Characterizing the Gas Transport and Mechanical Properties of Thermally Rearranged (TR) Polymers

Polyimides containing ortho-positional functional groups, such as hydroxyl and acetate groups, undergo thermal rearrangement to form polybenzoxazole structures upon heating between 350 and 450^oC.  These thermally rearranged (TR) polymers show excellent gas transport properties and have strong resistance to plasticization.  In this study, a series of TR homopolymer and copolymer were prepared with polyimide precursors based on HAB (3,3'-dihydroxy-4,4'-diaminobiphenyl), APAF (2,2'-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane), and 6FDA (2,2'-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride).  Pure gas permeabilities of H₂, N₂, O₂, CH₄, and CO_2­ were measured over pressure ranging from 3 atm to 17 atm at 35^oC.  Upon thermal rearrangement, APAF-6FDA has the best gas transport properties, presumably due to an enlarged cavity size and distribution of the TR polybenxazole structure.  In addition, this study systematically documents the mechanical properties of the polyimides and TR polymers.  HAB-6FDA polyimide has 10% elongation-at-break, whereas APAF-6FDA polyimide has only 4% elongation-at-break.