Polymerizations in oil-in-oil emulsions using 2D nanoparticle surfactants

Bradley J. Rodier , Al de Leon , Christina Hemmingsen and Emily Pentzer *
Department of Chemistry, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH, USA 44106. E-mail: ebp24@case.edu

Received 30th October 2017 , Accepted 17th November 2017

First published on 20th November 2017


Oil-in-oil emulsions are especially attractive for compartmentalized reactions with water-sensitive monomers which cannot be used with traditional oil/water emulsions. Herein, we demonstrate that alkylated graphene oxide nanosheets can be used as a surfactant for three different oil-in-oil emulsion polymerizations and to prepare closed-cell foams, hollow capsules, and functionalizable particles.


Stabilized emulsions are biphasic systems consisting of pockets of a discontinuous phase surrounded by a continuous phase, with the interfacial tension between the two liquids reduced by the presence of a surfactant. Typical surfactants are amphiphilic with hydrophobic and hydrophilic sections, for example sodium dodecyl sulfate (SDS); thus, kinetically stable suspensions of one liquid dispersed in another are prepared, for example, oil droplets in water.1–3 In addition to stabilizing the phase separation of two liquids, emulsions and mini-emulsions can be used for compartmentalized reactions or polymerizations. This is well established and widely explored as a tool in industrial applications for foods,4–6 cosmetics,7,8 biosensing,9–11 drug delivery,12–16 and coatings.17–19 Polymerizations in (mini)-emulsions can be used to prepare higher order structures and morphologies including foams or hydrogels,20,21 capsules,22–24 and solid particles.25–27 To achieve these structures and applications, oil-in-water (mini)-emulsions and small molecule surfactants are traditionally used. In recent years, particle surfactants have been explored to prepare hybridized polymer–nanoparticle materials, through so-called Pickering-type emulsions.28–32 Particle surfactants are complementary to small molecule surfactants, and are especially attractive due to their ability to be covalently functionalized and requirement of more energy for removal from the fluid–fluid interface. Of note, particle surfactants can have polar and non-polar halves (i.e., Janus particles), or can have neutral wettability to the two phases.21,33,34

An attractive complement and alternative to oil-in-water emulsions are oil-in-oil emulsions. These emulsions are composed of two organic solvents, typically one polar, such as N,N-dimethyl formamide (DMF), and one nonpolar, such as octane. Perhaps the most attractive feature of oil-in-oil emulsions is the use of water-free conditions and thus the ability to use water-sensitive functionalities. The limited reports on oil-in-oil emulsions can be attributed to the difficulty in accessing suitable surfactants. In fact, most surfactants for oil-in-oil emulsions are tailored co-polymers of fine-tuned stoichiometric composition. For example, Müllen, Klapper, and co-workers used polyisoprene-b-poly(methyl methacrylate) to stabilize the DMF/hexane interface of oil-in-oil emulsions for the preparation of polymer particles through metallocene-catalysed chemistry.35,36 In a similar vein, Lodge and co-workers used polystyrene-b-poly(ethylene oxide) and polybutadiene-b-poly(ethylene oxide) as surfactants to stabilize the interface of two chloroform phases, each containing a polymer immiscible with the other (polystyrene and polybutadiene).37,38 In addition to these examples of block copolymer surfactants, inorganic particles have been used to stabilize oil-in-oil emulsions: Binks and co-workers used silica particles of tailored hydrophobicity to stabilize emulsions of silicon oil and vegetable oil,39,40 while Tawfeek and co-workers demonstrated that clay nanosheets combined with a non-ionic, reactive surfactant stabilize paraffin-in-formamide, silicon oil-in-glycerin, and castor oil-in-glycerin emulsions.41 Recently, our group reported that oil-in-oil emulsions could easily be formed using alkylated carbon nanomaterials as surfactants.42 DMF/octane and ACN/octane emulsions were stabilized by graphene oxide nanosheets functionalized by primary alkyl amines. The continuous phase of these oil-in-oil emulsions is controlled by the length of the alkyl chain; specifically, GO nanosheets modified with relatively long alkyl amines (e.g., hexadecyl) are good surfactants for DMF-in-octane emulsions and GO modified with shorter alkyl amines (e.g., hexyl) is a good surfactant for octane-in-DMF emulsions.

Herein, we demonstrate that oil-in-oil emulsions stabilized only by alkylated graphene oxide nanosheets can be used as a platform for three different polymerization systems that give distinct macroscopic architectures. By introducing monomers into one or both the phases of the oil-in-oil emulsions, the location of the polymerization is controlled to be the outer phase, the inner phase, or the interface of the two oils to give closed cell foams, particles, and hollow shells, respectively. We demonstrate that this oil only system preserves water-sensitive moieties during polymerization, and that these functionalities can subsequently be used for modification. An overview of this work is shown in Fig. 1.


image file: c7py01819c-f1.tif
Fig. 1 Schematic overview of GO functionalization with two different alkyl amines to give C6-GO and C18-GO, and subsequent emulsion-based polymerizations to form closed cell foams, hollow capsules, or armoured particles.

Oil-in-oil emulsions of DMF and dodecane were used for emulsion polymerizations, stabilized by alkylated graphene oxide nanosheets. Both the synthesis of GO nanosheets and functionalization with alkyl amines were performed as previously reported.42 Functionalization of the GO nanosheets was verified by infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) (see Fig. S1), as well as the changes in solubility. For the work reported here, GO functionalized with hexylamine (C6-GO) and GO functionalized with octadecylamine (C18-GO) were used, and were dispersed in DMF and dodecane, respectively. We noted that conditions for polymerizations in oil-in-oil emulsions must ensure emulsion integrity; for example, our attempts to use radical polymerization to prepare closed cell foams were unsuccessful because elevated temperatures and/or oligomers destabilized or miscibilized the two phases.

