Preparation of Janus Particles Composed of Hydrophobic and

Jun 21, 2019 - Composite particles were prepared by the seeded polymerization of an ionic liquid monomer ([2-(methacryloyloxy)ethyl]trimethylammonium ...
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Preparation of Janus Particles Composed of Hydrophobic and Hydrophilic Polymers Takuto Ouchi, Ryuma Nakamura, Toyoko Suzuki, and Hideto Minami* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan

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S Supporting Information *

ABSTRACT: Composite particles were prepared by the seeded polymerization of an ionic liquid monomer ([2(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide) ([MTMA][TFSA]) in the presence of poly(methyl methacrylate) (PMMA) particles. The obtained particles have a core−shell morphology: PMMA core and P([MTMA][TFSA]) shell. To change the composite particle morphology from core−shell to Janus, we used the solvent-absorbing/releasing method with methyl isobutyl ketone, which is a suitable solvent for PMMA and P([MTMA][TFSA]) with nonionic surfactants (polyoxyethylene nonylphenyl ether, Emulgen 950) and Li[TFSA]. Based on the ultrathin cross-section observations, we found the core−shell PMMA/P([MTMA][TFSA]) composite particle morphology changed to a Janus structure. Moreover, anion exchange occurred in the obtained Janus particles. When using LiBr, we obtained PMMA hemisphere particles because of the changes in the PIL hemisphere polarity via anion exchange and dissolution in water. On the contrary, the use of poly(sodium styrenesulfonate) maintained the Janus structure because of ionic cross-linking and the changes in the PIL hemisphere to hydrophilic properties, which became swollen with medium water.

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INTRODUCTION Particles having two components with different chemical compositions are called “Janus particles”. Such particles have attracted much attention due to their anisotropic characteristics such as wettability and optical and electronic properties. Such particles are applicable to particulate surfactants for Pickering emulsion and electronic paper display materials.1−6 Studies have reported various preparation methods for Janus particles, such as Pickering emulsion,7−9 toposelective surface modification,10−12 macrophase separation induced by seeded polymerization,13−15 and microfluidic method.16−18 Okubo et al.19−21 proposed the solvent-absorbing/releasing method (SARM) as a means of controlling the thermodynamically stable morphology of composite polymer particles. In the presence of a surfactant, SARM tends to yield Janus particles consisting of polymers A/B owing to the decreasing difference in the interfacial tensions between polymer A/surfactant aq. and polymer B/surfactant aq., in which the surface free energy of polymer A/surfactant aq. is not significantly different from that of polymer B/surfactant aq., and the contact area of polymer A/polymer B becomes flat to minimize the interfacial tension between polymers. In our previous study, we prepared Janus particles comprising two hemispheres (polystyrene and poly(methyl methacrylate) (PMMA)) using SARM with sodium dodecyl sulfate.22 However, if either polymer has high hydrophilicity, problems may arise when attempting to © XXXX American Chemical Society

decrease the interfacial tension between the hydrophobic polymer/water to an identical level as that between a hydrophilic polymer/water even if a surfactant is present in the SARM process. Therefore, the preparation of Janus particles composed of two polymers types, which are characterized by significantly different polarities (i.e., hydrophilic and hydrophobic), using SARM remains a challenge. Ionic liquids, which are composed of organic ions and liquids at room temperature, have received much attention as new types of media due to their nonvolatility and nonflamability.23−25 Ionic liquids have also drawn attention from the viewpoint of functional materials utilizing their properties such as high CO2 solubility and ionic conductivity.26−28 Numerous studies have focused on the preparation of solidstate ionic liquids, such as ion gels29−31 and poly(ionic liquids) (PILs),32−38 which are characterized by mechanical stability and workability while maintaining the unique characteristics of ionic liquids. PILs are prepared by ionic liquid monomer polymerization with vinyl groups. Moreover, previous studies reported PIL particle preparation via suspension and Special Issue: Mohamed El-Aasser Festschrift Received: Revised: Accepted: Published: A

