Modification of Polysaccharides and their Use as Pickering Emulsifiers in Oil/Water Emulsions, Miniemulsion, and Microsuspension Polymerization
Emulsion and suspension systems are the basis for the formulation of a wide variety of products including pharmaceuticals, consumables such as food, personal care or cosmetics and commodity materials such as adhesives, paints or synthetic rubbers. These systems require surface active agents (surfactants) or emulsifiers to prevent phase separation. Conventional emulsifiers are derived from petroleum and can entail disadvantages such as ecotoxicity, skin irritation issues or formulation foaming problems, so the design of new emulsifiers that are biodegradable, non-toxic and, at the same time superior in performance, is worthwhile. Pickering emulsifiers (PE) obtained from polysaccharides are a promising new type of emulsion stabilizer and are the focus of this work. First, the hydrophobization of polysaccharide-based materials was investigated. Cellulose nanocrystals (CNC) were grafted with CO2-responsive polymers prepared by and grafted via reversible-deactivation radical polymerization (RDRP). The effect of the graft molecular weight on the graft density was investigated in detail and was shown to have a direct influence on the dispersion behavior of the grafted CNC. The results suggested that a particular pKaH, minimum graft density and graft length are required to effectively switch the CNC from a hydrophilic to a hydrophobic state. Graft molecular weights of ~5-10 kg/mol and graft densities between 2-6 x 10-2 chains/nm2 yielded CO2-switchable CNC which shuttled reversibly into the organic phase under N2 and into the aqueous phase under CO2. The modified polysaccharides were then employed to stabilize oil- water emulsions and in subsequent polymerizations. The surface and interfacial properties of the PE, the shelf life stability and droplet sizes of the miniemulsions were studied in detail. The chain length of the grafted polymer, the graft densities, the degree of substitution and the concentration of PE were correlated to the properties of the final Pickering emulsions. The type of initiator and type of PE was shown to directly affect the miniemulsion and microsuspension polymerization kinetics, polymer particle size, and molecular weight distribution. It was found that very small amounts of PE are required to give stable emulsions, which, along with the sustainability aspect, could make these materials highly attractive for industrial applications.