Particle Size Control in Miniemulsion Polymerization via Membrane

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Particle Size Control in Miniemulsion Polymerization via Membrane Emulsification Nida Nauman,†,§ Neomy Zaquen,†,‡,∥ Tanja Junkers,∥,⊥ Cyrille Boyer,†,‡ and Per B. Zetterlund*,† †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering and ‡Australian Centre for Nanomedicine, The University of New South Wales, High Street Gate 2, Kensington, 2033 Sydney, NSW, Australia § Department of Polymer and Process Engineering, University of Engineering and Technology, G.T. Road, 54890 Lahore, Punjab, Pakistan ∥ Institute for Materials Research (IMO-IMOMEC), Universiteit Hasselt, Agoralaan Building D, B-3590 Diepenbeek, Belgium ⊥ Polymer Reaction Design Group, School of Chemistry, Monash University, 19 Rainforest Walk, VIC 3800 Melbourne, Australia

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S Supporting Information *

ABSTRACT: Miniemulsion polymerization of methyl methacrylate has been conducted employing Shirasu porous glass (SPG) membrane emulsification for the generation of the initial miniemulsion. For the first time, submicron-sized monomer droplets and polymer particles have been prepared using membranes with pore sizes significantly smaller than those previously reported. Membrane pore sizes of diameters 100−400 nm were explored, demonstrating that the final particle size can be conveniently tuned within the diameter range of 250−1600 nm. The choice of radical initiator is crucial: a sufficiently hydrophobic initiator (lauroyl peroxide) is required to minimize the generation of bimodal particle size distributions via secondary nucleation. Given the advantages of low energy consumption, reduced shear stress (compared with conventional high-energy mixing approaches such as ultrasonication), and an easily adjustable particle size via the membrane pore size, membrane emulsification has significant potential for the synthesis of polymeric nanoparticles via miniemulsion polymerization.

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INTRODUCTION Synthesis of polymeric nanoparticles via radical polymerization in aqueous dispersed systems is of great importance in industry as well as in academia. Polymeric nanoparticles find major applications in paints/coatings1−3 and adhesives,4,5 but also in a wide spectrum of more sophisticated applications ranging from electrically conducting materials to optoelectronics and smart drug delivery, encapsulation,6−9 and biosensors. In general terms, a specific application is associated with a unique set of prerequisite polymer nanoparticle properties, and these properties are in turn to a large extent dictated by the mode of synthesis.10−12 The monomer type, particle size, and morphology are some of the major factors governing the choice of the synthesis method.12 Emulsion polymerization1,13 has been the main focus of industries for decades. Emulsion polymerization requires diffusional transport of monomers from micron-sized monomer droplets through a continuous aqueous phase to polymer particles that constitute the polymerization locus. Miniemulsion polymerization,14,15 also known as nanoemulsion polymerization, represents an alternative choice for polymer nanoparticle synthesis that is associated with some distinct advantages relative to emulsion polymerization. Due to the fact that miniemulsion polymerization entails direct transformation of monomer droplets to © XXXX American Chemical Society

polymer particles with no requirement for material transport across the aqueous phase, the technique offers access to hollow particles,16,17 organic/inorganic hybrid particles,15,18,19 and a relatively straightforward implementation of reversible deactivation radical polymerization.20−23 Miniemulsion polymerization proceeds via droplet nucleation of submicron-sized droplets typically in the range of 50−500 nm, normally obtained through droplet generation techniques such as ultrasonication, high-pressure valve homogenization, and static mixers.24−26 These techniques involve high shear forces, which are not suitable for some more-sensitive materials. Furthermore, the emulsification conditions of these techniques are challenging to precisely regulate, which results in the difficulty in controlling the droplet size, leading to broad droplet size distributions. Moreover, the scale up of these emulsification techniques is problematic because of their poor reproducibility as well as the high energy consumption.12,27 Various lowenergy emulsification approaches, including phase-inversion temperature, emulsion inversion point, and the in situ surfactant generation technique have been reported.28−30 Received: March 5, 2019 Revised: April 14, 2019

