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Langmuir 1999, 15, 2726-2732
Polymerization of Rodlike Micelles Steven R. Kline NIST Center for Neutron Research, Building 235, Room E151, Gaithersburg, Maryland 20899 Received October 16, 1998. In Final Form: February 4, 1999 A new polymerizable surfactant, cetyltrimethylammonium 4-vinylbenzoate (CTVB), has been prepared and investigated. In aqueous solution, highly entangled rodlike aggregates are formed at millimolar surfactant concentrations. Free-radical polymerization of the surfactant counterions results in nearly complete conversion to a stable, nonviscous solution. Small-angle neutron scattering reveals that the micellar diameter of 40 Å is unchanged during the polymerization process and that the final polymerized micelle length (400-1200 Å) is determined by the relative initiator concentration. In addition, the parent rodlike micellar aggregates rearrange into spherical micelles at elevated temperature, whereas the polymerized structures are insensitive to temperature changes or dilution.
Introduction Surfactant molecules in aqueous solution self-assemble into micellar aggregates at concentrations greater than their critical micelle concentration (cmc). A variety of aggregate structures can form such as spherical or rodlike micelles, vesicles, or lamellar phases.1 Most applications involving surfactants make use of this ability to selfassemble and their predictable, dynamic response to changing solution conditions. In particular, rodlike micelles display very rich rheological behavior, often at very low total surfactant concentrations (∼10-3 mol/L). Such solutions can be highly viscous and viscoelastic and can display an apparent yield stress.2-5 This provides for interesting investigations of the equilibrium micellar properties, as well as the transient and steady-state structures formed under applied flow fields.6-8 These entangled micellar solutions are similar to polymer solutions, and their unique behavior can be described by “living polymer” models.9,10 These macroscopic properties of micellar solutions arise from the microstructure of the highly elongated and entangled micelles. The equilibrium and dynamics of the micellar structures, however, are determined by a delicate balance of intermolecular forces that can be easily disrupted. For example, increasing the temperature reduces the average micelle length and results in a dramatic decrease of the solution viscosity. Addition of oils or polymers typically has the same destructive effect on the microstructure and loss of the useful macroscopic properties.11,12 (1) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: San Diego, 1994. (2) Candau, S. J.; Hirsch, E.; Zana, R.; Adam, M. J. Colloid Interface Sci. 1988, 122, 430. (3) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (4) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (5) Lin, Z. Langmuir 1996, 12, 1729. (6) Hayter, J. B.; Penfold, J. J. Phys. Chem. 1984, 88, 4589. (7) Kalus, J.; Hoffmann, H.; Ibel, K. Colloid Polym. Sci. 1989, 267, 818. (8) Butler, P. D.; Magid, L. J.; Hamilton, W. A.; Hayter, J. B.; Hammouda, B.; Kreke, P. J. J. Phys. Chem. 1996, 100, 442. (9) Cates, M. E. Macromolecules 1987, 20, 2289. (10) Cates, M. E.; Candau, S. J. J. Phys: Condens. Matter 1990, 2, 6869. (11) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (12) Brackman, J. C.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1990, 112, 2. (13) Fendler, J. H.; Tundo, P. Acc. Chem. Res. 1984, 17, 3.
