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Langmuir 1998, 14, 2670-2676
Polymer Gels with a Micron-Sized, Layer-like Architecture by Polymerization in Lyotropic Cocogem Phases Markus Antonietti,* Christine Go¨ltner, and Hans-Peter Hentze Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Kantstraeszette 55, D-14513 Teltow-Seehof, Germany Received September 23, 1997. In Final Form: February 20, 1998 Polymerization of hydrophilic monomers, such as acrylamide, in the aqueous lyotropic phases of counterioncoupled gemini surfactants (“cocogems”) results in highly ordered gels with a layerlike architecture between the submicrometer and micrometer length scale. The order of such gels is preserved even after removing the templating surfactant. The evolution of the gels is monitored using polarized light optical microscopy, rheology, small-angle X-ray scattering, and scanning electron microscopy (SEM). The synthesis envokes structural changes of the templating lyotropic phase structure as well as demixing of the gelling polymer and the surfactant phase. The overall gel morphology is, however, still controlled by the anisotropy of the surfactant assembly. Hence, the polymer gel is not a direct cast of the original phase structure, but its growth is indirectly controlled by the ordered arrays. The persistence of the lyotropic liquid crystalline phases of cocogems is high enough to preserve a layered order, however with the characteristic size of a demixing structure rather than that of the parental lyotropic phase. Chemical functionalization of these gels is easily achieved by copolymerizing functional comonomers, such as acrylic acid or dimethylaminoethyl methacrylate. It is expected that these ordered gels, simple to prepare and inexpensive, will find a number of applications in various fields of modern materials science, such as for the construction of size-selective membranes or high-performance material hybrides.
I. Introduction The implementation of polymerization reactions in organized media is a modern approach in colloid science.1 The diverse aspects summarized under this heading include the polymerization inside channels and between sheets of organic and inorganic solids (see, for instance, refs 2-4), the polymerization in microemulsion (e.g., refs 5 and 6), and the polymerization of micelles and vesicular phases (e.g., refs 7-9). Besides influencing particular molecular properties of the polymer chains, such as tacticity or copolymerization ratios, polymerization is intended to synthesize highly ordered polymeric structures with characteristic structure elements of the order of some nanometers, possibly via direct blueprinting of the original, nonpolymeric patterns. The main objective of structuring matter on a micrometer lengthscale is the introduction of new materials properties (e.g.) into chemically known compounds. From the viewpoint of materials science, the interest in polymerization in ordered media is focused on the creation of complex polymer structures from standard monomers by simple procedures. Among the routes mentioned above, polymerization in liquid crystalline, lyotropically ordered phases or microemulsions seems to be the most promising strategy. In these cases, the key problem is the thermo(1) Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon Science Publishers: Philadelphia, 1992. (2) Farina, M.; DeSilvestro, G. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1988; Vol. 12, p 486. (3) Blumstein, A. J. Polym. Sci. 1965, A3, 2653 and 2665. (4) Tieke, B.; Wegner, G. Makromol. Chem. Rapid Commun. 1981, 2, 543. (5) Candau, F. In Polymerization in organized Media; Paleos, C. M., Ed.; Gordon Science Publishers: Philadelphia, 1992; p 215. (6) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (7) Martin, V.; Ringsdorf, H.; Thunig D. Polymerization of Organized Systems. Midl. Macrom. Monogr. 1977, 3, 175. (8) Egorov, V. V.; Zubov, V. Russ. Chem. Rev. 1987, 56, 1153. (9) Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adv. Polym. Sci. 1985, 64, 1.
