Article pubs.acs.org/Macromolecules
Soft Multifaced and Patchy Colloids by Constrained Volume SelfAssembly Chris Sosa,† Rui Liu,† Christina Tang,† Fengli Qu,†,‡ Sunny Niu,† Martin Z. Bazant,§,∥ Robert K. Prud’homme,*,† and Rodney D. Priestley*,†,⊥ †
Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China § Chemical Engineering and ∥Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, United States ‡
S Supporting Information *
ABSTRACT: Soft colloidal particles with multiple surface patches of differing composition are critical to the development of complex macroscopic structures that can serve as interfacial catalysts, macroscale surfactants, electronically responsive materials, and drug delivery vehicles. Here, we present a continuous process for the scalable formation of soft colloidal particles with multiple surface domains that employs well-established principles of polymer precipitation and phase separation to controllably shape particle architectures. Our results illustrate the broad range of particle morphologies, including Janus and Cerberus structures, and surface compositions accessible to our versatile solution-based assembly system. We also identify polymer diffusion, precipitation, and vitrification as the primary determinants of particle structure for the first time.
C
size, surface functionality, and compositional anisotropy as the assembly process is scaled. We furthermore illustrate that tuning the molecular weight of the homopolymers and increasing the number of polymer components in the system can facilitate the formation of multifaced and multilobal nanocolloids, respectively. Realizing the full technological potential of multifaced colloids, though, will require their production at scales commensurate with other asymmetric molecules such as molecular surfactants and block copolymers, i.e., the kilogram scale or greater.3−7 In the current laboratory scale ∼3 kg/day of colloids would be produced by continuous production. We anticipate that our strategy, when scaled, will help access the transformative potential of soft multifaced nanocolloids in the rational design and large-scale production of functional nanomaterials. Janus nanocolloids, the simplest multiface particle named after the two-faced Roman god, can assemble into higher-order superstructures when induced to by various environmental stimuli.1,8 They can, for example, organize under magnetic9 or
olloids possessing patterned or structured surface domains of differing chemical composition can serve as nanoscale building blocks for the design of materials with molecular scale features that generate emergent behavior at the macroscopic scale.1−3 The multiscale functionality of such particles depends strongly on both the spatial topology and molecular properties of surface domains.1,2 Simultaneous control over these two features as well as overall particle size can consequently provide a means of tuning the macroscopic behavior of multifaced particles for applications in drug and gene delivery, electronic displays and sensors, and emulsion stabilization for cosmetic and home care formulations. Demonstrated here is a strategy for the self-assembly of multifaced nanocolloids through the continuous precipitationinduced, rapid demixing of polymer phases within a confined solution volume. Our results illustrate, for the first time, the fabrication of various multiface particles ranging from Janus and Cerberus to multilobal particles using a single assembly approach. Using a two-faced “Janus” colloid assembled from two simple homopolymers as a model system, we demonstrate the ability of our method to provide simultaneous control over particle © 2016 American Chemical Society
Received: April 7, 2016 Published: April 20, 2016 3580
DOI: 10.1021/acs.macromol.6b00708 Macromolecules 2016, 49, 3580−3585
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Figure 1. Schematic of the particle assembly and phase-separation process. (a) The confined impingement jet mixing system used to combine a solvent stream containing fully dissolved polymers with a nonsolvent stream for the polymers to drive polymer phase separation in a confined nanodroplet assembly volume. (b) SEM (top) and TEM (bottom) images of Janus particles composed of equal parts polystyrene (light region) and polyisoprene (dark region) fabricated by the solvent exchange process depicted in (a). (c) Proposed polymer−polymer phase separation mechanism by precipitation-induced surface nucleation and capillary instability. (d) Schematic and TEM images of particles with varying volume fractions of the two polymers.
