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Langmuir 1998, 14, 800-807
Polymerization of Styrene in Ternary Microemulsion Using Cationic Gemini Surfactants Michael Dreja and Bernd Tieke* Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburger Stasse 116, 50939 Ko¨ ln, Germany Received September 30, 1997. In Final Form: December 1, 1997 The use of cationic dimeric (“gemini”) surfactants in the oil in water (o/w) microemulsion polymerization of styrene is reported. Gemini surfactants of the alkanediyl-R,ω-bis(dimethylalkylammonium bromide) type (m-s-m) with m being 12 and s being 2, 4, 6, 8, 10, and 12 were used. The phase behavior of the microemulsion is decisively influenced by the spacer length s. All surfactants form single-phase o/w microemulsions with styrene in a temperature range from 25 to 60 °C. For s being 2, only very small stable microemulsion regions were observed, while for s being 4, 6, and 8, the clear and stable regions were gradually increased. The shape of the single-phase regions was very similar to that of the analogous single tail surfactant dodecyltrimethylammonium bromide. For s being 10, the microemulsion region was distinctly extended, while for s being 12, it was decreased again at low temperature, but even more increased at high temperature. Polymerization of the monomeric microemulsions led to spherical latex particles, whose size range could be easily controlled by the monomer/surfactant ratio. A significant dependence of the particle size on the surfactant spacer length was observed. At 25 °C, the particle size was maximum for s being 10, at 60 °C the particle size increased with s. Therefore, the particle size was directly correlated with the size of the single-phase microemulsion region. The molecular weight was maximum at medium spacer length. The experimental results are discussed by taking into account hydrophobic and electrostatic effects and the respective microdroplet structure resulting from the varying interfacial spontaneous curvature due to the different surfactant shapes.
Introduction Over the recent years, there has been an increasing interest in the polymerization of microemulsion systems.1-3 Microemulsions are dispersions of two immiscible liquids (e.g., water and an oil component) which are stabilized by surfactant molecules. They are formed in a spontaneous process resulting in an optically transparent and thermodynamically stable system. In globular oil in water (o/w) microemulsions containing styrene as the oil component and single-tail cationic surfactants as stabilizer, the styrene can be polymerized. Ultrasmall latex particles in the size range from 5 to 100 nm, so-called “nanolatex particles”, are usually obtained.4-9 The individual particles exhibit a relatively small polydispersity and consist of only a few polymer chains with high molecular weight. The particle size can be reduced by adding a cross-linker to the styrene.2 Due to the highly dispersed structure and a restricted reaction space, the polymerization rate is very high in microemulsion. Up to now only few * To whom correspondence should be addressed. E-mail: Tieke@ uni-koeln.de. (1) Candau, F. In Polymerization in organized media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, PA, 1992; p 215. (2) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (3) Sjo¨blom, J.; Lindberg, R.; Friberg, S. E. Adv. Colloid Interface Sci. 1996, 85, 125. (4) Antonietti, M.; Bremser, W.; Lohmann, S. Prog. Colloid Polym. Sci. 1992, 89, 62. (5) Perez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Langmuir 1990, 6, 1040. (6) Puig, J. E.; Perez-Luna, V. H.; Perez-Gonzales, M.; Macias, E. R.; Rodriguez, B. E.; Kaler, E. W. Colloid Polym. Sci. 1993, 271, 114. (7) Gan, L. M.; Chew, C. H.; Lim, J. H.; Lee, K. C.; Gan, L. H. Colloid Polym. Sci. 1994, 272, 1082. (8) Full, A. P.; Kaler, E. W.; Aranello, J.; Puig, J. E. Macromolecules 1996, 29, 2764. (9) Antonietti, M.; Nestl, T. Macromol. Rapid Commun. 1994, 15, 111.