10.1021/la981451o
A method for “locking in” the micellar structure so that it is less sensitive to environmental changes would be advantageous and would lead to novel applications.13-15 One approach to the stabilization of surfactant structures is through the use of polymerizable surfactants. Polymerizable groups can be incorporated into either end of the hydrophobic portion of the surfactant or into the counterion. The majority of these surfactants contain their polymerizable group in the surfactant tail, while few involve polymerization of the counterion. 14 It is worth noting that polymerized surfactant structures are in general similar in molecular structure to polymersurfactant complexes16 or hydrophobically modified polyelectrolytes,17,18 and comparisons can be drawn to these extensively studied systems. There are a large variety of surfactants and aggregate structures that have been polymerized, with varying levels of success in retaining the original structure. Considerable effort has been invested in the polymerization of vesicles for application such as drug delivery, model membranes, or microreactors.15 To a lesser extent, other surfactant structures have been polymerized such as spherical micelles,19 lamellae,20,21 or hexagonal arrays of cylinders.22,23 Other than in liquid-crystalline phases, rodlike micelles have not been polymerized, although elongated structures have been proposed as the polymerization product of globular micelles.24 Here, we capture the rodlike aggregate structure of the surfactant cetyltrimethylammonium 4-vinylbenzoate (CTVB), which contains a polymerizable counterion. Preparation and characterization of the surfactant will (14) Paleos, C. M. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach: Philadelphia, 1992; p 183. (15) Paleos, C. M. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach: Philadelphia, 1992; p 283. (16) Li, Y.; Dubin, P. L. In Structure and Flow in Surfactant Soltions; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; p 320. (17) Peiffer, D. G. Polymer 1990, 31, 2353. (18) Loppinet, B.; Gebel, G. Langmuir 1998, 14, 1977. (19) Larrabee, C. E.; Sprague, E. D. J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 749. (20) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 316. (21) McGrath, K. M. Colloid Polym. Sci. 1996, 274, 399. (22) Thundathil, R.; Stoffer, J. O.; Friberg, S. E. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2629. (23) Srisiri, W.; Sisson, T. M.; O’Brien, D. F.; McGrath, K. M.; Han, Y.; Gruner, S. M. J. Am. Chem. Soc. 1997, 119, 4866. (24) Lerebours, B.; Perly, B.; Pileni, M. P. Chem. Phys. Lett. 1988, 147, 503.
This article not subject to U.S. Copyright. Published 1999 by the American Chemical Society Published on Web 03/19/1999
Polymerization of Rodlike Micelles
be described, as well as its subsequent free-radical polymerization. Small-angle neutron scattering (SANS) will be used to probe the morphology of the polymersurfactant structure. The stability of the final product with respect to temperature and dilution will be compared to the parent micellar solution. Experimental Section Cetyltrimethylammonium hydroxide and 4-vinylbenzoic acid were purchased from Fluka (Ronkonkoma, NY). D2O, 99.9 mol % deuterium enriched, was purchased from Cambridge Isotope Labs (Andover, MA). The water-soluble free-radical initiator VA044 (2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride) was donated by Wako Chemicals (Richmond, VA). All materials were used as received. Water was distilled and deionized immediately before use. Cetyltrimethylammonium 4-vinylbenzoate was prepared by neutralization of 4-vinylbenzoic acid in the presence of a slight stoichiometric excess of cetyltrimethylammonium hydroxide (CTAOH). Freshly distilled water was boiled for 10 min to remove residual CO2 and allowed to cool to room temperature under nitrogen. The aqueous CTAOH stock solution was diluted to a mass fraction of 1%, and the 4-vinylbenzoic acid dissolved in a minimum amount of acetone was added. After thorough mixing, the resulting viscoelastic solution was refrigerated, yielding large, white crystals of CTVB (Tk ) 19 °C) and leaving excess CTAOH (Tk < 0 °C) in the supernatant. The CTVB obtained was recrystallized from absolute ethanol at an overall yield of 81%. Electrical conductivity measurements were performed using an ATI Orion (Boston, MA) conductivity cell (model 013010) with a cell