(784c) Synthesis and Gas Transport of Thermally Rearranged Polyimides for Applications in Gas Separations

Over the past several years, research on thermally rearranged (TR) polyimides has shown interesting separation properties for a number of gas separations such as CO₂/CH₄ and hydrogen separations.  TR polymers are traditionally synthesized via a solid state reaction of polyimides that contain reactive groups ortho-functional to the polyimide diamine.  The precursor polyimides can be synthesized via a number of pathways, including chemical and thermal imidization.  Furthermore, polybenzoxazole structures similar to those formed from polyimides can be synthesized by polyamides.  The scope of this work is address the differences in gas transport properties for H₂, N₂, O₂, CH₄, and CO₂ as they relate to polybenzoxazole backbone structure, ortho-position reactive group, and synthesis of polybenzoxazoles from either polyimides or from polyamides.

Three ortho-hydroxy polyimides were synthesized via thermal imidization.  The diamines for synthesizing these polyimides were 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB), and the dianhydrides were 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and 4,4'-oxydiphthalic anhydride (ODPA).  From these monomers, HAB-6FDA, APAF-6FDA, and APAF-ODPA were synthesized and converted to TR polymers by heating at 450°C for 30min.  Conversion under these conditions resulted in nearly complete thermal rearrangement as determined by sample mass loss.  The permeation properties of the precursor polyimides and TR polymers were determined for H₂, N₂, O₂, CH₄, and CO₂.  Out of all of the samples considered, the APAF-6FDA polyimide and the APAF-6FDA TR polymer had the most promising transport properties out of all of the polyimides and all of the TR polymers, respectively.  These samples had permeabilities near or above the 2008 Robeson upper bound for CO₂/CH₄ and H₂/CH₄separation.

A critical analysis on the effect of transport properties as they relate to the ortho-position leaving group was evaluated for HAB-6FDA.  An ortho-hydroxy HAB-6FDA sample was modified via an esterification reaction post-imidization.  This esterification reaction added different reactive functional groups to the ortho-position of the polyimide.  In addition to the ortho-hydroxy group, an acetate, propanoate, and pivalate group were added to the polyimide backbone structure.  These samples were thermally rearranged to different extents of conversion, and gas permeation was tracked as these samples were converted.  Samples with bulkier leaving groups showed more dramatic improvements to gas permeability than hydroxyl-functional polyimides.

Finally, a polyamide sample of APAF-6FDA was prepared for gas transport properties.  This sample can be converted into a polybenzoxazole via a condensation reaction at 350°C for 60min.  The APAF-6FDA polyamide had consistently lower permeability than the APAF-6FDA polyimide.  Furthermore, the polybenzoxazole synthesized from the polyamide had consistently lower permeability than the polybenzoxazole synthesized from the polyimide.  It is believed that these differences in transport properties relate to the non-equilibrium nature of polybenzoxazoles prepared from different synthetic pathways.