surfactants are known to form ternary globular microemulsions with styrene in an aqueous medium. Among them are n-alkyltrimethylammonium halides,2,4,5 n-alkyldiethanolamines9 and, as recently reported, polymerizable cationic single tail surfactants such as 11-acryloyloxyundecyltrimethylammonium bromide and n-alkyl (2-methacryloyloxyethyldimethyl)ammonium bromides.10,11 However, ternary microemulsions are desired for polymerization, because the use of cosurfactants is known to enhance chain transfer reactions and thus complicates the interpretation of experimental results.12 The search for new surfactants with improved emulsifying properties is also important for technical applications, since the common recipes still require large amounts of expensive materials. Recently, a surfactant modification by so-called “counterion variation” was described13 leading to assemblies of conventional cationic surfactants with multivalent counterions. These assemblies show enhanced stabilization properties for the polymerization of styrene in an o/w microemulsion. Using such “dimeric” or “trimeric” surfactant complexes, even smaller particles than with conventional surfactants were obtained. Unfortunately, a systematic variation of the surfactant structure has not been carried out. This is desirable since Chieng et al. showed that the microstructure of polymerizable microemulsions can be controlled via the surfactant chain length.14 Instead of the “dimeric” surfactant complexes discussed above also real dimeric surfactants, so-called “gemini” surfactants,15 might be useful in stabilizing (10) Dreja, M.; Tieke, B. Macromol. Rapid Commun. 1996, 17, 825. (11) Dreja, M.; Pyckhout-Hintzen, W.; Tieke, B. Macromolecules, in press. (12) Gan, L. M.; Chew, C. H.; Lye, I.; Ma, I.; Li, G. Polymer 1993, 34, 3860. (13) Antonietti, M.; Hentze, H. P. Adv. Mater. 1996, 8, 841. (14) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Polymer 1996, 37, 2801, 4823.
S0743-7463(97)01073-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/28/1998
Polymerization of Styrene in Ternary Microemulsion
microemulsions for polymerization. Unfortunately, up to now there is no experience in microemulsions with dimeric surfactants. We therefore prepared such microemulsions using cationic “gemini” surfactants of the alkanediyl-R,ω-bis(dimethylalkylammonium bromide) type (m-s-m surfactants), in which the surfactant structure can be systematically varied via the spacer chain length s:
While m was 12, s was 2, 4, 6, 8, 10, and 12. This type of surfactant has already been comprehensively characterized and shows interesting properties in aqueous solution.16-21 For example, the cmc of the m-s-m surfactants is much lower than that of the corresponding single tail surfactants. In our paper we report on phase and polymerization behavior of o/w microemulsions with the m-s-m surfactants and styrene. A major question is if, and in which case, the coupling between the two ionic headgroups results in a cooperative effect for the stabilization of the oil/water interface. This will be explained by examining the phase behavior of the ternary systems water/m-s-m surfactant/ styrene. A big advantage of the present system is that the surfactant shape, aggregation properties and thus the interfacial spontaneous curvature can be easily tuned by changing the spacer length s. Also, it is expected that gemini surfactants show reduced exchange dynamics in microemulsion, because two tails have to leave a microdroplet at the same time. A second important question is concerned with the polymerization behavior. It is still unclear how the polymerization process in simple ternary systems is influenced by the microemulsion microstructure. Therefore, polymerizations were carried out at two different temperatures (25 and 60 °C) using 60Co-γradiation as initiator. The use of γ-radiation is advantageous over chemical initiation because high-energy radiation is able to homogeneously penetrate the sample and thus avoids possible distribution inequalities of the initiator. Also, no additional component is introduced in the system which might influence the phase behavior. Cross-linked particles (“microgels”) are suitable for a correlation between size and effectiveness of the particle stabilization by the surfactant molecules during the polymerization process.2,13 We therefore also studied the influence of cross-linking on the polymerization and found that an effective single-phase stabilization and a small particle size after polymerization are inversely correlated with each other. Experimental Section Materials. Dodecyltrimethylammonium bromide (DTAB, Fluka) and cetyltrimethylammonium bromide (CTAB, Sigma) with purities of 99% were used as received. Styrene (Aldrich) was freshly distilled before use in order to remove the inhibitor (15) Zana, R. Curr. Opin. Colloid Interface Sci. 1997, 2, 312. (16) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 34, 780. (17) Devinsky, F.; Lacko, I.; Imam, T. J. Colloid Interface Sci. 1991, 143, 336. (18) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (19) Alami, E.; Levy, H.; Zana, R., Skoulios, A. Langmuir 1993, 9, 940. (20) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (21) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448.
