(645f) Fundamental Studies of Gas Diffusivity and Solubility On Molecular Transport In Thermally Rearranged Polyimides

Polyimides containing ortho-functional hydroxyl groups undergo thermal rearrangement to form polybenzoxazoles between 350°C and 450°C.  For certain separations, such as CO₂/CH₄ and O₂/N₂, these thermally-rearranged (TR) polymers have attractive combinations of permeability and selectivity.  This study seeks to develop a more detailed understanding of gas diffusivity and solubility in TR polymers, using a model ortho-functional polyimide synthesized from 3,3'-dihydroxy-4,4'-diamino-biphenyl diamine and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB-6FDA). 

By treating the polyimide at different thermal conversion temperatures between 350°C and 450°C for varying lengths of time, the sample was partially converted to its corresponding polybenzoxazole structure.  The diffusivity of CO₂ in a series of partially converted TR polymers based on HAB-6FDA increased by over an order of magnitude as the conversion of HAB-6FDA increased.  At the same extent of conversion, the solubility of CO₂ increased by a factor of approximately two.  Thus, a substantial increase in CO₂ permeability was observed, driven primarily by an increase in gas diffusion coefficients.

Gas permeability and sorption were determined as a function of temperature and used to obtain activation energies of diffusion and enthalpies of sorption for CO₂ and CH₄.  Further polymer characterization tests including thermogravimetric analysis coupled with mass spectrometry and differential scanning calorimetry were used to determine the regions of thermal rearrangement and thermal degradation, and to monitor the change in glass transition temperature at the onset of thermal rearrangement, respectively.