(4do) Gas Transport and Sorption in Polymer Membranes for Energy-Efficient Separations

In the petrochemical industry, gas separations are traditionally accomplished using energy-intensive processes such as absorption and distillation.  With proper synthesis and design, polymer membranes could provide an attractive alternative to these traditional separation techniques.  However, there are three major challenges that need to be addressed to improve currently available polymer membranes.  Polymers must have high permeability, high selectivity, and a resistance to plasticization.  Permeability is the gas flux normalized by the film thickness and pressure driving force, and selectivity helps set the purity ratio of gas mixtures.  Plasticization is the swelling of a polymer in the presence of condensable gases, which results in a decrease in gas selectivity during the operating lifetime of a polymer membrane.

In 2007, thermally rearranged (TR) polyimides were first investigated for separating CO₂ from CH₄.  These polymers showed exquisite combinations of CO₂ permeability and CO₂/CH₄ selectivity while still maintaining a resistance to CO₂ plasticization under mild conditions.  Because of these composite characteristics, this study aims to elucidate the mechanism of transport through TR polymers and to determine the relationship between polymer structure and gas separation properties.

TR polymers are formed via a solid state, thermally-induced decarboxylation reaction of polyimides containing reactive groups ortho-position to the polyimide diamine.  Between temperatures of approximately 350°C and 450°C, these polymers are thermally reacted from fully soluble, processable polyimides to crosslinked, insoluble polybenzoxazoles.  To understand the mechanism of transport for small molecules in TR polymers, the gas diffusivity and solubility was determined as a function of TR polymer conversion for H₂, N₂, O₂, CH₄, and CO₂.  For CO₂, conversion of a TR polymer known as HAB-6FDA resulted in an over 20-fold increase in gas permeability and an over 10-fold increase in diffusivity.  However, solubility only increased by a factor of approximately 2 during conversion.  Therefore, TR polymers achieve their separation properties through largely increased gas diffusivity. 

This increase in gas diffusivity indicates that TR polymers are potentially useful for separations typically governed by diffusion selective polymers such as olefin/paraffin separation.  To investigate the utility of TR polymers for these separations, several polyimide structures with varying chemical backbone functionality were synthesized and tested for ethylene/ethane and propylene/propane separation.  Similar to what has been observed for other gas pairs, conversion of TR polymers to their final polybenzoxazole structure resulted in increased permeability, a minimal loss in selectivity, and an increase in plasticization resistance.