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Concentrated Polymerized Cationic Surfactant Phases† Mark Summers and Julian Eastoe* School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
Robert M. Richardson Department of Physics, University of Bristol, Bristol BS8 1TS, United Kingdom Received February 3, 2003. In Final Form: March 18, 2003 Polymerizable surfactants (surfmers) 11-(methacryloyloxy)undecyltrimethylammonium bromide (A), dodecyl(11-(methacryloyloxy)undecyl)dimethylammonium bromide (B), 11-(acryloyloxy)undecyltrimethylammonium bromide (C), and dodecyl(11-(acryloyloxy)undecyl)dimethylammonium bromide (D) have been synthesized and characterized. Mixed micelles of A + B and C + D systems were polymerized by a thermal free-radical water-soluble initiator, to yield transparent solutions that remained stable for months. Copolymerization was followed by NMR, which was consistent with essentially 100% conversion. Lamellar mesophases (80% mass) were prepared by concentration of the polymerized micelles from dilute micellar systems. The mesophases were prealigned in a strong magnetic field prior to small-angle X-ray scattering (SAXS) and polarizing light microscopy studies. Detailed analysis of the SAXS data was consistent with stabilization of lamellar structures, comprising similar-sized internal domains, independent of the mixture compositions or surfactant type studied here. Hence, it is possible to synthesize polymerized liquid crystalline lamellar phases from such well-defined surfmer blends.
Introduction There is a need for easily attainable, robust reaction and templating media, which could offer selective control over domain size and structure for synthesis of nanoparticles and colloids. Owing to rich self-assembly on a nanometer length scale, surfactant phases, including lyotropic liquid crystalline mesophases, appear to be ideal candidates. Although conventional mesophases are more constrained than micellar and bilayer systems, physical or chemical forces can still disrupt the well-ordered structures. Mechanical stability could be enhanced by using polymerizable surfactants (surfmers), which are often denoted H-type or T-type, depending on whether the polymerizable group is located in the headgroup or hydrophobic tail region, respectively. There are numerous reviews of the area, to which the reader is referred for more detail (e.g., refs 1-5). One of the first reports on the polymerization of lyotropic liquid crystal phases was by Friberg et al.6 in 1980, and since then the work of O’Brien and co-workers (e.g., refs 7-11) has been among the most creative and significant in this exciting field. For example, O’Brien et al. carried * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +117 9289180. Fax: + 117 9250612. † Part of the Langmuir special issue dedicated to David O’Brien. (1) Gin, D. L.; Miller, S. A.; Ding, J. H. Curr. Opin. Colloid Interface Sci. 1999, 4, 338. (2) Guyot, A. Curr. Opin. Colloid Interface Sci. 1996, 1, 580. (3) Paleos, M. Chem. Soc. Rev. 1985, 14, 45. (4) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Polymerizable Surfactants. In Surfactants and Polymers in Aqueous Solution; John Wiley and Sons: New York, 1998; p 337. (5) Summers, M. J.; Eastoe, J. Adv. Colloid Interface Sci. 2003, 100102, 137. (6) Friberg, S. E.; Thundathil, R.; Stoffer, J. O. J. Polym. Sci. 1980, 18, 2629. (7) Dorn, K.; Klingbiel, R. T.; Specht, D. P.; Tyminski, P. N.; Ringsdorf, H.; O’Brien, D. F. J. Am. Chem. Soc. 1984, 106, 1627. (8) O’Brien, D. F.; Whitesides, T. H.; Klingbiel, R. T. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 95. (9) O’Brien, D. F.; Sisson, T. M.; Srisiri, W. J. Am. Chem. Soc. 1998, 120, 2322.
