Synthesis of Stable Polystyrene and Poly (methyl methacrylate

Oct 21, 2004 - The synthesis of polystyrene or poly(methyl methacrylate) in the presence of carboxymethyl cellulose (CMC), a cellulose derivative, was...
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MATERIALS AND INTERFACES Synthesis of Stable Polystyrene and Poly(methyl methacrylate) Particles in the Presence of Carboxymethyl Cellulose L. B. R. Castro, K. V. Soares, A. F. Naves, A. M. Carmona-Ribeiro, and D. F. S. Petri* Instituto de Quı´mica, Universidade de Sa˜ o Paulo, P.O. Box 26077, Sa˜ o Paulo SP 05513-970, Brazil

The synthesis of polystyrene or poly(methyl methacrylate) in the presence of carboxymethyl cellulose (CMC), a cellulose derivative, was carried out by emulsion polymerization using a cationic surfactant, cetyltrimethylammonium bromide (CTAB). First, the complex formation between CTAB and CMC was studied by surface tension measurements. The polymerization condition chosen was that corresponding to CMC chains fully saturated with CTAB and to the onset of pure surfactant micelle formation, namely, at 0.25 mmol L-1 CTAB and 1.0 g L-1 CMC. The hybrid particles were characterized by ζ potential and light scattering measurements and scanning electron microscopy. All dispersions were stable in the ionic strength of 2.0 mol L-1 NaCl at least for 4 days. The colloidal stability was attributed to the presence of a hydrated CMC layer around the particles. The present procedure brings the advantage of synthesizing and stabilizing particles with functional groups on the surface in a one-step method using very small amounts of surfactant, a friendly condition for the environment. Introduction The colloidal stability of complexes formed by charged polymeric particles and natural polyelectrolytes has been extensively studied.1-7 Generally, the particles are first synthesized, and then in a second step, they are stabilized by adding polymers, which adsorb on the particle surface. The stabilization might be driven by electrostatic, steric, or electrosteric repulsion.8 In the present work, we propose a strategy to obtain polymeric particles stabilized by natural polyelectrolytes during the synthesis. It is based on a standard emulsion polymerization recipe, except for the fact that the monomer is added to a solution containing surfactant and an oppositely charged polyelectrolyte. The presence of polyelectrolytes reduces the critical micelle concentration (cmc) and can lead to phase separation.9-13 For that reason, the binary system polyelectrolyte/surfactant was first studied to find the best synthesis conditions. This strategy might reduce an environmental problem related to the emulsion polymerization, which is the release of high amounts of surfactants to the wastewater. To our knowledge, emulsion polymerization reactions in complex polyelectrolyte/surfactant have seldom been studied.5,14 Marie and co-workers5 obtained stable polystyrene (PS) particles with a very small amount of coagulum using chitosan in combination with cetyltrimethylammonium chloride. Esquena and co-workers14 obtained stable PS and poly(methyl methacrylate) (PMMA) particles using a polyfructose-based surfactant in the polymerization. In the present work, the polymerization of styrene, methyl methacrylate (MMA), or * To whom correspondence should be addressed. Tel.: 0055 11 3091 3831. Fax: 0055 11 3818 5579. E-mail: dfsp@ iq.usp.br.