constant of 0.10 cm-1. Samples were stirred continuously and held at the desired temperature in a water bath thermostated to (0.02 °C. Samples for polymerization were prepared with water (or D2O) depleted of oxygen by bubbling with ultrahigh-purity nitrogen (grade five) for at least 30 min. After dissolution of the surfactant at the desired reaction temperature, the reaction was initiated by injecting a small volume of predissolved initiator into the equilibrated surfactant solution. The visual appearance of the reactions was identical whether they were stirred or unstirred during the reaction. The percentage of monomeric CTVB converted to polymer was determined by quantitative addition of bromine to the unreacted vinyl group, using the pale yellow end point of excess bromine.25 Under these titration conditions, no aromatic substitution by bromine could be detected. Verification of this method with known compounds (approximately 20 × 10-6 equiv) and blanks gave a relative standard uncertainty of (2% for the concentration of 4-vinylbenzoic acid. Poly(4-vinylbenzoic acid) was isolated from the polymerized micelles by concentrating the aqueous solution to near dryness, redissolving in ethanol/HCl (100:1), and repeating three times. The acidified material was vacuum-dried to remove excess HCl. After dissolving in a minimum volume of ethanol, poly(4vinylbenzoic acid) was precipitated with heptane (3×). CTACl remained soluble in the ethanol/heptane supernatant. Purified polymer was obtained at a typical yield of 70%. The purified polymer is insoluble in water and nonpolar solvents but soluble in base or alcohol. Samples of poly(4-vinylbenzoic acid) for neutron scattering were dissolved in deuterated methanol. Neutron scattering experiments were performed on the NG7 30 m SANS instrument at the NIST Center for Neutron Research in Gaithersburg, MD. Neutrons of wavelength λ ) 6 Å with a distribution of ∆λ/λ ) 10% were incident on samples held in quartz cells. Three different sample-to-detector distances were used to give an overall q range of 0.0040 Å-1 < q < 0.65 Å-1, where q ) (4π/λ) sin(θ/2) is the magnitude of the scattering vector. Sample scattering was corrected for background and empty cell scattering, and the sensitivity of individual detector pixels was normalized. The corrected data sets were circularly averaged (25) Skoog, D. A.; West, D. M. Fundamentals of Analytic Chemistry, 4th ed.; Saunders College Publishing: Phialdelphia, 1982. (26) NIST SANS Data Reduction and Imaging Software, 1998. (27) Barker, J. G.; Pedersen, J. S. J. Appl. Crystallogr. 1995, 28, 105.
Langmuir, Vol. 15, No. 8, 1999 2727 and placed on an absolute scale using standard samples and software supplied by NIST.26 Instrumental smearing was simulated27 for the instrument configurations used, eliminating smeared data points from the combined data set. For this study, model fitting was performed only on very dilute samples where interparticle interference is negligible, making a simplified expression for the scattered intensity appropriate.28 Therefore, model scattering functions were calculated as I(q) ) n〈P(q)〉 + b, where n is the number density of scatterers, 〈P(q)〉 is the intraparticle form factor averaged over a Schulz distribution of cylinder lengths, and b is the residual incoherent scattering. The Schulz distribution29 is characterized by a polydispersity, p ) σL/Ln, where σL2 is the variance of the distribution and Ln is the number-average length. The Schulz width parameter, z, is given as z ) 1/p2 - 1. Thus, models of polydisperse rigid cylinders28 or polydisperse semiflexible cylinders30 were used to quantify the shape of the polymerized micelles. Model fitting used a standard nonlinear least-squares method, minimizing the χ-squared error between model and data. The minimization procedure was restarted at several initial vectors to ensure that a global minimum was found. The scattering length density of the polymerized structure is an average of the entire CTVB molecule because there should be essentially complete poly(counterion) binding in the polymerized micelles. Using a molecular volume of 680 Å3 for CTVB gives a uniform scattering length density of 0.35 × 10-6 Å-2. A core-shell model for the micellar cross section did not significantly improve the quality of model fits or change the overall micelle diameter, so the simpler model of a uniform scattering length density was used.
