Manufacture of Asymmetric Membrane

These findings are helpful to design a new polyimide for the gas separation membrane. Further increase in the simulation accuracy as well as the expansion of the database are expected. 

In the support layer, the polymer matrix and the pores are co-continuously connected across the layer. The three-dimensionally continuous polymer network exhibits the sufficient mechanical strength, and the three-dimensionaUy continuous pores allow gases to flow through the layer without significant flow resistance. 

On the other hand, the skin layer is a continuous polymer layer without the pores. This layer covers the entire surface of the support layer and is considerably thin (typically less than 100 nm). Although the permeability of the skin layer is the same as that in bulk film, due to the thinness of this layer, the apparent flux of the permeant is significantly enhanced. In order to achieve large permeance, it is cmcial to prepare a very thin skin layer without defects. 

A number of methods has been developed for the manufacture of asymmetric membranes. However, the asymmetric membranes are mostly produced by the method developed by Loeb and Sourirajan (1962), namely the dry and wet method. This method is comprised of the following four steps (1) preparation of the polyimide solution (called dope), (2) molding (cast or spinning) of the dope, (3) coagulation of the dope by contact with nonsolvent of the polyimide to form the asymmetric structure, and (4) drying of the coagulated membrane. 

In the first step, solubility of the polyimide to the solvent, is important. Adequate solvent for the polyimide should be selected. In addition, viscosity of the dope should be adjusted to the range in which the dope is easy to cast or to spin. Further, it is important to select the coagulation solvent since the solvent exchanging rate in the coagulation process affects the structure of the membrane. 

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