Gelation phase Inversion membranes

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

A partial list of commonly encountered structural irregularities in phase Inversion membranes includes irregular gelation, wavemarks, macrovoids and blushing. 

Most ultrafiltration membranes are porous, asymmetric, polymeric stmctures produced by phase inversion, ie, the gelation or precipitation of a species from a soluble phase (see Membrane technology). 

Most ultrafiltration membranes are porous, asymmetric, polymeric structures produced by phase inversion, i.e., the gelation or precipitation of a species from a soluble phase. See also Membrane Separations Technology. Membrane structure is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc.). Commonly used polymers include cellulose acetates, polyamides, polysulfoncs, dyncls (vinyl chlondc-acrylonitrile copolymers) and puly(vinylidene fluoride). 

In this process phase inversion is introduced by lowering the temperature of the polymer solution. A polymer is mixed with a substance that acts as a solvent at a high temperature and the polymer solution is cast into a film. When the solution is cooled, it enters into an immiscible region due to the loss of solvent power. Liquid-liquid demixing occurs and the solution is separated into two phases, i.e., the polymer-lean phase is dispersed as droplets in the polymer-rich phase. Further, cooling causes gelation of polymer. Because the solvent is usually nonvolatile, it must be removed with a liquid that is miscible with the solvent but not miscible with the polymer. The membranes made by the TIPS method have pore sizes in the range of 0.1 and 1 pm and the pore structure is uniform in the depth direction. ... 

Water is, in most Instances, a strong nonsolvent making it a causative factor in the occurrence of blushing. After phase inversion and gelation but prior to capillary depletion, the membrane gel is still filled with the less volatile nonsolvent... 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

Membranes used in microfiltration, reverse osmosis, dialysis, and gas separation are usually prepared by the wet-extrusion process, since it can be used to produce almost every membrane morphology. In the process, homogeneous solutions of the polymers are made in solvent and nonsolvent mixtures, while phase inversion is achieved by any of the several processes, such as solvent evaporation, exposure to excess nonsolvent, and thermal gelation. In most formulations, polymer solutions of 15-40 wt% concentration are cast or spun and subsequently coagulated in a bath containing a nonsolvent (usually water). 

Khayet and Matsuura (2004) studied the separation of chloroform-water mixture via PVDF flat membrane by using both PV and VMD techniques. Both PV and VMD membranes were prepared using the phase inversion method and the same polymer material. VMD membranes with different pore sizes were prepared using pure water as a pore-forming additive in the PVDF/dimethylacetamide (DMAC) casting solution, whereas PV membranes were obtained with higher polymer concentration, without nonsolvent additives (water) and with solvent evaporation before gelation. A comparative study was made between both. 

As can be seen from the above description, there are many variables involved in the phase-inversion technique. Among others the composition of the polymer solution, the solvent evaporation temperature and evaporation period, the nature and the temperature of the gelation media, and the heat treatment temperature are the primary factors affecting the reverse osmosis performance of the membrane. When polymers other than cellulose acetate are used, solvents and nonsolvent additives appropriate to prepare membranes from the particular polymer must be found. Depending on the combination of variables, membranes of different polymeric materials with different pore sizes can be prepared. 

Equation 3.88 further indicates that 4>n and p are linearly related, which means that the concentration paths on the triangular diagram should be straight lines. This aspect of phase inversion was confirmed for the wet spinning of fibers, which shares many common features with the formation of membranes. Figures 3.22 and 3.23 Illustrate such concenU ation paths for some sets of k and P2 gelation media is water, and therefore p is 1.0 x 10 kg/m. The... 

This is one of the phase inversion processes, in which the cast film is immersed in a nonsolvent (gelation media). Polymeric membrane soUdifies as the nonsolvent defuses in and solvent defuses out of the film. This method is used to prepare asymmetric membranes. 

In a wet or dry-wet process of phase inversion, the thermodynamic properties of the polymer solution and gelation medium give us some information on the overall porosity of a final membrane but not on the pore size and its distribution. The pore size and its distribution are mainly controlled by kinetic effects. This means that upon the immersion of polymer solution into a coagulation bath, mass transfer mainly determines the asymmetric structure of the membrane. The mass transfer is normally expressed by the exchange rate of solvent/nonsolvent at the interface between the polymer solution and the gelation medium. This exchange rate depends upon the nonsolvent tolerance of the polymer solution, the solvent viscosity and so on [14]. 

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