Investigation On The Preparation And Performance Of Self-assembly Nanofiltration Membrane Based On Coordination Interaction

Layer-by-layer （LBL） self-assembly is a simple and versatile technique for formingcomposite membranes, and a number of studies have been carried out to investigatethe utility of LBL for preparation of pervaporation, nanofiltration （NF）, andgas-separation membranes. However, the driving force for the LBL assembly appliedso far is mostly electrostatic force of oppositely charged polymers in aqueous solution.Moreover, the driving force of electrostatic interaction is relatively weak, thereforemany hydrophilic polymers swell dramatically and are even destroyed in aqueoussolutions, and the formed composite membranes usually give large fluxes but little orno transport selectivity.In addition to electrostatic interaction, other driving force such as hydrogenbonding, coordination and chemical crosslinking can also be used for LBLself-assembly. The development and improvement in this field have enriched thisalternate deposition technology, and also have provided a much extensive space forconstructing versatile functional materials. Metal-ligand coordination is a more stableforce compared with the electrostatic interaction between polymer molecules, and hasbeen regarded as a promising driving force to build multilayer films.In this work, a coordination multilayer was explored by the alternate deposition of atransition-metal-containing polyelectrolyte (PSS（Cu）_1/2) and pyridine-containingpolymer （P4VP） on ultrafiltration（UF） membrane to investigate the effect ofmetal-ligand coordination on the performance of the prepared LBL membrane, andthe stability of the membrane. Two different preparation methods, static and dynamic,were investigated and compared, and the prepared LBL membranes werecharacterized by AFM, SEM and contact angles.Through the research of static self-assembly membrane preparation, the optimal condition and the performance of multilayer films were investigated. The resultshowed that the optimal conditions was: PSS（Cu）_1/2as the first deposition layer,0.4%（wt） PSS（Cu）_1/2,0.2%（wt） P4VP, each layer deposition time was15min. And theprepared4.5bilayers （n=4）(PSS（Cu）_1/2/P4VP)4/PSS（Cu）_1/2has a85%rejection ofNa₂SO₄and a solution flux of60L.m^-2.h^-1at1.0MPa and room temperature, while therejection ratio of NaCl was only20%and the solution flux was about85L.m^-2.h^-1atthe same situation. The significant difference of the rejection performance betweendifferent valent ions indicates that the prepared multilayer membrane has thecharacteristics of NF membranes and can be used for the separation of mono andbivalent ions. SEM pictures showed that the alternate self-assembly of the PSS（Cu）_1/2and P4VP polyelectrolytes turned the porous substrate UF membrane eventually todense NF membrane, which is also an indication of the successfully assembly. Theaverage roughness increases gradually with the increase of the membrane bilayernumber. Membrane contact angle decreases with the increase of the membrane bilayernumber which showed that membrane hydrophilic became better and the filmanti-pollution ability could be strengthened. The stability of two kinds ofnanofiltration membranes,(PSS（Cu）_1/2/P4VP)4/PSS（Cu）_1/2of coordination interactionand （PDADMAC/PSS）5of electrostatic interaction, was investigated and the resultsshow that the (PSS（Cu）_1/2/P4VP)4/PSS（Cu）_1/2multilayer is more suitable for watertreatment.Dynamic self-assembly membrane preparation method was investigated. Theprepared3.5bilayers （n=3）(PSS（Cu）_1/2/P4VP)4/PSS（Cu）_1/2has a88%rejection ofNa₂SO₄and a solution flux of43L.m^-2.h^-1at1.0MPa and room temperature, while therejection ratio of NaCl was only23%and the solution flux was about74L.m^-2.h^-1.3.5bilayers polyelectrolytes multilayer membrane prepared by dynamic method hasbetter rejection than4.5bilayers polyelectrolytes multilayer membrane by staticmethod, which shows that dynamic method has advantages in LBL self-assemblyprocess. The SEM images show that the dynamic method can successfully assemblypolyelectrolytes on the supporting membrane surface. Proper heating temperature canimprove the membrane rejection, but higher temperature will destroy the structure of the film and reduce the membrane rejection. The stability of the preparedself-assembly LBL NF membranes during two weeks time showed that the rejectionand flux of the dynamic (PSS（Cu）_1/2/P4VP)₃/PSS（Cu）_1/2NF membrane remainedstable, no occurrence of major fluctuations was found. This indicated that NFmembrane based on coordination interaction could resist the impact of the brinesolution and maintain excellent rejection performance and better stability.