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Copolymerization of Styrene with a Cationic Surfactant Monomer in Three-Component Lyotropic Mesophase† Dirk Pawlowski and Bernd Tieke* Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburgerstrasse 116, D-50939 Ko¨ ln, Germany Received January 29, 2003. In Final Form: March 7, 2003 Copolymerization, phase behavior, and structural properties of the ternary lyotropic mesophase system formed by the surfactant monomer (2-methacryloyl-oxyethyl)dodecyldimethylammonium bromide (1), styrene, and water are described. Lyotropic mesophases occur if the total monomer concentration is higher than 55 wt % and 1 is the major component. At low styrene content (1/styrene molar ratio of 10:1 or 5:1), hexagonal (HR) and cubic (QR) phases are formed, while at high styrene content (1/styrene ratio of 3:1) a lamellar phase exists. The lyotropic phases can be polymerized upon γ-irradiation. At low conversion, a styrene-rich copolymer is formed, whereas at high conversion, a surfactant-rich copolymer is obtained. Homopolymers are not detected. Small-angle X-ray scattering measurements and polarizing microscopy indicate that the structure of the HR-phase is preserved upon the polymerization. The LR-phase undergoes a transition into the HR-phase. Independent from the comonomer ratio, nanostructured gels with hydrophilic and hydrophobic compartments are formed. Structure models of the copolymerization process are presented.
1. Introduction The preparation of nanostructured materials is an active research area in materials chemistry. Unfortunately, only a few techniques are available for constructing man-made materials with a structural control in the nanometer size regime. One of these techniques is surfactant aggregation in aqueous solution leading to a variety of molecular assemblies with spherical, rodlike, and sheetlike structures.1-4 However, the disadvantage of surfactant assemblies is that they are intrinsically fluid in nature. Consequently they are easily disrupted or distorted by physical or chemical forces, which renders the assemblies inattractive for applications as functional materials. Several years ago, attempts were made to stabilize lamellar structures of surfactants such as monolayers, Langmuir-Blodgett films, or vesicles upon polymerization.5-7 For this purpose, polymerizable surfactants (“surfactant monomers” or “surfmers”) were used and polymerization was carried out after their organization in layered structures.5-7 The same concept of stabilization upon polymerization was also applied to three-dimensionally aggregated structures such as micellar solutions,8 microemulsions,9,10 or lyotropic liquid crystalline (llc) phases of surfactants.11-13 Especially the polymerization of llc †
Part of the Langmuir special issue dedicated to David O’Brien. * To whom correspondence should be addressed.
(1) Gruner, S. M. J. Phys. Chem. 1989, 93, 7562. (2) Seedon, J. M. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 380. (3) Hoffmann, H.; Ulbricht, W. Chem. Unserer Zeit 1995, 29, 76. (4) Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (5) Tieke, B. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach: Philadelphia, 1992; pp 105-181. (6) Tieke, B. Adv. Mater. 1990, 2, 222. (7) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (8) Paleos, C. M. In Polymerisation in Organized Media; Paleos, C. M., Ed.; Gordon and Breach: Philadelphia, 1992; pp 183-214. (9) Barton, J. Prog. Polym. Sci. 1996, 21, 399. (10) Candau, F. In Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999; pp 679-712. (11) Miller, S. A.; Ding, J. H.; Gin, D. L. Curr. Opin. Colloid Interface Sci. 1999, 4, 338. (12) O’Brien, D. F.; Armitage, B.; Benedicto, A. Acc. Chem. Res. 1998, 31, 861.
