Well-Defined Zwitterionic Microgels: Synthesis and Application as Acid

Sep 20, 2016 - This paper describes the synthesis, swelling behavior, and applications of well-defined narrowly dispersed zwitterionic (ZW) microgels ...
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Well-Defined Zwitterionic Microgels: Synthesis and Application as Acid-Resistant Microreactors Mohammad Vatankhah-Varnoosfaderani,*,† Maria Ina,† Hossein Adelnia,‡ Qiaoxi Li,† Aleksandr P. Zhushma,† Lee J. Hall,§ and Sergei S. Sheiko*,† †

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran § Halliburton Company, 3000 N. Sam Houston Parkway E., Houston, Texas 77032, United States ‡

S Supporting Information *

ABSTRACT: This paper describes the synthesis, swelling behavior, and applications of well-defined narrowly dispersed zwitterionic (ZW) microgels prepared by dispersion polymerization in aqueous media. Microgel stability was achieved through precise control of the dispersant composition, timely addition of a cross-linker after the nucleation stage, and the utilization of ionic initiators. Dispersion polymerization allowed for incorporation of both hydrophilic and hydrophobic comonomers, including acrylamide (AAm) and dopamine methacrylamide (Dopa-MA). The broad variety of compositions created many opportunities for practical applications such as encapsulation of mineral acids and synthesis of metal nanoparticles. The swelling behavior of ZW-co-AAm microgels in 6 M HCl was particularly interesting: whereas ZW moieties remained stable in contact with the strong acid, the amide groups underwent hydrolysis to carboxylic acid, resulting in microgel contraction and acid release. Zw-co-DopaMA microgels were employed as particulate microreactors, where the ZW moieties played a role of an osmotic pump delivering Ag ions to the DOPA moieties for conversion to silver nanoparticles uniformly dispersed inside the microgel particles.

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INTRODUCTION Hydrogel particles, also known as microgels, display a wide range of practical applications ranging from drug carriers1 to templates for the synthesis of nanoparticles.2 Adding the zwitterionic (ZW) functionality will significantly enhance these applications by imparting microparticles with many desirable features such as antifouling, high saline uptake, pH sensitivity, and antipolyelectrolyte effect.3−7 Due to the antifouling property, ZW microgels enable longer circulation in the bloodstream, which is vital for drug delivery.3 Due to the presence of both negative and positive charges in each repeat unit, ZW polymers display the so-called antipolyelectrolyte effect,6 which enables uptake of large quantities of both saline and acidic water without degradation. Despite these advantageous properties, the synthesis of ZW microgels possessing well-defined dimensions and chemical compositions remains challenging. Conventional methods for fabrication of microgels are based on inverse heterogeneous polymerization, except for N-isopropylacrylamide, which allows for direct polymerization due to its lower critical solubility temperature (LCST) characteristic at about 32−34 °C.8 To the best of our knowledge, there is only one paper on preparation of pure ZW microgels using inverse-emulsion polymerization,9 and so far no study has been reported on the synthesis of pure ZW © XXXX American Chemical Society

microgels in aqueous media. Inverse-emulsion polymerization can produce particles ranging from 50 nm to several 100s of nm, depending on formulation and reaction conditions. However, the use of organic solvents as polymerization media is unavoidable, which limits application of the final product in biomedical areas in particular. Moreover, incorporation of comonomers with different levels of polarity (hydrophilicity) is impossible via inverse-based methods due to phase separation. Herein we have addressed these problems through dispersion polymerization of ZW monomers in an aqueous environment, which combines two benefits. First, preparation of microgels in aqueous media eliminates the tedious process of medium substitution, which is commonly used in most heterogeneous polymerization techniques and frequently leads to broad size distributions.9,10 Second, dispersion polymerization allows for targeted incorporation of various comonomers at specific stages of the particle growth, which greatly enhances particles’ functionality. In a typical dispersion polymerization, the reaction starts from a homogeneous solution of monomers, medium, initiator, Received: August 5, 2016 Revised: September 4, 2016

