Article pubs.acs.org/Langmuir
Synthesis of “Hard−Soft” Janus Particles by Seeded Dispersion Polymerization Thomas S. Skelhon,† Yunhua Chen,† and Stefan A. F. Bon* Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom ABSTRACT: The majority of studies on Janus particles focus on those that show amphiphilicity, with distinct hydrophilic and hydrophobic domains. Here, we demonstrate the synthesis of a different class of Janus particles: “hard−soft” biphasic dumbbell- or peanut-shaped particles with distinct lobes of “soft” poly(n-butyl acrylate) and “hard” poly(styrene). The particles are made by seeded dispersion polymerization of butyl acrylate in the presence of poly(styrene) seed particles. Surface nucleation by capture of the oligoradicals onto the surface of the seed particles thereby forming a distinct new polymer phase is found to be the formation mechanism of these particles. The total available poly(styrene) seed surface area plays a significant role in the size and number of poly(butyl acrylate) lobes grown off a single particle. At particularly low values of the surface area, we observe the formation of multilobe particles. We further demonstrate that our synthesis method is versatile and can be extended to the submicrometer domains by using seed particles of 200 nm in diameter.
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To manufacture particles of “hard−soft” dumbbell geometry, a seeded emulsion polymerization system would be an appropriate choice. Several groups have reported anomalous particle morphologies, including raspberry-, half-moon-, and dumbbell-shaped particles, obtained when conducting emulsion polymerizations of acrylate monomers in the presence of seed particles, particularly the emulsion polymerization of styrene in the presence of a poly(butyl acrylate) (PBA, Tg = −54 °C) seed particles,17−19 or inversely, polymerizing butyl acrylate in the presence of polystyrene (PS, Tg = 100 °C) seed particles.20 The underlying reasons for the complex particle morphologies are ascribed to the phase separation whereby seed particles swollen with the secondary monomer phase separate upon polymerization. This phase separation throughout polymerization develops as a result of polymer/polymer incompatibility over the course of the reaction. To mediate the increase in interfacial energy upon formation of the separated domains within the particle, the morphology changes (if kinetically allowed) to achieve minimization in interfacial energy. A deciding factor between the observed geometries relies on the relative surface tensions between the two polymer phases and the interfacial tension between each polymer and the continuous phase.21 The use of surfactants, ionic comonomers, and initiator influences the ultimate particle morphology. An interesting approach to prepare a hard−soft dumbbell morphology in seeded emulsion polymerization is to make use of entropic phase separation of a secondary compatible “soft” polymer lobe from a lightly cross-linked polymer matrix and has been attempted by Mock et al.22 using polystyrene (PS) seed particles and N-isopropylacrylamide (NIPAm) as a second
INTRODUCTION Anisotropic “Janus” particles attract increasing interest because of their highly diverse and versatile nature, a key characteristic being that each particle features a specific property that is opposing in nature, for example, one side being hydrophilic while the other side is hydrophobic.1−3 This opposing anisotropy makes them attractive as colloidal objects for the fabrication of advanced materials and colloidal formulations. Predominant focus lies on the syntheses and applications of amphiphilic Janus particles, and the latter includes studies on how amphiphilic Janus particles adhere to and assembly on soft deformable interfaces, such as a liquid−liquid interface;4−7 how they can act as Pickering emulsifiers in emulsion polymerization;8 and how they can self-assembly into intricate supracolloidal structures.9−13 There is scope to investigate particles with alternative Janus type characteristics; for example, self-propelling microspheres that have one hemisphere covered by a thin deposited layer of platinum that catalyzes the decomposition of hydrogen peroxide from solution, triggering propulsion.14 A similar example is silica-based matchstick particles that have a catalytic head of manganese oxide and can undergo chemotaxis with low concentrations of hydrogen peroxide acting as attractant and fuel for propulsion.15 Herein, however, we are interested in the synthesis of dumbbell or peanut-shaped Janus particles that possess distinctive high and low glass transition temperature lobes and, therefore, are tentatively named “hard−soft” Janus particles. Owing to the differences of material behavior at each “side” of the particle, the ability for one lobe to deform under ambient conditions while the other remains a robust and solid entity presents an attractive feature of these Janus particles, which can be used for a variety of applications. We recently reported their hierarchical assembly into colloidal molecule-type clusters and their potential use as Pickering stabilizers.16 © 2014 American Chemical Society
Received: August 22, 2014 Revised: October 22, 2014 Published: October 24, 2014 13525
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filtered through a basic alumina column to remove inhibitor before use. Deionized water was used in all experiments. Synthesis of Micrometer-Sized Seed Particles. Styrene (25.0 g), AIBN (0.25 g), PVP (1.5 g), and methanol (70 g) were charged to a 250 mL round-bottom flask prior to stirring and nitrogen purge for 40 min, after which the flask was heated to 70 °C for 24 h while under stirring and slight nitrogen overpressure. Resulting particle dispersions were centrifuged and redispersed in methanol three times before analysis by optical microscopy and laser scattering analysis. Synthesis of “Hard−Soft” Particles by Seeded Dispersion Polymerization. A 27.0 g portion of the purified seed particle dispersion (solids content: 11 wt %), n-butyl acrylate (3.0 g), AIBN (0.1 g), PVP (0.03 g), water (5.7 g), and methanol (17.3 g) were charged to a round-bottom flask, stirred, and purged with nitrogen for 20 min prior to sealing and heating at 70 °C for 24 h. Kinetic Experiments of Seeded Dispersion Polymerization. For kinetic experiments, the same polymerization procedure was followed with varied quantities of 2.5 μm seed particles (0, 1.25, 2.5, 5.0, and 10.0 g, respectively) alongside n-butyl acrylate (5.0 g), AIBN (0.05 g), PVP (0.1 g), water (4.5 g), and methanol (40.5 g). During the polymerization, 2 mL aliquots were extracted at set time periods, aerated with oxygen, and cooled in ice to quench polymerization before measuring conversion gravimetrically and imaging by optical microscopy. Synthesis of Submicrometer Seed Particles. A 200 nm polystyrene seed dispersion was made first by emulsion polymerization. Briefly, to a 250 mL single jacketed glass reactor vessel fitted with a PTFE overhead stirrer with propeller blade (Radleys reactor duo), styrene (20.0 g), PVP-K30 (1.50 g), and deionized water (200 g) were added. The mixture was purged with nitrogen for 45 min while stirring prior to sealing with slight nitrogen overpressure. After heating to 70 °C, potassium persulfate (0.3 g) in 5 mL of degassed water was charged to the reactor. Polymerization ensued for 24 h before cooling to room temperature and dialysis against methanol using a cellulose dialysis membrane. Dialysis ensued for 7 days with solvent changing once a day. Excess PVP-K30 was removed afterward by three centrifugation cycles at 7882g for 60 min. The supernatant was disposed, and the particles were redispersed in methanol. Gravimetry was used to ascertain accurate solids content of the purified seed dispersion. Synthesis of Submicrometer “Hard−Soft” Particles. A 5.0 g portion of 200 nm seed particles in methanol (solids content: 12.3 wt %), n-butyl acrylate (0.35 g), AIBN (0.05 g), PVP (0.013 g), water (7.0 g), and methanol (22.5 g) were charged to a 50 mL round-bottom flask fitted with magnetic follower and rubber septum. The contents of the flask was purged with nitrogen for 20 min prior to sealing. Reactions were then heated to 70 °C for 24 h. During the polymerization, 2 mL aliquots were extracted at set time periods, aerated with oxygen, and cooled on ice to quench polymerization before measuring conversion gravimetrically and size analysis by DLS. Characterization. Particle sizing measurements of micrometersized colloids were undertaken using a Mastersizer 2000 (Malvern Instruments, Malvern, UK). Dilute particle suspensions in methanol were introduced using the μP dispersion unit. A laser was aligned to optimum transmission intensity, and a background spectrum was collected to eliminate any contaminants presenting in the final data. Dilute suspensions of 1 wt % were introduced dropwise by submerged syringe until 5% laser obscuration was reached. Dynamic light-scattering measurements were conducted using a Malvern instruments Zetasizer ZS, (Malvern, UK). Dilute 1 mL dispersions of particles in methanol were dispensed into plastic cuvettes and thermally equilibrated to 25 °C for 2 min prior to measurement of 3 sets of measurements, each of 10 subruns using 173° backscattering angle. Values for number (%) and volume (%) as a function of hydrodynamic diameter were used. Particle dispersions were imaged on a Leica DM2500 M in bright field transmission mode using a Nikon D5100 camera. Cryogenic scanning electron microscopy analyses were carried out using a Zeiss Supra 55-VP field emission gun scanning electron microscope with a Gatan Alto 2500 cryo transfer system and a Gatan