Langmuir, Vol. 14, No. 4, 1998 801 and oligomeric impurities. 1.3-Diisopropenylbenzene (Fluka) and hydroquinone (Merck) were used as received. Milli-QPLUS water with low conductivity (R > 18 MΩ) was used for all experiments. Surfactant Synthesis. The gemini surfactants were prepared according to the literature.16,17 All surfactants are of the alkanediyl-R,ω-bis(dimethylalkylammonium bromide) type and denoted as m-s-m, m being 12 and s being 2, 4, 6, 8, 10, and 12, respectively. For purification, the surfactants were recrystallized three times from either acetone or ethyl acetate. The products were received as white, crystalline solids. Their purity was checked by 300 MHz 1H NMR and surface tension measurements in aqueous solution (Wilhelmy plate method, Kru¨ss DigitalTensiometer K 10 T, 25 °C). The cmc values were found to be in the range described in the literature.20 Polymerization. Polymerization in microemulsion was performed in sealed glass vessels at room temperature (25 °C) and in a thermostated Dewar vessel (60 °C, Lauda K4). The transparent samples were prepared by titrating the desired amount of styrene with or without 1,3-diisopropenylbenzene as cross-linker (molar ratio cross-linker/styrene 1:20) into an aqueous surfactant solution, equilibrated and treated with ultrasound shortly before use in order to remove small gas bubbles. No attempts were made to remove dissolved oxygen, because a purging of the samples with an inert gas would have been accompanied by a partial evaporation of the styrene from the microemulsion. 60Co-γ-radiation was used for the polymerization. The dose rate was 53.1 krad/h ) 0.531 kGy/h (1 rad ) 10-5 J/g ) 0.01 J/kg ) 0.01 Gy). To achieve a complete conversion, a γ-ray dose of 10 kGy was applied to each sample. Methods. The clear single-phase regions of the microemulsion at 25 and 60 °C were determined visually by titrating styrene into water-surfactant mixtures in screw-capped glass tubes. Each sample was thoroughly homogenized using a Vortex mixer and thermostated in a water bath. The phase diagram at 60 °C was determined with styrene, to which a few parts per million of hydroquinone were added in order to prevent a thermal polymerization. Phase boundaries were checked by preparing samples by weight with compositions slightly below and above the phase boundaries determined by titration. The accuracy of the boundaries laid within (0.2% (w/w), and the obtained values were independent of titration speed and stirring rate. Particle size and size distribution were determined with a Nicomp C 370 particle sizer. For this purpose, γ-irradiated samples were diluted with water and thermostated during measurement at a temperature of 25 ( 0.1 °C. In this procedure, the intensity fluctuations of scattered light (HeNe laser, 632.8 nm) are measured with a photon correlator at a fixed angle of 90°. The measured correlation functions are fitted with a Gaussian distribution function for the relaxation times (method of cumulants). The resulting intrinsic diffusion coefficients were converted to the corresponding hydrodynamic intensity-weighted particle diameters with Gaussian distribution widths using the Stokes-Einstein equation, assuming that the solvent has the viscosity of pure water. Molecular weights were measured using gel permeation chromatography (Waters 501) equipped with a UV detector. A set of two Styragel columns (HR1, HR5, Waters) and a Eurogel column (GPC 1000, Knauer) was used after calibration with polystyrene standards (Polyscience). The eluent was degassed tetrahydrofuran (THF); the flow rate was 1 mL/min.
Results and Discussion Phase Behavior. The phase behavior of the ternary microemulsion systems water/m-s-m surfactant/styrene is shown in Figure 1 at 25 and 60 °C. All surfactants form single-phase o/w microemulsions with styrene at both temperatures. The extent of the single-phase region is decisively influenced by the spacer length s of the m-s-m surfactants. For s ) 2, the stable microemulsion regions are very small at both temperatures (Figure 1a). At 60 °C, the high concentration regime cannot be determined exactly due to the high viscosity of the samples. For s ) 4, 6, and 8, the clear and stable regions increase gradually.
