Free volume in polyimides:: Positron annihilation experiments and molecular modeling

We present a combined analysis of the free volume in polyimide membrane polymers by employing experimental measurements as well as computer simulations. The amount and distribution of free volume in polymeric membranes significantly determine the transport and separation properties. In the present work these free volume characteristics were determined directly from simulated atomistic packing models via a new method. Here a "virtual tracer sphere" probes the simulation cell to determine the unoccupied volume. As an experimental approach, positron annihilation lifetime spectroscopy was used to determine the average size of free volume cavities via a well-established correlation between orthopositronium lifetime and cavity size. We show that the results obtained from the combined analysis provide a valuable basis for the investigation of free volume properties; moreover, limitations of the standard model for the evolution of the positron lifetime data will be discussed.