Summary And Future Directions

The combination of experimental evidence and computational modeling show conclusively that stable, homogeneously blended (bulk-immiscible) mixed-polymer composites can be formed in a single microparticle of variable size. To our knowledge, this represents a new method for suppressing phase-separation in polymer-blend systems without compatibilizers that allows formation of polymer composite micro- and nanoparticles with tunable properties such as dielectric constant. Conditions of rapid solvent evaporation (e.g. small (<10 ^m) droplets or high vapor pressure solvents) and low polymer mobility must be satisfied in order to form homogeneous particles. While this work was obviously focused on polymeric systems, it should be pointed out that the technique is easily adaptable to making particles of small organic and inorganic (and hybrid composites) as well. A wide range of electronic, optical, physical and mechanical properties of single- and multi-component polymer nanoparticles remain to be explored.

Currently, we are investigating structure and dynamics of a number of polymerpolymer, and polymer-inorganic composite systems. The latter, for example, show an interesting time dependence in both size, morphology, and phase-separation behavior whose underlying microscopic mechanism is not yet completely clear. In other work, we are exploring the role of compatibilizers (short-chain copolymers) to increase the size range for producing homogeneous polymer-blend particles. Also, we are developing techniques for multi-color diffraction to extract information on ternary and higher-order composite particles. Some important issues relevant to commercialization remain to be resolved as well. Most importantly is the problem of particle throughput that is currently (optimistically) limited to a few milligrams per day. Other issues include hardware compatibility with various solvents that are commonly encountered in polymer solution work. Low-vapor pressure solvents such as methylene chloride are seriously problematic in acoustically driven droplet ejection devices such as ours. Effects such as cavitation, and clogging due to solvent evaporation will need to be confronted in order to expand the practical range of materials amenable with this technique.

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