Particulate Coacervation of Associative Polymer Brushes-Grafted

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Particulate Coacervation of Associative Polymer Brushes-Grafted Nanoparticles to Produce Structurally Stable Pickering Emulsions Taeseung Yang, Sang Koo Choi, Daehwan Park, Yea Ram Lee, Chan Bok Chung, and Jin Woong Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03203 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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Particulate Coacervation of Associative Polymer BrushesGrafted Nanoparticles to Produce Structurally Stable Pickering Emulsions Taeseung Yang†, Sang Koo Choi†, Daehwan Park†, Yea Ram Lee†, Chan Bok Chung‡, Jin Woong Kim*,†, § † ‡ §

Department of Bionano Technology, Hanyang University, Ansan 15588, Republic of Korea. SK Bioland Co. Ltd., Cheonan 31257, Republic of Korea. Department of Applied Chemistry, Hanyang University, Ansan 15588, Republic of Korea

KEYWORDS: Associative Nanoparticles, Living Radical Polymerization, Coacervation Agents, Hydrophobic Association, Complex Colloidal Film

ABSTRACT This study introduces a new type of associative nanoparticles (ANPs) that provides controlled chain-to-chain attraction with an associative polymer rheology modifier (APRM) to produce highly stable Pickering emulsions. The ANPs were synthesized by grafting hydrophobically modified hygroscopic zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-costearyl methacrylate (SMA)) brushes onto 20 nm-sized silica NPs via surface-mediated living radical polymerization. The ANP-stabilized Pickering emulsions show significant viscosity enhancement in the presence of the APRM. This indicates the ANPs act as particulate concertation agents at the interface due to their hydrophobic association with the APRM in the aqueous phase, which leads to the generation of an ANP-mediated complex colloidal film. Consequently, the described ANP-reinforced Pickering emulsion system exhibits the improved resistance to pH and salinity changes. This coacervation approach is advantageous because the complex colloidal layer at the interface provides the emulsion drops with a mechanically robust barrier, thus guaranteeing the improved Pickering emulsion stability against harsh environmental factors.

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1. Introduction Conventional emulsions are stabilized with low molecular weight surfactants or amphiphilic polymers that adsorb to liquid–liquid interfaces and impede the coalescence of emulsified droplets by taking advantage of electrostatic and steric repulsive forces. Pickering emulsions, on the other hand, are produced by the controlled assembly of solid particles on droplets suspended in an immiscible continuous liquid phase.1-3 The solid-phase interface is mechanically so robust that structurally stable emulsion drops can be achieved even to macroscopic length scales.4 Recent efforts to explore more practical applications by engineering the Pickering emulsion drops have been performed by chemically and physically modifying the particles-stabilized solid-phase interface. For instance, diverse functionalities have been incorporated into Pickering emulsions by grafting architecturally tailored polymers onto the solid particles.5,6 These surfaceactivated particles form the Pickering emulsion drops, which can be directly used for oil recovery, liquid-phase heterogeneous catalysis,7 and water purification.8 Furthermore, Pickering emulsions formed with interfaces that are assembled with uniform colloidal particles have been utilized as templates for the fabrication of more advanced colloidal materials, such as microscale catalytically active composite particles9 and hollow particulate capsules that are sometimes referred to as colloidosomes.10-12 These unique interfacial properties and resulting novel utilities make them more attractive in many industries seeking to avoid the use of conventional molecular surfactants.13 The spotlight of attention has been focused upon the Pickering emulsions mainly because they show the distinctively improved emulsion stability. In principle, the adhesion energy (E) of a particle at the interface is defined14 as, E=πa2γ(1±cosθ)2, where a is the radius of a particle, γ is the interfacial tension, and θ is the contact angle. Therefore, the removal of a particle from the interface requires a large amount of energy, which leads to the long-term maintenance of their initial droplet configuration. To put it simply, additional physical and/or chemical stresses should be imparted to the interface to disrupt this well-established emulsion system.15,16 However, if tiny particles-length scales less than tens of nanometers-are employed to produce the Pickering emulsion, the emulsion stability is readily deteriorated because the adhesion energy is proportional to the square of particle size.17,18 Alternatively, the surface chemistry of particles