dynamic influence of the growing polymer chain on its template, the microemulsion or the lyotropic phase. Specific interactions as well as the entropy loss of a polymeric chain in a confined geometry usually result in a loss of order on the nanometer scale, and nonordered products with nevertheless interesting properties are obtained. Some of these problems have been reviewed recently in context with polymerizations in a microemulsion;5,6 the loss of the lyotropic order and the appearance of a new structure hierarchy in the micrometer range were demonstrated within the polymerization of bicontinuous microemulsions, resulting in spongelike polymer networks.10-12 Attempts to use lyotropic liquid crystalline phases as media for polymerization have been covered in the literature;13-20 however, none of them reached the desired goal, namely direct casting of the mesophase into a solid polymer. Learning from these reports is the basis of the work presented here. Preparation of polymers and networks in the aqueous, continuous phases containing globular surfactant micelles or subphases, as reported by Stoffer and Bone,13,14 Vaskova et al.,15 and Friberg et al.,16-18 resulted in the disruption of the phase structure: (10) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1993, 34, 3305 and references cited therein. (11) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Lee, L.; Ng, S. C.; Pey, K. L.; Grant, D. Langmuir 1995, 11, 3321 and references cited therein. (12) Antonietti, M.; Hentze, H. P. Colloid Polym. Sci. 1996, 274, 696. (13) Stoffer, J. O.; Bone, T. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 264. (14) Stoffer, J. O.; Bone, T. J. Disp. Sci. Technol. 1980, 1, 37; 1980, 4, 393. (15) Vaskova, V.; Juranicova, V.; Barton, J. Macromol. Chem. Macromol. Symp. 1990, 131, 201. (16) Gan, L. M.; Chew, C. H.; Friberg, S. E. J. Macromol. Sci. Chem. 1983, A19, 739. (17) Gan, L. M.; Chew, C. H.; Friberg, S. E.; Higashimura, T. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1585. (18) Gan, L. M.; Chew, C. H.; Friberg, S. E. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 513. (19) Vrij, A. Pure Appl. Chem. 1976, 48, 471. (20) Menger, F. Angew. Chem., Int. Ed. Engl. 1991, 30, 1086.
S0743-7463(97)01054-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/23/1998
Polymerization in Lyotropic Cocogem Phases
Even in this simple case of a casting reaction, originally transparent reaction mixtures remain transparent only in very small regions of the phase diagram. The depletion force between the micelles or nanodroplets, caused by an osmotic pressure difference of the polymer chains between two droplets and outside this gap,19 causes this phase separation. Grinding the resulting solids, however, gave highly functionalized powders with very large specific surface areas, which were tested in specific adsorption experiments.20 The depletion force between lyotropic phases and the high-molecular weight polymer solution is obviously very strong. Therefore, it is expected that the introduction of some specific type of enthalpic interaction can assist in preserving the mutual adherence or compatibility between polymer and template. Such attractive interactions are known to play a crucial role in the formation of mesoporous silicas via the “true liquid crystal” approach, where a 1:1 copy of the original template is obtained.21,22 The development of inorganic materials with ordered pores was recently discussed in ref 23. Descriptions of “successful” polymerizations in cubic or other extended microemulsions and lyotropic phases are described in the literature (e.g., refs 24 and 25), but the analysis of the polymerization products relies on smallangle X-ray analysis only. It was recently shown by Burban, He, and Cussler via combination of a variety of instrumental techniques that such a procedure gives not enough evidence to prove direct structure imprinting into the polymer network, since most systems demix in a polymer subphase without local order and in a liquid crystalline subphase with high order, which is responsible for the X-ray peaks. There is usually no order left when the surfactant is removed.26 This was confirmed in our lab for a number of classical surfactant/monomer systems. In a recent paper, we showed that microemulsion formulations can be significantly improved by modifying classical ionic surfactants by coupling with appropriate organic counterions.27 Systems in which two or three surfactant tails were coupled by a polyfunctional organic couterion, so-called counterion-coupled gemini surfactants or cocogems, were of special interest, since it turned out that they form very stable microemulsions with extraordinarily large specific interface areas. In addition, it is possible to incorporate additional functional groups into the organic counterion, which results in functionalized surfactant arrays able to mediate interaction. So far, all studies of cocogem systems were focused on globular mesophases. It seems straightforward to employ cocogems to bypass some of the difficulties of the other surfactant-based recipes for the generation of ordered gels. Although the use of lyotropic phases of cocogems cannot prevent structure changes in the system during polymerizations, it allows preservation of a very high, anisometric order on an intermediate lengthscale, which is speculatively attributed to a higher stability and mechanical persistence of counterion-coupled surfactants. II. Experimental Part II.1. Synthesis of Cocogems and Polymerization Procedure. The synthesis of the cocogems was performed as described in ref 27. Surfactants with organic coun(21) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. Nature 1995, 378, 366. (22) Go¨ltner, C.; Antonietti, M. Adv. Mater. 1997, 9, 431. (23) Tanev, P.; Pinnavaia, T. J. Science 1995, 267, 242. (24) Strom, P.; Anderson, D. M. Langmuir 1992, 8, 961. (25) Laversanne, R. Macromolecules 1992, 25, 489. (26) Burban, J. H.; He, M.; Cussler, E. L. AIChE J. 1995, 41, 907. (27) Antonietti, M.; Hentze, H. P. Adv. Mater. 1996, 8, 840.
