Phase separation membranes

Figure 2.29 Scanning electron micrographs at approximately the same magnification of four microporous membranes having approximately the same particle retention, (a) Nuclepore (polycarbonate) nucleation track membrane (b) Celgard (polyethylene) expanded film membrane (c) Millipore cellulose acetate/cellulose nitrate phase separation membrane made by water vapor imbibition (Courtesy of Millipore Corporation, Billerica, MA) (d) anisotropic polysulfone membrane made by the Loeb-Sourirajan phase separation process...

Phase separation membranes. This category includes membranes made by the Loeb-Sourirajan technique involving precipitation of a casting solution by immersion in a nonsolvent (water) bath. Also covered are a variety of related techniques such as precipitation by solvent evaporation, precipitation by absorption of water from the vapor phase, and precipitation by cooling. 

Table 3.1 Phase separation membrane preparation procedures...

The first phase separation membrane was developed at UCLA from 1958 to 1960 by Sidney Loeb, then working on his Master s degree, and Srinivasa Sourirajan, then a post-doctoral researcher. In their process, now called the Loeb-Sourirajan technique, precipitation is induced by immersing the cast film of polymer solution... 

Figure 3.7 Process scheme used to form Loeb-Sourirajan water precipitation phase separation membranes [14]...

Since the discovery of the Loeb-Sourirajan technique in the 1960s, development of the technology has proceeded on two fronts. Industrial users of the technology have generally taken an empirical approach, making improvements in the technique based on trial and error experience. Concurrently, theories of membrane formation based on fundamental studies of the precipitation process have been developed. These theories originated with the early industrial developers of membranes at Amicon [19,22,24] and were then taken up at a number of academic centers. Unfortunately, much of the recent academic work is so complex that many industrial producers of phase separation membranes no longer follow this literature. 

Figure 3.11 Schematic of the three-component phase diagram often used to rationalize the formation of water-precipitation phase separation membranes...

Figure 3.19 Characteristic structure of a phase-separation membrane made by water vapor absorption and solvent evaporation. (Courtesy of Millipore Corporation, Billerica, MA)...

The first major application of microfiltration membranes was for biological testing of water. This remains an important laboratory application in microbiology and biotechnology. For these applications the early cellulose acetate/cellulose nitrate phase separation membranes made by vapor-phase precipitation with water are still widely used. In the early 1960s and 1970s, a number of other membrane materials with improved mechanical properties and chemical stability were developed. These include polyacrylonitrile-poly(vinyl chloride) copolymers, poly(vinylidene fluoride), polysulfone, cellulose triacetate, and various nylons. Most cartridge filters use these membranes. More recently poly(tetrafluo-roethylene) membranes have come into use. 

The products of the thermal phase-separation membranes form a wide range of styles and configurations. Three pore sizes are currently In commercial production In polypropylene flat stock, rated at 0.45, 0.2, and 0.1 micrometers, having maximum pore sizes of about 1.0, 0.55, and 0.3 micrometers, respectively. The last two are particularly attractive for depyrogenatlon work which has been described by J. R. Robinson, et al W. A similar membrane-manufacturing process Is also used for making hollow fibers and tubes which are especially useful in cross-flow applications (10) and plasmapheresis ( ). 

Bottino A., Roda G.C., CapanneUi G., Munari S.M. (1991),The formation of micro-porous polyvinylidene difluoride membrane by phase separation, / Membrane Sci., 57,1-20. 

We next consider the situation where the particle adheres only to a phase-separated membrane domain, or raff. This case has two important differences from the homogeneous adhesive membrane. First, the finite size of the raft implies that particle size plays an important role in determining the outcome. Second, line tension at the raft boundary, or the interface, can alter the dynamics that the membrane goes through as it reaches the point of complete wrapping of the particle. 

---