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Processing of Organic Microparticles During Capture in Aerogel
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Near-monodisperse, micrometre-sized poly (N-vinylpyrrolidone)-stabilized polystyrene latexes have been coa ted with polypyrrole by in situ deposition of the conducting polymer from aqueous solution. If the conducting polymer overlayer is sufficiently thin, it lies inside the steric stabilizer layer and the coated latexes retain reasonable colloid stability. The polypyrrole loading on the latex particles was systemically varied over a wide range (1-50 mass%) simply by changing the initial latex concentration. Pressed pellet conductivity measurements on the dried composites indicated an anomalously low percolation threshold of 1-2 vol. %, which is consistent with the conducting polymer component lying on the surface of(rather than within the interior of) the latex particles. IR spectroscopy studies of the composites confirmed the presence of several strong absorption bands due to the polypyrrole component. However, scanning electron microscopy (SEM) studies showed no evidence for the polypyrrole overlayer, which suggests that it must be very smooth and uniform. Disk centrifuge photosedimentometry (DCP) studies on the coated latexes revealed the presence of doublets and higher aggregates which indicates that these dispersions are weakly flocculated. However, if the polypyrrole overlayer is very thin (< 10 nm) the coated particles exist in solution mainly as singlets with relatively few doublets or larger floccs. Polystyrene latex can also be coated with other conducting polymers such as polyaniline and poly(3,4-ethylenedioxythiophene). Finally, both polyelectrolytes and physically adsorbed poly(N-vinylpyrrolidone) can provide an effective steric barrier to allow controlled deposition of the conducting polymer overlayer.
Improvements in the processability of polypyrrole-a relatively air-stable organic conducting polymer-can be obtained by producing colloidal dispersions. The particles reported here are ''inside out'', i.e., micrometer-sized, sterically stabilized polystyrene latex particles are coated with an outer layer of polypyrrole. Their synthesis and physical properties are described and ideas for further work and potential novel applications, for example in immunodiagnostic assays, suggested.
Aerogel is an ultra-low-density material that can be used to capture small particles incident upon it at speeds in excess of 1 km s−1. This permits capture of cosmic dust in space where the high speeds usually result in destructive impact events. The performance of aerogel in laboratory impact tests is described. Completely intact capture is rare; most studies show that between 10% to 100% of the incident particle's mass is captured. However, in all cases unaltered domains were found in the particles captured in the laboratory at speeds up to 6 or 7 km s−1. Several analytic techniques can be applied in situ to particles captured in aerogel, yielding data on the preimpact composition of the particle. Extraction techniques for removing small particles from aerogel are described, and after extraction, handling and analysis in the laboratory can proceed as for any small-sized particle. Coupled with the survival of intact regions in the captured particles, this allows detailed identification of the com...
Observations and experimental evidence are presented to support the hypothesis that high-speed impact into a parent body regolith can best explain certain textures and compositions observed for rims on some chondrules. A study of 19 interclastic rimmed chondrules in the Weston (H 3/4) ordinary chondrite shows that two main rim types are present on porphyritic olivine-pyroxene (POP) and porphyritic pyroxene (PP) chondrules: granular and opaque rims. Granular rims are composed of welded, fine-grained host chondrule fragments. Bulk compositions of granular rims vary among chondrules, but each rim is compositionally dependent on that of the host chondrule. Opaque rims contain mineral and glass compositions distinctly different from those of the host, partially reacted chondrule mantle components, and some matrix grains. Opaque rims are greatly enriched in FeO (up to 63 wt%). The original chondrule pyroxene compositional zonation patterns and euhedral grain outlines are discontinuous at the chondrule/rim interface. Opaque rims are dominated by fayalitic olivine (Fa92-56), with high Al2O3 content (0.78-3.15%), which makes them distinctly different from primary olivine, but similar to Fe-olivine in chondrule rims of other meteorites. Thin zones of chondrule minerals adjacent to the present rims are intermediate in FeO content between the Mg-rich interior and the Fe-rich rim, which indicates a reaction relationship. Regardless of conclusions drawn regarding other types of rims, granular and opaque rim characteristics appear to be inconsistent with nebular condensation, in that host and matrix fragments are included within the rim. We have initiated a series of experiments, using the Ames two-stage light gas gun, to investigate the hypothesis that the Weston chondrule rims are the result of thermal and mechanical alteration upon impact into a low-density medium. Clusters of approximately 200-micron-sized silicate particles were fired into aerogel (density = 0.1 g cm-3) at velocities of 5.6, 4.7, and 2.2 km sec-1. Recovered grains show characteristics that range from fragmented projectile grains mixed with melted aerogel that nearly rim the grains to grains that have melted aerogel clumps mixed with partially melted projectile. These experimental results demonstrate that rim-like thermal and mechanical alteration of projectiles can result from a high-velocity encounter with a low-density target. Therefore, experiments using appropriately chosen projectile and target materials can provide a test of the hypothesis that chondrule rims common to Weston and possibly other ordinary chondrites were formed by such a process.