electric fields10 to form patterned chains on solid substrates, undergo complex translational11 and rotational12 motion in alternating fields,13 migrate to the interface between two immiscible fluids in order to decrease the surface tension of macroscopic emulsions,14 and uniquely interact with cellular interfaces in order to facilitate the absorption of imaging or therapeutic agents.15 Janus colloids can be assembled from a broad variety of building blocks ranging from metals to polymers.14,16 The breadth of material properties exhibited by polymers as well as their ability to phase separate is particularly attractive for the generation of Janus as well as multiface colloids. While multiface colloids are challenging to create, specific methods have already been developed toward fabricating polymer Janus colloids. Scalability and comprehensive control over particle morphology, however, remain a technological challenge, especially when attempting to use the same fabrication approach for Janus, multiface, and patchy particles with no process modifications.17−19 One means of designing a facile and versatile route for the fabrication of both colloidal structures is through solutionbased self-assemblya strategy that is already used to create Janus colloids with high degrees of structural and compositional complexity.4,7,20,21 The primary means by which such particles are fabricated involves dissolving multiple, chemically distinct polymers in a mutually favorable solvent and gradually altering the solubility character of the solution until the polymer molecules coprecipitate into self-organized structures. Because of the slow time scale of solvent exchange, the final morphology adopted by the colloids via solution self-assembly is often at
thermodynamic equilibrium and unique to the particular processing conditions. Intermediate self-assembled structures that form on the way to the equilibrium particle morphology are therefore often difficult to capture. In addition, amphiphilic surfactant molecules or polymeric stabilizers can be used to establish the solution volume and improve particle morphological homogeneity.4,18,20 Despite significant progress and commercial demand, a challenge remains to develop a continuous, scalable, and simple particle fabrication system that offers comprehensive control over multiple particle features such as particle size, surface domain size, and surface topology.4,20 The phase separation of polymer blends, a self-directed physical process capable of generating multidomain structures at the nanoscale, is a promising route to fabricating structured multiface particles.22 Even though polymer phase separation has primarily been applied to bulk polymer films, the complex structures associated with it may be transferred to colloids in a controllable manner by confining the volume and time scale in which polymer demixing takes place. We demonstrate this concept by inducing the phase separation of dissimilar polymers precipitated from a common solvent via a confined impinging jet mixer, termed flash nanoprecipitation (FNP). In this manner, polymer phase separation is driven to occur within precipitating nanodroplets of polymer and solvent as the solvent rapidly exchanges (O(ms)) with an antisolvent during micromixing (see Figure 1).23 The process unit, FNP, has many distinctive advantages that render it a transformative route to Janus nanocolloids: (i) one-step and continuous process, (ii) 3581
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Macromolecules room temperature and low energy process, and (iii) proven scalability greater than 1400 kg/day of colloids. To illustrate the feasibility of the process, we first formed Janus nanocolloids of polystyrene (PS; Mw = 16 500 g/mol) and polyisoprene (PI; Mw = 11 000 g/mol) (XPS−PI = 0.07),24 where X, chi, is the Flory−Huggins interaction parameter between the two polymers. Here, tetrahydrofuran (THF) and water were selected as the solvent and antisolvent, respectively. The process conditions employed, e.g., jet velocity ∼1 m/s and 1 mm orifice, resulted in a Reynolds number ∼3500. As illustrated in Figure 1b, symmetric Janus nanocolloids with a diameter (d) ∼ 200 nm were formed. To demonstrate the versatility