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may be manipulated to maximize their wetting, thus endowing the particles with the higher adhesion energy.19 Even at the complete wetting conditions, particles are commonly detached from the interface because the high thermal fluctuation of the particles at the interface overwhelms the surface tension in such nanoscales.20,21 An approach to overcome this intrinsic issue is to provide another attractive force that can physically or chemically immobilize the particles at the interface. Thus, we sought to develop novel nanoscale particles that have attraction sites capable of interacting with components from the dispersion or the continuous phase. Consequently, the engineered colloidal interface cannot only fortify the endurance of the emulsion structure, but must also tolerate environmental stresses such as pH and salinity changes. The ultimate goal of this study was to synthesize a new type of associative nanoparticles (ANPs) and demonstrate that they play an important role in the production of highly stable Pickering emulsions. Thus, ANPs were synthesized by covalently grafting associative polymer brushes, poly(2-methacryloyloxyethyl phosphorylcholine-co-stearyl methacrylate) (poly(MPCco-SMA)) on silica NPs. Capable of hydrophobic interchain associations, these ANPs allowed us to modify the rheology of complex fluids. In particular, they have the ability to generate a complex particulate film at the oil-water interface by adhering to the interface while hydrophobically interacting with associative polymer rheology modifiers (APRM) dissolved in the aqueous phase, as schematically illustrated in Figure 1. The essence of this study is to take advantage of the hydrophobic interactions between the ANPs and APRM that have a similar hydrophobically modified water-soluble polymer. Moreover, the use of ANPs promises the ability to fine-tune surface wettability, which might possibly be done by simply adjusting the copolymerization ratio of SMA to MPC. Finally, we demonstrate that the ANP-stabilized Pickering emulsions exhibit a significant enhancement of viscosity and are insensitive to pH and salinity changes, as confirmed by suspension rheology studies.

2. Experimental Section 2.1.

Materials.

Silica

nanoparticles

(LUDOX-CL-X),

3-aminopropyltriethoxysilane,

molybdenumhexacarbonyl, divinylbenzene, toluene (99.8%), dibutyltin dilaurate, stearyl methacrylate, triethanolamine (TEA), n-decane, fluorescein isothiocyanate-dextran (FITCdextran), and sodium lauryl sulfate (SLS) were purchased from Sigma-Aldrich (USA).


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Trichloroacetyl isocyanate was supplied by TCI (Japan). Associative polymer rheology modifier (APRM) used in this study was a commercially available copolymer of acrylic acid and SMA (acrylic acid/SMA = 96.2/3.8 (w/w), MW = 400,000 g/mol). For controls, conventional polymer rheology modifier (PRM) was also used (poly(acrylic acid), MW = 400,000 g/mol). Both APRM and PRM were kindly supplied by KCI Co. (Korea). All other chemicals were reagent grade and used without further purification. Deionized double distilled (DI) water was used for all experiments.

2.2. Synthesis of ANPs by surface-mediated living radical polymerization. ANPs were synthesized via surface-mediated living radical polymerization. In the first step, 1 g of silica NPs were finely dispersed in 28.6 mL toluene in a round glass flask, and to this was added 1.5 mL of 3-aminopropyltriethoxysilane to functionalize the silica surface with primary amine groups. After sealing the flask, the mixture was stirred in a mantle at 100 °C for 8 h. Unreacted coupling agents were completely removed by repeated centrifugation with tetrahydrofuran and toluene at 4000 rpm for 5 min. In the next step, the amine-functionalized silica NPs were again dispersed in 40 mL toluene. Subsequently, 0.5 mL of trichloroacetyl isocyanate was added to induce the urea reaction between the amine group on the silica surface and the isocyanate group of trichloroacetyl isocyanate in the presence of 0.018 mL of dibutyltin dilaurate. The reaction mixture was refluxed at 80 °C for 8 h while purging with argon. The trichloroacetylfunctionalized silica NPs were recovered by repeated centrifugation with tetrahydrofuran and ethanol at 4000 rpm for 5 min. In the final step, 1 g of trichloroacetyl-functionalized silica NPs was dispersed in 40 mL ethanol containing MPC and SMA. Mo(CO)6 (0.02 g) was added as a catalyst. Living radical polymerization was then carried out at 70 °C for 12 h under argon atmosphere. The unreacted monomers and additives were completely removed by washing with methanol and water. After drying the recovered particles under vacuum at ambient temperature, fine ANP powder could be obtained.