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DOI: 10.1021/acs.macromol.9b00447 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules However, these approaches are typically also associated with issues related to particle size control. Membrane emulsification represents an alternative choice to high-energy droplet disruption methods. The technique involves the generation of droplets by passing the dispersed phase through a membrane comprising pores of a well-defined size into the continuous phase.31 This technique, which was first introduced by Nakashima et al.32 using Shirasu porous glass (SPG) membranes, enables one to control the droplet size by a judicious choice of the membrane pore size. Vladisavljević et al.,33 Oh et al.,3434 and Yuyama et al.35 investigated the effects of process parameters such as the membrane type, the average pore size and porosity, shear stress, temperature, transmembrane pressure, and emulsifier/ surfactant. Other reports have focused on specific aspects related to the process, such as stirred vessel membrane emulsification36 and premix membrane emulsification.37 Agner et al.38 compared the two-stage premix membrane emulsification and other emulsification devices for preparing submicronsized droplets of methyl methacrylate (MMA). Besides the low energy consumption, membrane emulsification provides the benefits of easy droplet size manipulation via the membrane pore size and improved reproducibility; these advantages are of utmost significance in the synthesis of polymeric nanoparticles via miniemulsion polymerization. However, to date, membrane emulsification accompanied by subsequent dispersed-phase polymerization has only been reported for the synthesis of large polymer particles in the 5−20 μm range in the form of suspension polymerization,39−45 that is, significantly above the particle size range typically associated with miniemulsion polymerization. In the present work, SPG membrane emulsification has been used for the first time in combination with miniemulsion polymerization for tuning the size of polymer particles as small as 200 nm in diameter. Radical polymerization of MMA has been performed in miniemulsion based on the generation of the initial miniemulsion via a single-stage direct SPG membrane emulsification involving membrane pore sizes markedly lower (100−400 nm) than those hitherto reported for the synthesis of polymer particles. Particular attention has been paid to the particle nucleation mechanism within the context of the nature of the radical initiator employed. It has been demonstrated that miniemulsion polymerization can be conducted successfully using membrane emulsification, tuning the polymer particle size in the range of 200−1600 nm by selecting the appropriate pore size.

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Figure 1. Schematic illustration of SPG membrane emulsification. Reprinted (adapted) with permission from ref 46 Copyright 2017 Royal Society of Chemistry. The organic phase consisted of the monomer, hexadecane, and an oil-soluble initiator (LPO, BPO, or AIBN), and the continuous phase comprised water and SDS. All miniemulsion formulations were based on 88 g of water, with 6 wt % of MMA relative to water and 10 wt % of hexadecane relative to MMA (Table 1). The two phases were first

Table 1. Recipes of Miniemulsions Prepared via Membrane Emulsificationa run

initiatorb (mol/L)

SDSc (wt %)

pore size (nm)

LPO 1 2 3 4 5 6 7

0.0165 0.0165 0.0165 0.01 0.01 0.01 0.01

8 9

0.01 0.01

10 11

0.01 0.01

2.75 2.75 2.75 7.25 7.25 7.25 7.25

100 200 300 100 200 300 400

7.25 7.25

100 400

7.25 7.25

100 400

BPO

AIBN

EXPERIMENTAL SECTION

a

Recipes based on 88 g of water, 6 wt % of MMA relative to water, and 10 wt % of hexadecane relative to MMA. bConcentration relative to the organic phase. cwt % relative to the organic phase.

Materials. Methyl methacrylate (MMA, 99%; Sigma-Aldrich) was purified by passing through an aluminum oxide column to remove the inhibitor. 2,2′-Azobisisobutyronitrile (AIBN, 98%; Sigma-Aldrich) was purified by recrystallization in methanol. Benzoyl peroxide (BPO, Sigma-Aldrich), lauroyl peroxide (LPO, 97%; Sigma-Aldrich), sodium dodecyl sulfate (SDS, 98%; Sigma-Aldrich), and hexadecane (HD, 99%; Sigma-Aldrich) were used as received. Deionized water was used for all experiments. Emulsification. Cylindrical hydrophilic SPG membranes (SPG Technology, Japan) of different pore sizes (100, 200, 300, and 400 nm) with an outer diameter of 10 mm and a length of 20 mm were used with an external pressure micro kit. Before emulsification, the SPG membrane was wetted by sonication in water for 30 min and, subsequently, clamped with O-rings and screwed into the membrane module of stainless steel. Figure 1 shows a schematic illustration of the SPG membrane emulsification setup.

