Pickering Emulsion Stabilized by Microporous Organic Polymer

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Pickering Emulsion Stabilized by Microporous Organic Polymer Particles for the Fabrication of a Hierarchically Porous Monolith Jieun Lee, and Ji Young Chang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02576 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Pickering Emulsion Stabilized by Microporous Organic Polymer Particles for the Fabrication of a Hierarchically Porous Monolith

Jieun Lee and Ji Young Chang* Department of Materials Science and Engineering College of Engineering, Seoul National University Seoul 08826, Korea. E-mail: [email protected]

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ABSTRACT A hierarchically porous monolith comprised of the melamine-based microporous organic polymer (MOP) particles was prepared by the Pickering emulsion templating method. The MOP

particles

were

synthesized

by

polycondensation

of

melamine

and

terephthaldicarboxaldehyde. Due to the balanced presence of hydrophilic nitrogen containing groups and hydrophobic benzene rings, the MOP particles showed good amphiphilicity. A Pickering emulsion was prepared, where cyclohexane droplets with an average size of about 25 µm were stabilized by the MOP particles (3.4 wt%) in an aqueous continuous phase. The cyclohexane internal phase fraction was slightly higher than 60 %. The emulsion showed no phase separation even after two weeks. The Pickering emulsion containing a small amount of PVA (1 wt%) in a continuous phase as a reinforcement was used as a template for the fabrication of a monolith of the MOP particles. The Pickering emulsion was freeze-dried to produce a hierarchically porous monolith. The MOP monolith possessed macropores templated by the oil droplets and micro- and mesopores in the MOP particles that constituted the macropore walls. The MOP monolith exhibited a high dye absorption ability in a solution of RhB in chloroform and a good absorption capacity for nonpolar organic solvents. After the absorption, the monolith could be regenerated by solvent exchange with cyclohexane and subsequent freeze-drying.

KEYWORDS: microporous organic polymer, Pickering emulsion, emulsion templating method, hierarchical porosity, absorbent

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INTRODUCTION Pickering emulsions1,2 are an interesting class of emulsions stabilized by solid particles which adsorb onto the interface between the two immiscible phases. Compared with conventional emulsions stabilized by surfactants, a Pickering emulsion demands a low concentration of solid particles for the stabilization against coalescence.3 In addition, the solid particles can be easily separated by filtration after use and recycled.4 A Pickering emulsion has been used as a template to fabricate 3D structures such as polymeric monoliths5,6 and microcapsules.7–9 For this reason, many studies on the structures and properties of Pickering emulsions have been conducted. The stability of a Pickering emulsion is significantly dependent on the surface nature of a particle emulsifier. The solid particles should have desirable hydrophilic-lipophilic balance (HLB)10 and be well dispersed in the continuous phase of the emulsion so as to stabilize the interface between the two immiscible phases, while preventing the coalescence of droplets. A wide variety of solid particles have been used for Pickering emulsions as such or after chemical modification to meet these requirements, including silica,11–13 metal-organic frameworks,7,14–16 graphene oxide,6,17,18 and metal oxides.8,19–21 Microporous organic polymers (MOPs) having micropores with diameters of smaller than 2 nm have drawn considerable interests as porous materials in recent years. They have promising characteristics for applications as absorbents and supports, including large specific surface areas, high chemical stabilities and tunable chemical structures.22–25 Most microporous organic polymers are prepared by the coupling reactions of multifunctional rigid building blocks and therefore are usually obtained as insoluble and infusible powders. This lack of processability restricts the range of MOP applications. Herein, we demonstrated the fabrication of a MOP monolith from a Pickering emulsion 3


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stabilized by MOP particles. This was the first example of a MOP used as a Pickering emulsifier to the best of our knowledge. MOPs have been synthesized from a variety of multifunctional compounds to have a wide range of chemical structures and properties. This flexibility in the structural design will make it possible to impart a suitable HLB to a MOP for a Pickering emulsion. In this study, a melamine-based MOP prepared by polycondensation of melamine with aldehydes was chosen as a Pickering emulsifier through screening.26,27 The melamine-based MOP showed amphiphilicity due to the presence of hydrophobic aromatic rings and hydrophilic nitrogen containing groups. The MOP monolith with a high surface area and hierarchical porosity could be obtained by removing the dispersed phase from the MOP-stabilized Pickering emulsion and used as a promising absorbent.