2.3. Characterization of ANPs. The particle size and morphology of the ANPs were confirmed by transmission electron microscope (TEM) analysis. TEM samples were prepared by dispersing 0.1 wt% of particles in methanol under ultrasonic treatment for 5 min. A drop of sample dispersion was then pipetted onto a carbon support film on a copper grid. After complete drying


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at ambient temperature for 24 h, the particles were observed with a high-resolution transmission electron microscope (HR-TEM, JEN 2100F, JEOL, Japan) operated at an accelerating voltage of 200 kV. The surface wettability of the particles was investigated by analysis of the contact angles. The particle samples produced with varying polymer chemistry were directly pelletized without the aid of binders. A drop of DI water was placed on the pellet and imaged by a CCD camera (Fainstec, STC-GEC83A, Korea). Then, contact angle was obtained by drawing a tangent. At least three measurements were carried out and an averaged contact angle was taken. The grafting mass of the polymer chains on the particle surface was determined via thermogravimetric analysis (Q500, TA Instruments, USA).

2.4. Preparation of ANPs-stabilized Pickering emulsions. The aqueous continuous phase was prepared by dispersing 1 wt% of ANPs in water. 10 wt% of n-decane against the total emulsion mass was added to form the oil dispersed phase. First, pre-emulsion drops having macro scales were produced via mild sonication (Power Sonic 510, Hwashin, Korea) for 5 min at room temperature. Then, they were down-sized to micron scales by applying probe-type sonication for 1 min at room temperature (VCX130, Sonic & Materials Inc., US). The Pickering emulsion produced was observed with a bright field microscope (3RS, Zeiss, Germany). The emulsion drop diameter was measured by analyzing the microscope image. More than 50 individual drop diameters were measured from a digital image and the average was taken. Surface topology of Pickering emulsion drops were observed with a scanning electron microscope (SEM, Hitachi, S4800, Japan) at an acceleration voltage of 20 kV. The dried particles were mounted on a copper stub and sputter-coated with platinum to minimize charging effect.

2.5. Suspension rheology of Pickering emulsions. For suspension rheological measurements, the ANP-stabilized Pickering emulsions were uniformly mixed with an aqueous solution containing the PRM. The pH of the solution was adjusted by varying the amount of TEA added. Also, the salinity of the aqueous continuous phase was controlled by addition of NaCl. Rheological measurements were conducted in the stress-control mode using a DHR-1 rheometer (TA Instruments, USA). In our rheology study, a cone-plate geometry with 40 mm diameter and 1 °angle was used and the sample gap was set to be 0.5 mm. The sample was loaded onto the rheometer plate and surrounded by a solvent trap to prevent any evaporation of water. The steady


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state viscosity was measured by changing the shear rate from 0.1 to 100 s-1 followed by repeated descending and ascending. All measurements were conducted at room temperature.

