Poly(ionic liquid) Composite Particles by

Aug 12, 2013 - ... Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan ... PS seed particles, secondary nucleated poly(ionic liquid)...
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Preparation of Polymer/Poly(ionic liquid) Composite Particles by Seeded Dispersion Polymerization Masayoshi Tokuda, Tatsunori Shindo, and Hideto Minami* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan ABSTRACT: Seeded dispersion polymerization of the ionicliquid monomer ([2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide) ([MTMA][TFSA]) was performed in ethanol by using either polystyrene (PS) or poly(methyl methacrylate) (PMMA) particles as seeds. In the presence of PS seed particles, secondary nucleated poly(ionic liquid) (PIL) particles were formed, and no PS/PIL composite particles were observed. In the case of PMMA seeds particles, the diameters of the obtained particles increased compared to those of PMMA seed particles (without formation of particles that were formed as byproducts), which indicates that the PMMA/PIL composite particles were successfully prepared. Transmission electron microscopy studies of ultrathin cross sections of the PMMA/ PIL particles revealed that the obtained particles had a sea-island structure consisting of PIL domains. These results are consistent with the theoretical considerations based on the spreading coefficients calculated from the interfacial tensions.

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INTRODUCTION Polymer/polymer composite particles prepared by multistep seeded polymerization (e.g., seeded emulsion polymerization1−4 and seeded dispersion polymerization5−8) have been used in various applications such as coatings and impact modifiers. The functionality of these particles strongly depends on particle morphology, which is controlled by a combination of kinetic and thermodynamic factors. Various morphologies, such as core−shell9−11 and polymeric oil-in-oil12,13 structures, have been reported during the preparation of composite polymer particles by seeded polymerization.14−17 Ionic liquids, which are room-temperature molten salts entirely composed of organic ions, have attracted great attention as environmentally friendly media because of their low vapor pressures, high thermal stability, and nonflammability. Ionic liquids also have other functional characteristics such as good ionic conductivity and high CO2 solubility, which make them interesting candidates for the development of functional materials. Poly(ionic liquid)s (PILs) combine the mechanical stability and processability of polymeric materials with the unique properties of ionic liquids.18−20 PILs maintain the above properties of ionic liquids as well as can be tuned the solubility and the properties by changing the anion of PIL in the same way of ionic liquids.21,22 They have attracted attention functional materials such as CO2 sorbents, polymer electrolytes, and microwave absorbing materials.23−25 PILs have been prepared by direct polymerization of ionic-liquid monomers containing vinyl groups on the cationic or anionic parts.26−33 Recently, PIL particles were synthesized by suspension polymerization and precipitation polymerization.34−37 In a previous study, we reported the preparation of micrometer© 2013 American Chemical Society

sized monodisperse PIL particles by dispersion polymerization and confirmed that the obtained particles maintained the properties of the ionic liquid.38,39 However, to the best of our knowledge, no reports are available on the preparation of polymer/PIL composite particles. These composite particles could have the advantage of low cost and easy processability as compared to PIL homopolymer particles for application. Moreover, in the case of the core−shell composite particle consisting of commodity polymer core and PIL shell, the films and pellets prepared from the composite particles should be promising for the design of ionic conductive materials, in which the ionic conductive PIL shell forms effective ion transport paths. In this Article, we obtained commodity polymer/PIL composite particles by seeded dispersion polymerization of a quaternary ammonium-based ionic-liquid monomer with seed particles of a commodity polymer. The morphology of the obtained particles was discussed from the viewpoint of spreading coefficients.

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EXPERIMENTAL SECTION

Materials. Styrene (S) and methyl methacrylate (MMA) were purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade 2,2′-azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. 2,2′-Azobis(4-methoxy-2,4dimethylvaleronitrile) (V-70, Wako Pure Chemical Industries, Ltd., Osaka, Japan), poly(vinylpyrrolidone) (PVP, K-30, weight-average Received: July 2, 2013 Revised: August 9, 2013 Published: August 12, 2013 11284

