Article pubs.acs.org/Langmuir
Theory-Guided Strategy for Nanolatex Synthesis Juliana de S. Nunes and José M. Asua* Institute for Polymer Materials (POLYMAT) and Grupo de Ingeniería Química, Departamento de Química Aplicada, University of the Basque Country UPV/EHU, Centro Joxe Mari Korta, Avda. de Tolosa 72, 20018 Donostia-San Sebastián, Spain ABSTRACT: The synthesis of waterborne nanocomposites in semicontinuous emulsion polymerization was investigated using a theory-guided strategy with the aim of achieving the best balance among small particle size, low surfactant concentration, and sufficiently high solids content. It was found that both kinetic (monomer feeding rate, radical generation rate, and temperature) and colloidal (ionic strength and polymer hydrophilicity) aspects were critical in the process. Waterborne nanoparticles as small as 13 nm were obtained with a solids content/(surfactant/polymer) ratio higher than 7.
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INTRODUCTION Waterborne dispersed polymers are used in a wide range of applications such as coatings, adhesives, additives for textiles, leather, and construction materials, impact modifiers for plastics, and rubber for tires.1−4 Most commercial waterborne polymers have an average particle size of between 80 and 250 nm. However, latexes with smaller sizes present advantages in many fields such as coatings,5 controlled released systems,6 polymer−carbon nanotube dispersions,7 and polymer−inorganic hybrids for applications in catalysis and photochromism.8 Therefore, there has been plenty of activity in developing strategies for synthesizing nanolatexes. The goal is to achieve a small particle size in a high-solids-content dispersion using a low concentration of surfactant. These are conflicting requirements because the surface area of the particles and the collision frequency among them increase as the particle size decreases and the solids content increases, and hence a higher concentration of surfactant is needed to provide colloidal stability to the system. However, viscosity (that affects mixing and heat removal) strongly increases as the particle size decreases and the solids content and surfactant concentration increase.9,10 A precursor of the strategies developed to achieve small-size, high-solids, low-surfactant latexes can be found in the work reported by Harada and Nomura, 11 in which it was demonstrated that in a batch emulsion polymerization the number of polymer particles increased by decreasing the amount of monomer in the formulation (maintaining constant the amounts of water and surfactant) below the point at which monomer droplets disappeared before the micelles. These results can be analyzed in terms of the prediction of Smith− Ewart12 theory for the number of particles ⎛ R ⎞0.4 Np ∝ ⎜ I ⎟ (aSS)0.6 ⎝μ⎠
where Np is the number of polymer particles, RI is the rate of generation of radicals (in the model it is assumed that the rate of particle nucleation is proportional to the rate of radical generation), μ is the rate of volumetric growth of a polymer particle, aS is the parking area, and S is the amount of surfactant in the system. μ is proportional to the propagation rate coefficient and to the concentration of monomer in the polymer particles. μ is also proportional to the average number of radicals per particle (n), ̅ and in Smith−Ewart theory, n̅ = 0.5 was considered. n̅ = 0.5 implies that radical desorption is negligible and termination occurs instantaneously upon the entry of one radical into a polymer particle already containing one radical. Nomura13 and Hansen and Ugelstad14 showed that for systems in which radical desorption is significant the number of polymer particles is given by ⎛ RI ⎞z Np ∝ ⎜ ⎟ (aSS)1 − z ⎝μ⎠
with z < 0.4 and decreasing as the significance of the radical desorption increased. Equations 1 and 2 establish that the number of particles is the result of the interplay between particle nucleation by the entry of radicals into micelles (only heterogeneous nucleation was considered) and the disappearance of micelles to provide surfactant to stabilize the growing polymer particles. The smaller the particle growth rate, the less surfactant required to stabilize the growing particles and hence the higher the number of micelles available for nucleation. Therefore, Np can be increased by lowering the rate of volumetric growth of the polymer particles (μ), which in ref 11 Received: February 15, 2012 Revised: April 13, 2012 Published: April 20, 2012
(1) © 2012 American Chemical Society