3. Results and Discussion The synthetic procedure begins with grafting hydrophobically modified hygroscopic zwitterionic polymer brushes onto 20 nm-sized silica nanoparticles by using surface-mediated living radical polymerization22 (Figure 2A). The copolymerization of SMA and MPC on trichloroacetyl-tagged silica NPs in the presence of catalytic Mo(CO)6 successfully grafted the poly(SMA-co-MPC) brushes. Specifically, SMA bearing a C18 alkyl chain was copolymerized to provide the polymer brush with hydrophobic dangling chains. We also copolymerized MPC with SMA to make the polymer brush hygroscopic, which favors an effective chain conformation with a large radius of gyration, thereby preventing chain folding or entanglement. The incorporation of poly(SMA-coMPC) brushes on the silica NPs was directly confirmed by the generation of a thin polymer shell layer, as shown by TEM observation (Figure 2B-D). TEM image analysis revealed the average thickness of the shell layer in a dried state to be approximately 5 nm. Thermo-gravimetric analysis showed that the grafting mass of polymer chains ranged between 14–22% when 0.2–0.8 weight fraction of SMA (φSMA) was copolymerized against MPC (Figure 3A). Copolymerization of SMA made the surface of ANPs controllably hydrophobic. This can be elucidated by observation of the increased contact angle of a water drop on an ANP-palletized solid sheet in a SMA-dependent manner (Figure 3B). In fact, when φSMA= 0.2–0.8, the contact angle could be controlled from 90° to 125°, thus allowing us to regulate the wettability of ANPs. After first producing the Pickering emulsions using only ANPs, the APRM and poly(acrylic acid-co-SMA) with 0.038 φSMA were added to the emulsion to avoid any adsorption before the ANPs fully cover the droplets. The C18 alkyl chain in the polymer backbone of APRM exhibits hydrophobic interactions with that of ANPs. For n-decane/water Pickering emulsions produced using the described formulation, the drop size and shape depended on the wettability of ANPs. They were easily controlled by varying the copolymerization ratio of SMA against MPC (Figure 4A). When φSMA = 0.2, the energy of attachment of a single particle to an oil-water interface passes through a maximum because of the contact angle close to 90°. This allows the ANPs to have best wettability at the O/W interface. Therefore, the oil droplets decreased in size and the emulsion stability could be markedly improved.23 When φSMA < 0.4, O/W emulsions were


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produced. On the other hand, when φSMA = 0.45–0.5, W/O emulsions were obtained. Above 0.5, W/O emulsions were initially produced but couldn’t maintain their stability due to the hydrophobic nature of the surface. These results imply that the emulsion type is determined by the surface property of ANPs. Simply following the Bancroft rule,24 the phase in which the ANPs wet more favorably constitutes the continuous phase; O/W emulsions are formed by hydrophilic ANPs with zeta potential of -30~-40 mV, whereas W/O emulsions are formed by hydrophobic ANPs. In this study, the most stable and spherical O/W and W/O emulsions were produced at φSMA= 0.2 and 0.5, respectively. As time goes, they showed higher drop viability compared to the case using ANPs with only MPC (Figure 5A). It means that the adhesion energy of the particles could be maximized at the certain copolymerization ratio of SMA against MPC, thus enhancing structural stability for a long term storage (Figure 5B, C). In these compositions, it appears the ANPs adhered to the interface and effectively associated with the APRM at the interface, resulting in a sort of particulate coacervation. Interestingly, when φSMA= 0.4–0.45, the ANPs of this Pickering emulsion system showed unusual interfacial behaviors during the generation of Pickering emulsions, such as the formation of non-spherical droplets and the sudden halt of coalescence (Figure 4B). Regarding this observation, it is rational to infer that the strong adhesion and dense packing of ANPs at the interface deformed the curvature of droplets25-27 and prevented further droplet coalescence,28,29 as can be seen in Figure 4C-F. When using ANPs only, such unusual drop formation was not observed. This result seems to arise from the generation of a complex particulate layer at the interface due to the hydrophobic interaction between ANPs and APRM. To test this hypothesis, the O/W Pickering emulsion was produced with a fluorescence probe, FITC-tagged ANPs, in which 1 wt% fluorescent ANPs was mixed with neat ANPs during emulsification. CLSM observation revealed that the Pickering emulsion drops were armored with ANPs (Figure 6A). Complete removal of n-decane from the emulsion produced a smooth, polymer-analogous thin film, which means the ANPs and APRM were fused to form a complex colloidal polymer film (Figure 6B). The average thickness of the film was measured to be approximately 50 nm (Figure 6C). It is therefore reasonable to conclude APRM likely bound ANPs at the interface, and this action is critical to improve emulsion stability. Having established the fabrication method to obtain ANP-stabilized Pickering emulsions by interfacial hybridization with APRM, we sought to characterize the flow properties of these


