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Shape-Tunable Synthesis of Sub-Micron LensShaped Particles via Seeded Emulsion Polymerization Wei-Han Chen, Fuquan Tu, Laura C. Bradley, and Daeyeon Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00494 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017
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Chemistry of Materials
Shape-Tunable Synthesis of Sub-Micron Lens-Shaped Particles via Seeded Emulsion Polymerization Wei-Han Chen †, Fuquan Tu,† ,‡ Laura C. Bradley, and Daeyeon Lee* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States * Corresponding Author email:
[email protected] †Authors contributed equally to this work. ‡Current address: Dow AgroSciences LLC, Actives to Products, Indianapolis IN 46268, USA ABSTRACT: We present a new method to synthesize highly uniform sub-micron lens-shaped particles based on seeded emulsion polymerization. Particles made of linear polystyrene are used as seeds for emulsion polymerization of a sacrificial monomer. The sacrificial polymer is removed through hydrolysis producing non-spherical lens-shaped particles. The shape of these particles is controlled by interfacial tensions among the three phases: polystyrene, sacrificial polymer, and the aqueous phase with a surfactant, and can be modeled by minimizing the surface energy of the particle. We show that various shapes including biconvex, planoconvex and concavo-convex particles can be obtained by changing the composition of the sacrificial polymer and the surfactant as well as the volume ratio of linear polystyrene and the sacrificial polymer during seeded emulsion polymerization. Our method presents a framework to produce large amounts of shape-tunable non-spherical particles that can be used in fundamental study of selfassembly and packing and in practical applications involving building blocks, emulsification and drug delivery.
Non-spherical particles present tremendous opportunities in addressing fundamental questions in self-assembly and packing and at the same time exploring new applications in biomedicine, coatings and structural materials. Anisotropic particles, for example, can be used as building blocks to construct diverse structures including rods, pyramids, chains and lockand-key structures.1-4 It has also been shown non-spherical particles can suppress the formation of coffee rings that are commonly observed in drying of particle dispersions; such uniform films of non-spherical particles can be used in ink-jet printing technologies.5 The shape of particles also has shown to influence their uptake by living cells, suggesting new strategies to enhance or suppress delivery of active agents in biomedicine.6 Furthermore, suprastructures with interesting optical and mechanical properties have been assembled with nonspherical particles.7 Disordered packings of ellipsoidal nanoparticles, for example, have increased toughness compared to packings of spherical particles at the same particle volume fraction.8 Numerous techniques including emulsion-based methods,910 nucleation and reshaping,11 mechanical deformation,12-13 photolithography,14 and microfluidics15-17 have been developed to synthesize non-spherical particles. Despite these advances, it remains a great challenge to synthesize uniform submicron non-spherical particles with shape tunability. In particular, the synthesis of biconvex or plano-convex submicron particles using potentially scalable methods has not been extensively demonstrated. These lens-shaped particles offer intriguing possibility in controlling their interactions with each other or with solid surfaces or interfaces.18-19 For example, a monolayer film of hemispherical particles on a solid surface has enhanced adhesion compared to spherical particles due to increased con-
tact area. Also flattened spheres assemble into chains because of anisotropy of interactions between these particles. Lensshaped particles can also be used as microlenses to enable super-resolution microscopy.20 Here, we present a framework for the scalable synthesis of lens-shaped particles that are both uniform and shape-tunable based on seeded emulsion polymerization, followed by the removal of a sacrificial polymer. We show that the shape of particles can be tailored by changing the interfacial tension among the three primary phases as well as by changing the ratio of the seed particle to the sacrificial monomer. Our work provides a robust method to synthesize large amounts of uni-
Figure 1. (a) Schematic illustration of the synthesis of lensshaped PS particles. (i) Seed particle PS swelled by tBA monomer; (ii) polymerization-induced phase separation and toluene is evaporated; (iii) acid hydrolysis to remove a sacrificial polymer phase. (b) and (c) shows SEM images of lens-shaped PS particles synthesized using 50 and 90 vol.% of toluene in the oil phase of the emulsion, respectively.