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larger, than those obtained by semicontinuous monomer addition. Nevertheless, it is an achievement considering that a batch process was used, and one wonders if the combination of catalytic chain-transfer polymerization and semicontinuous addition may further reduce the particle size. The presence of cobalt in the final latex may be of some concern. Very small nanolatexes (dp = 2.1−3.4 nm) have been produced by the photopolymerization of monomer-swollen micelles.33 In this process, it was claimed that polymerization occurs almost simultaneously in virtually all micelles, preserving their structure. The main drawbacks of this method are the low solids content (0.6−1.4 wt %) and the huge surfactant/polymer ratio (2−5). The reduction in the particle size observed in ref 29 when AIBN was substituted with benzoyl peroxide (BPO) was attributed to a more efficient nucleation because most of the radicals were produced within the polymer particles, which helped to nucleate the micelles rapidly. This analysis does not take into account the fact that the radicals are produced in pairs within the polymer particles, and hence in order to have a significant polymerization, one of them should desorb because otherwise bimolecular termination will occur rapidly.34 Because BPO is very hydrophobic, radical desorption is not favored and hence micellar nucleation will likely expand over a relatively long period of time. A possible reason for the enhanced micellar nucleation observed is that BPO, which is very hydrophobic, may act in a similar way as a costabilizer in miniemulsion polymerization,35 hence retaining the monomer in the non-nucleated micelles. However, it is worth pointing out that the very small particle size (3 nm) reported in ref 29 using benzoyl peroxide (BPO) at a low solids content/(surfactant/polymer) ratio is in conflict with the reported molecular weight of the polymer (750 000 g/ mol) because, assuming a density of 1.18 kg/L, the size of the particle needed to contain a single polymer chain is dp = 12.6 nm. Sajjadi36 extended the Smith−Ewart12 theory to semicontinuous processes, finding that
was achieved by lowering the concentration of monomer in the system. Another way of lowering the concentration of monomer in the polymer particles is by using a semicontinuous addition of monomer. Probably led by the fact that microemulsion polymerization usually yields smaller particles than conventional emulsion polymerization, much effort has been devoted to the synthesis of nanolatexes using semicontinuous microemulsion polymerization15−23 that involves the use of a cosurfactant that, when it is not reactive (e.g., n-pentanol), can provide undesirable characteristics to the nanolatex. This drawback can be overcome by using conventional semicontinuous emulsion polymerization. In this process, the initial charge of the reactor is formed by an aqueous solution of surfactant, eventually containing some monomer. The initiator (either water-soluble or oil-soluble) is usually included in the initial charge, and it can be further added during the semicontinuous addition of the rest of the monomer, which is carried out under starved conditions. The goal of this process is to nucleate the particles in the presence of a low concentration of monomer and later to make the particles grow. This process has been extensively used.22,24−31 The most representative results are summarized in Figure 1, which plots
⎛ RI ⎞2/3 Np ∝ ⎜ ⎟ (aSS) ⎝ Ra ⎠ Figure 1. Literature results for the synthesis of PMMA nanolatexes using semicontinuous emulsion polymerization.
(3)
where Ra is the rate of monomer addition to the reactor. The main difference between eqs 1 and 2 is that the volumetric growth rate of the particles is substituted by the rate of monomer addition to the reactor. The reason is that eq 3 assumes that the rate of volumetric growth of the particles is proportional to the polymerization rate and that the processes occurred under completely starved conditions, namely, that the rate of polymerization is equal to the rate of monomer addition. In addition, the model assumes that nucleation finishes when the micelles disappear, which occurs when the surface area of the particles is equal to aSS. This model also assumes that the rate of particle nucleation is proportional to the rate of radical generation. All of these assumptions limit the predicting capability of eq 3. Thus, although it describes the polymerization of styrene and butyl acrylate (BA)25,36 well, it underestimates the effect of Ra on Np for methyl methacrylate (MMA)37 and vinyl acetate (VA).36 The failure to account for the effect of Ra on Np for relatively water-soluble monomers may be due to the effect of monomer water solubility on both radical desorption and homogeneous nucleation. Radical desorption increases with the solubility of the monomers in water,38 and it reduces the polymerization rate in the particles,