prepared separately at room temperature. The membrane module setup was placed in a jar containing the continuous phase such that the upper part of the membrane was 1 mm below the water level (Figure S1). The continuous phase was subjected to magnetic stirring at 190 rpm. Prior to addition of the dispersed phase, the SPG module was checked for air leakage by applying nitrogen pressure to the empty dispersed-phase tank. The organic phase was subsequently transferred to the pressure tank while keeping the vertical vent tube (Figure 1) opened to remove any entrapped air. The organic-phase tank and vent tube were then closed and pressurized with nitrogen gas to allow droplet formation to take place. After the organic-phase tank was emptied, the resulting B


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Macromolecules emulsion was collected. All membrane emulsifications were performed at room temperature using a fixed volume of the dispersed phase of 5.8 mL. Polymerization. After completion of the membrane emulsification process, 10−15 mL of emulsion was transferred to a glass bottle (25 mL) and sealed with a rubber septum, parafilm, and copper wire. The emulsion was degassed with nitrogen for 20 min in an ice bath. The rubber septum was further sealed with vacuum grease, and polymerization was subsequently conducted at 70 °C in a preheated oil bath for 24 h. Samples of 1 mL ([MMA] 0.099 mol/L) were withdrawn at regular intervals, followed by the addition of one drop of 1 wt % aqueous solution of hydroquinone to prevent further polymerization. Characterization. The monomer droplet and polymer particle sizes were measured using dynamic light scattering (DLS) on a NanoZS instrument (Malvern). Measurements were conducted at the backscattering angle of 173° using a 633 nm laser with each data point being the average of three independent measurements performed in automatic mode (11 runs, each of 10 s). All samples were prepared by adding one or two droplets of emulsion to 1 mL of deionized water in disposable cuvettes to avoid multiple scattering effects. All measurements were conducted at 25 °C, with a 60 s equilibration period prior to each set of measurements. Molecular weights and molecular weight distributions (MWDs) of the polymer were determined by a gel permeation chromatographer (GPC) using a Shimadzu modular system with tetrahydrofuran as the eluent. The GPC was equipped with an LC-10ATVP solvent delivery system, an SIL-10ADVP auto sample injector, a CTO-10ACVP Shimadzu column oven, and an RID-10A Shimadzu refractive index detector. The setup also comprised a PLgel 5.0 μm bead-size guard column, followed by three linear PLgel 5.0 μm (103, 104, and 105 Å) columns. Calibration was conducted relative to PMMA standards with molecular weights of 500−106 g/mol. Transmission electron microscopy (TEM) was performed using a JEOL 1400 TEM instrument at the voltage of 100 V. The TEM samples were prepared by casting the miniemulsion (one or two droplets) onto a formvarcoated copper grid. Monomer-to-polymer conversions were calculated gravimetrically by withdrawing 1 mL of polymer latex from the reaction mixture in weighed aluminum dishes and drying for 24 h at room temperature, followed by 24 h of vacuum drying at 40 °C.

Figure 2. Effect of the pore size on the critical pressure (Pc) for 2.75 (runs 1−3; Table 1) and 7.25 wt % (runs 4−7; Table 1) SDS.

Increasing the SDS amount from 2.75 wt % (run 1) to 7.25 wt % (run 4) for a pore size of 100 nm led to a decrease in Pc from 390 to 320 kPa, consistent with the presence of SDS facilitating droplet disruption. The effect of SDS concentration on Pc decreased with increasing pore size. Droplet formation requires a disruptive force significantly greater than the interfacial tension to overcome the droplet retention at the pore site.31,47−49 Miniemulsion Stability during Membrane Emulsification. Table 1 shows the recipes and SPG pore sizes for all MMA miniemulsions prepared in this study. To investigate the effect of the surfactant concentration on emulsion stability, the droplet size was measured using DLS at the beginning and completion of all membrane emulsifications at 2.75 and 7.25 wt % SDS (Table 2). For the smallest pore sizes of 100 and Table 2. Effect of Surfactant Concentration on the Emulsification Time and Droplet Sizea run

pore size (nm)

