Computer Simulation Of The Formation Of Anti-fouling Polymeric Ultrafiltration Membranes With The Addition Of Amphiphilic Block Copolymers

The core problem of the development of modern chemical engineering industry ishow to achieve the goals of energy saving, emission reduction and fine manufacturing.Membrane separation method, as an emerging technology, is beneficial to make thesustainable development of modern chemical engineering industry possible. The keyof membrane separation technology is to prepare the excellent-properties polymericultrafiltration membrane materials with a good separation performance, a highwater-flux and a strong anti-fouling ability. In recent years, the blending method withthe addition of amphiphilic block copolymers to prepare polymeric membranes hasdrawn numerous attentions. In this work, the formation of an anti-fouling polymericultrafiltration membrane with the addition of amphiphilic block copolymers in thequaternary membrane-preparation system was simulated applying a polymer bondfluctuation lattice model with the Monte Carlo method. Aiming at the phaseseparation process by immersion precipitation, the effects of amphiphilic blockcopolymer additives on the thermodynamics and diffusion kinetics in themembrane-preparation system and the evolution process of phase structure andmorphology were studied in this paper. Our simulation results are of theoreticalsignificance for preparing polymeric ultrafiltration membranes with the blendingmethod.First, the isothermal phase diagram of membrane-preparation system wascalculated using the ternary approximation way based on mean field theory. It wasfound that the stable section and the meta-stable section of the isothermal phasediagram became bigger; the unstable however became smaller, which demonstratedthat the addition of amphiphilic block copolymer could enhance the thermodynamicstability of the membrane-preparation system. In addition, with the increase of theconcentration and the hydrophilic segment length of amphiphilic block copolymer,the diffusion rate of nonsolvent in the polymer solution increased.Second, the effects of amphiphilic block copolymer on the phase structure andmorphology in the membrane-preparation system were studied. It was concluded that,the addition of amphiphilic block copolymer could increase the amount of membranepore, and could suppress the formation of finger-like macrovoid structures; the increase of the concentration and the hydrophilic segment length of amphiphilic blockcopolymer could increase the average size of membrane pores and could induce theinner pores to merger and to form the interconnected pore structures; in addition,pentablock copolymers exhibited better performance for inducing the formation ofporous structures than triblock or diblock copolymers. M-N-M-N-M and M-N-Mstyle copolymers as pore-formation agents were better than N-M-N-M-N and N-M-Nstyle copolymers, respectively.Third, the surface segregation phenomenon in the interface of polymer phase andnonsolvent phase during the phase separation process was studied. It was found thatsurface segregation was beneficial to reduce the surface tension of polymer phase soas to promote gradually the formation and coalescence of membrane pores. Theamphiphilic block copolymer additives with different molecular structure stretchedwith different conformations on the surface of membrane pores. In addition, it wasfurther found via analyzing the change of average pore size of different sub-layers that,the evolution process of asymmetric structure in the membrane-preparation systemcould be divided roughly into be three stages and the diffusion behavior of nonsolventand the evolution of membrane structures owned different characters in every singlestage.