Polymerizable Cationic Micelles Form Cylinders at Intermediate

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Polymerizable Cationic Micelles Form Cylinders at Intermediate Conversions Khwanrat Chatjaroenporn, Robert W. Baker, Paul A. FitzGerald, and Gregory G. Warr* School of Chemistry F11, The University of Sydney, NSW 2006, Australia Received March 24, 2010. Revised Manuscript Received May 17, 2010 The structural evolution of micelles of the polymerizable surfactant ω-methacryloyloxyundecyltrimethylammonium bromide (MUTAB) during UV-initiated polymerization in aqueous micellar solution has been followed by small-angle neutron scattering. Although the micelles are short spheroids both before and after polymerization, a significant, distinct population of rodlike micelles develops during the reaction, which accounts for as much as 40 vol % of the micellized surfactant and coexists with the spheroids and dissolved monomer. These coexisting micelle populations are shown to remain in dynamic equilibrium throughout the reaction and can be understood by treating it as a ternary mixture of surfactant, amphiphilic polyelectrolyte, and water.

Introduction The problem of polymerizing micelles is an old one. Conceptually, one seeks to “lock in” the equilibrium micelle structure that is otherwise subject to change with dilution or increasing concentration, temperature, ionic strength, change of solvent, hydrophobe solubilization, etc.1,2 The advantages that have been proposed for such structures are numerous and include surface bound templates for nanostructured devices,3,4 dispersed nanoparticles of controlled geometry,5 elastic membranes at liquid-liquid interfaces,6 and templated mesoporous materials with well-defined geometry for catalysis and materials engineering.7 Despite decades of investigation,8-10 the apparent preservation of the self-assembled structure after polymerization has been achieved in only a few systems. Micelle polymerization has reportedly been successful for surfactants with a polymerizable moiety near the end of the alkyl tail (T-type surfactants) as well as by using a polymerizable counterion.8,10-16 The cross-linking of amphiphiles assembled into vesicles and cubic phases was *Corresponding author: Tel þ612 9351 2106, Fax þ612 9351 3329, e-mail [email protected]. (1) Gelbart, W. M.; Ben-Shaul, A.; Roux, D. Micelles, Membranes, Microemulsions, and Monolayers; Springer-Verlag: New York, 1994. (2) Hentze, H.-P.; Kaler, E. W. Curr. Opin. Colloid Interface Sci. 2003, 8, 164– 178. (3) Yang, Y.; Lu, Y.; Lu, M.; Huang, J.; Haddad, R.; Xomeritakis, G.; Liu, N.; Malanoski, A.; Sturmayr, D.; Fan, H.; Sasaki, D.; Assink, R.; Shelnutt, J.; van Swol, F.; Lopez, G.; Burns, A.; Brinker, C. J. Am. Chem. Soc. 2003, 125, 1269– 1277. (4) Biggs, S.; Walker, L. M.; Kline, S. R. Nano Lett. 2002, 2, 1409–1412. (5) Summers, M.; Eastoe, J. Adv. Colloid Interface Sci. 2003, 100, 137–152. (6) Kozlov, M. M.; Helfrich, W. Langmuir 1993, 9, 2761–2763. (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (8) Hyde, A. J.; Robb, D. J. M. J. Phys. Chem. 1963, 67, 2089–2092. (9) Hamid, S. M.; Sherrington, D. C. Br. Polym. J. 1984, 16, 39–45. (10) Michas, J.; Paleos, C. M.; Dais, P. Liq. Cryst. 1989, 5, 1737–1745. (11) Hamid, S. M.; Sherrington, D. C. Polymer 1987, 28, 332–339. (12) Kline, S. R. Langmuir 1999, 15, 2726–2732. (13) Kline, S. J. Appl. Crystallogr. 2000, 33, 618–622. (14) Summers, M.; Eastoe, J.; Davis, S.; Du, Z.; Richardson, R. M.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2001, 17, 5388–5397. (15) Gerber, M. J.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20, 8510–8516. (16) Liu, S.; Gonzalez, Y. I.; Danino, D.; Kaler, E. W. Macromolecules 2005, 38, 2482–2491. (17) Liu, S.; O’Brien, D. F. J. Am. Chem. Soc. 2002, 124, 6037–6042. (18) Sisson, T. M.; Srisiri, W.; O’Brien, D. F. J. Am. Chem. Soc. 1998, 120, 2322– 2329.

