Langmuir 2002, 18, 9609-9610
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Levitation Polymerization to Fabricate a Large Polymer Sphere Masafumi Yamato, Haruka Nakazawa, and Tsunehisa Kimura* Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan Received April 3, 2002. In Final Form: July 1, 2002
Introduction Polymer spheres are fabricated by means of suspension and emulsion polymerization. However, the size of spheres fabricated by these methods is limited to the order of micrometers or less because a stable suspension of monomer droplets and a resulting high sphericity are difficult to attain if the droplet size becomes larger. In this paper, we report fabrication of a polymer sphere with high sphericity, which first becomes possible by means of stable levitation of a droplet achieved by magnetoArchimedes levitation.1 Similar work has been reported on the fabrication of a glass sphere2 based on diamagnetic levitation.3,4 Diamagnetic materials, including water, organic materials, polymers, and so forth, can levitate in the air when a strong magnetic field gradient is applied.3 To levitate a water droplet in the air, for example, we need as high a value as B dB/dz ∼1400 T2/m, where B is the field strength and the derivative is with respect to the vertical coordinate z. This value is usually attained only by a magnet generating a field of 20 T in strength or more. On the other hand, if we use a paramagnetic gas, for example, a pressurized oxygen gas, instead of the air (slightly paramagnetic), a particle can be levitated with fields of moderate strength (magneto-Archimedes levitation1). This is due to the magnetic buoyancy by the paramagnetic gas, in addition to the magnetic repelling force directly acting on the levitating particle. Furthermore, if a liquid is used instead of a gas, the required field strength becomes much more moderate because the hydrostatic buoyancy provided by liquid is large. The balance of the forces is described as
-∆F g + µ0-1 ∆χ B dB/dz ) 0
(1)
where ∆F ) F1 - F2 and ∆χ ) χ1 - χ2 are the difference of the densities and that of the volumetric magnetic susceptibilities, respectively, between the particle (suffix 1) and the liquid medium (suffix 2); g and µ0 are the acceleration of gravity and the magnetic permeability of the vacuum, respectively. Alternatively, eq 1 is integrated to be written in terms of energy as follows:5
E ) ∆F gz - ∆χ B2/2µ0
(2)
* To whom correspondence should be addressed. Fax: +81 426 77 2821. Tel: +81 426 77 2845. E-mail:
[email protected]. (1) Ikezoe, Y.; Hirota, N.; Nakagawa, J.; Kitazawa, K. Nature 1998, 393, 749-750. (2) Kitamura, N.; Makihara, M.; Hamai, M.; Sato, T.; Mogi, I.; Awaji, S.; Watanabe, K.; Motokawa, M. Jpn. J. Appl. Phys. 2000, 39, L324L326. (3) Beaugnon, E.; Tournier, R. Nature 1991, 349, 470. (4) Motokawa, M. In Materials Science in Static High Magnetic Fields; Watanabe, K., Motokawa, M., Eds; Springer: New York, 2001; Chapter 18.
Figure 1. Experimental setup for “levitation polymerization”.