Langmuir, Vol. 14, No. 10, 1998 2671
terions were purified by a liquid-liquid extraction procedure from water/2-butanol. All systems were used as crystalline powders containing less than 5% inorganic salt. The polymerization procedure in lyotropic phases follows the classical recipes of radical polymerization. For instance, 5.00 g of CTMA2-tartrate was mixed with 5 mL of deionized water, and a mixture of 4.50 g of acrylamide, 0.50 g of N,N′-methylenebis(acrylamide) (cross-linker), and 50 mg of potassium peroxodisulfate (initiator) was added. After homogenizing with a high-speed stirrer, the lyotropic phase was equilibrated for 16 h at room temperature. The mixture was polymerized by heating the system to 60 °C for 16 h. The cross-linking density was varied between the ratios of 1/5 and 1/100 (w/w). II.2. Instruments. The phase behavior of the systems was studied by visual observation and polarized light optical microscopy before and during polymerization. To map the phase diagram, mixtures of varying constitutions were prepared. The birefringence of lyotropic hexagonal and lamellar mesophases was observed by placing the samples on glass supports between crossed polarizers of an Olympus BH-2 microscope. Small-amplitude oscillatory shear measurements were conducted during the polymerization using a Bohlin CVO50 rheometer with a cone-plate geometry (CP 1/40). A plate radius of 20 mm and a cone angle of 1° were used. The dynamic storage and loss moduli, G′ and G′′, were recorded at a polymerization temperature of 60 °C at a constant shear frequency of ω ) 2 Hz and a strain of 0.02. X-ray scattering measurements were obtained using a standard Kratky camera. The setup for the experiments has been previously described in detail.28 The measurements were performed in an s range of 1.0 × 10-2 nm-1 < s < 9.0 × 10- 1 nm-1 (the scattering vector s is defined as s ) 2/λ sin θ where 2θ is the angle between incident and scattered light). Peak positions and widths were determined by fitting a smeared Lorentzian function to the scattering curves. The morphologies of the polymer gels were examined using scanning electron microscopy (SEM). The porous polymers were dried by the critical-point technique after a gradient exchange of the water by acetone and subsequent solvent exchange to supercritical CO2. This technique is a standard method for SEM preparation of soft matter, originating from artifact-poor examination of biological matter,29 but being increasingly employed also for the characterization of polymer gels.30 To the best of our knowledge, this technique practically does not influence the superstructure of our gels, despite the fact that the gels are depicted in a dried instead of a water-swollen state. The dried and freeze-fractured samples were sputtered using a Pd/Ir target and examined with a Zeiss DSM 940 A scanning electron microscope. III. Results and Discussion Mesophase formation of the cocogems was characterized by polarization microscopy and SAXS and compared with the behavior of the parental cetyltrimethylammonium bromide (CTMA-Br) phase. Parts a and b of Figure 1 summarize these data by presenting the phase diagrams of CMTA-Br and CTMA2-tartrate in the range of 20-90 w % surfactant and temperatures between 25 and 90 °C. (28) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (29) Reimer, L.; Pfefferkorn, G. Scanning Electron Microscopy; Springer: Berlin, 1972. (30) Sawyer, L. C.; Grubb, D. T. Polymer Microscopy; Cambridge University Press: Cambridge, U.K., 1987.