vinyl acetate (VAc) was performed in the presence of cetyltrimethylammonium bromide (CTAB), a cationic surfactant, and carboxymethyl cellulose (CMC), a polyanion obtained from cellulose. Experimental Section Materials. MMA (Fluka, Buchs, Switzerland), styrene (S; Aldrich, Milwaukee, WI, S497-2), CTAB (Aldrich, Milwaukee, WI), VAc (Rhodia SA, Sa˜o Paulo, Brazil), potassium persulfate (K2S2O8; Merck, Munich, Germany), and CMC (Aldrich, Milwaukee, WI) sodium salt with a nominal mean degree of substitution (DS) of 0.7 and Mv of 90 000 g mol-1 were used in the polymerization. Interaction between CTAB and CMC. The interaction between CTAB and CMC was studied by means of surface tension measurements. CTAB was added to an aqueous solution of CMC in a concentration of 1.00 g L-1, so that the concentration of CTAB varied from 0.001 to 1.00 mmol L-1. The surface tension at the airwater interface was measured at 24.5 ( 0.5 °C with a Kru¨ss instrument and Du Nouy ring. Synthesis of Hybrid Particles. The synthesis of the hybrid particles followed a typical emulsion polymerization recipe,15 except that, instead of water, the medium was an aqueous solution of CMC at a concentration of 1.0 g L-1. CTAB was added to this solution, so that its final concentration amounted to 0.25 mmol L-1. The medium was purged with N2 for 30 min, while the temperature was brought to 82 ( 2 °C in the case of S and MMA or to 65 ( 2 °C in the case of VAc. Afterward, the initiator, K2S2O8, was added. Two minutes later monomer was thrown into the system without any particular procedure. The amounts used in two

10.1021/ie0495271 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/21/2004

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7775 Table 1. Formulations Used for Synthesis of Hybrid Particles reactant

MMA (g)

S (g)

formulation 1 formulation 2

9.32 37.76

9.06 36.24

VAc (g) water (g) K2S2O8 (g) 9.32 -

140 140

0.03 2.0

different formulations are described in Table 1. The polymerization was carried out under reflux and mechanical stirring (500 rpm). After 3 h, the system was cooled to room temperature and dialyzed (dialysis membrane 14 000 MW; Viskase Corp., Loudon, TN) against water with four changes daily for 1 week or until the conductivity of dialysis water reached 5 µS cm-1. In this process, no buffer was used. The dialyzed dispersions presented a pH in the range of 4.5-4.8. The hybrid particles are identified as PMMA/CMC, PS/CMC, and PVAc/CMC. At least three syntheses were performed for each system. Particle Characterization. The hybrid particles characterization was performed by means of a ZetaPlus zeta potential analyzer (Brookhaven Instruments Corp., Holtsville, NY) equipped with a 677-nm laser. The zeta potential value, ζ, was determined from the electrophoretic mobility, µ, in 0.001 mol L-1 KCl and Smoluchowski’s equation ζ ) µη/, where η is the medium viscosity and  the medium dielectric constant. The particle diameter D was obtained by dynamic light scattering at 90.0°. The dispersions were prepared in 0.001 mol L-1 KCl at 1.0 × 1011 particles mL-1. The particle size distribution from analysis of quasi-elastic light scattering (QELS) data was performed following well-established mathematical techniques.16 Scanning electron microscopy (SEM) was performed in a LEO 440-I microscope on droplets of dispersions at 1.0 × 1011 particles mL-1 after drying in air. A very thin carbon layer was deposited on the dried dispersions prior to the SEM analysis. The solid content and the conversion of monomer into polymer were determined by gravimetry. The mean particle number density (Np) was calculated considering the particle mean diameter determined by QELS, solid content in a 1.00-mL dispersion, and polymer density as 1.00 g cm-3. Infrared vibrational spectra (FTIR) of the dried hybrid particles PMMA/CMC and PS/CMC, PMMA, PS, and CMC were obtained with a Bomen MB100 equipment. The spectra obtained for the hybrid particles presented the characteristic bands observed in the pure components. The presence of strong acid-base interaction or hydrogen bonding between PMMA and CMC might have been evidenced by shifting in some of the characteristic bands, such as, for instance, 1730 cm-1 (CdO stretching, strong). However, no shift in the characteristic bands could be observed when the spectra of each pure component were compared to the hybrid particle spectra. The spectra are available under request. Colloidal Stability Tests. The colloidal stability was tested visually by adding 0.3 or 2.0 mol L-1 NaCl to the stock dispersions at pH 5.0. For comparison, the same tests were done with the commercial polystyrene sulfate (PSS) particles with a mean diameter of 85 nm and 2.0 × 1012 particles mL-1 provided by Interfacial Dynamics Corp. (Portland, OR). Results and Discussion Complex Formation between CTAB and CMC. The interaction between cationic surfactants and polyanions has been well reported in the literature.10,17-20

Figure 1. Surface tension as a function of log [CTAB] at 1.0 g L-1 CMC concentration. The line is a guide for the eyes.