monomer (as a toluene solution). However, high-resolution scanning electron micrographs of these particles reveal that the PNIPAm protrusion is not well-defined and that much of PNIPAm has encapsulated the seed particle as a result of the soft hydrophilic nature of the PNIPAm enveloping the hydrophobic poly(styrene) seed over time. Overall, seeded emulsion polymerization is a challenging technique as a route to synthesize controlled Janus particle structures with a welldefined dumbbell shape, being very sensitive to cross-link density and, thus, swelling characteristics; the surface chemistry of the seed latex; and the projected chemical nature of the two polymer lobes. An alternative effective route toward anisotropic polymeric particles with varying morphology would be seeded dispersion polymerization. One of the motivating factors for synthesizing Janus particles by this method is the lack of swelling of the seed particles. In a dispersion polymerization system, the monomers are completely soluble in the continuous phase and generally do not swell the seed particles. This eliminates problems arising from the phase separation within seed particles. In addition, the nonaqueous environment of the continuous phase lowers the difference in interfacial tension between the continuous phase and the particle, thereby reducing the effect of charged residues from surfactants, initiators, or charged comonomers on final particle morphology. Okubo and coworkers fabricated “snowman” shaped particles through the seeded dispersion polymerization of butyl methacrylate using micrometer-sized poly(styrene) seeds.23 Particles with a single “lobe” were generated with smaller seed particles of 1.28 μm diameter, whereas “confetti” particles possessing multiple lobes from single seed particle were found to form when a larger seed particle of 2.67 μm diameter was used. This difference in morphology was attributed to the absorption of multiple radical oligomers onto the larger particles. They also conducted a seeded dispersion polymerization of butyl methacrylate in the presence of a poly(styreneco-sodium styrenesulfonate) seed particles and dodecane droplets. After polymerization, the dodecane was evaporated leaving dimples in the surface of the micrometer-sized particles to resemble “golf balls”.24 El-Aasser and coworkers showed that seeded dispersion polymerization of butyl acrylate in the presence of a poly(styrene) seed under alcoholic conditions using poly(vinylpyrrolidone) as a protective colloid rendered biphasic micrometer-sized dumbell particles.25 The surface of poly(styrene) seed particles grafted with a layer of steric surfactant provide the locus of nucleation for butyl acrylate; however, this work was briefly reported, detailing only the observation of particle geometry. Herein, we aim to investigate the kinetics and mechanism of seeded dispersion polymerization of butyl acrylate using polystyrene micrometer-sized seed particles. The intention is that the mechanistic understanding in the fabrication of these hard−soft Janus particles will allow us to tune particle size and respective lobe size. In addition, we explore an adaptation of this polymerization system to enable the synthesis of submicrometer-sized hard−soft Janus particles.
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EXPERIMENTAL SECTION
Materials. Styrene (Reagent Plus, 99%), n-butyl acrylate (>99%), poly(vinyl pyrrolidine) and (PVP-K90, 360 000 g mol−1 and K30, 40 000 g mol−1) were supplied by Sigma-Aldrich, UK. Azobis(isobutyronitrile) (97%), ethanol (absolute AR), and methanol (AR) were purchased from VWR and used as received. Monomers were 13526
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C1002 liquid nitrogen cold stage. Dilute aqueous suspensions (1 wt %) of particles were drawn into a brass rivet and cooled in liquid nitrogen. Samples were heated to −95 °C under high vacuum for 15 min to remove contaminant ice through sublimation, followed by platinum sputter target coating in an argon atmosphere (25 s, 10 mA). Imaging was undertaken at −120 °C using a 1.5 kV accelerating voltage with a gold anticontaminator at −189 °C. All image analysis was conducted using open source software ImageJ (NIH, US).
poly(butyl acrylate) as separate particles. In one example, the seed particle mass was equal to the butyl acrylate mass charged to the reaction. The resultant particles after the seeded dispersion polymerization can be seen in Figure 1b. Note that the darker sphere represents the poly(styrene) seed particle, and the lighter part represents the newly formed poly(butyl acrylate) lobe. There appears to be a clear delineation between the two phases, indicating that no swelling and subsequent polymerization has occurred, ruling out entropic phase separation from within the particle. We may therefore attribute the formation mechanism of the second PBA lobe to the capture of oligomeric radicals from the solution phase onto the particle surface during the initial stage of the seeded dispersion polymerization, where it further propagates to form a lobe. This mechanism falls in line with other studies on seeded dispersion polymerization.23 We should expect the average number of lobes on each particle to be dependent on the total seed surface area available to nucleate from. To probe this, a series of seeded dispersion polymerizations of butyl acrylate in the presence of a varied quantity of seed particles were performed to investigate the influence of seed particle concentration, that is, the total seed surface area, on the polymerization kinetics and growth of the PBA lobe from the seed particles. Note that no changes in diameter of the polystyrene lobes were observed after the polymerization of BA. Four polymerizations of butyl acrylate (10 wt %) with a range of seed particle concentrations outlined below in Table 1 were conducted in a methanol/ water medium
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RESULTS AND DISCUSSION The synthetic procedure to fabricate “hard−soft” Janus particles involves a two-step reaction. Initially, polystyrene seed particles were synthesized via a dispersion polymerization procedure in methanol to make spherical particles of 2.5 μm average diameter and a monodisperse size distribution. A typical procedure involves batch addition of styrene, poly(vinylpyrrolidone) K90 (PVP, 360 000 g mol−1) as protective colloid, thermal radical initiator AIBN, and methanol as the continuous phase. The PVP content was fixed at 5.5 wt % to styrene and AIBN 1 wt % to styrene. After polymerization, the dispersion was cleaned by centrifugation and redispersed into fresh methanol to remove excess PVP in the continuous phase. The remaining solids content, deemed mostly PVP, in the continuous phase after polymerization was found to be 3.21 wt % as determined by gravimetric analysis of the first supernatant from centrifugation purification. Final monomer conversions of seed particle dispersion polymerizations exceeded 95% as determined by gravimetry before centrifugation. The supernatant after the first sedimentation of seed latex was transparent, indicating the absence of sub-500 nm diameter secondary nucleated particles, given the speed and duration of centrifugation. The resulting particles can be seen optically in Figure 1a.
Table 1. Percentage of Particles with Multiple PBA Lobes for Each Seeded Dispersion Polymerization PS seed concentration (g/L) 203.66 90.50 27.88 20.92
seed number per liter
total seed surface area (m2/L)
particles with multiple lobes in population
× × × ×
448 199 83 46
2.60% 7.04% 7.54% 35.74%
2.11 9.37 4.23 2.17
1013 1012 1012 1012
(90:10 w/w) using AIBN as thermal initiator (1 wt % to BA) and PVP-K90 as stabilizer (2 wt % to BA). Note that a dispersion polymerization of butyl acrylate was conducted in the absence of seed particles to be used as a control experiment for comparison. Aliquots of reaction mixture were extracted at set time intervals and analyzed by optical microscopy and gravimetry for monomer conversion. Final monomer conversions of all seeded particle dispersion polymerizations exceeded 90% as determined by gravimetry. Less than 0.75 wt % solids were found in the continuous phase after gravity separation of particles in all seeded dispersion polymerizations. BA monomer conversion data (XM) were obtained by the gravimetric analysis of the seeded dispersion polymerizations, as plotted in Figure 2a. When a free radical polymerization rate equation are applied and the initiator decomposition rate was taken into account (eq 1), a first-order relationship exists between the monomer concentration and polymerization rate, except at the start of the reaction, where a small induction period can be seen (Figure 2b). This is attributed to the reaction reaching thermal equilibrium. The rate of polymerization is relatively unaffected by the presence of polystyrene seeds at all concentrations. However, upon closer inspection, it appears that the polymerization without poly(styrene) seeds is
Figure 1. Optical micrographs of (a) poly(styrene) seed particles and (b) “hard−soft” Janus particles (weight ratio of charged BA monomer to PS seeds is 1:1). The darker sphere represents the poly(styrene) seed, whereas the lighter part represents the poly(butyl acrylate) lobe.