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Antonietti et al. Table 1
sample
surfactant
m(surf.), g
m(H2O), g
m(acrylamide), g
m(bis(acrylamide)), mg
m(KPS), mg
CG0A CG0B CGOC
CTAB CTMA2-tartrate CTMA2-terephthalate
0.86 (43 w %) 0.86 (43 w %) 0.86 (43 w %)
0.57 0.57 0.57
0.514 0.514 0.514
56 (1/10) 56 (1/10) 56 (1/10)
30 30 30
Figure 2. SAXS diffractograms of a lyotropic CTMA2-tartate phase through all stages of polymerization: (A) scattering of the pure lyotropic phase, consisting of water and 43 w % surfactant; (B) mixture of this phase with acrylamide and crosslinker (sample CG0B); (C) scattering of the system after polymerization of the acrylamide; (D) scattering of the system after removal of the surfactant from the gel, but swollen with water. The scattering vector (s) is defined as s ) 2/λ sin θ where 2θ is the angle between incident and scattered radiation.
Figure 1. Schematic phase diagrams of CTMA-Br (a) and CTMA2-tartrate (b), as revealed by polarization microscopy and SAXS. The spot and the arrow indicate the positions at which polymerization reactions were performed.
It is seen that counterion modification shifts the phase diagram toward higher surfactant concentrations, as expected for an increased surfactant ratio. For CTMA2tartrate, we observe between the micellar phase (L1) and the hexagonal phase (HI), an additional cubic phase, whereas for CTMA-Br, cylindrical micelles form a hexagonal mesophase at lower concentrations. The hexagonal mesophase of the cocogem is, however, more stable and ordered, as indicated by the brightness and quality of the polarization textures and the time dependence of their appearance. Polymerization reactions were performed at 60 °C and at compositions indicated by the spot and the arrow in the
phase diagrams. For CTMA-Br, reaction starts in the center of the hexagonal phase, whereas for CTMA2tartrate, the whole range from micellar phase to hexagonal phase is employed. Besides CTMA-Br (as the reference) and CTMA2-tartrate, we also used CTMA2-terephthalate for polymerization. The latter two cocogems have turned out in previous experiments to be the most efficient for polymer structure control.27 Table 1 summarizes the compositions of these polymerization mixtures. All polymerization reactions afforded slightly opaque polymer gels, which can be handled without difficulty in the solid state. The mechanical stability of these gels is rather high for a water-swollen polyacrylamide gel; i.e., they can be stretched or cut without ripping and fracturing. For all samples containing 43 w % surfactant, the lyotropic order prior to polymerization is hexagonal, as revealed by optical microscopy and contact preparations. Small-angle X-ray curves, as depicted in Figure 2, allow quantification of the type and degree of order. The monomer-free mixture (surfactant + water) shows a narrow scattering peak (A), the corresponding Bragg distance of which (d ) 4.65 nm) correlates well with the expectation of a slightly swollen hexagonal phase. In addition, we find a broad, additional peak at lower scattering angles (corresponding Bragg distance is about d ) 14.2 nm), the origin of which is as yet unclear. Adding the water-soluble acrylamide, which in principle is expected to be located in the aqueous domains, shifts the main scattering peak to higher scattering angles and causes it to broaden (B). This is in contradiction with the picture of simple monomer swelling of the phases (where the lattice parameters increase but do not decrease). Since the decrease of the long period is directly connected with an increase of the absolute interface area, we conclude that the monomer is also incorporated in the surfactant film and extends the interface. At the same time, the
Polymerization in Lyotropic Cocogem Phases