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form lens-shaped particles with shape tunability, which could have broad impacts in fundamental studies involving selfassembly and packing and also in practical applications including coatings, structural materials and drug delivery. The key feature of our method relies on seeded emulsion polymerization of linear polystyrene (PS) seed particles with a sacrificial polymer (SP), and the subsequent removal of SP via hydrolysis (Figure 1a). This method is different from our previous protocol for synthesizing polymeric Janus particles in that the current method does not use a crosslinker. Monodisperse seed particles made of linear PS (884.0 ± 36.1 nm diameter) are synthesized by dispersion polymerization. These seed particles are swollen by an emulsion made of a dispersed phase of tert-butyl acrylate (tBA) (monomer for SP), toluene, and 0.5 wt.% 2,2’-azobis(2, 4-dimethyl valeronitrile) (thermal initiator) in an aqueous phase containing 0.4 wt.% sodium dodecyl sulfate (SDS). Polymerization is carried out by placing the mixture in an oil bath at 70 °C for 24 hr. After polymerization, toluene is removed by evaporation resulting in Janus particles of which one side is PS and the other side the SP, poly(tert-butyl acrylate) (PtBA).21 Since no crosslinker is added with tBA, PtBA can be removed by performing acid hydrolysis to convert PtBA to poly(acryl acid) (PAA), which is highly water soluble. Previous studies have synthesized non-spherical particles similar in shape by first preparing spherical composite particles composed of PS and poly(methyl methacrylate) with different internal structures and then separating the two phases by solvent-driven cleavage.22 Nonspherical particles made using this method, however, were neither monodisperse nor uniform. Our use of PtBA circumvents such problems because its hydrolysis allows its complete removal by washing the particles with deionized water, producing uniform lens-shaped particles. One of the key innovations that allow for the synthesis of highly uniform lens-shaped particles is the addition of toluene to the monomer mixture. With insufficient toluene, nonspherical particles with significant shape irregularity are obtained (Figure 1b; also see Supporting Information). We believe that the high viscosity microenvironment within each particle during the polymerization of tBA likely impedes the diffusion of polymers and results in incomplete phase separation,23-24 producing multiple patches of PtBA; the removal of these patches via hydrolysis leads to the formation of irregularly shaped particles. Thus, toluene is increased to 90 vol.% in the monomer emulsion to facilitate phase separation and is subsequently evaporated off after seeded emulsion polymerization and prior to the hydrolysis of SP. Based on this approach, highly uniform biconvex particles are produced (Figure 1c). Our scheme suggests that the lens-shaped particles should be made of linear PS, which we confirm using Fourier transform infrared (FTIR) spectroscopy. The spectrum of particles before hydrolysis contains the characteristic signals for the benzene ring of PS at 1420-1620 cm-1 and the carbonyl of PtBA at 1730 cm-1 (Figure 2a). After hydrolysis and removal of the sacrificial PtBA, the FTIR spectrum contains only the characteristic signals for PS. The absence of PtBA in the final non-spherical particles confirms the polymers achieve complete phase separation forming a Janus structure consisting of a pure PS side and a pure PtBA side, and the sacrificial PtBA was removed completely. The fact that lens-shaped particles are made purely of linear PS can also be demonstrated by
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Figure 2. (a) FTIR spectra comparing the sacrificial polymer PtBA, PS seed particles, PS-PtBA Janus particles before hydrolysis, and non-spherical PS particles after removal of PtBA. (b) SEM images of lens-shaped PS particles after swelling with toluene to re-shape the particles to spherical shape. Inset: Size distribution compared with linear PS particles.
Figure 3. (a) Interfacial tensions among three phases: PS, SP and water phase. (b) Phase diagram based on three interfacial tensions (γPS/W, γSP/W, γPS/SP) to obtain different shapes of PS particles at equilibrium. swelling the particles with toluene (a good solvent for PS) and evaporating it off. Biconvex particles become spherical after the toluene treatment (Figure 2b). The average diameter of the toluene-treated particles (900.6 ± 38.1 nm) is similar to that of the original PS seed particles (884.0 ± 36.1 nm) (Figure 2c), again verifying that complete phase separation was achieved during seeded emulsion polymerization and that PtBA was completely removed via hydrolysis. We hypothesize that the shape of these lens-shaped particles derives from the structure of the intermediate PS-PtBA Janus particles. It has been previously shown that the morphology of Janus droplets (i.e., liquid droplets made of two immiscible phases) depends on the interfacial tensions among three liquid phases: the two immiscible liquids of the droplet and the continuous phase.16-17,25-26 Specifically, biphasic Janus droplets form when the contact angles at the three phase contact line (θPS and θSP; see Figure 3a for definition of each contact angle) are both less than 180° and together add up to be more than 180°.27 Conditions for forming different Janus structures can be summarized on a phase diagram relating the resulting morphology to the ratios of γ PS W γ PS SP and γ SP W γ PS SP (Figure 3b). Janus droplets form in the enveloped shaded region; outside this region droplets form either separated droplets or engulfed (core-shell) structures due to large differences in wetting between the three phases. The microstructure of individual Janus droplets can be tuned within the
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Chemistry of Materials
shaded region by carefully varying the two interfacial tension ratios, γ PS W γ PS SP and γ SP W γ PS SP .