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molecular weight: 3.6 × 105 g/mol), methanol and ethanol (Nakalai Tesque Inc., Kyoto, Japan), lithium bromide (LiBr) (99%, Aldrich), [2-(methacryloyloxy)ethyl]trimethylammonium chloride ([MTMA]Cl) solution (80 wt % in water, Aldrich), and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) (99.7%, Kanto Chemical Co., Inc.) were used as received. The water used in all the experiments was obtained from an ErixUV (Millipore, Japan) purification system and had a resistivity of 18.2 MΩ cm. The ionic liquid monomer (Figure 1) was prepared by mixing the aqueous solutions of [MTMA]Cl and Li[TFSA].38

The ionic liquid [MTMA][TFSA] (0.05 g) and the initiator V-70 (0.5 mg) were dissolved in ethanol (0.5 g) in a semibatch seeded dispersion polymerization system. This solution was then added to the dispersion [PMMA seed particles (0.2 g) and ethanol (2.0 g)] every 2 h (0.05 g × 4). The conversions of ionic liquid monomer were determined with 1H NMR as reported in our previous paper.38 The 1H NMR measurements were carried out with a Bruker Avance 500 MHz spectrometer at room temperature in CD3COCD3. In all polymerization systems, the conversions of ionic liquid monomer were higher than 99%. Contact Angle Measurements of Polymer Films. To calculate the interfacial tensions, the contact angles of water and CH2I2 were measured with a DropMaster 300 (Kyowa Interface Science, Saitama, Japan) instrument at room temperature. To prepare the PS and PMMA films, homogeneous toluene solutions of PS or PMMA were cast onto a glass substrate and then dried at room temperature for 24 h. The PIL film was similarly prepared using an acetone solution. Characterization. Scanning electron microscopy (SEM, JSM6510, JEOL, Tokyo, Japan) studies of the particles coated with platinum were performed at 20 kV. The number-average diameter (Dn) and coefficient of variation (Cv) were determined for 200 particles from the SEM images by using an image-analysis software (WinROOF, Mitani Co., Ltd., Japan). Transmission electron microscopy (TEM, JEM-1230, JEOL, Tokyo, Japan) characterization was performed at 100 kV. To observe the interior morphology of the particles, dry samples were embedded in an epoxy matrix, cured at room temperature overnight, and subsequently microtomed. The ultrathin cross sections (approximately 100 nm thick) were stained by floating on a 3 wt % aqueous phosphotungstic acid solution for 30 min at room temperature, and then observed with the transmission electron microscope. Phosphotungstic acid stains PIL but not PS and PMMA. The formation of by-produced particles was monitored by dynamic light scattering (DLS; FPAR-1000 RK, fiber-optics particle analyzer, Photal Otsuka Electronics, Osaka, Japan) at a light-scattering angle of 90° at room temperature using the Contin analysis routine. One to two droplets of the emulsion samples from a reactor were diluted in distilled water (approximately 8 mL) before performing measurements in the dilution mode.

Figure 1. Chemical structure of [MTMA][TFSA]. Preparation of Polystyrene (PS) and Poly(methyl methacrylate)(PMMA) Seed Particles. Monodispersed PS and PMMA seed particles were prepared by dispersion polymerization as follows: Starting solutions of PS: S (1.0 g), PVP (0.2 g), and AIBN (0.01 g) were dissolved in ethanol (6.0 g); and PMMA: MMA (1.2 g), PVP (0.12 g), and AIBN (0.012 g) were dissolved in methanol/water (7/3 w/w, 10.8 g). The mixtures were then poured into glass tubes and degassed using several vacuum/N2 cycles, and then sealed glass tubes were placed in a water bath at 60 °C for 24 and 5 h, respectively, with agitation at 80 cycles min−1 (3 cm strokes). Seeded Dispersion Polymerization of the Ionic-Liquid Monomer. Seeded dispersion polymerizations of [MTMA][TFSA] in the presence of PS and PMMA seed particles were performed under the conditions listed in Table 1 at temperatures of 60 and 30 °C, respectively, because PMMA is soluble in hot ethanol.