Polymerization of the continuous phase of an oil-in-oil emulsion stabilized with modified GO nanosheets resulted in the formation of closed cell foam. To this end, C6-GO was dispersed in DMF which contained a tetra-thiol and dialkyne; then a nonpolar oil, specifically dodecane or hexadecane, was added. Vortex mixing led to the formation of an emulsion, which was then placed in a UV box. Irradiation with UV light initiated the thiol–yne polyaddition polymerization between the tetra-thiol and dialkyne (Fig. 2A).43,44 Polymerization formed fused shells around the dispersed oil phase, resulting in closed cell foam, as shown by the optical images in Fig. 2B and C. Scanning electron microscopy (SEM) images of a cross section of the material are shown in Fig. 2D and E and they reveal a solid polymer studded with cavities, indicative of polymerization within the emulsion structure. While GO is assumed to line the cavities, the location cannot be determined by XPS or energy-dispersive X-ray spectroscopy (EDX), as GO nanosheets are difficult to differentiate from the polymer.


image file: c7py01819c-f2.tif
Fig. 2 (A) Schematic overview of UV light-induced thiol–yne ‘click’ polymerization in the continuous phase of an emulsion stabilized by C6-GO to give closed cell foam; (B, C) optical microscopy images of polythioether closed cell foam; (D, E) SEM images of polythioether closed cell foam.

Interfacial polymerizations using oil-in-oil emulsions stabilized by alkylated GO were also studied. C18-GO was dispersed in dodecane containing the water-sensitive di-functional small molecule hexamethylene diisocyanate (HMDI); a DMF solution of glycerol and ethylene diamine was then added and the mixture was briefly agitated by vortex. Of note, DMF was necessary for emulsion formation due to the high viscosity of glycerol. Subsequent interfacial polymerizations of the newly formed emulsion resulted in the formation of capsules (Fig. 3A), attributed to the reaction of the alcohols of glycerol and amines of ethylene diamine with isocyanates at the oil–oil interface. The capsules ranged in diameter from 25 to 150 μm (Fig. 3B and C). SEM images show that the capsules collapse when placed under vacuum and that the capsules have a rough and irregular surface (Fig. 3D). The SEM image in Fig. 3E shows that the capsule shell is ∼4 μm thick.


image file: c7py01819c-f3.tif
Fig. 3 (A) Schematic of capsules prepared by the interfacial polymerization of hexamethylene diisocyanate (HMDI) with glycerol and ethylene diamine using C18-GO as the surfactant; (B, C) optical microscopy images of capsules after an isopropanol and water wash; (D) SEM image of capsules; (E) higher magnification SEM image of capsules.

We then explored the preparation of solid polymer particles by polymerization of the inner oil phase of the oil-in-oil emulsion. To do so, C18-GO was used as the surfactant, dodecane was used as the continuous phase, and the discontinuous phase was a DMF solution of 2-isocyanoethyl methacrylate and catalytic AIBN (Fig. 4A). After the formation of an emulsion by vortex mixing, the system was heated to 55 °C without stirring. A distribution of particle sizes was observed by SEM (Fig. 4B), and the particles are solid, as demonstrated by cutting a particle using a focused ion beam (FIB) in the SEM (Fig. 4C). The FTIR spectra in Fig. S3 show that after the polymerization of the methacrylate of the monomer, the pendant isocyanate remains intact, and thus can be used as a handle for functionalization. The particles were modified by using a suspension in THF and addition of 7-mercapto-4-methylcoumarin, a fluorescence dye with a thiol that can undergo nucleophilic addition to isocyanates. Fig. 4E shows that the particles are modified effectively and uniformly by the reaction of the coumarin dye with the isocyanates (also see, Fig. S2). These data show that water-sensitive functionalities survive polymerizations in oil-in-oil emulsions and can be used for further modification, a feature not possible with water-based emulsions.


image file: c7py01819c-f4.tif
Fig. 4 (A) Scheme of polymerization of the inner phase of an oil-in-oil emulsion to prepare polymer particles with water-sensitive pendant groups that can be modified with a coumarin derivative; (B) SEM image of polymer particles; (C) SEM image of the particle cut, showing that they are solid; (D, E) optical images of particles reacted with 7-mercapto-4-methylcoumarin, under ambient and under UV light (365 nm).

In summary, we have demonstrated that oil-in-oil emulsions stabilized by alkylated GO nanosheets can be used as a facile and scalable platform for polymerizations to prepare three different architectures: closed-cell foams, hollow capsules, and polymer particles. The alkylated GO nanosheets assembled at the interface of DMF/dodecane oil-in-oil emulsions containing different monomers, and thiol–yne polymerization, amine-isocyanate polymerization, and radical polymerization were used to prepare the different architectures. The structures were characterized by using various microscopy techniques, and the location of the monomers dictated the structures formed. We further demonstrated that water-sensitive functional groups survive polymerizations in oil-in-oil emulsions and can be used for further modification. Thus, oil-in-oil emulsions are ideal platforms for the preparation of new material structures, as well as compartmentalized reactions, and are especially attractive for water-sensitive applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank CWRU College of Arts and Sciences and NSF CAREER Award #1551943 for financial support. B. J. R. is a NASA Harriett G. Jenkins Predoctoral Fellow (Grant #NNX13AR93H). XPS measurements were performed at the Swagelok Center for Surface Analysis of Materials (SCSAM) at CWRU. C. H. would like to thank the CWRU P-SURG program.

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Footnote

Electronic supplementary information (ESI) available: Synthetic techniques and characterization. See DOI: 10.1039/c7py01819c

This journal is © The Royal Society of Chemistry 2018