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Preparation of Low-Surface-Energy Poly[2-(perfluorooctyl)ethyl acrylate] Microparticles and Its Application to Liquid Marble Formation† Daisuke Matsukuma,‡ Hirohmi Watanabe,‡ Hiroki Yamaguchi,§ and Atsushi Takahara*,‡,§ ‡
Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO), Takahara Soft Interfaces Project, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan, and §Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Received October 11, 2010. Revised Manuscript Received December 28, 2010 We demonstrate the successful preparation of stable liquid marbles from various liquids. This is accomplished by using low-surface-energy poly[2-(perfluorooctyl)ethyl acrylate] (PFA-C8) as microparticles. The PFA-C8 microparticles were prepared by the spontaneous self-organized microparticulation of PFA-C8. The physical properties remained intact in the polymer morphology as confirmed by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) measurements. The extremely low surface energy of PFA-C8 provides a high solid-liquid spreading coefficient (SS/L) value for various combinations of liquids. As a result, liquid marbles were obtained from various liquids, unlike the case with other fluorine polymer particles such as poly(tetrafluoroethylene) (PTFE) and poly(vinilydene fluoride) (PVDF). These results suggest that the technique is widely applicable for preparing novel functional materials.
Introduction A liquid marble is a fluidic droplet encapsulated with small, low-surface-energy solid particles.1 Contrary to the simplicity of its composition, the liquid marble shows unique dynamic behavior as first demonstrated by Quere and co-workers.2-4 Liquid marbles work well as a nonwetting system because of their low adhesion to the substrate. Some liquid marbles can even float on a water surface.5-9 These unique characteristics offer a variety of applications such as low-energy transportation systems,10,11 microreactors,12-14 and microreservoirs.15 It is known that a combination of low-surface-energy particles and high-surface-energy liquids is required to fabricate stable liquid marbles.2-14 Although various nano- and microparticles such as poly(tetrafluoroethylene) (PTFE) and poly(vinilydene fluoride) (PVDF) have been applied so far, the only fluidic options are water and several other liquids. Most liquids cannot be encapsulated even by these particles. For widespread use, liquid marbles need to be fabricated from various liquids. † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding author. Tel: þ81-92-802-2517. Fax: þ81-92-802-2518. E-mail:
[email protected].
(1) Pike, N.; Richard, D.; Foster, W.; Mahadevan, L. Proc. Jpn. Acad., Ser. B 2002, 269, 895. (2) Aussillous, P.; Quere, D. Nature 2001, 411, 924. (3) Quere, D.; Aussillous, P. Chem. Eng. Technol. 2002, 25, 925. (4) Aussillous, P.; Quere, D. Proc. R. Soc. A 2006, 462, 973. (5) Bormashenko, E.; Bormashenko, Y.; Musin, A.; Barkay, Z. ChemPhysChem 2009, 10, 654. (6) Bormashenko, E.; Bormashenko, Y.; Musin, A. J. Colloid Interface Sci. 2009, 333, 419. (7) Bormashenko, E.; Musin, A. Appl. Surf. Sci. 2009, 255, 6429. (8) Dupin, D.; Armes, S. P.; Fujii, S. J. Am. Chem. Soc. 2009, 131, 5386. (9) Fujii, S.; Kameyama, S.; Armes, S. P.; Dupin, D.; Suzuki, M.; Nakamura, Y. Soft Matter 2010, 6, 635. (10) Bormashenko, E.; Pogreb, R.; Bormashenko, Y.; Musin, A.; Stein, T. Langmuir 2008, 24, 12119. (11) Zhao, Y.; Fang, J.; Wang, H.; Wang, X.; Lin, T. Adv. Mater. 2009, 22, 707. (12) Gao, L.; MacCarthy, T. J. Langmuir 2007, 23, 10445. (13) Tian, J.; Arbatan, T.; Li, X.; Shen, W. Chem. Commun. 2010, 46, 4734. (14) Bormashenko, E.; Balter, R.; Aurbach, D. Appl. Phys. Lett. 2010, 97, 091908. (15) Wang, W.; Bray, L. C.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 11608.
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Poly(perfluoroalkyl acrylate) with long fluoroalkyl side chains is a crystalline polymer that has superior material characteristics such as high chemical and thermal stability, a nonadhesive property, a low friction coefficient, and low solubility in various solvents.16 In particular, as demonstrated in our previous work, the molecular aggregation state of poly[2-(perfluorooctyl)ethyl acrylate] (PFA-C8) provides an extremely low surface energy among fluorinated polymers.17-19 The use of PFA-C8 may extend the range of usable liquids because of its superior physicochemical properties. The focus of this study is on two processes (Figure 1). One is the microparticle preparation of PFA-C8. Microparticles of PFA-C8 were prepared by self-organized microparticulation through the evaporation of polymer solution accompanying the crystallization of (perfluorooctyl)ethyl side chains. The other process is the encapsulation of various liquid droplets with these particles to form stable liquid marbles (even from methanol). The formation mechanism of liquid marbles was discussed in terms of the relationship of surface energy between particle and liquid. These results will help to construct novel functional liquid marbles.
