Asymmetric membrane formation

Tsay CS and McHugh AJ. Mass transfer modehng of asymmetric membrane formation by phase inversion. J. Polym. Sci. Polym. Phys. B 1990 28 1327-1365. 

G, B. Tanny, The Surface Tension of Polymar Solutious and Asymmetric Membrane Formation. 

Figure 2. Ternary phase diagram illustrating compositional trajectories during asymmetric membrane formation. The solid line represents the binodal and the dashed line the spinodal. The initial casting solution composition is represented by I. The transient changes in concentration at the top surface are indicated by the top arrow emanating from I and ending in II. The changes at the bottom surface are indicated by the bottom arrow emanating from I and ending in III.

BOO Boomgaard, Th. van den. Boom, R.M., and Smolders, C.A., Diffusion and phase separation in polymer solution during asymmetric membrane formation, Makromol. Chem., Macromol. Symp., 39, 271, 1990. 

LEB Leblanc, N., LeCerf, D., Chappey, C., Langevin, D., Metayer, M., and Muller, G., Influence of solvent and non-solvent on polyimide asymmetric membranes formation in relation to gas permeation, Separation Purification Technol, 22-23, 277, 2001. 

Tsay C.S. and McHugh, A.J. (1990) Mass-transfer modeling of asymmetric membrane formation by phase inversion. /. Poiym. Sci. Poiym. Phys., 28 (8), 1327—1365. 

Dense Films Considerable differences exist between formulations (dopes) for forming dense films versus asymmetric memlaunes. Typically, dense film polymer solutions consist of fewer components as compared to casting dopes for asymmetric structures. Draise films are made via the gradual removal of the solvent from the cast film, while asymmetric films are made through a phase separation mechanism. The diffoence in formation mechanisms reflects the difference in dope compositions for the two types of membranes. While in dense films, homogeneous mixing of solids and stresses at the film— support inteface are the major concerns, asymmetric membrane formation also includes complications introduced by phase separation, especially at the solid—polymCT inteface. 

Yilmaz, L., and McHugh, A. J. (1988). Modeling of asymmetric membrane formation. II. The effects of surface boundary conditions. J. Appl. Polym. Sci. 35, 1969. 

The use of synthetic polymeric membranes in separation processes [19,20] took off with the breakthrough of asymmetric membrane formation first developed by Loeb and Sourirajan [21]. These membranes have a thin dense polymer layer that governs the separation on top of a much thicker porous layer that provides mechanical support. They were first produced in the laboratory by spreading... 

Han, C. D., and L. Segal, A Study of Fiber Extrusion in Wet Spinning n. Effects of Spirming Conditions on Fiber Formation, J. Appl. Polym. Sci., 14, 2999-3019,1970. Tsay, C. S., and A. J. McHugh, Mass Transfer Modeling of Asymmetric Membrane Formation by Phase Inversion, J. Polym. Sci. Polym. Phys., 28, 1327-1365, 1990. Davankov, V A., and M. P. Tsyutupa, Structure and Properties of Hypercrosslinked Polystyrene—The First Representative of a New Class of Polymer Networks, React. Polym., 13, 27-42, 1990. 

Since Loeb and Sourlrajan s discovery (1 ) of a workable asymmetric reverse osmosis membrane, a lot of work has been done in order to elucidate and control the formation, structure and properties of a "skin", the surface layer of an asymmetric membrane. 

The aim of this study is to investigate the ability of ortho-phosphoric acid to function as the pore-producing additive for the preparation of CA reverse osmosis membranes. Ortho-phosphoric acid (PA) is known to be a strongly hydrogen bonded liquid, and it has been claimed ( ) to be a promising additive for the asymmetric CA membrane formation. To our knowledge, there has not been a thorough study on the problem. 

A few empirical and theoretical studies to postulate a general set of rules for the fabrication of asymmetric membranes by phase inversion mechanism (in which the polymer solution is coagulated within a nonsolvent bath) have been attempted. Thus, for example, from the literature which described the formation of asymmetric membranes, Klein and Smith (5) compiled working rules in the early 1970s regarding the requirements of a casting solution ... 

Wang, G-M., Chen, C-H., Ho, H-O., Wang, S-S., and Sheu, M-T. (2006), Novel design of osmotic chitosan capsules characterized by asymmetric membrane structure for in situ formation of delivery orifice, Int. J. Pharm., 319, 71-81. 

Tsai, F.-J. Torkelson, J. M., "Roles of Phase Separation Mechanism and Coarsening in the Formation of Poly(methyl methacrylate) Asymmetric Membranes," Macromolecules, 23, 775 (1990). 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

The thermodynamic description of the formation of mlcroporous systems by means of the phase diagrams, eis illustrated in Figures 1 to 3, is based on the assumption of thermodynamic equilibrium. It predicts under what conditions of temperature and composition a system will separate into two phases and the ratio of the two phases in the heterogeneous mixture. As related to the membrane formation procedure, the thermodynamic description predicts the overall porosity that will be obtained at specified states. However, no information is provided about the pore sizes, which are determined by the spatial distribution of the two phases. Equilibrium thermodynamics is not able to offer any explanation about structural variations within the membrane cross-section that is, whether the membrane has a symmetric or asymmetric structure or a dense skin at the surface. These... 

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