Chemistry Reference
In-Depth Information
water phase ratios, where the relative concentration of the more soluble mono-
mers in the aqueous reaction medium will greatly exceed that in the bulk organic
phase.
10.2.5
Heterogeneous Polymer Particles
[15,16]
The copolymer composition may drift during the course of an emulsion copoly-
merization because of differences in monomer reactivity ratios or water solubili-
ties. Various techniques have been developed to produce a uniform copolymer
composition. The feed composition may be continuously or periodically enriched
in a particular monomer to compensate for its lower reactivity. A much more
common procedure involves pumping the monomers into the reactor at such a
rate that the extent of conversion is always very high (
about 90%). This way,
the polymer composition is always that of the last increment of the monomer
feed.
This section, by contrast, focuses on techniques to synthesize latex particles
whose composition varies radially. Such materials are industrially important and
this topic is also of interest because its consideration provides useful insights into
emulsions technology. Particles with nonuniform structures are key ingredients in
surface coatings, adhesives, opacifiers, impact improvers for plastics, waxes,
chromatographic packings, components of polymer alloys, xerographic toners,
substrates for polypeptide syntheses, and other applications. Their sizes may be
controlled in a range from
.
100 nm to several microns. These products are often
called core-shell polymers, but their structures may vary from a “simple” core
and shell to a multilayer onion-like structure. Particle shapes are usually spherical,
but myriad other contours have also been observed.
Inhomogeneous particles can be made by dispersion polymerization and other
methods, but the most versatile processes for the production of such controlled
structures are variations of emulsion polymerization techniques. In essence, the
desired particle morphology is produced by sequential polymerizations of layers
of designed compositions. The aim in staged polymerizations that are intended to
produce particles with nonuniform structures requires that the polymers produced
in later stages envelope (or are enveloped by) polymer made in earlier process
steps. The whole operation is defeated if later-stage polymers form separate parti-
cles. For this reason, the amount of surfactant must be regulated in order to avoid
providing enough free emulsifier to stabilize the formation of such new particles.
At the same time, there must be enough surfactant to prevent coagulation of the
emulsion, in which the interfacial area between polymers and water phases is
growing. This is most conveniently accomplished by adding the later-stage mono-
mers
,
as
emulsions
containing the
required surfactant
at
the
calculated
concentration.
Practical applications do not always require tight control of particle size distri-
butions, but the necessity for avoiding generation of new particles at any stage
after the first polymerization process may necessitate this precaution. Analysis of
