In situ charge neutralization on governing particle coagulation nucleation and size distribution in macroemulsion polymerization

Abstract

Fabricating monodispersed polymer latex particles with ∼300 nm size at high monomer concentrations by batch macroemulsion polymerization remains significantly challenging because of latex stability. In this study, we developed a novel approach based on in situ charge neutralization to prepare 40 wt% solid content latex containing monodispersed sub-300 nm latex particles. The cationic initiator 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AIBA) was used to induce in situ charge neutralization in the particle nucleation period and shield the negative charges of surfactant sodium dodecyl sulfate molecules to further reduce the electrostatic repulsion between primary particles, resulting in primary particle coagulation nucleation. The primary particle coagulation promoted particle number decreased to ∼10¹⁶ L⁻¹, and the average particle size increased to ∼100 nm at a very low monomer conversion (<0.12). With increasing AIBA concentrations from 0.6 and 0.8 to 1.0 wt% (the molar ratio of AIBA/SDS is 0.64, 0.85 and 1.06, respective), the average particle size of the latex ultimately attained from 170.6 and 221.7 to 288.0 nm, respectively. Moreover, the addition of an electrolyte and copolymerization composition also governed the particle coagulation extent and affected the particle size distribution of the ultimate latex particles. To the best of our knowledge, this is the simplest, most efficient, and inexpensive approach to prepare large sized, monodispersed latex particles.

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Introduction

Particle coagulation as a thermodynamic behavior has been widely accepted as a novel approach to prepare large sized, narrowly dispersed and stable latex particles in dispersion/soap-free emulsion polymerization.^1–3 Different from traditional latex particle preparation methods such as two stage swelling, dynamic swelling and semi-continuous monomer-starved methods, particle coagulation is a simple process, that is, the unstable primary particles first aggregate into a large sized particle cluster by particle collision, and then the particle cluster gradually shifts to mature particles by a particle fusion process.⁴ Thus, the average particle size of latex particle size increases, because of the decrease in the particle number. However, the particle coagulation process in actual is very complicated, because of the reversible particle coagulation,^3,5–7 indicating that the particle clusters also could be divided into primary particles when primary particles aggregate into the particle cluster. Therefore, particle coagulation process could also be understood as a dynamic process as described by Ohshima et al.,⁸ in which the balance constant of particle aggregation and dissociation depends on the ratio of the rate constant for the association of several particles into the aggregated particle clusters and the rate constant for the dissociation of particle clusters into several primary particles. These rate constants were determined by the polymerization reaction composition including surfactant, initiator, electrolyte, and monomer concentrations.^3,9,10 Certainly, polymerization temperature and aqueous phase composition also play a very important role in governing the extent of particle coagulation.⁴

Particle coagulation has been extensively discussed. For example, Feeney et al. proposed a modified particle coagulation theory based on the Müller theory of particle coagulation and predicted the relationship between the particle size distribution and particle coagulation.¹¹ Peach et al. also stressed coagulative nucleation in surfactant-free emulsion polymerization using the data of Zhang et al.^12,13 However, these investigations on particle coagulation only concerned theoretical modeling or derivation, and a few experimental evidences of particle coagulation have been provided. In recent years, more and more experimental investigations about particle coagulation have been reported to prepare large scale, narrowly dispersed latex particles. Dobrowolska and Yamamoto directly visualized particle coagulation in soap-free emulsion polymerization by observing the particle morphology of ultimate latex particles by scanning electron microscopy (SEM) and atomic force microscopy (AFM).^14,15 These investigations provide sufficient theoretical and experimental evidences to support particle coagulation mechanism.