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Figure 1. Partial phase diagrams of the systems water/m-s-m surfactant/styrene for 25 and 60 °C (1 Φ and 2 Φ denote singleand two-phase regions): (a) 12-2-12; (b) 12-4-12; (c) 12-6-12; (d) 12-8-12; (e) 12-10-12; (f) 12-12-12.
Using gemini 12-4-12, an upper limit for the single-phase region was observed at both temperatures investigated (Figure 1b). In the case of 12-6-12, however, the upper limit disappears at 60 °C for the investigated concentration regime, whereas at 25 °C the single-phase region is very similar to that of 12-4-12 (Figure 1c). Increasing the length s of the gemini spacer chain to eight methylene units leads to a considerable growth of the single-phase region at 25 °C, while the solubilization limit is only slightly shifted at high temperature (Figure 1d). The shape of the singlephase region of the systems with s ) 4, 6, and 8 is very similar to that of the corresponding “monomeric” single
tail (m-)surfactant, dodecyltrimethylammonium bromide (DTAB).5,6,8 For s ) 10, the microemulsion region is distinctly extended at 25 °C (Figure 1e). The boundaries at 25 and 60 °C lie nearly parallel to each other, which means that the phase behavior is only slightly temperature dependent. If the 12-12-12 surfactant is used, the singlephase region is dramatically decreased at surfactant concentrations above 5% (w/w) and low temperature, but even more increased at high temperature (Figure 1f). This means that the phase behavior again becomes distinctly temperature dependent. Altogether, a short spacer chain length s supports solubilization of styrene at low surfactant
Polymerization of Styrene in Ternary Microemulsion
Figure 2. Plot of the molar solubilization ratio of styrene/ surfactant moles vs the surfactant concentration in % (w/w) at 60 °C.
concentration and low temperature, whereas a long spacer supports solubilization at high surfactant concentration and high temperature. In Figure 2 the solubilization properties of styrene by m-s-m surfactants and by DTAB are compared at 60 °C. The molar ratio of styrene/surfactant is plotted vs the surfactant concentration in percent (w/w). It can be seen that the solubilization behavior of the m-s-m surfactants with s g 4 is superior to that of DTAB. Up to about 12.5%(w/w), surfactant 12-12-12 is best suited for the microemulsification of styrene, five times better than DTAB. This means that the oligomethylene spacer s between the two surfactant tails in fact induces a cooperative stabilization of the oil-water interface compared with the single tail cationic surfactant molecules. Before discussing this result, it may be useful to recall some of the typical properties of binary m-s-m surfactant/ water systems. Zana and co-workers investigated the microstructure of the m-s-m surfactants in aqueous solution and in lyotropic mesophase.18-21 Using cryogenic temperature transmission electron microscopy (cryoTEM), they found that 12-2-12 forms entangled threadlike micelles very similar to those formed by cetyltrimethylammonium bromide (CTAB), whereas for the other m-s-m surfactants investigated here spherical or spheroidal micelles were reported, when the surfactant concentration in water was between 5 and 10% (w/w). Time-resolved fluorescence quenching showed that the aggregation number of 12-4-12 in aqueous solution grows relatively fast with increasing concentration in analogy to the m-s-m surfactants with shorter spacer chain. For s > 4, the variation of the aggregation number was only small.21 In an earlier work, it was reported that the solid m-s-m surfactants do not exhibit thermotropic liquid crystalline behavior but do form lyotropic mesophases. Upon an increase of the spacer chain length from s ) 4 to 8 the concentration range of the mesophase region decreased. For s ) 10 and 12, the mixtures remained micellar in the whole concentration range.19 This was explained by a mismatch of the packing of the spacer chains being maximum at about 10 methylene units, which is also revealed by a minimum in the melting point and a maximum in the surface area occupied at the air-water interface.20 Hence, the observed phase behavior results from a different packing demand of the m-s-m surfactants at the oil-water interface and from changes in the interfacial spontaneous curvature with the spacer length s. Regarding the molecular packing parameter P given
