Physical aging and plasticization of thick and thin films of the thermally rearranged ortho-functional polyimide 6FDA-HAB

It was recently shown that the rate of the thermal rearrangement process of an ortho-functional polyimide is thickness dependent. Here it is shown that the gas separation properties of thermally rearranged (TR) polymer films are also affected by thickness. In this work, the physical aging and plasticization behavior of TR polymers were tracked using thin (~1-2μm) and thick (~15-20μm) films derived from an ortho-functional polyimide based on 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) over hundreds of hours. The thin films experienced a much greater rate of physical aging than the thick films, as indicated by the rapid decline of gas permeabilities and the gradual increase of ideal selectivities. As the thermal rearrangement temperature was increased, the aging rate of the resultant thin film did not increase extensively and the plasticization curves of the thin and thick films became closer. Long term exposure of the TR films to CO₂ at 32atm over 500h shows that the CO₂ permeability of the thick TR films did not show significant decline over time; whereas, the CO₂ permeability of the thin TR films increased over the first several hours and decreased rapidly during further exposure time. Finally, the response of thick TR films to C₂H₄, C₂H₆ and C₃H₈ over long term exposure shows that physical aging is greatly accelerated when the polymer matrix is severely plasticized by the more condensable gases.