the particle diameter (dp) versus the solids content/ (surfactant/polymer) ratio, where the solids content was defined as polymer/(polymer + water). In some cases, the final monomer conversion is not reported, and it has been assumed that full conversion was reached. This plot helps to visualize how well the final goal of this strategy (achieving a small particle size in a high solids dispersion using a small amount of surfactant) is achieved. Ideally, one would like to be in the fourth quadrant: small particle size, high solids, and low surfactant/polymer ratio. Figure 1 illustrates the difficulty of producing high-solids nanolatexes using a low concentration of surfactant. Another way to reduce the particle size of the particle volumetric growth rate is to reduce the average number of radicals per particle by enhancing radical desorption using batch catalytic chain-transfer polymerization.32 The results achieved with this process are also represented in Figure 1. It can be seen that for the same solids content/(surfactant/ polymer) ratio the size obtained was similar, although slightly 7334
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which makes the particle growth lower than Ra/Np and hence increases the number of particles. Radical desorption also increases the radical concentration in the aqueous phase, enhancing micelle nucleation. In addition, homogeneous nucleation, which is more likely for water-soluble monomers, may contribute to the formation of new particles. However, this process should lead to a smaller effect of Ra on Np, which is not what was observed. In spite of its limitations, eq 3 highlights the main operational variables that affect the number of particles produced in semicontinuous emulsion polymerization carried out under monomer-starved conditions. This is valuable information in designing knowledge-based polymerization strategies to produce nanolatexes aimed at obtaining the best balance among the particle size, solids content, and surfactant concentration. Surprisingly, it has not been exploited for this purpose. This work is an attempt to obtain the best balance among the particle size, solids content, and surfactant concentration, making extensive use of the guidance of eq 3. The rate of radical generation was modified by varying the type and concentration of initiator and the reaction temperature. The rate of particle growth was varied by using different monomer feeding rates. The surface area that can be stabilized was modified by varying the type and concentration of surfactant and the monomer type (which affects the parking area). This allowed us to produce nanoparticles as small as 13 nm with solids content/(surfactant/polymer) ratios higher than 7.
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Table 2. Polymerization Conditions Used in the Synthesis of Nanolatexes latex A1 A2 A3 A4 A5 A6 B1 B2 B3 C1 C2 C3 D1 D2 D3 D4 D5 E1 E2 F1 F2 F3 F4 G1 G2 G3 G4
EXPERIMENTAL SECTION
Materials. Methyl methacrylate (MMA, technical grade, Quimidroga), styrene (S, technical grade, Quimidroga), butyl acrylate (BA, technical grade, Quimidroga), 2-hydroxy ethyl methacrylate (HEMA, ≥97%, Fluka), methacrylic acid (MAA, ≥98.0%, Sigma-Aldrich), sodium styrene sulfonate (NaSS, ≥90%, Sigma Aldrich), sodium dodecyl sulfate (SDS, ≥98.5%, Sigma-Aldrich), Dowfax 2A-1 (alkyldiphenyloxide disulfonate, 45 wt % solution, Dow), 2,2′azobisisobutyronitrile (AIBN, 98%, Wako), ammonium persulfate (APS, ≥98%, Aldrich), and hydroquinone (Merck) were used without previous purification. Deionized water was used in all experiments. Synthesis and Characterization of the Nanoparticles. Polymeric nanoparticles were prepared using semicontinuous emulsion polymerization. In this method, surfactant and initiator were added to a glass reactor equipped with a stainless steel stirrer, a reflux condenser, a sampling device, a nitrogen gas inlet tube, and a temperature probe. When the reaction temperature was reached, the monomer was fed very slowly over a certain time. At the end of monomer feeding, the system was maintained at the reaction temperature for 60 min in order to minimize the amount of residual monomer. A general formulation used in the polymerizations is presented in Table 1. Table 2 summarizes the polymerizations carried out. The reaction parameters studied were the monomer feeding rate (series A), temperature (series B), initiator type (series C) and concentration (series D), surfactant type (series E) and concentration (series F), and monomer type (series G). These formulations had a final solids content of about 18%.