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achieved by O’Brien et al.17,18 only after paying careful attention to the location of the polymerizable groups. There are few studies of the structures formed by polymerizable surfactants with the reactive group near the headgroup either in bulk or at interfaces, as these frequently lead to precipitation. Those that have succeeded have done so only partially,19 and then by operating at very low conversions20 or using surfactant mixtures to improve solubility.21 ω-Methacryloyloxyundecyltrimethylammonium bromide (MUTAB) (Figure 1) is a well-studied example of a tail-polymerizable (T-type) surfactant. Although it does not polymerize either in aqueous solution below its cmc or in nonaqueous solution,22 it forms almost spherical polymerized micelles that are similar in shape and size to its unpolymerized micelles above its cmc.14 Recently though, we demonstrated that at intermediate polymer conversions (∼50%) the micelles become significantly elongated.23 This unexpected and previously unreported behavior suggests that there is significant rearrangement of structure throughout the polymerization and that even the final state is an equilibrium micelle rather than a particle with a “locked in” structure. In this work we explore how the structure of aqueous MUTAB micelles evolves during solution polymerization. By irradiating micelle solutions for different times, we are able to track structure as a function of conversion using small-angle neutron scattering (SANS) and then test whether these are equilibrated.

Materials and Methods MUTAB was synthesized using a modified recipe from the literature10 as described previously.23 The critical micelle concentration was measured by conductivity to be 25 mM. MUTAB solutions were photopolymerized following the method of Michas et al.10 A 14.5 W, 53.8 mm length Pen-Ray Rare Gas Lamp (neon-mercury filled) was immersed in 4.0 mL of 50 mM (2  cmc) MUTAB in D2O, contained in a 10 mm diameter glass tube. Polymerization was conducted under a (19) White, K. A.; Warr, G. G. J. Colloid Interface Sci. 2009, 337, 304–306. (20) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 316–333. (21) Schwering, R.; Blom, A.; Warr, G. G. J. Colloid Interface Sci. 2008, 328, 227–232. (22) Hamid, S.; Sherrington, D. J. Chem. Soc., Chem. Commun. 1986, 936–938. (23) Chatjaroenporn, K.; Baker, R. W.; FitzGerald, P. A.; Warr, G. G. J. Colloid Interface Sci. 2009, 336, 449–454.

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Figure 1. The tail polymerizable (T-type) surfactant MUTAB.

Figure 3. MUTAB micelle dimensions vs polymer conversion. Symbols are length (b) and cross-sectional radius (O) for rodlike micelles and radius of rotation, rlong (9), and short radius, rshort (0), for spheroidal micelles. Uncertainties are commensurate with (or less than) the symbol sizes.

Figure 2. SANS data and two-shape model fits for MUTAB micelles at various polymer conversions. Curves offset from 0% conversion curve for clarity (fit values in Supporting Information). nitrogen atmosphere at 25 °C. Samples were polymerized by irradiating for various intervals from 6 to 8 min to yield a range of polymer conversions. Conversions were measured by the disappearance of the vinyl peaks in the 1H NMR spectrum at about 5.6 and 6.0 ppm. NMR samples were prepared by diluting ∼0.1 g of the polymerized aqueous micellar solution into ∼0.6 g of dimethyl sulfoxide (DMSO) to break up the micelles that would otherwise interfere with NMR interpretation. SANS experiments were performed on the NG7 beamline at the NIST Center for Neutron Research (NCNR) at the National Institute of Standards and Technology (NIST), Gaithersburg, MD.24 Measurements were performed at 25 °C using neutrons with an average wavelength of 6.0 A˚ and a wavelength spread of Δλ/λ = 0.124. Scattering was collected from 2.0 mm thick samples (in D2O) onto a 640  640 mm2 2D detector with 128  128 elements at three distances: 1, 5, and 13.5 m, with the detector offset by 20 cm at 1 m to increase the maximum accessible q. This gave a combined q range of 0.00453-0.518 A˚-1. Raw SANS data were reduced to 1D data and fit in Igor Pro 5.05A using the reduction and fitting procedures provided by NIST.25 A scattering length density difference of 6.09  10-6 A˚-2 was used for the MUTAB-D2O contrast (calculated using the NCNR SLD calculator: http://www.ncnr.nist.gov/resources/ sldcalc.html).