1 2 3 4 5 6

100 200 300 100 200 300

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RESULTS AND DISCUSSION Critical Pressure. SPG membrane emulsification involves the generation of a liquid dispersed phase (in this case the organic phase) by passage through the pores of a microporous membrane directly into the continuous phase (in this case the aqueous phase) by application of pressurized nitrogen gas. The generated droplets are stabilized by a surfactant present in the continuous phase. The critical pressure (Pc) refers to the minimum nitrogen gas pressure at which the dispersed phase starts to permeate through the membrane and droplets are formed. The pressure controls the dispersed-phase flux; to achieve a nearly monodisperse emulsion, the optimum pressure has been reported as 2−10 times that of Pc.31 In the present study, Pc was determined for two different concentrations of SDS, that is, 2.75 and 7.25 wt % (relative to the organic phase), by successively increasing the nitrogen pressure with intervals of 5 kPa every 30 min until the aqueous phase became slightly turbid. The onset of turbidity is an indication of emulsification, and the corresponding pressure is referred to as Pc. Figure 2 shows that Pc increases exponentially with decreasing membrane pore size. For the smallest pore size of 100 nm (run 4; Table 1), Pc was markedly higher compared with that for the largest pore size of 400 nm (run 7; Table 1). This inverse relationship of Pc with the pore size has been explained by the Laplace law describing capillary pressure.31

emulsification time (h)b

SDS (wt %)

± ± ± ± ± ±

2.75 2.75 2.75 7.25 7.25 7.25

20 42 37 19 39 24

5 5 5 2 3 5

Dzc (start) (nm) 330 436 674 274 436 782

± ± ± ± ± ±

35 57 43 5 13 25

Dzd (end) (nm) 550 780 800 239 450 722

± ± ± ± ± ±

72 80 55 10 15 38

a

Miniemulsion recipes are listed in Table 1. bEach value of emulsification time represents the mean (n = 3) ± SD. cZ-average diameter at the start of emulsification. dZ-average diameter at the completion of emulsification.

200 nm, the emulsification times (i.e., the time taken until all organic phase has been dispensed through the pores into the aqueous phase) were almost independent of the surfactant concentration. In the case of a pore size of 300 nm, the emulsification time decreased quite significantly as the surfactant concentration was increased from 2.75 to 7.25 wt %. Generally, an increase in the droplet size was observed during the emulsification. This is to be expected given that some miniemulsion degradation in the form of Ostwald ripening and coalescence is inevitable. The extent of miniemulsion degradation was generally less significant at the higher surfactant concentration, as anticipated. Particle Formation Mechanism. Initiator Type: Nucleation Mechanisms. Membrane emulsifications with 100 and 400 nm pore size membranes were conducted with 7.25 wt % SDS using the recipe in Table 1 (runs 10−11) with AIBN as the initiator. The miniemulsions obtained at the end of the C


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during the emulsification process, empty micelles would break up and SDS would diffuse to the newly generated oil−water interface. Moreover, it is also conceivable that some micelles would swell with the monomer and remain in the system, thus providing sites for secondary nucleation. However, secondary nucleation in monomer-swollen micelles would require the generation of initiator radicals in the aqueous phase, the likelihood of which depends on the water solubility of the initiator. It should be noted that emulsion-based systems of styrene are known to exhibit spontaneous generation of radicals at levels beyond that of the spontaneous initiation well-known in the case of homogeneous systems (bulk/ solution),52−56 and acrylates and methacrylates also undergo spontaneous initiation albeit to a significantly lesser extent than styrene.53 As such, it may be anticipated that secondary nucleation would be less prominent at room temperature using, for example, photoinitiation. It is also worth mentioning that particle size distributions can be affected by monomer transport driven by thermodynamics via diffusion across the aqueous phase between polymer particles (swollen with monomer) of different sizes and/or different monomer/ polymer ratios.57 To investigate the role of the initiator with regard to the nucleation mechanism, the initiators BPO and LPO were employed using the same pore sizes as above (100 and 400 nm). These initiators were selected based on their water solubilities: AIBN (0.04) > BPO (3 × 10−4) > LPO (2 × 10−9) (water solubilities in g/100 g water at 20 °C).58,59 In the case of BPO using 100 nm pore size, a monomodal particle size distribution similar to the droplet size distribution was obtained (run 8; Figure 4a). For the 400 nm pore size (run

membrane emulsification procedures were subsequently polymerized at 70 °C. Droplet and particle size distributions (i.e., before and after polymerization) measured by DLS are shown in Figure 3.

Figure 3. Size distributions with AIBN as the initiator: (a) 100 nm (run 10; Table 1); (b) 400 nm (run 11; Table 1). Dotted lines: droplets (before polymerization); full lines: particles (after polymerization at 70 °C for 24 h).