However, in these studies, the particle coagulation was usually induced by increasing the ionic strength to compress the thickness of the electric double layers. Moreover, the co-solvents such as methanol were usually added to reduce the interfacial energy of surfactant molecules adsorbed on the particle surface.¹⁶ Particle coagulation is well known to occur if the kinetic energy of the particles is larger than the potential energy of latex particles,¹¹ in which the potential energy equals the sum of the repulsive and attractive potential energy. In emulsion polymerization, the repulsive potential energy is usually supplied by the surfactant molecules adsorbed on the particle surface, the initiator chain ends, and the functional comonomers in main chains. In previous studies, these components possessed same type charges to increase the charge density of particle surface. Thus, particle coagulation in polymerization is usually limited, and relatively small sized particles (∼100 nm) have been obtained in the batch emulsion polymerization.^17–19 In this study, a novel cationic initiator 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AIBA) was used to replace the anionic KPS and nonionic AIBN to initiate the polymerization reaction of styrene (St) in the presence of anionic sodium dodecyl sulphate surfactant (SDS) at 40 wt% solid content. The water soluble cationic AIBA initiator decomposed into primary cationic radicals and gradually formed oligomeric radicals in the water phase, resulting in the oligomeric radicals with positive charges. When the oligomeric radicals with positive charges were captured by the anionic SDS micelles, the positive charges in the initiator chain ends shield the negative charges of the SDS micelles, further inducing in situ charge neutralization, resulting in particle coagulation in the nucleation period. In addition, the effect of various parameters such as monomer composition and ionic strength on the particle size distribution is also discussed.

Experimental

Materials

All the reagents were of analytical grade and supplied by Aladdin Chemistry Co., Ltd (Shanghai, China) unless mentioned otherwise. Styrene and butyl acrylate (BA) were purified by reduced pressure distillation under −0.1 MPa and stored at −5 °C before use. Potassium persulphate (KPS), SDS, and potassium carbonate (PCB) were directly used as the radical initiator, surfactant, and electrolyte, respectively. AIBA was also selected as the cationic radical initiator. Distilled deionized (DDI) water was used for all experiments.

Synthetic procedures

All the polymerization reactions were performed in a 500 mL, four-necked flask placed in a water-bath cauldron equipped with an anchor stirrer a reflux condenser, a nitrogen pipe, and a sample connection at 65 °C. The detail polymerization process is as follows: first, surfactant and electrolyte were directly added to the flask. Then, DDI was also added to the flask to dissolve the surfactant and electrolyte, followed by the addition of styrene. At the same time, nitrogen was directly bubbled at room temperature for 30 min to remove the oxygen in the system. After bubbling nitrogen, the reaction systems were heated, and the initiator (KPS or AIBA) dissolved in DDI was added to the flask to initiate polymerization reactions. The polymerization reactions were maintained at 65 °C for 6 h. The stirring rate was set to 250 rpm. The detail polymerization recipes are shown in Table 1.

Table 1 Standard recipe for preparation of PS latex particles (unit: g)^a

Runs	St	BA	DDI	SDS	AIBA	KPS	PCB
a Note: the molar ratio of AIBA to SDS in runs 2, 3, and 4 is 0.64, 0.85 and 1.06, respectively. The molar ratio of AIBA to SDS in runs from 5 to 9 is 0.64.
1	100	0	150	1.5	0	0.9	0.6
2	100	0	150	1.5	0.9	0	0.6
3	100	0	150	1.5	1.2	0	0.6
4	100	0	150	1.5	1.5	0	0.6
5	75	25	150	1.5	1.2	0	0.6
6	50	50	150	1.5	1.2	0	0.6
7	25	75	150	1.5	1.2	0	0.6
8	100	0	150	1.5	1.2	0	0.3
9	100	0	150	1.5	1.2	0	0.9

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Measurements

Dynamic light scattering. The average particle size, particle size distribution, and polydispersity index (PDI) of latex particles at different polymerization stages were determined by dynamic light scattering (DLS, Zetasizer-ZS90, Malvern, UK) using a 90° laser scattering angle at 25 °C. The text samples were prepared as follows: the latex samples obtained at different reaction times were first diluted with DDI to obtain an appropriate concentration (ca. 1/10 of the initial concentration), and then the diluted latex samples were centrifuged at 3000 rpm (room temperature, 5 min) to separate the unreacted monomer. The ultimate latex samples were further diluted with DDI to reach a final concentration (1/10 000 of the initial concentration). Three measurements were carried out for each sample to calculate the average particle size. The centrifugation step was not necessary for the latex samples obtained with high monomer conversion yields. The Z-average particle size (d_z) and PDI were obtained directly. The zeta potential of latex particles was also obtained using DLS according to previous measurement method.¹

Transmission/scanning electron microscopy. The particle size, distribution, and shape of latex particles were observed by transmission electron microscopy (TEM, 1210, JEOL, Japan). The latex samples were diluted with DDI to around 2‰ and dropped on a silicon wafer/copper net coating with carbon, and dried by freeze drying. The dried samples were observed by TEM at 100 KV. The particle morphology was observed by field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) after the samples were gold sputtering.

