Chemical quench, phase separation

This technique is called Chemically Induced Phase Separation (CIPS) as the thermodynamic origin of the phase separation can be regarded as a chemical quench, as will be explained below. 

The behavior of chemical phase-separated blends in the bulk after thermal quenching into the unstable region of the phase diagram is variable. In the bulk, the concentration fluctuations that govern the phase-separation process are random. As a result, the final morphology consists of mutually interconnected domain structures rich in a given blend component that coarsen slowly with time. 

Another traditional process for fabricating a porous scaffold is phase separation. In this process a polymer solution is quenched to induce phase separation and to form a two-phase solid a polymer-rich phase and a solvent-rich phase. The solvent is then removed (typically by sublimation), leaving behind a porous foam rich in polymer. The phase separation technique does not use of harsh chemicals or elevated tempera hires and is thus suitable for the incorporation of bioactive molecules. " ... 

Both networks are formed with high reaction rate. Here there is no time for phase separation, and the structures, corresponding to the liquid one-phase state, may be frozen (such a case should be typical of the reaction injection molding processes used for the production of IPNs). This case is a real chemical quenching. 

A mean field theory of the microphase separation was proposed by Shulz for the same case of chemical quenching and weakly cross-linked IPNs. It was established that microphase separation is related to the competition that exists between elasticity and repulse of the two network components. As a result of the mean field, approximation follows a characteristic size of the microphase. The ordered phase is characterized by a macrolattice with lamellar-, hexagonal-, or body-centered cubic lattice symmetry, which is dependent on the composition and the interaction parameter, %. 

The authors themselves emphasize the two main Hmitations of this theory. The first Hmitation concerns the assumption of chemical quenching , i.e., that the polymerization kinetics and cross-finking are so rapid that any phase separation during the time needed for these processes is negligible. Their estimate of a typical time constant for the development of instability in the linear regime of spinodal decomposition, 10 s , shows that this limit may not be applicable to all cases of experimental interest. We cannot help accepting this statement. The second limitation is that their treatment does not allow us to describe the periodically modulated structures. 

Membranes used for the pressure-driven separation processes, microfiltration, ultrafiltration and reverse osmosis, as well as those used for dialysis, are most commonly made of polymeric materials 11. Initially most such membranes were cellulosic in nature. These are now being replaced by polyamide, polysulphone, polycarbonate and a number of other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation. This process has four main steps (a) the polymer is dissolved in a solvent to 10-30 per cent by mass, (b) the resulting solution is cast on a suitable support as a film of thickness, approximately 100 11 m, (c) the film is quenched by immersion in a non-solvent bath, typically... 

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