Preparation and characterization of poly (N-vinyl imidazole) gel-filled nanofiltration membranes

Poly (N-vinyl imidazole) (PVI) gel-filled nanofiltration (NF) membranes were synthesized by UV-initiated graft polymerization of N-vinyl imidazole (VI) monomer , with and without crosslinker N,Nʹ-methylenbisacrylamide (MBAA) on/in a polysulfone (PSf) ultrafiltration (UF) substrate. A comprehensive characterization of the resulting composite membranes was conducted in terms of surface chemical composition (ATR-FTIR, XPS), surface morphology (SEM), surface charge (zeta potential), surface wettability (water contact angle) and barrier properties (water permeance, neutral solute/ions rejection and membrane stability). From rejection data of neutral solutes, including Vitamin B 12 and PEGs , the average pore sizes and pore size distributions of membranes were further determined. These results confirmed the prepared membranes as stable, defect-free, smooth, hydrophilic NF membranes with effective pore size between ~1.5 and 2   nm and water permeance between ~0.5 and 10   L   m −2   h −1   bar −1 . Moreover, owing to the weak polybase characteristic of the incorporated PVI gel, the membranes would carry slight positive charge in neutral or slightly acidic solution. The small pore size membranes displayed high salt rejections in the order of MgCl 2 >NaCl≈MgSO 4 >NaSO 4 at near-neutral condition (pH 5.9±0.3).This testified their NF separation capability based on size exclusion, facilitated by Donnan repulsion. At last, by comparison with another reported type of gel-filled NF membranes with microporous substrate, and by applying a gel model, we delivered a qualitative description of the underlying fundamental parameters controlling the performance of such gel-filled membranes. Overall, these novel weak polybase gel-filled NF membranes with desirable nanometers-size pores and potentially tunable charge characteristics could offer great versatility for molecular-level separations.