Chemistry Reference
In-Depth Information
interval begins when the monomer droplets disappear and ends when the poly-
merization reaction is completed.
In the ideal case in which both the monomer and emulsifier have negligible
water solubility, polymerization will begin in the micellar phase although radicals
are generated only in the aqueous phase. Most of the monomer is initially present
in the relatively large droplets, but there are normally about one million times as
many micelles as droplets. Thus, the micellar surface area available to capture
radicals from the aqueous phase is much greater than that of the larger monomer
droplets. (The soap/water interfacial area is 8
10
5
cm
2
/cm
3
aqueous phase if
there are 10
18
micelles in this volume, and each micelle has a diameter of only
50 cm
2
.) The droplets act mainly as reservoirs to supply the polymerization loci
by migration of monomer through the aqueous phase.
The foregoing considerations of relative capture surfaces and the available
experimental evidence rule out the monomer droplets as the polymerization loci.
The most realistic alternative assumption to micellar initiation is that the first
addition of monomer to primary radicals occurs in the aqueous phase. The oligo-
meric free radicals that are generated would be
2
O
3
SOMM
3
, if persulfate
initiators are used. These oligomeric species contain a polar end and a hydropho-
bic tail. According to this mechanism, they could be expected to add monomer
units until they encounter phase interfaces (current ideas include alternative fates
for these oligomers, as described later). For reasons mentioned, such interfaces
are likely to be those between the water and the monomer-swollen micelles.
Oligomeric radicals would be readily incorporated in a micelle and would initiate
polymerization of the monomer which is already solubilized therein. The micelle
is said to be “stung” when it is first entered by such an oligomeric radical or a
primary radical derived from initiator decomposition. Polymerization proceeds
rapidly inside a stung micelle because the monomer concentration there is large,
as noted earlier. The micelle is rapidly transformed into a small monomer-
swollen polymer particle. All the monomer which is needed for this growth pro-
cess is not contained within the initial micelle but diffuses to it from the droplets.
The flux of monomers diffusing from the droplets to the particles will decrease
as the former reservoirs are depleted, but reasonable calculations lead to the expec-
tation that the arrival rate at the particle
...
M
water interface will exceed the usage
rate even when more than 90% of the monomer has already left the droplets.
If the monomer is a good solvent for the polymer, the latex particles might be
assumed to expand indefinitely because of inhibition of monomer. An equilibrium
monomer concentration and swelling equilibrium is reached, however, because
the free energy decrease due to mixing of polymer and monomer is eventually
balanced by the increase in surface free energy which accompanies expansion of
the particle volume.
Once a micelle is stung, polymerization proceeds very rapidly. The particle can
accommodate more monomer as its polymer content increases and the water
polymer interfacial surface increases concurrently. The new surface adsorbs emul-
sifier molecules from the aqueous phase. This disturbs the equilibrium between