At constant polyelectrolyte concentration and increasing surfactant concentration, three critical surfactant concentrations, c1, c2, and c3, might be found. c1 is typically referred to as the critical aggregation concentration and corresponds to the concentration where the binding of the surfactant to the polyelectrolyte begins, while c2 is considered to be the surfactant concentration where the bulk polymer is saturated with surfactant. Additional surfactant molecules cannot bind to the polyelectrolyte and can therefore lower the surface tension until the formation of the pure micelles in the solution at c3. The surfactant forms micellelike clusters adsorbed to the polyelectrolyte chain that are smaller than the corresponding free micelles. The interaction depends on the surfactant chain length, on the polyelectrolyte persistence length, and on the size of the surfactant headgroup, but it does not depend on the polyelectrolyte molecular weight.21 The complexation between CTAB and CMC is expected because of attractive electrostatic interactions. CMC is supposed to lower the repulsion among the cationic headgroups and, therefore, the concentration at which aggregation takes place. The cmc of CTAB was determined by surface tension measurements at the air-water interface as being close to 1.0 mmol L-1. The surface tension γ decreased from 71.8 mN m-1 (pure water) to 37 ( 1 mN m-1. The same measurements were done in the presence of CMC 0.7-90 at a constant concentration of 1.0 g L-1. The polyanion alone reduced the surface tension of water to 62 ( 1 mN m-1. Upon addition of CTAB to the CMC 0.7-90 solution, the γ values decreased even more, as shown in Figure 1. Two plateaus are evident. The first plateau has begun at the CTAB concentration of 0.03 mmol L-1, which was attributed to c1, and has finished at 0.07 mmol L-1, which corresponds to c2. The surface tension value in the first plateau amounted to 54 ( 1 mN m-1. The onset of the second plateau at 0.25 mmol L-1 was attributed to c3. The binding between polyelectrolytes and surfactants is well reported in the literature.10-13 The reduction in the Gibbs free energy ∆G of micelles when bound to a polymer can be calculated by

∆G ) RT ln(c1/cmc)

(1)

In the present findings, the reduction in ∆G for the system CMC/CTAB amounted to 8.69 kJ mol-1, indicating that the interactions between CMC and CTAB are favorable. For comparison, the surfactant binding between cationic surfactants and CMC determined by means of a potentiometric technique13 led to a decrease

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Table 2. Characteristics of PMMA/CMC and PS/CMC Hybrid Particles Obtained by Two Different Formulationsa formulation 1 D (nm) index of polydispersity ζ (mV) Np × 1012 mL-1 solid content (mg mL-1) conversion (%) pH Λ (µS cm-1)

formulation 2

PMMA/CMC

PS/CMC

PMMA/CMC

PS/CMC

340 ( 10 0.076 ( 0.028 -(52 ( 2) 1.55 ( 0.06 32 ( 3 48 ( 4 4.8 ( 0.1 106 ( 7

227 ( 10 0.085 ( 0.022 -(39 ( 1) 3.9 ( 0.1 24 ( 3 37 ( 3 4.7 ( 0.1 110 ( 5

388 ( 23 0.097 ( 0.024 -(53 ( 2) 6.86 ( 0.01 210 ( 2 93 ( 2 3.7 ( 0.1 169 ( 5

612 ( 25 0.060 ( 0.028 -(58 ( 2) 1.73 ( 0.1 207 ( 3 94 ( 1 3.5 ( 0.1 195 ( 5

a D and ζ represent the mean particle diameter from QELS and zeta potential values, respectively. The indices of polydispersity were calculated from QELS software. Np is the mean particle number density considering the D values, solid content in a 1.00-mL dispersion, and polymer density as 1.00 g cm-3. The solid content in 1.00 mL and the conversion of monomer into polymer were determined by means of gravimetric methods. The pH and conductivity (Λ) values of the final dispersions are indicated. The mean values and respective standard deviations are results from three syntheses of each system.

