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Mechanisms involved in the stabilization of latex particles by adsorbed block copolymers in emulsion polymerization process
Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
Mechanisms involved in the stabilization of latex particles by adsorbed block copolymers in emulsion polymerization process Ludovic Beal a,∗ , Yves Chevalier a,b a
Laboratoire des Mat´eriaux Organiques a` Propri´et´es Sp´ecifiques, UMR 5041 CNRS, University of Savoie, BP24, 69390 Vernaison, France b Laboratoire d’Automatique et de G´ enie des Proc´ed´es, UMR 5007 CNRS, University of Lyon, ESCPE, 43 bd du 11 Novembre 1918, 69622 Villeurbanne, France Available online 5 July 2005
Abstract The efficiency of amphiphilic block copolymers composed of butyl methacrylate and methacrylic acid (PBMA-b-MAA) of various compositions was evaluated in terms of colloidal stability. These copolymers were adsorbed on a PBMA latex in quantities inferred from adsorption isotherm and their colloidal stability was investigated through the determination of the critical coagulation concentration CCC, by addition of either electrolyte or acid. The latexes covered with block copolymers were very stable against addition of either electrolyte or acid compared to the bare latex. The colloidal stability as evaluated by the CCC, increased with the relative coverage. The results were discussed in relation with the behavior obtained in emulsion polymerization. The overall trends were sorted correctly since a better coverage yielded a better colloidal stability and a smaller size at the end of emulsion polymerization. But no direct correlation could be found out between adsorption, colloidal stability and emulsion polymerization. Influence of stirring during the flocculation test was also evaluated. © 2005 Elsevier B.V. All rights reserved. Keywords: Amphiphilic block copolymer; Colloidal stability; Adsorption; Kinetic of coagulation; Emulsion polymerization
1. Introduction Colloidal dispersions [1] have to be stabilized against coagulation by means of a suitable surface chemistry which often involves adsorption of surface active materials, surfactants or polymers. A repulsive potential energy barrier is required in order to counterbalance the Van der Waals attractions between colloidal particles. Electrostatic repulsion by chemically attached or adsorbed charged materials is one way that has been formally described in the classical DLVO theory [2]. The second classical mean is the steric stabilization by polymeric surfactants [3]. Both mechanisms can be combined using polyelectrolytes as electrosteric stabilizers. In the case of emulsion polymerization, the role of the emulsifier is to ensure the stability of the final colloidal dispersion, but also to control the particle size during the polymerization process. There is well-established knowledge on ∗
Corresponding author. Tel.: +33 4 78 02 22 67; fax: +33 4 78 02 77 38. E-mail address: [email protected] (L. Beal).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.05.033
interactions between particles that govern the long-term stability of the polymer emulsion since the classical text books on electrostatic [2] and steric [4] stabilization. Regarding the action of emulsifiers in the course of the emulsion polymerization process, the mechanisms are much difficult to assess because the system is continuously transforming. Thus, transient out-of-equilibrium states may have a great relevance, in particular with respect to the particle nucleation phenomena; the kinetics of emulsifier adsorption have also to be considered, especially in the case of polymer adsorption. In a previous study [5], block copolymers composed of butyl methacrylate and methacrylic acid (PBMA-bMAA) have been used in emulsion polymerization of butyl methacrylate. The use of copolymers in emulsion polymerization has already been reported by many authors [3,6–11]. The effect of varying blocks lengths and monomer types was investigated through latex characteristics such as particles diameter, number of particles, amount of coagulum, average molar mass and its distribution. . . Block copolymers having a hydrophilic polyelectrolyte block have been observed as the
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
best stabilizers because their structure enables their adsorption or anchoring on the growing particles and to provide electrosteric stabilization thanks to the charged hydrophilic segments extended in water phase. As a result, the smallest particle diameters that could be reached at the end of emulsion polymerization were 60 nm against 380 nm for the emulsifierfree polymerization carried out according to the same recipe and temperature [5]. As evidence, even very small weight fractions of stabilizer with respect to the monomer (less than 1 wt.%) allow the preparation of particles with significantly smaller diameters, thus, a large number of particles. The stabilizing efficiency can be evaluated from the final number density of stabilized particles NP , which has been investigated as a function of copolymer and block length. A classical way to do so consist in looking at the block copolymer concentration dependence of NP , following the classical work of Smith and Ewart [12] who have first given a theoretical foundation of the linear relationships of log (NP ) versus log[surfactant] observed in many instances such as in the reference case of the emulsion polymerization of styrene in the presence of SDS as a model system. Our results with block copolymers did show such a power law relationship. They were close to those obtained by Burgui`ere et al. [13] in an earlier investigation on the emulsion polymerization of styrene in presence of block copolymers composed of styrene and acrylic acid. The α exponent ranged from 0.44 to 0.73, depending upon the copolymer composition. It increased as the hydrophilic content was increased and did not significantly depend on the length of the copolymer. The α values could be correlated by a competition between homogeneous and micellar nucleation but the mechanism proposed by Smith and Ewart did not hold because the copolymer exchange between micelles and growing particles was assumed to be very slow. Our previous work [5] has shown that small amounts of block copolymer were quite efficient for the stabilization of latex particles during the polymerization process, as compared to classical surfactants such as SDS. Thus, a special attention has been paid to the adsorption mechanism of the block copolymers from systematic measurements of equilibrium adsorption isotherms as measured with latex particles made of the same PBMA polymer prepared by means of emulsifier-free emulsion polymerization. The results were expressed by the diameter average of particles as a function of relative coverage (Fig. 1) and were compared to data pertaining to SDS taken from the literature [14,15]. As the relative coverage increased, particles average diameter first decreased very fast and thereafter hardly reached 60 nm at the about 40% relative coverage. Interestingly, the copolymers were able to stabilize particles at very low relative coverage (<10%) whereas coverages larger than 25% were required to obtain low diameter particles (∼100 nm) in the case of SDS. It might be possible to synthesize smaller particles by increasing the copolymer concentration of the polymerization recipe but only a small decrease of diameter could be expected according to the observed trends. Moreover, there was a significant residual concentration of copolymer in the
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Fig. 1. PBMA particle diameter determined by light scattering as a function of relative coverage for different block copolymers used as emulsifiers: () PBMA8 -b-MANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -b-MANa64 , () PBMA16 -b-MANa64 and ( ) SDS. The lines are guides for the eyes only.
