Regulating the size and molecular weight of polymeric particles by 1,1-diphenylethene controlled soap-free emulsion polymerization

Abstract

We herein report a facile soap-free emulsion polymerization (SFEP) method to prepare living particles with size and molecular weight that could be easily regulated. In this method, commercially available, odorless, and colorless 1,1-diphenylethene with no known toxicity was introduced to decrease the size and molecular weight of the particles. Using styrene as a model monomer, a series of monodisperse PS particles with a mean diameter ranging from 89 nm to 307 nm and a molecular weight ranging from 613 g mol⁻¹ to 11 760 g mol⁻¹ were produced via adjusting the amount of the monomer and DPE added into the SFEP system.

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1. Introduction

Soap-free emulsion polymerization (SFEP) has been extensively used to fabricate polymeric particles and hybrid particles with various morphologies and structures including conventional homogeneous particles,^1–3 core–shell particles,^4–6 yolk–shell particles,⁷ hollow particles,^8–10 asymmetric particles^11–13 and raspberry-like particles.^14–16 Owing to the monodispersity, ultraclean surface and broad applications of polymeric particles in biotechnology,^17–19 medical science,^20–22 energy,^6,23 environmental engineering^24–26 and information technology,^27–29 SFEP is still being used to develop various particles for either practical use or fundamental research. In relation to fundamental research, the focus of SFEP is to produce gas-responsive particles and regulate the size of particles. In order to separate colloidal particles from an emulsion in an environmentally friendly manner for the sustainability of the industry, Zhang et al.^30,31 synthesized an amidine-functionalized comonomer which could reversibly form amidinium bicarbonate with carbon dioxide to prepare N₂/CO₂ triggered reversibly coagulatable and redispersible latexes by SFEP. Meanwhile Fischer et al.³² utilized a monomer with a polymerizable methacrylic acid group and a carboxylate functionality to prepare particles that were able to undergo reversible coagulation and redispersion triggered by CO₂ and NH₃, respectively.

Particles produced by SFEP are usually submicron-sized, while in many cases, particles with a size larger than 1 μm or smaller than 100 nm have to be prepared by SFEP to fulfill specific applications. Yamamoto et al.^33,34 found that micron-sized particles could be synthesized in a soap-free emulsion system using an oil-soluble initiator and electrolyte. Similarly, Shibuya et al.³⁵ prepared micron-sized polystyrene (PS) particles in the presence of a high monomer concentration and strong base such as KOH. Meanwhile, in some situations, smaller particles need to be prepared using SFEP. Copolymerization with a hydrophilic monomer,¹ microwave irradiation^36,37 and ultrasonic irradiation^38,39 are the common approaches to decrease the particle size. Nevertheless, introducing a hydrophilic monomer into the SFEP system often led to heterogeneous structures where the hydrophilic components gathered toward the exterior of the particles and the hydrophobic components gathered in the interior of the particles.⁴⁰ SEFP that occurs under microwave and ultrasonic irradiation could efficiently decrease the particle size to dozens of nanometers, but this is unlikely to be economically applicable at an industrial scale since additional specialized equipment is needed. SEFP in a water–acetone mixture is another method to decrease the particle size, however, as high as a 50% volume fraction of acetone made this method environmental unfriendly.³ Very recently, Konno et al.⁴¹ reported that PS nanoparticles with a mean diameter less than 100 nm were synthesized via SFEP using an amphoteric initiator in the presence of submillimolar concentrations of an emulsifier. Sajjadi⁴² provided a monomer-starved semicontinuous SFEP method to achieve small particles, which is very promising in fabricating nanosized particles at an industrial scale. However, the aforementioned SEFP methods failed to control the molecular weight of the obtained particles.

