Computer Simulation On The Polymer Membrane Modified Via Surface Segregation

Membrane separation is a new energy-saving technology for separation and purification of multicomponent mixture.However,during the process of membrane separation,contaminants such as protein and oil pollution can easily form a continuous contamination layer covering the surface of the membrane,which reduces the separation efficiency.Modification of porous membranes by surface segregation is a research hotspot in membrane science.At present,some technological progress has been made in experiments;however,at the molecular level,the chemical structure and the mechanism of surface segregation of membrane materials have not been fully understood.Computer simulations can explain the experimental phenomena at microscale,explore the influencing factors of membrane formation,predict the experimental results and realize the molecular design of functional materials.The purpose of this thesis is to study the self-assembly behavior of polymers under non-solvent induced phase separation(NIPS)by dissipative particle dynamics(DPD)simulations and to study the morphological changes and the mechanism of NIPS process incorporated with surface segregationFirstly,the forced surface segregation process of triblock copolymer homogeneous membranes prepared by NIPS was studied by replacing the good solvent with non-solvent.The copolymer bears hydrophobic cellulose acetate(CA),hydrophilic poly(ethylene glycol)methacrylate(PEGMA)and low surface energy poly(hexafluorobutyl methacrylate)(PHFBM)segments.Homogeneous pore structure can only form at the polymer concentration range of 15~40%.Different degrees of phase separation happen under different PHFBM/PEGMA block ratios.The introduction of PEGMA segments promotes the migration of low surface energy PHFBM segments to the surface.The controlled morphology regulation of channel pore structure can be realized via adjusting CA-PHFBM-PEGMA/CA blend ratio in the range of 0~60%.Then,a model comprised of a non-solvent water bath and a casting solution was constructed.The casting solution contains the zwitterionic copolymer polyethersulfone-blockpolycarboxybetaine methacrylate(PES-b-PCBMA)as the blending additive and polyethersulfone(PES)as the bulk membrane material.The non-solvent bath is used for solvent/non-solvent exchange with the casting solution so as to mimick the NIPS process.Simulation results show that hydrophilic PCBMA enrich on the membrane surface through surface segregation.Because the carboxylic acid group of PCBMA is a weak electrolyte,it endows the membrane with pH-responsiveness characteristic.With the increase of protonation degree of carboxylic acid groups,the conformation of PCBMA chains on the membrane surface changes from collapse to extension.Finally,we take inorganic silica nanoparticles(SNPs)as the research object to explore its behavior in polymer matrix during surface segregation modification of polymer membrane.It is found that a polymer enrichment skin layer forms at the solvent/non-solvent interface during NIPS,which provide adhesion environment for SNPs.The grafted SNPs self-assemble into orderly structure on the membrane surface.The surface segregation effect of SNPs will be weakened by the increase of polymer concentration,while the elution of SNPs will happen at low polymer concentrations.The spacing of SNPs on the surface of the membrane can be adjusted through the adjustment of the grafting chain length.