out the first successful polymerization of inverse hexagonal H2 mesophases.10 McGrath et al. (e.g., refs 12-15) published a series of papers on polymerization of various mesophases. In general, it was found that a maximum polymerization of approximately 30% could be achieved with the single-chained surfmer, which agreed well with results of Rodrı´guez et al.16 This low conversion was attributed to a reduced molecular mobility favoring termination, as polymerization in dilute aqueous micelles was found to be essentially 100%.16 However, it was found that partial cross-linking of the mesophases enhanced their stability toward temperature changes. Taken together, the investigations published by O’Brien and McGrath (e.g., refs 7-15) demonstrate important influences of molecular structure on the efficiency of polymerization in surfactant mesophases. Gin et al. have also made significant advances in this area.17-20 Inverse hexagonal phases were also successfully polymerized, and various materials were encapsulated inside the internal aqueous nanochannels. The approach taken here is distinctly different to the previous published work, in that well-defined mixtures of single-chain and double-chain T-type surfmers, shown in (10) O’Brien, D. F.; Lee, Y.-S.; Yang, J.-Z.; Sisson, T. M.; Frankel, D. A.; Gleeson, J. T.; Aksay, E.; Keller, S. L.; Gruner, S. M. J. Am. Chem. Soc. 1995, 117, 5573. (11) Srisiri, W.; Sisson, T. M.; O’Brien, D. F.; McGrath, K. M.; Han, Y.; Gruner, S. M. J. Am. Chem. Soc. 1997, 119, 4866. (12) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 316. (13) McGrath, K. M. Colloid Polym. Sci. 1996, 274, 399. (14) McGrath, K. M. Colloid Polym. Sci. 1996, 274, 499. (15) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 612. (16) Rodrı´guez, J. L.; Soltero, J. F. A.; Puig, J. E.; Schulz, P. C.; Espinoza-Martı´nez, M. L.; Pieroni, O. Colloid Polym. Sci. 1999, 277, 1215. (17) Smith, R. C.; Fischer, W. M.; Gin, D. L. J. Am. Chem. Soc. 1997, 119, 4092. (18) Deng, H.; Gin, D. L.; Smith, R. C. J. Am. Chem. Soc. 1998, 120, 3522. (19) Gin, D. L.; Gray, D. H.; Smith, R. C. Synlett 1999, 10, 1509. (20) Ding, J. H.; Gin, D. L. Chem. Mater. 2000, 12, 22.
10.1021/la034184h CCC: $25.00 © 2003 American Chemical Society Published on Web 04/22/2003
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Figure 2. The cmc’s of binary surfactant mixtures composed of surfmers A and B as a function of the mole fraction of A. The solid line represents calculations assuming ideal mixing via eq 2. T ) 25 °C.
Figure 1. Polymerizable surfactant monomers (surfmers): 11(methacryloyloxy)undecyltrimethylammonium bromide (A); dodecyl(11-(methacryloyloxy)undecyl)dimethylammonium bromide (B); 11-(acryloyloxy)undecyltrimethylammonium bromide (C); dodecyl(11-(acryloyloxy)undecyl)dimethylammonium bromide (D).
Figure 1, have been employed. The compounds A and B are single- and double-chained counterparts, containing a polymerizable methacrylate group on the hydrocarbon chain tips. Surfmers C and D are the acrylate derivatives, which are expected to be more reactive. With appropriate mixtures of A + B and C + D, the hydrophobic volume V and effective headgroup area aeff can be varied such that different curvatures may be accessed. These surfactants are related to the inert analogues n-dodecyltrimethylammonium bromide (DTAB) and the double-chain di-ndodecyldimethylammonium bromide (DDAB). It has been shown previously that the mixing of DTAB + DDAB is essentially ideal both in aqueous micelles21 and in negatively curved films in water-in-oil (w/o) microemulsions.22 Therefore, owing to their chemical similarity it is reasonable to expect that the reactive surfactants A + B (and C + D) also behave ideally in their mixtures. Studies of these surfmer mixtures in aqueous micellar and w/o microemulsions have been published elsewhere,23-25 and the feasibility of forming polymerized surfactant concentrates has been established.23 The composition of these mixtures may be defined by a mole fraction XA ) [A]/[A] + [B] (or XC ) [C]/[C] + [D]) and the total surfactant concentration measured in mass percent (which would be [A% + B%] or [C% + D%] as appropriate). This paper represents a more comprehensive investigation of these mesophase-type systems than previously published. The (21) Lusvardi, K. M.; Full, A. P.; Kaler, E. W. Langmuir 1995, 11, 487. (22) Bumajdad, A.; Eastoe, J.; Griffiths, P.; Steytler, D. C.; Heenan, R. k.; Lu, J. R.; Timmins, P. Langmuir 1999, 15, 5271. (23) Summers, M.; Eastoe, J.; Heenan, R. K. Chem. Mater. 2000, 12, 3533; Summers, M. J.; Eastoe, J.; Davis, S. A.; Du, Z.; Richardson, R. M.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2001, 17, 5388. (24) Summers, M. J.; Eastoe, J.; Davis, S. A. Langmuir 2002, 18, 5023. (25) Summers, M. J.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. J. Dispersion Sci. Technol. 2001, 22, 597.
