Thermally Rearranged (TR) Polybenzoxazoles from o -Substituted Precursor Polyimides with Phenyl Pendant Groups: Synthesis, Properties, and Thermal Rearrangement Conditions

A series of polyimides (PIs) was synthesized from 6FDA and two -OH substituted diamines having bulky pendant phenyl, Ph, and trifluoromethyl, CF , groups as precursors for thermally rearranged polybenzoxazole, TR-PBO, membranes. One diamine had two pendant Ph substituents; in the other, the substituents were Ph and CF . Applying azeotropic and chemical cyclizations allowed the obtention of four -hydroxy ( -OH) or/and -acetoxy ( -OAc) substituted PIs depending on the imidization method. The PIs were labeled as 3Ph-OH, 4Ph-OH, or 3Ph-OAc and 4PH-OAc, respectively. Thermal rearrangements of all four precursors were investigated in the interval from 350 to 450 °C. The conversions to TR-PBO increased with temperature, and almost quantitative conversions were obtained at temperatures close to 450 °C, although -OH substituted PIs reached conversions slightly higher than those of -OAc PIs at a given temperature. The TR-polymers' fractional free volume (FFV) also increased with conversion but was higher for the -OAc substituted precursors. Despite the high TR-PBO conversions, self-supported uniform TR membranes with reasonable mechanical properties were obtained, except for 4Ph-OH. Gas separation behavior of the membranes significantly improved after the thermal treatment, and the final CO /CH permselectivities lay between the 1991 and 2008 Robeson upper bounds. Particularly, TR-membranes derived from -OAc precursors and with pendant CF group demonstrated better gas transport properties with values of = 1121 barrer and α = 29 for 3Ph-OAc derived membrane, which positioned it beyond the 2008 upper limit.