Table 1. Recipes for the Preparation of PIL Composite Particles by Seeded Dispersion Polymerizations of [MTMA][TFSA] in Ethanol with PS Seed Particles (1−3) and PMMA Seed Particles (4)a ingredients

1

2

3

PS seed particles (g)b PMMA seed particles (g)c [MTMA][TFSA] (g) AIBN (mg) V-70 (mg) ethanol (g) T (°C)

0.25

0.25

0.25

0.25 2.5

0.25 2.5

0.25 1.25

2.5 60

2.5 50

2.5 60

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RESULTS AND DISCUSSION Seeded Dispersion Polymerization of [MTMA][TFSA] Using PS Seed Particles. Figure 2a and b shows SEM images of polymer particles before and after the seeded dispersion polymerization of [MTMA][TFSA] by using PS seed particles (no. 1 in Table 1). The PS seed particles exhibited a high monodispersity (Dn, 1.7 μm; Cv, 2.3%), whereas the particles obtained after the polymerization process had a broad size distribution with many monodispersed particles having the same size as the starting PS seeds. After washing the obtained particles with toluene, which dissolves PS but not poly([MTMA][TFSA]), the particles exhibiting the same sizes as the PS seed particles disappeared, whereas those showing a broad size distribution remained (Figure 2c). These results

4 0.25 0.25 2.5 2.5 30

a

In a sealed glass tube; N2; 10 h; 80 cycles/min. bPrepared by dispersion polymerization; Dn = 1.7 μm; Cv = 2.3%. cPrepared by dispersion polymerization; Dn = 2.8 μm; Cv = 3.4%.

Figure 2. SEM images of PS seed particles (a) and particles prepared by seeded dispersion polymerization of [MTMA][TFSA] before (b) and after (c) extraction of PS with toluene. 11285

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values for Si because the seed particles cannot spread on the other components:

indicate that no composite particles were obtained, and secondary nucleation of the PIL occurred instead, which happens when polymerization of [MTMA][TFSA] takes place at a faster rate than the formation of the composite particles. To decrease the polymerization rate of [MTMA][TFSA], both polymerization temperature and the concentration of the initiator were reduced (Table 1, no. 2 and 3), but similar results were still observed in both cases with a mixture of PS seed particles and particles exhibiting a broad size distribution (Figure 3).

SPS < 0,

SPIL > 0,

Sethanol < 0

(2)

SPS < 0,

SPIL < 0,

Sethanol < 0

(3)

SPS < 0,

SPIL < 0,

Sethanol > 0

(4)

These three sets of relations correspond to the three different thermodynamically stable morphologies illustrated in Figure 5.

Figure 3. SEM images of particles prepared by seeded dispersion polymerization of [MTMA][TFSA] by using PS seed particles. Polymerization conditions: (a) 50 °C, AIBN (2.5 mg); (b) 60 °C, AIBN (1.3 mg).

Seeded dispersion polymerization of styrene by using PIL seed particles (2.0 μm) was also performed. Figure 4 shows the

Figure 5. Possible thermodynamically stable morphologies predicted by using the spreading coefficients.

The γ value for each polymer was determined by contactangle measurements and using the Young−Owens equation (eq 5): (1 + cos θ )γL = 2( γSdγLd +

(5)

where θ is the contact angle of a liquid droplet on a solid (polymer) surface, and γd and γp are the dispersive and polar components of the surface tension, respectively. The γd and γp values for each polymer were determined by contact-angle measurements using water (γd = 21.8 mN/m, γp = 51 mN/m)41 and CH2I2 (γd = 49.5 mN/m, γp = 1.3 mN/m).41 The contact angles of water and CH2I2 on the PIL film were 60.2° and 47.6°, respectively, and the calculated γdPIL and γpPIL values were 28.9 and 17.0 mN/m, respectively. The γdPS and γpPS values were calculated by the same method. The surface-tension values for each component are summarized in Table 2.

Figure 4. SEM images of PIL seed particles (a) and particles prepared by seeded dispersion polymerization of styrene (b).

SEM images of PIL seed particles and particles prepared by seeded dispersion polymerization. After the polymerization process, raspberry-like particles were observed (Figure 4b), indicating that composite particles were likely to be obtained. However, the obtained dispersion contained a large amount of 200-nm-sized particles according to dynamic light scattering measurements. This result indicates that a secondary nucleation of PS occurred, and that raspberry-like particles should be obtained during the drying process. These results suggest that composite particles consisting of PS and PIL could not be obtained, and that the formation of such structures is not influenced by kinetic factors but rather by thermodynamic factors. To discuss the influence of the thermodynamic factors, the spreading coefficient (S, mN/m), which is useful to predict the particle morphology40 and is given by eq 1, was estimated. Si = γjk − (γij + γik)

γSpγLp )

Table 2. Dispersive (γd), Polar (γp), and Total (γ) Components of the Surface Tension (mN/m) at 298 K poly([MTMA][TFSA]) PS ethanola a

γd

γp

γ

28.9 43.4 18.8

17.0 2.5 2.6

45.9 45.9 21.4

Reference 42.