local order is decreased by the monomer, as seen by the width of the peak. The reaction system after polymerization (C) shows a narrow scattering peak again, shifted to s ) 0.25 nm-1, corresponding to a Bragg distance of 4 nm. As compared to the structure of the parental surfactant solution, this corresponds to a volume shrinking of the lyotropic phase of no less than 35%, although the polymer should be included. Together with the SEM pictures shown below, this shrinking is explained by the decomposition of the reaction mixture into two subphases, one of them containing the majority of the surfactant molecules, but no polymer, the other containing the polymer network. Both subphases compete for the water, and the surfactant mesophase loses water, which shifts the repeat period to smaller values. The polymer gel in the purified state (D) contains no structure elements on the length scale of some nanometers after removal of the surfactant, in good agreement with the observations of Cussler, He, and Burban.26 This behavior found in X-ray analysis is supported by polarization microscopy: state (A) shows a distinctive fan texture, typical for hexagonal mesophases, whereas this texture is lost after addition of monomer (B); only a slight whitish decoration remains, indicating a weakly ordered, anisometric system. The fan texture reappears in similar strength after polymerization (C), whereas the purified polymer gel shows no birefringence at all (D). The morphology of the polymer gels was studied by using SEM. This method revealed that the three different surfactant systems examined influence the resulting morphology of the polymer network in very different ways. The gel derived from a lyotropic hexagonal mesophase of CTAB is essentailly nonordered (Figure 3a). While the standard surfactant CTAB seems to have no structuredirecting effect, it turns out that the cocogem systems CTMA2-terephthalate (Figure 3b) and CTMA2-tartrate (Figure 3c) induce a pronounced lamellar polymer morphology. The lamellae are partially bridged, which is an explanation for the good mechanical performance even after drying. In the case of the CTMA2-tartrate system, the sheetlike structure is even more pronounced and shows the characteristic grain boundaries found in lyotropic liquid crystal phases. The average thickness of the sheets is in the range of 500 nm, and they are bridged at distances of about 5 mm. This type of order is long range, and the gel sheets are about parallel until a grain boundary, presumably the one from the original lyotropic phase, is reached. The morphology found for these gels with its typical size cannot be a result of direct templating, i.e., a 1:1 blueprinting of the original microemulsion phase, as the characteristic length of the parental phase has increased by about 2 orders of magnitude. In this context, it must be underlined that at the beginning of the polymer all reaction mixtures are optically clear solutions, which turn opaque at short times into the reaction. Both observations give direct evidence that structure changes and demixing phenomena occur. To investigate possible demixing processes or phase transitions of the liquid crystalline mixture, the polymerization process was monitored rheologically. Dynamic mechanical shear experiments were employed in situ on one of the reaction mixtures. The same mixtures were polymerized on a heating stage and observed between the crossed polarizers of the optical microscope. The rheological curves of the CGA1 receipe are shown in Figure 4. Here, storage modulus G′, loss modulus G′′, and
Langmuir, Vol. 14, No. 10, 1998 2673
Figure 3. Scanning electron microscopy on critical-point dried gel samples generated in phases of three different surfactants. (a) Lyotropic CTAB phase as a structure-directing medium affords only disordered gels templating droplets are obtained. (b) The CTMA2-terephthalate phase results in the appearance of arrays with lamellar order. (c) The order is most pronounced when CTMA2-tartrate is used. Here, even the grain boundaries of the liquid crystalline arrays are cast in the final gel.
dynamic viscosity η* at 2 Hz are plotted versus reaction time t. G′ and G′′ of the original mesophase are found to be very low and out of the range of the chosen rheometer setup. The structure of the mixture at time zero is revealed by the X-ray diffractogram shown in Figure 2. As only a weak birefringence is detected in the monomer-swollen system, the degree of macroscopic order is low; i.e., the domains are on average smaller than the wavelength of visible light. During polymerization, the viscosity increases, and at the same time strong birefringence develops. About that time, the reaction mixture becomes optically turbid, which corresponds to earlier observa-
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Antonietti et al.