Due to the success in the controllability of the polymer composition, topological structure, molecular weight and molecular weight distribution, controlled/living radical polymerizations especially atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer (RAFT) polymerization have been exploited to perform in a soap-free emulsion system. Stoffelbach et al.⁴³ reported the soap-free miniemulsion polymerization of methyl methacrylate (MMA) using an amphiphilic molecule as both the surfactant and initiator under an activator generated by the electron transfer ATRP, and PMMA particles with diameters of 142 nm were produced. Ganeva et al.⁴⁴ demonstrated that amphiphilic RAFT-capped diblock copolymers with a proper length of hydrophilic and hydrophobic blocks could serve as stabilizers in the SFEP of S and produce particles with diameters of 33 nm. Generally, controlled radical polymerizations in a soap-free emulsion system have been well understood in the aspects of applicability, controllability of the polymerization process and the mechanism of nucleation.^45–47 The size of the particles produced by controlled radical SFEP could be decreased to dozens of nanometers, which is difficult to achieve by conventional water-based SFEP. However, it is impossible to ignore that the toxic and colored catalysts of ATRP cannot be removed from the polymers properly, and the RAFT agents are not commercially available and give their reaction products an unpleasant odor and color. Meanwhile, as a commercially available, odorless, colorless agent with no known toxicity, 1,1-diphenylethene (DPE) has been found to control the polymerization process, which was first reported by Wieland et al.⁴⁸ After that, a number of papers were carried out for applying DPE controlled radical polymerization to an emulsion system for industrial applications.^49–51 Our group developed DPE seeded emulsion polymerization and showed that it is a green and convenient way to prepare latex with a high solid content.^52–55 However, up to now, research in regulating the size and molecular weight of polymeric particles by DPE controlled SFEP is still absent.

Herein we report the regulation of polymeric particle size and molecular weight by DPE controlled soap-free emulsion polymerization. The morphologies, sizes and molecular weights of particles produced by this method were systematically studied by comparing with SFEP without DPE. By choosing styrene as the model monomer, monodisperse particles with mean diameters less than 100 nm were obtained.

2. Experimental section

2.1 Materials

Styrene (S, 99%, J&K Scientific Ltd.) was purified by distilling under a reduced pressure and stored in a refrigerator prior to use. Potassium persulfate (KPS, 99%, Sinopharm Chemical Reagent Co., Ltd.) was purified by recrystallization from water. 1,1-Diphenylethylene (DPE, 98%, Tokyo Chemical Industry Co., Ltd.) was used directly as received without further purification. Deionized water was used throughout the whole experiment.

2.2 Particle synthesis

The detailed experimental conditions in this work are listed in Table S1.† The particles were prepared as follows: monomers and water (80 mL) were added into a three-neck reactor with a stirrer and reflux condenser. The mixture was stirred at a speed of 200 rpm and heated to 80 °C. Then KPS in 20 mL water was added and stirred for 12 h to produce polymeric particles.

2.3 Characterization

Morphologies of PS particles were observed using scanning electron microscopy on a JSM 6700F (SEM, JEOL, Japan) and transmission electron microscopy on a JEM 2010 (TEM, JEOL, Japan). PS particles for SEM were deposited onto a clean silicon wafer and sputtered with platinum by a JFC-1600 auto fine coater at a current of 20 mA for 180 s before examination. PS particles for TEM were dropped on a copper grid and the water was evaporated at room temperature, leaving the particles on the copper grid. The size distribution of the particles was determined by a laser particle analyzer (DLS, LS13320, Beckman Coulter Instruments). Conversion was determined gravimetrically. Size exclusion chromatography (SEC) was carried out using a Waters 1515 GPC system with a Waters 2414 differential refraction detector and two Styragel HT THF columns. A six-point calibration curve was obtained with polystyrene standards and was used to obtain molecular weights and the polydispersity index (PDI) of the polymers. THF was used as the mobile phase with a flow rate of 0.5 mL min⁻¹. The conductivity of the particle suspensions was measured using a DDSJ-308F conductivity meter (Shanghai REX Instrument Factory). The zeta potential of the particle suspensions was assayed using a Beckman Coulter Delsa™ Nano C particle analyzer. All the zeta potential values were determined by averaging the values measured at least five times.

