A Study On The Preparation And Water Processing Properties Of Interfacially Crosslinked Porous Polyelectrolyte Membranes

Porous polyelectrolytes feature the combination of charge and pores,both of which confer them with significant potential in water treatment.Due to the hydrophilicity and high glass transition temperature of polyelectrolytes,porous polyelectrolytes are not directly accessible by conventional methods such as nonsolvent-induced phase separation and thermal-induced phase separation that are suited for uncharged polymers.Existing methods of preparing porous polyelectrolytes face several challenges.For example,the layer-by-layer assembly is incapable of engineering directional pores,not to mention the labor-demanding experimental involved.The freeze-drying method,widely exploited to prepare porous polyelectrolyte monolith,requires lengthy time and relatively high energy due to the slow kinetics of ice removal by sublimation.In addition,the pores of hydrophilic polyelectrolyte porous membranes are susceptible to excessive swelling or even dissolution in aqueous environments.Focusing on the creation and stabilization of pores in polyelectrolytes matrix,this thesis aims to tackle the above-mentioned problems by means of interfacial polyelectrolyte complexation and crosslinking,resulting in porous polyelectrolyte membranes for water processing applications such as solar-thermal desalination and nanofiltration.The main contents of this thesis are as follows:（1）Honeycombed Isoporous Polyelectrolyte Membranes（HIPMs）with hierarchical pores were prepared through the synergy of phase separation and electrostatic complexation,two steps account for pore creation and stabilization,respectively.Dependence of the hierarchical pore structures on the ratio of the two raw materials,i.e.,poly(3-cyanomethyl-1-vinylimidazolium（PCMVIm）and pillar[5]arene,was studied.It was found that the formation of honeycomb pores on the membrane surface is due to the“PCMVIm～pillar[5]arene”microphase separation and dissolving of pillar[5]arene-rich phase,while the cross-sectional pores were formed as a result of PCMVIm phase separation in water.In addition,the“PCMVIm～pillar[5]arene”electrostatic complexation was crucial for the stabilization of pores.This method was suitable for preparing HIPMs on various substrates including glass plates,glass beads,and glass fibers,broadening its application scenarios.Finally,a heavy metal ion sensor was created by coating the HIPMs on a glass fiber,whereby the hierarchical porous structures endowed the sensor with good responsiveness and low detection limit of Cu²⁺ions.The detection limit and response speed of the stable,reusable sensor were 0.075 n M and 10 s,which are 60 and 4.5 times improved compared to analogous sensors without pores.（2）An Ice-Dissolving-Complexation（IDCo）method,where ice-dissolving accounts for pore creation while electrostatic complexation accounts for pore stabilization,was established to prepare porous polyelectrolytes and their nanocomposite membranes embedded with three-dimensional connected pores.Aqueous solution of sodium carboxymethyl cellulose（CMCNa）was frozen,immersed in Cu²⁺/ethanol bath（-20 ^oC）for1 h at atmosphere pressure.During the immersion process,ice crystals were dissolved in ethanol while simultaneously,Cu²⁺ions diffused into the CMCNa matrix and underwent electrostatic complexation with carboxylate groups of CMCNa,improving the stability of pores in aqueous environment.It was found that the ice crystals dissolved in ethanol（-20^oC）within 1 h,which was 30 times quicker than the rate of ice sublimation.By combining the IDCo method with directional freezing,porous composite membrane with vertically aligned pores(CMC-Cu²⁺-CNT)were prepared.This porous hybrid featured 99%high absorption in UV-Vis-NIR light,allowing for good solar thermal evaporation rate(2.2 kg m^–2 h^–1)under 1-sun irradiation and promising stability in long term solar-thermal operation.（3）The research interest in ice dissolving was continued.An Ice-Dissolving-Crosslinking（IDCr）method,where ice-dissolving accounts for pore creation while covalent crosslinking accounts for pore stabilization,was proposed to improve the structure stability of porous polyelectrolytes in concentrated salt solution.Aqueous solution of CMCNa was frozen,immersed in acetone bath（-20 ^oC）to remove ice crystal templates.Then the porous CMCNa was cross-linked in trimesoyl chloride（TMC）solution,in which the“acyl chloride-hydroxyl”crosslinking occurred.The covalently cross-linked sodium carboxymethyl cellulose had resilience in aqueous solution and could restore its original shape after 20 compression cycles.CMC-TMC prepared by the IDCr method features advantages such as faster crosslinking kinetics,lower mass loss and stable morphology when immersed in water.In addition,the chemically cross-linked porous CMC-TMC engineered by the IDCr method was stable in Na Cl solutions（3.5 wt%）in 1-week immersion while that prepared by IDCo method suffered from pore collapse.After the incorporation of CNTs,the CMC-TMC-CNT porous monolith featured an evaporation rate of 2.02 kg m^–2 h^–1 and exhibited excellent salt stability in 10-h solar desalination.（4）Polyelectrolyte nanofiltration membranes with sub-nano pores were prepared by modulating the interfacial crosslinking of amine-functionlized polyelectrolyte（PILNH₂）and TMC.PILNH₂ was synthesized and exploited to prepare nanofiltration membranes.The preparation conditions were optimized by regulating the concentrations of PILNH₂ and the reaction times.Furthermore,the chemical structures,e.g.,pore size and distribution,surface morphology and surface zeta-potential,of the PILNH₂-TMC membranes were characterized.The PILNH₂-TMC membrane was comprised of～20 nm large nanoaggregates,while its effective mean pore size and the surface isoelectric points of the membrane were 0.54 nm and 6.5,respectively.Water permeance of the PILNH₂ membrane was 12.6 L m^–2 h^–1 bar^–1,which is 3 times as high as that of the polyethyleneimine nanofiltration membranes.In addition,the ion rejection of the PILNH₂ membrane remained stable with temperature（30～50 ^oC）and operating pressure（1～6 bar）increase.After 30-days immersion in water,chemical structures and surface morphology of the PILNH₂-TMC membrane remained stable.The rejections of PILNH₂ membrane for divalent cations such as Mg²⁺were greater than 90%while that for monovalent cations such as Li⁺were about 40%.When it comes to the separation of Mg²⁺/Li⁺mixture with a ratio of 50,water permeance and ion selectivity of PILNH₂ nanofiltration membrane were 11 L m^–2 h^–1 bar^–1 and 10.5,respectively.