Chemistry Reference
In-Depth Information
This results in formation of primary radicals. The primary radicals in turn react with the monomer
molecules dissolved in the water (though their number may be quite small). Additional monomer
molecules may add to the growing radicals in the water until the growing and propagating chains of
free radicals acquire surface-active properties. At that stage, the growing radicals consist of inorganic
and organic portions:
These growing radical-ions tend to diffuse into the monomer-water interfaces. The probability
that the diffusion takes place into monomer-swollen micelles rather than into monomer droplets
is backed by the considerations of the relative surface areas of the two. There are on the average
10
18
micelles in each milliliter of water. These are approximately 75
¯
in diameter and each
swollen micelle contains on the average 30 molecules of the monomer. At the same time, the
diameters of the monomer droplets are approximately 1
. and it is estimated that there are only
approximately 10
12
such droplets per milliliter of water. Thus, the micelles offer 60 times more
surfaces for penetration than do the droplets. The initiating radicals are almost always generated in
the water phase. After formation in the water phase, a number of free radicals may be lost due to
recombination. Termination is also possible after reaction of free radicals with some of the
monomers dissolved in the water.
Several theories tried to explain the entry process. Thus, a “diffusion control” model [
307
,
308
]
supposes that diffusion of aqueous-phase radicals into the particle surface is the rate-controlling step
for entry. Another theory suggests that displacement of surfactant from the particle surface is the rate-
determining step [
309
]. A third one assumes that the entry can be thought of as a colloidal interaction
between a latex particle and primary phase oligomeric aqueous-phase radical. These are the radicals
formed through reactions of initiating radicals and monomer molecules dissolved in water [
310
]. The
most accepted entry model appears to be the “control by aqueous-phase growth” model of Maxwells
et al. [
311
]. This theory postulates that free radicals generated in the aqueous phase propagate until
they reach a critical degree of polymerization (let us call it
m
), at which point they become surface-
active and their only fate is irreversible entry into a latex particle; the rate of entry of
z
into a
particle is assumed to be so fast as not to be rate-determining. An efficiency of less than 100% arises if
there is significant aqueous-phase termination of the propagating radicals.
The entry model of Maxwells et al. was derived from and/or supported by data on the influence of
particle surface characteristics (charge, size) on the entry rate coefficient [
312
]. It was assumed that
the aqueous radicals became surface active when the degree of polymerization reached 2-3. This was
based on thermodynamic considerations of the entering species.
Further data on the Maxwell et al. entry model was obtained by Gilbert and coworkers [
313
] who
studied the effects of initiator and particle surface charges. They obtained kinetic data for radical
entry in the emulsion polymerization of styrene and concluded that their data further supports the
Maxwell et al. entry model and refutes the alternative models mentioned above.
Once the radicals penetrate the micelles, polymerization continues by adding monomers that are
inside. The equilibrium is disturbed and the propagation process proceeds at a high rate due to the
concentration and crowding of the stabilized monomers. This rapidly transforms the monomer-
swollen micelles into polymer particles. The changes result in disruptions of the micelles by growths
from within. The amount of emulsifier present in such changing micelles is insufficient to stabilize the
polymer particles. In trying to restore the equilibrium, some of the micelles, where there is no
polymer growth, disintegrate and supply the growing polymer particles with emulsifier. In the process
many micelles disappear per each polymer particle that forms. The final latex usually ends up
containing about 10
15
polymer particles per milliliter of water. By the time conversions reach
10-20% there are no more micelles present in the reaction mixtures. All the emulsifier is now adsorbed
on the surface of the polymer particles. This means that no new polymer particles are formed.
All further reactions are sustained by diffusion of monomer molecules from the monomer droplets
into the growing polymer particles. The amount of monomer diffusing into the particles is always in
excess of the amount that is consumed by the polymerization reaction due to osmotic forces [
297
].
z
-
mers
