Research On Reverse Osmosis Composite Membrane Based On Amphiphilic Hyperbranched Polymer

Aromatic polyamide reverse osmosis composite membrane is widely used in seawater/brackish water desalination,(super)pure water preparation and wastewater treatment,but the separation performance of the membrane needs to be further improved to reduce the operating cost of reverse osmosis process.In this thesis,two hyperbranched polymers with high-density terminal functional groups and a core-shell hydrophilic / hydrophobic phase separation structure were synthesized,including hydroxyl-terminated hyperbranched polyether molecule(HBPO-star-PEO-OH)and terminal acyl chloride group hyperbranched polyether acyl chloride molecule(HBPO-star-PEO-COCl),and introduced into the polyamide separation layer through interfacial polymerization to regulate the microstructure of the reverse osmosis composite membrane and improve the separation performance of the membrane.Hyperbranched polyether HBPO-star-PEO-OH was synthesized by cationic ring-opening polymerization,then mixed with m-phenylenediamine(MPD)in aqueous solution,and finally introduced into the polyamide layer by conventional interfacial polymerization.By changing the addition amount of polymer and adding additives such as acetonitrile and lithium bromide into aqueous solution,the solvent composition was improved and the membrane production process was optimized.Experimental results showed that compared with the original reverse osmosis membrane,the separation performance and membrane structure of the reverse osmosis composite membrane which based on HBPO-star-PEO-OH have been improved.When the addition amount of HBPO-star-PEO-OH was 0.01 wt% and the content of lithium bromide in the water-phase assistant was 0.05 wt%,the separation performance of the modified membrane was the best.Its water flux was 28.0 LMH,and the desalination rate can reach 98.9%,meanwhile,the separation layer of the membrane was thinner and denser.However,compared with commercial membranes,their separation performance needs to be further improved.Therefore,in order to further improve the separation performance of the modified membrane,the hydroxyl-terminated hyperbranched polyether molecule HBPO-star-PEO-OH was first synthesized through an acid chloride reaction to obtain an acid-terminated chloride-branched polyether acid chloride molecule HBPO-star-PEO-COCl.Then,it was introduced into the polyamide separation layer of the membrane through a new secondary cross-linking method as the first oil-related bond functional monomer.By changing the addition amount of HBPO-star-PEO-COCl and the composition of the first oil phase mixed solvent,the film formation process of the secondary crosslinking method was optimized.The experimental results showed that when the additive amount of HBPO-star-PEO-COCl was 0.005 wt% and the organic phase solvent composition was 10 wt% tetrahydrofuran / n-hexane,it had the best membrane structure and separation performance.Its water flux can reach 69.8 LMH,about twice that of conventional polyamide commercial membrane(33.27 LMH),and the salt rejection rate can reach 99.22%.The structure of the composite membrane was studied by transmission electron microscopy,positron annihilation and other techniques.It was found that the modified reverse osmosis composite membrane had a thin(51 ± 2 nm)active layer and a dense,large free volume.Excellent membrane structure makes the separation performance of reverse osmosis membrane based on HBPO-star-PEO-COCl greatly improved.All in all,the improved separation performance of hyperbranched polyether-based reverse osmosis composite membranes can be attributed to the unique molecular structure of hyperbranched polymers.On the one hand,hyperbranched polyethers have a core-shell hydrophilic/hydrophobic phase separation structure.The ethylene oxide shell in the molecule is highly hydrophilic,while the internal HBPO polyether core is highly hydrophobic.It is easy to form a three-dimensional quasi-spherical structure in the aqueous solution,thereby promoting water molecules to quickly pass through the polyamide layer along the core-shell two-phase hydrophobic wall surface,and providing a water molecule transmission channel.On the other hand,the rich end groups and three-dimensional network structure of the hyperbranched polyether molecules can increase the crosslinking density of the polyamide layer,thereby increasing the salt rejection rate of the film.Therefore,this study provides an effective way for the development of new reverse osmosis composite membranes with good comprehensive separation performance.