Theory
All colloids prepared in this thesis were synthesized by either miniemulsion or surfactant free emulsion polymerization. In the next chapters, the mechanisms, differences and benefits of the two types of heterophase polymerizations will be discussed.
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Theory
3.1 Synthesis of Polymeric Colloidal Particles All colloids prepared in this thesis were synthesized by either miniemulsion or surfactant free emulsion polymerization. In the next chapters, the mechanisms, differences and benefits of the two types of heterophase polymerizations will be discussed.
3.1.1 Miniemulsion Polymerization A miniemulsion is a heterophase system that consists of small droplets (*30– 500 nm) of one liquid in an (immiscible) other. One may distinguish direct miniemulsions (oil in water) and indirect miniemulsions (water in oil) [1, 2]. Only direct miniemulsions will be discussed here as they were exclusively applied in this thesis. The miniemulsion is created by applying high shear forces (e.g. ultrasound or high pressure homogenization) [1–3] to a two phase mixture of oil (in a polymerization process typically the monomer) and water (Fig. 3.1). Though a miniemulsion is not in the thermodynamic equilibrium state, the droplets are critically stabilized. This stabilization requires the addition of two types of supplementary molecules to the emulsion. The water phase contains surfactant molecules while a second stabilization agent (costabilizer), termed ultrahydrophobe is added to the oil phase. The ultrahydrophobe is by definition the least soluble substance in the water phase. Two degradation mechanisms exist for a non-stabilized emulsion: collision and subsequent coalescence of individual droplets and Ostwald ripening (Fig. 3.2) [147–148]. The latter describes the growth of larger droplets on the account of smaller ones by diffusion of oil-phase molecules through the water phase. This is caused by the higher Laplace pressure in the smaller droplets that is defined as the difference of pressure (DP) between N. Vogel, Surface Patterning with Colloidal Monolayers, Springer Theses, DOI: 10.1007/978-3-642-35133-4_3, Springer-Verlag Berlin Heidelberg 2012
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3 Theory
Fig. 3.1 Schematic representation of the miniemulsion process
Fig. 3.2 Degradation of an emulsion by Ostwald ripening and prevention of the process in a miniemulsion. a Ostwald ripening leads to the growths of bigger particles on the account of the smaller particles. b By addition of an ultrahydrophobe, an osmotic pressure is build up upon diffusion that prevents the ripening process
the inside of a droplet (Pinside) compared to the outside (Poutside) and is given by the following relation: DP ¼ Pinside Poutside ¼
c R
As can be seen from the equation, the Laplace pressure is proportional to the surface tension of the liquid c and inverse proportional to the radius of the droplet R. In a miniemulsion, both degradation mechanisms are effectively counteracted. Coalescence of individual droplets is prevented by steric- or electrostatic repulsion caused by the surfactant molecules present at the droplet surface. The addition of the ultrahydrophobe induces an osmotic pressure that counteracts the Laplace pressure responsible for Ostwald ripening processes. Figure 3.2 schematically illustrates the process. Without an ultr
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