Figure 4. Rheological curves of sample CGA1 during polymerization at 60 °C, presenting the storage modulus G′, the loss modulus G′′, and the complex dynamic viscosity η*. After 6 min into the reaction, the system reaches its gel transition, and data can be acquired. The reaction is complete after 1 h, and the rheological characteristics remain constant.
tions.12 The opaqueness is an indication for demixing of the polymer gel and the surfactant template phase. After 6 min, G′ crosses G′′, and the system passes through its gel point. The birefringence and turbidity are maintained from this point on. A further stepwise increase of the storage modulus and the complex viscosity is not accompanied by any detectable phase transition of the lyotropic phase; we speculatively attribute this to the completion of the polymer gel structure, presumably by the interconnection between the single polymer layers. It should be pointed out that the mechanical strength of the final macroporous gel, despite its high porosity of 70%, is extraordinarily high (G′ ) 2 × 105 Pa), a crucial prerequisite for the application of such gels in separation devices.
From all these observations, we conclude that the surfactant/water mixture stays in a liquid crystalline state throughout the course of the reaction. The polymer gel phase, once formed, separates (for entropic reasons) from the templating mesophase (B). The demixing structure is governed by the anisotropy of viscosity as well as transport properties of the parental lyotropic liquid crystal; i.e., the kinetically controlled demixing results in anisometric droplets of the polymer phase (C). Due to the increasing amounts of polymer, the polymer and liquid crystalline phase finally underly a phase inversion (D), the polymer adopting an interconnected lamellar structure, the characteristic length of which is by far larger than the characteristic length of the lyotropic liquid crystalline system. It is seen that direct casting of the lyotropic liquid crystalline phase does not occur, because the polymer phase is already separated from the structuredirecting mesophase from the very beginning of the reaction. This is schematically shown in Figure 5. From this kinetic scenario, it becomes evident that the resulting gel structure sensitively depends on the amount of surfactant (determining the lyotropic phase structure and stability), the amount of initiator (controlling the relative rate of polymerization relative to the rate demixing), and the amount of cross-linker (adjusting the gel strength and the relative position of the gel point during polymerization). Therefore, all these parameters were varied over the widest possible range, as listed in Table 2. The decrease of the surfactant concentration (CGA14) expectedly causes the lamellar morphology to become more filigree and less ordered (Figure 6a) and finally to change into a “Swiss cheese” morphology (Figure 6b), in which the polymer gel grows around unconnected droplets or vesicles of the surfactant, thus resulting in 500 nm sized pores, which are homogeneously distributed throughout the gel. The surfactant concentration variation of these experiments goes along the line plotted in the phase
Figure 5. Scheme of the proposed reaction scenario. The reaction starts from a homogeneous lyotropic liquid crystalline phase (A). The polymer, once formed, demixes from the continuous lyotropic phase (B, polymer phase drawn in black)). The growth of the polymer domains is controlled by the anisotropic transport properties of the liquid crystalline phase (C). Increase of the amount of polymer leads to the interconnected gel structure (D), embedding the liquid crystalline phase with the majority of the surfactant. This subphase can be continuous, too. Table 2. Surfactant: CTMA2-Tartrate sample
m(surf.), g
CGA1 CGA2 CGA3 CGA4 CGA6 CGD1 CGD2 CGD3 CGD4 CGB1 CGB2 CGB3 CGB4
0.86 (43 w %) 0.76 (38 w %) 0.66 (33 w %) 0.54 (27 w %) 2.00 (50 w %) 0.86 (43 w %) 0.76 (38 w %) 0.66 (33 w %) 0.54 (27 w %) 0.86 (43 w %) 0.86 (43 w %) 0.86 (43 w %) 0.86 (43 w %)
a
R,a
%
60 55 50 45 67 60 55 50 45 60 60 60 60
m(H2O), g
m(acrylamide), g
m(bis(acrylamide), mg
m(KPS), mg
morphology
0.57 0.62 0.67 0.73 1.00 0.57 0.62 0.67 0.73 0.57 0.57 0.57 0.57
0.514 0.559 0.604 0.658 0.89 0.514 0.559 0.604 0.658 0.468 0.514 0.541 0.564
56 61 66 72 110 56 61 66 72 102 (1/5) 56 (1/10) 29 (1/20) 6 (1/100)
30 30 30 30 10 10 10 10 10 30 30 30 30
lamellar sheets with holes sheets with holes spherical pores lamellar layers and holes layers and holes layers and holes spherical pores lamellar lamellar scaffold-line scaffold-line
Relative amount of surfactant in the binary template (for comparison with the phase diagram, see Figure 1a,b).