where z ) 0 represents the center of a superconducting solenoid, for example. One of characteristic features of diamagnetic levitation is that the energy has a minimum for an appropriate combination of ∆F and ∆χ, as well as the profile of B.4,5 The existence of an energy minimum is in marked contrast to the suspension attained by the balance of only buoyancy and gravity. In addition, the location of the minimum is controlled by changing the strength and/or the profile of B. This can be done easily, for example, by changing the electric current supplied to the magnet. If a field profile such as that generated inside a solenoid is used, there are also minima in the horizontal directions, x and y, resulting in a three-dimensional trapping of a particle. In this work, all these advantages provided by the magneto-Archimedes effect are utilized to fabricate large-size polymer spheres. Experimental Section In the present study, benzyl methacrylate (diamagnetic) was used as a monomer and an aqueous solution of manganese chloride (paramagnetic) was used as a suspending liquid. Since the suspending medium used in this study is paramagnetic (χ2 > 0) and the monomer droplet is diamagnetic (χ1 < 0), then ∆χ ) χ1 - χ2 < 0. There are two cases depending on the sign of ∆F ) F1 - F2: (i) if ∆F > 0, the minimum is located at z > 0, that is, a droplet heavier than the medium levitates above the field center (z ) 0); (ii) if ∆F < 0, the minimum is located at z < 0, that is, a droplet lighter than the medium antilevitates6 (a droplet floating on the surface is pushed into the medium) below the field center. The sign of ∆F depends on the concentration of the MnCl2 solution. For the present combination of the monomer, the suspending liquid, and the field profile available by the magnet used, the antilevitation condition (ii) was appropriate. Namely, a stable levitation was maintained under condition (ii) throughout the polymerization process irrespective of the change of density of the droplet during polymerization. A Sumitomo Heavy Industry cryocooler-cooled superconducting magnet with a room-temperature bore of 10 cm in diameter was used, providing a vertical magnetic field of variable strength from 0 up to 10 T. A 7 M aqueous solution of manganese chloride (MnCl2) was filled in the inner space of a double-wall glass container, and the temperature was controlled at 70 °C by water circulation in the outer space. This container was inserted into the bore of the magnet (see Figure 1). Then, the field strength was elevated to ca. 1.4 T (strength in the center of the magnet) by which a droplet of benzyl methacrylate is to be trapped inside the aqueous solution. A ca. 1 mL benzyl methacrylate (as purchased from Aldrich, containing 50 ppm hydroquinone monomethyl ether as inhibitor and used without further puri(5) Ikezoe, Y.; Kaihatsu, T.; Uetake, H.; Hirota, N.; Nakagawa, J.; Kitazawa, K. Trans. MRS J. 2000, 25, 77-80. (6) Kimura, T.; Mamada, S.; Yamato, M. Chem. Lett. 2000, 12941295.
10.1021/la020316f CCC: $22.00 © 2002 American Chemical Society Published on Web 09/11/2002
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Langmuir, Vol. 18, No. 24, 2002
Notes started in time after the droplet was settled. The density of the droplet increases upon progress of polymerization, and then the force balance changes accordingly. A change in magnetic susceptibility of the droplet might occur, but it would be smaller than the change of the density. To prevent the droplet from sinking, it was necessary to reduce the repelling force exerted downward by the applied magnetic field. For this purpose, the field strength was reduced from ca. 1.4 to ca. 1.0 T around 60 min after the injection as shown in Figure 2. After a 5 h polymerization in the magnet, the droplet was taken out from the magnet and further polymerized in boiling water outside the magnet.
Figure 2. Field strength in the center of the magnet, which is controlled by changing the supplied electric current so that the droplet remains at the same position. No change of field strength was required after 180 min up to 300 min.
Figure 3. Photograph of the obtained poly(benzyl methacrylate) spheres. The size is about 7-9 mm in diameter. fication), with 0.7% benzoyl peroxide added as an initiator, was slowly injected into the aqueous solution of MnCl2 with a pipet. The droplet moved around in the solution for a while before it finally settled in a stable position. At 70 °C, polymerization did not start immediately because of the inhibitor contained, but
Results and Discussion Figure 3 shows a photograph of polymer spheres obtained after the completion of the polymerization outside the magnet. The size of these spheres is ca. 7-9 mm in diameter. The diameters of these spheres were measured with a micrometer. For each sphere, diameters were measured at about 10 different points. The average deviation with respect to the average diameter was evaluated to be ca. 0.6%. This value is of the same order as that reported for the glass sphere fabricated by diamagnetic levitation of glass in the air.2 A main advantage of the use of magneto-Archimedes levitation over other methods is trapping of a particle. The trapping is not possible by a simple adjustment of the densities of the particle and the suspending medium, nor under zero-gravity in space. In addition, the location of the minimum energy is easily fixed or changed through the control of the field strength or field profile. This is especially useful in the case in which a particle to be manipulated changes its density or magnetic susceptibility as in the present case. The technique presented here would also be available for the photopolymerization of a levitating monomer droplet. Acknowledgment. This work was partially supported by Japan Society for the Promotion of Science through the Research for the Future Program. LA020316F