Results The preparation of CTVB from an aqueous solution of CTAOH and 4-vinylbenzoic acid is itself an efficient purification step, yielding large crystalline flakes of CTVB and leaving excess CTAOH in the supernatant. After an additional recrystallization from ethanol, the identity of the product was confirmed by NMR in CDCl3. All peaks were identified when compared to reference spectra.31 Integration of the vinylic protons corresponded to a mole fraction of 96 ( 3% active material. The purified surfactant was characterized by electrical conductivity to determine the cmc and Krafft temperature. Figure 1 shows the conductivity of CTVB in water as a function of concentration, displaying a distinct change in slope indicating the onset of micellization. This break occurs at a cmc ) 1.6 × 10-4 mol/L, a mass fraction of 0.007%. The conductivity of CTVB at a mass fraction of 0.5% in H2O rises sharply as a function of temperature at the Krafft temperature of Tk ) 19 °C and coincides with the visual observation of sudden dissolution of the crystalline surfactant. Measurements repeated in D2O gave a slightly higher value of Tk ) 21 °C and an identical cmc ) 1.6 × 10-4 mol/L. The measured cmc and Krafft temperature are consistent with other cetyltrimethylammonium surfactants.32,33 The general appearance of CTVB micellar solutions is similar to that of cetyltrimethylammonium tosylate (CTAT), which also forms entangled micellar solutions without the addition of electrolyte.33,34 At concentrations above the cmc, CTVB solutions are more viscous than water and noticeably viscoelastic, becoming gellike at (28) Guinier, A.; Fournet, G. Small-Angle Scattering of X-rays; John Wiley and Sons: New York, 1955. (29) Schulz, G. V. Z. Phys. Chem. 1935, 43, 25. (30) Pedersen, J. S.; Schurtenberger, P. Macromolecules 1996, 29, 7602. (31) The Aldrich library of NMR spectra, 2nd ed.; Aldrich Chemical Co.: Milwaukee, WI, 1983. (32) Ingvarsson, A.; Flurer, C. L.; Riehl, T. E.; Thimmaiah, K. N.; Williams, J. M.; Hinze, W. L. Anal. Chem. 1988, 60, 2047. (33) Soltero, J. F. A.; Puig, J. E.; Manero, O.; Schulz, P. C. Langmuir 1995, 11, 3337. (34) Walker, L. M.; Moldenaers, P.; Berret, J.-F. Langmuir 1996, 12, 6309.
2728 Langmuir, Vol. 15, No. 8, 1999
Figure 1. Conductivity of CTVB in water as a function of concentration. The break in slope at a mass fraction of 0.007% indicates the critical micelle concentration. Measurements were done at 25 °C.
Figure 2. SANS intensity from unpolymerized CTVB micellar solutions. Concentrations are indicated as mass fractions, and data were taken at 30 °C (open symbols) and 60 °C (solid symbols). The low-q dependency shows the decrease in micelle size with increasing temperature.
concentrations greater than a mass fraction of approximately 1% (0.023 mol/L). Concentrations greater than a mass fraction of 1% were not studied as they were difficult to mix thoroughly during polymerization. SANS from these entangled, charged micellar solutions is also typical of cetyltrimethylammonium-based surfactants,8,35 showing a screened Coulomb interaction peak at low q values that arises from an average intermicellar spacing and a rollover at higher q values resulting from the well-defined cross-sectional diameter of the micelles. SANS data are shown in Figure 2 for two CTVB concentrations in D2O. Data were collected at 30 and 60 °C, showing the strong influence that temperature has on the equilibrium structure of the micelles and intermicellar interactions. This effect is most significant at low surfactant concentrations where the solution is less viscous. CTVB at a mass fraction of 0.1%, when heated to 60 °C, results in a dramatic decrease in the low-q scattering that
Kline
Figure 3. SANS of CTVB micelles at 60 °C before (solid symbols) and after polymerization (open symbols). The changes in the low-q region are indicative of the polymerization of the 4-vinylbenzoate counterions. The unchanged high-q region indicates preservation of the cross-sectional structure of the micelles.
is indicative that the micellar structure has changed from an elongated structure (strong q-1 dependence) at 30 °C to much shorter micelles at 60 °C. However, at a mass fraction of 1.0% the solution remains viscoelastic at 60 °C and still contains highly elongated micelles. The elongated micelles are polymerized at 60 °C by freeradical initiation. The progress of the reaction is followed visually, both from the turbidity increase and from the simultaneous decrease in viscosity. The reaction is also monitored quantitatively by titration of reaction aliquots. The conversion to polymer is further confirmed by the loss of vinyl protons in the NMR signal. As measured by bromine titrations, the reaction proceeds to typically greater than 95% conversion in approximately 1 h, with the actual time a function of the relative initiator concentration. After polymerization the solutions are bluish, visually much less viscous than their parent solutions, and not elastic. The SANS from CTVB solutions at two initial surfactant concentrations is shown in Figure 3, both before and after polymerization. In both cases, the initiator concentration is at a mole fraction of 5% relative to the CTVB concentration. Two structural features are readily apparent. First, at low q values the screened Coulomb interaction peak present in the micellar solutions has completely disappeared upon polymerization and is replaced by a power-law dependence of q-1, typical of cylindrical structures. This power-law dependence is less evident at a surfactant mass fraction of 1%, where there is a significant flattening of the low-q scattering because of interparticle interference effects. Therefore, any size characterization of the micelles should be done at lower concentrations where the structure factor is not significant. Second, at higher q values the scattering curves before and after polymerization coincide, indicating that the cross-sectional structure of the micelles is completely unchanged by the polymerization of the counterions. Once polymerized, these solutions can be frozen and thawed several times with no crystallization of surfactant. (35) Koehler, R. D.; Kaler, E. W. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; p 120.