3. Results and discussion

Fig. 1 presents the SEM and TEM images of PS produced by SFEP with different amounts of styrene with (Fig. 1a–e, runs 1–5 in Table S1†) and without DPE (Fig. 1a′–e′, runs 9–13 in Table S1†). It can be seen that PS in all cases aggregated and generated spherical particles except PS produced with 0.5 g of the monomer. The sizes of all these spherical particles increased with an increasing amount of added monomer into the SFEP system regardless of the existence of DPE. Meanwhile the size of PS produced with DPE was much smaller than that without DPE. The size distributions of PS in Fig. 2a and b and the coefficient of variation of the particles in Fig. S1† measured using DLS confirmed that monodisperse particles were obtained in the SFEP of S. The mean diameters of PS measured using DLS were about several to a dozen nanometers larger than that collected by SEM and TEM images, which was mainly attributed to the shrinkage of PS particles during the drying process of SEM and TEM sample preparation, while the DLS measured particles contained a hydrated layer around them in the aqueous medium. When no DPE was added into the soap-free emulsion system, the mean diameter of PS increased from 106 nm to 565 nm as the monomer increased from 0.5 g to 7.0 g. Meanwhile, the mean diameter of PS only increased from 89 nm to 307 nm as the monomer increased from 1.0 g to 7.0 g in the cases with DPE (Fig. 2c). Therefore, the introduction of DPE into the soap-free emulsion system could dramatically decrease the size of the particles in the hundreds of nanometers.

Fig. 1 SEM and TEM images of PS particles with 0.03 g DPE and different amounts of S: (a) 0.5 g, (b) 1.0 g, (c) 3.0 g, (d) 5.0 g, and (e) 7.0 g, and PS particles without DPE and different amounts of S: (a′) 0.5 g, (b′) 1.0 g, (c′) 3.0 g, (d′) 5.0 g, and (e′) 7.0 g.

Fig. 2 Size distributions of PS particles (a) with and (b) without DPE and different amounts of S and (c) the corresponding mean diameters.

It has been proven that DPE could effectively reduce the polymerization rate and conversion rate in bulk and emulsion polymerization systems.^48,55 Fig. 3 presents the conversion of the monomer with and without DPE in our SFEP system. The polymerization rate of S in the presence of DPE was much slower than that in the absence of DPE. And the ultimate conversion of styrene with DPE was 78.9%, which was also much lower than that without DPE. This could be attributed to the essential function of DPE in a radical polymerization system where DPE served as a chain transfer agent and combined with propagating radicals to form DPE-capped dormant species, reducing the concentration of living radicals and suppressing the polymerization rate. In a SFEP system, the reduction of the polymerization rate in particles was beneficial for rapidly achieving equilibrium between the monomers swelling in the particles and the monomers dissolved in the water, which reduced the conversion of styrene. In this work, the styrene was a hydrophobic monomer and the formation of PS particles followed a micellization-type mechanism,⁵⁶ when the amount of KPS was constant, the number of micelles formed by short-chain free radical oligomers was constant. Therefore, when the conversion of the monomer was reduced by DPE, the size of particles with DPE would be much smaller than that without DPE (Fig. 2). Meanwhile as the amount of feeding S increased, there was a much larger amount of monomers tending to swell in the particles; then more monomers would polymerize to generate PS, resulting in a larger size (Fig. 2c).

Fig. 3 Conversion of PS versus time in the presence and absence of DPE. The amount of styrene was 7.0 g.

In a soap-free emulsion polymerization system, electrostatic interactions between particles, including the ionic strength of the system and zeta potential of particles, also played a very important role in determining the size of the particles.^42,57 Fig. 4a presents the zeta potentials of particles diluted by a top clear layer obtained by centrifuging the particle suspensions produced with 7.0 g of the monomer in the absence (run 13 in Table S1†) and presence (run 5 in Table S1†) of 0.03 g DPE, which guaranteed that the diluted particle suspensions for the zeta potential measurement were in the same environment as the particles in the SFEP suspensions. By comparing the zeta potentials of PS particles without the removal of ions in the polymerization suspensions in the presence and absence of DPE (Fig. 4a), it could be found that under the same ionic strength (indirectly characterized by measuring conductivity in Fig. S2†), in the early stage of polymerization, the zeta potential of particles with DPE was much lower than that without DPE, which meant that the particles with DPE produced in the early stage of polymerization were more stable than that without DPE. This could be attributed to the fact that polymerization without DPE in the early stage produced more particles than with DPE (Fig. 4b) since DPE was able to effectively reduce the polymerization rate. Thus, the number of in situ produced emulsifiers that each particle without DPE could possess was much lower than the particles with DPE. The less stable particles without DPE were prone to aggregate with each other, forming larger particles and reducing the number of particles. As the polymerization continued, the number of particles in each SFEP system tended to be constant. The zeta potential of PS particles without DPE was finally lower than that with DPE, it was probably because the number of particles without DPE was smaller than that with DPE. The zeta potentials of cleaned particles after removing free movable ions (Fig. S3†) showed a very similar tendency as the uncleaned particles except that the zeta potential of cleaned particles without adsorbed emulsifiers was much higher.

