Formation Visualization Of Interfacially Polymerized Film And Preparation Of Composite Membrane For CO₂Separation

Interfacial polymerization （IP） has been a well-established method for thepreparation of commercially thin film composite （TFC） reverse osmosis （RO） andnanofiltration （NF） membranes. Moreover, it has been proved that IP is also aneffective way for the synthesis of gas separation membranes. However, little attentionhas been paid to the study of IP film formation. Meanwhile, no information has beenreported on studying the formation-structure-performance relationship for interfaciallypolymerized TFC gas separation membranes. Based on the above consideration, thisstudy makes attempts to develop two techniques to visualize the film formation by IP,as well as to elucidate the formation-structure-performance relationship forinterfacially polymerized TFC gas separation membranes based on the reaction ofN-methyldiethanolamine （MEDA） and trimesoyl chloride （TMC）.An optical contact angle measuring device （OCA） was used to visualize themorphology of pendant drop of aqueous solution immersed in organic solution andthe film formed during IP process. Firstly, the variations of the morphology ofpendant drop of aqueous solution were studied, the results offered in-situexperimental evidence for the growth of interfacial film towards the organic phase.Furthermore, the effects of various synthesis conditions on film morphology werestudied. It was found that the bubbles were generally appeared on the films. Thebubbles on the films were attributed to the instability arising from the formation of thenascent solid thin film with a huge surface energy. The bubbles on the films couldultimately result in the rough membrane surface.A digital microscope was used to visualize the film morphology during IPprocess. Firstly, the diffusion of the monomers in both aqueous and organic phaseswas analyzed, and the results show that the diffusion of the monomers in both phasescould cause the interfacial instability of the aqueous-organic interface. Moreover, itwas found that the morphology of interfacially polymerized film is of multi-scalecharacteristic, wherein small-scale bubbles were evenly distributed on large-scalerugged film surfaces. The small-scale bubbles on the films were attributed to theinstability arising from the formation of the nascent solid thin film with a huge surfaceenergy. And the large-scale rugged film surfaces, which reflected the fluctuations of the aqueous-organic interface, were attributed to the interfacial instability arising fromthe presence of very steep concentration gradient of aqueous monomer near theaqueous-organic interface.TFC membranes for CO₂/N₂separation were prepared by IP on crosslinkedpolydimethylsiloxane （PDMS） coating polysulfone （PS） support membrane. Thestructural properties of TFC membrane surfaces were characterized by severaltechniques. The relationships among the skin layer formation conditions, skin layerstructure, and membrane separation performance were investigated. Results show thatmembranes with higher CO₂permeance and good CO₂/N₂selectivity appeared toconsist of thinner, more crosslinked, and less crystalline skin layer structures. Suchhigh performance gas separation membranes were obtained by （1） increasingdiffusivity and decreasing solubility of the aqueous monomer in the organic solvent,and （2） reducing the concentration of the organic monomer and raising theconcentration of the aqueous monomer. These findings have great theoreticalsignificance for the controlled preparation of gas separation membranes.