Scalable Platform for Structured and Hybrid Soft Nanocolloids by

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Scalable Platform for Structured and Hybrid Soft Nanocolloids by Continuous Precipitation in a Confined Environment Victoria E. Lee,† Chris Sosa,† Rui Liu,‡ Robert K. Prud’homme,† and Rodney D. Priestley*,†,§ †

Department of Chemical and Biological Engineering and §Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, United States ‡ Ministry of Education Key Laboratory of Advanced Civil Engineering Material, School of Materials Science and Engineering and Institute for Advanced Study, Tongji University, Shanghai, China 201804 S Supporting Information *

ABSTRACT: Geometrically structured polymer nanocolloids, including Janus nanocolloids, have been widely investigated for their unique properties, which are derived from their anisotropy. Controlled surface decoration with inorganic nanoparticles could induce another level of functionality into structured nanocolloids that could enable applications in fields ranging from rewriteable electronics to biphasic catalysis. Here, we demonstrate flash nanoprecipitation (FNP) as a one-step, scalable process platform for manufacturing hybrid polymer− inorganic nanocolloids in which one phase is selectively decorated with a metal nanocatalyst by tuning the molecular interactions between the feed ingredients during the process. For instance, by modifying the polymer end-group functionality, we document the ability to tune the location of the metal nanocatalyst, including placement at the nanocolloid circumference. Moreover, the addition of molecular additives is shown to transform the Janus nanocolloid structure from spherical to dumbbell or snowman while maintaining the ability to control the nanocatalyst location. In considering the flexibility and continuous nature of the FNP process, it offers an industrial-scale platform for the manufacturing of nanomaterials that are anticipated to impact many technologies.

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inorganic materials would be attractive from an industrial viewpoint. Recently, flash nanoprecipitation (FNP) of a blend of homopolymers in solution against an antisolvent was shown to produce Janus nanocolloids as a result of the phase separation of the homopolymers within the confinement of droplets.21 The platform operates by inducing the precipitation of the polymers upon solvent exchange but differs from other precipitation-based approaches because of the significantly more rapid mixing within a confined volume, producing supersaturation on a millisecond time scale.22−25 As a result, polymer nanocolloids are produced with tunable sizes from 50 to 1000 nm and with narrow size distributions. In addition, injecting the process streams in a continuous manner would allow for industrial-scale production levels of kilograms of polymer nanocolloids per day.24 Through FNP, the size, surface anisotropy, and surface functionality of Janus nanocolloids can each be controlled independently. Moreover, FNP allows for the ability to generate even more complex structures including patchy and Cerberus nanocolloids (i.e., colloids with more than

here is significant interest in the development of geometrically structured polymer nanocolloids such as Janus particles (i.e., particles with two distinct “faces”), patchy particles, and ellipsoidal particles for their unique self-assembly behavior.1−8 Surface decoration with inorganic materials can introduce another level of functionality into structured nanocolloids for applications ranging from photonics to nanomedicine.9−12 Control over the geometry as well as the functionality of the nanocolloid surface therefore enables new applications. For instance, in biorefining, reactions at liquid− liquid interfaces carried out via emulsions that are stabilized by molecular surfactants often involve phase-transfer catalysts that are soluble in only one phase to initiate reactions.13 The yield and selectivity of these reactions could be dramatically improved by the development of amphiphilic Janus nanocolloids in which the nanocolloid acts as the emulsion stabilizer and one face is functionalized with the catalyst.14−16 Fabrication processes to prepare structured nanocolloids in which functionality is introduced by organizing inorganic nanomaterials atop the nanocolloid surface often require multiple steps and long residence times.17−20 Consequently, a one-step, efficient, cost-effective, and scalable process is highly desirable. Furthermore, flexibility within a single production platform to make a range of nanocolloids and functionalize them with © XXXX American Chemical Society

Received: January 23, 2017 Revised: March 15, 2017 Published: March 20, 2017 A

DOI: 10.1021/acs.langmuir.7b00249 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic of the FNP process. (b) Illustration of the mechanism of nanocolloid formation upon rapid mixing and solvent exchange using a blend of polystyrene (PS) and polyisoprene (PI) in the feed stream to form Janus nanocolloids (left) or a blend of amine-terminated PS (PSNH2) and PI in the feed stream to form core−shell nanocolloids (right).

