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systems, the intended vinyl acetate/butyl acrylate shell polymer was made in the
first polymerization stage. The surfaces of these particles were adjusted so as to
have a lower interfacial tension against water by copolymerizing with an ionic
comonomer, vinyl sulfonic acid. The core polymer, poly(vinyl acetate), was made
in the second-stage polymerization and its surface was rendered less hydrophilic
than in normal persulfate-initiated polymers by using a nonionic initiator. Such an
initiator would not have produced a stable poly(vinyl acetate) latex and so the
second-stage core polymer was made by performing a batch polymerization after
adding all the vinyl acetate monomer to the preformed shell latex polymer. A par-
allel method was used for the methyl methacrylate-based particles, except that the
ionic comonomer used in the shell polymer was methacrylic acid, which copoly-
merizes better with the other monomers used here.
It can be shown theoretically that the relative amounts of the two monomers
in a two-stage emulsion polymerization can affect the final particle structure
[19]
,
with core-shell morphology being favored thermodynamically, as the amount of
second-stage polymer is increased.
10.2.5.2
Influence of Polymerization Kinetics on Latex Particle Structure
Although the lowest free-energy structure is the most stable phase arrangement in
heterogeneous polymer particles, kinetic factors are also important because the
polymeric phases have limited mobility and the most favorable thermodynamic
state may not be achieved during the course of the polymerization or subsequent
storage. Note, in this connection, that thermodynamically unprofitable structures
are prone to rearrange on storage for long periods or under warmer conditions.
The manner in which the second-stage monomer is added to the polymeriza-
tion system has a strong influence on the final structure of the composite parti-
cles. Semibatch (or semicontinuous) operation is preferable to batch operation
when it is desired to have the second-stage polymer on the outside of the final
particles. In a batch second-stage polymerization all the monomer is added to the
first-stage particles at the beginning of the reaction. The second-stage monomer
has much higher mobility than the second-stage polymer; the second-stage mono-
mer is more likely to polymerize with the first-stage particles if this produces an
energetically favored state. Also, the plasticizing effect of the second-stage mono-
mer in the first-stage polymer enhances the ability of the latter molecules to rear-
range to the exterior of the particles, if this should reduce the overall surface
energy of the system.
When the second-stage monomer is added gradually to the first-stage polymer
particles the level of monomer can be kept to a minimum during the polymeriza-
tion. Thus, if the energetics of the system favor the second-stage polymer in a
core arrangement inside the first-stage polymer, batch polymerization of the shell
monomers will decrease the kinetic barrier to this inversion, while semibatch or,
better even, starve
feed polymerization imposes a higher kinetic barrier. On the
other hand, when it was desired to produce inverted structures, as in
[25]
, the
