Gas separation properties of aromatic polyimides with bulky groups. Comparison of experimental and simulated results

A series of new diamine monomers, with systematic variations in their chemical structure, by introducing various substituents onto the polymer backbone, have been made to react with two commercial dianhydrides: a very rigid one, pyromellitic dianhydride, (PMDA) and a more flexible one, 2,2'-bis(3,4-dicarboxyphenyl) hexafluompropane dianhydride (6FDA), to obtain a series of polyimide membranes with high fractional free volume (FFV), and their gas separation properties have been studied. In order to gain insight into the relationship between structure and gas separation properties of the polymers, a computer simulation of the polymers has been carried out, by a combination of molecular mechanics and molecular dynamics, to study the effect of the polymeric structure on the permeability and selectivity of the membranes. Moreover, the effect of the probe size on the accessible free volume, which cannot be experimentally determined, has been simulated and related with experimental gas separation parameters as permeability and selectivity. The use of FFV (that corresponds to a probe of zero-radius) cannot predict the behavior of several polymers, which have similar FFV values but different values of permeability and selectivity. However, simulation has permitted to correlate accurately permeability and selectivity with the accessible free volume to probes of different radii.