polymerizations were carried out in dilute (micellar) phases; evidence from NMR was consistent with a high conversion from monomer to polymer. Mesophases have been obtained from surfactant mixtures, consisting of highly polymerized aggregates by controlled concentration of the dilute systems. Direct structural evidence from small-angle X-ray scattering (SAXS) and polarizing light microscopy (PLM) has shown that well-defined polymerized lamellar phases can be stabilized by these mixed surfmers. Experimental Section Surfmer Synthesis and Characterization. The surfmers were synthesized and characterized as described elsewhere.23-25 Extensive additional detail is given in the Supporting Information for this paper. The chemical structures of the intermediates bromoundecyl methacrylate, the corresponding acrylate, and the surfmers A, B, C, and D were established by NMR (JEOL Lambda 300), mass spectroscopy (VG Analytical Autospec), and elemental analysis. The dilute aqueous phase behavior, critical micelle concentrations (cmc’s) (Figure 2), and adsorption parameters of these surfmers and the mixtures were determined by surface tension and electrical conductivity measurements, described in the Supporting Information. Polymerization of Aqueous Phases and Preparation of Surfmer Concentrates. Samples of the desired composition were made up in D2O (Fluorochem, 99.9% D-atom) and heated in sealed vials to ensure complete homogenization. Polymerization was initiated by 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V50, Aldrich 97%), and the samples were heated to 60 °C for 2 h. Although the macroscopic appearance of the samples did not immediately change (Figure 3), a slight increase in viscosity was often noted after the polymerization. The reaction was monitored by the disappearance of vinyl signals in the 1H NMR spectra (e.g., Figure 4). Once the reaction was complete, the mesophases were prepared gravimetrically by controlled evaporation of water, using a vacuum oven and a four-figure balance. Polarizing light microscopy studies were performed using a Nikon Optiphot-2 microscope equipped with polarizing filters and a Nikon Optizoom ×0.8 - ×0.2 lens. The samples were held between a coverslip and microscope slide and thermostated to 25 °C on a Linkam hot-cold stage. Optical textures were captured digitally by linking to a personal computer fitted with an ATI graphics card, running ATI video player software. Small-Angle X-ray Scattering. SAXS experiments were carried out on a purpose-built diffractometer at the University of Bristol, using KR X-rays (1.54 Å), from a 1.5 kW sealed tube. The beam was cleaned by a nickel filter and a graphite monochromator. The diffraction pattern was detected using a multiwire area detector, placed at 840 mm from the sample with an evacuated path covering a Q range of 0.03 f 0.5 Å-1. The sample-to-detector distance was calibrated using a silver behenate standard. Samples were prepared as a function of a mixing
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be expected that surfmers mix homogeneously in the micelles, so that eventual polymerization results in a random copolymer. On the other hand, nonideal mixtures may give rise to local phase-segregated zones inside the micelles, having consequences for the structure of the final copolymers. For an ideal mixture composed of surfactant 1 and surfactant 2 (activity coefficients assumed to be unity), the monomer concentration of surfactant 1 CM 1 is given by mic CM cmc1 1 ) X1
Figure 3. Micellar solutions composed of methacrylate surfmers A and B, before and after polymerization, 3 months after sample preparation. Total surfactant concentration ) 0.15 mol dm-3. XA is the mole fraction of surfmer A in the mixtures.
