Phase-Transfer Behavior of Cross-Linked Poly(acrylic acid) Particles

Article pubs.acs.org/Langmuir

Phase-Transfer Behavior of Cross-Linked Poly(acrylic acid) Particles Prepared by Dispersion Polymerization from Ionic Liquid to Water Hideto Minami,* Yusuke Mizuta, and Akira Kimura Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan ABSTRACT: The phase-transfer behavior of poly(acrylic acid) (PAA) particles from the hydrophobic ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide phase to the water phase in the particle state, which we reported previously, was examined in more detail. PAA particles were prepared in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Bmim][TFSA]) and the organic solvent chloroform and were extracted. The transfer of PAA particles to water in the particle state was also observed in [Bmim][TFSA] systems. In contrast, the transfer phenomenon was not observed in the chloroform system. It was clarified that water/oil interfacial tension γwo is an important parameter in the extraction of PAA in the particle state from the viewpoint of free energy. When the cationic surfactant tetradecyltrimethylammonium bromide, aqueous solution was used as the extraction medium, the PAA particles were extracted in the particle state from chloroform to water, in which γwo became as low as that of the ionic liquid. This suggests that the phase-transfer phenomenon of PAA particles in the particle state was induced by the ionic liquid’s unique property of low interfacial tension with water despite its high hydrophobic character.

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INTRODUCTION Ionic liquids (ILs) are salts but which are in a liquid state at ambient temperature. There has been increasing interest in the use of ILs as alternatives to organic solvents because of their nonvolatility and thermal stability. The physical properties of ILs have been extensively investigated.1−3 Many studies investigating applications of IL characteristics have been reported.4−32 In the field of polymer chemistry, anomalous behaviors of radical polymerization in ILs such as high polymerization rate and high molecular weight were observed in homogeneous systems due to the decrease in the termination rate due to the high viscosity of the ILs and, in some cases, the increase in the propagation rate coefficient.10,14,19,22,26,30 Various polymerizations (chemical oxidative, electropolymerization, and heterophase polycondensation) were also reported in heterogeneous systems in ILs, resulting in the formation of polymer particles.24,29,33,34 Recently, we reported the preparation of polystyrene (PS) particles by dispersion polymerization in the IL N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide ([DEME][TFSA]) for the first time.35 Furthermore, PS particles were successfully prepared by thermal polymerization in the absence of a radical initiator at 130 °C, and nylon 6 particles36 were prepared by hydrolytic polymerization of ε-caprolactam at 180 °C with a conventional reactor by using advantages of ILs such as thermal stability. We also demonstrated the preparation of poly(acrylic acid) (PAA) particles37 and PS/poly(methyl methacrylate) and PS/PAA composite polymer particles38 in ILs. © 2012 American Chemical Society

PAA particles prepared in [DEME][TFSA] were easily extracted from the [DEME][TFSA] phase to the water phase, and their particle state was maintained in the water phase, in which PAA particles had a cross-linked structure during polymerization without a cross-linker.37 A similar phenomenon has been reported by Lodge et al.,39−42 in which micelles consisting of poly(1,2-butadiene-b-ethylene oxide) block copolymers reversibly transferred between an IL and water by changing the temperature. It was clarified that the phenomenon was based on an entropy-driven process. However, the transfer phenomenon in our case seems to be due to another mechanism, because the particle size was quite different and there was no reversibility. In this study, we examined in more detail the phase-transfer behavior of PAA particles prepared by dispersion polymerization in ILs.

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EXPERIMENTAL SECTION

Materials. Acrylic acid (AA, Nacalai Tesque, Inc., Kyoto, Japan) was purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade 2.2-azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. Reagent grades of tetradecyltrimethylammonium bromide (TTAB, Tokyo Chemical Industry, Co., Ltd., Tokyo, Japan), chloroform (Nacalai Tesque, Inc., Kyoto, Japan), and poly(vinyl alcohol) (PVA; saponification degree, 35.4%; weight-average molecular weight, 1.9 × 105; The Nippon Synthetic Chemical Industry Co., Ltd., Osaka, Japan) were used as received. Ionic liquids [DEME][TFSA] (provided by Nisshinbo Received: December 5, 2011 Revised: January 5, 2012 Published: January 11, 2012 2523

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Figure 1. Visual appearances of polymerization mixture prepared by dispersion polymerization of AA in [DEME][TFSA] after the addition of water (no mixing; room temperature (rt); lower phase, [DEME][TFSA]; upper phase, water). Extraction time (hours): (a) 0; (b) 2; (c) 4; (d) 7; (e) 9; (f) 24.