aqueous phase at the end of the polymerization, which meant that copolymer was still available for reducing the particle size. Obviously, the particle size was no longer controlled by the copolymer concentration but by another concentrationindependent parameter in this concentration range. It was proposed that the final number of particles lower than the number of starting micelles was caused by limited coagulation during the nucleation period and/or easy monomer transfer from non-nucleated micelles to the growing particles by means of an Ostwald ripening effect. The present work is a continuation of the previous one [5] and aims at studying the effect of adsorption of block copolymers on the colloidal stability. Colloidal stability experiments were performed for two distinct purposes. The first goal is to infer the role of block copolymers in the emulsion polymerization mechanism. In particular, the high efficiency of block copolymers adsorbed at low coverages is an intriguing point. A second goal is to investigate structure–activity relationships of these copolymers by studying block copolymers of different block lengths. This investigation was carried out by means of classical coagulation measurements followed by turbidimetry. The series of block copolymers used previously in emulsion polymerization were adsorbed on a latex synthesized without surfactant and the critical coagulation concentration (CCC) of the covered and bare latexes were determined accordingly to coagulation rate measurements.
2. Experimental section The synthesis and the characterization of the block copolymers, the polymerization in emulsion and the determination of adsorption isotherm were described in details in our previous paper [5]. PBMA-b-MAA block copolymers were prepared in three steps. First, the PBMA-b-tBMA block copolymers were prepared by anionic polymerization of BMA and tBMA in THF initiated by Ph2 CHNa in the presence of crown ether DB18C6 [16] at −20 ◦ C by sequential addition of the monomers.
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
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Table 1 Composition of PBMAx -b-MAAy as analyzed by 1 H NMR. PBMAx -b-MAAy
x
y
Composition (mol% MAA)
8/16 8/32 8/64 16/64 32/64
8.9 9.2 8.3 20 33.3
16 33.8 62.2 70.7 60.5
64.5 78.6 88.2 77.9 64.5
Secondly, the modification of t-BMA block was achieved thermally at 200 ◦ C during 2 h leading to the formation of anhydride and acid groups [17]. Finally, hydrolysis of anhydride groups in THF–water mixture at reflux for 24 h afforded the PBMA-b-MAA block copolymers. The copolymers were characterized by 1 H NMR in CD3 OD and each block lengths were calculated from these data (Table 1). Batch emulsion polymerizations in presence or absence of block copolymers were conducted according to a conventional procedure involving degassed distilled water and a temperature of 70 ◦ C. Latexes were characterized by their solid content (9 wt.%) and the average diameter of particles determined by dynamic light scattering (DLS). Average diameter of PBMA latex synthesized by surfactant-free polymerization was 380 nm. Adsorption isotherms were determined by the solution depletion method. The adsorbed amount was determined by titration of the supernatant after sedimentation of the particles by ultracentrifugation. The titration of the supernatant containing the free block copolymers and other water-soluble species was carried out by pH-metry. The adsorbed amount Γ (mol m−2 ) was calculated from the concentrations of copolymer before and after adsorption and was plotted for each copolymer versus the residual concentration of copolymer. The colloidal stability of covered or bare latexes were investigated by the coagulation method. The kinetics of coagulation were followed by turbidimetry with a Perkin Elmer UV/visible spectrometer. A typical experiment was run as follows. An amount of copolymer solution was added to the suspension of latex particles at 0.02 wt.% concentration. The amount of copolymer was calculated such as to reach the desired coverage according to the equilibrium adsorption isotherm. After a 3 days delay allowing to reaching equilibrium adsorption, the stability of the dispersion was evaluated. Typically, 1 mL of a coagulating agent solution was added to 1 mL of dispersion in the cell and monitored during 3 min at λ = 800 nm. Three measurements were realized at each concentration of the coagulating agent in order to get an average value. Typical variations of absorbance as a function of time are plotted in Fig. 2. The homogenization of the solution was ensured either by magnetic stirring inside the measuring cell or by gentle hand-shaking the cell before introducing it inside the spectrometer. Gentle hand-shaking consisted in turning the measurement cell upside down twice; this was the general procedure employed because it ensured a faster initial stirring than that of a magnetic stir bar. Vigorous shaking introduced bubbles inside the sample, which precluded any
Fig. 2. Kinetics of coagulation: absorbance as a function of time for different concentrations of electrolyte.
turbidity measurement before the bubbles have been moved upwards. The mixing with the magnetic stir bar was less efficient at the beginning but it was kept running during the whole coagulation test. The stability ratio W was calculated according to the following relation [1]: W=
(dA/dt)t=0,C=CCC (dA/dt)t=0,C
where dA/dt is the slope of the linear part of the absorbance as a function of time and C the concentration of the coagulating agent.
3. Results and discussion 3.1. Adsorption isotherms on latex surface The adsorption isotherms, that is, the adsorbed amount (mol m−2 ) as a function of the residual concentration (mol L−1 ) have been built for each copolymer at concentrations above the cmc (Fig. 3). The adsorption isotherms were similar in shape: a very strong adsorption took place at low concentrations of copolymer, which was followed by a plateau at higher concentrations. Because of the very
Fig. 3. Adsorption isotherms, adsorbed amount versus residual concentration: () PBMA8 -b-MANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -bMANa64 , () PBMA16 -b-MANa64 and (♦) PBMA32 -b-MANa64 .