Fig. 4 (a) The zeta potentials of PS colloidal suspensions and (b) N_p of particles in 1 milliliter in the presence (run 5 in Table S1†) and absence of DPE (run 13 in Table S1†); the insets of (a) and (b) show the magnified plots of zeta potentials and N_p of particles in 1 milliliter, respectively.

By combining the data of the conversion and mean diameter (Fig. S4†) of PS versus time in the presence and absence of DPE, the number of particles (N_p) in each milliliter was calculated by the following formula:⁵⁸

N_p is the average number of particles in 1 mL polymer suspensions, W is the weight of the monomer added into the system, C is the conversion of the monomer in Fig. 3, ρ is the density of the polymer, the density of PS is 1.044 g cm⁻³,⁵⁹ d is the mean diameter of the particles (Fig. S4c†), and V is the total volume of the polymer suspensions.

The variation of N_p in the polymerization of 7.0 g of the monomer in the absence (run 13 in Table S1†) and presence (run 5 in Table S1†) of 0.03 g DPE at different times is presented in Fig. 4b. The N_p of PS particles without DPE in the early stage of polymerization was 1.27 × 10¹⁵. While the N_p of PS particles with DPE was 8.61 × 10¹⁴, which was much smaller. Owing to the inadequate surface-bound ions, the zeta potential of the particles was not low enough to prevent the aggregation of the particles. Therefore, both the N_p of particles with and without DPE decreased as the polymerization continued. Even so, the N_p of particles with DPE was 3.53 × 10¹³ and much higher than that without DPE, which was the main reason that the zeta potential of particles with DPE was higher than that without DPE. In the DPE controlled SFEP, the conversion was lower than that without DPE (Fig. 3), along with a relatively larger number of particles, thus, all these factors contributed to a smaller particle size in the presence of DPE.

The molecular weights and corresponding molecular weight distributions of PS were measured by SEC and presented in Fig. 5. As the monomer in the soap-free emulsion system increased, the SEC curves in Fig. 5a and b shifted to a high molecular weight direction and became unimodal, which indicated that the molecular weight of PS increased with increasing amount of the feeding monomer. The number-average molecular weight (M_n) calculated from the SEC curves in Fig. 5c clearly revealed that DPE could sharply decrease the molecular weight of PS, and the living particles with low molecular weights were conducive to serving as seeds and absorbing the monomer to fabricate particles with various morphologies.⁶⁰ The M_n of PS produced with DPE increased to 11 760 g mol⁻¹ as the amount of the monomer increased to 7.0 g, whereas the M_n of PS without DPE was 37 410 g mol⁻¹. The increase of M_n with the increase of the monomer could be explained by the equation proposed by Chiu et al.,⁶¹ where the degree of polymerization was proportional to the concentration of the monomer but inversely proportional to the concentration of DPE. It was noteworthy that the M_n of PS produced with 0.5 g styrene and 0.03 g DPE was only 613 g mol⁻¹, which meant that the number-average degree of polymerization (X̄_n) was 5.9. According to the calculation result of Vanderhoff,⁶² the critical X̄_n of PS to form primary polymer particles was 7.3. When the X̄_n was smaller than 7.3, the PS was a surface-active sulfate oligomer and soluble in water, thus the PS produced with 0.5 g styrene and 0.03 g DPE could not aggregate and form spherical particles, resulting in a thin film as shown in SEM and TEM images (Fig. 1a). The size of PS with X̄_n equal to 7.3 was 1.4 nm as calculated by Vanderhoff,⁶² and the size of PS with X̄_n equal to 5.9 produced in this work was 1.2 nm as measured using DLS (Fig. 2a), which was very close to the size of PS with a critical length. The molecular weight distribution (PDI) of the PS produced with DPE varied irregularly from 1.07 to 2.96 with alterations in the amount of the feeding monomer, but in most cases, the PDIs of PS with DPE was smaller than that without DPE (Fig. S5†). The relatively big difference in the molecular weights between the oligomers formed in the later stage of polymerization and polymers formed in the earlier stage of polymerization was probably the reason why the elution curve of the ultimate product showed a tailing peak, which broadened the PDI of the polymer and made the PDIs vary irregularly.