Langmuir, Vol. 14, No. 4, 1998 803
by the ratio v/Al, where A is the cross-sectional area per headgroup, v the volume of the alkyl chain and l its length,22 it is known that for conventional surfactants v/l is relatively constant so that P is predominantly determined by A. In the case of the m-s-m surfactants, A increases with s up to s ) 12.20 This means that the steric demand for curved aggregates is increased with increasing s in accordance with the experiments. (See note added in proof at the end of this paper.) Regarding the phase behavior of the microemulsions, the m-s-m surfactants with long spacer chains obviously do not gain enough free energy by forming a new separate (meso)phase as is the case for geminis with short s, so that the microemulsion droplets are stabilized over a wide concentration range. It has been shown theoretically as well as experimentally that the phase behavior of microemulsions is determined not only by the surfactant headgroup area but also by the spontaneous curvature and the bending energy of the interfacial film.23,24 The most striking result that can be drawn from the ternary phase diagrams is that a maximum spacer length s of 10 for the interfacial stabilization exists at the low temperature of 25 °C. For s ) 12 it is likely that the spacer chain is no longer arranged parallel to the oil droplet surface but refolds into the oil phase, in which the oligomethylene units are soluble. Therefore, the stabilized area abruptly decreases at this spacer length. At the airwater interface, it was observed that this process does not occur until s is 14.20 At high temperature, exchange mobility and counterion dissociation are increased, which favors the microdroplet stabilization. The phase diagrams suggest that then the spacer chain attains a more stretched conformation on average, so that even more oil than with 12-10-12 is solubilized. It is probable that if the spacer length of the m-s-m surfactant is increased to s > 12, at 60 °C a similar decrease of the single-phase region will occur as for s ) 12 at 25 °C. Polymerization. The ternary microemulsion systems were subjected to polymerization using 60Co-γ-radiation. The use of γ-irradiation is advantageous for microemulsions since polymerization can be conducted at variable and controlled temperature, so that the phase behavior of the monomeric systems can be easily adjusted. Samples with different surfactant-to-monomer weight ratio (WR) were prepared, according to a method reported by Antonietti et al.13 After polymerization, samples with high WR values were usually still transparent or slightly bluish, whereas samples with high monomer load gave bluish white dispersions. Though many monomeric samples were quite viscous, the viscosity of the polymerized samples was low. A surprising exception was found for systems with the 12-2-12 surfactant, for which a high viscosity was preserved during polymerization. The polymerization was performed at 25 and 60 °C in the presence of 1,3-diisopropenylbenzene as a cross-linker and at 25 °C without cross-linker. The resulting hydrodynamic particle radius Rh was determined by photon correlation spectroscopy. In Figure 3, Rh is plotted vs WR for the cross-linked particles. In addition, the accurate particle sizes and further characteristic data of the various γ-irradiated samples are compiled in Table 1. It can be seen from Figure 3 that the particle size increases with increasing monomer content (i.e., decreasing WR value), (22) Israelachvili, J.; Mitchell, D. J.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (23) Safran, S. A.; Turkevich, L. A.; Pincus, P. A. J. Phys. Lett. 1984, 45, L69. (24) Glatter, O.; Strey, R.; Schubert, K.-V.; Kaler, E. W. Ber. Bunsen. Phys. Chem. 1996, 100, 323.
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a
b
Figure 3. Plots of the hydrodynamic particle radius Rh vs the surfactant-to-monomer weight ratio WR after polymerization at (a) 25 °C and (b) 60 °C for systems with cross-linker.
Figure 4. Plots of the hydrodynamic particle radius Rh vs the m-s-m surfactant chain length s after polymerization at (a) 25 °C and (b) 60 °C for systems with cross-linker.