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Scheme 1. (a) Procedure and Mechanism of Zwitterionic Microgel Synthesis; (b) Multifunctional Microgels: Irreversible and Reversible Swelling in Acid Solution and Synthesis of Metal Nanoparticles

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and stabilizer. Upon initiation, the growing oligo-radicals become insoluble in the surrounding medium, resulting in the formation of primary particlesnuclei. This approach is straightforward for polymerization of hydrophobic particles in aqueous media,11−14 and it has been rarely exploited for dispersion polymerization of water-soluble monomers in aqueous media. For water-soluble polymers, their affinity to the surrounding medium hinders and eventually prohibits the nucleation process.15 Hence, choosing an appropriate medium for a given monomer is critical. Moreover, uncontrolled addition of a cross-linking agent (even at low amounts) may adversely affect the dispersion stability, causing coagulation and broadening of size distribution.16−18 In this study, the conventional dispersion polymerization has been advanced through accurate control of both the formulation composition and the protocol of ingredients addition, which enable the synthesis of narrowly dispersed ZW-based microgels in aqueous media (Scheme 1a). Furthermore, as mentioned above, dispersion polymerization readily allows addition of various comonomers [e.g., acrylamide (AAm) and dopamine methacrylamide (Dopa-MA)], yielding multifunctional microgels that can be used for encapsulation and release of strong acids and microreactor synthesis of nanoparticles (Scheme 1b).

MATERIALS AND METHODS

N-(Methacryloxypropyl)-N,N-dimethyl-N-(3-sulfopropyl)ammonium betaine (SBMA, zwitterionic monomer), acrylamide (AAm), 2ethylhexyl methacrylate (2-EHMA), butyl acrylate (BuA), methyl methacrylate (MMA), hydroxyethyl methacrylate (HEMA), ethylene glycol dimethacrylate (EGDM), N,N′-methylenebis(acrylamide) (MBA), ammonium persulfate (APS), silver nitrate (AgNO3), and azobis(isobutyronitrile) (AIBN) were obtained from Aldrich. Ethanol and other solvents were purchased from Fisher Scientific. Poly(vinylpyrrolidone) (PVP) with a molar mass of 40 000 g/mol was used as stabilizer (Aldrich). All the materials were used without further purification. The brine used for swelling studies was composed of 8 wt % NaCl and 2 wt % CaCl2. Synthesis of Dopamine Methacrylamide (Dopa-MA). Details of synthesis and characterization of Dopa-MA were reported in previous research performed by Glass et al.19 The product was gray powder with ca. 85% yield. 1H NMR confirmed the chemical structure of Dopa-MA. Deuterated dimethyl sulfoxide (DMSO-d6) was used as a solvent. 1H NMR (400 MHz, 273 K, DMSO-d6, δ): 6.4−6.6 (3H, m, Ph (phenyl)), 5.5 (1H, d, CH2C−), 5.25 (1H, d, CH2C−), 3.3 (2H, q, CH2−NH−), 2.5 (2H, t, CH2−Ph), 1.8 (3H, s, CH2C−) (Figure S1). Synthesis of ZW Microgels. A typical dispersion polymerization of ZW monomer was carried out as follows. Additives including SMBA (2.0 g), PVP (0.4 g), AIBN (15 mg), and 30 g of water/ethanol (30/ B

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obtained in a narrow range of ethanol mass fractions of ϕe = 60−80 wt % (Figure 1). The upper limit is dictated by the

