Chemistry Reference
In-Depth Information
micellar and dissolved soap, and micelles will begin to disintegrate as the concen-
tration of molecularly dissolved emulsifier is restored to its equilibrium value.
Thus, the formation of one polymer particle leads to the disappearance of many
micelles. The initial latex will usually contain about 10
18
micelles per milliliter
water, but there will be only about 10
15
particles of polymer in the same volume of
the final emulsion. When all the micelles have disappeared, the surface tension of
the system increases because there is little surfactant left in solution. Any tendency
for the mixture to foam while it is being stirred decreases at this time.
The transfer of free radicals out of the particles is assumed to be negligible.
Also, the rate of mutual termination of two radicals inside a particle is taken to be
very much faster than the free-radical entry rate, as is reasonable for small particles.
Then each particle will contain either zero or one radical. This radical will
add monomer until another radical enters the particle and terminates the kinetic
chain. The particle will be dormant until the cycle is started again by the entry of
another radical. Thus, on the average, each particle will be active for half the
time, or half the particles will be active and half dormant at any instant. Since the
number of particles is fixed in interval II, the addition of more initiator then will
have no effect on the polymerization rate, but the average lifetime of each grow-
ing macroradical and the mean molecular weight will be reduced. The average
number of radicals per particle will be equal to 0.5, if chain transfer can be
neglected. Transfer reactions are important in emulsion polymerizations, however,
because small free radicals, e.g., from transfer to monomer, are mobile enough to
diffuse out of the polymer particles. Hence, the mean number of radicals per par-
ticle is
C
M
values.
The foregoing mechanism is amenable to mathematical analysis, with the
salient results that during ideal inter
val
II polymerization, the rate of reaction is
proportional to [I]
2/5
and
,
0.5 for vinyl acetate and other monomers with significant
3/5
, while DP
n
depends on
3/5
and [I]
2
3/5
. (Here [I] is
S
S
the initiator molar concentration and
is the weight concentration of surfactant.)
In conventional solution, suspension, or low-conversion bulk free-radical reac-
tions, the rate of poly
mer
ization depends on [I]
1/2
while DP
n
is proportional to
[I]
2
1/2
. In these cases DP
n
cannot be increased at given [M] without decreasing
R
p
. In emulsion polymerization, however, both
S
R
p
and DP
n
can be changed in
parallel by controlling the soap concentration.
The Harkins
Smith
Ewart theory predicts that the number of polymer parti-
3/5
and [I]
2/5
. This is observed for some
batch polymerizations, as mentioned. In general,
cles formed,
N
, will be proportional to
S
N~ S
x
, but the value of the expo-
nent depends on the range of soap concentrations and the monomer solubility in
water. This topic is of more academic than practical interest, however, because
most useful polymerizations are not batch operations.
10.2.2
Role of Surfactants
Surfactants are employed in emulsion polymerizations to facilitate emulsification
and impart electrostatic and steric stabilization to the polymer particles. Steric