(1)
where cmc1 is the cmc of pure surfactant 1, and Xmic 1 is the mole fraction of surfactant 1 in the mixed micelle. A similar expression exists for the other surfactant 2, which can be combined with eq 1 and mass balance to eliminate Xmic 1 , yielding the Clint equation (eq 2)26 accounting for ideal mixed micelle formation.
Xbulk 1 - Xbulk 1 1 1 ) + CMC cmc1 cmc2
(2)
CMC represents the effective cmc of the mixture, and Xbulk is the overall mole fraction of surfactant 1. However, 1 when intermolecular interactions become significant surfactant mixtures may exhibit synergism or antagonism, so this picture breaks down. Corkill27 was the first to extend this theory to take into account nonideality in terms of regular solution theory (RST). An activity coefficient fi is introduced, and eq 1 may be expressed as mic f1 cmc1 CM 1 ) X1
(3)
2 f1 ) exp[β(1 - Xmic 1 ) ]
(4)
and
Figure 4. Example 1H NMR spectra at 25 °C for micelles in D2O formed by mixed surfmers A and B, before (a) and after (b) the polymerization. Polymerization was thermally initiated (60 °C, 2 h). Total surfactant concentration ) 0.15 mol dm-3, and the mole fraction XA ) 0.90. mole fraction XA ()[A]/[A] + [B], or XC )[C]/[C] + [D],) and total surfactant concentration measured in mass percent. Nonpolymerized samples were made up in 1.00 mm quartz capillary tubes. Prior to SAXS measurements, these samples were prealigned by placing in a magnetic field (9.4 T), heated to the isotropic melt, and then cooled at a typical rate of 1 °C/min. Polymerized samples were held in a flat 1 mm thick cell with polyimide film windows. Measurements were carried out at 25 °C. PC-WAVE software was employed for viewing and analyzing the SAXS data.
Results and Discussion Dilute Aqueous Systems. Mixed Micelle Formation. The thermodynamics of mixed micelle formation was investigated, to see whether (or not) these surfmers mix ideally in micelles. In the case of ideal mixing, it might
In the above, β is known as the interaction parameter: If β is negative this represents a net attraction between surfactants 1 and 2, which gives rise to so-called “synergy”. In contrast, if β is positive this represents a net repulsion between surfactants 1 and 2, which gives rise to antagonism; such behavior is found with hydrocarbon-fluorocarbon mixtures.28 A good example of an ideal mixture is DTAB/DDAB mixtures (the inert analogues of A and B), which have been investigated by Kaler et al.21 The main finding was ideal mixing behavior (β ) 0), which is to be expected for like-charged surfactants with very similar molecular structures. Following standard formalism,27 eqs 5 and 6 can be written 2 exp[β(1 - Xmic Xmic 1 1 ) ] )
CMC Xbulk 1 cmc1
mic 2 (1 - Xmic 1 ) exp[β(X1 ) ] )
Xbulk CMC 1 cmc2
(5)
(6)
These latter equations represent a pair of simultaneous equations that can be solved to give β and Xmic 1 . (26) Clint, J. J. Chem. Soc. 1975, 71, 1327. (27) Corkill, J. M. Internal Report of the Procter & Gamble Co., Cincinnati, OH, 1974. This work was published after the death of Dr. Corkill; see: Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, 337. (28) Mukerjee, P.; Yang, A. J. Phys. Chem. 1976, 89, 1388.
