Miniemulsions, consisting of submicron droplets of very hydrophobic lauryl methacrylate or
4-tert-butyl styrene, are successfully polymerized using water-soluble sodium persulfate.
Monitoring the calorimetric profile as well as the droplet and particle size distribution with
conversion manifests a process of monomer redistribution,
droplet disappearance, and narrowing of the
particle size distribution. The observed reaction
characteristics could be modeled adequately, using
thermodynamic principles. The results of the work
presented do not only have predicting value, but also
enfeeble the idea of a one-to-one translation of
monomer droplets into polymer particles in miniemulsion
polymerization.
1. Introduction
Miniemulsion polymerization is a heterogeneous polymerization
technique in which radical entry takes place in
monomer droplets rather than in monomer-swollen
micelles.[1–3] Monomer droplets, usually in the order of
50–500 nm, are created by the application of high shear, via
e.g., ultrasonication,[4] high shear homogenization,[5] or
more recently, static mixing.[5,6] This additional step,
reducing its attractivity for commercial use, is counterbalanced
by the claimed advantages of miniemulsion
polymerization, such as the use of very hydrophobic
monomers[7,8] or the ability to obtain higher solid
contents.[9,10] A property, often ascribed to miniemulsion
polymerization, is the so-called one-to-one copy of monomer
droplets into polymer particles. This means that
droplets do not change size, size distribution, or composition
upon polymerization. Therefore, each monomer
droplet would act as an individual nanoreactor, i.e., a
segregated system. The validity of this feature is, however,
highly disputable. Landfester et al.[11] claim, by using smallangle
neutron scattering, to have proven that there is no
difference between droplets and particles in the miniemulsion
polymerization of styrene (STY). Cheng et al.[12]
conclude, based on dynamic light scattering measurements
(DLS) that droplet identity preservationwasachieved in STY
miniemulsions. Nevertheless, the idea that an exact copy
is an exception rather than a common feature is much
more widespread. Miller et al.[13,14] demonstrated that the
average particle diameter in a miniemulsion polymerization
depends on the initiator concentration and increases with increasing conversion. This observation has been
made by Choi et al.[15] too, who also showed that a
hydrophobic initiator (2,20-azobis-(2-methyl butyronitrile)
orAMBN)gave broader particle size distributions (PSD)than
hydrophilic potassium persulfate. Lin et al.[16] showed that
the polydispersity of the particle size distribution narrows
with increasing conversion. Yildiz et al.[17] have shown that
the correspondence between droplets and particles in a
lauryl methacrylate (LMA) miniemulsion polymerization is
much better with AIBN (azobis-isobutyronitrile) initiation
than with potassium persulfate initiation. In the latter case,
an increase in particle number was observed, which was
attributed to droplet budding, i.e., the splitting of nucleated
particles throughout polymerization. Mixtures of separately
prepared miniemulsion droplets, each consisting of only
one monomer, showed a copolymer after polymerization
rather than two separate homopolymers.[18,19] Ugelstad
et al.[20] derived a theoretical relationship between the
particle diameter andthe volumetric growth rate for seeded
emulsion systems, thereby showing that concentration
differences between differently sized particles can occur
during polymerization, possibly resulting in monomer
redistribution.
