Chemistry Reference
In-Depth Information
power, although no clear-cut correlations were established between the chemical
structure of the polymer and its effect on solubilization. In general, larger effects
are observed for aromatic than aliphatic materials. The present state of knowledge
in this area is not sufficient to allow quantitative predictions to be made about the
solubilizing properties of surfactant-polymer complexes based solely on chemical
composition, although it is known that their effectiveness depends on the nature of
the polymeric component and the polymer : surfactant ratio.
7.6. SURFACTANT-POLYMER INTERACTIONS IN EMULSION
POLYMERIZATION
Surfactant-polymer systems have additional technological significance since sur-
factants are normally used in the emulsion polymerization of many materials,
often involving the solubilization of monomer in micelles prior to polymerization
and particle formation. Surfactants have also been shown to increase the solubility
of some polymers in aqueous solution. The combined actions of the surfactant as a
locus for latex particle formation (the micelle) in some cases, particle stabilization
by adsorbed surfactant, and as a solubilizer for monomer permit us to expect quite
complex relationships between the nature of the surfactant and that of the resulting
latex.
Within a given surfactant class, it is usually found that materials with high
cmc's produce latexes with larger particle sizes and broader size distributions.
For example, the particle sizes of polyethylacrylate latexes prepared with sodium
alkylsulfates ranged from 500 nm for the octyl to 50 nm for the dodecyl surfactant.
No similar trend was found for nonionic POE surfactants as a function of POE
chain length.
The ability of surfactants to form complexes with polymer chains may also
affect the ultimate properties and stability of the resulting polymer, especially
when the macromolecule exhibits some affinity for or reactivity with water. Perhaps
the best documented case of the effect of surfactant on latex stability is that of poly-
vinyl acetate. The stability of PVAc latexes has been found to vary significantly
depending on the surfactant employed in its preparation. It has also been found
that PVAc could be dissolved in concentrated aqueous solutions of SDS and that
it did not precipitate on dilution. The results suggest that, in this case at least, solu-
bilization did not occur in the micelle, but that extensive adsorption of surfactant
onto the polymer chain was required. They also indicate that a strong, stable PVAc-
SDS complex is formed that produces a water-soluble structure that is essentially
irreversible, unlike normal micelle formation. Cationic and nonionic surfactants
had little or no solubilizing effect under identical conditions, indicating the specific
nature of many, if not most, polymer-surfactant interactions.
PVAc is, of course, the precursor for the preparation of polyvinyl alcohol (PVA).
The fact that surfactants such as SDS can promote the solubilization of polyvinyl
acetate has been used to explain the observed increase in the rate of hydrolysis of
PVAc polymers prepared with that surfactant relative to materials prepared with a
