Functional polymer microspheres

A methodological description of heterogeneous polymerization processes, including suspension, emulsion, dispersion, and precipitation polymerization is presented. The discussion focuses on the initial state of the polymerization mixture, mechanism of particle formation, and the shape and size of polymer particles produced in different heterogeneous polymerization systems. The dependence of particle size and morphology on manufacturing parameters such as emulsifier, stabilizer, reactor design, and stirring speed is discussed. Special topics, including emulsifier-free (soapless) emulsion polymerization, seeded ponymerization, and the formation of core-shell particles are also briefly covered.

Suspension, emulsion, and dispersion polymerization : a methodological survey

Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties.

Self-assembly and rheology of ellipsoidal particles at interfaces.

A new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.

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Shear thickening (dilatancy) in concentrated dispersions

This article reviews the phagocytosis of polymer microspheres by macrophages with respect to the size and the surface characteristics of the microspheres. Since few investigations have been carried out using well-characterized polymer microspheres, the authors have emphasized own experimental results. The findings provide useful information on the development of polymer microspheres as carriers for drug delivery systems.

Phagocytosis of polymer microspheres by macrophages

A method for the formation of monodisperse polystyrene (PS) and polymethylmethacrylate (PMMA) spheres, in the micron diameter range, was developed. The polymerisation reaction was carried out in a series of alcohols, using homo- and co-polymers as steric stabilisers in combination with a quaternary ammonium salt which probably acts as an electrostatic co-stabiliser. Monodisperse PS spheres were formed in the particle size range of 1–6 μm, by a single step process. The size range could be extended by further addition of monomer to the reaction mixture during the polymerisation. The solubility parameter of the alcohol used and the nature of the surfactant determine the diameter of the resulting spheres. However, the concentration of surfactant did not seem to affect strongly the size of the spheres but only the dispersity.

Shimon Reich

Papers

Monodisperse polymeric spheres in the micron size range by a single step process