Polyamide Composite Nanofiltration Membranes Via Controllable Interfacial Polymerization

Polyamide composite membranes have been widely explored in water softening,brackish water purification and wastewater treatment because of their easy synthesis,facile operation and high selectivity.Generally,these membranes consist of a non-woven fabric support,a porous substrate and a dense polyamide layer.Most of the polyamide selective layers are synthesized via the interfacial polymerization between diamine monomers and triacid chloride in two immiscible solutions.This rapid and uncontrollable reaction raises the difficulties to control the structure of the polyamide layer and tailor the performance of the polyamide composite membranes.The key to improving the controllability of the interfacial polymerization is to control the reaction rate and the uniformity of the polymerization.In this thesis,we proposed four novel strategiess to effectively control the interfacial polymerization from the two key aspects.The controllable interfacial polymerization was used to prepare high-performance polyamide composite nanofiltration membranes.Firstly,we developed a novel method to improve the uniformity of interfacial polymerization by constructing cellulose nanocrystals(CNC)/grapheme oxides(GO)interlayer on the surface of the microfiltration support.1D nanorods CNC effectively improves the hydrophilicity of the nanocomposite interlayers and enlarges the packing space of the GO nanosheets,which significantly improves the permeability of the interlayers.The 2D nanosheets GO remarkably reduce the surface roughness of the microporous substrate,which provides a smooth and stable interface for interfacial polymerization.Thus,a uniform and integral polyamide selective layer with thickness less than 15 nm was synthesized at ultralow monomer concentration.The polyamide composite membranes exhibit extremely high water permeance(45.9 L/m2?h?bar)and high salt rejection of 95.3% to Na2SO4.Secondly,we proposed a vacuum-assisted strategy to prepare polyamide composite nanofiltration membranes with a diameter of 30 cm.This method realizes the uniform distribution and constant adsorption weight of diamine monomer solution in the porous substrates,which improves the uniformity of interfacial polymerization.Therefore,the polyamide selective layer was controllably synthesized with a smooth surface and high cross-linking degree.The polyamide composite membranes with a diameter of 30 cm show uniform nanofiltration performances.They exhibit ultrahigh rejection above 99.6% to Na2SO4 and high water permeance of 20 L/m2?h?bar.Thirdly,we established a new method to improve the controllability of interfacial polymerization by controlling the diffusion of amine monomers with glycerol.The highly viscous,water-soluble,non-toxic and environment-friendly glycerol was utilized to adjust the viscosity of the diamine solution.The process of the interfacial polymerization is well controlled by increasing the viscosity of the diamine solution to confine the diffusion of diamine from the aqueous solution to the reaction interface.Polyamide composite membranes with adjustable polyamide layer thickness from 5.6 to 32.3 nm were prepared by the controlled interfacial polymerization.When the thickness of the polyamide layers decreased from 32.3 nm to 15.1 nm,the water flux of the composite membrane increases by 51% while maintaining a high rejection of Na2SO4(> 99.4%).Lastly,we developed a facile method to control the process of interfacial polymerization by using Janus supports to tune the adsorption amount of the diamine monomer.The adsorption amount of the diamine solution can be easily adjusted by controlling the hydrophilic modification depth of Janus membranes.When the diamine monomer concentration is constant,the monomers participating in the interfacial polymerization are decreased with decreasing the adsorption amount of the diamine solution in the Janus support,which producing thinner polyamide layers.The polyamide layers with thickness less than 10 nm were synthesized on Janus supports and these polyamide composite nanofiltration membranes show ultrahigh water permeance(> 47.5L/m2?h?bar)and high salt rejection of 95.8% to Na2SO4.