EXPERIMENTAL SECTION Materials. Melamine was purchased from TCI. Terephthaldicarboxaldehyde was purchased from ACROS Organics. Polyvinyl alcohol (PVA, Mw 85,000 ~ 124,000) was obtained from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) was purchased from Junsei. Cyclohexane was purchased from Daejung Chemical. All chemicals were used without any further purification. Preparation of the Melamine-Based MOP and Characterization. Melamine-based MOP was synthesized according to the previously described procedure26 with slight modification. Melamine (3.13 g, 24.85 mmol) and terephthaldicarboxaldehyde (5 g, 37.28 mmol) were dissolved in DMSO (155 ml) and the solution was stirred at 180 ℃ for 72 h under an air atmosphere. After cooling to room temperature, the precipitated melamine-based MOP particles were isolated by filtration. They were washed with acetone, dichloromethane 4


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and was Soxhlet extracted with THF for 12 h to remove any remaining reactants. N2 adsorption-desorption isotherms were measured by a Belsorp-Max (BEL Japan, Inc.) apparatus. The contact angle was measured with a goniometer (Phoenix 300) by placing an approximately 10 µL of deionized water droplet on the surface of the sample at an ambient atmosphere. The sample was prepared in the form of a pellet by applying a pressure of 0.05 kgf/cm2 for 30 sec. The FT-IR spectrum was measured by a JASCO FT-IR 4200 spectrometer using KBr pellets. Solid-state

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C NMR spectrum was recorded on a Bruker Avance II

spectrometer (125 MHz) equipped with a CP-MAS probe. Preparation of Pickering Emulsions Stabilized by MOP and Characterization. Melamine-based MOP particles (200 mg) were dispersed in deionized water (2.5 mL) under stirring at 1000 rpm. Then, cyclohexane (4 mL) was slowly added to the aqueous dispersion at a constant stirring of 10000 rpm for 1 min, forming the medium internal phase emulsion of which internal phase fraction slightly higher than 60 %. The emulsification was performed using a homogenizer (WiseTis HG-15D). The microstructure of the Pickering emulsion was characterized by a confocal laser scanning microscopy (CLSM) using a Leica microscope (SP8 X), with an excitation wavelength of 561 nm. The emulsion containing Rhodamine 6G (10-4 M in the aqueous phase) was trickled on a 1 mm thick cover slip and covered with cover glasses with a thickness of 0.15 mm. Preparation of the MOP Monolith and Characterization. The Pickering emulsion for the fabrication of the MOP monolith was prepared in the same manner as described above except that a small amount of PVA (1 wt%) was mixed with water. The emulsion was freezedried to produce a free standing MOP monolith. The morphology of the MOP monolith was characterized by a scanning electron microscope (JEOL JSM-6330F). A compression test was 5


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carried out using a DMA Q800 instrument (TA Instruments) at a rate of 1 mm/min. The pore structure was characterized by N2 adsorption-desorption analysis (Belsorp-Max) and mercury porosimetry (PM33GT, Quantachrome). Absorption of Organic Liquids and Reusability. To analyze the absorption capacity of the MOP monolith, the monolith of known weight was placed in a vial filled with an oil. After 30 min, the wet monolith was drained for 1 min until no residual droplet was left on the surface. Each test was conducted three times. The absorption capacity was calculated via the following equation: q = (ms-m0)/m0

(1)

where q is the absorption capacity (g/g), ms is the weight of the wet monolith after 1 min of drainage (g), and m0 is the initial weight of the monolith (g). n-Hexane, n-heptane, cyclohexane, isopropyl alcohol (IPA), mineral oil, paraffin oil, silicon oil, and chloroform were used to measure the absorption capacity. Dye Absorption Measurement. The MOP monolith (100 mg) was immersed in an RhB solution in chloroform (0.03 mM, 15 mL) and the UV-vis absorption of the solution was measured at different time intervals. The same dye absorption experiment was performed with a MOP pellet (100 mg) having a diameter of 2 cm and a thickness of 0.7 mm. The MOP pellet was prepared by applying a pressure of 500 kgf/cm2 on the MOP monolith for 1 min. UV-Vis spectra were measured with a Sinco S-3150 spectrometer.

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RESULTS AND DISCUSSION

Scheme 1. (a) Structure of the melamine-based MOP and (b) schematic illustration for preparing a Pickering emulsion stabilized by the MOP particles.

