Multiphase Polymer Dispersions and Nanocomposites by Catalytic/Free Radical Emulsion Polymerization

Emulsion polymerization is one of the most important and versatile polymerization processes. About 10 million tonnes of polymer latices are produced annually for a variety of applications, such as coatings and paints. A key step in most applications is film formation upon evaporation of the dispersing medium, rendering aqueous dispersions particularly environmentally benign. Polymer dispersions containing not only one but two or more distinctly different types of polymer phases are frequently used in many applications. Multiphase latices can possess a multitude of morphologies, ranging from coreshell structures to hemispherical particles, ‘‘raspberry’’ structures (i.e. large particles with small particles on their surface) and separate particles of different polymers. Thermodynamic and kinetic techniques can determine the morphology of an individual system under a given set of polymerization conditions. While it is evident that various factors such as the surface tension of the various polymer phases towards water and towards each other, optionally cross-linking, the polymerization temperature, and the type of polymerization process play a role, the synthesis of multiphase latices is still empirical to a large degree. Polyolefin latices are attractive for a number of reasons. Amongst others, the raw materials are obtained directly from cracking of hydrocarbon feedstock, without the need for further energyand raw-material-consuming conversion to other monomers (such as styrene, acrylates or vinyl acetate), making polyolefin latices both economically and environmentally attractive. The crystallinity of a given polyolefin, for example, polyethylene, can be varied over a wide range. As for the accessibility of polyolefin latices, free radical emulsion polymerization of ethylene suffers from the necessity of working at very high pressures (2000 bar) and a low variability regarding tailoring of the polymer microstructure. The preparation of secondary dispersions usually requires large amounts of solvents, and is also viable only for low-molecular-weight polymers. Moreover, the necessity of a second dispersing step is Summary: Free radical emulsion polymerization of styrene (S) or butyl acrylate (BA) in the presence of latices of linear polyethylene (PE) prepared by catalytic emulsion polymerization affords colloidally stable multiphase latices. Coagulation of a PE/PS latex affords nanocomposites composed of small PE phases dispersed in a PS matrix, as evidenced by the large supercoolings of PE crystallization (by DSC). TEM of PE/PBA latices indicates a PBA phase around the PE particles under the emulsion polymerization conditions investigated. Films formed from these dispersions exhibit homogeneously dispersed PE particles. Multiphase latices are obtained by free radical emulsion polymerization of butyl acrylate in the presence of latices of linear polyethylene (PE) prepared by catalytic emulsion polymerization.