G5 G6
total charge (g)
monomer initiator SDS water
52.8 variable variable 240
T (°C) 70
10.6 wbm %
0.6% AIBN
MMA
0.587 0.440 0.352 0.293 0.220 0.176 0.293
10.6 wbm %
0.6% AIBN
MMA
0.293
60 70 80 80
10.6 wbm %
MMA
0.293
80
2.3 wbm %
MMA
0.293
80
MMA
0.293
80
S BA MMA MMA/MAA (95/5) MMA/HEMA (95/5) MMA/NaSS (95.5/4.5)
0.293
80
0.017 M (SDS) 0.017 M (Dowfax 2A1) 2.3 wbm % 3.6 wbm % 5.5 wbm % 10.6 wbm % 2.3
0.6% AIBN 0.6% APS 0.6% BPO 0.3% APS 0.6% APS 1.0% APS 1.5% APS 2.0% APS 0.3% APS
MMA
surfactant
initiator (wbm%)a
0.6% APS
0.3% APS
a
AIBN, in the initial charge; APS, added as a shot; and BPO, dissolved in monomer. wbm stands for weight based on monomer.
During the reaction, samples were withdrawn at regular intervals, and the reaction was stopped by the addition of 0.2 mL of a 1 wt % aqueous hydroquinone solution. The conversions were determined by gravimetry. The overall conversion was defined as the ratio of the polymer present in the reactor to the total monomer used in the formulation. The instantaneous conversion was defined as the ratio of the polymer present in the reactor to the monomer added until the moment in which the sample was withdrawn. The z-average particle size of the polymeric nanoparticles was measured by dynamic light scattering using a Zetasizer Nano ZS apparatus (Malvern Instruments). For this measurement, about 1 mL of the latex was placed in a vial and the reaction was immediately quenched with 0.2 mL of a 1 wt % aqueous solution of hydroquinone. Before the analysis, the samples were diluted with deionized water in order to avoid multiple scattering. The value was obtained from the average of two repeated measurements. The number of particles (Np) was calculated from the average diameter of particles, the overall conversion, and the amount of monomer added to the reactor, namely, assuming that in the highly diluted latex the monomer (if present) was in the aqueous phase. Some samples were also observed by transmission electron microscopy (TEM, Philips CM200). To perform this measurement, the latex was diluted and deposited on a grid; then a phosphotungstic acid aqueous solution was added, and the grid was dried overnight.
Table 1. General Polymerization Recipe Used in the Synthesis of Nanolatexes ingredient
feeding rate (g/min)
monomer
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The cmc of the surfactants was determined by means of surface tension measurements using a Du Noüy ring (KSV Sigma 70, KSV Instruments Ltd.). The same equipment was used to estimate the parking area, aS, of the surfactant on poly(methyl methacrylate) latexes by titrating a diluted latex with a solution of surfactant until micelles appeared in the system. aS =
1016 2 (Å /molecule) NA Γ1
Therefore, less surfactant is employed to stabilize the growing surface area of the particles; consequently, more micelles remain in the system and can participate in the nucleation of new particles. The rate of particle generation can be modified by varying the type and concentration of initiator. The surface area of the polymer particles that can be stabilized by the surfactant can be modified by varying the type and concentration of the surfactant and the hydrophilicity of the monomer. (The more hydrophilic the monomer, the higher the parking area of the surfactant (aS) on the corresponding polymer and hence the larger the area that can be stabilized.) Effect of the Particle Growth Rate. The particle growth rate was controlled by means of the monomer feeding rate. The evolution of the instantaneous conversion for latexes A1−A6 is presented in Figure 2. It can be seen that, for most of the process, polymerizations were carried out under starved conditions. The low conversion achieved in some cases at the beginning of the process might be due to the fact that an oilsoluble initiator, AIBN, was used in this series. Because it was included in the initial charge, which did not contain monomer, it could not be completely dissolved (water solubility is small (0.4 g/L)) and flakes of AIBN were observed in the system. The flakes disappeared after approximately 10 min of monomer feeding, but during this time, AIBN could not be efficiently used to initiate polymerization. The latexes obtained were stable for almost 1 year. The evolutions of the number of particles in series A are presented in Figure 3. It can be seen that most of the nucleation occurred during the first 30% of the process and that later the number of particles increased slowly. Figure 4 presents the effect of the monomer feeding rate on the final particle size and the number of particles. It can be seen that the final particle size decreased as the monomer feeding rate decreased, in agreement with eq 3 (i.e., Np ∝ Ra−0.67). This differs from the dependence reported by Sajjadi et al.37 (Np ∝ Ra−1.98). Effect of the Rate of Radical