then a core−shell morphology emerges as the equilibrium structure. In addition to thermodynamic forces driving nanocolloid morphology, kinetics also come into play because the nanocolloid structures can be kinetically arrested in nonequilibrium states as one or more of the polymers vitrifies during phase separation.21 Hence, FNP produces structural colloids via the phase separation of polymer mixtures within a confined volume. To explore the suitability of FNP for the production of structured hybrid nanocolloids, we first sought to generate structured polymer architectures of interest in the absence of inorganic nanoparticles. We thus began by demonstrating the formation of both Janus and core−shell nanocolloids by FNP, as illustrated in Figure 2. The size distribution analysis of the nanocolloids is shown in Figure S1a. We use polystyrene (PS) and polyisoprene (PI) as model polymers for this investigation.

two faces), expanding the number of accessible hybrid material architectures.21 Inspired by these developments and the challenge of producing emulsion-stabilizing, phase-selective catalytic materials, here we present the FNP platform as a new scalable process to generate hybrid polymer−inorganic Janus nanocolloids in which one phase is selectively decorated with a metal catalyst. By modifying the polymer end-group functionality, we also demonstrate our ability to control the location of the metal catalyst within the polymer Janus nanocolloids, such as at the colloid circumference. Furthermore, we document that the addition of simple molecular additives during processing can alter the nanocolloid geometry as well as modulate the location of the metal catalyst. Considering the continuous and facile operation and the variety of products formed, the FNP process offers a versatile approach to mass produce structured functional nanocolloids that are projected to enable new technologies. Figure 1a schematically illustrates the experimental setup for the FNP system used for the production of nanocolloids.22,26,27 Operationally, the solution stream and the antisolvent stream are manually expressed at equal flow rates and turbulently mixed within a confined volume. Rapid solvent/antisolvent exchange induces the spatially homogeneous precipitation of hydrophobic polymer colloids that are subsequently captured in a collection reservoir of pure water. It is significant to note that the FNP system is simple in design and robust in operation at room temperature, simplifying scale-up in production. A schematic of structured nanocolloid formation by FNP is shown in Figure 1b. Nanocolloids form upon rapid precipitation of polymers into condensed phases. When a solution of two polymers is fed into the FNP process, the final equilibrium structure is determined by the self-organization of the polymers to minimize the total interfacial energy between the ternary phases (i.e., the polymer−polymer−liquid phases).28,29 The Janus morphology emerges if the two polymers possess similar interfacial energies with the solvent/ water solution (γpolymer1−water ≈ γpolymer2−water) and a lower interfacial energy between themselves (γpolymer1−polymer2 < γpolymer1−water and γpolymer2−water) as shown on the left. However, if one polymer favors the solvent/water solution over the other,

Figure 2. TEM images of polymer nanocolloids formed via FNP by varying the ratio of homopolymers in the feed stream. Blends of PS and PI in the feed stream form Janus nanocolloids (a−c), and blends of PS-NH2 and PI form core−shell nanocolloids (d−f). The ratios of PS or PS-NH2 to PI are 1:1 (a, d), 2:1 (b, e), and 1:2 (c, f). PI, stained with OsO4, is the darker domain in the images and the blue domain in the schematics, and PS or PS-NH2 is lighter gray in the images and yellow in the schematics. The scale bar in each image is 200 nm. B

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Figure 3. SEM images and schematic illustrations of nanocolloid formation with a blend of homopolymers and a citrate-stabilized Au nanocatalyst. Nanocolloids of PS/PI (a) display the Au nanocatalyst aligned at the PS/PI interface because the neutral surface of the Au nanocatalyst does not favor interactions with either homopolymer (b). PS-NH2/PI nanocolloids (c) adopt a Janus morphology with the Au nanocatalyst dispersed exclusively on the PS-NH2 domain as a result of the attractive electrostatic interactions between the amine groups of the PS-NH2 polymer chains and the citrate groups on the Au nanocatalyst (d). In the SEM images, the stained PI is the light-gray region, the PS or PS-NH2 is the darker gray, and the small bright dots are the Au nanocatalyst. A polymer ratio of 1:2 PS or PS-NH2 to PI was used. The scale bar in all images is 200 nm.