Results and Discussion SANS data for 0.050 M MUTAB solutions in D2O are shown as a function of conversion in Figure 2. Scattering from the unpolymerized solution is typical of dilute charged micelles with a broad, weak interaction peak at ∼0.06 A˚-1. The data were fit well using a model of prolate spheroids26,27 interacting through a screened (24) Glinka, C.; Barker, J. G.; Hammouda, B.; Krueger, S.; Moyer, J. J.; Orts, W. J. J. Appl. Crystallogr. 1998, 31, 430–445. (25) Kline, S. R. J. Appl. Crystallogr. 2006, 39, 895–900. (26) Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 2461–2469. (27) Berr, S. S. J. Phys. Chem. 1987, 91, 4760–4765. (28) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109–118. (29) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651–656.

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Coulomb potential28,29 with a 24.2 A˚ radius on its axis of rotation and a short radius of 14.1 A˚. This is shorter than the fully extended surfactant tail length of ∼20 A˚ but is not unexpected since the methacrylate tail is known from interfacial tension measurements11,23 to be partially hydrophilic and thus is less likely to extend into the core of the micelle than a terminal methyl group. The fit also yields a fractional charge (degree of dissociation) of 0.35 per micellized surfactant ion, compared to 0.45 derived from conductivity, and an electrolyte concentration of 22 mM, which corresponds quite closely to the measured critical micelle concentration. Similarly, the scattering from the fully polymerized (100% conversion) solution is consistent with electrostatically interacting, compact micelles (or particles). The peak has moved to q near 0.03 A˚-1 and become significantly sharper, suggesting stronger interactions than for the unpolymerized MUTAB. These data are also fit well using a model of prolate spheroids with screened Coulomb repulsions, yielding radii of 90.3 and 18.9 A˚, revealing modest elongation of the aggregates at full conversion. The fractional charge per surfactant is 0.30 and the monovalent salt concentration is 6.5 mM, consistent with most of the free surfactant in solution being consumed. The SANS data for the intermediate polymer conversions are strikingly different, exhibiting intense low q scattering, which approaches the q-1 dependence expected for long rods.30 However, the data at intermediate conversions could not be fit either by rods31 or any other simple shape,31 even incorporating polydispersity.32 The simplest model that gave reasonable fits to the data consists of two populations: cylinders (fitted without interactions) and spheroids interacting through a screened Coulomb repulsion.25 As this two-population model contains a large number of fitting parameters, including volume fraction of spheroids, fs, the long (rotation axis) and short radii of the prolate spheroids, charge per micelle and the monovalent salt concentration, plus volume fraction, fr, radius and length of the rods, we have treated the scattering units themselves as homogeneous and have not sought to determine any internal structure. The scattering length densities of the hydrogenous micelles and D2O were fixed at 2.39  10-7 and 6.33  10-6 A˚-2, respectively. A table of all best-fit parameters and uncertainties for all samples is given in the Supporting Information. (30) Cotton, J. P. In Neutron, X-ray and Light Scattering, Lindner, P., Zemb, T., Eds.; North-Holland: Amsterdam, The Netherlands, 1991. (31) Guinier, A.; Fournet, G. Small-Angle Scattering of X-Rays; John Wiley and Sons: New York, 1955. (32) Lum Wan, J. A.; Warr, G. G.; White, L. R.; Grieser, F. Colloid Polym. Sci. 1987, 265, 528–534.