Polymerization of Rodlike Micelles
Figure 4. Temperature independence of the polymerized micelles. Data are shown for two surfactant concentrations. The scattered intensity at 30 °C (open symbols) is nearly identical to and obscures 60 °C data (closed symbols, behind), indicating no structural changes over length scales of approximately 10-2000 Å.
The polymerized micelles also retain their structure at elevated temperatures, as seen by SANS in Figure 4. Data are shown for two surfactant concentrations, each at 30 and 60 °C, with identical scattering patterns at each temperature. There is no change in structure over the entire q range, in sharp contrast to the structural changes that occur in the unpolymerized micelles of Figure 2. For free-radical polymerization, the degree of polymerization, N, of the final polymer is inversely proportional to the initiator concentration, and a higher degree of polymerization of the 4-vinylbenzoate counterion should produce a final product of longer cylindrical structures. To test this, polymerization of CTVB solutions at a mass fraction of 1.0% were carried out at initiator mole fractions 5.0%, 2.5%, 0.50%, and 0.25% relative to the CTVB monomer concentration. Figure 5 shows the SANS from these samples after polymerization, each diluted to a mass fraction of 0.10% in D2O to eliminate interparticle interactions. The scattered intensity is typical of dilute, uncharged cylinders and is plotted as a Holtzer plot to highlight the deviation of the scattering from ideal, rigid cylinders. The model fitted to the two highest initiator concentrations is, in fact, one of rigid, uncharged cylinders. The two lower initiator concentrations yield longer micelles that are flexible and thus are fitted to a model of semiflexible chains with excluded-volume effects, with the results of the fitting shown in Table 1. The fitted parameters for the rigid cylinder model are the diameter, number-average length, and polydispersity of length. The diameter, Kuhn length, number-average length, and polydispersity of length are fitted for the flexible cylinder model. Each model also includes a (small) residual incoherent background. As the initiator concentration is reduced, the fitted lengths, Ln, increase, as expected. The fitted values of the polydispersity of length, p, range from 0.32 to 0.58 and have a much larger relative standard deviation than the other fitted parameters. These polydispersities correspond to a value of Lw/Ln (or Mw/Mn) of approximately 1.2, which is low, but not unreasonable, for free-radical polymerization.36 (36) Odian, G. Principles of Polymerization, 2nd ed.; John Wiley and Sons: New York, 1981.
Langmuir, Vol. 15, No. 8, 1999 2729
Figure 5. Holtzer plot of polymerized CTVB as a function of initiator concentration. All samples have been diluted to a mass fraction of 0.10% poly(CTVB) (φ ) 9.0 × 10-4) and correspond to VA-044 mole fractions of (a) 5.0%, (b) 2.5%, (c) 0.50%, and (d) 0.25% relative to the CTVB concentration. Curves a and b are fitted to a rigid cylinder model. Curves c and d are modeled as semiflexible cylinders. Curve a is on an absolute scale, and successive curves have been offset by 0.002 for clarity. Results of the data fitting are listed in Table 1.