In the case of 100 nm pore size (run 10; Figure 3a), a distinctly bimodal particle size distribution was obtained after polymerization despite a monomodal initial droplet distribution (monomer conversion data are discussed below). The peak at the larger size corresponds quite well with the initial droplet size, whereas the “new” peak comprises much smaller particles with diameters well below 100 nm. For the 400 nm pore size (run 11; Figure 3b), the results were similar, although the new peak was even more pronounced in this case. If the polymerization proceeds exclusively via droplet nucleation, one would anticipate that the particle size distribution would be similar to the initial droplet size distribution. It can thus be summarized that the peak at the larger diameter does correspond to droplet nucleation, but the smaller peak is presumably a result of secondary nucleation. If the initial monomer droplets are too large in relation to the amount of SDS employed, there would be a significant amount of SDS located as free surfactant in the aqueous phase.50 The larger the monomer droplets, the lower the total oil−water interfacial area, and thus the higher the SDS concentration in the aqueous phase as the oil−water interfacial area is too small to accommodate all SDS. Based on an initial droplet diameter of 1661 nm (corresponding to run 11) and 7.25 wt % SDS, assuming full surface coverage (each SDS molecule occupying an area of 0.4 nm251 ), the SDS concentration in the aqueous phase at the end of the emulsification process would be 6.08 mM. This value is similar to the critical micelle concentration of SDS reported as 6.7 mM,50 and it can thus be concluded that the presence of SDS micelles is possible. The presence of such micelles would facilitate secondary nucleation, thus giving rise to a population of particles significantly smaller than the original monomer droplets. An additional consideration is the fact that at the beginning of the emulsification process, there are no monomer droplets in the aqueous phase and SDS micelles are certainly present at that stage. As monomer droplets are generated

Figure 4. Size distributions with different initiators: (a) BPO, using pore sizes of 100 (Dz,particle = 221 nm and PDI = 0.174) and 400 nm; runs 8−9; Table 1; (b) LPO, using pore sizes of 100 (Dz,particle = 256 nm and PDI = 0.275) and 400 nm (Dz,particle = 1559 nm and PDI = 0.290); runs 4 and 7; Table 1. Dotted lines: droplets (before polymerization); full lines: particles (after polymerization at 70 °C for 24 h).

9; Figure 4a), a bimodal particle size distribution was clearly observed, with one peak comparable to the droplet size distribution and another distribution corresponding to a particle size less than 100 nm. However, using LPO as the initiator, both 100 (run 4; Figure 4b) and 400 nm (run 7; Figure 4b) pore sizes resulted in monomodal particle size distributions, that is, particle generation via the nucleation of D


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than that of the traditional miniemulsion systems considered by Asua and co-workers, and as such it is reasonable exclude a much more dominant role of mechanism (i). The presence of radicals in the aqueous phase is a requirement for secondary nucleation to occur, and as such the results in terms of the polymerization rate and particle size distribution seem reasonable. Secondary nucleation leads to a population of smaller particles within which polymerization occurs at a higher rate due to compartmentalization effects (segregation of propagating radicals leading to less termination).13 LPO is considered to be water insoluble, but has been reported to generate radicals in the aqueous phase via a mechanism involving chain transfer to water in the droplet interfacial layer generating hydroxyl radicals able to undergo exit.63 Conditions Favoring Droplet Nucleation. Given that droplet nucleation was the main particle nucleation mechanism for LPO, the remainder of this work is focused on the use of this initiator. In addition to membrane emulsification using pore sizes of 100 and 400 nm (Figure 4b), pore sizes of 200 and 300 nm were also employed using 7.25 wt % SDS following the same protocol as stated in the emulsification section and polymerized at 70 °C (Table 1). The particle size distributions were in good agreement with the initial droplet size distributions for all four pore sizes (Figure 6), which

the initial monomer droplets as desired. The data clearly demonstrate that droplet nucleation is favored by (i) a small initial droplet size and (ii) highly hydrophobic initiators. The smaller the droplet size, the greater the overall oil−water interfacial area for SDS molecules to occupy, which leads to a lower concentration of SDS in the aqueous phase and thus a lower probability of the presence of micelles that can favor secondary nucleation. Moreover, the smaller the droplets, the greater the probability that radicals in the aqueous phase will enter existing droplets rather than cause secondary nucleation. As mentioned earlier, if monomer-swollen micelles are present, secondary nucleation will only occur if a radical is present in such a micelle. This can only occur if the initiator has sufficient water solubility such that it can diffuse across the aqueous phase from monomer droplets to micelles, or if the radicals generated have sufficient water solubility to undergo exit from monomer droplets (or polymer particles) and subsequently enter micelles. Such processes are minimized/eliminated if the initiator is sufficiently hydrophobic. Figure 5 shows conversion−time data (400 nm membrane) for polymerizations using the three different initiators at the

