A new analytical form for bond and angle potentials suitable for obtaining mesoscale effective force fields from target distributions is reported. Applications to realistic coarse-grained models of atactic polystyrene and polyamide-6,6 are described. The potential optimization procedure, despite its simplicity, allows the accurate reproduction of the target atomistic distributions. The procedure has been validated for both bond and angle potentials. Nonbonded numerical potentials have been optimized by pressure-corrected iterative Boltzmann inversion. Thus, the proposed coarse-graining strategy consists of hybrid analytical and numerical contributions to the mesoscale polymer force field. C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 871-885, 2005

To understand, at a molecular scale, the effect of water on the structure of the amorphous region of polyamide 6,6 (PA6,6), atomistic molecular dynamics simulations have been carried out. Our results concerning the very local water organization relative to PA moieties agree qualitatively with a two-step sorption model. The first sorption mode seems to be saturated well below the lowest water concentration studied (5% relative to the amorphous phase). Moreover, above this saturation, the overall water organization displays at 300 K larger clusters than the 2−3 molecules usually assumed in the literature. The temperature dependence of free volume, hole size, and hydrogen bonding has also been investigated. It shows a transition between plasticized and antiplasticized behavior.

The local dynamics of water and its effect on the segmental mobility in the amorphous phase of polyamide 6,6 have been investigated. At 300 K and below, where the polymer segments undergo no significant motion (displacements below 2 Å, no reorientation) in the time scale of the simulation, water molecules exhibit mostly orientational freedom, and a bimodal distribution of reorientation times is observed. As the temperature is increased, an increasing fraction of the water molecules also experience hopping behavior between polyamide 6,6 cavities, but true diffusion on a nanosecond time scale is observed only above 400 K. The analysis of amide and methylene group reorientation clearly shows lubrication of the intermolecular amide−amide hydrogen bond, detected above 350 K only at the time scale of 2 ns. This phenomenon goes beyond a simple free volume effect, as is shown through the analysis of the local hydration of individual amide groups. The motion of inner methylene segments is also affected by water, but in a more limited extent. The enhancement of their mobility with water content is probably an indirect effect of the lower reorientation times of neighboring amide groups.

The adsorption of indigo molecules on Cu(111) was investigated by low temperature (5 K) scanning tunneling microscopy from the isolated single molecule regime to one monolayer. Structural optimization and image calculations demonstrate that the molecules are in a physisorbed state. Because of the reduced symmetry at the surface, single molecules acquire a chiral character upon adsorption leading to a two-dimensional (2D) chirality. They adopt two adsorption configurations, related by a mirror symmetry of the substrate, each with a distinct molecular orientation. Consequently, the 2D chirality is expressed by the orientation of the molecule. For higher coverage, molecules self-assemble by hydrogen bonding in nearly homochiral molecular chains, whose orientation is determined by the orientation taken by the isolated molecules. When the coverage approaches one monolayer, these chains pack into domains. Finally, the completion of the monolayer induces the expulsion of the molecules of the wrong chirality that are still in these domains, leading to perfect resolution in enantiopure domains.