Plasticization of a novel polysulfone based mixed matrix membrane with high-performance CO2 separation studied by positron annihilation

Plasticization is one of the challenges encountered by polymer membranes in the application of gas separation. In this paper, a novel Schiff base network/polysulfone mixed matrix membrane (SNW-3/PSF MMM) was synthesized. The CO2 adsorption, permeation and plasticization behaviors of the membrane were investigated. The SNW-3/PSF MMM shows an obviously enhanced carbon capture capacity, and a more than 400% increase in CO2 permeability, compared with the pristine PSF. The CO2 plasticization pressure of SNW-3/PSF MMM also increases from 23.8 atm to 31.8 atm. Innovatively, the variation of free volume of the membranes after plasticization was investigated by positron annihilation spectroscopy. The results show that both free volume size and fractional free volume (FFV) increase after plasticization of MMM and pristine PSF, with a larger increase in pristine PSF. During the subsequent relaxation after removing CO2 exposure, the free volume size and FFV both show rapid decrease with time. For SNW-3/PSF MMM, the FFV recovers more nearly to the value before plasticization, than pristine PSF. Our results show that positron is a superb probe for the precise information about the microstructural changes in plasticization of gas separation polymer membranes.