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Figure 3 shows the dimensions of the spheroidal and rodlike micelles as a function of conversion. Long (3545 A˚), rodlike micelles are present in the solution at the very first sampling point, 7% conversion. Their cross-sectional radius of 17 A˚ is similar to that of both unpolymerized MUTAB micelles and to the structure of the coexisting population of spheroids at 7% conversion, which have the same 14 A˚ radius and are only slightly longer at 29 A˚. As conversion increases, the rodlike micelle length decreases while its radius grows gradually to 27 A˚. The long axis of the coexisting spheroidal micelles grows gradually at first and then with a somewhat abrupt transition from 35 to 80 A˚ at 30-40% conversion before resuming gradual elongation to its final length of about 90 A˚. This is accompanied by a similarly abrupt but smaller jump in their short radius. At very high conversions (>65%) the spheroid length, 2rlong, and the cylinder length almost converge, so the quantitative interpretation of the composition data is less certain. Figure 4a shows the distribution of surfactant between spheroidal and rodlike micelle populations, and Figure 4b shows the remaining fraction of free or dissolved MUTAB, both as a function of conversion. The free MUTAB is calculated by difference from the total volume fraction of MUTAB in the solution, fT, and the fitted spheroid and cylinder volume fractions ( fs and fr, respectively) ff ¼

fT - fr - fs dm - cp ðdp - dm Þ

ð1Þ

where dm and dp are the densities of the monomer and polymer, respectively, and cp is the conversion as measured by NMR. dm was estimated to be 1.063 g cm-3 from DTAB solution densities,33 and the density change during polymerization, dp - dm, was approximated to be 0.05 g mL-1 from the change in the molar volume of methyl methacrylate, which is about -0.020 ( 0.002 L/mol.34 This equates to a density change of 4-5% for MUTAB and is (just) within the tolerance of constant assumed SLDs. In Figure 4 the volume fractions are expressed as fractions of the total added solute, xi = fi/( ff þ fs þ fr). Note that at 100% conversion it was not possible to calculate ff (and thus xf and xs) from mass balance because of polymer depletion from solution (possibly due to deposition noted on the UV lamp). However, an upper limit can be estimated from light scattering and conductivity measurements of diluted polymerized MUTAB solutions, which both show that micelles are still present down to 4 mM (i.e., ff e 0.08). The composition data in Figure 4 comprise two distinct regions. At conversions up to 28%, the fraction of rodlike micelles increases, and that of the spheroidal micelles correspondingly decreases at about the rate of polymerized MUTAB formation. Over this range the volume fraction of free MUTAB remains approximately constant at ∼50%, which is expected given that our starting condition is twice the critical micelle concentration. Above 28% polymer conversion, the volume fraction of spheroids increases again until it consumes almost all of the surfactant (87-100%). The volume fraction of rodlike micelles plateaus or goes through a shallow maximum before it drops to 0%, and the amount of free MUTAB decreases to e8%. The boundary between these two regions coincides with the very distinct changes in micelle dimensions already noted in Figure 3: a large decrease in the length of the rodlike micelles from 3019 to580 A˚ and a small but distinct increase in the size of the spheroidal micelles; i.e., the (33) Yamanaka, M.; Kaneshina, S. J. Solution Chem. 1991, 20, 1159–1167. (34) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons: Hoboken, NJ, 1999.

