Generation, characterization, and modeling of polymer micro- and nano-particles

Polymer micro- and nano-particles are important in many technological appli cations, including polymer blends or alloys, biomaterials for drug delivery systems, electro-optic and luminescent devices, and polymer powder impregn ation of inorganic fibers in composites. They are also critical in polymer- supported heterogeneous catalysis. This article reviews recent progress in experimental and simulation methods for generating, characterizing, and mod eling polymer micro- and nano-particles in a number of polymer and polymer blend systems. A description of the use of gas atomization (of melts) and m icrodroplet (solution) approaches to generation and characterization of sph erical polymer powders and microparticles represents their unique applicati ons, giving the non-specialist reader a comprehensive overview. Using novel instrumentation developed for probing single fluorescent molecules in subm icrometer droplets, it is demonstrated that polymer particles of nearly arb itrary size and composition can be made with uniform size dispersion. This interesting finding is ascribed to new dynamic behavior, which emerges when polymers are confined in a small droplet of solution the size of a molecul e or molecular aggregates. Solvent evaporation takes place on a time scale short enough to frustrate phase separation, producing dry pure polymer or p olymer blend microparticles that have tunable properties and that are homog eneous within molecular dimensions. In addition, it shows how a number of o ptical methodologies such as Fraunhofer diffraction can be used to probe po lymer particles immobilized on two-dimensional substrates or levitated in s pace using a three-dimensional quadrupole (Paul) trap. Unlike conventional methods such as electron-beam microscopy, the optical diffraction methods p rovide a unique look inside a polymer particle in a measurement time scale of a few milliseconds, making it attractive to in-line production applicati ons. In particular, it shows that it is possible to use computational neura l networks, extensive classical trajectory calculations (i.e., classical mo lecular dynamics methods) in conjunction with experiments to gain deeper in sights into the structure and properties of the polymer microparticles. Ove rall, it is possible to use the new understanding of phase separation to pr oduce a number of useful, scientifically interesting homogeneous polymer bl ends from bulk-immiscible components in solution. Additionally, this new kn owledge provides useful guidelines for future experimental studies and theo ry development of polymer and polymer blend micro- and nano-particles, whic h are not widely studied.