Generation. Effect of the Polymerization Temperature. Equation 3 predicts that the number of particles will increase by increasing the rate of radical generation (RI). For a given formulation, one way to increase RI is to increase the polymerization temperature. It is worth pointing out that as far as the process is carried out under starved conditions, the temperature does not significantly affect the rate of particle growth. Figure 5 presents the effect of the reaction temperature on the evolution of the conversion and particle size. It can be seen that at 70 and 80 °C the polymerization proceeded under starved conditions, whereas this was not the case at 60 °C. Figure 5 also shows that the polymerization temperature strongly affects the particle size, which varied from 46 nm at 60 °C to 17 nm at 80 °C. Figure 6 presents the effect of the rate of radical generation (calculated as RI = 2f kI[I], with f = 0.6 and kI = 1.08 × 10−4 s−1, calculated by using the parameters given in ref 39) on the number of particles. It can be seen that in agreement with eq 3 Np increased with RI. However, the effect was stronger than predicted, perhaps because of the fact that the polymerization at 60 °C was not carried out under starved conditions. Considering only the reactions carried out at 70 and 80 °C, the experimental results (Np ∝ RI0.616) coincided quite well with the predictions of eq 3 (Np ∝ RI0.67). Effect of Initiator Type. An obvious way to modify RI is to use different initiators. Figure 7 presents the evolution of the
(4)
where NA is Avogadro’s number and Γ1 is the surface concentration. In turn, Γ1 is given by
Γ1 =
ΔCV (mol/cm 2) A sm
(5)
where V is the volume of the liquid phase (L), As is the surface area of the latex (cm2), m is the mass of solid adsorbent, ΔC = cmcp − cmc, where cmcp is the concentration of surfactant at the point at which micelles appeared in the diluted latex, and cmc is the critical micelle concentration.
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RESULTS AND DISCUSSION Equation 3 indicates that the number of particles increases (and, consequently, for a given solids content, the particle size
Figure 2. Instantaneous conversion vs polymerization time for the polymerization of MMA at different monomer feeding rates (series A).
Figure 3. Evolution of the number of particles for series A.
decreases) as the rate of addition of monomer to the reactor decreases, the rate of radical generation increases, and the surface area of the particles that can be stabilized by the surfactant increases. Under starved conditions, the decrease in the monomer addition rate leads to a slower growth of polymer particles. 7336
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Figure 4. Effect of the monomer feeding rate on (a) the final particle size and (b) the number of particles.
Figure 5. Evolution of the (a) instantaneous conversion and (b) particle size (measured by DLS) with the overall conversion of MMA at different temperatures (series B).
s−1), which would result in a smaller number of particles and a lower n̅. In addition, BPO is more hydrophobic than AIBN (AIBN, 0.04 g/L; BPO, 3 × 10−4 g/L40) and therefore less efficient than AIBN because the efficiency of an oil-soluble initiator increases with the desorption rate of the initiator radicals, namely, with the water solubility of the initiator radical. The observed effect of BPO was different than that reported in ref 29, where a decrease in the particle size was observed when BPO was used instead of AIBN. Simulations34 show that in a micellar system most of the radicals formed in pairs from an oil-soluble initiator within the micelles terminate rapidly. A small fraction of the radicals desorb, but for the case of AIBN, there are substantially fewer of them than the number produced in the aqueous phase from the small fraction of AIBN dissolved in that phase. On the other hand, APS is water-soluble and produces the radicals in the aqueous phase, where they are not subjected to fast termination. Therefore, APS is more efficient than AIBN in nucleating micelles. In addition, the sulfate groups of the polymer chains generated from APS can stabilize the polymer particles. Because smaller nanoparticles were produced with APS, this initiator was used in the rest of the study. Another way to increase the radical generation is to vary the initiator concentration in the system, and smaller particles are expected when the initiator concentration is increased. Figure 8a,b shows the effect of the initiator concentration on the evolution of conversion and particle size. It can be seen that the instantaneous conversions were slightly higher for the latexes with higher APS concentrations because of the higher
Figure 6. Effect of radical generation rate on the number of particles in latexes B1−B3 obtained at different temperatures.