as it has been observed before.2,13 It is obvious that in our case the particle size is also dependent on the gemini spacer length. At 25 °C, the particle size is larger for microemulsions with the 12-10-12 surfactant than for the other ones. The smallest latex particles are formed with 1212-12 and 12-2-12 surfactants. The slope of the curves is very similar for all systems. At 60 °C, the particle sizes for the same concentrations are usually larger than those at 25 °C, which is due to enhanced surfactant solubility and exchange dynamics in the system.25 The slope of the curves at 60 °C is smaller than that at 25 °C. This means that at high temperature, the particle size is less dependent on the WR value, i.e., the stabilization of high monomer loads is thermically enhanced during polymerization. Use of the 12-12-12 surfactant leads to the biggest particles, followed by those from the 12-10-12 microemulsion. 12-s-12 surfactants with s being 4, 6, and 8 yield very similar particle sizes. For a high surfactant concentration of the 12-2-12 surfactant, the particle size
is not uniform, which is a result of either a higher polydispersity of the size distribution or an anisotropy of the particle shape (see Table 1a). The shape of the latex particles formed with the 12-2-12 surfactant is currently under investigation using microscopic methods. Another reason for the apparently high polydispersity may result from scattering of the free surfactant aggregates in the solution. A high polydispersity was also reported for most latex dispersions obtained from “counterion coupled gemini” (“cocogems”) microemulsion systems.13 However, it should be emphasized that for all m-s-m systems except 12-2-12, the polydispersity is considerably smaller, i.e., in the range usually found for particles from conventional single tail surfactant systems. The systematic changes in the particle sizes are even more obvious, if the hydrodynamic particle radius Rh is plotted vs the spacer length s as shown in Figure 4. Now, the connection between the phase behavior and the polymerization process becomes evident. At 25 °C, the particle size increases with s for all concentrations until Rh becomes maximum for s being 10. At higher s values,
(25) Dreja, M.; Tieke, B. In press.
Polymerization of Styrene in Ternary Microemulsion
Langmuir, Vol. 14, No. 4, 1998 805 Table 1
(a) Characteristic Data of Polymerized m-s-m Systems with Cross-Linker polymerized at 25 °C
polymerized at 60 °C
m-s-m
WR
Rh/nm
σ
A/nm2
Rh/nm
σ
A/nm2
12-2-12
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
19.0 13.9 12.0 10.9 10.5 22.0 16.3 13.1 11.6 10.9 19.2 15.1 13.2 12.0 10.6 21.5 16.2 13.3 13.1 13.1 25.6 23.3 19.8 16.6 15.5 18.6 14.9 13.5 11.4 11.0
0.321 0.344 0.378 0.509 0.453 0.234 0.313 0.289 0.339 0.391 0.160 0.286 0.287 0.288 0.414 0.283 0.321 0.316 0.368 0.516 0.336 0.240 0.240 0.184 0.344 0.333 0.287 0.207 0.120 0.405
0.322 0.330 0.327 0.351 0.350 0.291 0.294 0.325 0.347 0.353 0.347 0.331 0.338 0.347 0.377 0.324 0.321 0.348 0.331 0.318 0.283 0.233 0.244 0.273 0.280 0.404 0.379 0.370 0.412 0.410
19.6 19.0 20.0 18.9 15.1 19.0 18.5 16.4 14.7 14.1 19.1 15.5 14.9 13.7 13.5 18.9 17.7 15.6 14.8 13.4 25.4 22.1 18.7 17.4 15.7 27.7 27.1 25.4 22.5 20.9
0.312 0.320 0.456 0.489 0.534 0.327 0.274 0.284 0.230 0.256 0.269 0.254 0.242 0.192 0.251 0.319 0.276 0.224 0.208 0.209 0.460 0.260 0.243 0.231 0.221 0.274 0.322 0.243 0.206 0.171
0.312 0.242 0.204 0.202 0.243 0.336 0.259 0.259 0.271 0.272 0.349 0.323 0.298 0.304 0.297 0.367 0.294 0.297 0.294 0.311 0.285 0.246 0.258 0.260 0.276 0.272 0.208 0.197 0.208 0.215
12-4-12
12-6-12
12-8-12
12-10-12
12-12-12
(b) Characteristic Data of Polymerized m-s-m Systems without Cross-Linker at 25 °C m-s-m
WR
Rh/nm
σ
Mw/106 g‚mol-1
m-s-m
WR
Rh/nm
σ
Mw/106 g‚mol-1
12-2-12
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
21.4 13.8 14.6 12.1 11.8 25.6 19.4 16.8 15.3 14.7 23.6 20.7 16.8 14.5 12.7
0.345 0.506 0.480 0.486 0.376 0.227 0.251 0.302 0.289 0.403 0.209 0.373 0.362 0.290 0.302
0.645 0.431 0.422 0.324 0.279 2.000 1.130 0.880 0.688 0.572 2.120 1.900 1.330 1.300 0.816
