Polymer depleted interface

III.1. Polymer Depletion at the Solid/Solvent Interface (Fig. 5) With... 

The amount of polymer depleted from the interface region, enriching the bulk of the phases, can be approximated by integrating the total polymer concentration along the distance from the center of the interface. The result for the example of Figure 9.2 is in Figure 9.6. 

FIGURE 9.6 Quantity of polymer depleted from one side of the interface area. For parameter values, see Figure 9.2. 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

Figure II. Initial mechanisms of phase inversion (I) polymer solution interface at zero time (II) initial depletion of solvent, inward contraction, and formation of the nodular layer (III) end of contraction and establishment of the nodular layer.

It is postulated that the main thermodynamic driving force for particle adsorption at the liquid-liquid interface is the osmotic repulsion between the colloidal particles and hydrophilic starch polymer molecules. This leads to an effective depletion flocculation of particles at the boundaries of the starch-rich regions. At the same time, the gelatin has a strong tendency to adsorb at the hydrophobic surface of the polystyrene particles, thereby conferring upon them some degree of thermodynamic... 

Polymers may cause either attraction or repulsion between parts. A few important mechanisms are shown in Figure Cl-7. If the polymer does not adsorb on the interface, but has a high concentration in the bulk fluid, it will pull water away from points where particles touch. This gives a lower pressure (often quite negative ) near these touching points and forces the particles together. This depletion binding is important in toothpaste. 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

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