Pentiptycene-based polyimides with hierarchically controlled molecular cavity architecture for efficient membrane gas separation

A series of new pentiptycene-containing diamines with systematically varied substituent groups were designed and synthesized with high purity and high yields. These diamines were used to prepare a series of new polyimides with 4,4'-hexafluoroisopropylidene bisphthalic dianhydride (6FDA) by conventional condensation polymerization. The obtained pentiptycene-containing polyimides possessed high molecule weights, excellent thermal stability, good solubility in a wide range of organic solvents and thus excellent processability for membrane fabrication. Because of the excess amount of internal free volume associated with the molecular cavities in the pentiptycene moieties and the consequently disrupted chain packing, all the polyimides exhibited high fractional free volume (FFV) leading to high gas permeabilities as well as good selectivities that are highly desired for gas separation membranes. In addition, the comparisons between the pentiptycene-containing polyimides bearing various substituent groups indicated that free volume, the vital structural parameter for fast and selective molecular transport, was very sensitive to the size of the substituent groups. Large substituent groups such as CF₃ led to increased FFV, while relatively small substituent groups such as CH₃ resulted in reduced FFV, possibly due to the mechanism of "partial filling" of the molecular cavities of pentiptycene units by the substituent groups. Gas permeability data also supported this unusual trend of the dependence of free volume on the substituent groups. This finding provides a completely new and potentially novel means of molecular-level manipulations to predictably construct preferred free volume architecture that may maximize the separation performance of polymeric membranes.