Molecular models of three fluorinated polyimides based on the 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA) have been studied using molecular dynamics (MD) simulations. The respective diamines were 4,4'-hexafluoroisopropylidene dianiline (6FpDA), 3,3'-hexafluoroisopropylidene dianiline (6FmDA), and 2,4,6-trimethyl-1,3phenylenediamine (DAM). Thirty independent samples were prepared using a hybrid pivot Monte Carlo-MD generation technique and average densities were found to be in very good agreement with experiment. Model structures also agreed with available wide-angle X-ray scattering data. Cohesive energies, Hildebrand solubility parameters, fractional free volumes (FFV), void space distributions and intermolecular as well as intramolecular interactions were analyzed. The differences in bulk properties between both 6FDA-6FpDA and 6FDA-6FmDA isomers remain fairly small, although the configurations of the former are more extended. 6FDA-DAM has a lower density, larger intermolecular distances, and higher free volume than the other two polyimides. Results are discussed with respect to their use as matrices for gas separation. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1166-1180, 2009