phases has recently become an active research area because new materials with large inner surfaces suitable as nanocomposites, catalyst carriers, or drug delivery devices may become accessible. However, only a limited number of systems were reported in which the structure of the monomeric system was retained after polymerization.14-17 In many systems, different mesostructures were obtained,18 the mesophase order got lost,19 or the polymerization was incomplete.18,20,21 Unfortunately, many authors characterized only the parent and final structures, whereas few investigations were concerned with the changes of the system during the reaction.22,23 Thus important information on the preparation of stable polymeric systems representing copies of the monomeric ones is still lacking. In our contribution, we report on the copolymerization of a cationic surfmer and styrene in a ternary lyotropic mesophase system. Phase behavior and structure are characterized before, during, and after the reaction. The system contains the surfactant monomer (2-methacryloyloxyethyl)dodecyldimethylammonium bromide (1) (for the chemical structure, see Chart 1), styrene, and water. The surfactant 1 carries the polymerizable methacrylate group near to the polar headgroup and thus may be denoted as an “H-type surfmer”. First attempts to polymerize 1 in llc phases were reported by McGrath and Drummond,24 but Pawlowski and Tieke25 were the first (13) Meier, W. Macromolecules 1998, 31, 2212. (14) Srisiri, W.; Benedicto, A.; O’Brien, D. F.; Trouard, T. P.; Ora¨dd, G.; Perssou, S.; Lindblom, G. Langmuir 1998, 14, 1827. (15) Srisiri, W.; Sissou, T. M.; O’Brien, D. F.; McGrath, K. M.; Han, Y.; Gruner, S. M. J. Am. Chem. Soc. 1997, 119, 4866. (16) Lee, Y.-S.; Yang, J.-Z.; Sissou, T. M.; Frankel, D. A.; Gleesou, J. T.; Aksay, E.; Keller, S. L.; Gruner, S. M.; O’Brien, D. F. J. Am. Chem. Soc. 1995, 117, 5573. (17) Gray, D. H.; Gin, D. L. Chem. Mater. 1998, 10, 1827. (18) McGrath, K. M. Colloid Polym. Sci. 1996, 274, 499. (19) Thundathil, R.; Stoffer, J. O.; Friberg, S. E. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2629. (20) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 316. (21) McGrath, K. M. Colloid Polym. Sci. 1996, 274, 399. (22) Leister, C. L.; Guymon, C. A. Macromolecules 2000, 33, 5448. (23) Pawlowski, D.; Tieke, B. Prog. Colloid Polym. Sci. 2001, 117, 182.
10.1021/la0341557 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/25/2003
Copolymerization in a Ternary Lyotropic Mesophase Chart 1. Chemical Structure of (2-Methacryloyloxyethyl)dodecyldimethylammonium Bromide (1)
who succeeded in the polymerization of 1 in hexagonal and cubic mesophases. The copolymerization of 1 and acrylamide in llc phases was also studied.23 Moreover, the copolymerization of 1 and styrene in an oil/water microemulsion was investigated in great detail.26 Copolymerization in llc phases or microemulsions is still a rather new topic, and only a few investigations have been reported yet.23,26-32 New nanostructured materials may be obtained as well as copolymer compositions not accessible by conventional polymerization methods. Thus it was of interest to also investigate the copolymerization of 1 and styrene in a highly concentrated surfactant solution showing llc properties and to characterize the system before, during, and after polymerization. The studies were carried out using polarizing microscopy, small-angle X-ray scattering (SAXS), and infrared spectroscopy. A kinetic study of the polymerization process was also carried out. The polymerization was initiated using γ-irradiation. High-energy radiation has several advantages over other methods: the samples are homogeneously penetrated, it is possible to initiate the polymerization at room temperature, and additional initiator compounds are not needed. 2. Experimental Section 2.1. Materials. The cationic surfactant monomer 1 (for the chemical structure, see Chart 1) was prepared by reaction of dimethylaminoethyl methacrylate with 1-bromododecane in acetone at 41 °C according to procedures described in the literature.25,33 A white solid was obtained which was recrystallized from ethyl acetate and dried in a vacuum (mp, 84 °C). Styrene (>99%) was purchased from Fluka and distilled before use. All experiments were carried out using deionized water (Milli-Q water). Lyotropic solutions of the 1/styrene/water ternary system were prepared by mixing the surfactant with previously degassed water, followed by stirring for 10 min under purging with nitrogen to keep the oxygen content as low as possible. Subsequently, styrene was added using a microliter syringe and the lyotropic solution was stirred again for 10 min. Then the samples were sealed and stored at 5 °C in the dark for 24-28 h. Polymerization was carried out by exposing the samples to 60Co γ-irradiation (dose rate, 0.388 kGy h-1). The conversion to polymer was determined gravimetrically. For this purpose, the samples were treated with hydroquinone and water after irradiation in order to inhibit the polymerization. The diluted samples were shaken for 24 h in order to extract water-soluble (24) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 612. (25) Pawlowski, D.; Haibel, A.; Tieke, B. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1865. (26) Dreja, M.; Pyckhout-Hintzen, W.; Tieke, B. Macromolecules 1998, 31, 272. (27) Dreja, M.; Tieke, B. Macromol. Rapid Commun. 1996, 17, 825. (28) Pyrasch, M.; Tieke, B. Colloid Polym. Sci. 2000, 278, 375. (29) Fu, X.; Qutubuddin, S. Langmuir 2002, 18, 5058. (30) Antonietti, M.; Hentze, H. P. Colloid Polym. Sci. 1996, 274, 696. (31) Summers, M.; Eastoe, J.; Davis, S.; Du, Z.; Richardson, R. M.; Heenan, R. K.; Steyther, D.; Grillo, I. Langmuir 2001, 17, 5388. (32) Li, F.; Zhang, Z.; Friberg, S. E.; Aikens, P. A. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2863. (33) Nagai, K.; Ohishi, Y. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 445.
