Computer Simulations On The Porous Membrane Formation By Block Polymer Self-assemble

Recently,polymer membranes have gained more and more attentions by academia and industry since its wide range of applications and easy grafting modification.Using simulation methods to study the mechanism and laws of membrane formation is useful to explain the microscopic phenomena in experiments and provide experimental guidance.In this thesis,we use dissipative particle dynamics(DPD)and multi-body dissipative particle dynamics methods(MDPD)to study the process of polymer self-assembling into porous membrane and the mechanism of micro-phase separation induced by non-solvent.Firstly,DPD was used to simulate the polystyrene-b-poly(4-vinyl pyridine)(PS-b-P4VP)block copolymer self-assemble with the non-solvent induced phase separation method(NIPS).We investigated the effects of polymer concentrations,block ratio and solvent exchange rate on the polymer self-assemble process,as well as on the structure of porous membrane.The affinity between polymer and non-solvent is an important factor determining the micro-phase separation,too high polymer concentration would limit the micro-phase separation while too low concentration would result in lamellar structures.Only under proper polymer concentration,can the channel membrane formed.Block ratio also plays an important role in shaping the pore structure.If the PS ratio is too high or too low,we could not obtain perfect morphology.Solvent exchange rate affects the degree of micro-phase separation in each simulation stage.It plays a vital role and impacts on the process of micro-phase separation.From the above conclusions,we can know the laws of membrane formation via non-solvent induced phase separation,so we investigate the self-assembly of tri-block copolymer PS-PAA-PEO into channel membrane by NIPS.It was found that the channel membrane could be obtained in the concentration range of 30% ~ 45%.The morphology evolution clearly showed the micro-phase separation process.The polymer chains of PS formed the matrix part of the channel membrane and the chains of PAA acted as the “open/close” switch,while the chains of PEO were distributed in the innermost.At pH lower than the pKa of PAA,the carboxyl groups of PAA are protonated,therefore the membrane pores are in the “open” state.In contrast,at neutral pH,the membrane pore is in the “closed” state because the repulsion between negative groups makes PAA chains extensively swelled.With pH values changing from 1 to 6.25,the membrane pore size decreased from 19.3 nm to 2.9 nm.The filtration of different sized nanoparticles were studied to justify the accuracy of the pore sizes under different dissociation degrees.The permeation results confirmed that the channel membrane,which was in the “close” state due to the swollen conformation of PAA,had the size-based filtration function.Finally,we used the MDPD method to simulate vapor-induced phase separation processes.We established the structure distribution figure at three factors,such as polymer concentration,solvent selectivity and block ratio.The effect of polymer concentration on the polymer self-assemble is the greatest.When the polymer concentration changes from high to low,the morphology changes from lamellar to porous,rod-like and spherical construction.The selective solvent and block ratio affect the distribution of the polymer.Each structure has its own formation conditions,and their application fields are different.