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Figure 2 shows the variation of mixed CMC with mole fraction of A in A + B mixtures, as determined by electrical conductivity measurements. The solid line on Figure 2 represents ideal mixing in terms of eq 2. Surprisingly, this plot illustrates a slightly positive deviation from ideality, which correlates with a positive β value, representing a slight antagonistic interaction. The resulting values of β were 0.13, 0.25, and 0.29 at XA equal to 0.40, 0.60, and 0.80, respectively. Compared with the standard methyl-ended chain system represented by DTAB/DDAB mixtures, this deviation from ideality can be rationalized by the incorporation of the reactive methacrylate group at the chain terminus. The hydrophilic headgroup region remains essentially unaffected, whereas the hydrophobic environment of the micelle is clearly altered by the slight hydrophilic nature of the methacrylate group. The repulsion may be caused between adjacent methacrylate groups by a shift in electron density through a combination of inductive and conjugate effects. Alternatively, the effective “bulk” of the methacrylate group may cause increased steric strain for molecular packing, resulting in a slightly positive value for β. Another interesting observation is that β does not remain constant throughout. This is possibly caused by a structural transition in the micellar state. Comparing β values with those for the most antagonistic H-carbon/F-carbon systems known (β ≈ +3),28 the effect seen here is (on an absolute scale) quite weak. Therefore, the mixing behavior of surfmers A and B may be considered as “essentially ideal”, with only a minor perturbation from ideality. Similar experiments and analyses were carried out with the acrylate analogues C and D. In this case, good agreement was found between ideal solution theory (eq 2) and the experimental data, suggesting that these mixtures behave ideally (β ≈ 0). The observation that β is reduced by replacing the methyl group in the reactive tip with hydrogen (methyacrylates versus acrylates) suggests that the weak “repulsive” interaction in A + B mixes is most likely of steric origin. Polymerization of Aqueous Mixed Micelles. Figure 3 shows a photograph of mixed micellar solutions of A + B mixtures, before and after polymerization. The picture was taken 3 months after preparation, demonstrating that these systems are stable to phase separation and precipitation. In the polymerized solutions, a weak orange coloration developed after 2 months, likely to be caused by slight oxidation to give trace Br2. 1H NMR spectra before and after polymerization for solutions in D2O are shown in Figure 4, respectively. Near complete conversion was confirmed by loss of the characteristic vinyl signals (δ ) 5.55 and 6.10 ppm), coupled with line broadening of the resulting spectrum, which is characteristic of polymerized micelles. Note that the sharp line at δ ∼ 4.7-4.8 ppm is due to HOD. Owing to the affinity of the V50 initiator for water and the relative “bulky” structure, it is reasonable to expect that the initiation takes place in the continuous aqueous phase. One hypothesis with respect to the mechanism of polymerization is that surfactant micelles act as a monomer reservoir (“feed”) for the growing polymer chain. However, this seems an unlikely route from a kinetic point of view, considering that monomer exchange between assemblies and the bulk occurs on a more rapid time scale than chain propagation.29 Furthermore, the local concentration of monomers in the aqueous phase is substantially lower than that in the micelle interior, and it is known that polymerization does not occur in the absence of micelles.23 In addition, this route could result in the (29) Aida, T.; Tajima, K. Chem. Commun. 2000, 24, 2399.
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Figure 5. SAXS data for XC ) 0 (i.e., all double-chain D) and a total surfactant concentration of 80% by mass in water. The main panel shows data obtained prior to polymerization, and the inset displays data corresponding to the polymerized system.
formation of high molecular weight polymers, which may cause dramatic structural changes and possible phase separation, both of which are not observed in these systems. A more realistic mechanism involves initiation of a monomer during exchange with the bulk, followed by rapid chain propagation in the hydrophobic cores. Owing to an artificially high local concentration of monomers in the hydrophobic cores, the rate of propagation may overlap with aggregate dissolution times and therefore influence any templating effect. Extensive structural work,23-25 using small-angle neutron scattering (SANS), has shown that the polymerization has little effect on the micelle size and shape. Furthermore, it was demonstrated that polymerized structures are insensitive to dilution effects, in particular (and crucially) that polymer micelles persist, even below the cmc of the nonpolymerized system.23 Therefore, taking together the NMR and SANS results it can be concluded that dilute polymerized micelles of these surfmers can be quite easily made. Concentrated Polymerized Surfmer Phases. As found by others with related systems,12-15 it proved difficult to completely polymerize concentrated mesophases with mixed A + B and C + D surfactants. Trial attempts using thermal and UV-initiated polymerization resulted in about 40% conversions, significantly lower than the near complete reactions that were achieved in the dilute systems. Hence, a different strategy was employed, by concentrating up a dilute solution that has previously been polymerized. In this way, it would be possible to obtain mesophases consisting of highly polymerized surfactant aggregates. Initial examples of polymerized mesophases made from methacrylates have been given elsewhere; here new data for the acrylate systems are presented. Detailed analyses of SAXS patterns from both types of surfmer can be found in Tables A11 and A12 in the Supporting Information. Figure 5 shows SAXS spectra for systems at XC ) 0 (i.e., all double-chain D) and 80% by mass surfactant. SAXS showed a single Bragg peak (0.224 Å-1), indicative of a system with long-range order. No repeat reflections could be detected, and it is most likely that this represents a polydomain lamellar phase, since it is formed from pure double-chain surfactant. PLM gave ill-defined birefringent textures consistent with this assumption. Note that the lamellar order peak appears at approximately the same Qmax for both the nonpolymerized and polymerized phases. This interesting observation suggests that, broadly, the phase structure is not changed by the polymerization and
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Figure 6. SAXS patterns for C + D mixtures in water at XC ) 0.75 and a total surfactant concentration of 60% by mass. The main panel shows data obtained prior to polymerization, and the inset displays data corresponding to the polymerized system.