Figure 4. Summary of lens-shaped PtBMA) and surfactant (SDS or CTAB). The volume ratio of SP to PS seed particles was 2:1 in all cases. The scale bars are 1 µm. To obtain different lens-shaped particles, we vary the relevant interfacial tensions by choosing different combinations of SP and surfactant. We choose either PtBA or poly(tert-butyl methacrylate) (PtBMA) as SP and either SDS and cetyltrimethylammonium bromide (CTAB) as the surfactant in the water phase. Figure 4 shows the results of different combination of SP and surfactant when the volume ratio of PS and SP is kept constant at 1:2. Hereafter, “(SM)n-X” is used to describe the composition of the particles where n denotes the volume ratio of sacrificial monomer (SM) to PS, and X indicates the surfactant used in the synthesis. PtBMA is chosen as a substitute for PtBA because hydrolysis converts PtBMA to water-soluble poly(methacrylic acid) (PMAA). By replacing tBA with tBMA, we expect γPS/SP to decrease and γSP/W to increase, because of the extra methyl moiety in each repeat unit. Based on the phase diagram, particles prepared with tBMA should start to adopt plano-convex shape (Figure 3b). Particles synthesized with tBMA indeed have such a shape as shown in Figure 4. Alternatively, we can tune γPS/W and γSP/W by replacing SDS with CTAB. Experimental determination of interfacial tension indicates that γPS/W becomes smaller, whereas γSP/W becomes larger, when SDS is replaced with CTAB in the aqueous phase (Supporting Information); thus we expect the particles to adopt concavo-convex shapes with CTAB. This hypothesis is consistent with our experimental results; when SDS is replaced with CTAB, both tBA- and tMBA-based particles adopt concavo-convex shapes. The shape of lens-shaped particles can also be controlled by changing the volume ratio of sacrificial monomer (SM) to PS while maintaining the composition of sacrificial monomer and the aqueous phase surfactant. With decreasing n, the particle shape changes while maintaining the overall particle geometry. For (tBA)n-SDS biconvex particles, the curvature at the Janus boundary increases with decreasing n, and the particles evolve from more symmetric biconvex to highly asymmetric biconvex particles (Figure 5a-d). For (tBMA)n-SDS plano-convex particles, decreasing n reduces the area of the flattened side, which, at the same time, leads to a shape transition from sub-hemispherical caps to truncated spheres (Figure
5e-h). Decreasing n for (tBA)n-CTAB concavo-convex particles decreases the area of the concave side and decreases its curvature such that the particle shape transitions from being plano-convex to spheres with well-defined cavities/dimples (Figure 5i-l). We use a surface energy minimization scheme to validate the changes in particle shape as a function of n. Surface Evolver minimizes the surface energy of the system (in this case, a biphasic particle in a liquid medium) via a gradient descent method to find the final particle shape. The ratios of γ PS W γ PS SP and γ PS W γ PS SP are estimated by qualitatively matching the simulated particle shapes to the experimentally observed shapes when n = 2 (Figure 5b, f, j). Particle shapes predicted for different values of n are in good agreement with experimentally obtained shapes, verifying that understanding the interplay between three interfacial tensions and the volume ratio of the sacrificial monomer to polystyrene provide a robust way of controlling and predicting particle shapes. In conclusion, we demonstrate that highly uniform lensshaped particles with shape tunability can be synthesized based on seeded emulsion polymerization of PS seed particles with a sacrificial monomer. The particle shape can be tuned by a number of parameters, such as the type of sacrificial polymer or surfactant, and volume ratio of sacrificial monomer to seed particles. The novelty of this synthesis lies in the fact that a framework for large-scale synthesis of submicron biconvex and plano-convex particles is presented, which has not been demonstrated in the past. The ability to produce highly uniform and tunable non-spherical particles will facilitate their application in fundamental study of self-assembly as well as in practical applications including drug delivery and coatings/structural materials.28 We believe this scheme can be generalized to synthesize lens-shaped particles made of a variety of polymers including those that are biodegradable and biocompatible.6,29 Our ongoing study focuses on understanding the packing behavior of these lens-shaped particles as well as their interactions with each other and solid or liquid surfaces.
Figure 5. Schematics, SEM images and Surface Evolver models showing changes in the shape of lens-shaped PS particles based on different sacrificial monomer or surfactants with increasing volume ratio n of SM to PS from 2, 1, to 0.5. (a)-(d) (tBA)n-SDS, (e)-(h) (tBMA)n-SDS, (i)-(l) (tBA)nCTAB.
