Evaporation induced phase separation

Evaporation induced phase separation (EIPS) - where the homogeneous polymer solution contains two or more solvents of different dissolution capacities, the more volatile solvent can be evaporated. 

Thermally Induced Phase Separation In the TIPS process, an initially homogeneous solution consisting of a polymer and solvent(s) phase separates due to a decrease in the solvent quality when the temperature of the casting solution is decreased. After demixing is induced, the solvent is removed by extraction, evaporation, or freeze drying. 

In the case of polystyrene blends with poly(vinyl methyl ether) two phase behaviour was found for blends from various chlorinated solvents whereas single phase behaviour was found for blends from toluene The phase separation of mixtures of these polymers in various solvents has been studied and the interaction parameters of the two polymers with the solvents measured by inverse gas chromatography It was found that those solvents which induced phase separation were those for which a large difference existed between the two separate polymer-solvent interaction parameters. This has been called the A% effect (where A% = X 2 Xi 3)-A two phase region exists within the polymer/polymer/solvent three component phase diagram as shown in Fig. 2. When a dilute solution at composition A is evaporated, phase separation takes place at B and when the system leaves the two phase region, at overall... 

Figure 1. Schematic plots of polymer/polymer/solvent phase diagrams for (a) two miscible polymers plus a solvent Inducing phase separation, (b) two immiscible polymers. The lines AB and CD show evaporation pathways.

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

Another scaffold fabrication technique is thermally induced phase separation (TIPS) [34,46,84,147-149]. TIPS involves decreasing the temperature of a polymer solution to obtain a polymer-rich and polymer-poor phase. Following phase separation, the solvent is removed using one of a number of methods (freeze drying, evaporation) resulting in the formation of pores in the polymer stracture. TIPS can also be combined with the use of porogens to increase void fraction, have better control over pore size, or improve pore interconnectivity. 

Phase inversion is known to be an effective way to create porous structures in membranes, where a competitive mutual diffusion between solvent and nonsolvent occurs to yield the porous structure. Phase inversion can be described as a demixing process whereby the initially homogeneous polymer solution is transformed in a controlled manner from a liquid to a solid state [24]. Apart from immersion in a nonsolvent bath, or immersion precipitation (IP), a variety of related techniques, such as precipitation by solvent evaporation, precipitation by absorption of water Irom the vapor phase, and precipitation by air cooling, corresponding to thermally induced phase separation (TIPS), vapor-induced phase separation (VIPS), and air-casting phase separation... 

Solvent-induced phase separation occurs when both liquid crystal and polymer are combined in a homogeneous solution the liquid crystal grows (as microdroplets) as the solvent evaporates. In such a segregation mechanism, the rate... 

Thermally-induced phase separation (TIPS) has been shown to be an excellent way to make microporous polymeric membranes. Microporous membranes are generally prepared by TIPS process, which is based on the phenomenon that the solvent quality decreases when the temperature is decreased. On removing the thermal energy by cooling or quenching, a polymer-diluent solution phase separation occurs. After the phase separation, the diluent is removed, typically by solvent extraction, and the extractant is evaporated to yield a microporous structure. Typically, the TIPS process has been used to produce isotropic structures that is, the pore size does not vary with direction in the membrane. A few studies have been reported on the formation of... 

It is expected [16-18] that thermally induced phase separation can be reversed when polymer blends are annealed below their respective LCST s if the system is in equilibrium. However, in the present PES/PI systems, as in the PBI/PI systems [19], reversibility is not observed. It is obvious that the data presented above represent only apparent phase boundary curves. Because of the ridigity of the PES and PI chains, the mobility of the segments is limited and the system is highly viscous. The observed one-phase system corresponds to a homogeneous "frozen" structure fomied from a solution of the constituents as the solvent evaporates. When relaxation times are sufficiently reduced, the chains have sufficient mobility to fomi a stable two-phase state. The location of the true phase boundary curve in the present case for PES/PI blends may lie below the Tg-composition line. 

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