Synthesis of Polymer Nanoparticles

The focus in this section will be laid on the synthesis of polystyrene particles via emulsifier-free emulsion polymerization.³⁰ This method was used to obtain PS latex beads in the size range of up to ~450 nm. Larger polymer particles were synthesized by using dispersion polymerization.

The schematic illustration of an emulsifier-free emulsion polymerization is depicted in Figure 3.1.

Figure 3.1. Schematic illustration of the emulsifier-free emulsion polymerization using AIBA as an initiator. Adapted from Vollmert⁹⁹ with permission from Springer Verlag.

The mechanism of the emulsifier-free emulsion polymerization was studied by Hansen and Ugelstad.¹⁰⁰ Macroscopic monomer droplets emerge by vigorous stirring in the continuous water phase. Due to the low, but finite solubility of the monomer, some molecules are dissolved in the aqueous phase (1). The water soluble initiator (e.g., AIBA) is thermally decomposed into charged radicals (2).

After initiation in the water phase (3), the radicals add monomer units, until a critical chain length is reached. These oligomers with a charged end group become

water insoluble, collapse into globules and form particle nuclei (4). This step is called homogeneous nucleation. Several of such nuclei assemble to form a stable primary particle, which is stabilized by ionic groups at the interface (5). In the following, the polymerization proceeds by diffusion of the monomer from the monomer droplets (6), forming a latex particle (7).

Emulsifier-free Emulsion Polymerization - Experimental Procedure

The synthesis was carried out in a three-necked flask, equipped with a reflux condenser and a gas inlet, under a slight argon flow. The flask was charged with 230 mL water, 26 mL styrene, 0-300 µL MTC, and 1.8 g PVP K-30. The reaction mixture was heated to the reaction temperature of 70 °C at a stirring speed of 850 rpm. After an equilibration time of 30 min, 0.6 g AIBA dissolved in 10 mL water, were added to initiate the polymerization. After nucleation, the stirring speed was reduced to 450 rpm. The reaction was carried out overnight. To terminate the polymerization, the mixture was exposed to ambient atmosphere.

Emulsifier-free Emulsion Polymerization - Results and Discussion

The emulsifier-free emulsion polymerization offers the opportunity to tune the overall particle size in a broad range. Therefore different parameters are adjusted:

the concentration of the (co)monomer and the initiator, the temperature, the stirring rate, the ionic strength, and the amount of steric stabilizer. Without disregarding the monodispersity, the best results can be affected using the comonomer concentration.

The particle diameter as a function of the comonomer concentration is shown in Figure 3.2.

Figure 3.2. The size of the polystyrene particle as a function of the volume of the comonomer solution MTC. The synthesis specifications are described in the main text. The black squares represent a down-scaling of the synthesis by the factor of 0.5. The right column shows scanning electron microscopy (SEM) images of the corresponding particles.

By adjusting the comonomer volume between 0 and 300 µL, the particle size can be varied between 130 and 440 nm with a low polydispersity of less than 5 %. The higher the comonomer concentration, and thus, the electrostatic stabilization is, the smaller the particle diameter gets. Nevertheless, larger particles are hardly achievable using emulsifier-free emulsion polymerization. Therefore, the dispersion polymerization technique was used.

Basics of Dispersion Polymerization

The dispersion polymerization is a type of precipitation polymerization. While the monomer and initiator are soluble in the reaction medium, the formed polymer is insoluble. The dispersion polymerization takes place in a homogeneous medium of monomers with a free-radical initiator and a polymeric stabilizer dissolved in an appropriate solvent. At an elevated temperature, the initiator decomposes, and

the radical adds solubilized monomer units, forming oligomeric radicals. This proceeds until a critical chain length is reached, leading to a phase separation and the formation of primary particles. This step is termed nucleation. In the following, the particle formation step starts. The particle nuclei are unstable and rapidly aggregate with each other. The formed particles are stabilized by the dispersing agent which adsorbs at the interface. In this step, all of the oligomeric radicals, and nuclei are captured by the mature particles and the particle formation step is completed. Subsequently, the polymerization proceeds within the particles, leading to monodisperse polymer latex beads.

Dispersion Polymerization - Experimental Procedure

In the following, the dispersion polymerization is described for the particles F, resulting in a particle diameter of 955.8 ± 24.8 nm (see Table 3.1).

Table 3.1. Parameters which can be tuned in a dispersion polymerization to change the particle diameter.

The dispersion polymerization was carried out in a single-neck flask connected to a KPG stirrer. First, 130 mL ethanol, 14 mL water, 10 mL styrene, and 5 g PVP were purged with argon, while heating the reaction mixture to 75 °C. After an equilibration time of 30 min, the gas inlet was removed, and 0.136 g AIBN dissolved in 6 mL ethanol were added to initiate the polymerization. At once, the stirrer was set to 60 rpm. After 1.5 h, 400 µL MTC were added. The reaction was

carried out overnight. To terminate the polymerization, the mixture was exposed to ambient atmosphere.

Dispersion Polymerization - Results and Discussion

The particle size can be tuned by several parameters in a dispersion polymerization.^101-102 In this thesis, the ethanol/water-ratio, the amount of comonomer, and initiator was varied, as depicted in Table 3.1. This enables the tuning of the particle diameter in the range of 490 and 1280 nm. The SEM images of the particles A, D, F, and G are shown in Figure 3.3.

Figure 3.3. Scanning electron microscopy (SEM) images of polystyrene particles, synthesized via dispersion polymerization.

Larger particle sizes can be achieved by reducing the comonomer (A and C, B and D) and the initiator concentration (E and F), and by reducing the ethanol/water ratio (F and G). The increase of the comonomer concentration increases the amine functional groups at the surface of the PS spheres, and therefore the surface electric charge.¹⁰¹ This reduces the size of the particles. The increase of the initiator concentration reduces the number of free radicals in the polymerization. This leads to lower molecular weight oligomers, which are more stable in the medium. Thus, fewer nuclei are generated, which grow to larger polymer spheres.¹⁰³ By changing the ethanol/water ratio, the polarity of the dispersion medium increases, leading to a reduction of the size of the polymer spheres.¹⁰¹

3.1.2 Synthesis of Hollow Silica Nanoparticles