Monomer conversion and particle number. Monomer conversion (X) was calculated according to the gravimetric method and was defined as the weight ratio of the polymer formed and initial monomers.²⁰ The particle number (N_p) was calculated according to the following equation:²¹where M₀ is the weight of the introduced monomer per liter of water, X is the monomer conversion determined by the gravimetrical method, ρ_p is the polymer particle density (1.044 g cm⁻³), and d_v is the average diameter of the latex particles.

Results

Particle size of ultimate latexes initiated by different initiators

In the current particle preparation technologies, macroemulsion polymerization is usually used to synthesize latex particles with ∼100 nm size.^17–19 The experimental results using KPS as the initiator according to the classical polymerization recipe are shown in Fig. 1(a). The average particle size and PDI of ultimate latex particles only attained to 84.56 nm and 0.058, respectively. This experimental result is in agreement with the previous reports.²² However, when AIBA was used as the initiator to replace KPS to initiate the macroemulsion polymerization reaction under same polymerization conditions (i.e., [I] = 0.6 wt%, [St] = 40 wt%, [SDS] = 1 wt%, [PCB] = 0.4 wt%), the average particle of ultimate latex particles attained to 170.6 nm. Moreover, the particle size distribution of latex particles shows a monodispersed distribution (Fig. 1(b)). With increasing AIBA concentrations from 0.6 wt% and 0.8 wt% to 1.0 wt%, the average particle size of ultimate latex particles also increased from 170.6 nm and 221.7 nm to 288.0 nm, respectively, as shown in Fig. 1(b)–(d). This experimental result is in contrast with the sequence of theoretical prediction by the Simith–Ewart theory,^23,24 in which the average particle size of the ultimate latex particles decreased with increasing initiator concentrations because of the increase in the particle number. In addition, we also carried out zeta potential measurements for ultimate latexes, the absolute value of zeta potential value presents a decreased trend as the addition of AIBA (run 1: −48.57 mV; run 2: −40.22 mV; run 3: −37.21 mV; run 4: −33.16 mV), indicting the addition of AIBA reduced the stability of latex particles. However, these results are larger than 30 mV, thus, the ultimate latexes using AIBA as initiator also are considered as stable latexes.

Fig. 1 TEM images of PS latex particles prepared with different initiator systems according to Table 1. Key: (a) 0.9 g KPS; (b) 0.9 g AIBA; (c) 1.2 g AIBA; (d) 1.5 g AIBA.

Evolution of particle size and morphology

In order to investigate the reasons of increased particle size of latex particles using AIBA as the initiator, the evolution of TEM/SEM images of latex particles as a function of polymerization time for 0.9 g AIBA initiator system was tracked as shown in Fig. 2. As expected, the latex particle size increases with polymerization time. Moreover, the distribution of latex particles also presents a relatively narrow size distribution in the entire polymerization reaction process. Notably, the initial average particle size of latex particles at 0.117 monomer conversion attained to 117.2 nm, which is much larger than the particle size of ultimate latex using KPS as the initiator. This is also an interesting experimental result. To exclude the effect of experimental errors, this polymerization reaction was repeated and the same results were obtained. In addition, SEM was used to observe this process, as shown in the insert figures in the Fig. 2. The SEM results confirmed that the average particle size of latex particles in the early polymerization period is much larger than 100 nm.

Fig. 2 TEM images of latex particles prepared by emulsion polymerization of St using 1.2 g AIBA as radical initiator at different polymerization times according to the 3rd recipe of Table 1. Key: (a): 10 min; (b) 20 min; (c): 30 min; (d): 60 min; (e) 120 min; (f): 300 min.

