Chemistry Reference
In-Depth Information
polymers may also compete for adsorbed surfactant. If the later-stage polymer is
more hydrophobic than the preformed polymer, some of the surfactant adsorbed
on the latter may desorb and stabilize the formation of particles of the later stage
polymer. This effect, which is consistent with the thermodynamic factors men-
tioned above, results in the production of some second crop particles of the later
stage material.
One of the difficulties is that there is no convenient theory for selecting the
appropriate surfactants
a priori
. The simplest procedure is to try to match the
HLB (hydrophile
lipophile balance) character of the surfactants to the polarity of
the particular polymers. Tabulated HLB values are available
[23,24]
(the higher
the HLB number the more hydrophilic is the soap), but the rating is essentially
empirical and effective use requires some experience and intuition.
Mention has been made of the fact that the polar character of polymer surfaces
is strongly affected by the ionic polymer end groups that are residues of initiator-
derived ion radicals, when persulfates are used in emulsion polymerizations.
Variation of the initiator type between those that yield ionic and nonionic end
groups is an effective way to control particle stability and avoid complications
due to migration of surfactant from one polymer surface to another
[25]
. This
method can also be supplemented by copolymerization with polar monomers to
affect surface hydrophilicity.
Poly(methyl methacrylate) (PMMA)/polystyrene (PS) latex particles made
using an ionic initiator in the first (PMMA) stage and a nonionic initiator in the
second (PS) polymerization have a strong tendency to invert to a PS core inside a
PMMA shell, because the PMMA is more hydrophilic than PS and as a result of
the presence of ionic groups from the initiator on the PMMA surface. The core-
shell structure for this system can be made more stable by using a nonionic initia-
tor, like an azo compound, in the first-stage PMMA polymerization and an ionic
initiator in the second stage, PS polymerization. This procedure may, however,
necessitate the presence of an anionic surfactant in the first-stage reaction, for
latex stability.
None of the factors that affect particle morphology operates alone. In particu-
lar, the mode of monomer addition is an interacting factor. This is illustrated by
procedures used to produce core-and-shell polymers for use in architectural paints
[26]
. Polymers used for this purpose are primarily copolymers of butyl acrylate
with either vinyl acetate or methyl methacrylate. The goal here was to make parti-
cles with conventional film-forming polymer shells and cores comprised of less
expensive monomers than were used in the shells. In practice, this could mean
poly(vinyl acetate) cores inside vinyl acetate/butyl acrylate shells or the same or
poly(methyl methacrylate) cores surrounded by methyl/methacrylate/butyl acry-
late shells. In all cases, the core polymers are more hydrophilic than the shell
materials. The procedure used to sidestep this difficulty illustrates many of the
principles discussed in this section.
Thermodynamically stable structures were made by modifying the surface
characteristics of the two polymer phases. To illustrate with respect to one of the