Langmuir, Vol. 19, No. 16, 2003 6499 components and subsequently filtered under pressure (overpressure, 1 bar) using a nylon membrane with a pore size of 0.1 µm. The residue was washed with water several times until the filtrate remained clear upon addition of silver ions. Finally, the residue was washed with toluene to remove styrene monomer and potential styrene homopolymer and dried to constant weight in a vacuum. 2.2. Methods. The composition of the copolymer was determined using IR spectroscopy. The ratio of the band intensities of the CdO stretching mode at 1650 cm-1 (ester group) and the aromatic out-of-plane bending mode at 698 cm-1 (styrene) was used to evaluate the composition. The phase behavior was studied using a standard Zeiss polarizing microscope equipped with a hot stage and a camera (Zeiss MC 80). To investigate the phase diagrams, a few drops of the lyotropic solution were placed between two glass slides, heated to the isotropic state, sheared, and recooled to room temperature. Subsequently, the phase transition temperature was determined by viewing the samples in the polarizing microscope during slow and constant heating at a rate of about 0.2 °C min-1. SAXS was carried out on sealed samples in fine glass capillary tubes (1 mm diameter) using a Kratky compact camera (Anton Paar) with block collimation. Ni-filtered Cu KR radiation with λ ) 0.154 nm was used. The acceleration voltage was 40 kV at an anode current of 30 mA. Reflexes were monitored in continuous scan mode. The scattering curves were corrected for slit-smearing effects by a computational desmearing procedure.25
3. Results and Discussion 3.1. Phase Behavior of the Monomeric System. The phase behavior of the ternary 1/styrene/water system was investigated using polarizing microscopy. Three different comonomer molar ratios 1/styrene of 10:1, 5:1, and 3:1, the total monomer concentration ranging from 50 to 90 wt %, were studied. Let us first discuss the partial phase diagram of the system with a 10:1 ratio. For representation of the phase behavior, we decided not to use the usual triangle representation, but plotted the temperature versus the total monomer concentration of surfmer plus styrene in the aqueous solution instead. The advantage of this representation is that the phase behavior can be easily compared with the binary 1/water system23,25 and the ternary 1/acrylamide/water system.23 The diagram of Figure 1a indicates a hexagonal and a cubic phase, which are also present in the binary system and the ternary system with acrylamide as comonomer.23 The HR-phase was identified from the typical fanlike texture appearing under crossed polarizers and from the first-order peak appearing in the SAXS pattern shown in spectrum A of Figure 2a. From the SAXS peak with a 1/d value of 0.2967 nm-1, a d spacing of 3.37 nm could be calculated. This value is 0.29 nm larger than for the styrene-free binary system due to the solubilization of styrene in the hydrophobic core of the cylindrical micelles. The bicontinuous QR-phase was identified from its SAXS pattern (not shown) and (indirectly) from the highly viscous, optically isotropic region appearing between crossed polarizers. For a solution containing 25 wt % of water, the phase transition from the HR to the isotropic microemulsion phase occurs at 46 °C. This is about 10 °C lower than for the corresponding binary 1/water mixture.23,25 It indicates that the mesophase stability becomes lower upon the solubilization of styrene. A similar result was found when hydrophilic, water-soluble acrylamide was added to the binary system.23 While styrene is solubilized in the hydrophobic region of the cylindrical micelles of 1, especially in the core of the rodlike micelles, acrylamide is physically bound to the headgroups of 1. In both cases, the effect is the same: according to the SAXS measurements,23,25 the
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Figure 2. SAXS patterns of the 1/styrene/water ternary system with a total initial monomer concentration of 70 wt % and a 1/styrene molar ratio of 10:1 (a) and 3:1 (b). Samples were measured before irradiation (A) and after exposure to 51.8 kGy (B) and 79 kGy (C). The polymer content of the irradiated samples can be estimated from the conversion vs dose curves of Figure 7.