in ∆G of 6.54 kJ mol-1, while in complexes formed by uncharged poly(ethylene oxide) and cationic surfactants, the reduction corresponded to 1.88 kJ mol-1.22 The effect of chain flexibility on the micelle-polyelectrolyte complex was studied by Monte Carlo simulations.13 Polyelectrolytes are rather stiff as a result of electrostatic repulsion among their charged segments, but an increase in the ionic strength makes them more flexible because of a screening effect. The persistence length of 5 nm was found for bare CMC (DS ) 0.88).23,24 A semiflexible conformation is expected for the CMC chains because in the polymerization medium the ionic strength is high because of the presence of cationic surfactant and initiator. Therefore, the polymerization medium might be depicted by the picture of CMC chains wrapped around the micelles with the carboxylate groups oriented to the positive heads of CTAB. For the polymerization, a CTAB concentration close to c3 was chosen because in this condition both CTAB/CMC complexes and free micelles might work as polymerization sites. Figure 2 shows the chemical structures of CTAB and CMC and a schematic representation of the coexistence of complexes formed by CTAB and CMC and free CTAB micelles. Hybrid Particles of PMMA and CMC. The PMMA/ CMC hybrid particles obtained by formulation 1 presented a mean diameter of 340 ( 20 nm and an index of polydispersity of 0.076 ( 0.028, as determined by QELS measurements (Table 2). The mean ζ potential value amounted to -(52 ( 2) mV, evidencing that the negatively charged hybrid surface is due to the presence of CMC. For comparison, CMC chains adsorbed onto polystyrene amidine particles led to mean ζ values of -(55 ( 5) mV.7 The mean particle number density Np of 1.55 × 1012 particles mL-1 was calculated considering the particle mean diameter and solid content. The resulting dispersions presented pH 4.8 and a conductivity close to 100 µS cm-1, which corresponds to the half of that measured before the polymerization. This conductivity decrease might have been caused by the dialysis process, which eliminated the low molecular weight electrolytes. The surfaces of PMMA/CMC particles are not as flat as those generally observed for particles polymerized by the conventional emulsion polymerization, as evidenced by the SEM image in Figure 3. The dried particles presented a mean diameter varying from 100 to 380 nm, which is comparable to that measured by QELS. For practical purposes, the solid content of 32 mg mL-1 obtained by formulation 1 (Table 2) might be unsatisfactory. For this reason, synthesis of PMMA/

Figure 2. Chemical structures of CTAB and CMC and a schematic representation of the coexistence of complexes formed by CTAB and CMC and free CTAB micelles in the bulk solution.

CMC particles following formulation 2 was also performed. In a comparison of both formulations, using the formulation 2 the conversion of monomer into polymer was practically complete, and the mean particle size and mean ζ-potential values practically did not change. The increase in the amount of initiator led to dispersions with higher ionic conductivity and lower pH. Typical

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Figure 3. SEM of PMMA/CMC at 1.0 × 1011 particles mL-1 after drying in air. Figure 5. SEM of PS/CMC at 1.0 × 1011 particles mL-1 after drying in air.

Figure 4. Typical particle size distributions obtained for PMMA/ CMC particles synthesized by formulation 1 (A) and formulation 2 (B) and PS/CMC particles synthesized by formulation 1 (C) and formulation 2 (D).

particle size distributions for PMMA/CMC particles obtained by means of formulations 1 and 2 are shown in parts A and B of Figure 4, respectively. A very small fraction of particles with a mean diameter of ca. 600 nm was observed in dispersions obtained by formulation 1 (Figure 4A), while formulation 2 (Figure 4B) led to particles with unimodal size distribution. Close to c3, a considerable amount of complexes and some free micelles coexist. The presence of these two kinds of polymerization sites might explain the bimodal distribution observed in some experiments. Upon addition of tetrahydrofuran (THF), a good solvent for PMMA, to dried hybrid particles, the formation of small gels and some solid particles was observed. The particles were attributed to pure CMC. The system was filtered against a poly(tetrafluoroethylene) (PTFE) membrane (Roth, Karlsruhe, Germany; 0.45-µm pore diameter). Methanol was added to the filtrate in order to check the presence of PMMA chains, which should precipitate if they were dissolved in THF. However, the system was limpid, indicating that even a good solvent was unable to extract the PMMA chains from the hybrid particles, which formed gels in the THF. On the basis of the presented findings, a model for the hybrid formation is proposed. Before the addition of monomer and initiator, the system is composed of complexes of CMC and CTAB. The complexation is driven by electrostatic interactions between the cationic heads of CTAB and the carboxylate groups of CMC along the chain. Such interactions should decrease the