12-8-12
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
25.5 19.1 16.5 14.6 13.5 29.9 21.1 20.4 17.9 16.2 17.7 15.0 14.6 13.6 12.9
0.367 0.273 0.364 0.397 0.211 0.187 0.219 0.288 0.216 0.282 0.241 0.164 0.318 0.200 0.166
1.400 0.970 0.575 0.351 0.451 0.726 0.925 0.894 0.646 0.403 0.166 0.125 0.126 0.147 0.164
12-4-12
12-6-12
the size is decreased again. A comparison with Figure 1 indicates that the latex particle size is obviously maximum, when the size of the one-phase microemulsion region in the ternary phase diagram is also maximum. The relation also holds for the polymerization at 60 °C. Here the latex particle size is maximum for s being 12, in analogy to the maximum size of the single-phase region, which is also found for the surfactant with s being 12. Thus the latex particle size is determined not only by the WR ratio but also by the structure of the parent microemulsion. Zana and co-workers reported that the surface area of the gemini surfactants is maximum for a spacer length s of 10 to 12 at 25 °C.20 Regarding the phase behavior of the microemulsions, this is true at s being 10. For s being 12, however, the situation in the oil-water interface of the microemulsion seems to be different to the air-water interface. The reason might be a preferred chain refolding into the oil phase at large s values, as already discussed above. A possible bridging between surfactant micelles, especially in the case of the surfactants with large s, could
12-10-12
12-12-12
also lead to an increase in the particle size and the polydispersity. Nevertheless, bridging has not been observed by Zana and co-workers in the case of short chain dimeric surfactants.27 According to Antonietti and Wu, the latex particle size can be correlated with an area A occupied by the surfactant molecule at the latex particle surface using a simple spherical geometric approach.2,24 The calculated values of A for each cross-linked sample are compiled in Table 1a. It is obvious that the stabilized area per molecule is larger than that of monomeric surfactants but still smaller than for the related “cocogems”.13 This is mostly a consequence of the absolute values of the particle sizes which are somewhat larger in our case for similar WR values. This may arise from a more effective interface stabilization of the polar spacers of the cocogems but also be due to the use of γ-rays for the initiation since there (26) Wu, C. Macromolecules 1994, 27, 298. (27) Frindi, M.; Michels, B., Levy, H.; Zana, R. Langmuir 1994, 10, 1140.
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Figure 5. Plot of the weight average Mw of the molecular weight vs the surfactant-to-monomer weight ratio WR after polymerization at 25 °C for systems without cross-linker (a) and vs the m-s-m surfactant chain length s after polymerization at 25 °C for systems without cross-linker (b).
is a distinct dependence of particle size and average molecular weight on the initiator concentration.2,6 For the non-cross-linked polymer particles, the weight average Mw of the molecular weight of polystyrene was determined by gel permeation chromatography. It is wellknown that the polymerization of simple monomers in o/w microemulsion yields very high Mw values.6-8,28,29 This is due to the highly dispersed medium and the confined reaction space, which leads to a small number of free radicals per particle. In Figure 5a, the Mw values for some of the polystyrene particles are plotted vs the surfactantto-styrene monomer weight ratio WR (see also Table 1b). It is obvious that the molecular weights are low at low styrene content and increase with the amount of styrene in the parent microemulsion. This means that according to the emulsion-based Candau-Leon-Fitch model (CLF model) the primary microemulsion droplets after initiation grow upon monomer transport from noninitiated microdroplets via the continuous water phase.1 This can (28) Gan, L. M.; Chew, C. H.; Lee, K. C., Ng, S. C. Polymer 1994, 35, 2659. (29) Lusvardi, K. M., Schubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 373.
