Maintaining hexagonal structures through interfacial positioning of crosslinkers for nanofiltration

Hypothesis: Optimizing interfacial positioning of crosslinkers within a reactive self-assembled hexagonal lyotropic liquid crystals (HLLC) system could assist in retaining the hexagonal structure during polymerization and thereby improving water filtration performances of the as-synthesized nanofiltration membranes.

Experiments: The positioning of the hydrophilic crosslinker, poly (ethylene glycol) diacrylate (PEGDA), within the reactive HLLC system was systematically investigated using ²H and ¹³C solid nuclear magnetic resonance (NMR) and small angle X-ray scattering (SAXS) techniques. The structural variation and water filtration performances of these HLLC systems with/without crosslinkers after polymerization were further studied using grazing incidence SAXS (GISAXS) and crossflow filtration tests, respectively.

Findings: The hexagonal structure can be effectively retained in the reactive HLLC system when an optimal PEGDA content is introduced and distributed well in the water pathways between the hexagonally packed cylinders. The system will otherwise transit into a lamellar structure if no PEGDA introduced even with reactive head groups. Crosslinkers with longer molecular chains are more effective in connecting the neighbouring cylinders and stabilizing the hexagonal structure during polymerization. The well-retained and aligned HLLC system results in a defect-free active layer with highly continuous water pathways, which improves both permeability and ion rejection compared to the system without PEGDA. Understanding the interfacial positioning of crosslinkers allows for precise control of the hexagonal structure during polymerization via molecular design for fabricating effective nanofiltration membranes.