Solvent-coagulant miscibility, coagulated polymers

Fig. 9. Effect of the solvent-coagulant miscibility on polymer structure, (a) ei 2 = 0, (b) si-2 = 0.5, (c) ei-2 = 0.77, and (d) si-2 = 1.5. The structures are for a coagulation time t = 0.02 s, except (d) which is for t = 0.03 s. All the other parameters are the same as in Figure 8. The extent of shrinking of the poljrmer-coagulant interfaces with respect to their initial positions have not been represented. Courtesy of Journal of Polymer Science.

Importance of the Solvent-Coagulant Miscibility. To recall, the miscibility between solvent and coagulant is, in this model, controlled by the interaction energy parameter i 2- A study of the effect of that parameter on the morphology of the precipitated polymer is presented in Figures 9a-d, for approximately the same coagulation time t = 0.02-0.03 s. For perfect mixing (ei 2 =... 

Wet spinning also involves pumping a solution to the spinneret. Now, however, the polymer is precipitated in an immiscible liquid. Polyacrylonitrile in dimethyl formamide, for example, can be precipitated by passing a jet of the solution through a bath of water, which is miscible with the solvent but causes the polymer to coagulate. Cellulose triacetate can be wet-spun from a methylene chloride-alcohol mixture into a toluene bath, where it precipitates. In other fibers, the precipitation can involve a chemical reaction. Viscose rayon is made by regenerating cellulose from a solution of cellulose xanthate. 

Most of the membranes listed in Table 5.20 are formed through phase separation processes, i.e., melt extrusion or coagulation of a polymer solution by a nonsolvent. In melt extrusion, a polymer melt is extruded into a cooler atmosphere which induces phase transition. The melt extrusion of a single polymer usually gives a dense, isotropic membrane. However, the presence of a compound (latent solvent) that is miscible with the polymer at the extrusion temperature but not at the ambient temperature, may lead to a secondary phase separation upon cooling. Removal of the solvent then yields a porous isotropic membrane. Anisotropic membranes may result from melt extrusion of a dope mixture of polymers containing plasticizers. 

How does the choice 6f the solvent now influence the membrane structure when water is used as the nonsolvent and cellulose acetate as the polymer The first interesting point is that the slope of the tie lines, which connect the two phases in equilibrium in the two-phase region, is less steep when the mumal affinity (or miscibility) between the solvent and the nonsolvent decreases [35,43]. The binodal and tie lines are depicted in figure in - 43 for the system water/solvent/CA, where the tie lines become steeper as the miscibility with water increases in the order DMF > dioxan > acetone >THE Light transmission measurements conducted on the same water/solvent/CA systems are shown in figure III - 44. When DMSO (e , DMF (d) and dioxan (c) are used as the solvent, instantaneous demixing occurs. Only when the solvent is added to the coagulation bath is... 

In this process, a polymer dissolved in a water miscible solvent is coated thinly onto the fabric. The porous structure is developed by passing through a coagulation bath where water displaces the solvent. For example, the textile substrate is coated with polyurethane solution, where DMF is used exclusively as the solvent, and the fabric is passed through a coagulation bath containing water, where water displaces the solvent to give a porous structure. 

Solubility parameters are indexes usually employed to evaluate the interactions between the polymer, solvent, and nonsolvent, such as the solvent s ability to dissolve a given polymer, the miscibility between solvent and nonsolvent, and the coagulation power of a nonsolvent toward a polymer of interest. These interactions will strongly affect the path followed by the P/S/NS system during PI, and hence the final membrane morphology. 

Macrovoids formation can be avoided by delayed demixing, and increasing polymer and/or nonsolvent concentration in the polymeric dope. Other studies suggested to introduce solvent into the coagulation bath [21-23], to increase solvent evaporation time [24], to work with a S-NS pair with low miscibility [25] or to use organic additives such as polyvinylpyrrolidone (PVP) [26,27]. 

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