Figure 5. Conversion as a function of the polymerization time for a pore size of 400 nm using AIBN (run 11; Table 1), BPO (run 9; Table 1), and LPO (run 7; Table 1).

same initiator concentration, revealing how the polymerization rate decreases in the order of AIBN > BPO > LPO. These differences in rates cannot be explained by the relatively minor differences in the values of the dissociation rate coefficients (kd) at 70 °C (AIBN (3.2 × 10−5 s−1) > LPO (2.9 × 10−5 s−1) > BPO (1.4 × 10−5 s−1)58,59). It is clear however that the polymerization rate increased significantly with increasing initiator water solubility. The higher the initiator water solubility, the more significant the extent of secondary nucleation as evidenced by the DLS data (Figures 3 and 4a). In the case of miniemulsion polymerization using oil-soluble initiators such as AIBN, initiation in droplets/particles can occur as a result of (i) AIBN decomposition within a droplet/ particle with initiation occurring before radical termination (the latter is rapid if the droplet/particle size is small due to the confined space effect60,61), (ii) AIBN decomposition within a droplet/particle followed by the exit of one radical, leaving the other to propagate, and (iii) entry of a radical from the aqueous phase (from the small fraction of AIBN located in the aqueous phase, entry of a radical having previously undergone exit). Asua and co-workers62 demonstrated that the entry of exited radicals is the main mechanism for droplets/particles with diameters less than 250 nm. In the present work, the droplet/particle size is greater (especially for larger pore sizes)

Figure 6. Size distributions for pore sizes of (a) 100 nm (b) 200 nm (c) 300 nm, and (d) 400 nm (runs 4−7; Table 1) with LPO as the initiator. Dotted lines: droplets (before polymerization); full lines: particles (after polymerization at 70 °C for 24 h). E


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Table 3. Summary of SPG Membrane Emulsification Dataa

implies that droplets were converted to polymer particles in accordance with predominant droplet nucleation. It can be confirmed from TEM images that uniformly sized polymer particles were achieved in all cases (Figure S2) in approximate agreement with the DLS data. Conversion−time data (Figure 7) reveal that the polymerization rates were largely

Figure 7. Monomer conversion (%) as a function of the polymerization time for pore sizes of 100, 200, 300, and 400 nm; initiated with LPO (runs 4−7; Table 1).

independent of the droplet size. The lack of dependence of the polymerization rate on the droplet/particle size is consistent with compartmentalization effects on the polymerization mechanism not being significant for such large droplets/particles. The molecular weight distributions (MWDs) were monomodal throughout the polymerization for all four membranes (Figure S3). One of the advantages of SPG membrane emulsification is the easy and precise controllability of the droplet size. Previous investigations have demonstrated that the droplet size exhibits a linear relationship with the membrane pore size.32 The mean ratio of the droplet size/membrane pore size is typically in the range of 2−10.31−34 DLS measurements showed that for a pore size of 100−300 nm (runs 4−6; Table 1), the ratio of the droplet to membrane pore size is 2.4 ± 0.2, whereas for a pore size of 400 nm (run 7; Table 1), the ratio was somewhat higher at 4.0 ± 1.25 in general accordance with the literature.31 Figure 8 summarizes the results using pore sizes of 100, 200, 300, and 400 nm in terms of Z-average diameters as a function of conversion using LPO (runs 4−7; see Table 3 for a data summary). In all cases, the particle size remains approximately

run

pore size (nm)

Dz b (nm)

PDIb

4

100

236

0.066

5

200

436

0.060

6

300

782

0.221

7

400

1661

0.285

Dz c (nm) 299 300 287 296 307 256 426 461 498 457 456 440 768 782 764 747 1023 625 1446 1424 1262 1015 1022 1599

PDIc

αd (%)

