(340d) Positron Annihilation Spectroscopy and Molecular Dynamics Study of the Membrane Formation Mechanism of Different Cellulose Derivatives Via Phase Inversion

With unique advantages including low fouling propensity, good mechanical property and wide availability, cellulose acetate (CA) has been extensively studied over the past 40 years as a model for the revelation of phase inversion and membrane formation mechanism. However, so far, nearly all existing studies about phase inversion focus on the formation of a dense selective layer on the top of asymmetric membranes. Much less attention has been paid to the structure at the bottom interface between polymer solution and casting substrate. Meanwhile, positron annihilation spectroscopy (PAS) has become a popular and powerful tool for the investigation of the layer structure of polymeric membranes in recent years. Tremendous research interest is therefore stimulated to study the formation mechanism of the membrane bottom layer of different CA polymers through PAS technology and molecular dynamics simulation.

It has been revealed in this work that the hydrophilicity of both the polymers and casting substrates plays a deterministic role in the porosity of the bottom layer of the membranes. The total energy of polymer-substrate pairs with similar hydrophilicity decreases significantly when they get close to each other, which initiates the formation of a dense selective layer at the bottom interface, while for polymer-substrate pairs with distinct hydrophobility, a fairly porous bottom surface is formed. An in-depth understanding of the resulted membrane structure has been elucidated with PAS. Other factors, such as membrane thickness, coagulant and solvent, have great influences on the formation process of the bottom layer as well. Slower phase inversion process usually favors thicker dense layer at the bottom. By careful choosing of membrane fabrication conditions, membranes with an ultra-thin selective bottom layer and an inter-connected, fully porous support layer can be formed, which may have wide application in various separation processes, especially in the novel forward osmosis process for the improvement of water flux and reduction of internal concentration polarization.