COOKING  WITH  NANOPARTICLES:  A  SIMPLE  METHOD  OF  FORMING 

ROLL, PANCACE,  AND  BREADED  POLYSTYRENE  MICROPARTICLES.

 

Anker J. N., Horvath T. D., Kopelman R.

 

The University of Michigan, Chemistry Department, Ann Arbor, MI 48109-1055 USA.

 

Introduction.

 

We report here simple methods of forming roll and disc-shaped (pancake) microparticles that require no unusual reaction conditions, templates, or molds. In addition, we have physically embedded small «breading» particles into the surface of larger particles to form hybrid particles of various compositions. We have breaded fluorescent nanospheres into rolled magnetic polystyrene particles. We have also breaded magnetic material into fluorescent polymer pancakes. Such magmnetic particles align in magnetic fields due to their magnetic anisotropy. Since a particle’s shape and composition affects its magnetic, optical, chemical and mechanical properties, our non-spherical particles may prove useful in a variety of optical, chemical and biomedical applications.

Nagy and Keller describe a physical method of forming elliptical polymer microparticles [1]. They formed a suspension of polystyrene microspheres in a polyvinyl alcohol (PVA) solution, and allowed it to dry into a thin film. The film was then stretched at a temperatute above the glass transition temperature of both the polystyrene (T_G 94°C), and the PVA (T_G 85°C). The embedded microspheres were deformed into ellipses while the film was stretched. The PVA was then dissolved leaving elliptical microspheres. The amount that the film was stretched controlled the aspect ratio of the particles.

Wang at al expanded Nagy and Keller’s method and formed oblate ellipticale particles by compressing particles in a PVA matrix above the glass transition temperature [2]. Jaing at al invented a method of forming a polystyrene mold around microspheres, dissolved the microspheres, and stretched the mold at an elevated temperature so that it had elliptical holes. They then filled the holes with a variety of polymers and sol gels to form solid and shell shaped elliptical microparticles [3]. Howerer, we are unaware of any room temperature deformations. In addition, we are unaware of any previous deformation of magnetic microspheres, although Gabrielson and Folkes tried unsuccessfully to deform magnetic polystyrene microspheres [4].

 

Methods.

 

Fluorescent polystyrene microspheres 3.4 mm in diameter were purchased from Bangs labs. Polystyrene microspheres containing ferromagnetic chromium dioxide 2 mm and 4.4 mm in diameter were purchased from Sperotech. Iron oxide nanoparticles were obtained from Magnox. Fluorescent decyl methacrylate and silica sol gel nanospheres were polymerized in our lab. Glass microscope slides were from Fisher Scientific.

A simple method of deforming microspheres into rolls and multirolls is illustrated in Figure 1. Polystyrene microspheres were deposite onto a microscope slide and the slide was clamped to a laser table. A second slide was placed on top to sandwich the particles. The top slide is then moved laterally while applying pressure with the fingers. With a low concecntration of particles and small lateral motions, single particle rolls are formed, while with a high concentration of particles and large lateral motions, the rolls form together into multirolls as shown in Figure 4. The rolling procedure can be performed with microspheres that are either suspended in solution, or dry. The preferred procedure was to suspend the microspheres in ethanol and deposite them on a microscope slide to dry before rolling.

Pancake-shaped microparticles are formed using a rolling pin (a ¼ diamater glass tube with a metal pin through it) to flatten deposited microspreres, as shown in Figure 2. This method cam also be used to form coupled pancakes and flattened rolls and multirolls.

Smaller particles could be breaded onto larger particles by applying small «breading» particles to the microscope slide before the larger microspheres are added. The breading particles are implanted into larger particle rolls or pancakes during the normal rolling or flattening procedure.