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emulsions. The Pickering emulsions produced in this study displayed a non-Newtonian shear thinning in which the viscosity decreased with the increase in shear rate. It was noticeable in our emulsion system that the Pickering emulsions made with APRM showed higher viscosities over the shear rate compared with those with conventional PRM (Figure 7A). Because the PRM and APRM showed the almost identical solution viscosity behavior in water (Figure 7B), the increase in the viscosity likely arises from the attraction between emulsion drops and APRM, which is presumably mediated by the ANPs at the interface. To verify this, the flow properties of the SLSstabilized emulsions made in the presence of APRM were observed. The viscosity of SLSstabilized emulsions lowered relative to that of the APRM solution (Figure 8A). By contrast, the ANP-stabilized emulsion rather showed a higher viscosity than the APRM solution, neat 100% at low shear regime (Figure 8B). In principle, emulsions are stabilized by imparting a repulsive force to the emulsion droplets. A combination of multiple interactions is involved in the stabilization mechanism of emulsions. In repulsive emulsion systems, when the droplet volume fraction is close to the random close packing density, each droplet is encased in a “cage” formed by surrounding droplets. Due to this cage effect, the emulsion exhibits elasticity.30,31 When the attraction between droplets are imparted, the elastic emulsion fluid behaviors could be achieved even for droplet volume fractions below the random cross packing. This mainly arises from formation of percolated droplet flocculates, thus resulting in a gel-like structure. From these understandings, it appears that the Pickering emulsion droplets produced in this study is percolated via APRM, which makes the emulsion droplets are attractive to each other. The natural next question was how the ANP-stabilized Pickering emulsions would respond to harsh environmental factors such as solution pH and salinity. The addition of salts to the ANPstabilized Pickering emulsion also lowered the viscosity (Figure 9A). However, our Pickering emulsions showed much stronger resistance to the salinity change, when compared to the SLSstabilized emulsion. We presume that because there is no structural deterioration of the Pickering emulsion drops under high-salinity conditions, this improved emulsion stability solely stems from the fact that the APRM is freely dissolved in the continuous phase. It is well known that APRM attracts water molecules and occupies the space even under the saline conditions, thereby retaining the structural robustness of a polymer gel fluid.32 Moving on to how varying the pH affects the viscosity behaviors of ANP-stabilized Pickering emulsions, we discovered the viscosity of the ANP-stabilized emulsions was enhanced under mildly acidic conditions (Figure


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9B). The highest viscosity enhancement was reproducibly obtained at pH 4–5. This is attributed to the fact that the chain extension of the grafted poly(MPC-co-SMA) brushes on the ANPs is more favorable in the mildly acidic conditions,22 which gives them more chances to hydrophobically interact with APRM. All of these results support the interpretation that the Pickering emulsions formed with interfaces that were made with ANPs and APRM could have such excellent emulsion stability against harsh environmental factors because of the formation of the complex ANPs/APRM thin film at the interface.