April 4, 2019 June 8, 2019 June 20, 2019 June 21, 2019 DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research dispersion polymerizations.39−42 In our group, we prepared PIL particles via the dispersion and emulsion polymerization of an ionic liquid monomer (e.g., [2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide ([MTMA][TFSA])).43−48 The obtained PIL particles were able to modify their polarity from hydrophobic to hydrophilic by using the exchange of an anion of P([MTMA][TFSA]). In this paper, we demonstrate the preparation of Janus particles consisting of two polymer phases with significantly different polarities (PMMA and hydrophilic PIL) via PIL ion exchange. Our strategy is as follows: (i) Composite core−shell particles composed of PMMA core and hydrophobic PILs shell were prepared via seeded dispersion polymerization of a hydrophobic ionic liquid monomer in the presence of PMMA particles. (ii) Janus particles were obtained via SARM by using the previously obtained core−shell particles. (iii) We then performed PIL hemisphere anion exchange in the Janus particles.

emulsified by an ultrasonic homogenizer (US-600T, NISSEI CORPORATION, 12 mm-diameter tip) for 10 min. The obtained emulsion was poured into the PMMA seed particle dispersion (6. 2 g, solid content was 3.3 wt %) in a 30 mL glass vial, which was shaken for 5 h at 70 cycles/min (3 cm strokes) for the absorption of [MTMA][TFSA] and AIBN. Thereafter, the dispersion of PMMA seed particles swollen with [MTMA][TFSA] was placed in a glass tube, and Li[TFSA] aq. (Li[TFSA] [0.64 g]/water [1.0 g]) was added. The reaction system was degassed using several vacuum/N2 gas exchanges. After sealing the glass tube, polymerization was performed at 60 °C for 24 h. Preparation of PMMA/P([MTMA][TFSA]) Janus Particles. The PMMA/P([MTMA][TFSA]) Janus particles were prepared using SARM. The procedure was as follows: MIBK (0.9 g), which is a suitable solvent for PMMA and P([MTMA][TFSA]), and Emulgen 950 (0.495 g) were added to the composite particle dispersion (0.33 wt % solid in water [15 g]) in a 50 mL glass vial, and the mixture was stirred with a magnetic stirrer for several hours in order to absorb MIBK. The adsorbing MIBK was slowly evaporated from the swollen composite particles at room temperature for 24 h in an uncovered glass vial with stirring by a magnetic stirrer. After releasing MIBK, the remaining Emulgen 950 in the obtained dispersion was removed by centrifugal washing five times with water. The residual amount of MIBK in the swollen composite particles after 24 h was measured by gas chromatography. Anion Exchange of PMMA/P([MTMA][TFSA]) Janus Particles. The water/methanol dispersion of PMMA/P([MTMA][TFSA]) Janus particles (Janus particles [0.01 g] and water/methanol [1 g/1 g]) was added to an electrolyte solution (LiBr [0.4 g] or PNaSS [0.451 g] and water/methanol [9 g/9 g]). Thereafter, the mixture was stirred at room temperature for 12 h to complete the anion exchange from [TFSA]. Characterization. The obtained PMMA/P([MTMA][TFSA]) composite and Janus particles were observed with an optical microscope (ECLIPSE 80i, Nikon). The obtained PMMA/P([MTMA][TFSA]) composite and Janus particles were also observed with a scanning electron microscope (SEM, JSM-6510, JEOL, Tokyo, Japan) at 20 kV after platinum coating. The number-average diameter (Dn) and the coefficient of variation (Cv) were estimated from more than 200 particles on the SEM images using image-analysis (WinROOF, Mitani Co., Ltd., Japan). The observation of transmission electron microscopy (TEM; JEM-1230, JEOL, Tokyo, Japan) was conducted at 100 kV. The ultrathin cross sections (approximately 100 nm thick) were prepared as follows: dried particles were embedded in an epoxy resin and cured at room temperature for 12 h. The cured block was microtomed using EM-UC6 (Leica) and stained with a 3 wt % aqueous phosphotungstic acid solution at room temperature for 30 min. The ultrathin cross sections of the particles were observed with TEM. The Janus particle surfaces polarity was estimated by using a confocal laser scanning microscope (C2si, Nikon Corp., Tokyo, Japan) in the presence of fluorescent dye with He−Ne laser excitation at 543 nm and an emission bandpass filter at 552−617 nm.

