Abstract
It was shown for the first time that the conditions of thermal treatment of the casting solution significantly affect the morphology and transport properties of porous, flat, and hollow fiber polysulfone (PSf) membranes. It is ascertained that the main solution components that are subjected to thermo-oxidative destruction are the pore-forming agent polyethylene glycol (PEG) and solvent N-methyl-2-pyrrolidone (NMP). It is proved that hydroxyl groups of PEG actively react in the process of the casting solution thermo-oxidative destruction. It is shown that despite the chemical conversion taking place in the casting solution, their stability towards coagulation virtually does not change. The differences in the membrane morphology associated with the increase of thermal treatment time at 120 °C are not connected to the thermodynamic properties of the casting solutions, but with the kinetics of the phase separation. It is revealed that the change of morphology and transport properties of membranes is connected with the increase of the casting solution viscosity. The rise of solution viscosity resulted in the slowdown of the phase separation and formation of a more densely packed membrane structure with less pronounced macropores. It is determined experimentally that with the increase of casting solution thermal treatment time, the membrane selective layer thickness increases. This leads to the decrease of gas permeance and the rise of the He/CO2 selectivity for flat and hollow fiber membranes. In the case of hollow fibers, the fall of gas permeance is also connected with the appearance of the sponge-like layer at the outer surface of membranes. The increase of selectivity and decline of permeance indicates the reduction of selective layer pore size and its densification, which agrees well with the calculation results of the average membrane density. The results obtained are relevant to any polymeric casting solution containing NMP and/or PEG and treated at temperatures above 60 °C.
Highlights
Polysulfone (PSf) and polyethersulfone (PES) are among the most widely used commercial bulk polymers in the fabrication of different kinds of separation membranes [1,2,3,4]
To overcome the problem of high viscosity and to reduce the time, the spinning solution is usually processed at elevated temperatures, and in this study, the temperature of 120 ◦ C was selected to conform to the maximal temperature used in literature to prepare casting solutions with polyethylene glycol (PEG) and NMP
The thermal treatment of the casting solution substantially affects the morphology and transport properties of porous flat-sheet and hollow fiber membranes made of polysulfone
Summary
Polysulfone (PSf) and polyethersulfone (PES) are among the most widely used commercial bulk polymers in the fabrication of different kinds of separation membranes [1,2,3,4]. This process is controlled by the different parameters including composition and viscosity of casting solution, the conditions of membrane fabrication (air gap height, polymers solution, and internal coagulant flow rates, etc.), the interaction between components of casting solution and coagulation bath It was shown [14] that the presence of components (1,4-dioxane, diethylene glycol or dimethyl ether, acetone) with lower miscibility with coagulant (water) rather than solvent (NMP) resulted in a more packed and dense skin layer of the asymmetric PSf membrane; while the membrane with a more open pore structure was obtained in the case of addition of γ-butyrolactone, possessing greater miscibility with water than NMP. Higher porosity of the drainage layer and permeance of resulted asymmetric membranes, can be achieved by the introduction of pore-forming components in the casting solution like polyethyleneglycol (PEG) and its derivatives [15,16,17], polyvinylpyrrolidone (PVP) [18,19,20], and surfactants [21,22]
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