electrostatic repulsion among the CMC charged monomers, and the polyelectrolyte chains might assume a more flexible conformation with loop formation beyond the nucleated complex. The addition of monomer leads to the formation of oligoradicals in the aqueous phase. These might polymerize in the complex nucleus and in the loops. The polymerization inside the loops might explain the particles’ rough surface. Because the hybrid particles formed gels in THF, this model assumes that the PMMA chain grows very close to the CMC layer, resulting in an entangled system at least very close to the surface. Hydrogen bondings between MMA carbonyl groups and CMC hydroxyl groups probably drive the favorable interaction between them. Besides the interaction between the monomer and polyelectrolyte, the interactions between the surfactant and polymer should also be considered. For instance, the area per molecule of sodium dodecyl sulfate increases with the polymer polarity.25 In microemulsion polymerization of MMA in octadecyltrimethylammonium chloride, the self-surfacting effect of MMA at the interface of the particles is supposed to reduce the bending rigidity of the interface, which might promote particle aggregation.26 In the present system, fluffy layers formed by highly hydrated CMC chains, which cover the particles, probably avoid aggregation. Hybrid Particles of PS and CMC. To study the effect of monomer hydrophilicity on the characteristics of the resulting particles, similar polymerizations were performed with a more hydrophobic monomer than MMA, namely, S. The polymerization of S took place in the presence of 1.0 g L-1 CMC and CTAB at a concentration of 0.25 mmol L-1, following formulations 1 and 2, as described in Table 1. PS/CMC particles obtained by means of formulation 1 presented mean values of D, ζ potential, and conversion rates lower than those obtained for PMMA/CMC particles (Table 2). A possible explanation for this might be the unfavorable interactions between hydrophobic styrene oligoradicals and CMC, which make them stay longer in the aqueous continuum, where they can be terminated, reducing the conversion of monomer into polymer. The SEM image in Figure 5 evidences that PS/ CMC hybrid particles present a diameter varying from 210 to 160 nm and a flat surface. The dimensions