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
low residual concentrations after adsorption from dilute solutions, it was not possible to measure with accuracy the steep rise of the adsorption at low concentrations. The residual concentration was zero within experimental accuracy, which meant irreversible adsorption for this part of the adsorption isotherm. The plateau which was observed at higher concentrations was not perfectly flat as it could be expected for a surface where surface sites were fully occupied by adsorbed molecules. This behavior was characteristic of a hindered adsorption phenomenon where the macromolecules already adsorbed prevented further adsorption. The present data were strongly reminiscent of the two regimes described by Alexander and de Gennes for the adsorption of end functional polymers and block copolymers [18,19]. The “mushroom” (dilute) regime with a strong affinity for the surface was observed at low concentrations where the surface was either bare or not too much crowded, leading to a steep increase of the adsorption. The “brush” (semi-dilute) regime at higher concentrations was characterized by a low affinity for the surface because lateral interactions between adsorbed macromolecules hindered the adsorption of additional macromolecules. The macromolecules got stretched in a direction perpendicular to the surface. The low affinity manifested by a low slope of the adsorption isotherm characterized the balance of the free energy of adsorption of the PBMA blocks on the PBMA latex and the entropic term of the conformational free energy of the PMANa blocks that stretched radially. Such a behavior has already been observed with block copolymers for various surfaces and solvents [20–23]. Depending on the copolymer structure, the adsorption strength and adsorbed amount at the plateau were quite different. For PBMA8 -bMANa16 and PBMA8 -b-MANa32 , the strength of the adsorption was very strong as indicated by high value of the adsorbed amount at vanishing residual concentrations. A slight but significant positive slope was observed at higher concentrations. Nevertheless, for PBMA32 -b-MANa64 which showed a lower adsorption, the accuracy of measurements was poorer, so that the slope in the brush regime was difficult to assess. The present results were at variance with adsorption isotherms described as Langmuir-type by some authors [24,25] where the adsorbed amount rapidly increased with polymer concentration up to a plateau region. These phenomena has already been described and explained by the stretching of macromolecules [26]. As the PMANa block length increased, the adsorbed amount on the “plateau” decreased. When the PBMA block length was increased, the adsorbed amount on the plateau remained constant. This meant that adsorption was controlled by steric effect at the level of the hydrophilic blocks: on the plateau region, the coverage by copolymers was limited by the PMANa chain radius. 3.2. Colloidal stability of dispersion 3.2.1. Stability against electrolytes The colloidal stability of bare latex have been investigated in diluted solution and the CCC has been determined by mea-
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Fig. 4. Stability ratio W as a function of the electrolyte concentration for the bare latex.
surements of the kinetics of coagulation by adding solution of sodium chloride and lithium sulfate of different concentrations. These determinations have been made at different concentrations of the latex ranging from 0.02 to 0.05 wt.%. Typical kinetic curves having a linear behavior at short times were always observed. The CCC did not depend on the latex concentration. It was 210 ± 10 mmol L−1 for lithium sulfate and 380 mmol L−1 for sodium chloride. Classical log–log representation of the stability ratio W versus the electrolyte concentration is given in Fig. 4: the two regions of slow and fast coagulation below and above the CCC were observed. This observation was in agreement with the classical theory of DLVO where the colloidal stability was mainly electrostatic in origin [2]. As the concentration of added salt increases, the electrostatic repulsion decreases and the rate of coagulation increases. When this repulsion vanishes, the rate of coagulation remains constant despite the increase of concentration of added salt. The colloidal stability of the latexes covered with block copolymers could not be evaluated by this way because no coagulation could be observed by adding either sodium chloride or even lithium sulfate, which allowed reaching higher salt concentrations. Even for concentrated latex suspensions (8 wt.% of latex particles), the latex particles did not aggregate in presence of high salt concentrations; only creaming (due to high density of the aqueous phase) could be observed after a long time was elapsed. This result shows how highly efficient stabilizers these block copolymers are. But no comparative evaluation of the stabilization by different copolymers or different coverages could be obtained. More powerful coagulating agents are required for that purpose. 3.2.2. Stability of dispersion against acid Since the stabilization was thought to be electrosteric, addition of a strong acid converts the anionic carboxylate groups into neutral carboxylic acid group, so that the stabilization turn from electrosteric to steric for low concentrations of added acid. The neutralization of the latex surface cannot reach completion, however, a residual electrical charge remains because polymethacrylic acid is known to bear a fraction of carboxylic acid groups that behave as strong acid.
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L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32 Table 2 Determination of colloidal stability of fully covered latexes, preparation of solutions according to adsorption isotherms
Fig. 5. Stability ratio W as a function of the acid concentration for the bare latex.
Whatsoever, a strong acid, such as HCl, is a stronger coagulating agent than a simple electrolyte. First, the colloidal stability of the bare latex has been evaluated against the concentration of hydrochloric acid (Fig. 5). The shape of the curve has the same features as for classical salt solutions and this observed trend would be accepted in accordance to the DLVO theory. Nevertheless, the acid concentration, necessary to observed fast coagulation, was 8.1 × 10−4 mol L−1 , that is, more than 10 times higher than the concentration of basic species. This observation suggests that strong acid species were present at the surface of the particles or that some carboxylic acid groups behave as strong acids. The excess of added acid should behave as an electrolyte that screens the electrostatic effects of this residual surface charge. In consideration, the CCC should be calculated as the difference between the added acid concentration and bases concentration and was 7.5 × 10−4 mol L−1 for the bare latex. The colloidal stability of fully covered latexes have been investigated by this way since a CCC could be reached. Classical curves of log W = f(log[HCl]) were obtained as shown in Fig. 6. The chemical composition of the studied samples and the residual concentration in the aqueous phase according to the adsorption isotherm are reported in Table 2. The CCC were again many times higher than the concen-
Fig. 6. Determination of the CCC for fully covered latexes: () PBMA8 -bMANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -b-MANa64 , () PBMA16 b-MANa64 and (♦) PBMA32 -b-MANa64 . Only one horizontal line is drawn to simplify the representation.
PBMAx -b-MAAy
Adsorbed amount (mol m−2 )
Residual concentration (mol L−1 )
8/16 8/32 8/64 16/64 32/64
0.1 0.045 0.015 0.015 0.01
250 150 100 100 100
trations of basic species (carboxylates to be acidified). The concentration of basic species was subtracted from the acid concentration in all cases. As expected, the adsorption of copolymer enabled a large enhancement of stability since the CCC were many times higher when particles were covered with copolymers. Moreover, the rates in the fast coagulation regime were slower than for the bare latex (excepted for the PBMA8 -b-MANa16 copolymer). According to the CCC values, the different copolymers gave similar stabilities within experimental accuracy. This coagulation test did not allow to discriminate between the various copolymers that gave significantly different results in emulsion polymerization. The average particles diameter of latexes synthesized at high block copolymer concentration (10−4 mol L−1 ) were comprised between 70 and 100 nm, a difference that dynamic light scattering could discriminate. The influence of the adsorbed amount on the colloidal stability of the latex was investigated in the same way in connection with the emulsion polymerization results. This has been investigated on the PBMA8 -b-MANa16 copolymer which presented a favorable adsorption isotherm where the adsorbed amount could be easily varied below 100% coverage. As expected, the CCC increases as the relative coverage increases (Fig. 7). At the low relative coverage of 10%, the increase of stability was significantly improved since the CCC was twice higher than for the bare latex. But the variation of the CCC looked quite smooth, almost linear as a function of the relative coverage. 10% coverage did not bring about a huge stability improvement of comparable magnitude with a full coverage, as was inferred from the emulsion poly-
Fig. 7. Influence of relative coverage on colloidal stability for latexes covered with PBMA8 -b-MANa16 .