The preparation procedures of the melamine-based MOP and the Pickering emulsion stabilized by the MOP particles are presented in Scheme 1. The melamine-based MOP was synthesized by polycondensation of melamine and terephthaldicarboxaldehyde in dimethyl sulfoxide according to the literature.26 The polymer precipitated during the reaction and was isolated as particles by filtration. In the solid state 13C cross-polarization/total suppression of spinning sidebands (CP/TOSS) NMR spectrum of the melamine-based MOP, the peak appeared at 53 ppm corresponding to the carbon atoms of aminal linkages (HN-C-NH).28 The condensation reaction between an 7


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aldehyde and a primary amine produces both imine and aminal linkages, but it was reported that aminal linkages were mainly formed under the reaction conditions.26,27 The strong peak at 166 ppm was assigned to the aromatic carbons of the triazine ring. The FT-IR spectrum (Figure S1) showed the C=N stretching vibration peaks at 1554 and 1484 cm-1, confirming the incorporation of triazine rings into the MOP. The melamine-based MOP particles had hydrophobic aromatic rings as major constituents, but hydrophilicity was also expected due to the high nitrogen content. The particles exhibited a contact angle of around 66.5⁰ (Figure S2), indicating that they had reasonably hydrophilic surfaces. The MOP particles with an average size of 120 nm (Figure S3) were dispersed in water and then cyclohexane was slowly added to the aqueous dispersion with fast stirring to form a stable emulsion. The emulsion appeared viscous because the internal phase fraction was slightly higher than 60 %.29 A low internal phase emulsion with an internal phase volume ratio of below 30 % usually shows high fluidity (Figure S4). In the Pickering emulsion, cyclohexane droplets were stabilized by a low concentration of the MOP particles (3.4 wt%) in an aqueous continuous phase. Figure 1a shows the photo images of the emulsion taken over time. The emulsion showed no phase separation even after two weeks indicating that the melamine-based MOP efficiently prevented the coalescence of cyclohexane droplets. We presumed that the balanced presence of hydrophilic nitrogencontaining groups and hydrophobic benzene rings led the MOP particles to assemble at the oil and water interface to form a long-time stable Pickering emulsion.30

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Figure 1. (a) Photographs of the Pickering emulsions stabilized by melamine-based MOP over time. (b) CLSM images of droplets dyed with Rhodamine 6G, and (c) the droplet diameter distribution.

The microstructure of the emulsion was characterized by confocal laser scanning microscopy (CLSM). An aqueous phase was marked by a fluorescent dye, Rhodamine 6G. In Figure 1b, a red color emission appears in the continuous phase, corroborating the formation of an oil-in-water type emulsion. The sizes of the cyclohexane droplets were about 20-30 µm (Figure 1c). The CLSM image of the Pickering emulsion measured after 2 weeks did not show any significant change in the morphology, suggesting the good emulsion stability. Even after 4 weeks, the emulsion maintained its stability although the slight coarsening of the oil droplets was observed (Figure S5). The Pickering emulsion could be used as a template for the fabrication of a MOP monolith with a hierarchically porous structure. A major issue in this process was that there were relatively weak interactions between the particles. The MOP particles assembled together at the interface of oil and water in the emulsion, but such an assembly could be 9


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disrupted in the dry state. We could not obtain a self-standing monolith when the MOPstabilized Pickering emulsion was freeze-dried (Figure S6). The stability of the monolith was improved by using poly(vinyl alcohol) (PVA) as a reinforcement. A Pickering emulsion was prepared in the same manner as described above except that an aqueous solution of PVA (1 wt%) was used as a continuous phase. The emulsion showed basically the same structure and behaviors as the emulsion prepared in pure water. An off-white colored monolith with a low density (0.03-0.04 g/cm3) was successfully obtained by the freeze-drying method and it could be cut into any shape with a knife (Figure 2a and S6). Figure 2b and c show the photo images of the MOP monolith taken during a loading and unloading cycle and the compressive stressstrain curve, respectively. The MOP monolith was compressed without crack up to the strain of 40 % (the stress of 0.024 MPa) and slightly recovered its original shape once the compression force was removed.

Figure 2. Photographs of (a) the melamine-based MOP particles and the MOP monolith prepared using the same mass of the MOP particles. (b) Photographs of the MOP monolith taken during a loading and unloading cycle and (c) its compressive stress-strain curves.

The porosities of the melamine-based MOP powders and the MOP monolith were investigated by N2 adsorption-desorption analysis at 77 K (Figure 3). The MOP monolith 10


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showed a BET surface-area of 241 m2/g, which was lower than that of the melamine-based MOP powders (396 m2/g) due to the presence of PVA. The both adsorption isotherms showed a sharp increase at low relative pressures, implying the existence of micropores.31 The pore size distribution of the MOP monolith determined by the non-local density functional theory (NLDFT) also showed the existence of micropores with a size of less than 2 nm and mesopores (Figure 3b). The pore structure of the monolith was further analyzed using a mercury porosimetry, which exhibited the presence of meso- and macropores (Figure S7).