4. Conclusions In summary, we introduced a facile and robust approach to fabricate a structurally stable Pickering emulsions. The key technology developed in our study is to use the ANPs modified with hydrophobically modified zwitterionic water-soluble polymers. Successful synthesis of the ANPs was confirmed using HR-TEM, TGA, and contact angle analyses. The surface wettability control of ANPs allowed us to regulate the emulsion type of the Pickering emulsions. Microscopic observations revealed that the Pickering emulsion drops were armored with a complex colloidal film made with ANPs and APRM. The ANP-stabilized Pickering emulsions showed the formation of non-spherical emulsion droplets, which is attributed to the strong adhesion and dense packing of ANPs at the interface. Rheological studies demonstrated that the ANP-stabilized Pickering emulsions fabricated in this study not only enhanced the viscosity of the emulsion fluid, but also imparted excellent resistance against the pH change and salt addition. These characteristics highlight the robustness and versatility of our interfacial colloid assembly approach in generating more advanced Pickering emulsion systems, which could be used to develop novel complex formulations for applications in drug delivery, chemical microreactors, and aesthetic therapy.

AUTHOR INFORMATION Corresponding Author * Telephone: +82 31 400 5499. E-mail: [email protected] Author Contributions T. Yang, S. K. Choi, and D. Park equally contributed to this work. Notes


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The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2008–0061891 and No. 2016R1A2B2016148).

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Figure 1. Schematic illustration of the hydrophobic interactions of ANPs with PRM at the oilwater interface of a Pickering emulsion drop.


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Figure 2. (A) Schematic presentation for the synthetic procedure of ANPs. (B) TEM images of bare silica NPs. TEM images of ANPs grafted with poly(SMA-co-MPC): (C) SMA/MPC=2/8 (w/w) and (D) SMA/MPC=8/2 (w/w).


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Figure 3. (A) TGA analysis for the determination of grafted polymer mass. (a) Bare silica NPs. ANPs grafted with poly(SMA-co-MPC): (b) SMA/MPC=2/8 (w/w) and (c) SMA/MPC=8/2 (w/w). (B) Contact angle of a water drop on pelletized ANP sheets.


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Langmuir

Figure 4. (A) Diameter changes of ANP-stabilized Pickering emulsions. Poly(SMA-co-MPC) brushes on ANPs were copolymerized with varying the weight fraction of SMA (φSMA) against total monomer mass. (B) Drop eccentricity of ANP-stabilized Pickering emulsion drops. W/O Pickering emulsions: (C) φSMA= 0.5 and (D) φSMA= 0.4. O/W Pickering emulsions: (E) φSMA= 0.3 and (F) φSMA= 0.4. For all cases, the concentration of ANPs was set to 1 wt%. The ratio of water to oil was 1/9 (v/v).


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Figure 5. (A) Drop viability at 25 ˚C. Microscope images of ANP-stabilized Pickering emulsion drops after 1 month storage: (B) ϕSMA=0, (C) ϕSMA=0.2.


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Figure 6. (A) CLSM image of ANP-stabilized Pickering emulsion drops. For this observation, ANPs were chemically labeled with FITC. (B) SEM image of the emulsion drops after removal of oil. (C) SEM image of a fractured ANP colloidal shell. The Pickering emulsions used in this observation were fabricated with 1wt% ANPs with φSMA= 0.2.


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Figure 7. (A) Rheological behaviors of ANP-stabilized O/W Pickering emulsions thickened with PRM and APRM respectively. The Pickering emulsions were produced with 0.25 wt% ANPs. The concentration of n-decane was set to 6.7 vol%. (B) Solution viscosity behaviors of PRM and APRM. The pH of the continuous phase was adjusted to 7.


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Figure 8. (A) Rheological behaviors of SLS-stabilized O/W emulsions thickened with APRM. The effect of oil concentration. The emulsions were made with 0.6 vol% APRM and n-decane as the oil at pH 5. (B) Rheological behavior of ANPs-stabilized Pickering emulsion. Viscosity changes of n-decane O/W emulsion as a function of shear rate. The emulsion was made with 0.6 vol% APRM and 13 vol% n-decane at pH 5.


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Figure 9. Rheological behaviors of ANPs-stabilized Pickering emulsions thickened with associative polymers. (A) Resistance to NaCl addition at pH 7. (B) Resistance to pH changes. The Pickering emulsions were produced with 0.6 vol% APRM and 6.7 vol% n-decane.


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Particulate Coacervation of Associative Polymer Brushes-Grafted

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