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ASSOCIATED CONTENT Supporting Information. Experimental methods and synthesis procedures, including linear polystyrene (LPS) seed particles, lens-shaped particles, and interfacial tension ratios used for Surface Evolver simulations, are provided.
AUTHOR INFORMATION Corresponding Author *Email:
[email protected] ACKNOWLEDGMENT We acknowledge funding from Penn MRSEC (DMR11-20901), Penn Provost’s Fellowship for Academic Diversity, and the Africk Family Post-Doctoral Fellowship. We thank Prof. Cherie R. Kagan for the use of FTIR and Prof. Kathleen J. Stebe for helpful discussion.
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(14) Zhao, K.; Harrison, C.; Huse, D.; Russel, W.; Chaikin, P. Nematic and Almost-Tetratic Phases of Colloidal Rectangles. Phys. Rev. E 2007, 76, 040401. (15) Kim, S.-H.; Abbaspourrad, A.; Weitz, D. A. Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids. J. Am. Chem. Soc. 2011, 133, 55165524. (16) Nisisako, T.; HATSUZAWA, T. Biconvex Polymer Microlenses Fabricated from Microfluidic Janus Droplets. J. Jpn. Soc. Precis. Eng. 2013, 79, 460-466. (17) Nisisako, T.; Torii, T. Formation of Biphasic Janus Droplets in a Microfabricated Channel for the Synthesis of Shape‐Controlled Polymer Microparticles. Adv. Mater.2007, 19, 1489-1493. (18) Ramírez, L. M.; Milner, S. T.; Snyder, C. E.; Colby, R. H.; Velegol, D. Controlled Flats on Spherical Polymer Colloids. Langmuir 2009, 26, 7644-7649. (19) Choi, H. K.; Yang, Y. J.; Park, O. O. Hemispherical Arrays of Colloidal Crystals Fabricated by Transfer Printing. Langmuir 2013, 30, 103-109. (20) Lee, J. Y.; Hong, B. H.; Kim, W. Y.; Min, S. K.; Kim, Y.; Jouravlev, M. V.; Bose, R.; Kim, K. S.; Hwang, I.-C.; Kaufman, L. J. Near-Field Focusing and Magnification through Self-Assembled Nanoscale Spherical Lenses. Nature 2009, 460, 498-501. (21) Tu, F.; Lee, D. Shape-Changing and AmphiphilicityReversing Janus Particles with pH-Responsive Surfactant Properties. J. Am. Chem. Soc. 2014, 136, 9999-10006. (22) Yamashita, N.; Konishi, N.; Tanaka, T.; Okubo, M. Preparation of Hemispherical Polymer Particles by Cleavage of a Janus Poly (methyl Methacrylate)/Polystyrene Composite Particle†† Part CCCLVII of the Series “Studies on Suspension and Emulsion”. Langmuir 2012, 28, 12886-12892. (23) Cho, I.; Lee, K. W. Morphology of Latex Particles Formed by Poly (methyl methacrylate) ‐ Seeded Emulsion Polymerization of Styrene. J. Appl. Polym. Sci. 1985, 30, 1903-1926. (24) Akiva, U.; Margel, S. Surface-Modified Hemispherical Polystyrene/Polybutyl methacrylate Composite Particles. J. Colloid Interface Sci. 2005, 288, 61-70. (25) Liu, S.-S.; Wang, C.-F.; Wang, X.-Q.; Zhang, J.; Tian, Y.; Yin, S.-N.; Chen, S. Tunable Janus Colloidal Photonic Crystal Supraballs with Dual Photonic Band Gaps. J. Mater. Chem. C 2014, 2, 9431-9438. (26) Roh, K.-H.; Martin, D. C.; Lahann, J. Biphasic Janus Particles with Nanoscale Anisotropy. Nat. Mater. 2005, 4, 759-763. (27) Guzowski, J.; Korczyk, P. M.; Jakiela, S.; Garstecki, P. The Structure and Stability of Multiple Micro-Droplets. Soft Matter 2012, 8, 7269-7278. (28) van Anders, G.; Klotsa, D.; Ahmed, N. K.; Engel, M.; Glotzer, S. C. Understanding Shape Entropy through Local Dense Packing. Proc. Natl. Acad. Sci. 2014, 111, E4812E4821. (29) Hans, M.; Lowman, A. Biodegradable Nanoparticles for Drug Delivery and Targeting. Curr. Opin. Solid State Mater. Sci. 2002, 6, 319-327.
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