Introduction

Recently we have developed an experimental technique for creating polymer particles of arbitrary composition and size.12 In the experiment, instrumentation for generation and characterization ofsingle molecules in solution ofdroplet streams with small ( 1 -2 um) average diameter and monodispersity, was used.3 This technique makes the initial volume of dilute solution sufficiently small so that the solvent evaporates on a very short time scale (few milliseconds). With a droplet generator specially designed and constructed in our laboratory, we have succeeded in generating and efficiently probing (i.e., reproducible delivery to a 1 ^m target) 1-^m diameter water droplets at rates ofup to 100 Hz. While the monodispersity is a function of initial droplet size, 1% relative standard deviation in particle size is routinely achievable with our on-demand droplet generator design. Using this technique, we generated polymerparticles ofarbitrary composition and size. Figure 1 shows Fraunhofer diffraction data from three different polyethylene oxide (PEO) microparticles, along with fits to the data using Mie theory.'') For micro- and nano-scale generated polymer particles, the refractive index is consistent with bulk (nominal) values, and the level ofagreement with Mie theory indicates that the particles are nearly perfect sphere.4 Figure 2(a) shows a schematic image ofthe generation ofthese polymer particles, and an optical image oftypical particles formed from the experiment a droplet generator is shown in Fig. 2b.

Polymerparticles in nano- and micrometer size range provide many unique properties due to size reduction to the point where critical length scales of physical phenomena become comparable to or larger than the size of the structure. Applications of such particles take advantage of high surface area and confinement effects, which leads to nano-structures with different properties than conventional materials. Clearly such changes offer extraordinary potential for development ofnew materials in the form ofbulk composites and blends which can be used for coatings, opto-electronic components, magnetic media, ceramics and special metals, micro- or nano-manufacturing, and bioengineering.5 In order to develop an understanding of properties influenced by the size scale ofthe materials, we have begun an extensive set of computationalexperiments.

A computational algorithm for generating and modeling polymer particles was developed

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