Experimental Section Materials. PFA-C8 polymer was synthesized by atom-transfer radical polymerization according to our previous paper.20 The weight-average molecular weight and polydispersity index of the PFA-C8 polymer were 3.6 105 and 1.44, respectively. Poly(tetrafluoroethylene) particles (PTFE: 1 μm average diameter) and poly(vinilydene fluoride) particles (PVDF: 130 nm average diameter) were purchased from Aldrich Chemical Co., Inc. All solvents in this study were used without further purification. (16) Pittman, A. G. In Fluoropolymers; Wall, L. A., Eds.; Wiley-Interscience: New York, 1972; p 419. (17) Honda, K.; Morita, M.; Otsuka, H.; Takahara, A. Macromolecules 2005, 38, 5699. (18) Honda, K.; Yakabe, T.; Koga, T.; Sasaki, S.; Sakata, O.; Otsuka, H.; Takahara, A. Chem. Lett. 2005, 34, 1024. (19) The bioaccumulation of the degraded product of perfluorinated acid with a long fluoroalkyl chain above the perfluorohexyl group is becoming a serious concern. Suja, F.; Pramanik, B. K.; Zain, S. M. Water Sci. Technol. 2009, 60, 1533. (20) Yamaguchi, H.; Honda, K.; Kobayashi, M.; Morita, M.; Masunaga, H.; Sakata, O.; Sasaki, S.; Takahara, A. Polym. J. 2008, 40, 854.
Published on Web 01/13/2011
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Figure 1. Schematic illustrations of (a) the preparation of PFA-C8 microparticles and (b) the fabrication of liquid marbles.
Figure 2. SEM images of PFA-C8 microparticles prepared from different polymer concentrations: (a) 0.5, (b) 1.0, and (c) 2.0 wt %, respectively. (d) Magnification of the image in b.
Preparation of PFA-C8 Microparticles. PFA-C8 microparticles were obtained by the slow evaporation of a PFA-C8 polymer solution (Figure 1a).21 PFA-C8 polymer was first dissolved in 3,3dichloro-1,1,1,2,2-pentafluoropropane (HCFC-225), and then acetic acid was added to the solution. The mixing ratio of HCFC-225 and acetic acid was adjusted to 3:1 v/v, and the polymer concentration was 0.5-2.0 wt % against the total amount of solution. After casting the polymer solution onto Si substrates, the sample was evaporated at ambient temperature to obtain polymer particles. The sample was freeze dried to remove residual solvents. Preparation of Liquid Marbles. The schematic procedure for preparing liquid marbles is shown in Figure 1b. Liquid marbles were obtained by gently shaking liquid droplets on a microparticle layer. The amount of all liquid droplets was adjusted to 5 μL, and the obtained liquid marbles were transferred onto another substrate by careful manipulation with a spatula. Digital camera images of the liquid marbles were captured using a Pentax Optio W80. (21) Yabu, H.; Higuchi, K.; Ijiro, K.; Shimomura, M. Chaos 2005, 15, 047505.
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Characterization. The morphology of the PFA-C8 microparticles was observed by scanning electron microscopy (SEM, Hitachi S-4300SE). The crystalline state of the PFA-C8 microparticles was evaluated by wide-angle X-ray diffraction (WAXD) measurements in a powder state. WAXD measurements were carried out at the BL02B2 beamline of SPring-8 (Japan Synchrotron Radiation Research Institute) using 0.100 nm X-rays. The scattering vector, q, is defined as q=(4π/λ)sin θ, where λ and θ are the wavelength and the incident angle of the X-ray beam, respectively. Thermal analysis was carried out by differential scanning calorimetry (Diamond DSC, Pyris, Perkin-Elmer) at a heating rate of 10 K/min. PFA-C8 in the bulk state was heated to 473 K in order to eliminate thermal history effects.