70 w/w) as medium were all mixed in a beaker and then transferred to a 100 mL reactor by passing through the filter. The solution was deoxygenated by bubbling of nitrogen gas for 30 min and then heated by a water bath at 50 °C while stirring at 150 rpm. The cross-linker (EGDM 1.5 and 3.0 mol % or MBA 1.5, 3.0, and 4.5 mol % respect to the monomers) was added after 30 min, which is approximately 5 wt % conversion from monomer to polymer. As the mixture became turbid (indication of initiation of nucleation stage), the cross-linker was fed into the reactor gradually using a syringe pump at a rate of 5 mL/h. The reaction was left overnight. The final product was cooled and then washed three times with ethanol to remove free stabilizer chains and unreacted monomers. Synthesis of Silver and Magnetic Nanoparticles in Microgels. We used dopamine moieties of ZW/Dopa-MA particles to reduce metallic ions and grow metal nanoparticles (NPs) in the microgels. As for silver nanoparticle (AgNP) synthesis, 5 mL of ZWco-Dopa-MA (90/10 mol/mol %) microgel (0.04 g/mL) solution and 1.25 mL of AgNO3 (10 mg/mL) solution were mixed, and then the mixture was diluted to 15 mL with water. The mixture was stirred at room temperature for 4 h. After reaction, the resulting ZW-co-DopaMA microgels with AgNPs in the microgels were separated from the suspension by centrifugation (4000 rpm, 10 min) and washed three times with water. As for magnetic particle synthesis, 2.5 mL of FeCl3 (20 mM) and 1.4 mL of FeSO4 (25 mM) were added dropwise into the solution, and the mixture was stirred at room temperature for 1 h to ensure complete absorption of ions by microgels. The ion-absorbed microgels were separated by centrifugation (5000 rpm, 15 min) and were washed with water to remove the unreacted ions. The microgels were redispersed in water, and 0.5 mL of ammonia was added to adjust the pH of the solution to 10. Subsequently, the mixture was heated to 80 °C and stirred for 30 min. From the addition of ammonia until the end of reaction, the mixture was purged with nitrogen. Finally, the dispersion was cooled down to room temperature, and the resulting microgels embedded with Fe3O4 were centrifuged (5000 rpm, 15 min) and washed three times with water. Characterization. Dimensions and surface morphology of synthesized particles were examined by scanning electron microscopy (Philips XL30) at an acceleration voltage of 20 kV. For these studies, thin films of microgels were cast from dilute dispersions (concentration onto a glass substrate followed by drying sputtering with gold). The average hydrodynamic diameter and dispersity index (Đ = 1 + RSD2) of the particles were determined by dynamic light scattering (DLS, Nano ZS, Malvern Instruments), where RSD is relative standard deviation. DLS was performed at an angle of 173° by using a He−Ne laser (4 mW) operated at 633 nm. Transmission electron microscopy (TEM) images were performed on a Hitachi H-7500 transmission electron microscope operated at 80 kV. Before TEM measurement, samples were prepared by dropping and drying on a copper gridsupported carbon film. Energy-dispersive X-ray (EDX) analysis was obtained with an EDAX detector installed on a Hitachi S-3000N scanning electron microscope operated at 15.0 kV (Figure S2).

Figure 1. (a−d) SEM images of un-cross-linked ZW particles prepared in an ethanol/water medium at different ethanol concentrations: 50, 60, 70, and 80 wt %. The reactions were carried out at 50 °C, using APS initiator (15 mg) and PVP stabilizer (0.4 g). The scale bar is 2 μm. (e) DLS distribution of hydrodynamic diameter of the corresponding microgels shows that an increase of ethanol fraction in the polymerization mixture leads to larger particles. Inset: diameter and dispersity index of the prepared particles.

solubility of ZW monomers that become insoluble at ϕe > 80 wt %. The lower limit is determined by particle nucleation due to discrete precipitation of oligo-radicals once they reach a critical size. At low ethanol fraction (ϕe < 60 wt %), we observe uncontrolled polymerization, ending with macroscopic coagulation of long polymer chains. Both composition boundaries are not rigid. There are other factors within the 60−80 wt % composition range that could affect the dispersion stability and narrow the range even further. The upper boundary is challenged (may be lowered) by the requirement of having sufficiently high dielectric constant for electrostatic stabilization of the growing particles by ionic initiators, such as ammonium persulfate (APS)20,21 (discussed below). The lower boundary may be pushed to a higher ethanol fraction to facilitate adsorption of polymer stabilizers on primary particles and thus enhance their steric stabilization. In our systems, we observed that an increase of the ethanol fraction (within the 60−80 wt % range) resulted in larger particles (Figure 1), which was attributed to the decreasing stabilization efficiency of APS, while undergoing faster nucleation. To verify the initiator effect on the electrostatic stabilization, we replaced the ionic APS with the neutral AIBN. As expected, a high yield of coagulum was obtained, suggesting that the ionic nature of APS is vital for electrostatic stabilization. We also claim that the concurrent