Evolution of particle size and number

Fig. 3 shows the evolution of average particle size and number as a function of polymerization reaction time for macroemulsion polymerization of St in different initiator systems. The evolution of average particle size and number for KPS and AIBA present a similar trend, in which the average particle size increased with polymerization time, and particle number first increased with polymerization time and then became constant after the particle nucleation. However, it is noted that the initial particle number of KPS initiator systems at 0.062 monomer conversion reached to 1.32 × 10¹⁸ L⁻¹, which is ∼100 times the one initiated by AIBA at the same monomer conversion. The final latex particle number also presents a similar trend.

Fig. 3 The evolution of Z-average particle size of latex particles (obtained from DLS) and particle number as a function of monomer conversion for the macroemulsion polymerizations of St according to the recipes 3 and 11 of Table 1 at different initiator systems. Key: (a): 1.2 g KPS; (b): 1.2 g AIBA.

Particle size of ultimate latexes prepared in different ionic strengths

In previous studies on the preparation of large sized latex particles by macroemulsion polymerization, electrolyte also was added to initial polymerization recipes to increase the ionic strength of aqueous phase.^3,4,9 The increase in the ionic strength of aqueous phase can compress the thickness of the electric double layer and further promote the particle coagulation, increasing the average particle size of the ultimate latex. Therefore, in this study, the effect of electrolyte concentration on the particle size of the ultimate latex was also investigated. Fig. 4 shows the TEM images of ultimate latex particles prepared by macroemulsion polymerization using 1.2 g AIBA as the initiator at different electrolyte concentrations. In contrast, the final latex particle size decreases with increasing electrolyte (potassium carbonate) concentrations from 301.1 nm and 188.4 nm to 175.2 nm, corresponding to 0.2, 0.4, and 0.6 wt%, respectively. Discussing the role of electrolyte in determining the particle size of the ultimate latex in the presence of AIBA is very interesting.

Fig. 4 TEM images of PS latex particles prepared using 1.2 g AIBA as initiator in different electrolyte concentrations ((a): 0.2 wt%; (b): 0.6 wt%). Note: the TEM images of latex particles using 1.2 g AIBA as initiator in 0.4 wt% electrolyte concentration was shown in Fig. 1(c).

Effect of monomer composition

It is well known that polystyrene latex particles are hard, because the glass transition temperature (T_g) is higher than the reaction temperature (65 °C). If soft monomer such as butyl acrylate is added to the polymerization recipe, the copolymer of St and BA shifts affords soft latex particles at the same reaction temperature, affecting the particle growth. Some BA was added to the initial polymerization recipe, and the effect of monomer composition on ultimate latex particle size was investigated. Unfortunately, clear TEM images were not obtained for the systems with BA/St ratios larger than >50/50. This is attributed to the low film forming temperature of copolymer latexes. Thus, DLS was used to analyze the particle size distribution of the ultimate latex as shown in Fig. 5, and the effect of monomer composition on the particle size distribution of ultimate latex particles was investigated. The average particle size of ultimate latex particles increased from 246.5 nm and 255.2 nm to 276.5 nm at the BA/St ratios of 25/75, 50/50, and 75/25, respectively. Moreover, the latexes showed a narrow particle size distribution irrespective of varying BA/St ratios, because the PDI corresponding to all the systems were much smaller than 0.1.

Fig. 5 The particle size distribution of ultimate latexes prepared using 1.2 g AIBA as initiator at different BA/St monomer ratios ((a): BA/St = 25/75; (b): BA/St = 50/50; (c): BA/St = 75/25).

Discussion

It is difficult to explain the increase in the particle size of ultimate latex particles using macroemulsion polymerization theories in the presence of cationic AIBA initiator. Therefore, the particle formation and growth process in macroemulsion polymerization were reviewed using KPS as the anionic radical initiator. In the macroemulsion polymerization, the surfactant concentration is larger than the critical micelle concentration (CMC) (CMC = 0.151 g L⁻¹ in water ²⁵), and the particle nucleation mainly occurs in the interior of the micelles. The detail polymerization process can be described as follows: the anionic initiator first decomposes into primary radicals in the aqueous phase, then the primary radicals react with monomer such as styrene dissolved in the aqueous phase to form oligomeric radicals. The oligomeric radicals are captured by swollen micelles and further initiate the monomer polymerization in the micelle interior to form polymer latex particles. The end of the particle nucleation was marked by the depletion of all the uninitiated micelles. The particle growth was carried out in the particle/micelle interior, and monomer droplets functioned as monomer. In this process, the particle number was constant until the completion of the reaction. When monomer droplets disappeared, the second interval of the polymerization reaction finished. After that, the polymerization reaction only occurred within the particle interior.