Figure 2. Optical micrographs of polymerization mixture prepared by dispersion polymerization of AA in [DEME][TFSA] before (a) and after the addition of water (no mixing; rt; lower side, [DEME][TFSA]; upper side, water or air) (b−f). Extraction time (minutes): (b) 0; (c) 5; (d) 20; (e) 25; (f) 35. Arrows indicate the interface between water (or air) and [DEME][TFSA]. Holdings Inc., Tokyo, Japan) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Bmim][TFSA], Nippon Synthetic Chemical) were used as received.

izations of AA in chloroform and [Bmim][TFSA] were carried out under the same conditions. Extraction of PAA Particles with Water. The same volume of water (1 mL) was added to the PAA dispersion in a glass vessel. The extraction of PAA particles was carried out for 24 h at room temperature without mixing. Characterization. PAA particles were observed by scanning electron microscopy (SEM, S-2460, Hitachi Science Systems Ltd., Ibaraki, Japan). The extraction process of PAA particles was observed with an optical microscope (ECLIPSE 80i, Nikon, Japan). Monomer conversions were determined by gas chromatography (GC-18A, Shimadzu Corp., Kyoto, Japan) by employing helium as the carrier

Dispersion Polymerization of AA. Dispersion polymerization of AA (0.25 g) in [DEME][TFSA] (2.5 g) was carried out in a 10 mL glass vessel at 70 °C for 3 h with magnetic stirring at 400 rpm in a nitrogen atmosphere. AIBN (2.5 mg) and PVA (12.5 mg) were used as the initiator and the stabilizer, respectively. Dispersion polymer2524

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Figure 3. SEM photographs of PAA particles prepared by dispersion polymerizations in [DEME][TFSA] (a), [Bmim][TFSA] (b), and chloroform (c). gas, N,N-dimethylformamide as the solvent, and p-xylene internal standard. Interfacial tensions between water and chloroform were measured by the pendant drop method DropMaster 500 (Kyowa Interface Science Co., Ltd., Saitama,

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as the ILs or with a Japan).

The obtained particles were observed by SEM as shown in Figure 3. PAA particles of 100−300 nm were successfully prepared by dispersion polymerizations in the [Bmim][TFSA] and chloroform systems. The size of these PAA particles was similar to that in the [DEME][TFSA] system (Figure 3a). We have reported that PAA particles prepared by dispersion polymerization in [DEME][TFSA] had a cross-linked structure without a cross-linker due to the formation of acid anhydride of the carboxyl group of PAA.43 Although we have speculated that the IL may have a significant influence on the formation of the cross-linked structure, in this study, PAA particles prepared in chloroform also had a cross-linked structure, in which the PAA particles prepared in chloroform were also insoluble in water. The cross-linked structure could be formed not only because of the formation of acid anhydride but also because of hydrogen bonding between the carboxyl group of PAA and the hydroxyl group of PVA as the stabilizer.44,45 After the extraction of PAA particle, PVA should remain in the IL phase or be buried in the PAA particles (thus not being available for colloidal stabilization) and act as cross-linker because low hydrolysis grade (35.4%) PVA was insoluble in water. Figure 4 shows visual appearances of the extraction of PAA particles from [Bmim][TFSA] and chloroform to water. In the

