(640d) Polybenzoxazole (PBO) Gas Separation Membranes: New Synthesis and Gas Transport Properties

Membrane materials featuring ultrafast and highly selective transport properties combined with good chemical, mechanical, and thermal stability in the process environment are highly desired for energy-efficient membrane gas separations. Polybenzoxazole (PBO) polymers possess many of the above mentioned attributes and are considered to be good candidates for gas separation membranes. For example, the well-known thermally rearranged (TR) polymer membranes that feature complicated PBO structures are so far the best performing membranes for natural gas purification (i.e., CO₂ removal from methane). However, there are two major practical barriers in producing the TR-PBO membranes: very high conversion temperature (> 400 C) and strict requirement of inert atmosphere or vacuum. In this paper, we investigate the feasibility and efficiency of a new approach of thermal cyclodehydration of polyhydroxyamide (PHA) precursors to produce high performance PBO membranes. In depth comparisons between the PHA-PBO and TR-PBO routes show that PHA cyclodehydration route can produce fully converted PBO membranes at a temperature at least 100 C lower than the TR route. Besides the very comparable transport properties exhibited by the PBOs from these two routes, PHA-PBOs are considerably more mechanically robust than the TR polymers because the low PHA-to-PBO conversion temperature eliminates the possibility of thermal degradation that is very likely in TR route. More importantly, it is demonstrated for the first time in this paper that PBO films can be produced equally well in both air and inert atmospheres via PHA cyclodehydration route, i.e., PBOs formed in air have statistically the same physical and transport properties as of those formed in inert atmosphere. In this talk, synthesis and characterization of these new PBO membranes derived from PHA cyclodehydration will be presented. Fundamental transport properties including pure gas permeation data (with H₂, N2, O₂, CO₂, and CH₄) will be discussed to illustrate the fundamental relationship between microscopic structures with macroscopic transport properties for these new PBO membranes.