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
Fig. 8. Influence of stirring mode on CCC for bare and fully covered latexes. Degrees of polymerization of block copolymers PBMAx -b-MAAy are referred to as x/y. Full bars: initial hand-shaking; dashed bars: continuous magnetic stirring.
merization results. Although the samples and measurement method were quite different, a similar smooth variation of the colloidal stability has been observed by Laible and Hamann by means of sedimentation measurements on silica particles dispersed in organic solvent with the help of grated polymers at different surface densities [27]. For the emulsion polymerization in the presence of increasing amounts of adsorbed copolymer (Fig. 1), the variation of the particle size as a function of relative coverage was far from linear whereas the CCC variation was approximately linear. Even if the trends are correct, there are no direct relationship between the adsorption, the colloidal stability as evaluated from the present method and the particle size obtained by emulsion polymerization. All the measurements described above have been performed by an initial mixing by means of hand-shaking the latex and HCl solution before recording the turbidity as a function of time. This method allowed a better and faster homogeneity of the sample at the beginning of the coagulation test but the sample was no more stirred during the course of the coagulation. Magnetic stirring was used and the results allow to discuss on the influence of stirring during coagulation. Figs. 8 and 9 show the influence of stirring on the CCC and the rates of coagulation above the CCC
Fig. 9. Influence of stirring mode on the kinetics of coagulation of bare and fully covered latexes. Degrees of polymerization of block copolymers PBMAx -b-MAAy are referred to as x/y. Full bars: initial hand-shaking; dashed bars: continuous magnetic stirring.
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([dA/dt]C=CCC ). Generally, the CCC was not influenced by stirring; the CCC values differed only by 2–7% which should be small as compared to experimental accuracy. Conversely, large differences of coagulation rates were observed, which amounted up to 55% for the bare latex. The rates were higher when magnetic stirring was used (excepted for the PBMA8 -b-MANa16 ). Once coagulation has started, the suspension is depleted around the particles that have coagulated and Brownian motion alone might be inefficient in bringing new particles in close contact with aggregates. This effect is more pronounced when aggregation has proceeded because aggregated particles are less mobile. Aggregate–aggregate coagulation is difficult in the absence of stirring. The influence of stirring is to renew the depleted region immediately after a coagulation event. That way, the coagulation rates are faster under stirring, as was observed. The coagulation behavior of the bare latex was more sensitive to stirring; the CCC value was significantly changed and the rates were the most accelerated. This observation was confirmed by visual observation of the samples: the particles of bare latex coagulated rapidly under stirring to form large particles of approximately 1 mm diameter and the aqueous phase became perfectly transparent. The behavior for covered latexes is much less sensitive to the stirring mode. This difference suggests that the copolymer covered latexes show a limited aggregation; a behavior that remains to be explained.
4. Conclusions Our previous study have shown that amphiphilic block copolymers composed of butyl methacrylate and methacrylic acid were very efficient in reducing the average diameter of particles in the emulsion polymerization of butyl methacrylate, even for very low concentration of copolymers (<10−4 mol L−1 ) corresponding to very low relative coverage [5]. This result led us to question the role of the block copolymers in the emulsion polymerization process, in particular regarding the colloidal stability of the formed particles. This question deserved also some attention because experimental data dealing with the colloidal stability improvement brought about by block copolymer adsorption are quite scarce in the literature, excepted those using the surface force apparatus. The colloidal stability was evaluated by determining the critical coagulation concentration, CCC, from turbidity measurements of the kinetics of coagulation of diluted latex against addition of a flocculating agent. Block copolymers provided a very strong stabilization since it was not possible to coagulate the covered latexes with classical electrolytes. Neutralization of the surface charges by addition of an acid was necessary in order to evaluate the colloidal stability. The CCC with HCl as an acidic electrolyte were much higher for latex particles covered with copolymer and the kinetics of coagulation were slower, which was in agreement with our earlier results. However, it was difficult to sort the data with respect to the influence of the composition of the copolymers.
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L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
Decreasing the relative coverage of the latex decreased the stability, as expected, but the variation of the CCC was almost linear with the relative coverage, which differed from the variation of the particles diameter in emulsion polymerization. In conclusion, there are no direct relationship between the particle size obtained by emulsion polymerization and the adsorption or the related improvement of the colloidal stability, as evaluated from the present method. It appears from this investigation that the adsorption of copolymers allows to reach very high stabilities, suggesting that block copolymers are indeed able to prevent coagulation during the emulsion polymerization process. This is in agreement with the observation of small particle size at low copolymer coverage [5]. The emulsion polymerization mechanisms in the presence of block copolymers are far to be elucidated however. Since the adsorption and desorption of block copolymers are very slow, and no coagulation takes place during the process, the number of particles (thus their final size) is determined during the nucleation step. Nucleation necessarily takes place inside the block copolymer micelles for the final size to be smaller than for the surfactant-free polymerization process. But the final number of latex particles is less than the initial number of micelles, meaning that part of the monomer swollen micelles have not been nucleated. We suggest that the nucleation taking place by means of the entry of anionic initiator or anionic macroradicals formed in the aqueous phase is hindered by the large negative charge of the micelles. The nucleated micelles (primary particles) are larger than the empty ones, but contain the same amount of copolymer because they originate from an empty micelle and the block copolymer neither adsorb nor desorb. The electrostatic repulsion is therefore larger for small empty micelles than for nucleated micelles because the surface charge density of the later is less. Empty micelles are kept empty in that way; butyl methacrylate monomer diffusion through the aqueous phase allows for particle growth. Notice that because styrene monomer is less soluble in water, slower diffusion from the empty micelles to the primary particles favors the nucleation of a larger number of micelles; the exponent α of the power law was close to one in this case [10,13]. Finally, the difference with small surfactant molecules like SDS is huge because such emulsifiers adsorb and desorb very fast; micelles are lost during the emulsion polymerization pro-
cess because the surfactant molecules adsorb onto growing particles.
