The dependence of gas permeation on system size in glassy polymers has been tested by creating several models containing different numbers of molecules of the same chain length for the ODPA-ODA homopolyimide and by subsequently determining the permeation characteristics of helium. Eight "standard size" 4150-atom, a 6225-atom, and a much more expensive computationally 56025-atom systems were generated using hybrid pivot Monte Carlo (PMC)-molecular dynamics (MD) single-chain sampling. Following the careful relaxation of the polyimide matrices, helium atoms were then inserted into these systems, and MD simulations were carried out at the same applied external conditions of constant temperature and pressure tensor. Average densities for the pure matrices all fell within 0.7% of the experimental value. Energetic and structural properties as well as solubilities and characterization of the void space were also found to be number independent, thus showing that the preparation procedure gives reproducible and reliable results. Helium diffusion was analyzed over periods up to 20 ns using different approaches, such as observation of the individual trajectories, mean-square displacements, distributions of penetrant displacements components, and van Hove correlation functions. No number dependence could be detected, whether the gas molecules were in the anomalous or in the Fickian regime.

Fully atomistic molecular dynamics (MD) simulations have been carried out on a series of bulk melt pure PEO oligomers and PEO oligomer−silica systems, which differed by their macromolecular end groups. A realistic hybrid model of a silica nanoparticle combining an ionic core as well as the fine-tuning of the surface thickness and number of OH groups per unit surface area was used. The PEO oligomers were decorrelated in all systems under study in order to prevent any artifacts related to the preparation procedure. Significant changes were found to occur in the immediate vicinity of the interface with flattened PEO backbones arranged in densily packed shells and stabilized by the added PEO−silica interactions. Their conformations were also more coiled in order to better adapt to the surface structure. While methyl end groups did not show special characteristics other than their steric effect, hydroxyl end groups had a much higher affinity for the surface and tended to position themselves perpendicular to the surface, thus forming dynamic hydrogen-bonding complexes between the hydroxyl oxygens and the silanol hydrogens. The range of influence of the nanoparticle was evident for structural properties only up to two or three molecular layers, 10−15 Å, but was approximately twice that for dynamical ones.

No abstract