Molecular modeling investigation of free volume distributions in stiff chain polymers with conventional and ultrahigh free volume: Comparison between molecular modeling and positron lifetime studies

The present paper deals with a molecular modeling-based characterization of the distribution of free volume in a number of different stiff chain glassy polymers including ultrahigh free volume and conventional materials. The free volume distribution was analyzed for the validated packing models and compared with respective positron annihilation data (if available). It will be shown that the molecular modeling approach permits a more detailed insight into free volume distributions. There the observed distributions reach from more or less symmetric monomodal (with maximum probability at free volume element radii of about 3 angstrom) via monomodal distributions with extended tails toward larger radii up to distinctly bimodal distributions with a tendency toward a continuous nanoporous free volume phase besides "conventional", free volume organized in isolated holes of radii up to a few angstroms. The described sequence roughly corresponds to the trend of measured permeabilities for small molecules in the investigated materials.