morphology of the hexagonal phase seems to be unaffected by the presence of polymer, and the polymer molecules cannot be unambiguously identified in the micrographs. For higher polymer concentrations (1.6 wt % and above) the FFEM images show lamellar ordering, in agreement with SAXS results. In the case of a lamellar phase, the surfaces exposed by cleveage are midsections through the hydrophobic center of the bilayer. The stair-step effect represents points where the fracture plane jumps to adjacent bilayer. In contrast to the hexagonal phase, the micrographs of the lamellar phase show polymer molecules embedded in the hydrophobic interior of bilayers (Figure 2C), similar to the situation reported before for a more surfactant-concentrated region of the lamellar phase in this system.7-9 Furthermore, there is only one phase (either hexagonal or lamellar) visible in the FFEM micrographs for all the polymer concentrations investigated here. The interesting case of a sample with 1.6 wt % of polymer (Figure 2B) reveals a lamellar structure with discernible steps and large, although crumpled, hydrophobic surfaces, broken in all directions and containing irregular rounded lumps. The texture seen in the micrograph is so crumpled that it is difficult to unequivocally identify the images of polymer molecules. The microstructure of the consecutive stages of the hexagonal-to-lamellar transition was also recorded using polarization microscopy. For the binary C12E5/water mixture containing 50 wt % of surfactant, a typical fanlike structure characteristic of hexagonal phase is observed (Figure 3A). Different domains are clearly separated by sharp boundaries. The radial lines, originating at the cross section points of the disclination lines of the structure and the optical axes of individual domains, serve as evidence of thermal undulations of the cylinders.19 The structure of a sample containing 0.6 wt % of polymer consists of much smaller domains than those in the binary hexagonal system, mixed with some large domains (Figure 3B). Further addition of the polymer leads to hexagonal texture with small domains only (see Figure 3C for a sample containing 1 wt % of polymer). Eventually, a gradual evolution of the sample texture from the fanlike (hexagonal) to a focal conic pattern of smectic A structure20 is observed in agreement with SAXS results (see Figure 4). (18) Nallet, F.; Laversanne, R.; Roux, D. J. Phys. II 1993, 3, 487. (19) Saupe, A. J. Colloid and Interface Sci. 1977, 58, 549-558. (20) Hartshorne, N. H. In Liquid Crystals and Plastic Crystals; Ellis Horwood, Ltd.: New York, 1974.

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Figure 3. Polarization micrographs of fanlike texture in the hexagonal phase for a series of samples of the ternary C12E5/ water/PS0.7NaPSS0.3 system with low polymer content: (A, top) no polymer added; (B, middle) 0.6 wt % of polymer; (C, bottom) 1.02 wt % of polymer. The micrographs were taken at the temperature 21 °C between crossed polarizers at magnification 100×. For all samples the surfactant-to-water ratio is 1.0. Note the decrease of the domain size as the polymer concentration increases.

Freeze fracture electron microscopy has proven to be a valuable tool for microstructure determination as well as detection of the polymer molecules. Often the combination of the three methods, SAXS, FFEM, and polarizing microscopy, is quite enough to fully characterize the phase diagram. Unfortunately, here we face a more difficult situation. The closely overlapping Bragg spectra of the hexagonal and lamellar phase, broadened peaks, and the high scattering background lead to interpretative uncertainties. Over the 2 year period of sample equilibration we observed no macroscopic phase separation. Also, there

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Figure 4. Polarization micrographs for a series of samples of the ternary C12E5/water/PS0.7NaPSS0.3 system of high polymer content taken at 21 °C (magnification 100×): (A, top left) 1.62 wt % of polymer; (B, bottom left) 2.97 wt % of polymer; (C, top right) 3.9 wt % of polymer; (D, bottom right) 7.7 wt % of polymer. For all samples the surfactant-to-water ratio is 1.0.

is no sign of coexistence of two topologically different phases, as confirmed by both the electron microscopy and optical microscopy observations. Such a coexistence has been observed before in this system for larger than these studied here surfactant-to-water ratios,9 where the SAXS and FFEM data indicated the presence of microdomains of two different lamellae, lamellar and cubic or lamellar and a polymer-rich phases. Unfortunately, at this stage of our study we are not able to rule out the possibility of the presence of a narrow two-phase region and, therefore, determine the order of the phase transition. Moreover, the surfaces exposed by cleavage in a normal hexagonal phase are at the water-surfactant interface along the side of the layer of closely packed cylinders. They have a very symmetrically striated appearance with no granular structure, same as observed in the HII phase in phosphatidylethanolamine-deficient Escherichia coli cells.21 Experimental findings presented above support the visualization of the HI-LR transition as a progressive deformation of the surfactant-built cylinders brought about by the increased polymer concentration. We postulate at least partial incorporation of large polymer molecules into the hydrophobic core of the cylinders. The gradual change from a well-defined 2D hexagonal microstructure of long cylindrical objects toward the wellaligned 1D bilayer structure seems to occur homogeneously in the entire sample volume. As the polymer concentration increases the cylinders fragmentate or deform, which leads to several effects in the hexagonal (21) Rietveld, A. G.; Verkleij, A. J.; de Kruijff, B. Biochim. Biophys. Acta 1997, 1324, 263-272.

