Introduction

We describe a new method for production and characterization of polymer and polymer-composite particles from solution using microdroplet techniques. In particular, 2-dimensional optical diffraction - an experimental tool that has long been used for size characterization of liquid droplets - is presented as a sensitive probe of phase separation behavior and particle dynamics in polymer-blend microparticles generated from microdroplets of dilute polymer solution. As we discuss further, this technique is sensitive to the presence of sub-domains with dimensions on the order of 30 nm, thus capable of providing information on material homogeneity on a (macro)molecular length scale. Under conditions of rapid solvent evaporation (i.e., very small droplets) and relatively low polymer mobility, homogeneous particles can be formed using different polymers that ordinarily undergo phase-separation in bulk preparations. This important result paves the way for producing new materials from polymer alloys with tunable material properties.

Over the last several years, an enormous amount of experimental and theoretical effort has been focused on multi-component polymer systems as a means for producing new materials on the micron and nanometer scale with specifically tailored material, electrical and optical properties. Composite polymer particles, or polymer alloys, with specifically tailored properties could find many novel uses in such fields as electro-optic and luminescent devices,"' conducting materials? and hybrid inorganic-organic polymer alloy.4 A significant barrier to producing many commercially and scientifically relevant homogeneous polymer blends, 567, 8 however, is the problem of phase separation from bulk-immiscible components in solution that has been studied in detail by several different groups.9 101 1 The route typically taken in trying to form homogeneous blends of immiscible polymers is to use compatibilizers to reduce interfacial tension. Recently, a number of different groups have examined phase-separation in copolymer systems to achieve ordered meso- and micro-phase separated structures with a rich variety of morphologies.1213 For solvent-cast composites, phase separation and related morphologies depend on the time scale for solvent evaporation relative to molecular organization.

Our interest is in using small droplets ( 5 - 10 ^m diam.) of dilute mixed-polymer solution to form homogeneous polymer composites without compatibilizers as a possible route to new materials with tunable properties. Over the last several years, advances in microdroplet production technology for work in single-molecule detection and spectroscopy in droplet streams has resulted in generation of droplets as small as 2 - 3 microns with a size dispersity of better than 1%. In the context of polymer particle generation, droplet techniques are attractive since particles of essentially arbitrary size (down to the single polymer molecule limit) can be produced by adjusting the size of the droplet of polymer solution, or the weight fraction of the polymer in solution. While droplet production in the size range of 20 - 30 microns (diameter) is more or less routine (several different on-demand droplet generators are now available commercially), generation of droplets smaller than 10 microns remains non-trivial-especially under the added constraint of high monodispersity. Small droplets (< 10 ^m) are especially attractive as a means for producing multi-component polymer-blend and polymer-composite particles from solution since solvent evaporation can be made to occur on a millisecond time scale, thus inhibiting phase-separation in these systems.

The primary condition for suppression of phase separation in these systems is that solvent evaporation must occur on a time scale that is fast compared to self-organization times of the polymers. This implies time scales for particle drying on the order of a few milliseconds implying droplet sizes 10 ^m (depending on solvent, droplet environment, etc). We have shown recently that a microdroplet approach can be used to form homogeneous composites of co-dissolved bulk-immiscible polymer14 using instrumentation developed in our laboratory for probing single fluorescent molecules in droplet stream.1516 In addition to a new route to forming nanoscale polymer composites, a microparticle format offers a new tool for studying multi-component polymer blend systems confined to femtoliter and attoliter volumes where high surface area-to-volume ratios play a significant role in phase separation dynamics.

Here we describe in some detail the basis of optical diffraction in spherical dielectric particles as a probe of material homogeneity in polymer composites, and discuss limitations on domain size (in multi-phase composites), and dielectric constant. We show how this measurement technique can be used to recover information on drying kinetics, inter-polymer dynamics, and material properties. In the following chapter, we describe some results of detailed molecular dynamics modeling that can be used to connect experimental observables with microscopic dynamics within the particle as well as to suggest future experiments. The organization of this chapter on synthesis and characterization of polymer and polymer-composite particles is as follows: First we describe our instrumentation for production, manipulation, and characterization of polymer particles from microdroplets of solution. Next we summarize some of the important results. Finally, we discuss some exciting possible future directions and applications

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