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EXPERIMENTAL METHODS Materials. Methyl methacrylate (Nakalai Tesqu, Inc., Kyoto, Japan) and 2,2′-azobis(isobutyronitrile) (AIBN; Wako Pure Chemical Industries, Ltd., Osaka, Japan) were purified by vacuum distillation in N2 and recrystallization in methanol, respectively. Poly(sodium styrenesulfonate) (PNaSS) was prepared by the solution polymerization of sodium styrenesulfonate (NaSS). Poly(vinylpyrrolidone) (PVP; K-30, weight-average molecular weight: 4.0 × 104 g/ mol), commercial grade poly(oxyethylene)nonylphenylether (Emulgen 950, average ethylene oxide unit is 50.6, Kao Co., Japan), [2-(methacryloyloxy)ethyl]trimethylammonium chloride ([MTMA]-Cl) solution (80 wt % aqueous solution, Aldrich), Nile red (Tokyo Chemical Industry Co., Ltd.,), methylisobutylketone (MIBK; Nakalai Tesque Inc., Kyoto, Japan), and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) (99.7%, Kanto Chemical Co., Inc.) were used without further purification. The water was purified with an ErixUV (Millipore, Japan); the resistivity of purified water was 18.2 MΩ cm. The ionic liquid monomer (Figure 1) was

Figure 1. Chemical structure of [MTMA][TFSA].

synthesized by just mixing the aqueous solutions of [MTMA]Cl and Li[TFSA]. After mixing, phase separation occurred, and the lower oil phase (ionic liquid monomer, [MTMA][TFSA]) was washed with water. Preparation of PMMA Seed Particles. PMMA particles were synthesized by the dispersion polymerization of MMA; 1.0 g of MMA, 0.2 g of PVP, and 0.1 g of AIBN were dissolved in a methanol/water medium (7/3 w/w, 10 g). The homogeneous solution was degassed by several vacuum/N2 gas exchanges in a round-bottomed flask sealed with a silicone rubber cap. The polymerization was carried out at 60 °C for 5 h with stirring by a magnetic stirrer (200 rpm). Preparation of PMMA/P([MTMA][TFSA]) Composite Particles. The oil phase of [MTMA][TFSA] (0.4 g) containing dissolved AIBN (4.0 mg) was mixed with an Emulgen 950 aqueous solution (0.35 wt %, 4.0 g) and

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RESULTS AND DISCUSSION Preparation of PMMA/P([MTMA][TFSA]) Janus Particles by SARM. Figure 2(a) and (b) shows the SEM images B

DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. SEM photographs of PMMA seed particles (a) and obtained particles (b) prepared by the seeded polymerization of [MTMA][TFSA]. TEM photograph (c) of the ultrathin cross section of the obtained particles stained with a 3 wt % phosphotungstic acid.

Scheme 1. Thermodynamically Stable Morphologies of Composite Particles Predicted Using Spreading Coefficients

SPMMA < 0, SPIL < 0, Swater > 0

of the PMMA seed particles and obtained particles prepared by seeded polymerization. The PMMA seed particles were highly monodisperse. After seeded polymerization, we did not observe second nucleated particles, and the diameter of the obtained particles (2.8 μm) by the seeded polymerization increased compared to that of the PMMA seed particles (2.1 μm) while maintaining the monodispersity of the obtained particles (Cv: PMMA seed, 4.96%; obtained particle, 4.33%). These results indicate that seeded polymerization proceeded successfully, and the diameters of the obtained particles were consistent with the calculated value (∼2.9 μm) based on the recipe. Moreover, to check the internal morphology of the composite particles, we prepared ultrathin cross sections of the obtained particles stained with phosphotungstic acid. The PIL phase was stained with phosphotungstic acid, thus resulting in a darker color for the PIL phase than for the PMMA phase. On the basis of TEM observations of ultrathin cross sections (Figure 2c), we observed that the obtained particles exhibit a core−shell structure that consists of PMMA cores and PIL shells. The cross sections were observed to be ellipsoidal shape due to the compressive stress of the cutting process. To discuss the morphology of PMMA/PIL composite particles from a thermodynamic perspective, we predicted the morphology by using the spreading coefficient (S, mN/m). A previous study reported that the spreading coefficient is useful for understanding the thermodynamically stable morphology of composite particles.49 The spreading coefficient is defined as Si = γjk − (γij + γik), where γjk, γij, and γik are the interfacial tensions between each component. The number of Si patterns is described by the following expressions: SPMMA < 0, SPIL > 0, Swater < 0