Figure 8. Comparison of SAXS patterns for concentrated polymerized surfmer phases as a function of XC at 80% surfactant by mass.
Figure 7. Polarizing light microscopy optical textures from nonpolymerized (upper picture, magnification ×20) and polymerized (lower picture, magnification ×10) samples with XC ) 0.75 and a total surfactant concentration of 60% by mass.
that flattened lamellae are formed in the process of concentrating up the dilute polymerized system. Figure 6 shows the resulting SAXS patterns from 60% by mass samples at XC ) 0.75 (i.e., 75 mol % single-chain C and 25 mol % double-chain D). A single sharp Bragg peak was observed for the nonpolymerized sample (d001 ) 30 Å). Interestingly, the polymerized sample exhibits a second-order Bragg peak (d002 at 0.38 Å-1), giving clear evidence for a lamellar phase; the calculated layer thickness (33 Å) is similar to that of the unreacted system. The corresponding PLM images for these samples are given in Figure 7. The top picture for the unreacted sample is consistent with a focal conic lamellar phase texture;
the lower panel shows birefingent characteristics of a poorly ordered LR phase. Figure 8 compares the diffraction patterns of polymerized mesophases, at a common surfactant concentration of 80%, as a function of mixture compositions for values XC ) 0, 0.5, and 0.75. This change corresponds to swapping all double-chain D (XC ) 0) for 25 mol % D and 75 mol % single-chain C. For all these samples, the primary peak remains in approximately the same position (with a slight shift to lower Q as the single-chain compound is added). However, there is a clear buildup in intensity of the second reflection with single-chain concentration, suggesting improved local ordering as the fraction of single-chain surfmer increases. For lamellar LR phases, the first and second diffraction peaks should be related by Qo; Q ) 1:2, and as described in the footnote the following parameters may be inferred from peak positions:30 the repeat distance between layers d001; the effective hydrocarbon layer thickness dhc; the effective polar channel thickness, also containing surfactant headgroups δ; and the average surface area per headgroup aav h . Table 1 lists values of these parameters derived from the SAXS patterns for C + D acrylate mesophases shown in Figure 8. The layer thickness is approximately constant (∼30 Å), the hydrocarbon chain thickness is consistent with C12 chains (expected value ∼ 17 Å), and the polar region thickness δ is similar. The average molecular areas are only slightly
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Table 1. SAXS Data Analysis of Lyotropic Liquid Crystal Phases Composed of Acrylate Surfmers C and D, after (p) Polymerizationa XC
mass %
phase
d/Å
dhc/Å
δ/Å
2 aav h /Å
0p 0.50 p 0.75 p
80 80 80
LR LR
28 32 33
18 17
14 16
81 77
a X ) mole fraction of C; mass % ) total surfactant concentration C in mass percent; d ) unit cell thickness; dhc ) characteristic hydrocarbon layer thickness ((3 Å); δ ) water and headgroup thickness ((3 Å); aav h ) average surface area per headgroup ((15 Å2).
