Development of High-performing Polydimethylsiloxane-based Membranes for Carbon Dioxide Separation

Membrane separation is highlighted as one of the most promising approaches to mitigate the excessive CO₂ [carbon dioxide] emission, due to its significant reduction of energy cost compared with many conventional separation techniques. Unfortunately, the separation performance of current membranes does not meet the practical CO₂/N₂ [nitrogen] separation requirements. And due to the huge volume of industrial flue gas, membranes with exceptionally high permeability are needed for practical reasons.

Currently, the separation mechanism of most polymeric membranes is based on size-sieving. However, this method is not sufficient for CO₂/N₂ separations due to the similar kinetic diameters of CO₂ (3.30 Å [angstrom]) and N₂ (3.64 Å). Thus, developing a new method based on a non-size sieving mechanism could offer a solution to the improvement of CO₂/N₂ separation efficiency.

In this dissertation, (bicycloheptenyl) ethyl terminated polydimethylsiloxane (PDMSPNB) membranes were firstly studied. The developed polymer membranes show higher permeability and better selectivity than those of conventional PDMS [polydimethylsiloxane] membrane. The achieved performance (CO₂ [CO₂ permeability] 6800 Barrer and α[CO₂/N₂] [CO₂/N₂ selectivity] 14) is very promising for practical applications. The key to achieving this high performance is the use of an in-situ cross-linking method of the difunctional PDMS macromonomers, which provides lightly crosslinked membranes. By combining positron annihilation lifetime spectroscopy, broadband dielectric spectroscopy and gas sorption measurements, we have elucidated the key parameters for achieving their excellent performance.

In the following work, two CO₂-philic groups, amidoxime (AO) and polyethylene oxide (PEO), were successfully incorporated into the rubbery PDMS and glassy poly(1- trimethylsilyl-1-propyne) (PTMSP) systems. By the careful tuning of functional group composition, the membranes showed controlled CO₂/N₂ solubility selectivity. The combination of CO₂-philic groups and highly permeable polymer matrix showed one of the highest CO₂/N₂ separation performance among all polymeric membranes. The overall gas separation performance (P_CO2 6800 Barrer and α[_CO2/N2] 19 for AO-PDMSPNB; P_CO2 3400 Barrer and α[_CO2/N2] 19 for PEO-PDMSPNB; P_CO2 6000 Barrer and α[_CO2/N2] 17 for AO-PTMSP) of the highest performing membranes have exceeded/achieved the Robeson upper bound line. These studies provide a roadmap to enhancing gas separation performance in rubbery and glassy polymers by tuning gas solubility selectivity.