TEM images of the nanocolloids confirms that ∼50% of the nanocolloid is composed of each polymer. The thickness of the shell layer can be tuned by adjusting the ratio of PS-NH2 to PI in the feed stream, as demonstrated in Figure 2e,f; see Figure S1c for an analysis of the polymer ratio in the colloids. Following the study on polymer nanocolloid morphology, we focus on incorporating Au nanocatalyst into the nanocolloids in a controlled fashion. Au nanocatalyst is dispersed in the aqueous antisolvent stream prior to performing FNP. Operationally, the FNP system is unchanged; only the composition of the aqueous feed stream is modified. With a 1:2 PS/PI mixture in the polymer solution stream, Janus nanocolloids are formed with approximately the same composition, and remarkably, Au nanocatalysts are aligned at the PS/PI/water contact line, as shown in Figure 3a. Such a structure is difficult to achieve, and we do so here using a one-step process. Insights from investigations on the addition of surfactant nanoparticles to polymer blends and block copolymer systems provide a framework in which to rationalize our results for the case of polymer blends.34−36 Nanoparticles have been shown to adsorb to liquid−liquid interfaces, thereby minimizing interfacial tension between two immiscible liquids, and can also effectively compatibilize polymer blends by stabilizing interfaces between the two polymers.37−41 In the similar case of diblock copolymer thin films, nanoparticles with a surface that was neutral with respect to the two blocks assembled at the interface between the phase-separated microdomains.34 In the PS/PI system presented here, the hydrophilic citrate-stabilized Au nanocatalyst localizes to the contact line between the two polymers and the aqueous environment, as schematically illustrated in Figure 3b. In this manner, the Au nanocatalyst can be viewed as nanoparticle surfactants reducing the interfacial energy between the PS and PI domains. Next, we investigate the morphology of nanocolloids composed of PS-NH2 and PI formed in the presence of the Au nanocatalyst. Interestingly, Janus nanocolloids are formed in

However, as highlighted in our recent contribution, a variety of polymers can be transformed into structured nanocolloids using FNP.21 When an equal-mass PS/PI mixture at an overall concentration of 0.5 mg/mL in THF is injected at approximately 5 m/s against an aqueous stream, symmetric Janus nanocolloids with a mean diameter of ∼400 nm are formed, as illustrated in Figure 2a. As recently documented, the anisotropy of the Janus nanocolloids can be controlled by changing the ratio of the two polymers in the feed stream (Figure 2b,c).21 The ease of tuning Janus anisotropy is one hallmark of the FNP process. To direct the assembly of a metal catalyst, (i.e., gold (Au) nanocatalyst) atop only one domain of a Janus nanocolloid, we sought to selectively functionalize one of the domains. The simplest strategy to achieve this is to use a polymer end-group functionalized with the appropriate moiety.9 In our case, we use amine-terminated PS (PS-NH2), which electrostatically interacts with citrate groups stabilizing the Au nanocatalyst. We thus perform FNP using a blend of PS-NH2 and PI in the feed stream. Figure 2d shows that the nanocolloids formed using an equal-mass mixture of PS-NH2 and PI in the feed stream have a core−shell morphology in which a PI core is encapsulated by a PS-NH2 shell; size distributions are provided in Figure S1b. Two hypotheses for the mechanism of this transition from a Janus to core−shell morphology are proposed. The first is that the presence of the amine group increases the hydrophilicity of the PS-NH2 chain to the point at which it becomes energetically favorable for it to form a shell around the more hydrophobic PI core. The second is that the positive charge of the amine group is drawn to the experimentally observed negative charge present at the surface of the polymer nanocolloids. The origin of this negative surface charge is unclear and is a topic of debate beyond the scope of this work.30−33 Investigations are currently ongoing in an effort to elucidate the mechanism of core−shell nanocolloid formation upon addition of an amine functional group. An analysis of C

