(696g) Comparison of the Permeabilities and Selectivities of Polybenzoxazole Membranes Thermally Rearranged from Hydroxyl-Polyimides Formed from Different Dianhydride Precursors

Membrane separation is an excellent method for purification of natural gas. It is much cheaper and easier to implement than the alternatives. Multiple polymers have been studied for use in natural gas separation, and two stand above the rest: polyimide and polybenzoxazole. Originally, polybenzoxazole hadnâ??t been discovered yet, so researchers focused on polyimides. They continuously worked to overcome the limitations of membrane separation, which are the tradeoff between selectivity and permeability, physical aging, and plasticization. It was found that the selection of monomer precursors could influence the separation properties of the resultant polyimide membranes. Other methods of increasing resistance to plasticization are thermal annealing and cross-linking.

Recently, it was discovered that polyimides containing hydroxyl groups in a specific position could be thermally rearranged into polybenzoxazole, which has even better separation properties than polyimide due to the size and distribution of its free volume elements. Methods to influence these properties are similar to those used with polyimides. Polybenzoxazole is also very easy to mold into different forms, such as hollow fibers, because the polyimides are easy to make into these shapes and retain their shape when thermally rearranged.

The intent of this research is to address the egregious lack of permeation testing performed to compare different dianhydride precursors. Many studies have been done to determine the effects of the structure of diamine precursor on the separation properties of polybenzoxazole, but none regarding the effect of the structure of the dianhydride. In this work we compare the permeabilities and selectivities of polybenzoxazole membranes thermally rearranged from hydroxyl-polyimides formed from three dianhydride precursors.

Thus far, we have synthesized samples of polyimides from each of the following three dianhydrides: 1) 4,4â??-oxydiphthalic anhydride (ODPA), 2) 3,3â??,4,4â??-benzophenone tetracarboxylic dianhydride (BTDA), and 3) 3,3â??,4,4â??-biphenyl tetracarboxylic dianhydride (BPDA). Each dianhydride was combined with 2,2â??-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) to form the polyimide. Multiple membranes have been made from the polyimides made from ODPA and BTDA, and work is on-going to make membranes from polyimides made from BPDA. A few of the formed membranes were successfully thermally rearranged as well.