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RESULTS AND DISCUSSION Similar to poly(acrylamide),15 solubility of zwitterionic (ZW) polymers in water/alcohol mixtures is readily tunable by adjusting the mixture composition. In addition to the composition control, colloidal stabilization is affected by ionization of an initiator, adsorption of polymeric stabilizers on the surface of growing particles, and interparticle crosslinking. We have examined the individual effects of these parameters on dispersion stability and particle-size distribution, starting our studies with the effect of medium composition. As shown in Scheme 1, dispersion polymerization has two basic prerequisites: solubility of monomers and precipitation of oligomers. Respectively, these prerequisites set the upper and lower bounds on the medium composition. In the case of ethanol/water mixtures, stable, well-defined ZW particles were C

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Figure 2. (a−d) SEM images of ZW particles with different concentrations of EGDM and MBA cross-linkers. The reactions were carried out in ethanol/water 70/30 medium at 50 °C, using APS (15 mg) and PVP (0.4 g) as initiator and stabilizer, respectively. The lower panel shows DLS results of the corresponding microgels cross-linked with different amounts of MBA (e) and EGDM (f). The insets in both graphs show the obtained diameter and dispersity index of the prepared microgels. The scale is 2 μm.

in the presence of MBA, resulting in higher number of nuclei and smaller particle size. However, higher kp of MBA also favors interparticle cross-linking and eventually results in larger particles. Regardless of the kp values, interparticle cross-linking occurs at larger concentrations of the cross-linker as observed for EGDM and MBA at molar fractions exceeding 3 and 4.5 mol %, respectively. Note also that cross-linking did not affect the particle dispersity, maintaining low values around Đ = 1.01 (RSD = 0.1). Dispersion polymerization allows incorporation of a broad range of comonomers at different stages of the reaction. This is a significant advantage with respect to inverse-based methods (e.g., inverse emulsion polymerization), which impede addition of comonomer, especially when using highly hydrophobic monomers. As a proof-of-concept, the following monomers, ranging from highly hydrophobic to highly hydrophilic, were copolymerized with ZW: 2-ethylhexyl methacrylate (2-EHMA), butyl acrylate (BuA), methyl methacrylate (MMA), hydroxyethyl methacrylate (HEMA), acrylamide (AAm), and dopamine methacrylamide (Dopa-MA). All of the studied systems formed stable dispersions, yielding well-defined microgel particles as evidenced by SEM images in Figure 3a− f. Incorporation of different monomers in one particle significantly expands the range of practical applications for ZW-based microgels. Two potential applications are discussed below. We also found that by decreasing monomer concentration, the size of particle decreases (Figure S3). We have synthesized ZW-based microgels with different fractions of acrylamide (AAm) comonomers ranging from 0 to 100 mol % and studied their swelling behavior in three different liquids: water, brine, and 6 M HCl. Given the small size of