The interfacial tension between the polymer and ethanol was calculated by the Fowkes equation (eq 6), and that between PS and poly([MTMA][TFSA]) was calculated by the Wu equation (eq 7):

(1)

where γ is the interfacial tension (mN/m) among three components (ij, jk, ik) and the suffixes (i, j, k) refer to PS, PIL, and the medium (ethanol), respectively. In the case of seeded dispersion polymerization, there are three possible sets of

γij = γi + γj − 2( γidγjd + 11286

γi pγi p )

(6)

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Figure 6. SEM images of PMMA seed particles (a) and particles prepared by seeded dispersion polymerization of [MTMA][TFSA] (b). TEM image of ultrathin cross sections of the obtained particles stained for 30 min with 3 wt % aqueous phosphotungstic acid solution (c).

Figure 7. SEM images of PIL seed particles (a) and particles prepared by seeded dispersion polymerization of MMA (b). TEM image of ultrathin cross sections of the obtained particles stained for 30 min with 3 wt % aqueous phosphotungstic acid solution (c).

⎛ 4γ dγ d 4γi pγjp ⎞ i j ⎟ + γij = γi + γj − ⎜⎜ d p p⎟ d γ + γ γ + γ i j ⎠ ⎝ i j

the PMMA system were as follows: SPMMA = −3.1 < 0, SPIL = −6.3 < 0, and Sethanol = −8.4 < 0. These values suggest that composite polymer particles can be prepared without secondary nucleation of the PIL. Figure 6a and b shows the SEM images of PMMA seed particles and particles prepared by seeded dispersion polymerization (no. 4 in Table 1). After polymerization, the diameter of the obtained particles (3.4 μm) was larger than that of the PMMA seed particles (2.8 μm) and the monodispersity was maintained. This result indicates that the polymerization smoothly proceeded. The morphological characteristics of composite particles have been estimated with a variety of analytical techniques such as differential scanning calorimetry45 and surfactant titration.46−48 TEM observation of microtomed sections of composite particles is the most useful technique for the observation of direct information about the internal structure.49 Figure 6c shows a TEM image of the ultrathin cross sections of the obtained particles stained with phosphotungstic acid. The stained PIL phase appears darker than the PMMA phase, and it can be seen that the obtained particles exhibit a sea-island structure, in which many small PIL domains dispersed in the PMMA matrix. These results indicate that PMMA/PIL composite particles were successfully prepared. The PMMA seed particles are swollen by an amount of [MTMA][TFSA], and PIL domains are trapped as the viscosity of the swollen particles increases during polymerization. This results in the formation of a thermodynamically unstable sea-island structure. Contrarily, seeded dispersion polymerization of MMA by using PIL seed particles was performed. After the polymerization, the diameter of the obtained particles (Figure 7b) was larger than that of the PIL seed particles (Figure 7a) with maintaining monodispersity. TEM image of phosphotungsticacid-stained ultrathin cross sections of PIL/PMMA particles indicates that dark PIL cores surrounded by light PMMA shells were observed although the shell was likely to be imperfect (Figure 7c).

(7)