Figure 5. Schematic representation of the microstructural evolution of the C12E5/water/PS0.7NaPSS0.3 system from the HI to LR phase upon addition of polymer.

phase: smaller domain size (as seen in polarizing microscopy), broadening of the SAXS peaks, and the disappearance of second-order harmonics (reflections from (20,0) planes). The simultaneous increase of the intensity of a second Bragg peak (this peak originates from the reflections from (11,0) planes) is indicative of the increased ordering of first neighbors, which is consistent with the notion of cylinders becoming deformed and tending to connect. A schematic representation of this process is shown in Figure 5. Eventually, the hexagonal order is lost owing to the extensive deformation and progressing fusion of cylinders. Finally, a fully developed lamellar phase is formed with the same period as the distance

Phase Transition by Polymer Content

Figure 6. Polarization micrograph of a peripheral region in the ternary C12E5/water/PS0.7NaPSS0.3 system with the polymer concentration gradient taken at 21 °C (magnification 100×). The polymer concentration decreases from right to left. Note two well-defined regions of different texture: mosaic structure of a lamellar phase (right-hand side) and fan texture of a hexagonal phase (left-hand side). There is no additional phase present at the intermediate polymer concentrations.

between the planes of highest packing density in the hexagonal phase, and the second-order harmonic due to scattering by the 1D layered structure reappears. The latter observation remains in agreement with the characteristic distances calculated for both phases from the position of Bragg reflections. For the hexagonal phase we obtain the distance between the amphiphilic cylinders a ) 58.2 Å, which corresponds to the spacing between the (10,0) planes d ) 50.4 Å. For the lamellar phase, the measured period is D ) 49.4 Å. The agreement between d and D is excellent considering the different compositions of corresponding phases: the HI phase exists in the binary system and the LR phase forms in the ternary system containing 7.7 wt % of polymer. In a pioneering series of papers Ranc¸ on and Charvolin have shown that in the homologous C12E6/water mixtures there are epitaxial relations between the planes of highest density of matter in the three consecutive phases: HI, V, and LR.22,23 Also, they have shown that cylindrical deformations of locally hexagonal structure appear in the lamellar phase before the equilibrium cubic phase is formed. A similar process of fragmentation and rupture of hexagonal phase has been observed on the cubic/ hexagonal boundary. These relations appear to be quite general. A convenient way to study the concentration dependence of the phase sequence in surfactant lyotropic systems is the microscopic observation of peripheral regions.24 We used a variation of this method by exposing the original hexagonal phase of the binary surfactant-water system (S/W ) 1.0) to the polymer concentration gradient. This was done in situ by placing a flake of polymer into a sample sealed between the microscopic slide and a cover slip. The micrograph in Figure 6 clearly shows the development of a typical mosaic pattern of the lamellar phase as the polymer concentration increases. The absence of additional phases in the transition region provides further evidence of the direct H1-LR phase-transition mechanism. The melting pattern of the HI phase provides an additional argument supporting the notion of a gradual (22) Ranc¸ on, Y.; Charvolin, J. J. Phys. Chem. 1988, 92, 2646-2651. (23) Ranc¸ on, Y.; Charvolin, J. J. Phys. Chem. 1988, 92, 6339-6344. (24) Rogers, J.; Winsor, P. A. J. Colloid and Interface Sci. 1969, 30, 500-510.

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Figure 7. Melting of the hexagonal phase in the binary C12E5/ water with 50 wt % of the surfactant: crossed polarizers; T ) 22 °C; magnification 100×.

Figure 8. Melting of the hexagonal phase in the ternary C12E5/ water/PS0.7NaPSS0.3 system containing 1.0 wt % of the polymer: crossed polarizers; T ) 22 °C; magnification 100×. Note the texture of the new phasesthin needles approximately oriented in one direction.

smectic ordering induced in the hexagonal phase by polymer molecules prior to the HI-LR phase transition. The hexagonal phase of the binary C12E5/water system melts into an isotropic phase as the temperature is elevated to about 22 °C. A polarizing micrograph of the two-phase region taken during melting of this system is shown in Figure 7. In contrast to this, during melting at 22 °C the hexagonal phase of a sample containing 1 wt % of polymer shows an assembly of thin light needles predominantly oriented in one direction (Figure 8), which is the direction of smectic ordering. An analogous example for a hexagonal-cubic phase transition showing four preferred growth directions has been discussed in reference 22. Conclusions We have investigated ternary aqueous solutions of a nonionic surfactant C12E5 and a highly hydrophobic random heteropolymer, poly(styrene-co-styrenesulfonate) of sulfonation degree 30%, along the path of the surfactantto-water ratio equal to unity. We present evidence that at a temperature of 21 °C the increase of polymer content causes a phase transformation from the hexagonal microstructure of long cylindrical objects to a bilayer structure of the lamellar phase. The observed phase evolution is consistent with the notion of polymer molecules being embedded, at least partially, in the hydrophobic part of the liquid-crystalline

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matrix. The specific path from the HI to LR phase observed here may be the consequence of the size of macromolecules: they are too large to facilitate a gradual change of the hydrophobic/hydrophilic interface topology, as is the case in the binary surfactant/water system. Nevertheless, the existence of a cubic phase has been demonstrated before in the ternary system with a large surfactant content.9 This could be seen as evidence that the ratio of free-to-bound water molecules (in this case equal to 2) plays an important role in microstructure formation in this system, as postulated before.9

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Acknowledgment. We thank L. Bon Nguyen for the preparation of polymer and J. C. Dedieu for excellent technical assistance with the electron microscopy. We are grateful to R. Ober for making his SAXS equipment available to us, for his assistance, and for useful discussions. We express our gratitude to C. E. Williams, W. Urbach, J. Charvolin, and P. Pincus for stimulating discussions. LA980193S