 

Figure 1. Method of rolling microspheres into rolls and multirolls. Microspheres are deposited onto a microscope slide and the slide is clamped to a laser table. A second slide is placed on top to sandwich the particles. The top slide is then moved laterally while applying pressure with the fingers as shown in (a). With a low concentration of particles and small lateral motions, single particle rolls are firmed, while with a high concentration of particles and large lateral motions, the rolls form together into multirolls (b).

Figure 2. Method of flattening microspheres into « micropancakes». A rolling pin,compressing a ¼ glass tube with a metal pin through it, is rolled over particles deposited on a microscope slide.

 

Results.

 

The rolling process works remarkably well for a process literally done by hand. It works wet or dry, with large concentrations of particles or small concentrations, with polystuyrene particles, and magnetic polystyrene particles, and in the presence of breading. Howerer, hand rolling has its limiteations. The shape of particles formed by hand is hard to control; for example, rolled particles tend to be thinner and longer where direct pressure from the fingers is applied, than elsewhere. When the particles begin to deform, one feels the rolling resistance decrease, but otherwise one has to check under the microscope to see particle shape. If too much pressure is applied, the particles, it is important to use a sufficient volume of water: insufficient water leads to droplets that shrink with evaporation, causing particles to aggregate. This aggregation interferes in the rolling process as shown in Figure 3. However, we have demonstrated that particles can be rolled and breaded. We are designing simple machines to apply uniform pressure and control the shape of the particles formed.

Rolls and multirolls of magnetic polystyrene particles breaded with fluorescent polystyrene, sol gel, and decyl methacrylate have been formed. Figure 4 shows a CCD image of fluorescent breaded magnetic microspheres. Due to their magnetic shape anisotropy, these rolls align with external magnetic field when placed in solution.

Fluorescent polystyrene pancakes have also been formed, and pancakes have been breaded with magnetic material. The magnetically breaded fluorescent pancakes align with external magnetic field, as shown in Figure 5. These magnetically breaded pancakes were stable in water for at least five days.

 

 

 

Figure 3. Wer-rolled 3.4 mm polystyrene microspheres forced together in an evaporating droplet containing a soap bubble.     Figure 5. Fluorescent 3.4 mm polystyrene microspheres were flattened onto iron oxode nanoparticles (magnetic breadings). The magnetically breaded fluorescent pancakes align with external magnetic fields: a) , b) and c) show fluorescent images of one particle aligned in three different magnetic field orientations.

Figure 4. Fluorescent image of breaded multirolls. They were formed from 2 mm CrO2 containing magnetic polystyrene microspheres and breaded with ~ 500 nm decyl methacrylate nanospheres containing a fluorescent dye. These particles were rolled dry.

 

Discussion and concluisions.

 

We have discovered a method of forming non-spherical particles that requires no template, unusual reaction conditions, or elevated temperatures. Our early efforts have used polystyrene microspheres  and polystyrene containing chromium dioxide nanoparticles. We expect the method to apply to a number of other polymer and metal micro/nanospheres. Non-spherical magnetic particles can be oriented in magnetic fields due to their shape anisotropy. We plan to eludicate the effects of temperature, solvent, stress, and rate of strain on particle formation, and to develop a machine to uniformly deform particles.

 

References.

 

1.      Nagy M., Keller A. // Polymer Communications, 30 (5) (1989), 130 – 132.

2.      Wang S., Xu P., Mark J. E. // Macromolecules, 24 (22) (1991), 6037 – 6039.

3.      Jiang P., Bertone J. F., Colvin V. L. // Science, 291 (5503) (2001), 453 – 457.

4.      Gabrielson L., Folkes M. J. // Journal pf Materials Science, 36 (1) (2001), 1 – 6.

 

Acknowledgement

 

We wish to thank Murphy Brasuel for providing the fluorescent Decyl methacrylate nanosphere breading, and Yong-Eun Koo for providing fluorescent silica based sol gel nanosphere breading. This research was supported by NSF grant DMR 9900434 and NIH/NCI contract № 01 – CO – 07013.