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Figure 4. Volume fraction distribution of MUTAB among (a) spheroidal and cylindrical aggregates and (b) dissolved monomer as a function of polymer conversion. Symbols represent spheroids (9), rodlike micelles (b), and free (or nonscattering) MUTAB dissolved in solution (2). The open symbols are for mixtures of MUTAB and polymerized MUTAB at an equivalent mole ratio. Solid and dashed line in (a) are a guide to the eye only. Dotted line in (b) shows the fraction of dissolved, unpolymerized MUTAB in solution in the immiscibility limit (see text).

axis of rotation increases from ∼30 to ∼80 A˚, and the short radius increases from ∼14 to ∼17 A˚. We can attribute the unanticipated occurrence of two types of amphiphilic aggregates to a kinetic effect, as in emulsion polymerization,35 or to an equilibrium coexistence, such as in certain mixed polymer-micelle systems.36-38 A kinetic model in which micelles are consumed and converted to rodlike particles is inconsistent with the data, which shows that rodlike micelles are longest at low conversions and disappear completely near full conversion. The maximum degree of polymerization of MUTAB at any point during the reaction cannot possibly exceed the number of MUTAB monomers in each globular micelle (or particle) ultimately formed at 100% conversion, which we determine from spheroid and monomer molar volumes to be 314. As the aggregation numbers of the elongated cylindrical micelles formed at intermediate conversions are much greater than this, each such micelle must contain many polymer chains. This in turn implies that the micellar solution is continuously reorganizing itself throughout the polymerization and is therefore close to being in dynamic equilibrium. Of course, it is not necessary that the final micelles formed at 100% conversion consist of a single polymerized chain of MUTAB. As we will see below, they probably consist of many chains of polymerized MUTAB. To verify our conclusion that the polymerizing system remains in equilibrium during the reaction, we examined small-angle neutron (35) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley & Sons: New York, 1997. (36) Svensson, A.; Piculell, L.; Karlsson, L.; Cabane, B.; J€onsson, B. J. Phys. Chem. B 2003, 107, 8119–8130. (37) Zhao, G.; Chen, S. B. Langmuir 2007, 23, 9967-9973. (38) Holyst, R.; Staniszewski, K.; Patkowski, A.; Gapinski, J. J. Phys. Chem. B 2005, 109, 8533-8537.

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Figure 5. SANS data comparing mixtures of MUTAB and polymerized MUTAB ((), polymerized MUTAB reaction mixture ()), and mixtures of DTAB and polymerized MUTAB (O) at (a) 25%, (b) 50%, and (c) 75%. Curves are offset from the polymermonomer mixtures (() for clarity.

scattering from solutions containing mixtures of unpolymerized and 100% polymerized MUTAB in molar ratios of 25:75, 50:50, and 75:25. These are shown in Figure 5, together with results for the polymerizing systems with conversions closest to these overall compositions, i.e., at 28%, 50%, and 77% conversion. The results clearly show that the structures of the mixed samples are virtually identical to the polymerized samples, reflecting their reorganization into the same equilibrium population as the reacting mixture. Most striking is the 25:75 mixture, which shows the formation of a significant population of long, rodlike micelles by mixing solutions of unpolymerized and fully polymerized micelles, both of which are spheroidal. The best-fit parameters for these mixed solutions are shown in Figure 4, together with those of the polymerized solutions. The micelle dimensions and distribution between populations prepared by the two routes are in excellent accord. Even mixtures of polymerized MUTAB with solutions of a nonpolymerizable cationic surfactant dodecyltrimethylammonium 11718 DOI: 10.1021/la101159g