Because the polymerized structures do not change upon dilution, a standard Zimm analysis can be carried out at each initiator concentration. Extrapolation of the low-q (linear) regions of a dilution series of SANS measurements to zero q and zero concentration give a common intercept of Mw-1, independent of the geometry of the aggregate and free from any residual structure factor effects. This yields the (weight-average) molecular weight of the polymer-surfactant species and the degree of polymerization, N, assuming there is a single polymer chain per polymerized cylinder. A representative Zimm plot is shown in Figure 6. The molecular weight provides an estimate of the cylindrical micelle length (a weight-average length) when combined with the molecular volume of the surfactant and an average diameter of 40 Å. The results of the Zimm analysis for the four initiator concentrations are shown in Table 2. The degree of polymerization and lengths derived confirm that the fitted models are proper descriptions of the polymerized structures. The fitted number-average lengths (from Table 1) are corrected to weight-average values and listed in Table 2 for easy comparison to the weight-average lengths derived from the Zimm analysis. Additionally, the length of the final polymerized micelles is not sensitive to the initial surfactant concentration but rather is controlled by the relative initiator concentration. Using initial CTVB mass fractions of 1.0% and 0.10% with a mole fraction of 5% (relative) initiator yields identical polymerized structures. This is evident in Figure 7, when the polymerized 1.0% CTVB solution is diluted 10-fold and compared to the polymerized 0.10% solution. The initial micellar solutions are quite different, with much stronger intermicellar interactions at the higher concentration. The polymerized products, however, are identical with an average length of 400 Å. Discussion The physical properties and scattering behavior of CTVB solutions are as expected, based on the behavior of other
2730 Langmuir, Vol. 15, No. 8, 1999
Kline
Table 1. Results of the Cylindrical Form Factor Modeling of the Polymerized CTVB Micellesa initiator (mol %)
D (Å)
pL
Ln(cyl) (Å)
5.0 2.5 0.50 0.25
39.4 ( 0.06 40.2 ( 0.06 40.4 ( 0.06 41.2 ( 0.06
0.44 ( 0.15 0.32 ( 0.12 0.37 ( 0.11 0.58 ( 0.16
400 ( 4 880 ( 25
a
Ln(contour) (Å)
1200 ( 35 900 ( 80
L(Kuhn) (Å)
xχ2/N
680 ( 25 570( 50
2.0 1.9 1.8 2.0
Reported uncertainties are 1 standard deviation.
Figure 6. Representative Zimm plot of the polymerized micelles corresponding to curve a of Figure 5. Dilutions a-d correspond to mass fractions of 0.1%, 0.2%, 0.5%, and 1.0% poly(CTVB), respectively. The contrast factor is k ) 5.45 × 10-3 cm2 mol g-2. Extrapolation yields a weight-average molecular weight for the polymerized micelle of 380 000 g/mol. Results for other initiator concentrations are listed in Table 2. Table 2. Molecular Weight and Weight-Average Length of Polymerized Micelles Derived from Zimm Analysisa initiator (mol %)
Mw (g/mol)
N
Lw(Zimm) (Å)
Lw(fit) (Å)
5.0 2.5 0.50 0.25
0.38 × 106 0.70 × 106 1.3 × 106 1.2 × 106
880 1600 3000 2700
460 850 1600 1400
480 970 1360 1200
Figure 7. Comparison of polymerized structures resulting from different initial conditions. The upper curve was reacted at a mass fraction of 1.0% CTVB, while the lower curve (open symbols) was reacted at a mass fraction of 0.10% CTVB. Both reactions were initiated with a mole fraction of 5% (relative) VA-044. The lower set of solid data points is a 10-fold dilution of the 1.0% reaction. The dilution overlays the lower concentration reaction, and both have identical structures of rigid cylinders of L ) 400 Å and D ) 40 Å.