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Mechanisms involved in the stabilization of latex particles by adsorbed block copolymers in emulsion polymerization process Ludovic Beal a,∗ , Yves Chevalier a,b a
Laboratoire des Mat´eriaux Organiques a` Propri´et´es Sp´ecifiques, UMR 5041 CNRS, University of Savoie, BP24, 69390 Vernaison, France b Laboratoire d’Automatique et de G´ enie des Proc´ed´es, UMR 5007 CNRS, University of Lyon, ESCPE, 43 bd du 11 Novembre 1918, 69622 Villeurbanne, France Available online 5 July 2005
Abstract The efficiency of amphiphilic block copolymers composed of butyl methacrylate and methacrylic acid (PBMA-b-MAA) of various compositions was evaluated in terms of colloidal stability. These copolymers were adsorbed on a PBMA latex in quantities inferred from adsorption isotherm and their colloidal stability was investigated through the determination of the critical coagulation concentration CCC, by addition of either electrolyte or acid. The latexes covered with block copolymers were very stable against addition of either electrolyte or acid compared to the bare latex. The colloidal stability as evaluated by the CCC, increased with the relative coverage. The results were discussed in relation with the behavior obtained in emulsion polymerization. The overall trends were sorted correctly since a better coverage yielded a better colloidal stability and a smaller size at the end of emulsion polymerization. But no direct correlation could be found out between adsorption, colloidal stability and emulsion polymerization. Influence of stirring during the flocculation test was also evaluated. © 2005 Elsevier B.V. All rights reserved. Keywords: Amphiphilic block copolymer; Colloidal stability; Adsorption; Kinetic of coagulation; Emulsion polymerization
1. Introduction Colloidal dispersions [1] have to be stabilized against coagulation by means of a suitable surface chemistry which often involves adsorption of surface active materials, surfactants or polymers. A repulsive potential energy barrier is required in order to counterbalance the Van der Waals attractions between colloidal particles. Electrostatic repulsion by chemically attached or adsorbed charged materials is one way that has been formally described in the classical DLVO theory [2]. The second classical mean is the steric stabilization by polymeric surfactants [3]. Both mechanisms can be combined using polyelectrolytes as electrosteric stabilizers. In the case of emulsion polymerization, the role of the emulsifier is to ensure the stability of the final colloidal dispersion, but also to control the particle size during the polymerization process. There is well-established knowledge on ∗
Corresponding author. Tel.: +33 4 78 02 22 67; fax: +33 4 78 02 77 38. E-mail address: [email protected] (L. Beal).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.05.033
interactions between particles that govern the long-term stability of the polymer emulsion since the classical text books on electrostatic [2] and steric [4] stabilization. Regarding the action of emulsifiers in the course of the emulsion polymerization process, the mechanisms are much difficult to assess because the system is continuously transforming. Thus, transient out-of-equilibrium states may have a great relevance, in particular with respect to the particle nucleation phenomena; the kinetics of emulsifier adsorption have also to be considered, especially in the case of polymer adsorption. In a previous study [5], block copolymers composed of butyl methacrylate and methacrylic acid (PBMA-bMAA) have been used in emulsion polymerization of butyl methacrylate. The use of copolymers in emulsion polymerization has already been reported by many authors [3,6–11]. The effect of varying blocks lengths and monomer types was investigated through latex characteristics such as particles diameter, number of particles, amount of coagulum, average molar mass and its distribution. . . Block copolymers having a hydrophilic polyelectrolyte block have been observed as the
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
best stabilizers because their structure enables their adsorption or anchoring on the growing particles and to provide electrosteric stabilization thanks to the charged hydrophilic segments extended in water phase. As a result, the smallest particle diameters that could be reached at the end of emulsion polymerization were 60 nm against 380 nm for the emulsifierfree polymerization carried out according to the same recipe and temperature [5]. As evidence, even very small weight fractions of stabilizer with respect to the monomer (less than 1 wt.%) allow the preparation of particles with significantly smaller diameters, thus, a large number of particles. The stabilizing efficiency can be evaluated from the final number density of stabilized particles NP , which has been investigated as a function of copolymer and block length. A classical way to do so consist in looking at the block copolymer concentration dependence of NP , following the classical work of Smith and Ewart [12] who have first given a theoretical foundation of the linear relationships of log (NP ) versus log[surfactant] observed in many instances such as in the reference case of the emulsion polymerization of styrene in the presence of SDS as a model system. Our results with block copolymers did show such a power law relationship. They were close to those obtained by Burgui`ere et al. [13] in an earlier investigation on the emulsion polymerization of styrene in presence of block copolymers composed of styrene and acrylic acid. The α exponent ranged from 0.44 to 0.73, depending upon the copolymer composition. It increased as the hydrophilic content was increased and did not significantly depend on the length of the copolymer. The α values could be correlated by a competition between homogeneous and micellar nucleation but the mechanism proposed by Smith and Ewart did not hold because the copolymer exchange between micelles and growing particles was assumed to be very slow. Our previous work [5] has shown that small amounts of block copolymer were quite efficient for the stabilization of latex particles during the polymerization process, as compared to classical surfactants such as SDS. Thus, a special attention has been paid to the adsorption mechanism of the block copolymers from systematic measurements of equilibrium adsorption isotherms as measured with latex particles made of the same PBMA polymer prepared by means of emulsifier-free emulsion polymerization. The results were expressed by the diameter average of particles as a function of relative coverage (Fig. 1) and were compared to data pertaining to SDS taken from the literature [14,15]. As the relative coverage increased, particles average diameter first decreased very fast and thereafter hardly reached 60 nm at the about 40% relative coverage. Interestingly, the copolymers were able to stabilize particles at very low relative coverage (<10%) whereas coverages larger than 25% were required to obtain low diameter particles (∼100 nm) in the case of SDS. It might be possible to synthesize smaller particles by increasing the copolymer concentration of the polymerization recipe but only a small decrease of diameter could be expected according to the observed trends. Moreover, there was a significant residual concentration of copolymer in the
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Fig. 1. PBMA particle diameter determined by light scattering as a function of relative coverage for different block copolymers used as emulsifiers: () PBMA8 -b-MANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -b-MANa64 , () PBMA16 -b-MANa64 and ( ) SDS. The lines are guides for the eyes only.