Compared to initiator KPS, AIBA is not only a water soluble radical initiator, but also a cationic initiator. When KPS was replaced by AIBA, the AIBA initiator also decomposed into primary radicals and reacted with St dissolved in the aqueous phase to form oligomeric radicals with positive charges. When positive charged oligomeric radicals were captured by SDS micelles, the positive charges of AIBA initiator chain ends shield the negative charges of swollen micelles, further decreasing the electrostatic repulsion among swollen micelles. Empty micelles first dissolve into surfactant molecules to stabilize swollen micelles. However, with the progress of the polymerization reaction, more and more oligomeric radicals were captured by swollen micelles, enhancing the positive charges for anionic surfactant SDS molecules; therefore, primary particle coagulation occurred. The primary particle coagulation reduced the particle number and further increased the particle size. The process that positive charged oligomeric radicals shielded negative charged SDS was called in situ charge neutralization. It is noted that the positive charges of initiator ends not only adsorbed on the particle surface, but also wrapped in particle interior, resulting in obtaining a negative zeta potential value in run 4. The detailed process of in situ charge neutralization is shown in Fig. 6.

Fig. 6 Schematic representation of primary particle coagulation induced by in situ charge neutralization in macroemulsion polymerization using AIBA as initiator.

Because in situ charge neutralization promotes primary particles/swollen micelles, the initial particle number reduced, forming large sized latex particles in the early polymerization period as shown in Fig. 2. In addition, the decrease in the particle number in the early nucleation period limits particle coagulation in the particle growth period and narrowed the particle size distribution of the ultimate latex particles. This was confirmed by the evolution of particle size and number as shown in Fig. 3. With increasing AIBA concentrations, the positive charges enhanced, further increased the extent of primary particle coagulation, resulting in enlarged particle size, as shown in Fig. 1. In addition, the in situ charge neutralization decreased the thickness of the electric double layer, thus, the addition of electrolyte did not play a critical role in determining the thickness of the electric double layer. In contrast, the addition of electrolyte decreases the CMC of SDS molecules and increases the number of the micelles. As a result, the particle size of the ultimate latex decreased with the addition of electrolyte as shown in Fig. 4.

When BA as a comonomer was added to the initial polymerization recipe, the copolymerization process of St and BA was similar to the homopolymerization of St. When the BA/St ratio reached to 50/50, the latex particles were soft at the same reaction temperature, enhancing the extent of particle coagulation in the early nucleation period. Therefore, the average particle size of latex particle increased with increasing BA/St ratios.

Conclusions

Cationic AIBA initiated macroemulsion polymerization was developed for preparing sub-300 nm, monodispersed polymer latex particles with 40 wt% solid content. Primary particle coagulation nucleation was proposed as a novel nucleation mechanism to explain the role of cationic initiator AIBA in determining the particle size distribution of the ultimate latex. As the oligomeric radicals with positive charges were captured by negatively charged SDS micelles, in situ charge neutralization occurred in the swollen micelle/primary particle interior, further decreasing the electrostatic repulsion between primary particles. As a result, primary particle coagulation occurred. The particle coagulation in the early nucleation period decreased the particle number and increased the average particle size. With increasing AIBA concentrations, the charge neutralization function enhanced, increasing the particle size of the ultimate latex increased with increasing initiator AIBA concentrations. In addition, the primary particle coagulation was also controlled by the monomer composition. Soft monomer such as BA was favorable in promoting primary particle coagulation and preparing large sized latex particles.

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The authors appreciate the financial support from the National Natural Scientific Foundation of China (No. 51573022).

Notes and references

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