Skin formation, polymer coagulation

This method was developed by Zsigmondy in the early part of the last century. A cast polymer solution that consists of polymer and solvent is brought into nonsolvent vapor environment saturated with solvent vapor. While saturated solvent vapor suppresses the evaporation of solvent from the film, nonsolvent vapor diffuses into the film causing polymer coagulation. The formation of the dense skin layer does not occur. Pore sizes are uniform, perpendicular to the membrane surface. 

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. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [414], resulting in the formation of a crystalline fiber with a surface skin. Variations in the structure produced by annealing at elevated temperature are known to increase the fiber modulus due to a more perfect alignment of the molecules [472]. The chemistry and physics of the aromatic polyamide fibers have been reviewed [419]. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [337], resulting in the formation of a crystalline fiber with an exterior surface skin. 

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