To form the secondary “soft” lobe, a seeded dispersion polymerization of n-butyl acrylate was undertaken in the presence of the micrometer-sized polystyrene seed particles. This was conducted under batch conditions by addition of a cleaned seed particle dispersion, PVP K90 surfactant, n-butyl acrylate, and AIBN in a methanol−water medium. Methanol is used specifically in this reaction because longer chain alcohols are good solvents for poly(butyl acrylate). The PVP content was reduced to 1 or 2 wt % with respect to butyl acrylate. This was done to promote the growth of poly(butyl acrylate) particles off the polystyrene seed particles rather than to provide sufficient steric stabilization for the growth of 13527
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seen to progress at a slightly elevated rate in comparison with the seeded polymerizations during the first 150 min of the reaction. A potential explanation can be as follows: The elevated polymerization rate arises from the dependence of solution polymerization kinetics during the early stages of the polymerization on initiator concentration. The seed particles absorb some of the radicals, causing a temporary depression in polymerization rate. Arguably, increasing the seed particle concentration should enhance this effect, something that cannot be observed clearly from the data. To unravel the exact reason(s) for the observed slight retardation, a more detailed kinetic study is required. −ln(1 − XM) = 2k p
f [I]0 (1 − kdk t
e−kdt ) (1)
where XM is BA monomer conversion, kp is radical propagation rate, kt is radical termination rate, f is radical initiation efficiency, [I]0 is initiator AIBN concentration, kd is initiator decomposition rate (3.2 × 10−5 s−1 at 70 oC), and t is polymerization time. In comparison of our observations with that of previously reported literature, we find that our results appear to be inconsistent with that of the kinetics of seeded dispersion polymerizations of methyl methacrylate (MMA) in the presence of micrometer-sized PMMA seeds.18 In the PMMA system, the initial polymerization rate was observed to be higher by a factor of 1.3 compared with an identical nonseeded reaction. We attribute this disparity to the fact that a far higher monomer/polymer mass ratio (28:1) was employed in the PMMA system (compared with the 4:1 mass ratio used in our highest BA containing reaction), and an overall higher starting concentration of monomer in the continuous phase, which has been shown to increase initial polymerization rate. Conversely, Perro et al. report that the emulsion polymerization rate of styrene in the presence of silica seeds in manufacturing hybrid colloidal Janus particles by surface nucleation is not affected by
Figure 2. (a) Monomer conversion as a function of time in the seeded dispersion polymerizations at various seed particle concentrations given as seed surface area concentration; (b) First-order kinetic plot taking into account initiator decomposition of the seeded dispersion polymerizations.
Figure 3. Optical micrographs of particles during a seeded dispersion polymerization of BA in the presence of polystyrene seed particles (seed particle surface area: 199 m2/L). 13528
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the presence of seed particles in accordance with our observations.26 Optical micrographs of the seeded dispersion polymerizations were taken at set intervals throughout the polymerization. An example set of optical micrographs imaging the “hard−soft” particles can be seen in Figure 3. The PBA lobes grow gradually with the increasing polymerization time. On the basis of these images, quantitative measurements regarding the evolution of lobe volume can be taken to analyze particle growth kinetics. The volume of the growing poly(butyl acrylate) lobe was modeled as a spherical cap, whereby the PBA lobe was treated as a hemisphere of a sphere. Taking the height of the lobe (h) and radius (R), as displayed in Figure 4, the lobe volume can be calculated by following equation: V=
πh2 (3R − h) 3
Figure 5. Calculated PBA lobe volume based on optical micrographs as a function of monomer conversion during seeded dispersion polymerizations.
(2)
Figure 4. Schematic of the measurements taken to calculate volume of the poly(butyl acrylate lobe) (red) growing off a polystyrene seed particle (green).
It is worth noting that one assumption taken in this measurement is that the area of contact between the poly(styrene) and poly(butyl acrylate) lobe is treated as planar rather than a 3D curved surface. Despite the relatively negligible effect that seed particle concentration has on the rate of polymerization, its influence on lobe volume is far more significant. Values of the calculated volume for the poly(butyl acrylate) lobes are plotted as a function of monomer conversion in each reaction as depicted in Figure 5. Because the monomer concentration in each reaction is equivalent, yet the number of seed particles is varied, an increase in lobe volume for lower seed particle concentrations is expected. However, the lobe growth profile for the polymerization conducted with 83 m2/L seed surface area overlaps that of the polymerization for 199 m2/L seed surface area. It would be expected that the profile of 83 m2/L should exceed that of 199 m2/L. In addition, the observed lobe volume of the PBA lobes for the lowest seed surface area reactions of 46 and 83 m2/L do not reach the expected volume based on the mass of monomer at the set seed particle concentration, as expressed in eq 3 mBA ρpBA Vlobe = Np (3)
Figure 6. Optical micrographs of final seed particle dispersions after 24 h polymerization time for each of the seeded dispersion polymerizations. The seed surface area for each reaction is indicated on the micrographs.
poly(styrene) seed particles, each possessing one lobe of poly(butyl acrylate). However, the lowest seed surface area polymerizations of 46 and 83 m2/L indicate the presence of a large quantity of secondary nucleation. In addition, some poly(styrene) seed particles possessing more than one PBA lobe can be seen. The growth of the PBA lobe size can be calculated to allow the expected lobe volume based on BA monomer mass charged to the system and seed particle concentration to be compared with experimentally observed volume values. When the PBA lobe volume fraction is measured as a function of conversion, a linear dependence can be seen for all reactions, as shown in Figure 7. Lobe volume fraction is calculated by dividing the observed lobe volume during the polymerization by the final measured lobe size. It can also be seen that when high seed
where mBA is the mass of butyl acrylate, ρPBA is the density of PBA and Np is the number of seed particles. Optical micrographs of the final particle dispersions reveal why these disparities are observed for the reactions at lower seed surface area, as seen in Figure 6. The polymerizations containing the highest seed surface area reveal monodisperse 13529
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Figure 8. PBA lobe volume of Janus particles (black) and size of secondary nucleated particles (red) versus conversion in the seeded dispersion polymerization at 46 m2/L of seed surface area.
Figure 7. Measured lobe volume fractions normalized against expected lobe volume as a function of conversion. Black line represents the theoretical fractional growth profile for the reactions based on conversion data.
contrast to these works, our system is susceptible to secondary nucleation, therefore preventing controlled nucleation of a larger number of PBA lobes at consistent volume. The multilobed particles were reanalyzed by optical microscopy (Figure 9) after 7 days and found to have shown
concentration (with 199 and 448 m2/L of total seed surface area) applied, seeded dispersion polymerizations showed a consistent fit with the expected linear volume growth profile. The polymerizations incorporating a lower seed surface area (46 and 83 m2/L), however, deviate significantly from the linear growth curve as a result of secondary nucleation. This result provides insight into the particle formation mechanism for the seeded dispersion polymerization. All dispersion polymerizations start as a homogeneous solution whereby homogeneous nucleation occurs in the continuous phase. In our case of seeded dispersion polymerizations, as the oligoradicals grow toward a critical threshold size, they may be captured by the surface of a seed particle and commence nucleation on the surface of the seed particle. In reactions with high concentrations of seed particles, there is a large surface area available for capture; however, when the available seed surface area is too low, capture by a seed particle surface may not occur before the oligoradical reaches a critical threshold size and precipitates out into a secondary primary particle. Figure 8 shows secondary nucleated PBA particles present from the very early stage of the reaction (i.e. at 10% conversion) in the case of our lowest seed surface area (46 m2/L). To quantify the prevalence of seed particles possessing multiple lobes, populations of particles in each of the seeded dispersion reactions of varying seed surface area were counted for particles with multiple lobes, with over 150 particles being counted per sample particle dispersion (see Table 1). As expected, the number population of particles with multiple lobes increases as the seed surface area available to nucleate from decreases. This observation correlates with results reported from the seeded emulsion polymerization of styrene in the presence of silica seed particles, whereby varying the surface area of silica seed in the reaction determines the number of poly(styrene) lobes grown off a single seed particle.26 In this example, the total seed surface area in the reaction was varied by altering seed particle size; therefore, larger seed particles would reliably accommodate more poly(styrene) nodules, allowing the controlled nucleation of up to six lobes and, in extreme cases, many lobes, with the final particle resembling a raspberry structure.27,28 However, in
Figure 9. Optical micrograph of multilobe particles 7 days after synthesis indicating little change in particle morphology.