RESULT AND DISCUSSION

Extraction of PAA Particles from Ionic Liquid to Water. Figure 1 shows the visual appearances of the extraction process of PAA particles from [DEME][TFSA] (lower phase) to water (upper phase) without mixing. Just after the addition of water to the [DEME][TFSA] dispersion of PAA, the upper phase was clear (Figure 1a). As time passed, creaming was observed (Figure 1b). The particles remained at the interface for several hours and gradually diffused to the water phase. The extraction of PAA in the particle state was achieved at approximately 24 h as shown in Figure 1f. When the [DEME][TFSA] phase was stirred, the transfer of PAA particles was accelerated to 3 h although the interface between [DEME][TFSA] and water became unclear. Figure 2 shows the optical micrographs of the extraction process of PAA particles from [DEME][TFSA] (lower side) to water (upper side). Before the extraction, the size of the PAA particle in the [DEME][TFSA] was about 100 nm (from SEM observation). As soon as water was added to the [DEME][TFSA] dispersion of PAA, PAA particles were observed to be swollen with water (around 10 μm) in the [DEME][TFSA] phase and formed aggregates (Figure 2b,c). Despite the high hydrophobic character of [DEME][TFSA], a small amount of water could be soluble in [DEME][TFSA]. The PAA particles should be swollen with saturated water in the [DEME][TFSA] phase. The swollen PAA particles diffused to the interface (Figure 2d,e) and then gradually transferred to the water phase in the particle state (Figure 2f). Because this phenomenon was observed on a horizontal slide glass with the optical microscope, the driving force of the transfer of PAA particles would not be due to the difference in density between the PAA particles swollen with water (d ≈ 1.0) and [DEME][TFSA] (d = 1.42). To examine whether [DEME][TFSA] affected the phasetransfer phenomenon, the same extractions were carried out using another IL, [Bmim][TFSA], and chloroform, which have sufficient hydrophobicity and higher densities than water. In addition, [Bmim][TFSA] and chloroform are suitable media for dispersion polymerization of AA because AA is soluble but PAA is insoluble in both media. The conversions of AA were 96 and 97% after 3 h in the [Bmim][TFSA] and chloroform systems, respectively.

Figure 4. Visual appearances of polymerization mixtures prepared by dispersion polymerization in [Bmim][TFSA] (a) and chloroform (b) 24 h after the addition of water (no mixing; rt; lower phase, (a) [Bmim][TFSA] and (b) chloroform; upper phase, (a, b) water).

case of the [Bmim][TFSA] system, PAA particles were extracted to water in the particle state, a behavior similar to that observed in the [DEME][TFSA] system. In contrast, in the case of the chloroform system, PAA particles remained at the interface and did not transfer to the water phase even after 120 h. To determine the quantity of PAA extracted from the chloroform, the solid content in the water phase was measured by gravimetry. Although the water phase was observed to be 2525

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surfactant. The adsorption behavior of a particle surfactant to an water/oil interface can be explained using free energy calculated by interfacial tensions (water/oil, oil/particle, and water/particle) as shown in Scheme 1 and eq 1.

transparent, 91.4% of PAA was extracted. This result suggests that PAA extracted from the chloroform phase to the water phase in the molecular state, in which the cross-linked structure of PAA particles would have been cleaved, and PAA dissolved in the water phase. To clarify the difference in the extraction behavior of chloroform dispersion of PAA particles, the same extraction procedure was carried out after replacing the chloroform medium with the [Bmim][TFSA] medium. The same volume of [Bmim][TFSA] was added to the chloroform dispersion of PAA particles, and then, only chloroform was evaporated by heating at 65 °C for 3 days by taking advantage of ILs nonvolatility. After complete replacement, the PAA dispersion ([Bmim][TFSA] medium) had good colloidal stability, and an equal volume of water was added to the dispersion to extract PAA particles. As shown in Figure 5, the PAA particles dispersed in [Bmim][TFSA], which were prepared in chloroform, trans-

ΔG = G2 − G1 = πr 2[ − γwo sin 2 θ + 2(γwp − γop) (1 + cos θ)]

(1)

where γwo, γop, and γwp are the interfacial tensions of water/oil, oil/particle, and water/particle, respectively; r is the particle radius; and ΔG is the difference in free energies between state 1 (θ = 180°) and state 2 (180° > θ ≥ 0°). In the case of a particle surfactant, ΔG should have a minimum value at 180° > θ > 0°. In contrast, when PAA particles are extracted in the particle state with water, ΔG should have a minimum value at θ = 0°. Water/oil interfacial tension, γwo, between water and chloroform is 31.6 mN/m, which is level comparable to that of common organic solvents such as toluene. In the case of the chloroform system, the PAA was extracted to the water phase as molecular state, which would affect the interfacial tension between water and chloroform. However, the PAA particles dissolve in water phase after the adsorption at the chloroform/ water interface. Thus, the dissolving PAA could be neglect before the adsorption. In contrast, γwo between water and [DEME][TFSA] and [Bmim][TFSA] are 10.7 and 13.1 mN/ m, respectively, which are relatively low despite the high hydrophobic properties of [DEME][TFSA] and [Bmim][TFSA] (solubility in water: [DEME][TFSA], 0.92 g/100 g; [Bmim][TFSA], 0.77 g/100 g; chloroform, 0.82 g/100 g). Because PAA particles were highly swollen with water, the water/particle interfacial tension, γwp, is considered to be approximately 0 mN/m. Furthermore, the oil/particle interfacial tensions γop are presumed to be approximations of interfacial tension between a PAA solution and oil. Therefore, the oil/particle interfacial tensions of [DEME][TFSA]/PAA and chloroform/PAA particles were set as 9.8 and 20.7 mN/m, respectively, which were measured with a 5 wt % PAA solution by the pendant drop method. Figure 6 shows the variations in free energy ΔG of a 10 μm radius PAA particle at various contact angles. The contact angles indicate the positions of the PAA particles (θ = 180°: oil phase, 180° > θ > 0°: water/oil

