Development of Polybenzoxazole (PBO)-Based Polymer Membranes for Gas Separation

Polybenzoxazoles (PBOs) are highly attractive membrane materials because of their superior gas separation parameters (high permeability and high selectivity) and excellent mechanical and thermal stability. However, the practical application of PBO-based membranes is frequently frustrated by their poor processability due to insolubility problem. Current PBO synthesis approach addresses the insolubility issue by solid-state thermal rearrangement (TR) of aromatic polyimides (APIs) with ortho-positioned hydroxyl group, wherein the soluble API precursors are solution cast into membranes before being thermally converted to TR membranes.

However, the API-TR process poses some major challenges, complicating the implementation of TR membranes for practical industrial applications. First of all, the temperatures required for the TR conversion are very high (> 400 °C), which severely limits the choice of porous supporting polymers used in common membrane modules such as hollow fibers. Moreover, the high thermal conversion temperature causes collateral thermal degradation in the TR polymers, causing brittleness and defects in the resulting membranes. In addition, the thermal conversion process requires strict inert media (N₂ or vacuum) for the imide-to-benzoxazole conversion to occur, which makes the API-TR process of PBO membranes production highly cost-intensive and complex.

This work explores the feasibility of alternative routes to produce high performance PBO-based membranes as well as investigate new synthesis approach of PBOs with much improved processability to overcome the abovementioned challenges in the API-TR route. Specifically, four approaches to form PBO-based membranes have been investigated in this work:

• Developing an alternate process to produce PBO membranes via thermal cyclodehydration (CD) of polyhydroxyamide (PHA) precursors:
• Investigating the interplay between the TR and CD processes in a series of blends of API and PHA in varying ratios:
• Developing a novel one-pot, solution cyclization synthesis route to produce organo-soluble PBOs (S-PBOs) via a silylation assisted process of in situ amide-to-benzoxazole conversion:
• Synthesizing PBO-based copolymers via a combination of in situ solution cyclization and solid-state TR conversion to maximize separation performance: