Experimental

Diffraction-Based Probes of Material Homogeneity in Polymer-Composite Microparticles

Our primary experimental tool for probing phase-separation behavior and material homogeneity in polymer composites is essentially an interferometric technique that has been used for a number of years as a method for sizing liquid microdroplets (size range ^ 2 - 20 microns. Recently, we have shown that this measurement technique can be used to recover information on drying kinetics, inter-polymer dynamics, and material properties such as dielectric constant." The basis of the technique involves illumination of a .dielectric sphere with a plane-polarized laser to produce an inhomogeneous electric field intensity distribution, or grating, within the particle that results from interference between refracted and totally-internally-reflected waves within the particle. The angular spacing between intensity maxima, as well as the intensity envelope is a highly sensitive function of particle size, and refractive index (both real and imaginary parts). Unlike conventional microscopy approaches with diffraction-limited X/2) spatial resolution, two-dimensional diffraction (or, angle-resolved scattering) is sensitive to material homogeneity on a length scale of ® X/20 or about 20 - 30 nm for optical wavelengths. This dimension is comparable to single-molecule radii of gyration for relative large molecular weight (> 100 k) polymers, and thus provides molecular scale "resolution" of material homogeneity in ultrasmall volumes 1 - 100 femtoliters).

Light scattering from micron-sized spherical dielectric droplets or particles18 has been used for a number of years as a method of szing'192021 and analysis of various physical and chemical properties.22,23 While various light scattering techniques from spherical droplets have been very well characterized for particle sizing and refractive index determination.24,25,2«,27 use of 1 - and 2-dimensional angle-resolved elastic scattering has very recently begun to be utilized as a tool for characterizing in situ polymerization in micro droplet,28 29 and probing multi-phase30 31 and homogeneous14 composite particles. As shown in Ref 14, the fringe contrast (and intensity fluctuations along an individual fringe) is also very sensitive to material homogeneity on a length scale of 33 X/20 or about 20 - 30 nm for optical wavelengths. In our experimental configuration, this intensity grating is projected in the far-field using (f/1.5) collimating optics and detected with a CCD camera.32 The scattering angle (center angle and width) is established by means of an external calibration, and is used for high-precision Mie analysis of one-dimensional diffraction data.

For optical diffraction studies, individual particles were studied using droplet levitation techniques. Details of the apparatus and CCD calibration procedure are described in Ref 32. The nominal scattering angle was 90 degrees with respect to the direction of propagation of the vertically polarized HeNe laser, and the useable full plane angle (defined by the f/1.5 achromatic objective) was 35 degrees. The CCD (SpectraSource Instruments) was thermoelectrically cooled and digitized at 16 bits. Details ofthe droplet generator used are described in Ref. 16. Aqueous solutions were handled by

angle (degrees)

Figure 1. Two-dimensional slices of 4-dimensional error surface (varying Re(n)) for Me theory match to diffraction from PEG particle (Im(n) fixed). The lower trace shows the best match to the experimental scattering data angle (degrees)

Figure 1. Two-dimensional slices of 4-dimensional error surface (varying Re(n)) for Me theory match to diffraction from PEG particle (Im(n) fixed). The lower trace shows the best match to the experimental scattering data simply loading the Pyrex tip by vacuum aspiration and re-installing into the generator. For the work done on co-dissolved polymers in tetrahydrofuran (THF), the entire droplet generator chamber and ballast reservoir were backfilled with THF, and the tip was loaded with the polymer solution of interest.

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