little difference in morphology or number population (with respect to single lobed particles). This would suggest that the poly(butyl acrylate) lobes are somewhat anchored to the surface of the poly(styrene) seed particle; otherwise, we would expect the multilobe particles to migrate into a single lobe by time to minimize unfavorable interfacial energies between the PBA and PS surface and equally to minimize the PBA− continuous phase interfacial tension. Once we achieved a good understanding of how to synthesize micrometer-sized “hard−soft” Janus particles, we were interested to see if we could scale-down our method into the regime of submicrometer-sized particles. As aforementioned, the synthesis of hard−soft Janus particles through seeded emulsion polymerization has proved difficult. The synthesis of micrometer-sized “hard−soft” Janus particles is simple through dispersion polymerization, as demonstrated above; however, the lower size limit of a reliable monodisperse seed made by dispersion polymerization is ∼1 μm. We therefore made our seed latex by emulsion polymerization, but with the condition that we required a submicrometer seed 13530
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Figure 10. (a) Conversion and hydrodynamic diameter (DLS) as a function of time for the styrene emulsion polymerization to synthesize the submicrometer seed particles; (b) BA monomer conversion as a function of time for a seeded dispersion polymerization of BA in the presence of submicrometer PS seed particles; (c) volume-averaged hydrodynamic diameter as a function of conversion for Janus particle samples extracted during the seeded dispersion polymerization.
particle with the same surface properties as the larger micrometer-sized system. Du et al. reported an emulsion polymerization procedure to render submicrometer poly(styrene) latex particles stabilized by PVP, thereby presenting an ideal route to fabricate our seed particles.29 The emulsion polymerization to prepare our seed latex was followed both by gravimetry and dynamic light scattering, and as such, the polymerization kinetics data and particle growth can be found plotted in Figure 10a. Monodisperse poly(styrene) particles were obtained with an average diameter of 200 nm. The use of PVP-K30 as a steric stabilizer ensures a layer of stabilizer on the surface of the particles, which acts as a favorable nucleation site for the subsequent seeded dispersion polymerization in the same manner as the larger micrometer-sized particles. Prior to the seeded dispersion polymerizations, the PS seed particles were dialyzed against methanol until dispersed in a pure methanol environment, after which the dispersion is cleaned of any remaining PVP by repeated centrifugation cycles in methanol. A similar strategy to the seeded dispersion polymerizations involving micrometer-sized PS particles was applied to nucleate and grow a secondary lobe of PBA from the surface of the submicrometer seed particles. Owing to the seed surface area dependence of the polymerization, the seed particle concentration was reduced to match the seed surface area per liter with that studied in the micrometer-sized system. This effectively reduced the seed solid content from 10 to 1.05 wt %. The total seed surface area here is ∼246 m2/L. Throughout the reaction, aliquots of reaction mixture were extracted from the reaction and analyzed for conversion gravimetrically, as can be seen in Figure 10b. The conversion−time data for the seeded dispersion polymerization indicates a reaction profile similar to that of the micrometer-sized seeded dispersion polymerization, reaching high conversion over 24 h of reaction time. A closer analysis of the volume-averaged hydrodynamic diameter of particles by dynamic light scattering was conducted during the dispersion polymerization. Peak values are plotted as a function of conversion in Figure 10c. An increase in particle size from the initial volume weighted seed diameter of 250 nm (note that the media here contains PVP) can be seen during the polymerization up to a final value of ∼280 nm in a manner similar to the results seen for the micrometer-sized seeded dispersion polymerizations. Most importantly, Cryo-SEM images reveal the characteristic dumbbell morphology of Janus particles (Figure 11). It clearly shows that smaller lobes of PBA were grown from the PS seed surface. Note that half the mass of butyl acrylate was added in comparison with the mass of seed particles for this reaction.
Figure 11. Cryo-SEM images of (a) submicrometer poly(styrene) seed particles and (a′) at high magnification; (b)submicrometer hard− soft Janus particles and (b′) at high magnification.
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CONCLUSION We demonstrate a reliable method to fabricate “hard−soft” Janus particles by a two-step seeded dispersion polymerization. By various optical and kinetic analyses, we find that the rate of polymerization during the seeded dispersion polymerization of butyl acrylate in the presence of PS seed particles is unaffected by the concentration of seed particles. However, a profound effect on the volume and number of PBA lobes per seed particle is observed. A low seed surface area concentration results in the formation of multilobed particles and significant secondary nucleation. As a result of this work, the optimum parameters for synthesizing “hard−soft” Janus particles can be found without undesired effects such as secondary nucleation and multilobed particles. In addition, we translate the synthetic procedure to allow the formation of submicrometer-scale “hard−soft” particles verified by cryo-SEM, therefore opening the range of accessible size for these particles from nano to micrometer scale. We believe that the “hard−soft” dumbbell particles are an exciting class of Janus particles, in addition to the more commonly made and used amphiphilic Janus particles and that their use can find exciting areas of application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions †
T.S.S. and Y.C. contributed equally.
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Notes
(20) Okubo, M.; Kanaida, K.; Matsumoto, T. Production of Anomalously Shaped Carboxylated Polymer Particles by Seeded Emulsion Polymerization. Colloid Polym. Sci. 1987, 265, 876−881. (21) Sundberg, D. C.; Casassa, A. P.; Pantazopoulos, J.; Muscato, M. R.; Kronberg, B.; Berg, J. Morphology Development of Polymeric Microparticles in Aqueous Dispersions. I. Thermodynamic Considerations. J. Appl. Polym. Sci. 1990, 41, 1425−1442. (22) Mock, E. B.; Zukoski, C. F. Emulsion Polymerization Routes to Chemically Anisotropic Particles. Langmuir 2010, 26, 13747−13750. (23) Okubo, M.; Fujibayashi, T.; Yamada, M.; Minami, H. MicronSized, Monodisperse, Snowman/Confetti-Shaped Polymer Particles by Seeded Dispersion Polymerization. Colloid Polym. Sci. 2005, 283, 1041−1045. (24) Fujibayashi, T.; Komatsu, Y.; Konishi, N.; Yamori, H.; Okubo, M. Effect of Polymer Polarity on the Shape of “Golf Ball-Like” Particles Prepared by Seeded Dispersion Polymerization. Ind. Eng. Chem. Res. 2008, 47, 6445−6449. (25) Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. Seeded Dispersion Polymerization. J. Appl. Polym. Sci. 2002, 84, 2710−2720. (26) Perro, A.; Reculusa, S.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Synthesis of Hybrid Colloidal Particles: From Snowman-like to Raspberry-like Morphologies. Colloids Surf. A 2006, 284-285, 78−83. (27) Reculusa, S.; Poncet-Legrand, C.; Perro, A.; Duguet, E.; Bourgeat-Lami, E.; Mingotaud, C.; Ravaine, S. Hybrid Dissymmetrical Colloidal Particles. Chem. Mater. 2005, 17, 3338−3344. (28) Perro, A.; Duguet, E.; Lambert, O.; Taveau, J.-C.; BourgeatLami, E.; Ravaine, S. A Chemical Synthetic Route towards “Colloidal Molecules”. Angew. Chem., Int. Ed. 2009, 48, 361−365. (29) Du, X.; He, J. Facile Size-Controllable Syntheses of Highly Monodisperse Polystyrene Nano- and Microspheres by Polyvinylpyrrolidone-Mediated Emulsifier-Free Emulsion Polymerization. J. Appl. Polym. Sci. 2008, 108, 1755−1760.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Part of the equipment used in this research was supported by Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF).