aqueous phase at the end of the polymerization, which meant that copolymer was still available for reducing the particle size. Obviously, the particle size was no longer controlled by the copolymer concentration but by another concentrationindependent parameter in this concentration range. It was proposed that the final number of particles lower than the number of starting micelles was caused by limited coagulation during the nucleation period and/or easy monomer transfer from non-nucleated micelles to the growing particles by means of an Ostwald ripening effect. The present work is a continuation of the previous one [5] and aims at studying the effect of adsorption of block copolymers on the colloidal stability. Colloidal stability experiments were performed for two distinct purposes. The first goal is to infer the role of block copolymers in the emulsion polymerization mechanism. In particular, the high efficiency of block copolymers adsorbed at low coverages is an intriguing point. A second goal is to investigate structure–activity relationships of these copolymers by studying block copolymers of different block lengths. This investigation was carried out by means of classical coagulation measurements followed by turbidimetry. The series of block copolymers used previously in emulsion polymerization were adsorbed on a latex synthesized without surfactant and the critical coagulation concentration (CCC) of the covered and bare latexes were determined accordingly to coagulation rate measurements.
2. Experimental section The synthesis and the characterization of the block copolymers, the polymerization in emulsion and the determination of adsorption isotherm were described in details in our previous paper [5]. PBMA-b-MAA block copolymers were prepared in three steps. First, the PBMA-b-tBMA block copolymers were prepared by anionic polymerization of BMA and tBMA in THF initiated by Ph2 CHNa in the presence of crown ether DB18C6 [16] at −20 ◦ C by sequential addition of the monomers.
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Table 1 Composition of PBMAx -b-MAAy as analyzed by 1 H NMR. PBMAx -b-MAAy
x
y
Composition (mol% MAA)
8/16 8/32 8/64 16/64 32/64
8.9 9.2 8.3 20 33.3
16 33.8 62.2 70.7 60.5
64.5 78.6 88.2 77.9 64.5
Secondly, the modification of t-BMA block was achieved thermally at 200 ◦ C during 2 h leading to the formation of anhydride and acid groups [17]. Finally, hydrolysis of anhydride groups in THF–water mixture at reflux for 24 h afforded the PBMA-b-MAA block copolymers. The copolymers were characterized by 1 H NMR in CD3 OD and each block lengths were calculated from these data (Table 1). Batch emulsion polymerizations in presence or absence of block copolymers were conducted according to a conventional procedure involving degassed distilled water and a temperature of 70 ◦ C. Latexes were characterized by their solid content (9 wt.%) and the average diameter of particles determined by dynamic light scattering (DLS). Average diameter of PBMA latex synthesized by surfactant-free polymerization was 380 nm. Adsorption isotherms were determined by the solution depletion method. The adsorbed amount was determined by titration of the supernatant after sedimentation of the particles by ultracentrifugation. The titration of the supernatant containing the free block copolymers and other water-soluble species was carried out by pH-metry. The adsorbed amount Γ (mol m−2 ) was calculated from the concentrations of copolymer before and after adsorption and was plotted for each copolymer versus the residual concentration of copolymer. The colloidal stability of covered or bare latexes were investigated by the coagulation method. The kinetics of coagulation were followed by turbidimetry with a Perkin Elmer UV/visible spectrometer. A typical experiment was run as follows. An amount of copolymer solution was added to the suspension of latex particles at 0.02 wt.% concentration. The amount of copolymer was calculated such as to reach the desired coverage according to the equilibrium adsorption isotherm. After a 3 days delay allowing to reaching equilibrium adsorption, the stability of the dispersion was evaluated. Typically, 1 mL of a coagulating agent solution was added to 1 mL of dispersion in the cell and monitored during 3 min at λ = 800 nm. Three measurements were realized at each concentration of the coagulating agent in order to get an average value. Typical variations of absorbance as a function of time are plotted in Fig. 2. The homogenization of the solution was ensured either by magnetic stirring inside the measuring cell or by gentle hand-shaking the cell before introducing it inside the spectrometer. Gentle hand-shaking consisted in turning the measurement cell upside down twice; this was the general procedure employed because it ensured a faster initial stirring than that of a magnetic stir bar. Vigorous shaking introduced bubbles inside the sample, which precluded any
Fig. 2. Kinetics of coagulation: absorbance as a function of time for different concentrations of electrolyte.
turbidity measurement before the bubbles have been moved upwards. The mixing with the magnetic stir bar was less efficient at the beginning but it was kept running during the whole coagulation test. The stability ratio W was calculated according to the following relation [1]: W=
(dA/dt)t=0,C=CCC (dA/dt)t=0,C
where dA/dt is the slope of the linear part of the absorbance as a function of time and C the concentration of the coagulating agent.
3. Results and discussion 3.1. Adsorption isotherms on latex surface The adsorption isotherms, that is, the adsorbed amount (mol m−2 ) as a function of the residual concentration (mol L−1 ) have been built for each copolymer at concentrations above the cmc (Fig. 3). The adsorption isotherms were similar in shape: a very strong adsorption took place at low concentrations of copolymer, which was followed by a plateau at higher concentrations. Because of the very
Fig. 3. Adsorption isotherms, adsorbed amount versus residual concentration: () PBMA8 -b-MANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -bMANa64 , () PBMA16 -b-MANa64 and (♦) PBMA32 -b-MANa64 .