larger than the literature value for the inert analogue DDAB.31 Similar results for the methacrylates A + B are given in the Supporting Information, and it can be seen that the lamellar phase is dominant over a broad range of mixture compositions and concentrations. Furthermore, comparing the behavior of acrylates and methacrylates shows little influence of the chemical nature of this reactive-end group on the mesophase structure. Hence, it appears that polymerized mesophases can be prepared from surfmer mixtures, and the final structures are quite robust to changes in mixture composition. Summary and Conclusions The reactive surfactants shown in Figure 1 can be blended to form mixed micelles. The thermodynamics of mixing is essentially ideal; a slight deviation from ideality noted for the methacrylate analogues may be ascribed to steric interactions, whereas the less hindered acrylates give rise to ideal mixing behavior. The mixed micelles can be polymerized by a thermal free-radical water-soluble initiator (V50), to yield stable solutions that remain stable for months. Proton NMR studies show that copolymerization proceeds efficiently in the mixed micelles, with conversions of essentially 100% without phase separation. Previous SANS experiments23,25 show that micelle con(30) The repeat distance between layers d00l can be obtained from Qo ) (2π/d001). The hydrocarbon layer thickness dhc can be calculated using dhc ) φAd001, where φA is the volume fraction of alkyl chains () φS(VA/ VS), where φS is the volume fraction of surfactant, known from composition, and VA and VS are the volumes ()1024Mw/(NAF)) of the alkyl chain and the whole surfactant molecule, respectively; F ≈ 1 g cm-3). Consequently, this implies layered water channels (containing surfactant headgroups) of dimension δ ()d - dhc). Furthermore, an approximate value for the surface area per headgroup (aav h ) can be obtained by dividing the volume of two alkyl chains by the thickness 24 of the hydrocarbon layer: aav h ) [(2MA × 10 )/(NAFAdhc)] where MA and FA are the mass and density of the alkyl chains, respectively. (31) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774.
centrations, sizes, and morphologies are broadly retained after polymerization. Thus, the average number of monomer units per cluster does not change much after polymerization. Although the mechanism is not strictly a topochemical process, results strongly suggest that it is a “template-assisted” process. Although direct polymerization of concentrated mesophases was unconvincing, it was possible to prepare lamellar phases by concentration of polymerized micelles, synthesized in dilute micellar systems. In these concentrated mesophases, it was found that exchange of the reactive acrylate tip with a methacrylate group had little consequence for phase behavior and final mesophase structure. As the polymerization reaction was carried out in the dilute phase, it seems likely that the final assemblies form from aggregation of individual polyelectrolyte-like chains into structures exhibiting extended long-range order. An effective way of improving molecular alignment in the nonpolymerized phases was to prealign samples in the presence of a strong magnetic field. SAXS and polarizing light microscopy clearly demonstrate retention of the lamellar (LR) phase after reaction. Detailed analysis of the SAXS data was consistent with stabilization of lamellar structures, comprising similar-sized internal domains, independent of the mixture compositions studied here. O’Brien et al.10 have successfully polymerized a reverse H2 hexagonal phase by introducing a radical initiator into the oil continuous phase. Introducing an oil-soluble initiator into hydrophobic regions of preassembled liquid crystal phases of the cationic studied here might offer an alternative method of preparation. However, the sterically restricted environment may also cause problems for reaction. A possible solution would be to employ an H-type surfmer, or mixtures of H- and T-type surfmers, and carry out the polymerization in the water continuous phase. In conclusion, preservation of long-range order in these systems is evident, suggesting that it is possible to synthesize polymerized liquid crystalline lamellar phases from such well-defined surfmer blends. Acknowledgment. M.S. thanks EPSRC for a Ph.D. studentship. Martin Murray (University of Bristol) is thanked for assistance with NMR experiments. Supporting Information Available: Surfmer synthesis, structural characterization of intermediates and surfmers, properties of dilute aqueous solutions, and small-angle X-ray diffraction data. This material is available free of charge via the Internet at http://pubs.acs.org. LA034184H
