Synthesis of monodispersed polymethylmethacrylate spheres
Procedure modified by George Lisensky and Jacob Horger, Beloit
College, from the Inverse Opal Photonic Crystals Laboratory Guide by R. Schroden and
N. Balakrishnan, University of Minnesota MRSEC, 2001.
Monodispersed polymethylmethacrylate spheres are synthesized from a rapidly stirred aqueous suspension of methyl methacrylate. The small uniform diameter particles appear irridescent since their size is similar to the wavelength of visible light.
Procedure
Wear eye protection
Chemical gloves recommended
Fumehood recommended
Under a slow flow of nitrogen, stir 16 mL pure water with a 20x10
mm oval-shaped magnetic stir bar in a 25 mL round bottom flask. The nitrogen
enters through a long needle and exits through a short needle passing
through a septum in the top of a condenser and then to a bubbler to monitor
flow. A Corning stirrer-hotplate spins the stir bar at maximum speed and
keeps the temperature at 70 degrees C. A crystallizing dish containing
water helps minimize temperature fluctuations.
Add 3.0 mL methyl methacrylate. Maintain the 70 degree C temperature
and rapid stirring. The size of the spheres produced depend on temperature,
stir rate, and concentration. (Click here
to see an image of poorly stirred product.) The methyl methacrylate should
form a suspension with uniformly sized spherical particles in the water.
Add 0.015 to 0.020 g 2,2-azobis(2-methylpropionamidine) which decomposes
with heat to produce a free radical initiator for the polymerization reaction.
(The initiator is added through the condensor since adding it to a joint
exposed to methyl methacrylate vapor twice resulted in a permanently polymethylmethacrylate
fused joint.)
A milky white suspension is observed as the polymerization proceeds.
Keep adjusting the temperature to maintain 70 degrees C for the next 40
minutes.
After 40 minutes of heating, remove the condenser, and transfer the
suspension to 1.5 mL microcentrifuge tubes. There should not be a noticeable
odor if the polymerization was successful.
Spin your tubes at 5000 rpm for 15 minutes. Always balance the centrifuge
by pairing tubes such that opposing tubes have the same amount of solution.
(Spinning at 10,000 rpm appears to crush the product.)
Remove and discard the water above the polymerized polymethylmethacrylate
spheres.
Successful product will change colors with the angle of observation
or lighting; the product will look iridescent.
Transfer product to an open container and let the water evaporate
overnight to leave a dry powder.