Polymerization in Lyotropic Cocogem Phases
Langmuir, Vol. 14, No. 10, 1998 2675
Figure 6. Scanning electron micrographs, demonstrating the influence of surfactant concentration and degree of cross-linking on the gel structure. (a) The decrease of the surfactant concentration from 43% to 38% (CGA2) makes the lamellar structure very filigree. (b) A further decrease to 27% (CGA4) results in a “Swiss cheese” morphology. (c) Decreasing the cross-linking density from 1/10 to 1/20 (CGB3) leads to an enhanced importance of bridges between the lamellae. (d) For a cross-linking density of 1/100, the same amount of material is included in both platelets and bridges, and a scaffoldlike morphology is obtained.
diagram, Figure 1b; i.e., the gel shown in Figure 6a arises from the transition region between the hexagonal and micellar phases, whereas Figure 6b depicts the situation arising from micellar and nonextended phases. Decreasing the cross-linking density results in a different morphology, where the basic building principle of a layer-like architecture is, however, essentially preserved (CGB1-4). Due to the higher molecular weight, the probability of bridging between the primary sheets is apparently increased, and a scaffold-like superstructure with variable bridging density is obtained (Figure 6c,d). Decreasing the rate of polymerization causes an expected loss of order; i.e., we use the selectivity in this kinetically controlled environment with a lower reaction rate (CGD1-4 as compared to CGA1-4). In this context, it is important to bear in mind that the formation and re-formation of surfactant assemblies are dynamic processes that cannot put up resistance against elastic forces for long. To obtain highly ordered gels, it is therefore advisable to use high cross-linking densities and large amounts of initiator. Encouraged by the wide variability of possible structures, functional comonomers were introduced into such an optimized reaction mixture, which, in principle, would allow the one-step synthesis of an extended, functionalized macroporous polymer gel, as it is desirable for a variety of chromatographic purposes. For that, 10 w % of some hydrophilic comonomers were added to the standard recipe (CGA1), containing carboxy moieties, alkanesulfonic acid, tertiary amino, or simple hydroxy groups. The choice of comonomers and the stoichiometries of these experiments are summarized in Table 3 (CGC1-5).
Table 3. Surfactant: CTMA2-Tartrate sample CGC1 CGC2 CGC3 CGC4 CGC5 CGC6
comonomer (remainder acrylamide) 10 w % acrylic acid 10 w % 2-(acrylamido)-2-methyl-1propansulfonic acid 10 w % 2-(N,N-dimethylamino)ethyl methacrylate 10 w % 2-carboxyethyl acrylate 10 w % 4-vinylvenzoate 90% hydroxyethyl methacrylate 10% ethylene glycol dimethacrylate
morphology lamellar lamellar lamellar lamellar lamellar curved platelets
In all cases, highly ordered gels with a basic lamellar morphology were obtained. Some of the finest morphologies are depicted in Figure 7a-c. Acrylic acid (Figure 7a), for instance, is easily incorporated and even improves the degree of order as well as the stability of the gel bridges. The fine structure of gel sheets functionalized with dimethylamino groups is demonstrated in Figure 7b. Hydroxyethyl methacrylate was polymerized even as the structure-forming, major component, thus testing the sensitivity of the overall procedure to the variation of the monomer (CGC6). The influence of a complete exchange of the monomer by acrylate- or methacrylate-based monomers is demonstrated in Figure 7c. Using hydroxyethyl methacrylate as the monomer results in an anisotropic platelet-like gel morphology, the sheet seems to disintegrate into randomly shaped platelets.