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
low residual concentrations after adsorption from dilute solutions, it was not possible to measure with accuracy the steep rise of the adsorption at low concentrations. The residual concentration was zero within experimental accuracy, which meant irreversible adsorption for this part of the adsorption isotherm. The plateau which was observed at higher concentrations was not perfectly flat as it could be expected for a surface where surface sites were fully occupied by adsorbed molecules. This behavior was characteristic of a hindered adsorption phenomenon where the macromolecules already adsorbed prevented further adsorption. The present data were strongly reminiscent of the two regimes described by Alexander and de Gennes for the adsorption of end functional polymers and block copolymers [18,19]. The “mushroom” (dilute) regime with a strong affinity for the surface was observed at low concentrations where the surface was either bare or not too much crowded, leading to a steep increase of the adsorption. The “brush” (semi-dilute) regime at higher concentrations was characterized by a low affinity for the surface because lateral interactions between adsorbed macromolecules hindered the adsorption of additional macromolecules. The macromolecules got stretched in a direction perpendicular to the surface. The low affinity manifested by a low slope of the adsorption isotherm characterized the balance of the free energy of adsorption of the PBMA blocks on the PBMA latex and the entropic term of the conformational free energy of the PMANa blocks that stretched radially. Such a behavior has already been observed with block copolymers for various surfaces and solvents [20–23]. Depending on the copolymer structure, the adsorption strength and adsorbed amount at the plateau were quite different. For PBMA8 -bMANa16 and PBMA8 -b-MANa32 , the strength of the adsorption was very strong as indicated by high value of the adsorbed amount at vanishing residual concentrations. A slight but significant positive slope was observed at higher concentrations. Nevertheless, for PBMA32 -b-MANa64 which showed a lower adsorption, the accuracy of measurements was poorer, so that the slope in the brush regime was difficult to assess. The present results were at variance with adsorption isotherms described as Langmuir-type by some authors [24,25] where the adsorbed amount rapidly increased with polymer concentration up to a plateau region. These phenomena has already been described and explained by the stretching of macromolecules [26]. As the PMANa block length increased, the adsorbed amount on the “plateau” decreased. When the PBMA block length was increased, the adsorbed amount on the plateau remained constant. This meant that adsorption was controlled by steric effect at the level of the hydrophilic blocks: on the plateau region, the coverage by copolymers was limited by the PMANa chain radius. 3.2. Colloidal stability of dispersion 3.2.1. Stability against electrolytes The colloidal stability of bare latex have been investigated in diluted solution and the CCC has been determined by mea-
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Fig. 4. Stability ratio W as a function of the electrolyte concentration for the bare latex.
surements of the kinetics of coagulation by adding solution of sodium chloride and lithium sulfate of different concentrations. These determinations have been made at different concentrations of the latex ranging from 0.02 to 0.05 wt.%. Typical kinetic curves having a linear behavior at short times were always observed. The CCC did not depend on the latex concentration. It was 210 ± 10 mmol L−1 for lithium sulfate and 380 mmol L−1 for sodium chloride. Classical log–log representation of the stability ratio W versus the electrolyte concentration is given in Fig. 4: the two regions of slow and fast coagulation below and above the CCC were observed. This observation was in agreement with the classical theory of DLVO where the colloidal stability was mainly electrostatic in origin [2]. As the concentration of added salt increases, the electrostatic repulsion decreases and the rate of coagulation increases. When this repulsion vanishes, the rate of coagulation remains constant despite the increase of concentration of added salt. The colloidal stability of the latexes covered with block copolymers could not be evaluated by this way because no coagulation could be observed by adding either sodium chloride or even lithium sulfate, which allowed reaching higher salt concentrations. Even for concentrated latex suspensions (8 wt.% of latex particles), the latex particles did not aggregate in presence of high salt concentrations; only creaming (due to high density of the aqueous phase) could be observed after a long time was elapsed. This result shows how highly efficient stabilizers these block copolymers are. But no comparative evaluation of the stabilization by different copolymers or different coverages could be obtained. More powerful coagulating agents are required for that purpose. 3.2.2. Stability of dispersion against acid Since the stabilization was thought to be electrosteric, addition of a strong acid converts the anionic carboxylate groups into neutral carboxylic acid group, so that the stabilization turn from electrosteric to steric for low concentrations of added acid. The neutralization of the latex surface cannot reach completion, however, a residual electrical charge remains because polymethacrylic acid is known to bear a fraction of carboxylic acid groups that behave as strong acid.
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L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32 Table 2 Determination of colloidal stability of fully covered latexes, preparation of solutions according to adsorption isotherms
Fig. 5. Stability ratio W as a function of the acid concentration for the bare latex.
Whatsoever, a strong acid, such as HCl, is a stronger coagulating agent than a simple electrolyte. First, the colloidal stability of the bare latex has been evaluated against the concentration of hydrochloric acid (Fig. 5). The shape of the curve has the same features as for classical salt solutions and this observed trend would be accepted in accordance to the DLVO theory. Nevertheless, the acid concentration, necessary to observed fast coagulation, was 8.1 × 10−4 mol L−1 , that is, more than 10 times higher than the concentration of basic species. This observation suggests that strong acid species were present at the surface of the particles or that some carboxylic acid groups behave as strong acids. The excess of added acid should behave as an electrolyte that screens the electrostatic effects of this residual surface charge. In consideration, the CCC should be calculated as the difference between the added acid concentration and bases concentration and was 7.5 × 10−4 mol L−1 for the bare latex. The colloidal stability of fully covered latexes have been investigated by this way since a CCC could be reached. Classical curves of log W = f(log[HCl]) were obtained as shown in Fig. 6. The chemical composition of the studied samples and the residual concentration in the aqueous phase according to the adsorption isotherm are reported in Table 2. The CCC were again many times higher than the concen-
Fig. 6. Determination of the CCC for fully covered latexes: () PBMA8 -bMANa16 , () PBMA8 -b-MANa32 , (䊉) PBMA8 -b-MANa64 , () PBMA16 b-MANa64 and (♦) PBMA32 -b-MANa64 . Only one horizontal line is drawn to simplify the representation.
PBMAx -b-MAAy
Adsorbed amount (mol m−2 )
Residual concentration (mol L−1 )
8/16 8/32 8/64 16/64 32/64
0.1 0.045 0.015 0.015 0.01
250 150 100 100 100
trations of basic species (carboxylates to be acidified). The concentration of basic species was subtracted from the acid concentration in all cases. As expected, the adsorption of copolymer enabled a large enhancement of stability since the CCC were many times higher when particles were covered with copolymers. Moreover, the rates in the fast coagulation regime were slower than for the bare latex (excepted for the PBMA8 -b-MANa16 copolymer). According to the CCC values, the different copolymers gave similar stabilities within experimental accuracy. This coagulation test did not allow to discriminate between the various copolymers that gave significantly different results in emulsion polymerization. The average particles diameter of latexes synthesized at high block copolymer concentration (10−4 mol L−1 ) were comprised between 70 and 100 nm, a difference that dynamic light scattering could discriminate. The influence of the adsorbed amount on the colloidal stability of the latex was investigated in the same way in connection with the emulsion polymerization results. This has been investigated on the PBMA8 -b-MANa16 copolymer which presented a favorable adsorption isotherm where the adsorbed amount could be easily varied below 100% coverage. As expected, the CCC increases as the relative coverage increases (Fig. 7). At the low relative coverage of 10%, the increase of stability was significantly improved since the CCC was twice higher than for the bare latex. But the variation of the CCC looked quite smooth, almost linear as a function of the relative coverage. 10% coverage did not bring about a huge stability improvement of comparable magnitude with a full coverage, as was inferred from the emulsion poly-
Fig. 7. Influence of relative coverage on colloidal stability for latexes covered with PBMA8 -b-MANa16 .