Figure 1. Phase diagram of the 1/styrene/water ternary system with a 1/styrene molar ratio of 10:1, without irradiation (a) and after exposure to 14 kGy (b) and 99 kGy (c). Gray areas indicate two-phase regions.
diameter of the cylindrical micelles increased, that is, the interfacial curvature decreased and thus the HR-phase became less stable. In the system with a 5:1 molar ratio of 1/styrene, the styrene concentration is higher and the mesophase stability is further decreased (Figure 3a). At room temperature, the HR-phase exists from 62 to 74 wt %. For a total monomer concentration of 70 wt %, the phase transition into the isotropic microemulsion phase has decreased to 36 °C. Heating of the QR-phase no longer leads to a phase transition into the microemulsion phase, but in the temperature range from 43 to 48 °C an oily streak texture occurs, indicating the formation of a lamellar structure. Its phase region is only small and reaches from 78 to 83 wt %. The heating increases the molecular mobility so that the styrene is now less effectively stabilized in the rodlike micelles than in the sheetlike structure of the lamellar phase.
A further increase of the styrene content causes a decrease in the interfacial curvature of the micelles until finally the lamellar structure becomes more stable. At a comonomer ratio of 3:1, only the LR-phase is left at room temperature. Its concentration range is from 62 to 87 wt % (Figure 4a). At higher monomer concentration, the surfactant 1 forms hydrated crystals in addition to the lamellar phase. The presence of the lamellar phase is evident from the oily streak texture appearing under crossed polarizers (Figure 5a,b) and from the SAXS pattern shown in Figure 2b (diagram A). The X-ray diagram shows four nearly equidistant peaks with 1/d values of 0.1035, 0.2105, 0.3137, and 0.4060, which can be ascribed to Bragg reflections from the smectic layers. It is striking that the third-order peak is more intense than the first-order one. The origin for this is unclear. From the four 1/d values, an average d spacing of 9.64 ( 0.08 nm could be calculated. Assuming a maximum length x of 3.2 nm for the surfmer 1 in the stretched conformation, one obtains a bilayer thickness 2x of 6.4 nm. The distance x can be easily calculated from usual bond lengths and angles. As shown in the structure model of the lamellar phase in Figure 6, the residual spacings y and z of altogether about 3.2 nm can be ascribed to the intercalation of water and styrene in the hydrophilic and hydrophobic interlayers, respectively. Some styrene might also be among the hydrophobic
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Figure 4. Phase diagram of the 1/styrene/water ternary system with a 1/styrene molar ratio of 3:1, without irradiation (a) and after exposure to 75 kGy (b). Gray areas indicate two-phase regions.
Figure 3. Phase diagram of the 1/styrene/water ternary system with a 1/styrene molar ratio of 5:1, without irradiation (a) and after exposure to 14 kGy (b) and 92 kGy (c). Gray areas indicate two-phase regions.