It has been reported that Wu equation precisely predicts the interfacial tension in the case of solid/solid interface.43 The calculated spreading coefficients for the studied system were SPS = −11.4 < 0, SPIL = −16.0 < 0, and Sethanol = 1.2 > 0. These relations indicate that a thermodynamically stable morphology should involve the formation of individual particles in a seeded dispersion polymerization system containing PS seed particles in ethanol, which is consistent with the experimental results (Figures 2−4). At low conversion, the medium is not pure ethanol but a mixture of ethanol and [MTMA][TFSA] (10/1, w/w), in which the values of spreading coefficient would be affected. The surface tension of the ethanol/[MTMA][TFSA] mixture (the weight fraction of ethanol/[MTMA][TFSA] was 10/1) was measured by pendant drop measurement. The γ value was 20.7 mN/m, which was close to the γ of pure ethanol (21.4 mN/m). This difference did not affect the magnitude relationship of the spreading coefficient. Recently, Lacroix-Desmazes et al. reported thermodynamic prediction of particle morphology based on the graphical representation.44 The morphology in our system calculated from Lacroix-Desmazes’s procedure also indicates the individual particle. This behavior can be explained by the higher polarity (high γp) of the PIL although the surface tension γ is the same as PS (Table 2), which results in a higher interfacial tension between PS and PIL (13.5 mN/m) as compared to that observed between commodity polymers such as PS and PMMA (3.4 mN/m).9 Seeded Dispersion Polymerization of [MTMA][TFSA] by Using PMMA Seed Particles. The seeded dispersion polymerization of [MTMA][TFSA] in the presence of PMMA particles was performed in a similar way but at a polymerization temperature of 30 °C. PMMA (γd = 40.4 mN/m, γp = 8.5 mN/m) has a higher polarity than PS, and the spreading coefficients calculated for 11287

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CONCLUSION Seeded dispersion polymerization of the ionic-liquid monomer [MTMA][TFSA] in ethanol using PS seed particles led to the secondary nucleation of the PIL particles but not to the formation of composite structures. The spreading coefficients calculated from the interfacial tensions suggest that a thermodynamically stable morphology for the obtained materials involves the formation of individual particles. In the case of PMMA seed particles, which have a higher polarity than the PS particles, composite structures exhibiting a sea-island morphology were successfully prepared. We also synthesized core−shell PMMA/PIL composite particles with a PMMA core and a PIL shell by seeded dispersion polymerization in the semibatch system. These results can be explained by the unique property of PIL, which has hydrophobicity as well as high polarity, and shows new insights of controlling morphologies of PIL composite particles from the viewpoint of the particle design.

To control the morphology of the PMMA/PIL composite particles, semibatch seeded dispersion polymerization was performed, in which [MTMA][TFSA] was fed dropwise (0.05 g × 4) to the polymerization reactor in the presence of PMMA seed particles. Figure 8a and b shows SEM images of PMMA seed particles and particles prepared by seeded dispersion polymerization in

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

Figure 8. SEM images of PMMA seed particles (a) and particles prepared by seeded dispersion polymerization of [MTMA][TFSA] in a semibatch system with (b) and without (d) platinum coating. TEM image of ultrathin cross sections of the obtained particles stained for 30 min with a 3 wt % aqueous phosphotungstic acid solution (c).

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (Grant No. 23350114) from the Japan Society for the Promotion of Science (JSPS) and by a Research Fellowship of JSPS for Young Scientists (given to M.T.).

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the semibatch system. The diameter of the obtained particles (3.3 μm) after seeded dispersion polymerization in the semibatch system was larger than that of the original PMMA seed particles (2.8 μm), and the monodispersity was maintained, which indicated that PMMA/PIL composite particles were obtained. Figure 8c shows a TEM image of phosphotungstic-acidstained ultrathin cross sections of PMMA/PIL particles after seeded dispersion polymerization in the semibatch system. Light PMMA cores surrounded by dark PIL shells are observed. In the case of the semibatch system, the viscosity inside the PMMA seed particles is very high at 30 °C, and therefore, the PIL is unable to diffuse into the particles. As a result, core−shell composite structures consisting of a PMMA core and a PIL shell can be prepared. Such a core−shell composite particles are expected to apply to a building block for the functional materials as described in the Introduction. To confirm whether the composite particles exhibit ionic conductivity (the surface of particles should be PIL), they were observed by SEM without platinum coating. Previously, we reported that the PIL particles without coating could be observed by SEM because of PIL conductivity.38 As shown in Figure 8d, the PMMA/PIL core−shell particles can be clearly observed even without coating. This result strongly suggests that the surface of the composite particles maintain the properties of ionic liquid, indicating that these composite particles have potential for various applications such as ionic conductive materials and CO2 selective permeable membrane.50 The investigation of the properties of the film prepared from the PMMA/PIL composite particles is in progress and will be reported in future publications.

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dx.doi.org/10.1021/la402486n | Langmuir 2013, 29, 11284−11289