L. Beal, Y. Chevalier / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 26–32
Fig. 8. Influence of stirring mode on CCC for bare and fully covered latexes. Degrees of polymerization of block copolymers PBMAx -b-MAAy are referred to as x/y. Full bars: initial hand-shaking; dashed bars: continuous magnetic stirring.
merization results. Although the samples and measurement method were quite different, a similar smooth variation of the colloidal stability has been observed by Laible and Hamann by means of sedimentation measurements on silica particles dispersed in organic solvent with the help of grated polymers at different surface densities [27]. For the emulsion polymerization in the presence of increasing amounts of adsorbed copolymer (Fig. 1), the variation of the particle size as a function of relative coverage was far from linear whereas the CCC variation was approximately linear. Even if the trends are correct, there are no direct relationship between the adsorption, the colloidal stability as evaluated from the present method and the particle size obtained by emulsion polymerization. All the measurements described above have been performed by an initial mixing by means of hand-shaking the latex and HCl solution before recording the turbidity as a function of time. This method allowed a better and faster homogeneity of the sample at the beginning of the coagulation test but the sample was no more stirred during the course of the coagulation. Magnetic stirring was used and the results allow to discuss on the influence of stirring during coagulation. Figs. 8 and 9 show the influence of stirring on the CCC and the rates of coagulation above the CCC
Fig. 9. Influence of stirring mode on the kinetics of coagulation of bare and fully covered latexes. Degrees of polymerization of block copolymers PBMAx -b-MAAy are referred to as x/y. Full bars: initial hand-shaking; dashed bars: continuous magnetic stirring.
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([dA/dt]C=CCC ). Generally, the CCC was not influenced by stirring; the CCC values differed only by 2–7% which should be small as compared to experimental accuracy. Conversely, large differences of coagulation rates were observed, which amounted up to 55% for the bare latex. The rates were higher when magnetic stirring was used (excepted for the PBMA8 -b-MANa16 ). Once coagulation has started, the suspension is depleted around the particles that have coagulated and Brownian motion alone might be inefficient in bringing new particles in close contact with aggregates. This effect is more pronounced when aggregation has proceeded because aggregated particles are less mobile. Aggregate–aggregate coagulation is difficult in the absence of stirring. The influence of stirring is to renew the depleted region immediately after a coagulation event. That way, the coagulation rates are faster under stirring, as was observed. The coagulation behavior of the bare latex was more sensitive to stirring; the CCC value was significantly changed and the rates were the most accelerated. This observation was confirmed by visual observation of the samples: the particles of bare latex coagulated rapidly under stirring to form large particles of approximately 1 mm diameter and the aqueous phase became perfectly transparent. The behavior for covered latexes is much less sensitive to the stirring mode. This difference suggests that the copolymer covered latexes show a limited aggregation; a behavior that remains to be explained.
4. Conclusions Our previous study have shown that amphiphilic block copolymers composed of butyl methacrylate and methacrylic acid were very efficient in reducing the average diameter of particles in the emulsion polymerization of butyl methacrylate, even for very low concentration of copolymers (<10−4 mol L−1 ) corresponding to very low relative coverage [5]. This result led us to question the role of the block copolymers in the emulsion polymerization process, in particular regarding the colloidal stability of the formed particles. This question deserved also some attention because experimental data dealing with the colloidal stability improvement brought about by block copolymer adsorption are quite scarce in the literature, excepted those using the surface force apparatus. The colloidal stability was evaluated by determining the critical coagulation concentration, CCC, from turbidity measurements of the kinetics of coagulation of diluted latex against addition of a flocculating agent. Block copolymers provided a very strong stabilization since it was not possible to coagulate the covered latexes with classical electrolytes. Neutralization of the surface charges by addition of an acid was necessary in order to evaluate the colloidal stability. The CCC with HCl as an acidic electrolyte were much higher for latex particles covered with copolymer and the kinetics of coagulation were slower, which was in agreement with our earlier results. However, it was difficult to sort the data with respect to the influence of the composition of the copolymers.
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Decreasing the relative coverage of the latex decreased the stability, as expected, but the variation of the CCC was almost linear with the relative coverage, which differed from the variation of the particles diameter in emulsion polymerization. In conclusion, there are no direct relationship between the particle size obtained by emulsion polymerization and the adsorption or the related improvement of the colloidal stability, as evaluated from the present method. It appears from this investigation that the adsorption of copolymers allows to reach very high stabilities, suggesting that block copolymers are indeed able to prevent coagulation during the emulsion polymerization process. This is in agreement with the observation of small particle size at low copolymer coverage [5]. The emulsion polymerization mechanisms in the presence of block copolymers are far to be elucidated however. Since the adsorption and desorption of block copolymers are very slow, and no coagulation takes place during the process, the number of particles (thus their final size) is determined during the nucleation step. Nucleation necessarily takes place inside the block copolymer micelles for the final size to be smaller than for the surfactant-free polymerization process. But the final number of latex particles is less than the initial number of micelles, meaning that part of the monomer swollen micelles have not been nucleated. We suggest that the nucleation taking place by means of the entry of anionic initiator or anionic macroradicals formed in the aqueous phase is hindered by the large negative charge of the micelles. The nucleated micelles (primary particles) are larger than the empty ones, but contain the same amount of copolymer because they originate from an empty micelle and the block copolymer neither adsorb nor desorb. The electrostatic repulsion is therefore larger for small empty micelles than for nucleated micelles because the surface charge density of the later is less. Empty micelles are kept empty in that way; butyl methacrylate monomer diffusion through the aqueous phase allows for particle growth. Notice that because styrene monomer is less soluble in water, slower diffusion from the empty micelles to the primary particles favors the nucleation of a larger number of micelles; the exponent α of the power law was close to one in this case [10,13]. Finally, the difference with small surfactant molecules like SDS is huge because such emulsifiers adsorb and desorb very fast; micelles are lost during the emulsion polymerization pro-
cess because the surfactant molecules adsorb onto growing particles.
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