presence of steric stabilizers, e.g., poly(vinylpyrrolidone) (PVP), is required to achieve stable dispersions. This was confirmed by removing PVP from the polymerization mixture, which resulted in spontaneous coagulation. As such, we have identified an optimal formulation composition for dispersion polymerization of narrowly dispersed microparticles, which consists of 70/30 wt/wt % ethanol/water polymerization medium, APS initiator (1.4 mol %), and PVP steric stabilizer (29 wt %). This particular composition yields particles with a hydrodynamic diameter of 620 nm and dispersity of Đ = 1.012 (RSD = 0.11). In the experiments discussed so far, we purposely did not use any cross-linker in order to exclude the effect of cross-linking on solubility of oligo-radicals. This effect is particularly strong at the nucleation stage as cross-linking reduces monomer diffusion upon nuclei vitrification and may also cause interparticle crosslinking.16,17 As anticipated, even at an extremely low concentration (0.05 wt %) of the cross-linker (MBA and EGDM) in the starting feed of the reaction, the dispersion precipitated. Therefore, to prevent coagulation, the addition of cross-linker was delayed until after the nucleation stage (∼5% conversion), which produced well-defined microgels as evident from SEM and DLS measurements (Figure 2). The observed variations of the particle size with cross-linker concentration (insets in Figure 2e,f) are consistent with the previously reported correlations between the propagation rate of crosslinking (kp) and particle size.22 The kp values of EGDM and MBA are 650−800 M−1 s−1 (50 °C) and 20 000−30 000 M−1 s−1 (20 °C), respectively (i.e., MBA is more reactive than EGDM).23,24 Therefore, at early stages of nucleation after adding cross-linker, the growing oligo-radicals precipitate faster D

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increase of the swelling ratio with the AAm molar fraction. In brine (8 wt % NaCl and 2 wt % CaCl2), the swelling behavior exhibited the same trend as in pure water (i.e., swelling enhancement upon increasing the AAm fraction). This trend was not expected, since ZW hydrogels are well-known for their high saline uptake, and ascribed to the hydrophobic nature of the methacrylate backbone of the ZW strands (Scheme 1). Of particular interest was the swelling behavior in 6 M HCl, which displayed several distinct features. First, the 100% ZW hydrogel imbibed a massive amount of HClca. 25 times its own dry weight, which was about 20 times larger than the swelling degree of pure AAm hydrogel in the same medium. Second, the acid uptake was precisely controlled in a broad range from α = 2 to 25 by adjusting the AAm fraction (Figure 4a). Third, the 100% ZW gels were absolutely stable in the presence of the strong acid and maintained the high swell ratio for 17 days (maximum tested). Fourth, the ZW-AAm gels exhibited a very unusual swelling behavior at long time periods. As seen in Figure 4b, the quick (seconds; not shown) swelling was followed by slow deswelling while the extent of shrinkage increased with the AAm fraction. This is caused by the hydrolysis of amide groups of AAm to carboxylic acid, which “loses” the imbibed water resulting from ion screening of acrylic acid units in high ionic strength media (i.e., polyelectrolyte effect). Moreover, acrylic acid units are not able to deprotonate in the strong acidic environment at pH < pKa = 4.25. The extraordinary high swelling capacity of ZW hydrogels in strong concentrated acids along with their chemical stability in harsh environments distinguishes them from other types of hydrogels. This presents interesting opportunities for encapsulation of heavy metal ions and stimuli-responsive release of chemical materials in corrosive and acidic media. Figure 3f shows a SEM image of the ZW/Dopa-MA microgels, which can be used as templates for the synthesis of metallic nanoparticles (MPNs). The intraparticle synthesis enables size control of MNPs and prevents their aggregation.2,28,29 The combination of the ZW and Dopa moieties allows for the concurrent accomplishment of two tasks: (i) the ZW units work as osmotic pumps attracting metal ions from a surrounding medium and (ii) the Dopa units aid in reduction (conversion) of the dispersed metal ions to free-metal nanoparticles. As a proof of concept, silver nanoparticles (AgNPs) and magnetic nanoparticles (Fe3O4) (Figure S4 and Movie S1) were synthesized inside the ZW/Dopa-MA microgels. In Figure 5a, a TEM image of the nanocomposite ZW microgels shows uniformly dispersed nanoparticles with no sign of aggregation. The dark contrast observed in the TEM image in Figure 5b suggests that the AgNPs have been synthesized inside the microgels (Figure S2). Employing microgels prevents aggregation of the highsurface-energy MNPs, which enables easy processing, long-term

Figure 3. SEM images ZW microgels containing 10 mol % of different comonomers: (a) AAm, (b) HEMA, (c) MMA, (d) BuA, (e) 2EHMA, and (f) Dopa-MA. All reactions were carried out in 70/30 ethanol/water at 50 °C, using APS initiator (15 mg) and PVP stabilizer (0.4 g). The scale bar is 2 μm.

