The Molecular Structure and Dynamics of Short Oligomers of PMDA-ODA and BCDA-ODA Polyimides in the Absence and Presence of Water

Four constant-pressure molecular dynamics (MD) simulations were undertaken at room temperature to compare, at the molecular level, the structure and dynamics of short model oligomers of two polyimides based on the same diamine, 4,4’-diaminophenyl ether (ODA). The dianhydrides were, respectively, pyromellitic dianhydride (PMDA) and bicyclo(2.2.2)-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA). The systems under study were first analyzed in the pure state. In a second stage, 3.3% water, by weight, was inserted into the simulation boxes to mimic the highly hygroscopic nature of polyimides. The models are in the glassy state and reproduce many features found experimentally. The PMDA-ODA and BCDA-ODA oligomers have similar intramolecular flexibility, similar trends in the energy and density variations when water is added, as well as identical probe-accessible volumes and oligomer-water interaction sites. However, the choice of the dianhydride clearly affects the density, the cohesion, the intermolecular interactions, the morphology of the void space, and the degree of water clustering in these short-chain systems. The diffusion of water in the anomalous regime is characterized by two distinct behaviors.