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bromide (DTAB) exhibit the same general behavior (Figure 5). DTAB differs from MUTAB only in that the terminal methacrylate group is replaced by a methyl, making it slightly shorter and slightly more hydrophobic. The only visible effect of this difference is slightly reduced scattering at low q, which is probably due to the slight length differences in the tail structure. DTAB, like MUTAB and polymerized MUTAB, produces spheroidal micelles on its own. Yet, when mixed with 100% polymerized MUTAB it too produces an equilibrium population of rodlike micelles, clear from the large scattering at low q in Figure 5. Table 1 compares the major best-fit parameters to the SANS results for these different mixed systems. The formation of rodlike micelles is not due to a specific interaction of the unpolymerized methacrylate group of MUTAB with the polymer. The equilibrium coexistence of two distinct self-assembly structures cannot be explained either by simple models of ideal mixing;which lead to a single average mixed micelle structure and composition;or complete immiscibility, which would lead to two distinct populations of spheroids. The rodlike micelles must form because of mixing of polymerized and unpolymerized surfactants. This itself is unusual, as coexistence of two types of micelles is usually associated with an immiscibility or repulsive interaction.39-41 The fact that the longest rods form in low conversion (monomeric surfactantrich) systems and that shorter spheroids are formed at 100% conversion both imply that the rods must also contain high amounts of unpolymerized surfactant. Before polymerization, the concentration of MUTAB is twice the cmc, so about half of the MUTAB is in solution and half in micelles, which agrees well with the volume fractions derived from SANS at 0% conversion (Figure 4). In an ideally mixed system, each ensuing composition has its own cmc, so the mixed cmc and equilibrium monomer concentration will decrease quickly with increasing conversion as more of the less-soluble polymer is formed.42 Typically, attractive interactions between two components (synergism) amplify this effect and further lower the cmc. In the immiscibility limit we would expect polymerized MUTAB to be expelled from micelles into separate particles, while the equilibrium between monomer and micellar MUTAB is dynamically maintained until all of the micelles are consumed, after which dissolved MUTAB begins to be incorporated into the polymeric species. That is, the amount of dissolved MUTAB should remain constant until about 50% conversion. Figure 4b clearly shows that the monomer MUTAB concentration is initially constant as the reaction proceeds, decreasing only at high conversions. The dotted line sketches the monomer concentration expected for the immiscibility limit. We can deduce from this that the chemical potential of the dissolved monomer in solution and in micelles is unchanged at low conversions and therefore that the spheroidal micelles contain no polymerized MUTAB. The rodlike micelles cannot be pure polymer as these would be compact spheroids; hence, they must constitute a third coexisting “phase” in which the chemical potential of MUTAB is also the same. That is, we can treat the reacting system in the same way as a three-component system or mixed polymer-surfactant solution and regard the rods as a polymerized MUTAB “phase” saturated with monomeric MUTAB. This fixes their composition as long as there are pure MUTAB micelles with the same chemical potential. (39) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283–5287. (40) Davey, T. W.; Warr, G. G.; Asakawa, T. Langmuir 2003, 19, 5266–5272. (41) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1996, 12, 1204–1207. (42) Clint, J. H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327–1334.

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Table 1. Comparison of Micelle Structures Found in Polymerizing MUTAB Micellar Solution and Mixtures of Polymerized MUTAB with Unpolymerized MUTAB and DTAB Obtained from Best Fits of a Two-State Model to the Data of Figure 5a 25% polymer

50% polymer

75% polymer

reaction MUTAB mixture DTAB mixture reaction MUTAB mixture DTAB mixture reaction MUTAB mixture DTAB mixture 0.0034 0.0038 0.0029 0.0026 0.0026 580 545 455 390 387 19.0 17.8 18.3 20.4 20.4 0.0052 0.0037 0.0025 0.0067 0.0059 34.9 36.5 71.0 83.0 99.7 13.4 10.9 13.9 16.3 16.2 a Uncertainties for volume fractions are (5% and for lengths and radii are (0.5 A˚.

