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.
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