Mne (×105) (g/mol)

Đe

0.240 0.220 0.127 0.197 0.223 0.275 0.062 0.081 0.101 0.083 0.102 0.080 0.229 0.211 0.174 0.154 0.399 0.413 0.228 0.215 0.241 0.221 0.230 0.290

17 45 57 73 79 98 19 32 60 70 76 97 27 48 56 68 79 99 11 30 54 67 76 86

2.59 2.92 2.83 2.29 1.87 2.20 1.65 2.49 2.50 3.04 3.71 1.28 1.56 2.77 2.45 2.31 1.99 1.14 0.41 0.87 1.28 1.60 1.60 0.93

3.17 2.87 2.87 3.19 3.31 3.60 2.90 2.88 3.08 3.02 2.71 5.86 2.82 2.49 2.79 2.80 2.98 5.35 3.17 3.21 4.12 4.32 4.28 6.24

a Monomer droplets were prepared with 100−400 nm pore sizes with 0.01 mol/L LPO in 7.25 wt % SDS aqueous solution at 190 rpm and polymerized at 70 °C for 24 h. bZ-average diameter and polydispersity index of monomer droplets analyzed by DLS. cZ-average diameter and polydispersity index of particles analyzed by DLS. dMonomer conversions were determined via gravimetric calculation. eNumber average molecular weight and dispersity measured by GPC.

constant with monomer conversion as expected for an ideal miniemulsion polymerization where particles are formed directly via the transformation of monomer droplets. It is further illustrated that this approach allows convenient tuning of the particle size in the approximate range of 200 nm to 2 μm. Such a control and versatility of the particle size is very challenging to achieve using conventional methods of miniemulsion generation, and as such the present work represents a significant step forward.

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CONCLUSIONS A novel approach has been developed for miniemulsion polymerization using Shirasu porous glass (SPG) membrane emulsification for the generation of the initial miniemulsion. Previous work has focused on the synthesis of micron-sized polymer particles via membrane emulsification followed by radical polymerization. In the present work, submicron-sized monomer droplets and polymer particles have been prepared using membranes with pore sizes significantly smaller than those previously reported, that is, 100−400 nm in diameter. Miniemulsion polymerization of methyl methacrylate was conducted using radical initiators of different water solubilities. It has been demonstrated that the water solubility of the initiator plays a crucial role: to achieve predominant monomer droplet nucleation, the initiator must be sufficiently hydrophobic such that secondary nucleation is minimized. By using lauroyl peroxide (LPO) as the initiator, predominant droplet

Figure 8. Z-average diameter as a function of the conversion for pore sizes of 100 (red), 200 (blue), 300 (green), and 400 (black) nm using LPO as the initiator (runs 4−7; Table 3). The data points at zero conversion and the dotted lines correspond to initial droplet diameters. F


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Macromolecules

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nucleation occurred for pore sizes of 100, 200, 300, and 400 nm, demonstrating that the final particle size can be conveniently tuned within the diameter range of 250−1600 nm. The use of the less hydrophobic initiators AIBN and BPO led to significant secondary nucleation as manifested by bimodal particle size distributions. Overall, the results demonstrate the potential for the use of membrane emulsification in tandem with miniemulsion polymerization as a platform for the synthesis of a wide range of polymeric nanoparticles with convenient control over the particle size, and with the additional advantages of low energy consumption and reduced shear stress. However, before feasible scale up and industrial implementation, the issue of the long miniemulsification time needs to be addressed, which as a consequence leads to the relatively low solid content employed in this initial study (longer times needed to pass a greater amount of monomer through the membrane). In principle, the technique is applicable to the synthesis of all polymeric nanoparticle types accessible via “conventional” miniemulsion polymerization approaches using high-energy mixing (e.g., ultrasonication).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00447. Photo of the experimental setup, TEM images of polymer particles, and molecular weight distributions (PDF)

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AUTHOR INFORMATION

ORCID

Tanja Junkers: 0000-0002-6825-5777 Cyrille Boyer: 0000-0002-4564-4702 Per B. Zetterlund: 0000-0003-3149-4464 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the European Union Horizon 2020 research and innovation program under the Marie SkłodowskaCurie grant agreement No. 665501 with the research Foundation Flanders (FWO) (N.Z.).

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Particle Size Control in Miniemulsion Polymerization via Membrane

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