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REFERENCES
(1) Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into Complex Structures. Nat. Mater. 2007, 6, 557− 562. (2) Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, SelfAssembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194−5261. (3) Hu, J.; Zhou, S.; Sun, Y.; Fang, X.; Wu, L. Fabrication, Properties and Applications of Janus Particles. Chem. Soc. Rev. 2012, 41, 4356− 4378. (4) Kim, J.-W.; Lee, D.; Shum, H. C.; Weitz, D. A. Colloid Surfactants for Emulsion Stabilization. Adv. Mater. 2008, 20, 3239− 3243. (5) Cheung, D. L.; Bon, S. A. F. Stability of Janus Nanoparticles at Fluid Interfaces. Soft Matter 2009, 5, 3969−3976. (6) Kumar, A.; Park, B. J.; Tu, F.; Lee, D. Amphiphilic Janus Particles at Fluid Interfaces. Soft Matter 2013, 9, 6604−6617. (7) Ruhland, T. M.; Gröschel, A. H.; Ballard, N.; Skelhon, T. S.; Walther, A.; Müller, A. H. E.; Bon, S. A. F. Influence of Janus Particle Shape on Their Interfacial Behavior at Liquid−Liquid Interfaces. Langmuir 2013, 29, 1388−1394. (8) Walther, A.; Hoffmann, M.; Mueller, A. H. E. Emulsion Polymerization Using Janus Particles as Stabilizers. Angew. Chem., Int. Ed. 2008, 47, 711−714. (9) Hong, L.; Cacciuto, A.; Luijten, E.; Granick, S. Clusters of Amphiphilic Colloidal Spheres. Langmuir 2008, 24, 621−625. (10) Whitelam, S.; Bon, S. A. F. Self-Assembly of Amphiphilic Peanut-Shaped Nanoparticles. J. Chem. Phys. 2010, 132, 074901. (11) Chen, Q.; Whitmer, J. K.; Jiang, S.; Bae, S. C.; Luijten, E.; Granick, S. Supracolloidal Reaction Kinetics of Janus Spheres. Science 2011, 331, 199−202. (12) Chen, Q.; Bae, S. C.; Granick, S. Directed Self-Assembly of a Colloidal Kagome Lattice. Nature 2011, 469, 381−384. (13) Vissers, T.; Preisler, Z.; Smallenburg, F.; Dijkstra, M.; Sciortino, F. Predicting Crystals of Janus Colloids. J. Chem. Phys. 2013, 138, 164505. (14) Howse, J.; Jones, R.; Ryan, A.; Gough, T.; Vafabakhsh, R.; Golestanian, R. Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk. Phys. Rev. Lett. 2007, 99, 048102. (15) Morgan, A. R.; Dawson, A. B.; Mckenzie, H. S.; Skelhon, T. S.; Beanland, R.; Franks, H. P. W.; Bon, S. A. F. Chemotaxis of Catalytic Silica−Manganese Oxide “Matchstick” Particles. Mater. Horizons 2014, 1, 65−68. (16) Skelhon, T. S.; Chen, Y.; Bon, S. A. F. Hierarchical SelfAssembly of “Hard−Soft” Janus Particles into Colloidal Molecules and Larger Supracolloidal Structures. Soft Matter 2014, 10, 7730−7735. (17) Okubo, M.; Katsuta, Y.; Matsumoto, T. Studies on Suspension and Emulsion. LI. Peculiar Morphology of Composite Polymer Particles Produced by Seeded Emulsion Polymerization. J. Polym. Sci. Polym. Lett. Ed. 1982, 20, 45−51. (18) Min, T. I.; Klein, A.; El-Aasser, M. S.; Vanderhoff, J. W. Morphology and Grafting in Polybutylacrylate−Polystyrene Core− Shell Emulsion Polymerization. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 2845−2861. (19) Chen, Y.-C.; Dimonie, V.; El-Aasser, M. S. Interfacial Phenomena Controlling Particle Morphology of Composite Latexes. J. Appl. Polym. Sci. 1991, 42, 1049−1063. 13532
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Synthesis of “Hard−Soft” Janus Particles by Seeded Dispersion Polymerization Thomas S. Skelhon,† Yunhua Chen,† and Stefan A. F. Bon* Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom ABSTRACT: The majority of studies on Janus particles focus on those that show amphiphilicity, with distinct hydrophilic and hydrophobic domains. Here, we demonstrate the synthesis of a different class of Janus particles: “hard−soft” biphasic dumbbell- or peanut-shaped particles with distinct lobes of “soft” poly(n-butyl acrylate) and “hard” poly(styrene). The particles are made by seeded dispersion polymerization of butyl acrylate in the presence of poly(styrene) seed particles. Surface nucleation by capture of the oligoradicals onto the surface of the seed particles thereby forming a distinct new polymer phase is found to be the formation mechanism of these particles. The total available poly(styrene) seed surface area plays a significant role in the size and number of poly(butyl acrylate) lobes grown off a single particle. At particularly low values of the surface area, we observe the formation of multilobe particles. We further demonstrate that our synthesis method is versatile and can be extended to the submicrometer domains by using seed particles of 200 nm in diameter.
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To manufacture particles of “hard−soft” dumbbell geometry, a seeded emulsion polymerization system would be an appropriate choice. Several groups have reported anomalous particle morphologies, including raspberry-, half-moon-, and dumbbell-shaped particles, obtained when conducting emulsion polymerizations of acrylate monomers in the presence of seed particles, particularly the emulsion polymerization of styrene in the presence of a poly(butyl acrylate) (PBA, Tg = −54 °C) seed particles,17−19 or inversely, polymerizing butyl acrylate in the presence of polystyrene (PS, Tg = 100 °C) seed particles.20 The underlying reasons for the complex particle morphologies are ascribed to the phase separation whereby seed particles swollen with the secondary monomer phase separate upon polymerization. This phase separation throughout polymerization develops as a result of polymer/polymer incompatibility over the course of the reaction. To mediate the increase in interfacial energy upon formation of the separated domains within the particle, the morphology changes (if kinetically allowed) to achieve minimization in interfacial energy. A deciding factor between the observed geometries relies on the relative surface tensions between the two polymer phases and the interfacial tension between each polymer and the continuous phase.21 The use of surfactants, ionic comonomers, and initiator influences the ultimate particle morphology. An interesting approach to prepare a hard−soft dumbbell morphology in seeded emulsion polymerization is to make use of entropic phase separation of a secondary compatible “soft” polymer lobe from a lightly cross-linked polymer matrix and has been attempted by Mock et al.22 using polystyrene (PS) seed particles and N-isopropylacrylamide (NIPAm) as a second
INTRODUCTION Anisotropic “Janus” particles attract increasing interest because of their highly diverse and versatile nature, a key characteristic being that each particle features a specific property that is opposing in nature, for example, one side being hydrophilic while the other side is hydrophobic.1−3 This opposing anisotropy makes them attractive as colloidal objects for the fabrication of advanced materials and colloidal formulations. Predominant focus lies on the syntheses and applications of amphiphilic Janus particles, and the latter includes studies on how amphiphilic Janus particles adhere to and assembly on soft deformable interfaces, such as a liquid−liquid interface;4−7 how they can act as Pickering emulsifiers in emulsion polymerization;8 and how they can self-assembly into intricate supracolloidal structures.9−13 There is scope to investigate particles with alternative Janus type characteristics; for example, self-propelling microspheres that have one hemisphere covered by a thin deposited layer of platinum that catalyzes the decomposition of hydrogen peroxide from solution, triggering propulsion.14 A similar example is silica-based matchstick particles that have a catalytic head of manganese oxide and can undergo chemotaxis with low concentrations of hydrogen peroxide acting as attractant and fuel for propulsion.15 Herein, however, we are interested in the synthesis of dumbbell or peanut-shaped Janus particles that possess distinctive high and low glass transition temperature lobes and, therefore, are tentatively named “hard−soft” Janus particles. Owing to the differences of material behavior at each “side” of the particle, the ability for one lobe to deform under ambient conditions while the other remains a robust and solid entity presents an attractive feature of these Janus particles, which can be used for a variety of applications. We recently reported their hierarchical assembly into colloidal molecule-type clusters and their potential use as Pickering stabilizers.16 © 2014 American Chemical Society
Received: August 22, 2014 Revised: October 22, 2014 Published: October 24, 2014 13525