2676 Langmuir, Vol. 14, No. 10, 1998
Figure 7. Scanning electron micrographs of functionalized gels: (a) gel sheets containing 10 w % of acrylic acid; (b) lamellar cationic gel, functionalized with 10 w % dimethylamino groups; (c) gel consisting of hydroxyethyl methacrylate instead of acrylamide. Here, the whole gel consists of randomly shaped platelets.
Antonietti et al.
terion-coupled gemini surfactants (cocogems) can be used to generate highly ordered polymer gels by polymerizing water-soluble monomers within the confined geometry of a microphase-separated medium. For a certain composition range, gels with a lamellar architecture were obtained, the characteristic size of which is, however, by 2 orders of magnitude larger than the lengthscale of the parental lyotropic phases. Instead of elements in the 5-50 nm range, micron-sized pores and gel sheets are formed. It is concluded that due to phase separation (for entropic reasons) of the gel from the lyotropic surfactant assembly, a direct cast of the original structure is impossible. The demixing structure is governed by the anisotropy of viscosity and transport properties of the initial lyotropic liquid crystal; therefore, lamellae of a much larger dimension are obtained. Variation of the cross-linking density and the surfactant concentration showed that the gel structure depends on both: the morphology can be changed in a certain range to show both pores within the layers and bridges between the layers, the extreme situation being a scaffold-like architecture at low crosslinking densities. Templating at lower surfactant concentrations in the isotropic range of the phase diagram results in the expected formation of spherical pores in a continuous gel matrix, the product of a nonanisotropic demixing process. It was also shown that the gels can be easily functionalized by copolymerizing appropriate functional comonomers. In this study, carboxy, sulfonate, hydroxy, and tertiary amino groups were incorporated, giving rise to gels with similar or even more pronounced structure. Complete exchange of the monomer (hydroxyethyl methacrylate instead of acrylamide) results in a different, but still ordered, gel structure. Due to this change, the mixing thermodynamics as well as the polymerization kinetics, such a dependence on chemical constitution had to be expected. Optimization of the polymerization recipe for other monomers was not performed. Polymer gels with well-defined micron-sized pores, defined connectivity and geometry, high mechanical stability, and adjustable chemical functionality fulfill important prerequisites for modern DNS and cell fragment analysis, all of which are based on hydrophilic polymer gels with pore sizes in the range of the species to be separated. A comparative examination in 2D gel electrophoresis is currently under way. With respect to application, it is pointed out that the presented gel synthesis is a simple one-step procedure performed within short time, being possible also on larger scales. Mesoand microconstruction of polymer gels using the structuredirecting influence of specialized surfactants is an intriguing tool to generate supermolecular structures and soft materials, of which promising developments can be expected.
IV. Conclusion and Outlook The properties of surfactants and their assemblies can be significantly modified by counterion exchange,27,31,32 which allows circumventing some of the weaknesses of classical surfactant systems as templates for supramolecular synthesis. The lyotropic phases of such coun(31) Antonietti, M.; Go¨ltner, C. G.; Hentze, H. P. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1699. (32) Bijma, K.; Engberts, J. B. F. N. Langmuir 1997, 13, 4843.
Acknowledgment. We thank B. Klein for technical assistance during surfactant and gel synthesis and M. Micha and I. Zenke for help with the SAXS measurements. We also thank Martin Neese for his help connected with the rheological measurements and Erich C. for endless mental support. Financial support by the Fonds der Chemischen Industrie and the Max Planck Society is gratefully acknowledged. LA971054Y