cetyltrimethylammonium-based surfactants with strongly binding counterions.8,35 By analogy, CTVB micelles should be of similar length, which can be microns or longer. The micelles may or may not be branched at these conditions, but that is relatively unimportant concerning the final polymerized structure. Branching may have implications on kinetics of the reaction, but the linear polymer formed is unlikely to support a branched network structure. No detailed modeling of the SANS spectra from these charged rodlike micelles is presented because proper screened Coulomb structure factors are not easily calculated for nonspherical colloids.37 Instead, attention is focused on the structure of the polymerized micelles. It is clear from Figure 3 that locally the cross-sectional structure of the micelles is preserved by the polymerization of the counterions. However, this linking of the counterions alters the intermicellar interactions and sets the length of the polymerized micelles. Macroscopically, this alters
the rheology of the solution. The polymerized micelle solutions are fluid and not elastic and have a viscosity consistent with uncharged cylinders.38 Thus, the polymerization does not truly represent a “freezing” of the entangled micellar structure. The reaction occurs over a time of approximately 103 s, while molecular motion of the surfactant is on time scales 106-109 times shorter. Local to each monomer, the propagation may be fast, but the micelles have ample time to rearrange during the reaction. The final product, however, does retain the cylindrical structure rather than adopt a different morphology. Quantitative modeling of SANS data was done only at concentrations below n ∼ L-3. At higher concentrations there are interparticle interference effects, and because any such interactions will have the largest effect at low q values, they will severely affect the accuracy of any lengths derived from modeling. The upturn at low q in the Holtzer plot (Figure 5) is an indication of micelle flexibility and dictates the choice between a rigid or semiflexible cylinder model. Application of an improper model resulted in significantly inferior χ-squared values and unphysical parameters. Using the rigid cylinder model for the longest polymerized micelles (curves c and d in Figure 5) gave results that were insensitive to the cylinder length and could not reproduce the low-q slope of the data. Attempting to fit the shorter cylinders (curves a and b) with a semiflexible cylinder model resulted in contour lengths
(37) Weyerich, B.; D’Aguanno, B.; Canessa, E.; Klein, R. Faraday Discuss. Chem. Soc. 1990, 90, 245.
(38) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986.
a The relative uncertainty in the molecular weight and degree of polymerization, N, is 10%. Lw(fit) is calculated using the fitted polydispersity and Ln of Table 1.
Polymerization of Rodlike Micelles
equal to the fitted rigid cylinder lengths but with Kuhn lengths much larger than the contour length, a clear indication that the cylinders are rigid. The close agreement (see Table 2) between the curve-fitting results and the structure-independent Zimm analysis again confirms the validity of the models. Because the polymerized counterions are completely condensed on the micelle surface, persistence lengths (Lp ) LKuhn/2) obtained for the semiflexible polymerized cylinders of Lp ∼ 300-350 Å should be representative of an uncharged structure. Recent literature values for the persistence length of uncharged, flexible micelles are significantly lower, with Lp ∼ 170 Å.39 The persistence lengths of the polymerized micelles are much more comparable to those of charged micelles, which have reported values of Lp from 280 Å39 to more than 400 Å.40 Whether this discrepancy in persistence lengths is due to the presence of a nonzero charge on the polymerized micelles or an increased “intrinsic” stiffness resulting from the poly(counterion) is unclear from these limited measurements. The polymerized micelles are stable as prepared and remain stable for more than 1 year after polymerization. The polymerized micelles can be evaporated to dryness and then redispersed in water, retaining their original polymerized dimensions. The stability and solubility are in sharp contrast to the insoluble 1:1 precipitates that are typically formed when oppositely charged surfactants and ionomers are mixed.41 This enhanced stability may be, in part, due to the nature of the polymerized material. The polymer produced is certainly polydisperse and not at 100% conversion. This small fraction of unpolymerized material and oligomers present may solubilize the polymer-surfactant complex. The presence of these minor components, however, does not seem to contribute any significant net charge to the polymerized structures as shown by the modeling. The exact conformation of the polymer has yet to be determined but is likely confined to the surface of the micelle. The cetyltrimethylammonium tails aggregate and must be shielded from water. By electroneutrality, the polymerized 4-vinylbenzoic acid must be close to the surface. The electrostatic (noncovalent) association to the hydrophobic tails allows the polymer to adopt a favorable conformation. Whether this conformation is an orderly wrapping around the micelle surface or a more random coverage is unknown. Polymerization of 4-vinylbenzoic acid alone (5 × 10-3 mol/L of VBA + 5 × 10-5 mol/L of VA-044 in 2 cm3 of 2-propanol, 60 °C) gives a glassy, brittle polymer that is insoluble in water. This polymer dissolves slowly, over a period of a few weeks, without agitation in a stoichiometric solution of CTAOH. Although SANS measurements to characterize the microstructure present have not yet been performed, the final solution is bluish, nonviscous, and identical macroscopically to those formed when CTVB is polymerized, providing evidence that the polymerized structures are, in fact, equilibrium structures. Poly(4-vinylbenzoic acid) was isolated from the polymerized micelles to determine the molecular weight and degree of polymerization. Small sample sizes and the ionic character of the polymer precluded the use of gel permeation chromatography. Instead, the isolated polymer was dissolved in deuterated methanol, and its radius of (39) Jerke, G.; Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. Langmuir 1998, 14, 6013. (40) Butler, P. D.; Magid, L. J. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; p 250. (41) Antonietti, M.; Maskos, M. Macromolecules 1996, 29, 4199.