Figure 5. Visual appearances of PAA dispersion prepared by dispersion polymerization in chloroform, which was replaced with [Bmim][TFSA] just after (a) and 24 h after (b) water was added (no mixing; rt; lower phase, [Bmim][TFSA]; upper phase, water).

ferred to the water phase in the particle state when the medium was replaced. This result indicates that there was no difference between the PAA particles prepared in [Bmim][TFSA] and chloroform, but ILs should have a significant influence on the extraction of PAA particles in the particle state. Influence of Interfacial Tension on the Extraction of PAA Particles. It is well-known that colloidal particles adsorb to an interface such as an water/oil interface and act as a

Scheme 1. Free Energy of Particle Transfer from the Oil Phase to the Water Phase

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Figure 6. Free energies of 10 μm radius PAA particle for transfer from [DEME][TFSA] phase to water phase (green solid line) and from chloroform to water phase (red dashed line) at various contact angles.

interface, θ = 0°: water phase). The red dashed line shows ΔG required for a PAA particle to transfer from the chloroform phase to the water phase, and ΔG has a defined minimum value at θ = 49°. From this calculation, PAA particles are expected to remain at the interface between water and chloroform when water is added to the chloroform dispersion of PAA. In the case of the [DEME][TFSA] system, the green solid line indicates a monotonic decrease to 0°, at which the PAA particles are expected to transfer from the [DEME][TFSA] phase to the water phase. These calculations are consistent with the experimental results, and this phenomenon was observed as noted above. These results suggest that interfacial tension influences the extraction of PAA particles in the particle state. High γwo of water/chloroform prevents PAA particles from transferring from the chloroform phase to the water phase in the particle state. Figure 7 shows experimental images of the extraction of PAA particles from chloroform to water at various dissolved concentrations of TTAB ((a) 0.06, (b) 0.45, and (c) 3.80 mM). TTAB aqueous solution is used as the extraction medium to decrease the water/chloroform γwo ((a) 21.1, (b) 14.0, and (c) 0.8 mN/m). The higher the concentration of TTAB aqueous solution, the more the PAA particles transferred to the water phase in the particle state. These results indicate that decreased water/chloroform interfacial tension caused PAA particles to transfer to the water phase. Figure 7d shows that PAA could maintain the particle state in water. These results suggest that the extraction of PAA particles in the particle state is induced by the unique physicality of ILs that have a low γwo despite their high hydrophobic character.

Figure 7. Visual appearances (a−c) of PAA dispersion prepared by dispersion polymerization in chloroform 1.5 h after addition of various concentrations of TTAB(aq) (no mixing; rt; lower phase, chloroform; upper phase, TTAB(aq)) and optical micrograph (d) of PAA particles in TTAB(aq) after extraction. Concentration of TTAB (mM): (a) 0.06; (b) 0.45; (c, d) 3.80.

particles transferred from chloroform to water in the particle state. These results suggest that extraction of PAA particles was a phenomenon induced by the unique physicality of ILs that have a low γwo despite having a high hydrophobic character.

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AUTHOR INFORMATION

Corresponding Author

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

ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (Grant 23350114) from the Japan Society for the Promotion of Science (JSPS). The authors thank Nisshinbo Holdings Inc. and The Nippon Synthetic Chemical Industry Co., Ltd., for supplying [DEME][TFSA] and [Bmim][TFSA], respectively.

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CONCLUSION The phase-transfer behavior of PAA particles from IL to water was examined in detail. The difference in densities was not the only driving force of the extraction of PAA particles in the particle state. ILs had a significant influence on the extraction behavior of the PAA particles. The importance of ILs can be observed in the measurement of interfacial tension with water. Moreover, free energy calculations of the PAA particle required for the transfer from the oil phase to the water phase showed that a low water/oil interfacial tension, γwo, induced extraction of PAA particles in the particle state. In the case of using a TTAB aqueous solution as the extraction medium, PAA 2527

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