microgel particles, swelling was quick and leveled off within a few seconds. Figure 4a shows a histogram of the equilibrium

Figure 4. (a) Histogram of the equilibrium swelling ratios of ZW/ AAm microgels in pure water, brine, and 6 M HCl acid as a function of molar fraction of the ZW monomer. (b) Swelling kinetics of ZW/AAm microgels with different molar ratios (as indicated) in 6 M HCl acid.

volumetric swelling ratio (α = V/V0) of the synthesized gels in water, brine, and 6 M HCl acid. As anticipated, 100% ZW particles demonstrated weak swelling in pure water and extensive swelling in the strongly ionized brine and HCl solution. The observed behavior is well-known and ascribed to ionic association of the ZW groups in low-ionic-strength media (e.g., water) and their dissociation upon ions addition (e.g., from brine and 6 M HCl).25−27 In ZW-co-AAm particles, the addition of the AAm units promotes solubility in water by disrupting the ZW-ionic associations and forming hydrogen bonds with water molecules. This was evidenced by the steady

Figure 5. TEM images of 90/10 mol/mol ZW-Dopa-MA microgels with Ag nanoparticles at different magnifications and lattice parameter of Ag. E

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(3) Zhang, L.; Cao, Z.; Li, Y.; Ella-Menye, J.-R.; Bai, T.; Jiang, S. Softer zwitterionic nanogels for longer circulation and lower splenic accumulation. ACS Nano 2012, 6 (8), 6681. (4) Kabiri, K.; Faraji-Dana, S.; Zohuriaan-Mehr, M. J. Novel sulfobetaine-sulfonic acid-contained superswelling hydrogels. Polym. Adv. Technol. 2005, 16 (9), 659. (5) Ogawa, K.; Nakayama, A.; Kokufuta, E. Preparation and characterization of thermosensitive polyampholyte nanogels. Langmuir 2003, 19 (8), 3178. (6) Nisato, G.; Munch, J.; Candau, S. Swelling, structure, and elasticity of polyampholyte hydrogels. Langmuir 1999, 15 (12), 4236. (7) GhavamiNejad, A.; Park, C. H.; Kim, C. S. In Situ Synthesis of Antimicrobial Silver Nanoparticles within Antifouling Zwitterionic Hydrogels by Catecholic Redox Chemistry for Wound Healing Application. Biomacromolecules 2016, 17 (3), 1213. (8) Pelton, R.; Chibante, P. Preparation of aqueous latices with Nisopropylacrylamide. Colloids Surf. 1986, 20 (3), 247. (9) Cheng, G.; Mi, L.; Cao, Z.; Xue, H.; Yu, Q.; Carr, L.; Jiang, S. Functionalizable and ultrastable zwitterionic nanogels. Langmuir 2010, 26 (10), 6883. (10) Oh, J. K.; Tang, C.; Gao, H.; Tsarevsky, N. V.; Matyjaszewski, K. Inverse miniemulsion ATRP: a new method for synthesis and functionalization of well-defined water-soluble/cross-linked polymeric particles. J. Am. Chem. Soc. 2006, 128 (16), 5578. (11) Paine, A. J.; Luymes, W.; McNulty, J. Dispersion polymerization of styrene in polar solvents. 6. Influence of reaction parameters on particle size and molecular weight in poly (N-vinylpyrrolidone)stabilized reactions. Macromolecules 1990, 23 (12), 3104. (12) Peng, B.; van der Wee, E.; Imhof, A.; van Blaaderen, A. Synthesis of monodisperse, highly cross-linked, fluorescent PMMA particles by dispersion polymerization. Langmuir 2012, 28 (17), 6776. (13) Adelnia, H.; Riazi, H.; Saadat, Y.; Hosseinzadeh, S. Synthesis of monodisperse anionic submicron polystyrene particles by stabilizerfree dispersion polymerization in alcoholic media. Colloid Polym. Sci. 2013, 291 (7), 1741. (14) Adelnia, H.; Gavgani, J. N.; Soheilmoghaddam, M. Fabrication of composite polymer particles by stabilizer-free seeded polymerization. Colloid Polym. Sci. 2015, 293 (8), 2445. (15) Ray, B.; Mandal, B. M. Dispersion polymerization of acrylamide. Langmuir 1997, 13 (8), 2191. (16) Song, J.-S.; Tronc, F.; Winnik, M. A. Two-stage dispersion polymerization toward monodisperse, controlled micrometer-sized copolymer particles. J. Am. Chem. Soc. 2004, 126 (21), 6562. (17) Song, J.-S.; Winnik, M. A. Cross-linked, monodisperse, micronsized polystyrene particles by two-stage dispersion polymerization. Macromolecules 2005, 38 (20), 8300. (18) Horák, D.; Karpíšek, M.; Turková, J.; Beneš, M. HydrazideFunctionalized Poly (2-hydroxyethyl methacrylate) Microspheres for Immobilization of Horseradish Peroxidase. Biotechnology progress 1999, 15 (2), 208. (19) Glass, P.; Chung, H.; Washburn, N. R.; Sitti, M. Enhanced reversible adhesion of dopamine methacrylamide-coated elastomer microfibrillar structures under wet conditions. Langmuir 2009, 25 (12), 6607. (20) Adelnia, H.; Pourmahdian, S. Soap-free emulsion polymerization of poly (methyl methacrylate-co-butyl acrylate): effects of anionic comonomers and methanol on the different characteristics of the latexes. Colloid Polym. Sci. 2014, 292 (1), 197. (21) Adelnia, H.; Gavgani, J. N.; Riazi, H.; Bidsorkhi, H. C. Transition behavior, surface characteristics and film formation of functionalized poly (methyl methacrylate-co-butyl acrylate) particles. Prog. Org. Coat. 2014, 77 (11), 1826. (22) An, Z.; Tang, W.; Hawker, C. J.; Stucky, G. D. One-step microwave preparation of well-defined and functionalized polymeric nanoparticles. J. Am. Chem. Soc. 2006, 128 (47), 15054. (23) Beuermann, S.; Buback, M. Rate coefficients of free-radical polymerization deduced from pulsed laser experiments. Prog. Polym. Sci. 2002, 27 (2), 191.

