A recently developed molecular model for diffusion in dense aromatic polymers, which attempts to explain penetrant jump frequencies in terms of phenyl ring partial flips, is investigated via molecular simulation. The model polymer system under consideration in this paper corresponds to the interfacial domains generated by the lateral chain invariant (LCI) grain boundaries between crystallites of a stiff chain polyamide, poly(p-phenylene terephthalamide) (PPTA), and the low molecular weight penetrant selected for study is water. Fully atomistic constraint molecular dynamics simulations are conducted with interatomic and intramolecular interactions described by the DREIDING potential [S. L. Mayo, B. D. Olafson, and W. A. Goddard III, J. Phys. Chem. 94, 8897 (1990).] The coupling of the diffusive motion of water with the local polymer chain dynamics is examined at two temperatures and over a range of grain boundary densities.