of the process, we generated PS−PI Janus nanocolloids of similar size but varying anisotropy in a systematic manner (see Figure 1d). The PS/PI interface is smooth as illustrated in Figure S1. Simultaneous control over Janus particle size and spatial anisotropy was achieved by simply altering the homopolymer feed ratio and overall feed concentration. This was accomplished without the need for additional process modifications or surfactant interfacial stabilizers. The self-assembled nanocolloids instead acquired their stability from a colloidally stabilizing surface charge of −33 mV that appears to have resulted from the unique interactions between the surrounding aqueous media and the hydrophobic particle surface.25 Importantly, the absence of surfactants allows for fully Janus interior and exterior structures, unlike most surfactant-based particle formation processes. An indispensable feature of the precipitation-induced selfassembly by FNP (PISA-FNP) method is that key process parameters can be independently manipulated to understand their influence on particle size and morphology as well as gain insight into the mechanism of Janus nanocolloid formation. For instance, the representative images of PS−PI Janus nanocolloids processed as a function of overall feed concentration and polymer ratio in Figure 2a illustrate that increasing the overall feed concentration from 0.1 to 1.0 mg/mL systematically increases the size of the Janus nanocolloids from ∼125 to ∼540 nm in diameter (all particle sizes are based on light scattering measurements) (see Figures S2 and S3). The particle anisotropy can furthermore be tuned independently at each feed concentration by altering the PS−PI polymer ratio from 20:80 to 80:20. As the overall feed concentration is increased to 2 mg/mL, the ability to form Janus nanocolloids depends on the feed ratio of PS to PI. At low PS/PI feed ratios, Janus colloids are observed. However, as the PS/PI feed ratio increases, multifaceted colloids are observed. Wide-field TEM images (Figure S4) confirm the morphological homogeneity among numerous particles. The phase diagram presented in Figure 2A suggests a competition between the time scales of polymer phase separation in confined environments and the vitrification time of PS, as set by the volumetric flow rate. According to this hypothesis, manipulating the time scale of either polymer phase separation or solvent exchange can shift the phase boundary between Janus and multifaceted internal structures in a controlled manner. To investigate this effect, we elected to operate the FNP process under identical conditions but increase the polymer molecular weight. Figure 2B show representative images of PS−PI Janus nanocolloids processed as a function of polymer ratio and overall feed concentration in which the PS and PI Mw were 1500 and 1000 kg/mol, respectively. At a feed concentration of 0.1 mg/mL, Janus nanocolloids were observed, illustrating that even high-Mw
Figure 2. Phase diagrams of two-component particles fabricated under varying feed stream conditions. TEM images of (A) low-Mw and (B) high-Mw two-component particles prepared from varying overall solvent feed stream concentration and ratio of polystyrene and polyisoprene in the feed stream. (C) Collapse of the data from (A) and (B), where multiface and Janus particles are separated by a simple scaling relation (see eq 1). 3582
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Figure 3. Varying surface domain composition and properties of Janus and patchy colloids. TEM images of (a−c) Janus particles prepared with PS and hydroxy-terminated PB (a), hydroxy-terminated PS and hydroxy-terminated PB (b), and hydroxy-terminated PS and PB (c). (d, e) New Janus particles formed from PS and poly(lactic acid) (d) as well as poly(lactic acid) and PB (e) homopolymer pairings in the solvent feed stream. (f) Novel trilobal Cerberus particles possessing three phase-separated polymer surface domains composed of polystyrene, polybutadiene (dark middle region), and polyvinylcyclohexane. Insets (g, h): TEM images of the polymers used to create particle (f) in the form of two-component polyvinylcyclohexane−polybutadiene (g) and polystyrene−polybutadiene (h) Janus particles.