tails of the surfactant. Depending on the water content of the system, the lamellar phase is stable up to 48-65 °C. At these temperatures, a transition into the isotropic microemulsion phase takes place. 3.2. Polymerization. For polymerization, the mesophase samples were exposed to different γ-irradiation doses. Subsequently the conversion to polymer was determined and the effect of the irradiation on the structure and phase behavior of the samples was studied. During γ-irradiation, the viscous lyotropic solutions turned into water-insoluble polymer gels, which were swellable in water or toluene. To determine the conversion to polymer, the residual styrene and surfactant monomers were leached out from the polymer gel by washing with toluene and water, respectively. Then the gel was dried and the conversion was determined gravimetrically. In Figure 7, the conversion versus dose curves of ternary mixtures with a total monomer concentration of 70 wt % and different comonomer molar ratios of 10:1, 5:1, and 3:1
are shown. All samples could be polymerized. After exposure to 300 kGy, the conversion to polymer was between 80 and 95%, the highest value being reached for the sample with the lowest styrene content. The initial reaction rate was highest for the sample with a 3:1 ratio, in which the lamellar phase was present, followed by the samples with 10:1 and 5:1 ratios, in which the HR-phase was present. The polymerization in the hexagonal mesophase of the binary 1/water system proceeds even faster than in the 10:1 mixture.25 In the binary system, 90% conversion was already reached after exposure to a γ-ray dose of 50 kGy. Thus we conclude that in the hexagonal phase the reaction rate decreases with increasing styrene concentration of the system and that in the lamellar phase the reactivity is higher than in the hexagonal phase. The latter may be due to a higher mobility of the styrene molecules in the layered structure of the lamellar phase than in the core of the cylindrical micelles in the hexagonal phase. All conversion versus dose curves show an initially slow reaction which is accelerated once a radiation dose of 100 kGy is exceeded and slows again at radiation doses higher than 180 kGy. The latter is due to hindered monomer diffusion in the highly viscous, increasingly polymerized system and to monomer depletion at the end of the reaction. The initial rise can be ascribed to an increase of the number of growing chain ends due to the continuous irradiation. Moreover, it can be assumed that the molecules of 1 undergo the same structural transition in the headgroup region as in the binary mixture at low conversion. In the binary mixture, 1 exhibits a confor-
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Figure 7. Conversion vs dose behavior for γ-ray polymerization of the 1/styrene/water system with different 1/styrene molar ratios. The initial total monomer concentration was 70 wt %.
Figure 5. Optical textures from the 1/styrene/water ternary system with a 1/styrene molar ratio of 3:1: total monomer concentration of 70 wt % at room temperature (a) and total monomer concentration of 90 wt % at 60 °C (b). The oily streak texture indicates the presence of the lamellar phase.
Figure 6. Model of the structure of the lamellar phase found in the ternary 1/styrene/water system at a high styrene content.
mational transition from a cyclic24,34 to an elongated structure in the headgroup region at about 5% conversion, which is accompanied by an acceleration of the polymerization, because the monomer units attain a more reactive conformation in the new phase.23 3.3. Composition of the Copolymer. To get more information on the polymerization process, we analyzed the copolymer composition and the composition of the filtrate obtained after washing out the residual monomer. (34) Hamid, S. M.; Sherrington, D. C. Polymer 1987, 28, 332.
