We construct an approximation to the partition function for hindered rotors based entirely on their asymptotic behavior and no fitting parameters. The approximant is shown to be quite accurate in all temperature ranges. Explicit auxiliary functions are derived for the Helmholtz free energy, internal energy, heat capacity, and entropy. We apply this function to estimating the heat capacity and unimolecular dissociation rate for ethane.

The use of block copolymer micelles as novel nucleating agents for foaming thermoplastics is explored. Diblock copolymers can self-assemble into a large number of spherical micelles, making them promising nucleants for the production of microcellular foams with >10 9 cells/cm 3 . Different types of A-B diblock copolymers were added to a polystyrene matrix in low concentrations, and the resulting blends were foamed in a batch process using carbon dioxide as the blowing agent. Polystyrene-b-poly-(ethylene propylene) and polystyrene-b-poly(methyl methacrylate) diblocks were not effective as nucleants. Diblocks containing poly(dimethylsiloxane) as the core block showed a bimodal cell size distribution and a small increase in cell concentration. The increased solubility of carbon dioxide in poly(dimethylsiloxane) (PDMS), and the reduced surface tension of PDMS, lowered the minimum work of bubble formation and increased the cell concentration. None of the foams showed an increase in cell concentration proportional to the large number of potential nucleants present. Block copolymer micelles as nucleants were not able to sufficiently lower the work of formation for cell nucleation. They are too small in size, can still agglomerate into larger, nonequilibrium structures, and have large contact angles due to the high surface tension of most polymers.

Ž. We describe the implementation of the cell multipole method CMM in a Ž . Ž . complete molecular dynamics MD simulation program MPSim for massively parallel supercomputers. Tests are made of how the program scales with size Ž . Ž . linearly and with number of CPUs nearly linearly in applications involving up to 10 7 particles and up to 500 CPUs. Applications include estimating the surface tension of Ar and calculating the structure of rhinovirus 14 without requiring icosahedral symmetry.

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