instantaneous conversion and particle size using AIBN, ammonium persulfate (APS), and benzoyl peroxide (BPO). The amount of initiator used was fixed at 0.6 wbm % for comparison. It can be seen that, when AIBN and APS were used as initiators, the type of initiator had no effect on the monomer conversion (likely because the polymerization was carried out under starved conditions). BPO led to a slower polymerization rate because of the combined effect of a smaller number of particles and a lower value of the average number of radicals per particle. In comparison with the other oil-soluble initiator, AIBN, BPO had a lower radical generation rate (RI AIBN = 1.06 × 10−6 mol L−1 s−1, RI BPO = 3.17 × 10−7 mol L−1 7337
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Figure 7. Evolution of the (a) instantaneous conversion and (b) particle size (measured by DLS) with the overall conversion of MMA using different initiators (series C).
Figure 8. Evolution of the (a) instantaneous conversion and (b, c) particle size (measured by DLS) with the overall conversion of MMA using different APS and Na2SO4 concentrations (series D).
concentration of radicals in the system. However, contrary to the predictions of eq 3, the particle size increased with the concentration of initiator. To determine if the increase in ionic strength that accompanied the increase in initiator concentration was responsible for these results, the effect of the
initiator concentration on particle size at constant ionic strength was investigated using Na2SO4 as an electrolyte. Figure 8c shows that when the latexes were obtained at constant ionic strength, the particle size decreased with initiator concentration, which agrees with the predictions of eq 3. It is worth mentioning that, because of the effect of the sulfate groups of the initiator, the number of anionic groups on the particle surface was different in each polymerization of this series. This difference may affect the latex stability, hiding the effect of the rate of radical generation. Effect of Particle Stabilization (aSS). Effect of Surfactant Type. An important factor that influences the number of particles is the parking area (aS) of the emulsifier. Equation 3 predicts that the greater the aS, the greater the number of
Table 3. Surface Properties of the Conventional Surfactants Used in This Work surfactant SDS Dowfax 2A-1 a
cmc (mol/L) −3
7.1 × 10 4.8 × 10−4
aS (Å2/molecule)a 70 ± 1.5 38 ± 1.3
Obtained by titrating the latex with SDS and Dowfax 2A-1 solutions. 7338
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Figure 9. Effect of the type of surfactant on (a) the particle size (measured by DLS) and (b) the number of particles with the overall conversion of MMA (series E).
Figure 10. Effect of SDS concentration on dp and Np (series F).
Figure 11. Micrographs of final latex F4 prepared by the polymerization of MMA using semicontinuous emulsion polymerization.
particles of the system. A way to modify aS is to use different surfactants. Therefore, the performances of SDS and Dowfax 2A-1 were compared. Table 3 summarizes the properties of these surfactants. Figure 9 shows that in agreement with the predictions of eq 3 larger particles were obtained with Dowfax 2A-1. Figure 9 also shows that new particles were produced during an important part of the process. Although micelles are present at the beginning of the polymerization, they completely disappear early in the process (at about Xtotal = 0.1 and 0.08 for
SDS and Dowfax 2A-1, respectively). However, new particles are nucleated above these critical conversions. This means that the surfactant had to desorb from the surfaces of the existing particles to stabilize the new particles formed by homogeneous nucleation. SDS was used in the rest of this study. Effect of Surfactant Concentration. Figure 10 presents the effect of the SDS concentration on the particle size and number of particles for the semicontinuous emulsion polymerization of MMA using a monomer feeding rate of 0.293 g/min and a 7339
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Figure 12. Evolution of the number of particles during polymerization and the final particle size obtained for the latexes synthesized using different monomer compositions (serial G). From left to right: PS, PBA, PMMA, P(MMA-co-MAA) (95/5), P(MMA-co-HEMA) (95/5), and P(MMA-coNaSS) (95.5/4.5).
From a practical point of view, the results presented in Figure 12b show that very small nanoparticles were obtained for a given solids content and surfactant/polymer ratio by simply modifying the hydrophilicity of the monomer mixture. Actually, this allowed us to produce nanoparticles as small as 13 nm with a solids content/(surfactant/monomer) ratio higher than 7.