The composition was studied using infrared spectroscopy. In Figure 8, the compositions of the copolymer and the comonomers in the filtrate are plotted versus the radiation dose. Samples with initial comonomer ratios of 10:1, 5:1, and 3:1 were investigated. At irradiation doses below 100 kGy, the polymer content still was very low and did not exceed 20%. So it was not possible to obtain a reliable analysis of the copolymer composition. Copolymer obtained after irradiation with 130 kGy always contained styrene in excess, while in the filtrate the surfactant monomer 1 was enriched. With increasing conversion, the copolymer composition gradually approached the initial comonomer composition. This means that at high conversion the surfmer is incorporated in the polymer in excess. Homopolymers of styrene or 1 were not detected, in agreement with previous results obtained for the (co)polymerization of 1 and styrene in an oil-in-water microemulsion.26,27 Since the initially formed copolymer contains styrene in excess, it can be concluded that the reaction starts in the core of the cylindrical micelles of the HR-phase, and in the hydrophobic interlayers of the lamellar phase, where the styrene is solubilized. Due to the rapid exchange of the surfactant molecules in the mesophase, the whole system is very mobile and the growing chain ends easily come into contact with the reactive units of the surfmer molecules. As a consequence, the chain growth jumps between the methacrylate groups near to the aqueous phase and the styrene in the hydrophobic core of the mesophase. Since the styrene is more rapidly consumed than 1, the chain growth progressively shifts to the headgroup region of the surfmer molecules during the reaction. Since the outer surfaces of adjacent micelles are in close contact, the growing chain ends are exchanged and thereby the system gets cross-linked. A gel is formed, as confirmed by the observations. A structure model showing the initial copolymer formation in the cylindrical micelles of the HR-phase and the cross-linking at higher conversion is represented in Figure 9a,b, respectively. The model draws an analogy to the polymerization behavior of recently investigated H-type surfmer/styrene/water systems in microemulsions,26-29 which are also converted into nanostructured gels upon polymerization. 3.4. Changes of Structure and Phase Behavior during Polymerization. The changes of structure and phase behavior during polymerization were investigated using polarizing microscopy and SAXS. The change of the phase behavior of samples with comonomer ratios of 10:1, 5:1, and 3:1 is indicated in Figures 1b,c, 3b,c, and 4b. For the samples with a 10:1 ratio, phase diagrams were determined after exposure to 14 and 99 kGy, corresponding to a conversion to polymer of less than 5 or approximately 10%, respectively. In Figure 1b,c, it can be seen that the
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Figure 9. Structure model of the 1/styrene/water system in the HR-phase (a) before polymerization (left) and after inital polymerization primarily inside the hydrophobic core (right) and (b) after complete polymerization. A copolymer gel is obtained, in which the polymerized surfactant molecules form an amphiphilic network, whereas the polystyrene is located in the hydrophobic cores of the cylindrical micelles.
Figure 8. The 1/styrene molar ratio in copolymer and residual monomer after various γ-ray doses for samples with different initial 1/styrene ratios of 10:1 (a), 5:1 (b), and 3:1 (c). The initial total monomer concentration was always 70 wt %.
irradiation increases the width and the height of the hexagonal phase region. After exposure to 99 kGy, the transition temperature into the micellar phase increased from 45 to 70 °C for a sample with 70 wt % total monomer plus polymer concentration. The HR-phase was extended to a concentration range from 55 to 86 wt % at room temperature, while the QR-phase was no longer detectable. SAXS diagrams of samples with a 10:1 comonomer ratio and 70 wt % total monomer plus polymer concentration are shown in Figure 2a,b. After exposure to 51.8 kGy, the 1/d value of the reflection of the HR-phase increased to 0.3118 nm-1 (Figure 2a, diagram B). This corresponds to a d value of 3.27 nm, which still is 10% larger than for the styrene-free binary mixture of the same total monomer plus polymer concentration.23 The difference is due to the presence of the partially polymerized styrene in the micellar cores. Upon further irradiation, the 1/d value decreased again; at a γ-ray dose of 79.0 kGy, a d value of
0.290 nm-1 was reached, which corresponds to a d spacing of 3.48 nm (Figure 2a, diagram C). The change can be ascribed to an increase of the diameter of the micellar rods as was analogously found for the binary 1/water system.19 In the binary system, the increase was ascribed to a conformational transition of the surfmer molecules taking place when a few percent of the surfactant molecules were polymerized. With increasing conversion, the X-ray peak becomes broader and less intense, indicating an increasing polydispersity of the system, as was also found when 1 and styrene were copolymerized in a microemulsion. The polydispersity is a result of monomer diffusion in the system in order to compensate the monomer consumption in some of the micelles caused by the polymerization process. So the monomer micelles shrink, while the partially polymerized ones increase in diameter. For the samples with a 5:1 comonomer ratio, the size of the HR-phase region also increases upon γ-irradiation (Figure 3b,c). After exposure to a γ-ray dose of 92 kGy, the HR-phase occurred between 57 and 82 wt % of the total monomer plus polymer concentration, the transition to the isotropic phase occurring at about 56 °C. Simultaneously, the small LR-phase appearing at elevated temperatures and high total monomer content of 80 wt % rapidly disappeared upon γ-irradiation. The phase region of the cubic phase decreased, but its transition temperature to the microemulsion phase slightly increased to 45 °C, if the radiation dose was 92 kGy. We also investigated the polymerization of the styrenerich samples with a comonomer ratio 1/styrene of 3:1.
