Thermally rearranged polybenzoxazoles membranes with biphenyl moieties: Monomer isomeric effect

A nucleophilic monomer (3,3′-diamino-4,4′-dihydroxybiphenyl, mHAB), isomer of the commercial 3,3′-dihydroxybenzidine (pHAB), has been synthesized in good yield and high purity by a two-step synthesis. This monomer was polymerized with 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6FDA) to form a new ortho-hydroxy polyimide (mHAB-6FDA) followed by thermal rearrangement to polybenzoxazole in solid state. mHAB-6FDA polyimide film showed excellent film-forming ability and good mechanical properties, which were similar to those found for the isomeric polymer pHAB-6FDA. A detailed thermal study was carried out for both polymers (mHAB-6FDA and pHAB-6FDA) to discover the relationship between the thermal treatment employed and the degree of conversion to polybenzoxazole (PBO). Thermal rearrangement (TR) to PBO for the polymer derived from mHAB started from a lower temperature than for the polymer pHAB-6FDA. Final TR conversion was higher for pHAB-6FDA when the employed temperature and residence time were high enough; also, for any treatment temperature, fractional free volume (FFV) was higher for the polymer derived from mHAB. Gas separation properties were measured and it was found that gas permeability (and particularly CO2 permeability) was much better for the material made from mHAB. The high gas permeability along with good selectivity qualifies mHAB-6FDA as a good candidate in carbon capture applications.