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Langmuir which the Au nanocatalysts are selectively located on the PSNH2 domain, as illustrated in Figure 3c. Interfacial energy arguments guide our interpretation of this phenomena. Without Au nanocatalyst in the system, the ionized (in water) amine end-groups of the PS-NH2 chains drive the formation of core− shell nanocolloids with PS-NH2 as the outer shell. However, upon addition of Au nanocatalyst, which is no longer neutral with respect to both polymers as a result of attractive electrostatic interactions between the amine groups on the polymer chains and citrate groups on the Au nanocatalyst, the Au nanoparticles are engulfed in the PS-NH2 phase (Figure 3d). This is analogous to the use of amine surfactants to transfer negatively charged Au colloids from an aqueous phase into an organic solvent phase.42 Once the amine groups of PSNH2 have surrounded the Au nanocatalyst, the remainder of the PS-NH2 chain, which displays characteristics of pure PS, is oriented outward toward the solvent mixture. The polymer− polymer interactions are then effectively those between PS and PI, and the nanocolloid assumes a Janus morphology. The anisotropy of the nanocolloids is still controlled by the feed ratio of the two polymers. This demonstrates that the FNP platform is capable of producing hybrid polymer−inorganic Janus nanocolloids by simply adjusting the formulation of the feed streams without any change in the process. The results detailed in this report have thus far been obtained by incorporating the Au nanocatalyst into the aqueous antisolvent stream of the FNP system. However, the same results were obtained when the Au nanocatalyst was added to the collection reservoir instead. At the point when the effluent reaches the reservoir, the mixing of the streams is complete and the nanocolloids have adopted a morphology that minimizes their interfacial energies. However, because a 1:1:1 volume ratio of polymer solution/antisolvent/reservoir is typically used, a significant fraction of solvent remains in the collection vessel. The polymers in the system retain some mobility due to this residual solvent and can transition from one stable state to another upon addition of new components. For example, PSNH2/PI core−shell nanocolloids transition to Janus morphology upon addition of Au nanocatalyst to the reservoir. We have so far chosen to focus on the incorporation of the Au nanocatalyst in the antisolvent stream to emphasize the scalable nature of the FNP process, but the ability to add components at another point in the system demonstrates the flexibility of FNP. During the FNP process, nanocolloid morphology is determined by the minimization of the total interfacial energy, and additives such as surfactants or low-molecular-weight block copolymers have been studied in polymer blend and block copolymer colloid systems as agents that affect the particle morphology by changing the energy interactions in the system.43−45 To further manipulate the interactions in the FNP system, we therefore added a surfactant molecule to the FNP process. We chose to use Tween 80, a nonionic surfactant that does not contribute electrostatic effects. Interestingly, with Tween 80 present in the system, added either to the antisolvent stream or to the antisolvent reservoir, nanocolloids all adopt a snowman Janus morphology whether they are prepared with PS/PI (Figure 4a) or PS-NH2 /PI (Figure 4b). Such morphologies have been predicted using surface tension arguments.46 Amphiphilic Tween 80 interacts with the hydrophobic polymer chains and the aqueous environment to reduce the interfacial energy penalty of the polymer−solvent interface, leading to the increase in surface area between the polymer and surrounding solvent and a reduction of the

Figure 4. TEM images of PS/PI (a) and PS-NH2/PI (b) snowman Janus nanocolloids after the addition of Tween 80. PI, stained with OsO4, is the darker domain in the images and the blue domain in the schematics, and PS or PS-NH2 is lighter gray in the images and yellow in the schematics. A polymer ratio of 1:2 PS or PS-NH2 to PI was used. The scale bar in all images is 200 nm.

polymer−polymer contact area. The result is the snowman Janus structure with two lobes where the relative sizes of the two lobes depend on the relative volumes of the two polymers in the feed stream. We next investigate the morphology of hybrid polymer− inorganic nanocolloids upon the introduction of Tween 80. Although the addition of Tween 80 again leads to the formation of snowman Janus nanocolloids for mixtures of both PS/PI and PS-NH2/PI, it has a significant effect on determining where the Au nanocatalyst particles are located on the nanocolloid. For nanocolloids prepared from a PS-NH2/PI blend in the feed stream, the Au nanocatalyst particles remain distributed on the PS-NH2 domain due to the electrostatic attractions between the citrate groups on the Au nanocatalyst surface and the amine groups on the PS-NH2 chains, as shown in Figure 5a. These

Figure 5. SEM images of nanocolloids prepared with a PS-NH2/PI blend showed that the Au nanocatalyst is exclusively dispersed in the PS-NH2 domain (a), whereas nanocolloids prepared with a PS/PI blend had the Au nanocatalyst dispersed exclusively in the PI domain (b). PI, stained with OsO4, is the light-gray domain in the images and the blue domain in the schematics, and PS or PS-NH2 is darker gray in the images and yellow in the schematics. The Au nanocatalyst is shown by the small bright dots. A polymer ratio of 1:2 PS or PS-NH2 to PI was used. The scale bar in all images is 500 nm.