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filtered through a basic alumina column to remove inhibitor before use. Deionized water was used in all experiments. Synthesis of Micrometer-Sized Seed Particles. Styrene (25.0 g), AIBN (0.25 g), PVP (1.5 g), and methanol (70 g) were charged to a 250 mL round-bottom flask prior to stirring and nitrogen purge for 40 min, after which the flask was heated to 70 °C for 24 h while under stirring and slight nitrogen overpressure. Resulting particle dispersions were centrifuged and redispersed in methanol three times before analysis by optical microscopy and laser scattering analysis. Synthesis of “Hard−Soft” Particles by Seeded Dispersion Polymerization. A 27.0 g portion of the purified seed particle dispersion (solids content: 11 wt %), n-butyl acrylate (3.0 g), AIBN (0.1 g), PVP (0.03 g), water (5.7 g), and methanol (17.3 g) were charged to a round-bottom flask, stirred, and purged with nitrogen for 20 min prior to sealing and heating at 70 °C for 24 h. Kinetic Experiments of Seeded Dispersion Polymerization. For kinetic experiments, the same polymerization procedure was followed with varied quantities of 2.5 μm seed particles (0, 1.25, 2.5, 5.0, and 10.0 g, respectively) alongside n-butyl acrylate (5.0 g), AIBN (0.05 g), PVP (0.1 g), water (4.5 g), and methanol (40.5 g). During the polymerization, 2 mL aliquots were extracted at set time periods, aerated with oxygen, and cooled in ice to quench polymerization before measuring conversion gravimetrically and imaging by optical microscopy. Synthesis of Submicrometer Seed Particles. A 200 nm polystyrene seed dispersion was made first by emulsion polymerization. Briefly, to a 250 mL single jacketed glass reactor vessel fitted with a PTFE overhead stirrer with propeller blade (Radleys reactor duo), styrene (20.0 g), PVP-K30 (1.50 g), and deionized water (200 g) were added. The mixture was purged with nitrogen for 45 min while stirring prior to sealing with slight nitrogen overpressure. After heating to 70 °C, potassium persulfate (0.3 g) in 5 mL of degassed water was charged to the reactor. Polymerization ensued for 24 h before cooling to room temperature and dialysis against methanol using a cellulose dialysis membrane. Dialysis ensued for 7 days with solvent changing once a day. Excess PVP-K30 was removed afterward by three centrifugation cycles at 7882g for 60 min. The supernatant was disposed, and the particles were redispersed in methanol. Gravimetry was used to ascertain accurate solids content of the purified seed dispersion. Synthesis of Submicrometer “Hard−Soft” Particles. A 5.0 g portion of 200 nm seed particles in methanol (solids content: 12.3 wt %), n-butyl acrylate (0.35 g), AIBN (0.05 g), PVP (0.013 g), water (7.0 g), and methanol (22.5 g) were charged to a 50 mL round-bottom flask fitted with magnetic follower and rubber septum. The contents of the flask was purged with nitrogen for 20 min prior to sealing. Reactions were then heated to 70 °C for 24 h. During the polymerization, 2 mL aliquots were extracted at set time periods, aerated with oxygen, and cooled on ice to quench polymerization before measuring conversion gravimetrically and size analysis by DLS. Characterization. Particle sizing measurements of micrometersized colloids were undertaken using a Mastersizer 2000 (Malvern Instruments, Malvern, UK). Dilute particle suspensions in methanol were introduced using the μP dispersion unit. A laser was aligned to optimum transmission intensity, and a background spectrum was collected to eliminate any contaminants presenting in the final data. Dilute suspensions of 1 wt % were introduced dropwise by submerged syringe until 5% laser obscuration was reached. Dynamic light-scattering measurements were conducted using a Malvern instruments Zetasizer ZS, (Malvern, UK). Dilute 1 mL dispersions of particles in methanol were dispensed into plastic cuvettes and thermally equilibrated to 25 °C for 2 min prior to measurement of 3 sets of measurements, each of 10 subruns using 173° backscattering angle. Values for number (%) and volume (%) as a function of hydrodynamic diameter were used. Particle dispersions were imaged on a Leica DM2500 M in bright field transmission mode using a Nikon D5100 camera. Cryogenic scanning electron microscopy analyses were carried out using a Zeiss Supra 55-VP field emission gun scanning electron microscope with a Gatan Alto 2500 cryo transfer system and a Gatan
monomer (as a toluene solution). However, high-resolution scanning electron micrographs of these particles reveal that the PNIPAm protrusion is not well-defined and that much of PNIPAm has encapsulated the seed particle as a result of the soft hydrophilic nature of the PNIPAm enveloping the hydrophobic poly(styrene) seed over time. Overall, seeded emulsion polymerization is a challenging technique as a route to synthesize controlled Janus particle structures with a welldefined dumbbell shape, being very sensitive to cross-link density and, thus, swelling characteristics; the surface chemistry of the seed latex; and the projected chemical nature of the two polymer lobes. An alternative effective route toward anisotropic polymeric particles with varying morphology would be seeded dispersion polymerization. One of the motivating factors for synthesizing Janus particles by this method is the lack of swelling of the seed particles. In a dispersion polymerization system, the monomers are completely soluble in the continuous phase and generally do not swell the seed particles. This eliminates problems arising from the phase separation within seed particles. In addition, the nonaqueous environment of the continuous phase lowers the difference in interfacial tension between the continuous phase and the particle, thereby reducing the effect of charged residues from surfactants, initiators, or charged comonomers on final particle morphology. Okubo and coworkers fabricated “snowman” shaped particles through the seeded dispersion polymerization of butyl methacrylate using micrometer-sized poly(styrene) seeds.23 Particles with a single “lobe” were generated with smaller seed particles of 1.28 μm diameter, whereas “confetti” particles possessing multiple lobes from single seed particle were found to form when a larger seed particle of 2.67 μm diameter was used. This difference in morphology was attributed to the absorption of multiple radical oligomers onto the larger particles. They also conducted a seeded dispersion polymerization of butyl methacrylate in the presence of a poly(styreneco-sodium styrenesulfonate) seed particles and dodecane droplets. After polymerization, the dodecane was evaporated leaving dimples in the surface of the micrometer-sized particles to resemble “golf balls”.24 El-Aasser and coworkers showed that seeded dispersion polymerization of butyl acrylate in the presence of a poly(styrene) seed under alcoholic conditions using poly(vinylpyrrolidone) as a protective colloid rendered biphasic micrometer-sized dumbell particles.25 The surface of poly(styrene) seed particles grafted with a layer of steric surfactant provide the locus of nucleation for butyl acrylate; however, this work was briefly reported, detailing only the observation of particle geometry. Herein, we aim to investigate the kinetics and mechanism of seeded dispersion polymerization of butyl acrylate using polystyrene micrometer-sized seed particles. The intention is that the mechanistic understanding in the fabrication of these hard−soft Janus particles will allow us to tune particle size and respective lobe size. In addition, we explore an adaptation of this polymerization system to enable the synthesis of submicrometer-sized hard−soft Janus particles.
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EXPERIMENTAL SECTION
Materials. Styrene (Reagent Plus, 99%), n-butyl acrylate (>99%), poly(vinyl pyrrolidine) and (PVP-K90, 360 000 g mol−1 and K30, 40 000 g mol−1) were supplied by Sigma-Aldrich, UK. Azobis(isobutyronitrile) (97%), ethanol (absolute AR), and methanol (AR) were purchased from VWR and used as received. Monomers were 13526
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C1002 liquid nitrogen cold stage. Dilute aqueous suspensions (1 wt %) of particles were drawn into a brass rivet and cooled in liquid nitrogen. Samples were heated to −95 °C under high vacuum for 15 min to remove contaminant ice through sublimation, followed by platinum sputter target coating in an argon atmosphere (25 s, 10 mA). Imaging was undertaken at −120 °C using a 1.5 kV accelerating voltage with a gold anticontaminator at −189 °C. All image analysis was conducted using open source software ImageJ (NIH, US).