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increasingly take place, when more monomer is supplied at a constant initiator concentration or γ-ray dose, respectively. Thus, a high molecular weight is an indication for a high quality of stabilization of the system during polymerization. Regarding the different surfactant systems, there are large differences in the absolute values of Mw. In Figure 5b, the dependence of the molecular weight on the gemini spacer length s is plotted for a constant WR value of 2. A maximum molecular weight can be found at medium spacer length. Similar curve shapes were found at the other WR values. Since neither the particle size nor the stabilization of the parent microemulsion were maximum at an s value of 6, one must conclude that there are two opposite effects leading to such a result. Let us remember that the thermodynamically stable monomer droplet microemulsion is converted upon the polymerization process into a latex dispersion which is stabilized by the surfactant molecules. The first of the two opposite effects is concerned with the preferred spontaneous curvature of the m-s-m surfactant which determines the structure of the monomeric droplets. A small spacer length s leads to micelles which are able to stabilize only minor amounts of the monomer, whereas long spacers lead to the stabilization of large amounts. The packing parameter P suggests a transition from small elongated to large spherical microdroplets. This effect is similar to that originating from a decrease in the alkyl chain length of single tail surfactants.25 For example, on going from CTAB to DTAB, a transition from rodlike to spherical micelles takes place in aqueous solution. During the course of the polymerization, a short spacer s therefore promotes the formation of small particles, i.e., polymer of low molecular weight. The second effect results from the diffusive exchange between the micellar droplets, which is high for those with large s values, i.e., low bending rigidity and low viscosity. A high entry rate of growing chain radicals favors a low molecular weight of the polymer at a simultaneously higher chain concentration per particle. The high exchange dynamics at large s lead to the formation of many small polymer chains, which can coagulate very fast to form large latex particles. These considerations are consistent with the model of a heterocoagulation mechanism recently proposed by Napper et al. for the polymerization of styrene in o/w-microemulsion.30,31 This model comprises the entry of oligomeric styrene radicals from the aqueous phase into the monomerfilled microemulsion droplets. The entry should be directly related to the mobility of the microdroplets. Therefore, a medium spacer chain length of s being 6 constitutes a compromise for the surfactant between its ability to stabilize the parent microemulsion droplets and its ability to support the polymerization process by a high diffusive exchange mobility. Conclusions The phase behavior and the polymerization of styrene in ternary o/w microemulsions with cationic gemini surfactants of the 12-s-12 type is reported. The surfactants promote the formation of o/w microemulsions similar to their single chain analogue DTAB, but the solubilization properties of the m-s-m surfactants are much better. The single-phase region is strongly dependent on the spacer chain length s, being maximum for s ) 10 at 25 °C and s ) 12 at 60 °C. Polymerization of the monomeric (30) Kim, D. R.; Napper, D. H. Macromol. Rapid. Commun. 1997, 17, 845. (31) Kim, D. R.; Napper, D. H. Langmuir 1996, 12, 3139.
Polymerization of Styrene in Ternary Microemulsion
microemulsion leads to spherical latex particles, whose overall size is controlled by the monomer/surfactant ratio. A correlation between the particle size and the size of the single-phase region was found. This suggests that an important influence of the microemulsion microstructure on the polymerization process exists, resulting from the different spontaneous curvature of the m-s-m molecules induced by the different spacer length s. The optimum microemulsion formulation leading to both small particle size and high molecular weight was found for a medium gemini spacer length of s being about 6. Noted Added In Proof. The aggregation of gemini surfactants on solid interfaces was reported most recently (Manne, S.; et al. Langmuir 1997, 13, 6382). On an anionic
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mica plane, the aggregates tend to favor a lower curvature than in solution but follow the same general variation with surfactant geometry. This correlates with observations in surfactant-silica mesophases and might also be valid for ternary microemulsion phases. Acknowledgment. We thank C. Sulitzki for help during surfactant and particle synthesis. Support of PA Partikel Analytik GmbH, Ko¨ln, and Mundipharma GmbH, Limburg, is gratefully acknowledged. We also thank the Deutsche Forschungsgemeinschaft for financial support (Project Ti219/5-1). LA9710738