Fig. 5 SEC elution curves of PS (a) with and (b) without DPE and (c) the corresponding number-average molecular weights.

Fig. 6 shows the SEM and TEM images of PS particles produced by the polymerization of 3.0 g S with (Fig. 6a–d, runs 3 and 6–8 in Table S1†) and without (Fig. 6a′–d′, runs 11 and 14–16 in Table S1†) different amounts of DPE. Monodisperse and spherical particles were obtained in all cases. The particle size decreased with the increase of DPE and KPS added into the soap-free emulsion. Similar to the PS particles produced with different amounts of the monomer but constant DPE, the size of PS particles produced with DPE was also much smaller than that without DPE. The size distributions of PS with and without DPE in Fig. 7a and b demonstrated that all of these particles possessed a monodisperse and narrow size distribution. The mean diameter of PS particles with DPE decreased from 228 nm to 157 nm as the DPE increased from 0.03 g to 0.09 g, while the mean diameter of PS particles without DPE decreased from 419 nm to 291 nm as KPS increased from 0.05 g to 0.15 g (Fig. 7c). When the amount of KPS is increased, there would be an increasing number of micelles, and more particles were then produced, resulting in a decrease in the size of each particle. Thus, increasing the amount of DPE in a SFEP system could effectively decrease the particle size without reducing the monodispersity of the particles (Fig. S6†).

Fig. 6 SEM and TEM images of PS with different amounts of DPE: (a) 0.03 g, (b) 0.05 g, (c) 0.07 g and (d) 0.09 g, and PS without DPE but with different amounts of KPS: (a′) 0.05 g, (b′) 0.08 g, (c′) 0.12 g and (d′) 0.15 g.

Fig. 7 Size distributions of (a) PS with 3.0 g of the monomer and different amounts of DPE, (b) PS with 3.0 g of the monomer and different amounts of KPS, and (c) the corresponding mean diameters.

The SEC elution curves in Fig. 8a and b are unimodal and shifted to the low molecular weight direction with increasing amounts of DPE and KPS, indicating a decrease in the molecular weight. The introduction of DPE into the soap-free emulsion system prolonged the retention time for about 0.5–2 min compared with PS without DPE, which meant that PS with DPE had a smaller molecular weight than that without DPE. The M_n calculated from the SEC elution curves in Fig. 8c demonstrated that the molecular weight of PS with DPE decreased from 5938 g mol⁻¹ to 1176 g mol⁻¹ as DPE increased from 0.03 g to 0.09 g, while the molecular weight of PS without DPE decreased from 34 760 g mol⁻¹ to 6805 g mol⁻¹ as KPS increased from 0.05 g to 0.15 g. The decrease of M_n was attributed to the increasing amount of primary free radicals, when the feeding monomer was constant, then the amount of the monomer added to each primary free radical was decreased. And because of the lower conversion resulting from DPE, the M_n of PS with DPE would be much smaller than that without DPE. The PDIs of PS with DPE and without DPE also showed irregular variations with an increase of KPS (Fig. S7†).

Fig. 8 SEC elution curves of (a) PS with 3.0 g of the monomer and different amounts of DPE, (b) PS with 3.0 g of the monomer and different amounts of KPS, and (c) the corresponding number-average molecular weights.

4. Conclusions

By combining the mechanism of SFEP and DPE controlled radical polymerization, the influences of the amount of the feeding monomer, DPE and KPS, on the morphology, size and molecular weight of PS particles were investigated. A series of monodisperse PS particles with a mean diameter ranging from 89 nm to 307 nm and a molecular weight ranging from 613 g mol⁻¹ to 11 760 g mol⁻¹ was produced via altering the amount of the monomer and DPE added into the SFEP system. In conclusion, this work provides a facile method to produce living, monodisperse and ultraclean particles whose size and molecular weight could be easily regulated, which is applicable to precursor particles for seeded polymerization to produce particles with various morphologies.

Acknowledgements

This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant No. 2012AA02A404), the Key Program of the National Natural Science Foundation of China (Grant No. 51433008), the General Program of the National Natural Science Foundation of China (Grant No. 51173146) and the Basic Research of Northwestern Polytechnical University (Grant No. 3102014JCQ01094, JC20120248, 3102014ZD).

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental conditions, coefficient of variation and molecular weight distribution of PS particles. See DOI: 10.1039/c5ra17156c

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