Results Characterization of PFA-C8 Microparticles. PFA-C8 microparticles were prepared by the solvent evaporation of a polymer solution containing a small amount of poor solvent (Figure 1a). In this procedure, the difference in the evaporation rate of the two solvents is the key to inducing the self-organization of polymer chains and subsequent polymer particle formation.21 Langmuir 2011, 27(4), 1269–1274
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Figure 3. SEM images of PFA-C8 microparticles prepared from different solvent compositions. The v/v ratios of HCFC-225 to acetic acid are (a) 2:1 and (b) 4:1. The polymer concentration is fixed at 1.0 wt %.
Microparticles can be obtained only when the boiling point of the poor solvent is much higher than that of the good solvent. HCFC225 (boiling point: 54 °C) and acetic acid (boiling point: 114 °C) are a good and a poor solvent, respectively, for PFA-C8, and the combination produces uniform PFA-C8 microparticles with high reproducibility. The morphology of PFA-C8 microparticles was observed by SEM. Figure 2 shows SEM images of microparticles on a Si substrate. The mixing ratio was fixed at 3:1 v/v for HCFC-225 and acetic acid, and the measurement was performed after complete removal of the solvents. The sizes of the particles gradually increased with increasing polymer concentration. Microparticles with an average diameter of 1.4 ( 0.28 μm were obtained from a 1.0 wt % PFA-C8 solution in mixed solvents (Figure 2b). Further increases in PFA-C8 concentration caused the aggregation of microparticles, as shown in Figure 2c. The solvent ratio affected the size and size distribution of the microparticles (Figure 3). Although monodisperse, small microparticles were obtained at a high concentration of acetic acid (Figure 3a), the solubility of PFA-C8 itself was limited in this solution. As a result, a small number of polymer particles were obtained. However, large aggregates were obtained at a high fraction of HCFC-225 (Figure 3b). Subsequently, the WAXD measurement was performed to evaluate the crystalline state of PFA-C8 microparticles. Figure 4 shows WAXD profiles of PFA-C8 microparticles together with PFA-C8 in the bulk state. The obtained diffraction profile of the PFA-C8 microparticles was the same as that of the bulk PFA-C8. The peaks at q=3.83 nm-1 (d=1.64 nm) and 5.76 nm-1 (d=1.09 nm) were assigned as second- and third-order diffractions from the lamellar structure of (perfluorooctyl)ethyl side chains.17,22-24 The presence of higher-order diffraction indicates relatively high order of lamellar structure. On the other hand, the peak at q = 12.5 nm-1 (d = 0.50 nm) corresponded to the diffraction from hexagonally packed (perfluorooctyl)ethyl groups of PFAC8.17,25,26 No difference in the diffraction profile was observed among samples. The melting point and the area of melting endothermic peak were also independent of the microparticle size, as confirmed by DSC measurements (Figure S1). These results clearly suggest that the bilayer lamella structure of (perfluorooctyl)ethyl side chains of PFA-C8 is present in the microparticle. It has been reported that (perfluorooctyl)ethyl groups were highly oriented at the outermost layer of the PFA(22) Takahara, A.; Morotomi, N.; Hiraoka, S.; Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1989, 22, 617. (23) Volkov, V. V.; Plate, N. A.; Takahara, A.; Kajiyama, T.; Amaya, N.; Murata, Y. Polymer 1992, 33, 1316. (24) Volkov, V. V.; Fadeev, A. G.; Plate, N. A.; Amaya, N.; Murata, Y.; Takahara, A.; Kajiyama, T. Polym. Bull. 1994, 32, 193. (25) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549. (26) Corpart, J. M.; Girault, S.; Juhue, D. Langmuir 2001, 17, 7237.
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Figure 4. WAXD profile of PFA-C8 microparticles together with the polymer in the bulk state. The microparticles were prepared from different polymer concentrations: (a) 0.5, (b) 1.0, (c) 2.0 wt %.
C8 film and contributed to its low surface free energy.17,18 Because the radius of the microparticle is larger than the molecular scale, it can be expected that the orientation of (perfluorooctyl)ethyl side chains in the surface region induces the low surface energy of the particles. Liquid Marble Formation. Liquid marbles were prepared by the encapsulation of 5 μL liquid droplets with PFA-C8 microparticles. Figure 5 shows digital camera images of liquid marbles after their careful transfer onto Si substrates. Stable liquid marbles were obtained from various liquids including methanol. PFA-C8 microparticles with an average diameter of 1.4 ( 0.28 μm were used for the fabrication. Conversely, in the case of PTFE and PVDF polymer particles, most liquid droplets penetrated the polymer particle layer instead of being encapsulated. The shape of the liquid marbles was determined by their contact length. Spherical liquid marbles were obtained when the contact length was not very long. Distortion occurred according to the increase in the contact length of the liquid.3 The contact length itself is directly linked to the capillary length of the encapsulated liquid. The capillary length (κ-1) is a physical value of liquids that is determined by the surface energy (γ) and the density (F) (κ-1 = (γ/Fg)1/2, where g is the acceleration of gravity).27 Figure 6 shows the relationship between the contact length of liquid marbles deposited on the Si substrate and the (27) See Supporting Information.