storage, and well-defined performance of nanoparticles. Moreover, the microreactor approach allows for the synthesis of hybrid particles possessing a combination of properties toward a much broader spectrum of applications. In addition to being an excellent reducing agent, it has been reported that dopamine and its derivatives mimic the composition of mussel foot proteins aiding in strong adhesion to a wide variety of substrates.30−32

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CONCLUSIONS This report describes the preparation of narrowly dispersed zwitterionic-based microgels by dispersion polymerization in ethanol/water media. The impact of various experimental conditions on colloidal stability and particle size as well as size distribution is investigated. It was found that the presence of even as low as 0.5 mol % of cross-linker in the polymerization feed gave rise to coagulation. However, stable microgels were achieved when the introduction of cross-linker was delayed until after the nucleation stage. Successful preparation of the microgels through dispersion polymerization in aqueous media provides a vital capability to introduce a wide range of comonomers with different polarity levels. Finally, dopamine methacrylamide was successfully incorporated to endow the microgels with attractive features of mussel-inspired chemistry and the microreactor functionality.

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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.6b01713. Supplementary data related to this article including 1H NMR of dopamine methacrylamide (Figure S1), EDX result of microgels with Ag nanoparticles (Figure S2), DLS results of microgels in different monomer concentration (Figure S3), and TEM image of microgels with Fe3O4 nanoparticles (Figure S4) (PDF) Absorption of microgels containing Fe3O4 nanoparticles on the magnet (Movie S1) (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.S.S.). *E-mail [email protected] (M.V.-V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from Halliburton and National Science Foundation (DMR 1407645 and DMR 1436201). The authors also acknowledge contributions of Dr. Amar Kumbhar and Wallace Ambrose at CHANL (UNC-CH), Sean Olson and Chad Tabikh at UNC-CH.

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