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Abstract
A reactive cosurfactant (stearyl methacrylate (SMA) or dodecyl
methacrylate (DMA)) was proposed for
stabilizing styrene miniemulsion polymerization . Polystyrene
latex particles were produced via both
monomer drolet nucleation and homogeneous
nucleation in the miniemulsion polymerization using cetyl
alcohol (CA) or DMA as the cosurfactant,
with the result of a quite broad particle size distribution. On
the other hand, the miniemulsion
polymerization with hexadecane (HD) or SMA showed a
predominant monomer droplet nucleation. The resultant particle
size distribution was relatively narrow. In
miniemulsion polymerization, the less hydrophobic DMA is
similar to CA, whereas the more hydrophobic SMA is similar to
HD. A nonionic surfactant,
nonylphenol polyethoxylate with an average of 40 ethylene oxides
per molecule (NP-40), in combination
with a reactive cosurfactant such as DMA or SMA
was also used to prepare stable styrene miniemulsions in this
study. The rate of Ostwald ripening for
these miniemulsions in the deceasing order is: CA > DMA > HD
∼ SMA. This trend correlates well with the water solubility of
cosurfactants. Polystyrene latex
particles were produced via both monomer droplet nucleation and
homogeneous (or micellar) nucleation for the system exhibiting
strong Ostwald ripening and/or
droplet coalescence during the very early stage of
polymerization.. On the other hand, monomer
droplet nucleation becomes more important for the system
showing weak Ostwald ripening. Temperature has an important
influence on the shelf- life of
miniemulsion and subsequent polymerization. This is attributed
to the fact that hydrogren bonds between the
polyethylene oxide part of NP-40 and water are quite
sensitive to changes in temperature.
The water solubility data of cosurfactants are not available in
the literature except HD. Based on a simple
procedure established in this work, the solubilities of these
cosurfactants in water were estimated by the extended Lifshitz-
Slyozov-Wangner (LSW) theory. The
water solubility of cosurfactant in the decrasing order is:
CA(5.77x10-8 ml/ml) > DMA(1.38x10-8 ml/ml) > SMA (3.23x10-9 ml/
ml) > HD (1.14x10-9 ml/ml) . In addition, the
LSW theory can be used to calculate the monomer droplet-water
interfacial tension. This feature may provide
useful information on the degree of droplet
surface coverage by surfactant. The rate of Ostwald ripening for
the miniemulsion stabilized by SDS in
combination with dye (Blue 70) is about 1 order of magnitude
greater than the others stabilized by DMA or SMA.. The data of
UV absorption for the final products indicate that about
50% ∼ 70% of polystyrene latex particles produced via
monomer droplet nucleation in the miniemulsion polymerization.
n the miniemulsion polymerization.
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