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Figure 5. Digital camera views of liquid droplets encapsulated with PFA-C8 microparticles. The liquid volume is 5 μL.
When the SS/L value is positive, successive particle spreading may occur on the surface of a liquid. As a result, the liquid droplet is encapsulated with the polymer particles to form a stable liquid marble. However, when the SS/L value is negative, the formation of a liquid marble is unsuccessful because of the dewetting of the polymer particle layer. The SS/L value is defined as follows SS=L ¼ - 2γS þ 2ðγL d γS d Þ1=2 þ 2ðγL p γS p Þ1=2
Figure 6. Size of the contact of a liquid marble deposited on a Si substrate as a function of the capillary length of various liquids. The liquid volume is 5 μL.
capillary length of liquids. Because all liquid drops were adjusted to 5 μL, the capillary lengths of various liquid marbles can be comparable with each other. As shown in Figure 6, the contact length linearly decreases according to the increase in capillary length. Consequently, a distorted liquid marble was obtained with a long capillary length. This result suggests that the physical properties of the liquid determined the shapes of the liquid marbles. If the liquid marble stability is governed by the surfaceinterface interaction, then the formation of liquid marbles can be described by the spreading coefficient between the particles and the liquid. The spreading coefficient (SB/A) describes the spontaneous spreading of liquid B placed on the surface of liquid A.28 Liquid B can spread on liquid A only when the SB/A value is positive (spreading wetting). This concept could be used to form liquid marbles as a solid-over-liquid spreading coefficient (SS/L).29,30 (28) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (29) Hapgood, K. P.; Khanmohammadi, B. Powder Technol. 2009, 189, 253. (30) Eshtighi, N.; Liu, J. S.; Shen, W.; Hapgood, K. P. Powder Technol. 2009, 196, 126.
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ð1Þ
where γLd and γLp are the dispersive and polar components of the surface energy of a liquid and γSd and γSp are those of the polymer particle. γS is the sum of both components (Fowkes’s equation: γS =γSd þ γSp).31 Table 1 summarizes the calculated SS/L values for various combinations of polymer particles and liquids. Because the physical properties remain intact in the polymer morphology, the surface energy of the PFA-C8 microparticles was considered to be the same as in the film state, as shown in our previous paper.32 The surface energies of PTFE, PVDF, and various liquids were taken from the literature.33,34 As summarized in Table 1, the combination of PFA-C8 with liquids shows relatively high SS/L values among these three polymers. Because the surface energy of a liquid is an intrinsic characteristic, polymer particles having a low surface energy are required merely to produce high SS/L values. It is clear that the extremely low surface energy of PFA-C8 enabled the successful formation of liquid marbles. However, the magnitude of the surface energy of PTFE and PVDF is not low enough to give high values of SS/L. As a result, the fabrication of stable liquid marbles was unsuccessful with these polymer particles.
Implications and Prospects Stable liquid marbles were prepared by the encapsulation of liquid droplets with PFA-C8 polymer particles. The polymer particles were obtained by self-organized microparticulation through solvent evaporation. The proposed mechanism of selforganized microparticulation is as follows (Figure 7).21,35,36 PFAC8 polymer is dissolved in a random coil conformation in a (31) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40. (32) Honda, K.; Morita, M.; Sakata, O.; Sasaki, S.; Takahara, A. Macromolecules 2010, 43, 454. (33) Kreveren, D. W. V. Properties of Polymers, Elsevier: Amsterdam, 1990. (34) Fowkes, F. M.; Riddle, F. L., Jr.; Pastore, W. E.; Weber, A. A. Colloids Surf. 1990, 43, 367. (35) Yabu, H.; Tajima, A.; Higuchi, T.; Shimomura, M. Chem. Commun. 2008, 4588. (36) Zhang, K.; Serizawa, T. J. Nanosci. Nanotechnol. 2008, 8, 1.
