303879 The Effect of Synthesis Route and Structure On Transport Properties for Thermally Rearranged Polymers
Thermally rearranged (TR) polymers are polybenzoxazoles synthesized via a decarboxylation reaction of polyimides that contain leaving groups ortho-position to the diamine. These polymers have shown interesting separation properties for a number of gas separations such as CO2/CH4 separation and hydrogen separation. The aim of this study is to investigate the effect of polyimide precursor structure on the transport characteristics for polyimides and TR polymers. Additionally, we seek to elucidate the effect of precursor and polybenzoxazole synthesis route on TR polymer transport properties.
Two diamine and two dianhydride monomers were used to synthesize a series of polyimides. The monomers consisted of 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB), 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), and 4,4'-oxydiphthalic anhydride (ODPA). From these monomers, hydroxyl –functional polyimides HAB-6FDA, APAF-6FDA, and APAF-ODPA were synthesized. These samples were characterized using 1H and 13C solution NMR, TGA coupled with mass spectrometry, FT-IR, density measurements, and DSC. Additionally, several samples were characterized by positron annihilation lifetime spectroscopy and solid state 13C NMR. Samples were thermally reacted at 450°C for 30min to form their analog TR polymer structure. Gas permeability of H2, N2, O2, CH4, and CO2 were determined up to 50 bar to compare the effect of polyimide and TR polymer structure gas transport properties. In general, conversion of these samples caused changes in permeability and selectivity that parallel the upper bound for CO2/CH4, O2/N2, N2/CH4, and H2/CH4 separation. Incorporation of the fluorinated monomers (i.e., APAF and 6FDA) increased gas permeability while only slightly decreasing gas selectivity.
Two comparisons of synthesis route were examined for this study. First, the effect of the polyimide synthesis route was compared by synthesizing HAB-6FDA from thermal, chemical, and solid state imidization. These studies showed that samples synthesized from thermal and solid state imidization (i.e., samples of identical chemical structure) showed similar transport properties; however, samples synthesized via chemical imidization, which contain acetate instead of hydroxyl leaving groups, had higher gas permeability. Second, the effect of TR polymer synthesis route was compared by testing transport properties of chemically similar polybenzoxazoles prepared from unique precursors. For this comparison, a TR polymer prepared from the APAF-6FDA polyimide was compared to a chemically similar polybenzoxazole prepared via the solid state condensation reaction of an APAF-6FDA polyamide. Permeability was consistently higher in the polyimide than in the polyamide for all gases. Moreover, conversion of the polyimide resulted in a greater increase in gas permeability than conversion of the polyamide.
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