A consortium of chemical companies under the direction of the U.S. government
first produced styrene-butadiene rubber in 1942. The Mutual Recipe was
used during World War II for the continuous emulsion polymerization of
styrene butadiene [Saunders 435]:
Butadiene 75 parts by weight comonomer
Styrene 25 parts by weight comonomer
Water 180 parts by weight carrier (don't say "solvent")
Fatty Acid Soap 5.0 parts by weight emulsifier
n-Dodecyl Mercaptan 0.50 parts by weight modifier
potassium persulfate 0.30 parts by weight initiator
The reaction proceeds at 50 degrees C for 12 hours until the conversion
reaches 72%, then the reaction is terminated by the addition of hydroquinone
to minimize crosslinking.
Harkins and his
coworkers describe a model for the emulsion process in:
J. Am. Chem. Soc. 69, 1428 (1947)
Smith-Ewart: growing monomer-polymer particles contain a free radical only
half the time.
It looks like there may be multiple models to deal with. Once this is all
sorted out, it may be appropriate to have a Harkin page, a Smith-Ewart
page, and maybe others. It may be that the discussion below mixes one
persons idea with another, and if this is the case, later on it will be
necessary to sort out the ideas.
The "ingredients" for an emulsion polymerization include:
- a water soluble initiator
- an emulsifier, and
- a monomer that is only slightly soluble in water.
- water
There are three phases in the emulsion system (in the beginning):
- aqueous phase
- monomer droplets
- micelles (there will be micelles in only the first of three intervals
or stages...)
The monomer consumed by the polymerization in the micelle is replaced as more
monomer diffuses from the aqueous solution into the micelle. As monomer leaves
the aqueous solution, more monomer from monomer drops leaves the monomer
droplets and thus maintains a steady concentration of monomer in the
aqueous solution.
To sum this up, here is the "life cycle" of a monomer
molecule:
- The monomer is added to the water, and most of it goes into "globes"
or droplets of monomer because of the low water solubility. There is a
high probability our one monomer molecule will start out in a monomer
droplet.
- The monomer eventually leaves the droplet and goes into the media.
- The monomer diffuses into the micelle.
- The monomer becomes part of the polymer.
The first free radical to enter a monomer swollen micelle starts the
polymerization. The second free radical to enter the micelle terminates
the polymerization. When the third free radical enters the micelle, the
process is repeated. As this process repeats, the micelle becomes larger
and larger. The micelles are disrupted to form particles of polymer
swollen with monomer which are stabilized by soap molecules around the
periphery.
The monomer must have a low solubility, less than 0.004 moles of monomer per
liter of water. With too high of a solubility, there would be
solution polymerization in the water phase, which is undesirable.
Monomer droplet vs. Micelle:
Size- a swollen micelle has a diameter of 50 to 100 angstroms, but an
emulsified monomer droplet has a diameter on the order of 10,000 angstroms.
Because of this size difference, there is a much greater micelle surface
area than monomer droplet surface area.
Illustration of a micelle