Figure 3. (a) N2 adsorption-desorption isotherms and (b) NL-DFT pore size distributions of the melamine-based MOP powders and the MOP monolith measured at 77 K.

Figure 4. SEM images of the MOP monolith

The cross-sectional morphology of the MOP monolith measured by SEM (Figure 4) 11


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showed the dominant presence of macropores with thin walls. The sizes of the macro pores were in the range of 20-30 µm, which were consistent with the oil droplet diameters of the emulsion measured from the CLSM image (Figure 1b). This result indicated that the cyclohexane droplets acted as a template for the macropore formation. The MOP monolith was hierarchically porous as the macropore walls were comprised of the melamine-based MOP particles possessing micro- and mesopores. Figure 4c shows the macropore wall image, where PVA spreads over the wall to reinforce the monolithic structure. We presumed that the hydroxyl groups of PVA formed hydrogen bonds with the melamine-based MOP and acted as a reinforcement to improve the mechanical stability of the monolith.32 The MOP monolith had a high absorption capacity for nonpolar organic liquids and could absorb 18-36 times of its own weight (Figure 5a). The absorption capacity of the monolith was linearly correlated with the liquid density, indicating that the accessible pore volume was not changed by these liquids (Figure 5b).33,34

Figure 5. (a) Absorption capacity of the MOP monolith for organic liquids and (b) the relationship between the density of the organic liquid and the adsorption capacity of the MOP monolith.

The MOP monolith maintained its microstructure after use for the organic liquid absorption. The MOP monolith used for the chloroform absorption was regenerated by 12


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solvent exchange with cyclohexane and subsequent freeze-drying. The shape of the MOP monolith was not changed significantly by the chloroform absorption (Figure 6a). The SEM image of the regenerated MOP monolith showed the same microstructure as that of the original MOP monolith (Figure 6b).

Figure 6. (a) The photograph of the chloroform absorbed MOP monolith and (b) the photograph and SEM images of the regenerated MOP monolith after use.

Microporous materials have been widely used to remove small molecules such as aromatic compounds,35 gases,36,37 and organic dyes.38,39 We investigated the dye absorption behavior of the MOP monolith. The MOP monolith (100 mg) was immersed in an RhB solution in chloroform (0.03 mM, 15 mL). Figure 7a shows the UV-vis spectra of the RhB solution measured during the absorption test. The absorption intensity of RhB decreased gradually and became almost zero after 15 min, indicating that RhB molecules were removed by the MOP monolith. For comparison, the same dye absorption experiment was performed with a MOP pellet (100 mg) having a diameter of 2 cm and a thickness of 0.7 mm (Figure S8). The MOP pellet was prepared by pressing the MOP monolith and contained no macropores templated by the oil droplets. Figure 7b shows that the MOP pellet absorbed RhB much 13


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slowly compared to the MOP monolith. This result could be attributed to the hierarchical pore structure of the MOP monolith, which facilitated mass transfer.40,41

Figure 7. (a) UV-vis spectral changes of the RhB solution in chloroform (initial concentration = 0.03 mM, 15 mL) during the absorption test with the MOP monolith (100 mg) and the visible color change of the RhB solution after the dye absorption for 15 min. (b) UV-vis spectral changes of the RhB solution in chloroform (initial concentration = 0.03 mM, 15 mL) during the absorption test with the MOP pellet (100 mg) and the visible color change of the RhB solution after the dye absorption for 15 min.

CONCLUSIONS We presented the formation of a Pickering emulsion stabilized solely by MOP particles for the first time. The melamine-based MOP particles with amphiphilicity could stabilize the oil droplets dispersed in the water phase, resulting in the formation of a Pickering emulsion. A hierarchically porous monolith was fabricated by the emulsion templating method. The monolith possessed macropores formed from the oil droplets as well as micro- and mesopores of the MOP particles, which constituted the macropore walls. This work provides a useful method to consolidate MOP particles into a bulk shape, which will broaden the range of MOP applications. 14


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ASSOCIATED CONTENT Supporting Information Characterization data of the melamine-based MOP and the MOP particles, photographs and CLSM images of the MOP-stabilized Pickering emulsions, photographs of the freeze-dried Pickering emulsions.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Ji Young Chang: 0000-0003-2695-1210 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A01006585).

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Jieun Lee and Ji Young Chang* Pickering Emulsion Stabilized by Microporous Organic Polymer Particles for the Fabrication of a Hierarchically Porous Monolith

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Pickering Emulsion Stabilized by Microporous Organic Polymer

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