The morphology phase diagram for the particles (see Figure 2) is qualitatively consistent with a simple scaling theory based on surface nucleation.26 The nearly uniform particle size R(c) for different PS:PI mixtures at the same feedstream solvent concentration (c) suggests that phase separation occurs mainly after the flash precipitation of homogeneous particles. Phase separation due to spinodal decomposition would lead to random snake-like structures27 or coherent stripes28 that are not observed under these process conditions. A potential mechanism is heterogeneous nucleation by surface wetting.26 Relaxation to the Janus structure occurs if the PS diffusion distance (Dτ)1/2 during the vitrification time τ exceeds the surface layer coalescence distance, scaling as
polymers have sufficient mobility at dilute concentrations to self-organize into fully segregated polymer domains prior to kinetic trapping. As the overall feed concentration was increased, multifaced nanocolloids formed, particularly at high PS/PI feed ratios. The structural features observed in the processed nanocolloids are consistent with the suggestion that internal particle formation proceeds via the phase separation of viscous fluids in a confined environment (Figure 1c). The equilibrium Janus structure adopted by PS and PI at low feed concentrations suggested that the two polymers self-organized into two phaseseparated hemispherical domains in order to minimize the total interfacial energy of the ternary phase (polymer−polymer− liquid) system. The Janus morphology, therefore, emerged because the two polymers possessed similar interfacial energies with the THF/water solution (γPS−water ∍ γPI−water) and a low interfacial energy between themselves (γPS−PI < γPS−water and γPI−water) while still forming a stable contact angle. When either the feed concentration or molecular weight of the PS and PI was significantly increased, the time scale over which the polymers phase separated during the assembly process was sufficiently increased above the vitrification time of PS to trap the internal colloidal structure in a nonequilibrium multifaceted state. Since the rate of phase separation decreases by ∼N2, where N is the degree of polymerization, the high-Mw polymers could generate multifaceted structures at lower polymer feed concentrations than their low-Mw counterparts (see Supporting Information). The role of PS as a structural trapping agent, moreover, allowed for the enhanced capture of nonequilibrium structures in particles with a high PS content.
−1/2 ⎛ PI ⎞⎟ R (c ) = ⎜1 + ⎝ PS ⎠
(1)
This dimensionless criterion for Janus particle formation, which scales as R̃ =
R < Dτ
1+
PI PS
(2)
successfully predicts (dotted line is scaling prediction) the formation of Janus versus patchy particles in Figure 2C by collapsing the experimental data from Figures 2A and 2B. While the surface structure of nanocolloids strongly influences functionality, the material composition of surface domains determines the types of interactions the colloids exhibit with external environments. We have thus extended the PISA−FNP methodology to two other systems: (i) PS−PI Janus nanocolloids with varying polymer end-group functionality and (ii) Janus nanocolloids with new polymer 3583
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solution of the same concentration to generate the solvent feed stream for the PISA−FNP process. The solvent stream was then mixed with an equal volume of nonsolvent (filtered DI water) in our impingement jet mixers at jet velocities of approximately 1 m/s before pouring into a large, rigorously stirred nonsolvent bath. The ratio between the volumes of the feed solvent stream, nonsolvent stream, and nonsolvent bath was 1:1:1. TEM Sample Preparation and Imaging. 500 μL of the precipitated particle solution was transferred to a centrifuge tube and stained with a 0.2 wt % aqueous solution of OsO4 (Electron Microscopy Sciences) as described by Higuchi et al.20 3.5 μL of the stained particle solution was then deposited on a carbon-coated, copper TEM grid (CF-300-Cu, Electron Microscopy Sciences) and allowed to dry under ambient conditions. The sample was then imaged on a CM200 FEG-TEM with a Gatan 678 imaging filter using an accelerating voltage of 200 kV. Scaling Relationship. The scaling relationship described in the text was prepared with diffusion coefficients of 14 × 107 and 6 × 107 nm2/s for low- and high-MW PS, respectively, as well as a vitrification/ mixing time of 1 ms. Dynamic Light Scattering Measurements of Particle Size. The sizes of the aqueous particle dispersions were measured on a ZetaSizer Malvern dynamic light scattering instrument (Malvern Instruments, Malvern, UK) with a 633 nm laser and backscatter detection angle of 173°.