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observed by SEM are in agreement with those observed by QELS. The mean ζ potential values indicate that, similarly to the PMMA/CMC particles, the CMC chains are on the particle surface, with the carboxylate groups oriented toward the aqueous medium. Upon an increase in the contents of monomer and initiator (formulation 2), the PS/CMC particles became larger, the mean ζ potential values became more negative, and the conversion rate reached 94%. These features indicate that the CMC chains are on the particle surfaces and they might represent a barrier only under mild polymerization conditions. Upon the addition of THF (1 mL), a good solvent for PS, to dried PS/CMC (10 mg) hybrid particles, the solid particles appeared dispersed in a limpid liquid. The system was filtered against a PTFE membrane (Roth, Karlsruhe, Germany; 0.45-µm pore diameter). Upon addition of methanol, a poor solvent for PS, to the filtrate, it turned turbid, indicating the presence of PS. A new aliquot of filtrate was prepared and analyzed by gel permeation chromatography using a column calibrated with PS standard samples and two detectors (UV spectrophotometer and refractometer). A bimodal size distribution was obtained. The first population presented Mw ) 124 686 g mol-1 and Mn ) 77 928 g mol-1, while the second presented Mw ) 11 858 g mol-1 and Mn ) 3236 g mol-1. These findings suggest that, contrary to the PMMA/CMC, PS can be easily separated from CMC in the presence of a good solvent because the interactions between CMC and PS chains on the particle surface are only marginal. VAc and CMC. The synthesis of a more hydrophilic monomer than MMA, namely, VAc, in the presence of 1.0 g L-1 CMC and CTAB at a concentration of 0.25 mmol L-1 did not take place. However, under the same conditions but in the absence of CMC, the polymerization of VAc is very quick. This means that the oligoradicals are terminated in the aqueous phase or the level of free monomers in the aqueous phase decreases enormously when CMC is present. Both situations could be possible. If the interactions between VAc and CMC are very favorable, two consequences are expected: (i) instead of diffusing to the micelle, the oligoradicals adsorb on the CMC layer in the aqueous phase, where their termination is very fast; (ii) the solubility of monomer in the aqueous phase is diminished. Another important aspect concerns the monomer reactivity. The radicals formed in S and MMA monomers by H abstraction are well stabilized by a resonance effect, and then radical formation in both cases will be favored. In contrast, VAc is not stabilized by a resonance effect. The VAc radicals strive to change to radicals with higher resonance stabilization either by reaction with a different monomer or by chain transfer with the same monomer. Colloidal Stability. The use of polymeric particles in industrial processes requires stable colloidal dispersions under the desired conditions. For instance, latex dispersions are suitable for use as binders in paint systems if they are stable at ionic strengths above 2.5 mol L-1 NaCl.27 The stability of stock dispersions of PMMA/CMC and PS/CMC hybrid particles obtained by means of formulation 1 in the presence of 0.3 and 2.0 mol L-1 NaCl was in both cases observed during periods of 1 month and 1 week, respectively. Figure 6A shows the colloidal stability of a stock PMMA/CMC dispersion obtained

Figure 6. Images of stock dispersions of PMMA/CMC at 1.55 × 1012 particles mL-1 in 0.3 mol L-1 NaCl (A), PMMA synthesized by formulation 1 in the absence of CMC (B), and dispersions of commercial PSS at 2 × 1012 particles mL-1 in 0.3 mol L-1 NaCl (C).

with formulation 1 in 0.3 mol L-1 NaCl. To give proof that the CMC chains play a crucial role in the colloidal stability of hybrid particles, a control polymerization of pure PMMA was done following the formulation but in the absence of the polyelectrolyte CMC. The resulting material was a coagulum, as shown in Figure 6B. Electrostatic attraction between sulfate terminal groups and neighbor cationic heads probably caused the aggregation process.5 For this reason, another colloidal stability experiment was performed using the commercial PSS latex particles at 2.0 × 1012 particles mL-1 in the presence of 0.3 mol L-1 NaCl. The PSS particles precipitated immediately, as shown in Figure 6C. Therefore, the observed colloidal stability for the hybrid particles was attributed to the hydration of CMC chains, which promoted electrosteric effects and a hydration cushion around the particles. Synthesizing particles in the presence of CMC chains also favored a stronger interaction between the particle and electrosteric stabilizer than that commonly achieved by incorporating the stabilizer after the polymerization. The stability of PMMA/CMC and PS/CMC particles obtained by means of formulation 2 in the presence of 2.0 mol L-1 NaCl was observed for 4 days, in both cases. In comparison to the particles obtained by formulation 1, these particles showed lower stability, which can be explained a the basis of a significant increase of mean diameter D, as observed for PS/CMC particles, or of concentration Np, as observed for PMMA/CMC dispersions. Upon an increase of the conversion rates, both Np and D increased, causing a decrease in the mean interparticle distances. The interaction potential V between two charged particles varies as a function of the mean separation distance.28,29 When the mean separation distance values decreased, the secondary minimum was favored, leading to reversible aggregation processes. This novel synthesis method presents the advantage of yielding stable particles with functional surfaces in only one step using much lower surfactant concentrations. The surface enrichment of synthetic particles with polysaccharides opens the applicability field for such materials. For instance, they can be excellent protein,30 drugs,31 or bilayer membrane carriers or the base for paints with biocidal action.6 Conclusions Stable hybrid particles of PMMA/CMC and PS/CMC were polymerized in an aqueous medium containing very small amounts of CTAB and CMC chains by means of two different formulations. Low monomer and initia-

