Phase separation/inversion process

The majority of polymeric membranes can be formed via a phase separation/inversion process by which a polymer solution (in which the solvent is the continuous phase) inverts into a swollen three-dimensional macromolecular network or gel (where the polymer is the continuous phase). Phase separation can be induced by solvent evaporation (the dry-cast process), nonsolvent/solvent exchange (the wet-cast process), cooling (the thermal-cast process), and polymer leaching (the polymer-assisted process). 

The thermoplastic-rich phase may be separated in the course of polymerization (Sec. 13.4.2) or can be incorporated as a dispersed powder in the initial formulation (Sec. 13.4.3). A strong drawback of the in situ-phase separation for processing purposes is the high viscosity of the initial solution which results from the much higher average molar mass of the TP compared with the liquid rubbers. Also, for the same reason, the critical concentration crit has a smaller value (phase inversion is observed at smaller concentrations of modifier). 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

One particularly interesting system is the epoxy 2,6-dimethyl-4-heptanone as up to 40 wt % of this solvent can be easily mixed together with the epoxy precursors to generate a phase separation process. This allows one to verify experimentally the possible morphologies which were predicted based on the schematic phase diagram at concentrations below the phase inversion (see Fig. 7). Shown... 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Composite MIP membranes can also be obtained by incorporation of MIP particles into the membrane polymer matrix by mixing the particles in an appropriate solvent with the membrane-forming polymer that is then solidified by the phase inversion process. Membranes thus prepared were used for separation, with targets such as tetracycline [257], theophylline [255], methylphosphonic acid [258], bisphenol [259], indole derivatives [260], propanolol [261], luteolin [262] and norfloxacin... 

The preparation of ASA in bulk or bulk-suspension polymerization processes has been described by McKee et al. [14-18]. The system is similar to that used in the preparation of high-impact polystyrene (HIPS) and in bulk-produced ABS. Thereby the rubber was prepared using free radical polymerization, dissolved in the SAN monomers which were then polymerized using free radicals. Phase separation between the rubber and PSAN occurred, followed by phase inversion. 

Compared with the liquid-phase inversion method, this phase separation process has several advantages [59] ... 

The first moment q t) of the structure factor is also computed in order to observe the coarsening processes in the later stage of phase separation more clearly. This quantity q t), the inverse of which is a measure of the average domain size, is defines as... 

Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique and the temperature induced phase separation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development... 

Relying heavily on their established expertise In the field of polymer solution thermodynamics, Ramlde and Manabe (12) take a more detailed look at the mechanism of phase-separation which occurs during membrane formation. In this context, phase separation Is the process which takes place during phase Inversion. A theory Is developed which allows for the prediction of pore characteristics... 

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