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Polydiacetylenes are a unique class of highly conjugated organic polymers that are of interest for both electronic and photonic applications. Many of these applications require high quality thin polydiacetylene films, i.e., films possessing minimal impurities and inhomogeneities, uniform thicknesses, and few defects such as light scattering centers. The growth of such films is not trivial. Photodeposition from solution is a novel process, discovered in our laboratory, which shows excellent potential for producing high quality polydiacetylene films, superior to those grown by conventional techniques. The basic method is quite simple; a diacetylene monomer solution is irradiated with UV light through quartz, glass, or other transparent substrate and a thin polydiacetylene film results. During the course of this process thermal density gradients arise due to uneven heat generation throughout the monomer solution. These gradients can then, under the influence of gravity, give rise to buoyancy-driven convection. Additionally, changes in monomer concentration may induce solutal convection. This convection affects transport of material to and from the growing polydiacetylene film, and thus affects the dynamics of the growth process, and thereby also the microstructure, morphology, and properties of the film. Evidence of this is seen when the films are viewed under a microscope; they exhibit small particles of solid polymer which form in the bulk solution, get transported by convection to the surface of the growing film, and become embedded. Also convection tends to cause the film thicknesses to be less uniform, and may even affect the molecular orientation of the films. We have been actively studying the fundamental science of diacetylene polymerization in solution and polydiacetylene film photodeposition from solution, including the kinetics, mechanism, and photo-chemistry of the process, and the nature and properties of the films obtained. Initial results of this work have just been published in J. Am. Chem. Soc. The thrust of our research is to investigate in detail, both in 1-g and in low-g, the effects of convection (and the lack thereof) on this novel and interesting reaction.
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