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tor concentrations led to smaller and more stable particles but to low conversion rates, while high monomer and initiator concentrations yielded very high conversion rate and dispersions either with large particles or with high particle density. Regardless of the formulation, the colloidal stability of stock dispersions of PMMA/CMC and PS/CMC in the presence of 2.0 mol L-1 NaCl was observed for a period of at least 4 days. The stability is probably due to the presence of CMC hydrated layers surrounding the particles, which are strongly bound to the particle surfaces. These are very interesting findings because they show that colloidal stability requirements can be fulfilled with environmentally friendly formulations. Acknowledgment The authors acknowledge FAPESP and CNPq for financial support. Literature Cited (1) Ashmore, M.; Hearn, J. Langmuir 2000, 16, 4906. (2) Ashmore, M.; Hearn, J.; Karpowicz, F. Langmuir 2001, 17, 1069. (3) Smith, N. J.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1. 1995, 91, 1483. (4) Yager, T. D.; McMurray, C. T.; van Holde, K. E. Biochemistry 1989, 28, 2271. (5) Marie, E.; Landfester, K.; Antonietti, M. Biomacromolecules 2002, 3, 475. (6) Vieira, D. B.; Lincopan, N.; Mamizuka, E. M.; Petri, D. F. S.; Carmona-Ribeiro, A. M. Langmuir 2003, 19, 924. (7) Reis, E. A. O.; Caraschi, J. C.; Carmona-Ribeiro, A. M.; Petri, D. F. S. J. Phys. Chem. B 2003, 107, 7993. (8) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (9) Thalberg, K.; Lindman, B. J. Phys. Chem. 1989, 93, 1478. (10) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893. (11) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (12) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642.

(13) Wallin, T.; Linse, P. Langmuir 1996, 12, 305. (14) Esquena, J.; Domingues, F. J.; Solans, C.; Levecke, B.; Booten, K.; Tadros, Th. F. Langmuir 2003, 19, 10463. (15) Gilbert, R. Emulsion Polymerization: A Mechanistic Approach; Academic Press: London, 1995. (16) Grabowski, E. F.; Morrison, I. D. Particle Size Distribution from Analysis of Quasi-Elastic Light Scattering Data. In Measurements of Suspended Particles by Quasi-Elastic Light Scattering; Dahneke, B., Ed.; Wiley-Interscience: New York, 1983; Chapter 7. (17) Jain, N. J.; Albouy, P. A.; Langevin, D. Langmuir 2003, 19, 5680. (18) Monteux, C.; Williams, C. E.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 57. (19) Guillot, S.; Delsanti, M.; De´sert, S.; Langevin, D. Langmuir 2003, 19, 237. (20) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (21) Francois, J.; Dayantis, J.; Sabaddin, J. Eur. Polym. J. 1985, 21, 165. (22) Mya, K. Y.; Sirivat, A.; Jamieson, A. M. J. Phys. Chem. B 2003, 107, 5460. (23) Kamide, K.; Saito, M.; Suzuki, H. Makromol. Chem., Rapid Commun. 1983, 4, 33. (24) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Smit, J. A. M.; van Dijk, J. A. P. P.; van der Horst, P. M.; Batelaan, J. G. Macromolecules 1998, 31, 6297. (25) Vijayendran, B. R. J. Appl. Polym. Sci. 1979, 23, 733. (26) Balladur, V.; Theretz, A.; Mandrand, B. J. Colloid Interface Sci. 1997, 194, 408. (27) Butler, L. N.; Fellows, C. M.; Gilbert, R. G. Ind. Eng. Chem. Res. 2003, 42, 456. (28) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1997. (29) Carmona-Ribeiro, A. M. J. Phys. Chem. 1989, 93, 2630. (30) Sierakowski, M. R.; Freitas, R. A.; Fujimoto, J.; Petri, D. F. S. Carbohydr. Polym. 2002, 49, 167. (31) Carmona-Ribeiro, A. M. Curr. Med. Chem. 2003, 10, 1241.

Received for review June 1, 2004 Revised manuscript received September 3, 2004 Accepted September 11, 2004 IE0495271