(1)

SPMMA < 0, SPIL < 0, Swater < 0

(2)

(3)

When the relationship of Si satisfies eqs 1, 2, and 3, the thermodynamically stable morphologies are “core−shell morphology,” “particle engulfing,” and “individual morphology,” respectively49 (Scheme 1). The value of Si for each component was calculated on the basis of the interfacial tensions between the polymers and medium. Each interfacial tension was calculated with the Fowkes (eq 4) and Wu equations (eq 5) γij = γi + γj − 2( γidγjd +

γi pγjp )

(4)

ij 4γ dγ d 4γi pγjp yzz j i j zz + γij = γi + γj − jjjj d z j γi + γjd γi pγjp zz k {

(5)

The dispersive and polar components of surface tension (γd and γp, mN/m, respectively) for PMMA and PIL were calculated with the contact angle measurement and the Young−Owens equation. Table 1 summarizes the calculated surface tension values for each component. On the basis of this surface tension data, the calculated interfacial tensions are γPMMA/water = 20.7, γPIL/water = 15.3, and γPMMA/PIL = 3.5. Table 1. Dispersive (γd), Polar (γp), and Total (γ) Components of Surface Tension (mN m−1) at Room Temperature

PMMA P([MTMA][TFSA]) Water C

γd

γp

γ

40.5 25.6 21.8

8.5 10.6 51.0

48.9 36.2 72.8

DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Scheme 2. Anion Exchange from [TFSA] to HCO3− on PIL Surface

Thereafter, S values were calculated for each system by using these interfacial tension values. The obtained spreading coefficient values for this system are SPMMA = −9.0 < 0, SPIL = 1.94 > 0, and Swater = −32.4 < 0. These relationships suggest that the thermodynamically stable morphology is a core−shell morphology, thus indicating that the obtained PMMA/P([MTMA][TFSA]) composite particles had a thermodynamically stable morphology. To change the morphology of the composite particles from core−shell to Janus, we require a SPIL value below zero. The SPIL value is expressed as SPIL = γPMMA/water − (γPIL/water+ γPMMA/PIL). To decrease the γPMMA/water, a nonionic surfactant (Emulgen 950) was added during the SARM process (i.e., ionic surfactants induce anion exchange between PIL and ionic surfactants44). MIBK was used as a suitable solvent for both polymers, which was added to the core−shell particle dispersion and was stirred with a magnetic stirrer for several hours so that it can be absorbed into the composite particles. Adsorbing MIBK was then slowly evaporated at room temperature. After 24 h, the residual amount of MIBK was less than 1%. Figure 3 shows a TEM image of phosphotungstic acidstained ultrathin cross sections of the composite particles. We

Figure 4(a) and (b) show the SEM and TEM images of the phosphotungstic acid-stained ultrathin cross sections for

Figure 4. SEM (a) and TEM (b) photographs of the ultrathin cross sections stained with phosphotungstic acid showing the PMMA/ P([MTMA][TFSA]) composite particles after SARM in the presence of Li[TFSA]. Figure 3. TEM photographs of ultrathin cross sections (stained with 3 wt % aqueous phosphotungstic acid solution for 30 min) showing the PMMA/P([MTMA][TFSA]) composite particles after SARM.

PMMA/P([MTMA][TFSA] composite particles after the SARM process in the presence of Li[TFSA]. After the SARM process, the particle shape was still spherical, but the composite particle morphology changed from core−shell to a different structure, thus indicating that the PMMA/P([MTMA][TFSA]) Janus particles were successfully prepared via SARM in the presence of Li[TFSA]. Polarity Change in PMMA/P([MTMA][TFSA]) Janus Particles Using Anion Exchange. In our previous study, we reported that PIL particle solubility can be easily modified by anion exchange.43 Therefore, we performed the anion exchange of the Janus particle P([MTMA][TFSA]) phase. Figure 5(a) and (b) show an optical micrograph and SEM image of the obtained particles after anion exchange with LiBr. In the optical micrograph and SEM image, we observed hemispherical particles. This result indicates that PIL hemispherical phase should dissolve in water; i.e., the solubility of the PIL phase changed to hydrophilic after anion exchange. To maintain the Janus structure after anion exchange, we performed the anion exchange process by using PNaSS, which should maintain the Janus structure via ionic crosslinking with PIL. Figure 6(a) and (b) shows the optical micrographs of the obtained particles after anion exchange. Even after anion exchange, the composite particles maintained a Janus structure even though the PIL phase was swollen, in which PNaSS worked as counteranions for plural PIL molecules resulting in formation of a cross-linking structure.