poly(butyl acrylate) as separate particles. In one example, the seed particle mass was equal to the butyl acrylate mass charged to the reaction. The resultant particles after the seeded dispersion polymerization can be seen in Figure 1b. Note that the darker sphere represents the poly(styrene) seed particle, and the lighter part represents the newly formed poly(butyl acrylate) lobe. There appears to be a clear delineation between the two phases, indicating that no swelling and subsequent polymerization has occurred, ruling out entropic phase separation from within the particle. We may therefore attribute the formation mechanism of the second PBA lobe to the capture of oligomeric radicals from the solution phase onto the particle surface during the initial stage of the seeded dispersion polymerization, where it further propagates to form a lobe. This mechanism falls in line with other studies on seeded dispersion polymerization.23 We should expect the average number of lobes on each particle to be dependent on the total seed surface area available to nucleate from. To probe this, a series of seeded dispersion polymerizations of butyl acrylate in the presence of a varied quantity of seed particles were performed to investigate the influence of seed particle concentration, that is, the total seed surface area, on the polymerization kinetics and growth of the PBA lobe from the seed particles. Note that no changes in diameter of the polystyrene lobes were observed after the polymerization of BA. Four polymerizations of butyl acrylate (10 wt %) with a range of seed particle concentrations outlined below in Table 1 were conducted in a methanol/ water medium
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RESULTS AND DISCUSSION The synthetic procedure to fabricate “hard−soft” Janus particles involves a two-step reaction. Initially, polystyrene seed particles were synthesized via a dispersion polymerization procedure in methanol to make spherical particles of 2.5 μm average diameter and a monodisperse size distribution. A typical procedure involves batch addition of styrene, poly(vinylpyrrolidone) K90 (PVP, 360 000 g mol−1) as protective colloid, thermal radical initiator AIBN, and methanol as the continuous phase. The PVP content was fixed at 5.5 wt % to styrene and AIBN 1 wt % to styrene. After polymerization, the dispersion was cleaned by centrifugation and redispersed into fresh methanol to remove excess PVP in the continuous phase. The remaining solids content, deemed mostly PVP, in the continuous phase after polymerization was found to be 3.21 wt % as determined by gravimetric analysis of the first supernatant from centrifugation purification. Final monomer conversions of seed particle dispersion polymerizations exceeded 95% as determined by gravimetry before centrifugation. The supernatant after the first sedimentation of seed latex was transparent, indicating the absence of sub-500 nm diameter secondary nucleated particles, given the speed and duration of centrifugation. The resulting particles can be seen optically in Figure 1a.
Table 1. Percentage of Particles with Multiple PBA Lobes for Each Seeded Dispersion Polymerization PS seed concentration (g/L) 203.66 90.50 27.88 20.92
seed number per liter
total seed surface area (m2/L)
particles with multiple lobes in population
× × × ×
448 199 83 46
2.60% 7.04% 7.54% 35.74%
2.11 9.37 4.23 2.17
1013 1012 1012 1012
(90:10 w/w) using AIBN as thermal initiator (1 wt % to BA) and PVP-K90 as stabilizer (2 wt % to BA). Note that a dispersion polymerization of butyl acrylate was conducted in the absence of seed particles to be used as a control experiment for comparison. Aliquots of reaction mixture were extracted at set time intervals and analyzed by optical microscopy and gravimetry for monomer conversion. Final monomer conversions of all seeded particle dispersion polymerizations exceeded 90% as determined by gravimetry. Less than 0.75 wt % solids were found in the continuous phase after gravity separation of particles in all seeded dispersion polymerizations. BA monomer conversion data (XM) were obtained by the gravimetric analysis of the seeded dispersion polymerizations, as plotted in Figure 2a. When a free radical polymerization rate equation are applied and the initiator decomposition rate was taken into account (eq 1), a first-order relationship exists between the monomer concentration and polymerization rate, except at the start of the reaction, where a small induction period can be seen (Figure 2b). This is attributed to the reaction reaching thermal equilibrium. The rate of polymerization is relatively unaffected by the presence of polystyrene seeds at all concentrations. However, upon closer inspection, it appears that the polymerization without poly(styrene) seeds is
Figure 1. Optical micrographs of (a) poly(styrene) seed particles and (b) “hard−soft” Janus particles (weight ratio of charged BA monomer to PS seeds is 1:1). The darker sphere represents the poly(styrene) seed, whereas the lighter part represents the poly(butyl acrylate) lobe.
To form the secondary “soft” lobe, a seeded dispersion polymerization of n-butyl acrylate was undertaken in the presence of the micrometer-sized polystyrene seed particles. This was conducted under batch conditions by addition of a cleaned seed particle dispersion, PVP K90 surfactant, n-butyl acrylate, and AIBN in a methanol−water medium. Methanol is used specifically in this reaction because longer chain alcohols are good solvents for poly(butyl acrylate). The PVP content was reduced to 1 or 2 wt % with respect to butyl acrylate. This was done to promote the growth of poly(butyl acrylate) particles off the polystyrene seed particles rather than to provide sufficient steric stabilization for the growth of 13527
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seen to progress at a slightly elevated rate in comparison with the seeded polymerizations during the first 150 min of the reaction. A potential explanation can be as follows: The elevated polymerization rate arises from the dependence of solution polymerization kinetics during the early stages of the polymerization on initiator concentration. The seed particles absorb some of the radicals, causing a temporary depression in polymerization rate. Arguably, increasing the seed particle concentration should enhance this effect, something that cannot be observed clearly from the data. To unravel the exact reason(s) for the observed slight retardation, a more detailed kinetic study is required. −ln(1 − XM) = 2k p
f [I]0 (1 − kdk t
e−kdt ) (1)
where XM is BA monomer conversion, kp is radical propagation rate, kt is radical termination rate, f is radical initiation efficiency, [I]0 is initiator AIBN concentration, kd is initiator decomposition rate (3.2 × 10−5 s−1 at 70 oC), and t is polymerization time. In comparison of our observations with that of previously reported literature, we find that our results appear to be inconsistent with that of the kinetics of seeded dispersion polymerizations of methyl methacrylate (MMA) in the presence of micrometer-sized PMMA seeds.18 In the PMMA system, the initial polymerization rate was observed to be higher by a factor of 1.3 compared with an identical nonseeded reaction. We attribute this disparity to the fact that a far higher monomer/polymer mass ratio (28:1) was employed in the PMMA system (compared with the 4:1 mass ratio used in our highest BA containing reaction), and an overall higher starting concentration of monomer in the continuous phase, which has been shown to increase initial polymerization rate. Conversely, Perro et al. report that the emulsion polymerization rate of styrene in the presence of silica seeds in manufacturing hybrid colloidal Janus particles by surface nucleation is not affected by
Figure 2. (a) Monomer conversion as a function of time in the seeded dispersion polymerizations at various seed particle concentrations given as seed surface area concentration; (b) First-order kinetic plot taking into account initiator decomposition of the seeded dispersion polymerizations.
Figure 3. Optical micrographs of particles during a seeded dispersion polymerization of BA in the presence of polystyrene seed particles (seed particle surface area: 199 m2/L). 13528
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the presence of seed particles in accordance with our observations.26 Optical micrographs of the seeded dispersion polymerizations were taken at set intervals throughout the polymerization. An example set of optical micrographs imaging the “hard−soft” particles can be seen in Figure 3. The PBA lobes grow gradually with the increasing polymerization time. On the basis of these images, quantitative measurements regarding the evolution of lobe volume can be taken to analyze particle growth kinetics. The volume of the growing poly(butyl acrylate) lobe was modeled as a spherical cap, whereby the PBA lobe was treated as a hemisphere of a sphere. Taking the height of the lobe (h) and radius (R), as displayed in Figure 4, the lobe volume can be calculated by following equation: V=
πh2 (3R − h) 3
Figure 5. Calculated PBA lobe volume based on optical micrographs as a function of monomer conversion during seeded dispersion polymerizations.
(2)
Figure 4. Schematic of the measurements taken to calculate volume of the poly(butyl acrylate lobe) (red) growing off a polystyrene seed particle (green).
It is worth noting that one assumption taken in this measurement is that the area of contact between the poly(styrene) and poly(butyl acrylate) lobe is treated as planar rather than a 3D curved surface. Despite the relatively negligible effect that seed particle concentration has on the rate of polymerization, its influence on lobe volume is far more significant. Values of the calculated volume for the poly(butyl acrylate) lobes are plotted as a function of monomer conversion in each reaction as depicted in Figure 5. Because the monomer concentration in each reaction is equivalent, yet the number of seed particles is varied, an increase in lobe volume for lower seed particle concentrations is expected. However, the lobe growth profile for the polymerization conducted with 83 m2/L seed surface area overlaps that of the polymerization for 199 m2/L seed surface area. It would be expected that the profile of 83 m2/L should exceed that of 199 m2/L. In addition, the observed lobe volume of the PBA lobes for the lowest seed surface area reactions of 46 and 83 m2/L do not reach the expected volume based on the mass of monomer at the set seed particle concentration, as expressed in eq 3 mBA ρpBA Vlobe = Np (3)
Figure 6. Optical micrographs of final seed particle dispersions after 24 h polymerization time for each of the seeded dispersion polymerizations. The seed surface area for each reaction is indicated on the micrographs.
poly(styrene) seed particles, each possessing one lobe of poly(butyl acrylate). However, the lowest seed surface area polymerizations of 46 and 83 m2/L indicate the presence of a large quantity of secondary nucleation. In addition, some poly(styrene) seed particles possessing more than one PBA lobe can be seen. The growth of the PBA lobe size can be calculated to allow the expected lobe volume based on BA monomer mass charged to the system and seed particle concentration to be compared with experimentally observed volume values. When the PBA lobe volume fraction is measured as a function of conversion, a linear dependence can be seen for all reactions, as shown in Figure 7. Lobe volume fraction is calculated by dividing the observed lobe volume during the polymerization by the final measured lobe size. It can also be seen that when high seed
where mBA is the mass of butyl acrylate, ρPBA is the density of PBA and Np is the number of seed particles. Optical micrographs of the final particle dispersions reveal why these disparities are observed for the reactions at lower seed surface area, as seen in Figure 6. The polymerizations containing the highest seed surface area reveal monodisperse 13529
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Figure 8. PBA lobe volume of Janus particles (black) and size of secondary nucleated particles (red) versus conversion in the seeded dispersion polymerization at 46 m2/L of seed surface area.