fc L/A˚ rc/A˚ fs rlong/A˚ rshort/A˚

0.0017 385 21.7 0.0062 104 17.0

0.0006 210 27.6 0.0106 96.1 18.1

0.0007 331 25.8 0.0097 98.4 18.1

0.0002 1880 42.1 0.0097 92.8 18.5

As conversion proceeds the spheroidal MUTAB micelles are eventually consumed, leaving the rodlike mixed polymer/monomer micelles in equilibrium with their dissolved monomer. Further polymerization consumes MUTAB from the mixed aggregates, changing the composition of the mixed rods and decreasing the equilibrium monomer concentration. More polymer-rich mixtures shorten in length, gradually transforming into the slightly elongated polymeric MUTAB micelles observed at 100% conversion. The critical micelle concentration of 100% polymerized MUTAB was too low to measure. The ease with which it mixes with monomeric surfactant and dynamically rearranges suggests that the degree of polymerization is quite low. These observations are self-consistent. It is known from studies of Gemini and oligomeric surfactants that cmcs decrease sharply with the degree of oligomerization,43,44 so it would not take a high degree of polymerization of MUTAB to lower the effective cmc out of the experimentally accessible range. This leads us to conclude that the structures formed at 100% conversion are true micelles, each comprised of many polymerized MUTAB molecules. This is consistent with the degree of polymerization of around 10 derived from vapor pressure osmometry as reported by Nika et al.45 This would imply that the particles formed at 100% conversion are micelles comprised of ∼30 such polymerized MUTAB chains. Such relatively short polymer chains would remain capable of dynamically re-equilibrating when mixed with a second amphiphile. It is not necessary for the equilibrium model that the mixed aggregates are rodlike, but their formation in mixtures of polymerized MUTAB with both MUTAB and DTAB suggests that their occurrence may be expected quite generally. Why? We expect a linear polyelectrolyte to prefer a coiled or elongated conformation over the collapsed state that exists in micelles of polymerized MUTAB on configurational entropy grounds, but the hydrophobicity of the alkyl chains restricts possible contact with water. A miscible, monomeric surfactant present in sufficiently high concentrations allows the polymerized MUTAB chains to access these more extended conformations within the core of such rodlike mixed aggregates. Electrostatic repulsions between adjacent charged head groups of polymerized MUTAB are probably not very significant, as the monomer carries an identical charge. In this scenario the monomer component is more or less indifferent to the shape of the mixed aggregate, as long as its environment is sufficiently micelle-like. This can be tested by examining the effects of modifying various structural features of the polymerizable surfactant or

monomer component of some rather more different mixtures (or, equivalently, structure evolution in polymerizations carried out in mixed surfactant systems). We would predict that changing counterion or headgroup structure of the cationic surfactant will alter the energy cost of incorporating surfactant into a rodlike mixed aggregate and therefore alter the equilibrium coexistence composition.46,47 Mixing a nonionic surfactant, which will not screen repulsions effectively, might favor elongated polymerized MUTAB conformations and hence rods.21,48 At the opposite extreme, we would expect that a monomeric fluorocarbon-chained surfactant added to polymerized MUTAB should approximate the immiscibility limit discussed above39-41 and hence suppress rod formation in a mixed system. Some of these possibilities will be explored in a future study.

(43) Zana, R. Langmuir 1996, 12, 1208–1211. (44) FitzGerald, P. A.; Carr, M. W.; Davey, T. W.; Serelis, A. K.; Such, C. H.; Warr, G. G. J. Colloid Interface Sci. 2004, 275, 649–658. (45) Nika, G.; Paleos, C. M.; Dais, P.; Xenakis, A.; Malliaris, A. Prog. Colloid Polym. Sci. 1992, 89, 122–124.

(46) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236–10244. (47) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853–1859. (48) Blom, A.; Warr, G. G. Langmuir 2006, 22, 6787–6795.

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Conclusions UV-initiated polymerization of a MUTAB micellar solution generates a dynamic evolution of self-assembled structures that remain equilibrated at all conversions up to 100%. At low conversions, spheroidal micelles of unpolymerized MUTAB coexist with a distinct population of mixed rodlike micelles of MUTAB-swollen polymer chains. At higher conversion the initial monomer spheroids are replaced by a growing population of aggregates of polymerized MUTAB, to which the structure and composition of the mixed micelles gradually converge as monomer is consumed. The micelle shapes at any conversion can be regenerated simply by mixing unpolymerized and polymerized MUTAB at an equivalent ratio, indicating that the final MUTAB product has a low degree of polymerization and can still readily undergo dynamic exchange in solution. Acknowledgment. We are grateful for financial support through an Australian Research Council Discovery Grant and travel funds from the Access to Major Research Facilities Program. This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR9986442. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. Supporting Information Available: Full fit values for the fits in Figures 2 and 5 in tabular form. This material is available free of charge via the Internet at http://pubs.acs.org.

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