components. As illustrated in Figures 3a−c, PS−PI Janus nanocolloids were prepared with polymer surfaces containing varying amounts of hydrogen or hydroxyl moieties. This was achieved by using homopolymers with different end-group functionalities in the feed stream rather than chemically altering the particles postfabrication. The surface functionality of the Janus colloids can thus be systematically tuned accordingly and allows for further chemical modification of the particles as needed for specific applications. PS−PI Janus nanocolloids with carboxyl functionalities were also demonstrated (Figure S5). The alteration of full domain composition, on the other hand, is illustrated in Figures 3d,e with new Janus particles formed from PS−poly(lactic acid) (PLA) and polybutadiene (PB)−PLA polymer pairings. The process is therefore capable of producing Janus nanocolloids from varied polymer combinations, including those consisting of biodegradable materials, despite the significant dissimilarity between the properties of paired polymer components. One of the advantages of the PISA−FNP system is that more than two homopolymers can be simultaneously fed into the system, opening the possibility of generating complex nanocolloids by a facile bottom-up self-assembly approach. We found that feeding an equal ratio of three immiscible polymers PS (Mw = 16 500 g/mol), PB (Mw = 18 000 g/mol), and polyvinylcyclohexane (PVCH) (Mw = 25 000 g/mol) dissolved in THF and coprecipitated with an aqueous antisolvent led to the formation of patchy, trilobal nanocolloids (XPS−PI = 0.07, XPS−PVCH = 0.32)29 (Figure 3f, see Supporting Information). To our knowledge, this is the first observation of such a threecomponent, nanocolloid often described as a Cerberus or patchy particle processed in scalable manner. The ability to easily create Cerberus as well as Janus and multifaced nanocolloids from two or more homopolymers not only attests to the versatility of our process but also affords opportunities to construct more sophisticated multisurface colloids in the future. Through PISA−FNP, colloidal size, anisotropy, and surface functionality can be independently controlled, providing a rapid, solution-based strategy for the formation of soft multifaced nanocolloids. The simplicity and scalability of the process, furthermore, provide a platform for Janus particle production commensurate with current technological interest. We anticipate that this discovery will lead to further advancements in the bottom-up design of heterogeneous materials from the nano- to macroscopic scale and ultimately to their commercial realization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00708.
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Additional electron microscopy images and DLS data (Figures S1−S6) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected] (R.K.P.). *E-mail
[email protected] (R.D.P.). Author Contributions
C.S., R.L., C.T., R.K.P, and R.D.P. designed experiments. C.S. conducted experiments and analyzed data. C.S., R.L., C.T., F.Q., R.K.P., and R.D.P. discussed and interpreted the results while M.Z.B. provided the scaling argument. C.S., F.Q., S.N., R.K.P., and R.D.P. prepared the manuscript. Notes
The authors declare no competing financial interest.
MATERIALS AND METHODS
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Materials. Polystyrene (Mw = 16 500 g/mol, PDI = 1.03; Mw = 1500 kg/mol, PDI = 1.1), polyisoprene (Mw = 11 000 g/mol, PDI = 1.06; Mw = 1000 kg/mol, PDI = 1.05), polybutadiene (Mw = 18 000 g/ mol, PDI = 1.04), polyvinylcyclohexane (Mw = 25 000 g/mol, PDI = 1.09), hydroxyl-terminated polybutadiene (Mw = 12 500 g/mol, PDI = 1.03), hydroxyl-terminated polystyrene (Mw = 16 000 g/mol, PDI = 1.09), carboxy-terminated polystyrene (Mw = 16 500 g/mol, PDI = 1.06), and carboxy-terminated polybutadiene (Mw = 10 400 g/mol, PDI = 1.04) were all purchased from Polymer Source Inc. Polylactic acid (Mw = 11 000 g/mol) was obtained from Lakeshore Biomaterials. The tetrahydrafuran (THF) used to dissolve the polymers was purchased from Fisher-Scientific. The DI water for the experiments was filtered through a 0.2 μm filter (Nanopure Diamond, Barnstead International, Dubuque, IA) to remove any potential contaminants. Nanoparticle Assembly. Homopolymers were dissolved in THF to create a solution of a given polymer concentration and then combined in a 1:1, 4:1, or 1:4 ratio with a second homopolymer
ACKNOWLEDGMENTS C.S. acknowledges support from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Contract DE-AC05-06OR23100. R.D.P. acknowledges support of the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860), the NSF through a CAREER Award (DMR-1053144), and the AFOSR through a YIP Award (FA9550-12-1-0223). We thank Dr. Nan Yao, John Schreiber, and Yao-Wen Yeh from Princeton’s Imaging and Analysis Center for their electron microscopy assistance. 3584
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