TITAN HAZE - STRUCTURE AND PROPERTIES OF CYANOACETYLENE AND CYANOACETYLENE-ACETYLENE PHOTOPOLYMERS

The structure and morphological properties of polymers produced photoc hemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution t o the haze layers found on Titan. A structural analysis of these polym ers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosp here of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolys is) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase HC3N at 185 and 254 nm. The resulting br own to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon-carbon double bonds, which, o n exposure to heat, cyclizes to form a graphitic structure. Similar me thods of analysts were used to show that when HC3N is photolyzed in th e presence of Titan's other atmospheric constituents (CH4, C2H6, C2H2, and CO), a copolymer is formed in which the added gases are incorpora ted as substituents on the polymer chain. Of special significance is t he copolymer of HC3N and acetylene (C2H2). Even in experiments where C 2H2 was absorbing nearly all of the incident photons, the ratio of C2H 2 to HC3N found in the resulting polymer was only 2:1. Scanning electr on microscopy was used to visually examine the polymer particles. Whil e pure polyacetylene particles are amorphous spheres roughly 1 mu m in diameter, polycyanoacetylenes appear to be strands of rough, solid pa rticles slightly smaller in size. The copolymer of HC3N and C2H2 exhib its characteristics of both pure polymers. This is particularly import ant as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorpo ration of other minor atmospheric constituents, like HC3N, into the po lyacetylenes are expected to have optical constants more comparable to those of the Titan haze. (C) 1997 Academic Press.