Impact of crosslinking on organic solvent nanofiltration performance in polydimethylsiloxane composite membrane: Probed by in-situ low-field nuclear magnetic resonance spectroscopy

Polydimethylsiloxane (PDMS) has been widely used in gas separation, pervaporation, and organic solvent nanofiltration. The crosslinking and penetration behavior of PDMS has been of great importance for fabricating composite membranes. In this study, an in-situ low-field nuclear magnetic resonance (LF-NMR) spectroscopy was employed to probe the crosslink density of PDMS during the crosslinking process. The relationship between crosslink density and nanostructure of PDMS selective layer was systematically investigated. It was found when the PDMS network had low crosslink density (υe < 1.32 × 10-3 mol cm-3), the un-crosslinked PDMS chains could easily infiltrate into the pores of the substrate, leading to a defective active layer. Therefore, the rejection for Evans blue from organic solvents was below 3.9%. With the increasing crosslink density to 1.57 × 10-3 mol cm-3, the integrity of the PDMS selective layer was remarkably reinforced with a rejection of 99.0%. Moreover, the as-prepared membrane exhibited good performance for concentrating trace vitamin B12 from different solvents. The swelling property was responsive monitored with T2 relaxation time using LF-NMR.