Langmuir, Vol. 15, No. 8, 1999 2731
gyration was measured, ranging from Rg ) 80 to 140 Å as the initiator concentration decreased. For Gaussian coils, the degree of polymerization can be expressed as42 N ) 6Rg2/a2. Without any knowledge of the actual solvent quality and using a ) 7 Å as the statistical segment length, this yields corresponding values of N from 800 to 2400. Given the rough level of approximation, these values are reasonable in comparison to the results of the Zimm analysis in Table 2. Both analyses indicate a high degree of polymerization of the counterions, and only one or a few polymer molecules per polymerized micelle, rather than an aggregate of oligomers. A low degree of polymerization in a micellar polymerization can result when the growing polymer chain is severely sterically hindered, but the mobility of the 4-vinylbenzoate counterions at the micelle surface eliminates this steric restriction. The insensitivity of the polymerized micelles to temperature and dilution confirms the formation of high molecular weight polymer, because individual surfactants and oligomer-like Gemini surfactants43 show structural changes with changing solution conditions. An order of magnitude difference in the initial surfactant concentration produces identical polymerized structures. This is because the concentration at the micellar surface is the relevant concentration and is relatively constant above the cmc. 4-Vinylbenzoic acid alone can be successfully polymerized at high concentration (2.5 mol/L). To test whether micelle formation is necessary for reaction at millimolar concentrations, an attempt was made to polymerize CTVB in CDCl3 (0.035 mol/L) at similar reaction conditions (50 °C, with an initiator mole fraction of 5% VA-044). The physical appearance as well as SANS from this solution before and after polymerization was identical to that of the pure solvent, with no indication of micelle formation or polymerization. Although the polymerization of the counterions is most likely not a true two-dimensional surface polymerization, the failure of polymerization in a nonmicellar solution indicates that the high concentration of 4-vinylbenzoate counterions near the surface of the micelle promotes polymerization. A more detailed investigation of the reaction kinetics will be presented in a future paper. Conclusions The preparation of a surfactant, CTVB, with a polymerizable counterion is simple. Its polymerization proceeds easily to nearly complete conversion, producing high molecular weight polymer. The structure of the parent micelle is partially preserved upon polymerization. The cross-sectional diameter is retained, but the overall length is reduced. This shorter average length and the confinement of the polymerized counterions to the micelle surface both contribute to the loss of viscoelasticity of the final product. The resulting solution is one of discrete cylindrical structures with one or a few polymer chains per particle. Higher molecular weight polymer results in longer average cylinder lengths that become semiflexible. The polymerized cylinders are remarkably stable and redispersible, in sharp contrast to typical polymerized surfactants or 1:1 polymer-surfactant complexes. Acknowledgment. The work was supported by the National Science Foundation under agreement no. DMR9423101. The author thanks B. Hammouda and B. Bauer (42) Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1982. (43) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 185, 84.
2732 Langmuir, Vol. 15, No. 8, 1999
for useful discussions and B. Coxon for the use of NMR facilities. Certain trade names and company products are identified to adequately specify the experimental procedure. In no case does such identification imply recom-
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mendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily best for the purpose. LA981451O
