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Most chemically stable organic materials age through changes on the mesoscopic scale (from 10 nm to 10 mm). Examples include the slow crystallization of polymers, changes in particle size distributions with age, and the swelling of pressed powders. We began the Mesochem project to develop a new mesoscopic modeling capability for organic materials, including polymers, molecular crystals, and filled polymer composites. Our goal was to develop and validate novel mesoscopic modeling techniques that are well suited for materials of interest to LLNL's nation security mission, such as high explosives, binding agents, and foams.

The interaction of LiD with water vapor in the partial pressure range of 10{sup -7} Torr to 20 Torr has been investigated. The reaction probability of water with pure LiD cleaved in an ultra high vacuum environment was obtained using the modulated molecular beam technique. This probability was 0.11 and independent of LiD surface temperature suggesting a negligible activation energy for the reaction in agreement with quantum chemical calculations. The value gradually reduced, however, to .007 as the surface concentration of oxygen containing product (LiOH), which was monitored in-situ by Auger electron spectroscopy on the reaction zone, approached full coverage. As the hydroxide film grew beyond a monolayer, the phase lag of hydrogen product increased from zero to 20 degrees and the reaction probability reduced further until it approached our detection limit ({approx} 10{sup -4}). This phase lag was attributed to a diffusion limited process in this regime. In separate experiments, the film growth has been studied in nitrogen atmosphere with 100% relative humidity using thermogravimetric analysis (TGA) and in air with 50% relative humidity utilizing scanning electron microscopy (SEM). For exposures to environment with high water concentrations and for micrometer thick films, the reaction probability reduced to 4 xmore » 10{sup -7} and was independent of exposure time, The lattice diffusion through the film was no longer controlling the transport of water to the LiD/LiOH interface. Microcracks generated in the film to release stress provided easier pathways to the interface. A modified microscope, capable of both atomic force microscopy (AFM) and nanoindentation, was employed to investigate the surface morphology of LiOH.H{sub 2}O grown on LiOH at high water vapor partial pressures and the kinetics of this growth.« less

Coherent medium theory for a master equation description of transport on random lattices is extended to includes scattering in clusters larger than two sites (or one bond) exactly. Bond and site-averaged numerical calculations for the square lattice show some features of transport are improved by this method but that larger clusters are needed in order to obtain exact long-time properties.

It is apparent that the kinetic rate kD influences the effective rate of reaction and that in certain circumstances (kr � kr) it becomes the rate­ limiting step [keff = kD]. Accordingly, the rate coefficient kD' which describes the mass transport rate at which reactants encounter each other, is of central concern to solution kineticists. There is little doubt that the subject of diffusion-controlled reactions has broad application beyond this prototype example. Diffusion-controlled reactions play a prominent role in heterogeneous catalysis (after all, the perfeot catalyst leads to mass transport limitations), polymer chain growth kinetics, colloid or crystal growth, and biophysical chemistry.