Table 4. Parking Area of the Latexes Obtained in This Work latex PS PBA PMMA P(MMA-co-MAA) P(MMA-co-HEMA) P(MMA-co-NaSS) a
aS (Å2/molecule)
41
47 62 70 ± 1.5a 252 ± 2.84a 93 ± 0.44a 129 ± 0.08a
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Obtained by titrating a diluted latex with a SDS solution.
CONCLUSIONS
The synthesis of waterborne nanoparticles via semicontinuous emulsion polymerization was investigated, with the goal of reducing the particle size without increasing the surfactant concentration and lowering the solids content. Theory predicts that the particle size should decrease with the rate of radical generation and the total surface area that can be stabilized by the surfactant as well as by lowering the particle volumetric growth rate.36 The rate of radical generation was modified by varying the type and concentration of initiator and the reaction temperature. The rate of particle growth was varied by using different monomer feeding rates. The surface area that can be stabilized by the surfactant was modified by varying the type and concentration of surfactant and the monomer type. It was found that the experimental results agreed well with the theoretical predictions of the effect of the monomer feeding rate and the surfactant concentration and, under some circumstances, with that of the radical generation rate (e.g., increasing temperature). In addition, it was found that there were colloidal aspects that were critical to the process: (i) water-soluble initiator that contributed to particle stabilization yielded smaller polymer particles than oil-soluble initiators; (ii) the increase in ionic strength caused by the initiator concentration compensated the increase in the radical generation rate, yielding larger particles; (iii) increasing the monomer hydrophilicity led to a substantial reduction of the particle size; and (iv) water-soluble monomers may cause limited particle coagulation, which led to an increase in the particle size. A consideration of these effects allowed us to obtain a good balance among the particle size, solids content, and surfactant concentration, and waterborne nanoparticles as small as 13 nm were obtained with a solids content/(surfactant/polymer) ratio higher than 7.
concentration of APS of 0.6 wbm %. The reactions proceeded under starved conditions (not shown). It can be seen that the particle size decreased with the concentration of surfactant. In addition, the effect on Np (Np ∝ 1.1) agreed well with the predictions of eq 3 (Np ∝ 1.0). Figure 11 shows representative pictures of latex F4. It can be seen that the size agreed well with the value obtained by dynamic light scattering and that latex F4 presented a narrow particle size distribution. Effect of Monomer Composition. For a given surfactant, the parking area (aS) increases with the hydrophilicity of the polymer particle, which can be easily varied by using different monomeric compositions. Therefore, in series G, monomers of increasing hydrophilicity were used: styrene (S), butyl acrylate (BA), MMA, MMA/hydroxyl ethyl methacrylate (MMA/ HEMA 95/5 w/w), MMA/MAA (95/5 w/w), and MMA/ NaSS (95.5/4.5 w/w). Figure 12 presents the effect of the hydrophilicity on both the evolution of the number of particles and the final particle size. It can be seen that particle nucleation occurred at the very beginning of the process and that beyond Xtotal = 0.5 the number of particles was virtually constant. Values of the parking area are presented in Table 4. Figure 12b shows that the monomer hydrophilicity strongly affected the final particle size. Initially, the particle size decreased with the hydrophilicity of the monomer, but when a highly water-soluble monomer (NaSS) was included in the formulation, although the particle size was small at the beginning of the process (Figure 12a), limited particle coagulation occurred in the second part of the process, leading to a larger final particle size (37 nm, Figure 12b). A possible cause of the limited coagulation is the formation of polyelectrolytes by the polymerization of NaSS in the aqueous phase, which may act as flocculants. It is worth pointing out that NaSS likely forms a hydrated shell that may affect the particle size measured by dynamic light scattering. 7340
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support provided by the Industrial Liaison Program in Polymerization in Dispersed Media of Polymat (Arkema, AkzoNobel, BASF, Comex, Cytec Surface Specialties, Nuplex Resins, ICI Paints, Solvay, Stahl, Euroresin, Rohm and Haas, Synthomer, and Wacker Chemie).
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