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Table 1. Summary of Experimental Results on the Formation of Liquid Marbles and the Corresponding Calculated SS/L Values of the Combination PTFEa
PFA-C8a liquids
liquid marble formation
SS/L (mN/m)
liquid marble formation
PVDFa SS/L (mN/m)
liquid marble formation
SS/L (mN/m)
water yes 18.4 yes 18.1 yes 22.3 methylene iodide yes 23.3 yes 21.1 no 8.2 b 11.5 dimethylsulfoxide yes 18.3 yes 15.8 b 5.9 dimethylformamide yes 17.4 yes 13.6 1,4-dioxane yes 16.0 no 9.7 no -4.7 toluene yes 14.1 no 7.7 no -3.3 chloroform yes 12.8 no 4.7 no -10.3 tetrahydrofuran yes 12.5 no 4.2 no -10.9 ethanol yes 10.6 no 2.1 no -9.8 methanol yes 9.8 no 1.4 no -8.6 triethylamine no 9.2 no -1.0 no -16.7 trifluoroethanol no 7.8 no -1.3 no -10.1 a The values of the surface energy (mN/m) of PFA-C8, PTFE, and PVDF are γS = 7.82 (7.46 and 0.36 for γSd and γSp), 20.0 (18.4 and 1.6 for γSd and γSp), and 30.3 (23.3 and 7.0 for γSd and γSp), respectively.33 b PVDF particles were dissolved in a solvent.
Figure 7. Schematic representation of the formation of PFA-C8 microparticles during solvent evaporation.
mixture of HCFC-225 and acetic acid. Because HCFC-225 is a good solvent for PFA-C8, the evaporation of HCFC-225 leads to the desolvation of the polymer in solution. Nucleation and selforganization (crystallization) of the (perfluorooctyl)ethyl side chains occur at this time. Further evaporation of solvent advances the crystallization to the particle state through the adsorption of polymer chains. As a result, microparticles of PFA-C8 are produced after the complete removal of solvents. In this procedure, the molecular aggregation state of PFA-C8 may act as an important driving force to fabricate regular microparticles. In preliminary experiments, in situ monitoring of the crystallization using synchrotron radiation WAXD measurements supports the formation mechanism of the self-organized microparticulation of PFA-C8. The PFA-C8 microparticles encapsulated various liquids including water, chloroform, ethanol, and even methanol. These experimentally observed behaviors are correlated with the calculated SS/L value, which is simply determined by the individual surface energies of the liquid and polymer particles. The extremely low surface energy of PFA-C8 provides high SS/L values, and liquid marbles were easily formed from various liquids. However, PTFE and PVDF were inefficient for this purpose as predicted by the SS/L calculation. The calculation of SS/L values is a useful criterion for predicting the formation of a liquid marble. It is important that the liquid marble formed only when the SS/L value was high enough. If spreading wetting is the only physicochemical Langmuir 2011, 27(4), 1269–1274
criterion for the formation of stable liquid marbles, then all combinations that give a positive SS/L value should form liquid marbles. However, in the present case, SS/L > 9.8 seems to be the threshold value for the formation of liquid marbles. This is also adaptable in the case of PTFE and PVDF. These experimental results clearly indicate that the conventional spreading wetting does not sufficiently describe the formation of liquid marbles, which is more complicated than the conventional, simple spreading wetting of two liquids. The results of the present study clearly revealed that the wetting phenomena between two phases cannot explain the formation of liquid marbles well enough. The encapsulation of various liquids enables an adequate discussion of the driving force of the liquid marble. For a precise explanation of liquid marble formation, geometrical interfacial interactions such as the capillary interaction between individual polymer particles on a liquid surface should be considered. In the case of liquid marbles, capillary interaction is a phenomenon that is provided by the intermolecular attractive forces between the particles on the liquid surface.37 The attraction force may also contribute to the stabilization of liquid marbles. Because the size and the size distribution of the polymer particles are directly related to the capillary interaction, an estimation of the capillary interaction is difficult using the PFA-C8 (37) Kralchevsky, P. A.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 230.
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microparticles prepared in this study. It is known that the PFA-C8 polymer can be grown on a solid surface such as silica particles.20 The use of PFA-C8-grafted monodisperse silica particles may help to elucidate the contribution of these forces in detail. Liquid marbles can now be fabricated from various liquids by using PFA-C8 microparticles. This result will find many novel applications, such as using liquid marbles as a microreactor. Many liquids can carry a solid material and act as a reaction medium. The controlled preparation of various liquid marbles may open up a new area of research.
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Acknowledgment. Synchrotron radiation experiments were performed on BL02B2 and BL40B2 at SPring-8 with the approval of the JASRI (proposal nos. 2010A1454 and 2010A1532). We thank Dr. M. Morita (Daikin Industries Co., Ltd.) for the generous gift of PFA-C8 monomer. Supporting Information Available: DSC curves of PFA-C8 microparticles together with the polymer in the bulk state. Surface tension, density, and capillary length of some liquids used in this study. This material is available free of charge via the Internet at http://pubs.acs.org.
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