Binodal region

On the contrary, in the second binodal region (b) in Fig. 20.1-7, where the local mixing point finds the polymer-rich solution as the continuous phase, dispersed spharaids of nesrly polymer-free fluid are nucleated. These do meins then coalesce to p red nee a foam structure whose walls are composed of the solidified dispersed polymer phase. To obtain an open-cell foam with low resists ace to flow, defects clearly must occur in the waits of (be cells.39 Soch a structure is shown in Fig. 20.1-66. Tbe dense film on the surface can be promoted by a brief exposure of the cast or apan nascent membrane to air to obtain a more concentrated region at the surfhee prior to immersion in the nonsolvent precipitation hath, which then sets the dease layer in place and proceeds to nucleate the subetracture as described above, This evaporation step, however, is not required in all cases to produce acceptuble skins.56,65,66... 

Binodal points represent the points of contact of a common tangent to A vs. V at constant temperature and composition when a region of negative curvature exists between two regions of positive curvature. The locus of binodal points, known as the binodal curve or two-phase envelope, represents the experimentally observed phase boundary under normal conditions. For example, saturated liquid and saturated vapor represent states on the binodal curve. The binodal region exists between the binodal and spinodal curves, where p/ V)T,aa jv < 0. 

Stable states defined by positive curvature on the A-V diagram at constant temperature and composition, where (dp/dV)T,aa Ni < 0. Stable equilibrium states exist outside the binodal region where single-phase behavior prevails. Subcooled liquids and superheated vapors represent examples of stable states. 

Metastable states equilibrium states within the binodal region where (dp/dV)T,aa Ni < 0. Superheated liquids and subcooled vapors represent examples of metastable states. They can be isolated and studied because small fluctuations in system properties that normally trigger phase separation are quenched. 

Metastable states equilibrium states that exist within the binodal region, where (92Agmixmg/9y )r. > 0. 

Generally, it is agreed that two fundamentally different mechanisms of phase separation can occur, as shown in Fig, 20.1-7. The so-called binodal regions in the two-phase envelope mark metastable regimes in which a single phase can exist until a nucleus (homogeneous or heterogeneous) presents itself and initiates a precipitation process. This process is similar in many respects to classical crystallization from... 

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