Figure 7. Measured lobe volume fractions normalized against expected lobe volume as a function of conversion. Black line represents the theoretical fractional growth profile for the reactions based on conversion data.
contrast to these works, our system is susceptible to secondary nucleation, therefore preventing controlled nucleation of a larger number of PBA lobes at consistent volume. The multilobed particles were reanalyzed by optical microscopy (Figure 9) after 7 days and found to have shown
concentration (with 199 and 448 m2/L of total seed surface area) applied, seeded dispersion polymerizations showed a consistent fit with the expected linear volume growth profile. The polymerizations incorporating a lower seed surface area (46 and 83 m2/L), however, deviate significantly from the linear growth curve as a result of secondary nucleation. This result provides insight into the particle formation mechanism for the seeded dispersion polymerization. All dispersion polymerizations start as a homogeneous solution whereby homogeneous nucleation occurs in the continuous phase. In our case of seeded dispersion polymerizations, as the oligoradicals grow toward a critical threshold size, they may be captured by the surface of a seed particle and commence nucleation on the surface of the seed particle. In reactions with high concentrations of seed particles, there is a large surface area available for capture; however, when the available seed surface area is too low, capture by a seed particle surface may not occur before the oligoradical reaches a critical threshold size and precipitates out into a secondary primary particle. Figure 8 shows secondary nucleated PBA particles present from the very early stage of the reaction (i.e. at 10% conversion) in the case of our lowest seed surface area (46 m2/L). To quantify the prevalence of seed particles possessing multiple lobes, populations of particles in each of the seeded dispersion reactions of varying seed surface area were counted for particles with multiple lobes, with over 150 particles being counted per sample particle dispersion (see Table 1). As expected, the number population of particles with multiple lobes increases as the seed surface area available to nucleate from decreases. This observation correlates with results reported from the seeded emulsion polymerization of styrene in the presence of silica seed particles, whereby varying the surface area of silica seed in the reaction determines the number of poly(styrene) lobes grown off a single seed particle.26 In this example, the total seed surface area in the reaction was varied by altering seed particle size; therefore, larger seed particles would reliably accommodate more poly(styrene) nodules, allowing the controlled nucleation of up to six lobes and, in extreme cases, many lobes, with the final particle resembling a raspberry structure.27,28 However, in
Figure 9. Optical micrograph of multilobe particles 7 days after synthesis indicating little change in particle morphology.
little difference in morphology or number population (with respect to single lobed particles). This would suggest that the poly(butyl acrylate) lobes are somewhat anchored to the surface of the poly(styrene) seed particle; otherwise, we would expect the multilobe particles to migrate into a single lobe by time to minimize unfavorable interfacial energies between the PBA and PS surface and equally to minimize the PBA− continuous phase interfacial tension. Once we achieved a good understanding of how to synthesize micrometer-sized “hard−soft” Janus particles, we were interested to see if we could scale-down our method into the regime of submicrometer-sized particles. As aforementioned, the synthesis of hard−soft Janus particles through seeded emulsion polymerization has proved difficult. The synthesis of micrometer-sized “hard−soft” Janus particles is simple through dispersion polymerization, as demonstrated above; however, the lower size limit of a reliable monodisperse seed made by dispersion polymerization is ∼1 μm. We therefore made our seed latex by emulsion polymerization, but with the condition that we required a submicrometer seed 13530
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Figure 10. (a) Conversion and hydrodynamic diameter (DLS) as a function of time for the styrene emulsion polymerization to synthesize the submicrometer seed particles; (b) BA monomer conversion as a function of time for a seeded dispersion polymerization of BA in the presence of submicrometer PS seed particles; (c) volume-averaged hydrodynamic diameter as a function of conversion for Janus particle samples extracted during the seeded dispersion polymerization.
particle with the same surface properties as the larger micrometer-sized system. Du et al. reported an emulsion polymerization procedure to render submicrometer poly(styrene) latex particles stabilized by PVP, thereby presenting an ideal route to fabricate our seed particles.29 The emulsion polymerization to prepare our seed latex was followed both by gravimetry and dynamic light scattering, and as such, the polymerization kinetics data and particle growth can be found plotted in Figure 10a. Monodisperse poly(styrene) particles were obtained with an average diameter of 200 nm. The use of PVP-K30 as a steric stabilizer ensures a layer of stabilizer on the surface of the particles, which acts as a favorable nucleation site for the subsequent seeded dispersion polymerization in the same manner as the larger micrometer-sized particles. Prior to the seeded dispersion polymerizations, the PS seed particles were dialyzed against methanol until dispersed in a pure methanol environment, after which the dispersion is cleaned of any remaining PVP by repeated centrifugation cycles in methanol. A similar strategy to the seeded dispersion polymerizations involving micrometer-sized PS particles was applied to nucleate and grow a secondary lobe of PBA from the surface of the submicrometer seed particles. Owing to the seed surface area dependence of the polymerization, the seed particle concentration was reduced to match the seed surface area per liter with that studied in the micrometer-sized system. This effectively reduced the seed solid content from 10 to 1.05 wt %. The total seed surface area here is ∼246 m2/L. Throughout the reaction, aliquots of reaction mixture were extracted from the reaction and analyzed for conversion gravimetrically, as can be seen in Figure 10b. The conversion−time data for the seeded dispersion polymerization indicates a reaction profile similar to that of the micrometer-sized seeded dispersion polymerization, reaching high conversion over 24 h of reaction time. A closer analysis of the volume-averaged hydrodynamic diameter of particles by dynamic light scattering was conducted during the dispersion polymerization. Peak values are plotted as a function of conversion in Figure 10c. An increase in particle size from the initial volume weighted seed diameter of 250 nm (note that the media here contains PVP) can be seen during the polymerization up to a final value of ∼280 nm in a manner similar to the results seen for the micrometer-sized seeded dispersion polymerizations. Most importantly, Cryo-SEM images reveal the characteristic dumbbell morphology of Janus particles (Figure 11). It clearly shows that smaller lobes of PBA were grown from the PS seed surface. Note that half the mass of butyl acrylate was added in comparison with the mass of seed particles for this reaction.
Figure 11. Cryo-SEM images of (a) submicrometer poly(styrene) seed particles and (a′) at high magnification; (b)submicrometer hard− soft Janus particles and (b′) at high magnification.
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CONCLUSION We demonstrate a reliable method to fabricate “hard−soft” Janus particles by a two-step seeded dispersion polymerization. By various optical and kinetic analyses, we find that the rate of polymerization during the seeded dispersion polymerization of butyl acrylate in the presence of PS seed particles is unaffected by the concentration of seed particles. However, a profound effect on the volume and number of PBA lobes per seed particle is observed. A low seed surface area concentration results in the formation of multilobed particles and significant secondary nucleation. As a result of this work, the optimum parameters for synthesizing “hard−soft” Janus particles can be found without undesired effects such as secondary nucleation and multilobed particles. In addition, we translate the synthetic procedure to allow the formation of submicrometer-scale “hard−soft” particles verified by cryo-SEM, therefore opening the range of accessible size for these particles from nano to micrometer scale. We believe that the “hard−soft” dumbbell particles are an exciting class of Janus particles, in addition to the more commonly made and used amphiphilic Janus particles and that their use can find exciting areas of application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions †
T.S.S. and Y.C. contributed equally.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Part of the equipment used in this research was supported by Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF).
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REFERENCES
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