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These samples exhibit a lamellar phase at room temperature, if the total monomer content exceeds 62 wt % (Figure 4a). If these samples are exposed to a radiation dose of 75 kGy, the lamellar phase almost completely disappears and instead a hexagonal and a cubic mesophase are observed (Figure 4b). The hexagonal phase is stable up to 57 °C, where a transition into an unknown isotropic phase occurs followed by the appearance of a lamellar phase at 59 °C. This phase is stable up to 75 °C. It exists only in the small concentration range from 73 to 77 wt %. Above 75 °C, the optically isotropic microemulsion phase is formed. At concentrations above 86 wt %, a highly viscous, optically isotropic phase is found at room temperature, which likely is the bicontinuous cubic phase. It is stable up to 45 °C. At a water content of less than 10%, the surfactant is not sufficiently hydrated anymore and crystals appear under crossed polarizers. The radiation-induced transition from the lamellar to the hexagonal phase is also evident from the SAXS measurements. SAXS diagrams of samples with 70 wt % total monomer concentration, exposed to 51.8 or 79.0 kGy, are shown in Figure 2b, diagrams B and C. The two samples exhibit a single reflection, the 1/d value being 0.333 nm-1 after a radiation dose of 51.8 and 0.324 nm-1 after 79 kGy. The values correspond to d spacings of 3.0 and 3.08 nm, respectively. Especially at the higher irradiation dose, an intense, sharp reflection is obtained, which indicates a well-ordered, hexagonal array of the rodlike micelles, the order being higher than in the samples exclusively polymerized in the hexagonal phase (compare with Figure 2a). We believe the lamellar-to-hexagonal phase transition can be ascribed to the immiscibility of the polystyrene formed upon the radiation process in the paraffin matrix of the surfactant molecules. The transition from the sheetlike to rodlike structure is therefore driven by a minimization of the interface between the two species. In addition, depletion of styrene at higher conversion may also favor the transition. Nevertheless, a complete separation of polystyrene from the residual material is not observed. This can be ascribed to the exclusive formation of a copolymer in a highly organized gel structure, in which the polymerized surfactant molecules form an amphiphilic network structure, whereas the polystyrene portions are located in the hydrophobic core of the cylindrical micelles.
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4. Summary and Conclusions Our studies indicate that the ternary system 1/styrene/ water forms lyotropic mesophases once the total monomer concentration exceeds about 55 wt %. At low styrene content, hexagonal and cubic mesophases are formed, while at higher styrene content with a molar ratio of surfmer/styrene of 3:1, a lamellar phase is apparent. γ-Irradiation of the system leads to exclusive formation of a copolymer; a high conversion of more than 80-90% can be reached. While at low conversion styrene is preferentially incorporated in the copolymer, the polymer chains formed at high conversion contain the surfmer in excess. Polymerization stabilizes the HR-phase by extending the phase region and increasing the phase transition temperature, but the order of the mesophase becomes somewhat reduced. In a small concentration range, the QR-phase is also stabilized. Polymerization of the LR-phase proceeds under transition into the HRor QR-phase depending on the composition of the system. The HR-phase obtained after the phase transition is well-ordered; the SAXS peaks are sharper than for the system polymerized exclusively in the hexagonal phase. The irradiation process leads to a physically cross-linked copolymer gel, in which the polymerized surfactant molecules form an amphiphilic network structure, whereas the polystyrene is located in rodlike shapes in the hydrophobic cores of the cylindrical micelles. Since both parts are copolymerized and the individual cylindrical rods are cross-linked by chains extended over two or more micelles, the system is very stable to chemical or physical perturbations such as heating or addition of solvents. A more detailed characterization of the materials properties will be the subject of future investigations. Due to the mesoscale order, a large inner surface is present in the gel, making it attractive for a variety of applications such as drug delivery devices, chromatographic resins, or catalyst carriers. Acknowledgment. The authors thank Astrid Haibel, II. Physikalisches Institut der Universita¨t zu Ko¨ln, for the SAXS measurements. LA0341557