interactions are not affected by the addition of Tween 80, despite the change in the overall morphology of the nanocolloid. However, nanocolloids made from a PS/PI blend with the Au nanocatalyst in the antisolvent stream, which without Tween 80 would display the Au nanocatalyst particles at the PS/PI interface, now display the Au nanocatalyst dispersed on the PI domain (Figure 5b) when they come into contact with Tween 80 in the antisolvent reservoir. In addition to the amphiphilic Tween 80 promoting D

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interactions of the more hydrophilic Au nanocatalyst with the more hydrophobic polymer in the system (PI), the lower glasstransition temperature and liquidlike behavior of the PI allow for the Au nanocatalyst to disperse in the PI domain more easily than in the more glassy PS domain. We therefore show that the addition of a surfactant not only can contribute to changes in nanocolloid morphology but also can direct the placement of nanocatalyst materials within the colloid. In this article, we present FNP as a scalable platform for the production of hybrid polymer−inorganic nanocolloids with a range of morphologies and functionalities. We have shown that control over nanocolloid morphology can be achieved by tailoring the interactions between the polymers and the solvents, either by incorporating functional groups into one of the polymers or by adding a surfactant to the system. Beyond changing the morphology of the nanocolloids and in a move toward adding functionality, these factors can also be used to direct the location of added nanocatalytic particles on the nanocolloids, enabling the facile formation of nanocolloids, which could show promise in biphasic catalytic applications upon scale-up to industrial levels.

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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.langmuir.7b00249. Size distribution and nanocolloid composition. TEM and SEM images of nanocolloids prepared with polybutadiene in place of polyisoprene. Size distribution and TEM of the Au nanocatalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rodney D. Priestley: 0000-0001-6765-2933 Notes

The authors declare no competing financial interest.

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REFERENCES

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METHODS AND MATERIALS

Materials. Polystyrene (Mw = 13.2 kg/mol, PDI = 1.06) was purchased from Fluka Analytical. Amine-terminated polystyrene (Mn = 10 kg/mol, PDI ≤ 1.3) was purchased from Aldrich. Poly(1,4isoprene) (Mw = 11.6 kg/mol, PDI = 1.06) was purchased from Polymer Source Inc. Tetrahydrofuran was purchased from FisherScientific. DI water was filtered through a 0.2 μm filter using a NANOpure Diamond filtration system. Tween 80 (polyoxyethylene (80)) was purchased from Fisher Scientific. Citrate-stabilized Au nanocatalyst was prepared according to the method introduced by Turkevich and Frens.47,48 Gold(III) chloride trihydrate purchased from Sigma-Aldrich was dissolved in DI water and brought to a vigorous boil while stirring. Sodium citrate tribasic dihydrate, also purchased from Sigma-Aldrich, was dissolved in DI water and quickly added to the flask. The mixture was kept boiling and stirring until a deep red color was achieved, and the suspension was cooled to room temperature overnight. The Au nanocatalyst was approximately 20 nm in diameter, as confirmed by dynamic light scattering and TEM. Nanocolloid Assembly. Homopolymers were dissolved in THF at 0.5 mg/mL and were mixed over a range of desired volumetric ratios. FNP was performed using a confined impinging jet (CIJ) system where the polymer solution was injected against an equal volume of the antisolvent stream, which was composed of DI water, an aqueous suspension of prepared citrate-stabilized Au nanocatalyst, or a 1% v/v aqueous solution of Tween 80. The effluent stream emptied into a reservoir of either DI water or a 1% v/v aqueous solution of Tween 80 with the volume ratio of polymer solution to antisolvent to reservoir being 1:1:1, resulting in a final solvent composition of 33.3% THF and 66.6% water. Sample Preparation and Imaging. The nanocolloids were aged for 1 h after FNP before staining. When staining, 500 μL of the polymer nanocolloids with gold was mixed with 500 μL of an aqueous 0.2 wt % OsO4 solution (Electron Microscopy Sciences) in a centrifuge tube and incubated for 2 h before centrifuging and washing with DI water three times. To prepare samples for electron microscopy, 7 μL of the stained nanocolloid suspension was deposited on a carbon-coated copper TEM grid (CF-200-Cu, Electron Microscopy Sciences), and 80 μL of the stained nanocolloid suspension was deposited on carbon tape mounted on an aluminum SEM stand (Electron Microscopy Sciences). Samples were left to dry overnight under ambient conditions. Imaging was performed on a CM100 TEM with an accelerating voltage of 100 kV and on an FEI Quanta 200 FEG Environmental SEM with an accelerating voltage of 10 kV. E

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