Gas transport properties of polyimide–POSS nanocomposites

A mixed matrix membrane material was formed by mixing molecular scale inorganic cages, octaamino functional oligomeric silsesquioxane (OAPS) in a fluorine containing polyimide, 6FDA–MDA. These materials formed a miscible blend at loadings up to 20 wt% OAPS with phase separation occurring with the formation of small nanoscale OAPS particles at higher loadings (∼50 wt%). The pure gas transport properties were measured as a function of OAPS loading. There was a general decrease in permeabilities for all gases studied except carbon dioxide with increasing OAPS loading. This reduction was due to decreases in both solubility and diffusivity which indicate that the OAPS cages were not accessible to gas molecules at conditions used for the study. The amino groups on the surface of the OAPS interact with CO2 resulting in slight increase in solubility and no net decrease in permeability at all OAPS loadings. An increase in permselectivities was observed for most gas pairs including He/CH4, CO2/CH4 and O2/N2.