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 the emulsion polymerization of a rapidly stirred aqueous suspension of methyl methacrylate.
2,2-Azobis(2-methylpropionamidine) dihydrochloride is a heat activated water soluble free radical initiator so the emulsion droplets polymerize starting from their outer edge.
The small uniform diameter particles should appear irridescent if they are close-packed and their size is similar to the wavelength of visible light.
Add a 20x10 mm oval-shaped magnetic stir bar to a 25 mL round bottom flask.
(It is important to use a stir bar designed for a round bottom flask for maximum stirring.)
Add a teflon sleeve to the end of a condenser.
(Not using a teflon sleeve usually results in a permanently polymethylmethacrylate fused joint at the end of the experiment.)
Add 16 mL pure water and stir under a slow flow of nitrogen. 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 °C. A crystallizing dish containing
water helps minimize temperature fluctuations.
Add 3.0 mL methyl methacrylate. Maintain the 70 °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.
When conditions are stable, quickly add 0.015 to 0.020 g 2,2-azobis(2-methylpropionamidine) dihydrochloride and resume nitrogen flow. This compound decomposes
with heat to produce a free radical initiator for the polymerization reaction.
A milky white suspension is observed as the polymerization proceeds.
Keep adjusting the temperature to maintain 70 °C for the next 40
minutes.
After 40 minutes of heating, remove the condenser. There should not be a noticeable
odor if the polymerization was successful. Be sure to return the Teflon joint sleeve.
If you will be measuring the sphere size by optical diffraction, transfer part of the suspension to a 1.5 mL microcentrifuge tube to save for use in that experiment.
If you will be measuring the sphere size by Stoke's Law, transfer the rest of the suspension to a test tube. Monitor the height of the spheres in the solution for at least a week.
Plot the height in mm as a function of the number of settling hours. Obtain these measurements before doing the next step.
Spin your sample 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 should look irridescent.
Transfer product to a pre-weighed open container and let the water evaporate overnight to leave a dry powder. Record the weight of product obtained.
Materials
Methyl methacrylate, Aldrich M55909
2,2-Azobis(2-methylpropionamidine) dihydrochloride, Aldrich 440914
Stirrer/hotplate
20x10 mm oval-shaped magnetic stir bar, VWR 58949-006
Reusable Teflon joint sleeves, VWR 56608-XXX
25 mL round bottom flask, condenser, crystallizing dish
Nitrogen, septum, hoses, syringe needles
Centrifuge and tubes