observed that the composite particle morphology slightly changed. However, the particle surfaces still consisted of P([MTMA][TFSA]). However, we still observed a similar morphology for the additional amounts of surfactant in the SARM systems, thus suggesting that decreasing the interfacial tension between PMMA and medium was insufficient. In a previous study, we reported that hydrogen carbonate (HCO3−) ions derived from carbon dioxide in water exchange with the [TFSA] anions on the P([MTMA][TFSA]) particle surface (Scheme 2).44 Before the anion exchange, the surface of the PIL particles should be relatively hydrophobic (zeta potential of the PIL particles should be almost zero) due to the formation of ion pairs between the poly[MTMA] cation and hydrophobic [TFSA] anion. However, this anion exchange in the [TFSA] anion with HCO3− on the PMMA/P([MTMA][TFSA]) surface enhanced P([MTMA][TFSA]) hydrophilicity due to the dissociation of the hydrogen carbonate ions from the PIL surface, thus inhibiting a decrease in the difference in interfacial tension between the PMMA/medium and P([MTMA][TFSA])/medium. To prevent anion exchange, Li[TFSA] was added to the SARM systems. An excess amount of the [TFSA] anion in the medium would suppress the anion exchange of PIL with HCO3−. D

DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Optical micrograph (a) and SEM image (b) of the obtained particles after PMMA/P([MTMA][TFSA]) Janus particle anion exchange using LiBr.

Figure 7. FT-IR spectra of PMMA/PIL Janus particles before and after anion exchange using PNaSS.

On the contrary, we observed hemisphere particles after anion exchange using NaSS (monomer) with identical conditions (Figure 6c). This result indicates that PNaSS acts as a polyanion and a cross-linker and maintained the Janus structure. The FT-IR spectra confirmed ionic exchange. The FT-IR spectra (Figure 7) show the disappearance of the band that corresponds to the [TFSA] anion (SO, 1355 cm−1), thus indicating the completion of anion exchange from [TFSA] to Br− anions. Finally, to evaluate the changes in polarity for the PIL phase from hydrophobic to hydrophilic, Nile red (hydrophobic fluorescent dye) was added to the dispersion of PMMA/PIL Janus particles before and after anion exchange and was observed with confocal laser scanning microscopy (CLSM) (Figure 8). For Janus particles before anion exchange, Nile red became completely covered on the Janus particles, thus indicating that the entire surface of the Janus particle should be hydrophobic. On the contrary, for Janus particles after anion exchange, Nile red was adsorbed on only one side of the PMMA surface. These results indicate that PIL changed from hydrophobic to hydrophilic during anion exchange.

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Figure 8. Optical micrographs (a, b) and CLSM images (a′, b′) of PMMA/PIL Janus particles before (a, a′) and after (b, b′) anion exchange using PNaSS dispersed in a Nile red solution.

CONCLUSIONS We successfully prepared PMMA/P([MTMA][TFSA]) composite particles with a Janus structure using SARM in the presence of Li[TFSA]. PIL polarity changed from hydrophobic to hydrophilic during anion exchange by using PNaSS, and the obtained particles maintained a Janus structure via ionic crosslinking between PIL and PNaSS, which were Janus particles composed of polymers with large differences in polarity. This Janus particle preparation strategy using PIL properties should provide a new perspective on particle design.

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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.iecr.9b01856. Visual images of contact angles on PMMA and PIL films, measured with water and methylene iodide for

Figure 6. Optical micrographs of PMMA/P([MTMA][TFSA]) Janus particles after anion exchange with PNaSS (a, b) and NaSS (c). E

DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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PMMA and hexadecane and methylene iodide for PIL. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: (+81) 78 803 6197). ORCID

Hideto Minami: 0000-0001-6173-6597 Funding

This work was partially supported by JSPS KAKENHI (Grant No. 17H03116). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.9b01856 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX